Molecular and morphological analyses clarify species delimitation in section Costatae and reveal Betula buggsii sp. nov. (sect. Costatae, Betulaceae) in China

BACKGROUND AND AIMS

Delineating closely related and morphologically similar species is difficult. Here, we integrate morphology, genetics, ploidy and geography to resolve species and subspecies boundaries in four trees of section Costatae (genus Betula): Betula ashburneri, B. costata, B. ermanii and B. utilis, as well as multiple subspecies and polyploid races.

METHODS

We genotyped 371 individuals (20-133 per species) from 51 populations at 15 microsatellite markers, as well as a subset of individuals, using restriction-site associated DNA sequencing and nuclear internal transcribed spacers. We determined the ploidy level of eight individuals using flow cytometry and characterized leaf variation for a subset of 109 individuals by morphometric analysis.

KEY RESULTS

Integration of multiple lines of evidence suggested a series of revisions to the taxonomy of section Costatae. Betula costata and B. ermanii were found to be valid. Molecular and leaf morphology analyses revealed little differentiation between diploid B. albosinensis and some samples of B. utilis ssp. utilis. By contrast, other B. utilis ssp. utilis samples and ssp. albosinensis formed a morphological continuum but differed based on genetics. Specifically, B. utilis ssp. albosinensis was divided into two groups with group I genetically similar to B. utilis ssp. utilis and group II, a distinct cluster, proposed as the new diploid species Betula buggsii sp. nov. Phylogenomic analysis based on 2285 620 single nucleotide polymorphisms identified a well-supported monophyletic clade of B. buggsii. Morphologically, B. buggsii is characterized by elongated lenticels and a distinct pattern of bark peeling and may be geographically restricted to the Qinling-Daba Mountains.

CONCLUSIONS

Our integrated approach identifies six taxa within section Costatae: B. ashburneri, B. buggsii, B. costata, B. utilis ssp. utilis, B. utilis ssp. albosinensis and B. ermanii. Our research demonstrates the value of an integrative approach using morphological, geographical, genetic and ploidy-level data for species delineation.

Experimental evidence for species-dependent responses in leaf shape to temperature: Implications for paleoclimate inference

In many woody dicot plant species, colder temperatures correlate with a greater degree of leaf dissection and with larger and more abundant leaf teeth (the serrated edges along margins). The measurement of site-mean characteristics of leaf size and shape (physiognomy), including leaf dissection and tooth morphology, has been an important paleoclimate tool for over a century. These physiognomic-based climate proxies require that all woody dicot plants at a site, regardless of species, change their leaf shape rapidly and predictably in response to temperature. Here we experimentally test these assumptions by growing five woody species in growth cabinets under two temperatures (17 and 25°C). In keeping with global site-based patterns, plants tend to develop more dissected leaves with more abundant and larger leaf teeth in the cool treatment. Overall, this upholds the assumption that leaf shape responds in a particular direction to temperature change. The assumption that leaf shape variables respond to temperature in the same way regardless of species did not hold because the responses varied by species. Leaf physiognomic models for inferring paleoclimate should take into account these species-specific responses.

Leaf structural and hydraulic adjustment with respect to air humidity and canopy position in silver birch (Betula pendula)

Climate change scenarios predict an increase in air temperature and precipitation in northern temperate regions of Europe by the end of the century. Increasing atmospheric humidity inevitably resulting from more frequent rainfall events reduces water flux through vegetation, influencing plants' structure and functioning. We investigated the extent to which artificially elevated air humidity affects the anatomical structure of the vascular system and hydraulic conductance of leaves in Betula pendula. A field experiment was carried out at the Free Air Humidity Manipulation (FAHM) site with a mean increase in relative air humidity (RH) by 7% over the ambient level across the growing period. Leaf hydraulic properties were determined with a high-pressure flow meter; changes in leaf anatomical structure were studied by means of conventional light microscopy and digital image processing techniques. Leaf development under elevated RH reduced leaf-blade hydraulic conductance and petiole conductivity and had a weak effect on leaf vascular traits (vessel diameters decreased), but had no significant influence on stomatal traits or tissue proportions in laminae. Both hydraulic traits and relevant anatomical characteristics demonstrated pronounced trends with respect to leaf location in the canopy-they increased from crown base to top. Stomatal traits were positively correlated with several petiole and leaf midrib vascular traits. The reduction in leaf hydraulic conductance in response to increasing air humidity is primarily attributable to reduced vessel size, while higher hydraulic efficiency of upper-crown foliage is associated with vertical trends in the size of vascular bundles, vessel number and vein density. Although we observed co-ordinated adjustment of vascular and hydraulic traits, the reduced leaf hydraulic efficiency could lead to an imbalance between hydraulic supply and transpiration demand under the extreme environmental conditions likely to become more frequent in light of global climate change.

Is xylem of angiosperm leaves less resistant to embolism than branches? Insights from microCT, hydraulics, and anatomy

According to the hydraulic vulnerability segmentation hypothesis, leaves are more vulnerable to decline of hydraulic conductivity than branches, but whether stem xylem is more embolism resistant than leaves remains unclear. Drought-induced embolism resistance of leaf xylem was investigated based on X-ray microcomputed tomography (microCT) for Betula pendula, Laurus nobilis, and Liriodendron tulipifera, excluding outside-xylem, and compared with hydraulic vulnerability curves for branch xylem. Moreover, bordered pit characters related to embolism resistance were investigated for both organs. Theoretical P50 values (i.e. the xylem pressure corresponding to 50% loss of hydraulic conductance) of leaves were generally within the same range as hydraulic P50 values of branches. P50 values of leaves were similar to branches for L. tulipifera (-2.01 versus -2.10 MPa, respectively), more negative for B. pendula (-2.87 versus -1.80 MPa), and less negative for L. nobilis (-6.4 versus -9.2 MPa). Despite more narrow conduits in leaves than branches, mean interconduit pit membrane thickness was similar in both organs, but significantly higher in leaves of B. pendula than in branches. This case study indicates that xylem shows a largely similar embolism resistance across leaves and branches, although differences both within and across organs may occur, suggesting interspecific variation with regard to the hydraulic vulnerability segmentation hypothesis.

Linking fine root morphology, hydraulic functioning and shade tolerance of trees

Background and Aims

Understanding root traits and their trade-off with other plant processes is important for understanding plant functioning in natural ecosystems as well as agricultural systems. The aim of the present study was to determine the relationship between root morphology and the hydraulic characteristics of several orders of fine roots (<2 mm) for species differing in shade tolerance (low, moderate and high).

Methods

The morphological, anatomical and hydraulic traits across five distal root orders were measured in species with different levels of shade tolerance and life history strategies. The species studied were Acer negundo, Acer rubrum, Acer saccharum, Betula alleghaniensis, Betula lenta, Quercus alba, Quercus rubra, Pinus strobus and Pinus virginiana.

Key Results

Compared with shade-tolerant species, shade-intolerant species produced thinner absorptive roots with smaller xylem lumen diameters and underwent secondary development less frequently, suggesting that they had shorter life spans. Shade-tolerant species had greater root specific hydraulic conductance among these roots due to having larger diameter xylems, although these roots had a lower calculated critical tension for conduit collapse. In addition, shade-intolerant species exhibited greater variation in hydraulic conductance across different root growth rings in woody transport roots of the same root order as compared with shade-tolerant species.

Conclusions

Plant growth strategies were extended to include root hydraulic properties. It was found that shade intolerance in trees was associated with conservative root hydraulics but greater plasticity in number of xylem conduits and hydraulic conductance. Root traits of shade-intolerant species were consistent with the ability to proliferate roots quickly for rapid water uptake needed to support rapid shoot growth, while minimizing risk in uncertain environments.

Convergence in leaf size versus twig leaf area scaling: do plants optimize leaf area partitioning?

BACKGROUND AND AIMS

Corner's rule states that thicker twigs bear larger leaves. The exact nature of this relationship and why it should occur has been the subject of numerous studies. It is obvious that thicker twigs should support greater total leaf area ([Formula: see text]) for hydraulical and mechanical reasons. But it is not obvious why mean leaf size ([Formula: see text]) should scale positively with [Formula: see text] We asked what this scaling relationship is within species and how variable it is across species. We then developed a model to explain why these relationships exist.

METHODS

To minimize potential sources of variability, we compared twig properties from six co-occurring and functionally similar species: Acer grandidentatum, Amelanchier alnifolia, Betula occidentalis, Cornus sericea, Populus fremontii and Symphoricarpos oreophilus We modelled the economics of leaf display, weighing the benefit from light absorption against the cost of leaf tissue, to predict the optimal [Formula: see text] combinations under different canopy openings.

KEY RESULTS

We observed a common [Formula: see text] by [Formula: see text] exponent of 0.6, meaning that [Formula: see text]and leaf number on twigs increased in a specific coordination. Common scaling exponents were not supported for relationships between any other measured twig properties. The model consistently predicted positive [Formula: see text] by [Formula: see text] scaling when twigs optimally filled canopy openings. The observed 0·6 exponent was predicted when self-shading decreased with larger canopy opening.

CONCLUSIONS

Our results suggest Corner's rule may be better understood when recast as positive [Formula: see text] by [Formula: see text] scaling. Our model provides a tentative explanation of observed [Formula: see text] by [Formula: see text] scaling and suggests different scaling may exist in different environments.

Leaf anatomy, BVOC emission and CO2 exchange of arctic plants following snow addition and summer warming

BACKGROUND AND AIMS

Climate change in the Arctic is projected to increase temperature, precipitation and snowfall. This may alter leaf anatomy and gas exchange either directly or indirectly. Our aim was to assess whether increased snow depth and warming modify leaf anatomy and affect biogenic volatile organic compound (BVOC) emissions and CO₂ exchange of the widespread arctic shrubs Betula nana and Empetrum nigrum ssp. hermaphroditum METHODS: Measurements were conducted in a full-factorial field experiment in Central West Greenland, with passive summer warming by open-top chambers and snow addition using snow fences. Leaf anatomy was assessed using light microscopy and scanning electron microscopy. BVOC emissions were measured using a dynamic enclosure system and collection of BVOCs into adsorbent cartridges analysed by gas chromatography-mass spectrometry. Carbon dioxide exchange was measured using an infrared gas analyser.

KEY RESULTS

Despite a later snowmelt and reduced photosynthesis for B. nana especially, no apparent delays in the BVOC emissions were observed in response to snow addition. Only a few effects of the treatments were seen for the BVOC emissions, with sesquiterpenes being the most responsive compound group. Snow addition affected leaf anatomy by increasing the glandular trichome density in B. nana and modifying the mesophyll of E. hermaphroditum The open-top chambers thickened the epidermis of B. nana, while increasing the glandular trichome density and reducing the palisade:spongy mesophyll ratio in E. hermaphroditum CONCLUSIONS: Leaf anatomy was modified by both treatments already after the first winter and we suggest links between leaf anatomy, CO₂ exchange and BVOC emissions. While warming is likely to reduce soil moisture, melt water from a deeper snow pack alleviates water stress in the early growing season. The study emphasizes the ecological importance of changes in winter precipitation in the Arctic, which can interact with climate-warming effects.

Waterlogging in late dormancy and the early growth phase affected root and leaf morphology in Betula pendula and Betula pubescens seedlings

The warmer winters of the future will increase snow-melt frequency and rainfall, thereby increasing the risk of soil waterlogging and its effects on trees in winter and spring at northern latitudes. We studied the morphology of roots and leaves of 1-year-old silver birch (Betula pendula Roth) and pubescent birch (Betula pubescens Ehrh.) seedlings exposed to waterlogging during dormancy or at the beginning of the growing season in a growth-chamber experiment. The experiment included 4-week dormancy (Weeks 1-4), a 4-week early growing season (Weeks 5-8) and a 4-week late growing season (Weeks 9-12). The treatments were: (i) no waterlogging, throughout the experiment ('NW'); (ii) 4-week waterlogging during dormancy (dormancy waterlogging 'DW'); (iii) 4-week waterlogging during the early growing season (growth waterlogging 'GW'); and (iv) 4-week DW followed by 4-week GW during the early growing season ('DWGW'). Dormancy waterlogging affected the roots of silver birch and GW the roots and leaf characteristics of both species. Leaf area was reduced in both species by GW and DWGW. In pubescent birch, temporarily increased formation of thin roots was seen in root systems of GW seedlings, which suggests an adaptive mechanism with respect to excess soil water. Additionally, the high density of non-glandular trichomes and their increase in DWGW leaves were considered possible morphological adaptations to excess water in the soil, as was the constant density of stem lenticels during stem-diameter growth. The higher density in glandular trichomes of DWGW silver birch suggests morphological acclimation in that species. The naturally low density of non-glandular trichomes, low density of stem lenticels in waterlogged seedlings and decrease in root growth seen in DWGW and DW silver birch seedlings explain, at least partly, why silver birch grows more poorly relative to pubescent birch in wet soils.

The effects of short- and long-term air pollutants on plant phenology and leaf characteristics

Pollution adversely affects vegetation; however, its impact on phenology and leaf morphology is not satisfactorily understood yet. We analyzed associations between pollutants and phenological data of birch, hazel and horse chestnut in Munich (2010) along with the suitability of leaf morphological parameters of birch for monitoring air pollution using two datasets: cumulated atmospheric concentrations of nitrogen dioxide and ozone derived from passive sampling (short-term exposure) and pollutant information derived from Land Use Regression models (long-term exposure). Partial correlations and stepwise regressions revealed that increased ozone (birch, horse chestnut), NO2, NOx and PM levels (hazel) were significantly related to delays in phenology. Correlations were especially high when rural sites were excluded suggesting a better estimation of long-term within-city pollution. In situ measurements of foliar characteristics of birch were not suitable for bio-monitoring pollution. Inconsistencies between long- and short-term exposure effects suggest some caution when interpreting short-term data collected within field studies.

Climate change alters leaf anatomy, but has no effects on volatile emissions from Arctic plants

Biogenic volatile organic compound (BVOC) emissions are expected to change substantially because of the rapid advancement of climate change in the Arctic. BVOC emission changes can feed back both positively and negatively on climate warming. We investigated the effects of elevated temperature and shading on BVOC emissions from arctic plant species Empetrum hermaphroditum, Cassiope tetragona, Betula nana and Salix arctica. Measurements were performed in situ in long-term field experiments in subarctic and high Arctic using a dynamic enclosure system and collection of BVOCs into adsorbent cartridges analysed by gas chromatography-mass spectrometry. In order to assess whether the treatments had resulted in anatomical adaptations, we additionally examined leaf anatomy using light microscopy and scanning electron microscopy. Against expectations based on the known temperature and light-dependency of BVOC emissions, the emissions were barely affected by the treatments. In contrast, leaf anatomy of the studied plants was significantly altered in response to the treatments, and these responses appear to differ from species found at lower latitudes. We suggest that leaf anatomical acclimation may partially explain the lacking treatment effects on BVOC emissions at plant shoot-level. However, more studies are needed to unravel why BVOC emission responses in arctic plants differ from temperate species.

Effect of wood ash on leaf and shoot anatomy, photosynthesis and carbohydrate concentrations in birch on a cutaway peatland

Trees in cutaway peatland are growing in difficult conditions. Fertilization with nutrient-rich wood ash helps improve growth conditions. Photosynthesis and carbohydrate concentration along leaf anatomy were studied on plots treated with 10 and 5 t ha(-1) wood ash (WA10 and WA5) and on untreated (Control) plot to explain the physiological background of the differences in tree growth. The leaves from WA10 had the largest leaf area, total thickness, the thickest mesophyll and also significantly larger average values of all anatomical parameters of the shoots. The photosynthetic assimilation was significantly higher on treated plots at 200 and 400 ppm CO2 levels. In leaves on the treated plots, the sucrose concentration was lower while that of starch was higher than in trees on untreated soil. The differences in the maximum photosynthesis were relatively small. At unit ground, the leaf area provided for a wood ash-treated tree an efficient surface for CO2 assimilation, light interception and some starch storage during the growing period.

Thermal acclimation of shoot respiration in an Arctic woody plant species subjected to 22 years of warming and altered nutrient supply

Despite concern about the status of carbon (C) in the Arctic tundra, there is currently little information on how plant respiration varies in response to environmental change in this region. We quantified the impact of long-term nitrogen (N) and phosphorus (P) treatments and greenhouse warming on the short-term temperature (T) response and sensitivity of leaf respiration (R), the high-T threshold of R, and associated traits in shoots of the Arctic shrub Betula nana in experimental plots at Toolik Lake, Alaska. Respiration only acclimated to greenhouse warming in plots provided with both N and P (resulting in a ~30% reduction in carbon efflux in shoots measured at 10 and 20 °C), suggesting a nutrient dependence of metabolic adjustment. Neither greenhouse nor N+P treatments impacted on the respiratory sensitivity to T (Q10 ); overall, Q10 values decreased with increasing measuring T, from ~3.0 at 5 °C to ~1.5 at 35 °C. New high-resolution measurements of R across a range of measuring Ts (25-70 °C) yielded insights into the T at which maximal rates of R occurred (Tmax ). Although growth temperature did not affect Tmax , N+P fertilization increased Tmax values ~5 °C, from 53 to 58 °C. N+P fertilized shoots exhibited greater rates of R than nonfertilized shoots, with this effect diminishing under greenhouse warming. Collectively, our results highlight the nutrient dependence of thermal acclimation of leaf R in B. nana, suggesting that the metabolic efficiency allowed via thermal acclimation may be impaired at current levels of soil nutrient availability. This finding has important implications for predicting carbon fluxes in Arctic ecosystems, particularly if soil N and P become more abundant in the future as the tundra warms.

Genetic diversity of F1 and F2 interspecific hybrids between dwarf birch (Betula nana L.) and Himalayan birch (B. utilis var. jacquemontii (Spach) Winkl. 'Doorenbos') using RAPD-PCR markers and ploidy analysis

Crosses between Betula nana and B. utilis 'Doorenbos' were undertaken in order to obtain interspecific hybrids which could be characterized by wide spreading stems, strong branching habit, decorative clear white bark and an interesting shape of purple leaves. The research purpose was to examine genetic diversity of the 16 F1 and F2 putative progenies by using the RAPD-PCR method and the ploidy analysis. A total of 242 RAPD markers were scored with 24 primers and 220 (90.9%) polymorphic bands were found. In the NJ dendrogram, cluster I consisted of the female parent--B. nana and 12 hybrids and cluster II grouped the male parent--B. utilis 'Doorenbos' with 4 hybrids (F2/2, F1/8, F1/7 and F2/1). The 2-D scaling by PCoA was in agreement with the similarity index, i.e. two hybrids (F1/8, F2/2) grouped with the male parent while others with female parent. Classification of the hybrid plants by chromosome counting demonstrated that 13 hybrids were confirmed with accurate chromosome counts as being diploid (2n=2x=28) and 3 plants (F1/7, F1/8, F2/2) as triploid with 42 chromosomes.

Wood properties of Populus and Betula in long-term exposure to elevated CO₂ and O₃

We studied the interactive effects of elevated concentrations of CO2 and O3 on radial growth and wood properties of four trembling aspen (Populus tremuloides Michx.) clones and paper birch (Betula papyrifera Marsh.) saplings. The material for the study was collected from the Aspen FACE (free-air CO2 enrichment) experiment in Rhinelander (WI, USA). Trees had been exposed to four treatments [control, elevated CO2 (560 ppm), elevated O3 (1.5 times ambient) and combined CO2 + O3 ] during growing seasons 1998-2008. Most treatment responses were observed in the early phase of experiment. Our results show that the CO2- and O3-exposed aspen trees displayed a differential balance between efficiency and safety of water transport. Under elevated CO2, radial growth was enhanced and the trees had fewer but hydraulically more efficient larger diameter vessels. In contrast, elevated O3 decreased radial growth and the diameters of vessels and fibres. Clone-specific decrease in wood density and cell wall thickness was observed under elevated CO2 . In birch, the treatments had no major impacts on wood anatomy or wood density. Our study indicates that short-term impact studies conducted with young seedlings may not give a realistic view of long-term ecosystem responses.

[Simulation of leaf inclination angle distribution of main tree species in Daxing'an Mountains of China based on the Campbell ellipsoid distribution function]

Leaf inclination angle distribution directly decides the amount of radiation interception by vegetation canopy, and also, decides the size and direction of the incident radiation, being the key parameter in quantitative remote sensing. This paper simulated the leaf inclination angle distribution of the main tree species in Daxing'an Mountains forest region based on the Campbell ellipsoid distribution model and iterative method, and quantitatively analyzed the fitting results of canopy with and without leaf stratification as well as the effects of tree age group on the leaf inclination angle distribution. For the test 6 main tree species, the leaf inclination angle distribution was in planophile shape, and the mean leaf inclination angle was smaller for coniferous tree than for broadleaved tree. Whether with or without stratify, the fitting result and the measured result were basically identical. For Betula platyphylla and Larix gmelinii, the correlation coefficient between the simulated and measured values was 0.8268 and 0.8192, and the root mean square error was 3.7% and 4.3% respectively, indicating that the Campbell model was reliable applied for forest canopy. Considering the effects of tree age group, though the leaf inclination angle distribution trend with leaf stratification had no correlation with age group, the mean leaf inclination angle of young L. gmelinii was relatively smaller than that mature one, suggesting that age group had positive effects on the numerical design of leaf inclination angle distribution and negative effects on the numerical design of extinction coefficient.

Role of axis reversal from the short-shoot to long-shoot habit for crown maintenance in slow-growing Betula maximowicziana trees

PREMISE OF THE STUDY

Branch growth and its spatial arrangement determine crown architecture, leaf display, and, thus, the productivity of trees. Branch axes elongate by the sequential production of shoots with differing morphology and function, such as short shoots and long shoots. This study investigated ontogenetic changes in axis growth in Betula maximowicziana and quantified the role of axis reversal between the short-shoot and long-shoot habits, particularly reversal from the short-shoot to the long-shoot habit.

METHODS

From 8 trees with varying levels of growth vigor, 716 branch axes forming the basic crown architecture were sampled. Past growth of the branch axes was reconstructed from leaf and bud scale scars and compared between slow-growing and vigorously growing trees.

KEY RESULTS

Branch axes reversed more frequently between the long- and short-shoot habits in slow-growing trees than in vigorously growing trees. Short-shoot-origin axes that reversed to the long-shoot habit lived for longer periods and grew larger than axes that remained in the short-shoot habit. Short-shoot-origin axes reversed as they grew away from branch apices, typically >6 yr after they had originated.

CONCLUSIONS

Reversal of short-shoot-origin axes to the long-shoot habit is an endogenous growth process of trees with reduced vigor. Like epicormic branching, the reversal may contribute to the maintenance of productivity of large old trees by prolonging axis longevity and filling the inner part of the crown. This study presents an ontogenetic change in branch growth, which broadens our perspectives on the growth and survival of long-living trees.

[Short-term death dynamics of trees in natural secondary poplar-birch forest in Changbai Mountains of Northeast China]

Taking the 5 hm2 sampling plot in the natural secondary poplar-birch forest in Changbai Mountains as test object, and based on the two census data in 2005 and 2010, an analysis was made on the main tree species composition and quantity, size class distribution of dead individuals, and regeneration characteristics of the main tree species in different habitat types of the plot in 2005-2010. In the five years, the species number of the individuals with DBH> or = 1 cm increased from 46 to 47, among which, 3 species were newly appeared, and 2 species were disappeared. The number of the individuals changed from 16509 to 15027, among which, 2150 individuals died, accounting for 13% of the whole individuals in 2005, and 668 individuals were newly increased. The basal area of the trees increased from 28.79 m2.m-2 to 30.55 m2.m-2, with that of 41 species increased while that of 6 species decreased. The decrease of the basal area of Betula platyphylla and Populus davidiana accounted for 72.3% of the total decrease. Small individuals had higher mortality, as compared with large ones, and the mortality of the individuals with DBH<5 cm occupied 65% of the total. B. platyphylla and P. davidiana contributed most in the dead individuals with large DBH. No difference was observed in the tree mortality among different habitat types, but the mortality of the individuals with different size classes showed greater variation.

Changes in crown architecture as a strategy of mountain birch for survival in habitats disturbed by pollution

Although trees in polluted areas often exhibit modified growth habits, the immediate causes of changes in crown architecture and their consequences for persistence of plant populations in disturbed habitats are not well understood. We compared individuals of mountain birch, Betula pubescens ssp. czerepanovii, growing in severely disturbed habitats (industrial barrens) surrounding a nickel-copper smelter in north-western Russia, with birches growing in unpolluted habitats. They were found to have shorter heights, a shrubby growth habit, lower depth/width and surface/foliar mass ratios of the crown, higher numbers of dead branches and twisted trunks and higher branching resulting from increased numbers of long shoots and more densely spaced buds than individuals in unpolluted forests. The increased production of long shoots was enabled by their formation not only from the axillary buds of previous-year long shoots but also from the apical buds of short shoots. These latter long shoots develop in the inner part of the crown, thus increasing the crown density. Additionally, birches from industrial barrens better compensated for mechanical damage, such as trunk/shoot removal, compared to birches from unpolluted forest and mountain tundra habitats, presumably due to the larger number of buds formed annually. The specific crown architecture of these birches can be explained by the direct effects of pollution combined with changes in microclimate due to pollution-induced forest decline. The seed progenies of birches from an industrial barren reared in a benign environment produced higher numbers of long shoots than seedlings from other habitats, suggesting that adaptive changes in crown architecture are partially shaped by the selection imposed by long-term pollution impacts. Nearly spherical and compact crowns minimise the impacts of unfavourable environmental conditions on trees and are therefore adaptive. We concluded that the development of specific crown architecture allows mountain birch to dominate in habitats that are severely disturbed by pollution.

[Estimation of leaf area index of three forest types in Xiaoxing'an Mountains of Northeast China]

From early July to early November 2009, the effective leaf area index (LAIe) of secondary Betula platyphylla forest, spruce-fir valley forest, and mixed broadleaved-Korean pine forest in Xiaoxing'an Mountains were measured with Winscanopy2006 Plant Canopy Analyzer, and the LAIe measured in early November and calibrated with the woody-to-total ratio as well as the clumping index (for clumping beyond the shoots) and the needle-to-shoot area ratio (for clumping within the shoots) was derived as the true leaf area index (LAIt). In combining with litterfall method, the LAIt and its seasonal dynamics of the three forest types were estimated. The LAIe of the secondary B. platyphylla forest reached its peak in July, with a value of 2.21, and that of the spruce-fir valley forest and mixed broadleaved-Korean pine forest peaked in August, with the values of 2.57 and 2. 68, respectively. All the three forest types had the highest LAIt in July, with the values of 3.44, 3.86, and 6.93 for the secondary B. platyphylla forest, spruce-fir valley forest, and mixed broad-leaved-Korean pine forest, respectively. In comparison with the method proposed in this study, the peak time LAIe of the secondary B. platyphylla forest, spruce-fir valley forest, and mixed broad-leaved-Korean pine forest obtained by the optical instrument method was underestimated by 33.1%, 32.9% and 66.0%, respectively, and the LAIe of the three forest types in the entire study period was underestimated averagely by -13.2%, 22.8%, and 56.5%, correspondingly.

Defining an adequate sample of earlywood vessels for retrospective injury detection in diffuse-porous species

Vessels of broad-leaved trees have been analyzed to study how trees deal with various environmental factors. Cambial injury, in particular, has been reported to induce the formation of narrower conduits. Yet, little or no effort has been devoted to the elaboration of vessel sampling strategies for retrospective injury detection based on vessel lumen size reduction. To fill this methodological gap, four wounded individuals each of grey alder (Alnus incana (L.) Moench) and downy birch (Betula pubescens Ehrh.) were harvested in an avalanche path. Earlywood vessel lumina were measured and compared for each tree between the injury ring built during the growing season following wounding and the control ring laid down the previous year. Measurements were performed along a 10 mm wide radial strip, located directly next to the injury. Specifically, this study aimed at (i) investigating the intra-annual duration and local extension of vessel narrowing close to the wound margin and (ii) identifying an adequate sample of earlywood vessels (number and intra-ring location of cells) attesting to cambial injury. Based on the results of this study, we recommend analyzing at least 30 vessels in each ring. Within the 10 mm wide segment of the injury ring, wound-induced reduction in vessel lumen size did not fade with increasing radial and tangential distances, but we nevertheless advise favoring early earlywood vessels located closest to the injury. These findings, derived from two species widespread across subarctic, mountainous, and temperate regions, will assist retrospective injury detection in Alnus, Betula, and other diffuse-porous species as well as future related research on hydraulic implications after wounding.

Variation in intra-annual wood formation, and foliage and shoot development of three major Canadian boreal tree species

PREMISE OF THE STUDY

In a warming climate, boreal trees may have adjusted their growth strategy (e.g., onset and coordination of growth among different organs such as stem, shoot, and foliage, within and among species) to cope with the extended growing seasons. A detailed investigation into growth of different organs during a growing season may help us assess the potential effects of climate change on tree growth in the boreal forest.

METHODS

The intra-annual growth of stem xylem, shoot tips, and foliage area of Pinus banksiana, Populus tremuloides, and Betula papyrifera was monitored in a boreal forest in Quebec, Canada during the growing season of 2007. Xylem formation was measured at weekly intervals, and shoot elongation and foliage expansion were measured three times per week from May to September. Growth indices for stem, shoot, and foliage were calculated and used to identify any climate-growth dependence.

KEY RESULTS

The time periods required for stem growth, branch extension, and foliage expansion differed among species. Of the three species, P. banksiana had the earliest budburst (20 May) yet the latest completion date of the foliage growth (2 August); P. tremuloides had the latest budburst (27 May) yet the earliest completion date of the foliage growth (10 July). Air temperature positively affected shoot extension growth of all three species. Precipitation positively influenced stem growth of the two broadleaf species, whereas growing season temperature positively impacted stem growth of P. banksiana.

CONCLUSION

The results show that both the timing of growth processes and environmental dependences differ among co-occurring species, thereby leading to different adaptive capability of these boreal tree species to climate change.

Species-specific effect of UV-B radiation on the temporal pattern of leaf growth

Recent molecular and physiological studies have demonstrated that ultraviolet-B radiation (UV-B) can affect some of the processes involved in leaf growth, but the phases of leaf growth affected have not been clearly delimited. We used functional growth analysis to assess the effects of UV-B radiation on the time course of leaf growth in seedlings of two birch species (Betula pendula and Betula pubescens). Our aim was to identify the phase(s) of leaf development affected by UV-B radiation. In a greenhouse study, 1-year-old birch seedlings were subjected to three daily doses of supplemental UV-B radiation treatments (UV-B⁺) and no UV-B radiation controls (UV-B⁻). Leaf growth measurements every 2 days were complemented by assessment of other functional traits over a 4-week period at the start of the growing season. Using fitted curves, we were able to determine that the rate of leaf expansion was slowed by the UV-B⁺ treatment in leaves of B. pendula because of a slower maximum leaf growth rate compared with plants under the UV-B⁻ controls, but that compensation toward the end of the period of expansion negated this difference when leaves reached their final size. UV-B⁺ had little effect on the rate of B. pubescens leaf growth despite a larger reduction in leaf final size due to UV-B⁺ than occurred in B. pendula leaves. In conclusion, effective regulation ameliorated the effects of UV-B radiation on leaf and seedling growth in B. pendula, whereas in B. pubescens, reductions in leaf final size under UV-B⁺ were consistent with a slightly reduced rate of height growth.

Leaf-traits and growth allometry explain competition and differences in response to climatic change in a temperate forest landscape: a simulation study

BACKGROUND AND AIMS

The ability to simulate plant competition accurately is essential for plant functional type (PFT)-based models used in climate-change studies, yet gaps and uncertainties remain in our understanding of the details of the competition mechanisms and in ecosystem responses at a landscape level. This study examines secondary succession in a temperate deciduous forest in eastern China with the aim of determining if competition between tree types can be explained by differences in leaf ecophysiological traits and growth allometry, and whether ecophysiological traits and habitat spatial configurations among PFTs differentiate their responses to climate change.

METHODS

A temperate deciduous broadleaved forest in eastern China was studied, containing two major vegetation types dominated by Quercus liaotungensis (OAK) and by birch/poplar (Betula platyphylla and Populus davidiana; BIP), respectively. The Terrestrial Ecosystem Simulator (TESim) suite of models was used to examine carbon and water dynamics using parameters measured at the site, and the model was evaluated against long-term data collected at the site.

KEY RESULTS

Simulations indicated that a higher assimilation rate for the BIP vegetation than OAK led to the former's dominance during early successional stages with relatively low competition. In middle/late succession with intensive competition for below-ground resources, BIP, with its lower drought tolerance/resistance and smaller allocation to leaves/roots, gave way to OAK. At landscape scale, predictions with increased temperature extrapolated from existing weather records resulted in increased average net primary productivity (NPP; +19 %), heterotrophic respiration (+23 %) and net ecosystem carbon balance (+17 %). The BIP vegetation in higher and cooler habitats showed 14 % greater sensitivity to increased temperature than the OAK at lower and warmer locations.

CONCLUSIONS

Drought tolerance/resistance and morphology-related allocation strategy (i.e. more allocation to leaves/roots) played key roles in the competition between the vegetation types. The overall site-average impacts of increased temperature on NPP and carbon stored in plants were found to be positive, despite negative effects of increased respiration and soil water stress, with such impacts being more significant for BIP located in higher and cooler habitats.

Effects of the hydraulic coupling between xylem and phloem on diurnal phloem diameter variation

Measurements of diurnal diameter variations of the xylem and phloem are a promising tool for studying plant hydraulics and xylem-phloem interactions in field conditions. However, both the theoretical framework and the experimental verification needed to interpret phloem diameter data are incomplete. In this study, we analytically evaluate the effects of changing the radial conductance between the xylem and the phloem on phloem diameter variations and test the theory using simple manipulation experiments. Our results show that phloem diameter variations are mainly caused by changes in the radial flow rate of water between the xylem and the phloem. Reducing the hydraulic conductance between these tissues decreases the amplitude of phloem diameter variation and increases the time lag between xylem and phloem diameter variation in a predictable manner. Variation in the amplitude and timing of diameter variations that cannot be explained by changes in the hydraulic conductance, could be related to changes in the osmotic concentration in the phloem.

Browsing affects intra-ring carbon allocation in species with contrasting wood anatomy

Current knowledge on tree carbon (C) allocation to wood is particularly scarce in plants subjected to disturbance factors, such as browsing, which affects forest regeneration worldwide and has an impact on the C balance of trees. Furthermore, quantifying the degree to which tree rings are formed from freshly assimilated vs. stored carbohydrates is highly relevant for our understanding of tree C allocation. We used (13)C labelling to quantify seasonal allocation of stored C to wood formation in two species with contrasting wood anatomy: Betula pubescens Ehrh. (diffuse-porous) and Quercus petraea [Matt.] Liebl. (ring-porous). Clipping treatments (66% shoot removal, and unclipped) were applied to analyse the effect of browsing on C allocation into tree rings, plus the effects on tree growth, architecture, ring width and non-structural carbohydrates (NSCs). The relative contribution of stored C to wood formation was greater in the ring-porous (55-70%) than in the diffuse-porous species (35-60%), although each species followed different seasonal trends. Clipping did not cause a significant depletion of C stores in either species. Nonetheless, a significant increase in the proportion of stored C allocated to earlywood growth was observed in clipped birches, and this could be explained through changes in tree architecture after clipping. The size of C pools across tree species seems to be important in determining the variability of seasonal C allocation patterns to wood and their sensibility to disturbances such as browsing. Our results indicate that the observed changes in C allocation to earlywood in birch were not related to variations in the amount or concentration of NSC stores, but to changes in the seasonal availability of recently assimilated C caused by modifications in tree architecture after browsing.

The effect of tree architecture on conduit diameter and frequency from small distal roots to branch tips in Betula pendula, Picea abies and Pinus sylvestris

We studied the effect of tree architecture on xylem anatomy in three Betula pendula Roth., three Picea abies (L.) H. Karst. and three Pinus sylvestris (L.) trees (mean age 35 years). First, the analysis of conduit anatomy in different tree parts showed that conduits tapered and their frequency increased from roots (≥ 2 mm) to stem, from stem to branches and further to leaf petioles in B. pendula. Conduit anatomy in lateral and main roots, as well as lateral and main branches, significantly differed from each other in all the studied species. The increase in conduit diameter and decrease in frequency from the pith to the bark were clear aboveground, but variable patterns were observed belowground. In the leaf petioles of B. pendula, conduit diameter increased and conduit frequency decreased with increasing individual leaf area. Second, the results concerning the scaling of conduit diameter were compared with the predictions of the general vascular scaling model (WBE model) and Murray's law. The scaling parameter values at the tree level corresponded with the predictions of the WBE model in all the studied trees except for one tree of both conifer species. However, the scaling parameter values changed from one tree compartment to another rather than remaining uniform inside a tree, as assumed by the WBE model. The assumptions of the WBE model of a constant conductivity ratio, constant tapering and an unchanged total number of conduits were not fulfilled. When the conductivity ratio and relative tapering were plotted together, the results aboveground corresponded quite well with Murray's law: the conductivity ratio increased when relative tapering decreased. Our results support the theory that trees adjust both their macro- and microstructure to maximize their water transport efficiency, but also to prevent embolism and ensure mechanical safety.

Wood anatomical analysis of Alnus incana and Betula pendula injured by a debris-flow event

Vessel chronologies in ring-porous species have been successfully employed in the past to extract the climate signal from tree rings. Environmental signals recorded in vessels of ring-porous species have also been used in previous studies to reconstruct discrete events of drought, flooding and insect defoliation. However, very little is known about the ability of diffuse-porous species to record environmental signals in their xylem cells. Moreover, time series of wood anatomical features have only rarely been used to reconstruct former geomorphic events. This study was therefore undertaken to characterize the wood anatomical response of diffuse-porous Alnus incana (L.) Moench and Betula pendula Roth to debris-flow-induced wounding. Tree microscopic response to wounding was assessed through the analysis of wood anatomical differences between injured rings formed in the debris-flow event year and uninjured rings formed in the previous year. The two ring types were examined close and opposite to the injury in order to determine whether wound effects on xylem cells decrease with increasing tangential distance from the injury. Image analysis was used to measure vessel parameters as well as fiber and parenchyma cell (FPC) parameters. The results of this study indicate that injured rings are characterized by smaller vessels as compared with uninjured rings. By contrast, FPC parameters were not found to significantly differ between injured and uninjured rings. Vessel and FPC parameters mainly remained constant with increasing tangential distance from the injury, except for a higher proportion of vessel lumen area opposite to the injury within A. incana. This study highlights the existence of anatomical tree-ring signatures-in the form of smaller vessels-related to past debris-flow activity and addresses a new methodological approach to date injuries inflicted on trees by geomorphic processes.

Differential anatomical responses to elevated CO2 in saplings of four hardwood species

To determine whether an elevated carbon dioxide concentration ([CO(2)]) can induce changes in the wood structure and stem radial growth in forest trees, we investigated the anatomical features of conduit cells and cambial activity in 4-year-old saplings of four deciduous broadleaved tree species - two ring-porous (Quercus mongolica and Kalopanax septemlobus) and two diffuse-porous species (Betula maximowicziana and Acer mono) - grown for three growing seasons in a free-air CO(2) enrichment system. Elevated [CO(2)] had no effects on vessels, growth and physiological traits of Q. mongolica, whereas tree height, photosynthesis and vessel area tended to increase in K. septemlobus. No effects of [CO(2)] on growth, physiological traits and vessels were seen in the two diffuse-porous woods. Elevated [CO(2)] increased larger vessels in all species, except B. maximowicziana and number of cambial cells in two ring-porous species. Our results showed that the vessel anatomy and radial stem growth of Q. mongolica, B. maximowicziana and A. mono were not affected by elevated [CO(2)], although vessel size frequency and cambial activity in Q. mongolica were altered. In contrast, changes in vessel anatomy and cambial activity were induced by elevated [CO(2)] in K. septemlobus. The different responses to elevated [CO(2)] suggest that the sensitivity of forest trees to CO(2) is species dependent.

Changes of hydraulic conductivity during dehydration and rehydration in Quercus serrata Thunb. and Betula platyphylla var. japonica Hara: the effect of xylem structures

Xylem cavitation and its recovery were studied in 1-year-old stems of ring-porous Quercus serrata Thunb. and diffuse-porous Betula platyphylla var. japonica Hara. The Q. serrata had 5-100 microm vessel diameter in the functional current xylem and 5-75 microm in nonconducting 1-year-old xylem; B. platyphylla had a narrower range of vessel diameters of 5-55 microm and more than double the number of vessels in both functional growth rings. Although hydraulic conductivity of Q. serrata appeared to decrease after release of moderate water stress of a half loss of native hydraulic conductivity--about -2 MPa in xylem water potential--no significant recovery of hydraulic conductivity was observed, probably because of intraspecific variation in vessel diameter distribution, which induced variable vulnerability to cavitation. Furthermore, in terms of xylem anatomy, larger and more efficient vessels of the current xylem did not show obvious refilling. In B. platyphylla, after release of water stress, rapid (1 h) recoveries of both hydraulic conductivity and water potential were apparent after rewatering: so-called 'novel refilling'. During that time, a high degree of vessel refilling was observed in both xylems. At 12 h after rewatering, embolized vessels of the current xylem had refilled completely, although about 20% of vessels were still embolized in 1-year-old xylem. This different pattern of vessel refilling in relation to xylem age for B. platyphylla might be attributable to structural faults in the 1-year-old xylem, such as pit degradation or perhaps xylem aging itself. Results show that Q. serrata performs water conduction using highly efficient large vessels instead of unclear vessel refilling. In contrast, B. platyphylla transports water via less efficient but numerous vessels. If cavitation occurs, B. platyphylla improves water conduction by increasing the degree of vessel refilling.

Senescence-related changes in nitrogen in fine roots: mass loss affects estimation

The fate of nitrogen (N) in senescing fine roots has broad implications for whole-plant N economies and ecosystem N cycling. Studies to date have generally shown negligible changes in fine root N per unit root mass during senescence. However, unmeasured loss of mobile non-N constituents during senescence could lead to underestimates of fine root N loss. For N fertilized and unfertilized potted seedlings of Populus tremuloides Michx., Acer rubrum L., Acer saccharum Marsh. and Betula alleghaniensis Britton, we predicted that the fine roots would lose mass and N during senescence. We estimated mass loss as the product of changes in root mass per length and root length between live and recently dead fine roots. Changes in root N were compared among treatments on uncorrected mass, length (which is independent of changes in mass per length), calcium (Ca) and corrected mass bases and by evaluating the relationships of dead root N as a function of live root N, species and fertilization treatments. Across species, from live to dead roots, mass decreased 28-40%, N uncorrected for mass loss increased 10-35%, N per length decreased 5-16%, N per Ca declined 14-48% and N corrected for mass declined 12-28%. Given the magnitude of senescence-related root mass loss and uncertainties about Ca dynamics in senescing roots, N loss corrected for mass loss is likely the most reliable estimate of N loss. We re-evaluated the published estimates of N changes during root senescence based on our values of mass loss and found an average of 28% lower N in dead roots than in fine roots. Despite uncertainty about the contributions of resorption, leaching and microbial immobilization to the net loss of N during root senescence, live root N was a strong and proportional predictor of dead root N across species and fertilization treatments, suggesting that live root N alone could be used to predict the contributions of senescing fine roots to whole-plant N economies and N cycling.

Mycorrhizal colonisation of mountain birch (Betula pubescens ssp. czerepanovii) along three environmental gradients: does life in harsh environments alter plant-fungal relationships?

Environmental stress affects ectomycorrhizal communities (ECM), but it is not known how general the detected ECM responses are. We investigated ECM fungi on roots of mountain birch, Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti, along three environmental gradients, two natural (altitude, seashore) and one human-induced (pollution), within the Kola Peninsula, NW Russia. Chlorophyll fluorescence of birch leaves indicated no environmental stress even in the conditions that were presumed most stressful in terms of abiotic environment, where the biomass and population density of birches were strongly reduced. Although neither overall ECM colonisation nor root fungal biomass showed stress-related patterns, colonisation by Cenococcum geophilum tended to decrease with abiotic stress. ECM morphotype diversity declined with abiotic stress, and along altitudinal gradient this decline was related to an increase in proportion of morphotypes with high fungal biomass. Polycormic birches had higher ECM colonisation than monocormic birches at high stress sites only. ECM morphotype diversity increased with foliar nitrogen concentration at low stress sites, but not at high stress sites. Birches with higher chlorophyll fluorescence had lower chitin concentration in their roots (indicating lower proportion of fungal structures) at high stress sites only. Our results suggest that at high stress sites (1) mechanical shelter created by polycormic trees may favour ECM fungi and (2) mountain birches maintain lower ECM diversity than at low stress sites.

Sap flux in pure aspen and mixed aspen-birch forests exposed to elevated concentrations of carbon dioxide and ozone

Elevated concentrations of atmospheric carbon dioxide ([CO2]) and tropospheric ozone ([O3]) have the potential to affect tree physiology and structure and hence forest water use, which has implications for climate feedbacks. We investigated how a 40% increase above ambient values in [CO2] and [O3], alone and in combination, affect tree water use of pure aspen and mixed aspen-birch forests in the free air CO2-O3 enrichment experiment near Rhinelander, Wisconsin (Aspen FACE). Measurements of sap flux and canopy leaf area index (L) were made during two growing seasons, when steady-state L had been reached after more than 6 years of exposure to elevated [CO2] and [O3]. Maximum stand-level sap flux was not significantly affected by elevated [O3], but was increased by 18% by elevated [CO2] averaged across years, communities and O(3) regimes. Treatment effects were similar in pure aspen and mixed aspen-birch communities. Increased tree water use in response to elevated [CO2] was related to positive CO2 treatment effects on tree size and L (+40%). Tree water use was not reduced by elevated [O3] despite strong negative O3 treatment effects on tree size and L (-22%). Elevated [O3] predisposed pure aspen stands to drought-induced sap flux reductions, whereas increased tree water use in response to elevated [CO2] did not result in lower soil water content in the upper soil or decreasing sap flux relative to control values during dry periods. Maintenance of soil water content in the upper soil in the elevated [CO2] treatment was at least partly a function of enhanced soil water-holding capacity, probably a result of increased organic matter content from increased litter inputs. Our findings that larger trees growing in elevated [CO2] used more water and that tree size, but not maximal water use, was negatively affected by elevated [O3] suggest that the long-term cumulative effects on stand structure may be more important than the expected primary stomatal closure responses to elevated [CO2] and [O3] in determining stand-level water use under possible future atmospheric conditions.

Morphological variation among Betula nana (diploid), B. pubescens (tetraploid) and their triploid hybrids in Iceland

BACKGROUND AND AIMS

Introgressive hybridization between two co-existing Betula species in Iceland, diploid dwarf birch B. nana and tetraploid downy birch B. pubescens, has been well documented. The two species are highly variable morphologically, making taxonomic delineation difficult despite stable ploidy levels. Here an analysis is made of morphological variation within each ploidy group with an aim to establishing a reliable means to distinguish the species.

METHODS

Plant materials were collected from 14 woodlands in Iceland. The plants were identified based on 2n chromosome numbers. Morphological variation in species-specific characters within each ploidy group was analysed qualitatively and quantitatively. The morphological index was based on eight discrete characters, whereas the multivariate analysis was based on nine leaf variables.

KEY RESULTS

Of the 461 plants examined, 9.5 % were found to be triploid hybrids. The three ploidy groups were morphologically distinguishable but their variation overlapped. The diploid, triploid and tetraploid groups had average scores of 1.3, 4.1 and 8.3, respectively, in the morphology index scale from 0 (B. nana) to 13 (B. pubescens). A linear discriminant analysis also revealed significant separation among the three ploidy groups and the model assigned 96 % and 97 % of the B. nana and B. pubescens individuals correctly. The triploid hybrids were difficult to predict since only half of them could be assigned correctly. Leaf length was the most useful variable identifying triploid hybrids. Geographical patterns within the ploidy groups could partly be explained by differences in mean July temperature.

CONCLUSIONS

Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.

Lorentzian model of roots for understory yellow birch and sugar maple saplings

Total 66 small (<50m(2)), 24 medium (101-200m(2)) and 36 large (201-500m(2)) canopy gaps at the three sites of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh) forests were established in southern Québec, Canada. Half of the gaps were covered by 8x8m(2) shading cloths to mimic a closed canopy. From these gaps, 46 understory yellow birch and 46 sugar maple saplings with different tree ages and sizes were sampled. Single- and multi-variable linear and nonlinear models of root biomass and traits (root surface area, volume, length and endings) were developed and examined. Lorentzian model as a multi-variable nonlinear model was firstly applied to the simulations using both base diameter and height, and performed the best fit to total root biomass in both species with the highest correlation coefficients (R(2)=0.96 and 0.98) and smallest root mean squared deviations (RMSD=7.85 and 7.02) among all the examined models. The model also accurately simulated small fine root (2.0mm in diameter), coarse fine root (>2.0-5.0mm) and coarse root (>5.0mm) biomass (R(2)=0.87-0.99; RMSD=2.24-6.41), and the root traits (R(2)=0.71-0.99; RMSD=0.19-19.38). The study showed yellow birch roots were longer, larger, had more endings (tips) and grew faster than sugar maple roots. The root traits were largely distributed to small fine roots, sharply decreased from small fine roots to coarse fine roots, the fewest in coarse roots except for root volume. When trees were large, coarse root biomass increased more rapidly than fine root biomass, but vise versa when the trees were small.

CROWN RATIO INFLUENCES ALLOMETRIC SCALING IN TREES

Allometric theories suggest that the size and shape of organisms follow universal rules, with a tendency toward quarter-power scaling. In woody plants, however, structure is influenced by branch death and shedding, which leads to decreasing crown ratios, accumulation of heartwood, and stem and branch tapering. This paper examines the impacts on allometric scaling of these aspects, which so far have been largely ignored in the scaling theory. Tree structure is described in terms of active and disused pipes arranged as an infinite branching network in the crown, and as a tapering bundle of pipes below the crown. Importantly, crown ratio is allowed to vary independently of crown size, the size of the trunk relative to the crown deriving from empirical results that relate crown base diameter to breast height diameter through crown ratio. The model implies a scaling relationship in the crown which reduces to quarter-power scaling under restrictive assumptions but would generally yield a scaling exponent somewhat less than three-quarters. For the whole tree, the model predicts that scaling between woody mass and foliage depends on crown ratio. Measurements on three boreal tree species are consistent with the model predictions.

Spacing of silver birch seedlings grown in containers of equal size affects their morphology and its variability

Silver birch (Betula pendula Roth) seedlings were grown individually in containers arranged in rows radiating from a central point (Nelder plot) at densities spanning the range from 207 to 891 plants m(-2). Height of one set of seedlings was measured at weekly intervals and additional seedlings were harvested each week for dry mass and leaf area measurements. Height and shoot:root dry mass ratio increased with plant density. Seedling-to-seedling variability in dry mass, but not height, increased with increasing plant density. The red to far-red (R:FR) photon ratio in horizontally propagated radiation decreased with increasing density, even when plant densities and leaf area index values were low. In a separate experiment, elongating internodes of seedlings were irradiated locally by red and far-red light emitting diodes and stem elongation measured. Far-red light markedly increased stem elongation, suggesting that changes in light quality sensed by growing internodes are involved in the observed responses to growth density.

Sexual reproduction advances autumn leaf colours in mountain birch (Betula pubescens ssp. czerepanovii)

Autumnal change in leaf colour of deciduous trees is one of the most fascinating displays in nature. Current theories suggest that autumn leaf colours are adaptations to environmental stress. Here I report that the number of ripening female catkins altered timing of yellow autumn leaf colours in mountain birch. The tree's autumnal colour change was brought forward if the tree matured plenty of female catkins. Since yellow colour pigments in leaves are unmasked as leaf nitrogen is re-translocated, sexual reproduction may alter resource allocation at times of leaf senescence. Thus, our current view on the reasons for leaf senescence has to be re-examined, and a novel evolutionary explanation is needed for the appearance of yellow autumn leaf colours.

Transitions in the functioning of the shoot apical meristem in birch (Betula pendula) involve ethylene

In many trees, a short photoperiod (SD) triggers substantial physiological adjustments necessary for over-wintering. We have used transgenic ethylene-insensitive birches (Betula pendula), which express the Arabidopsis ethylene receptor gene ETR1 carrying the dominant mutation etr1-1, to investigate the role of ethylene in SD-induced responses in the shoot apical meristem (SAM). Under SD, the ethylene-insensitive trees ceased elongation growth comparably to the wild-type. In contrast, the formation of terminal buds, which in trees is typically induced by SD, was abolished. However, although delayed, endo-dormancy did eventually develop in the ethylene-insensitive trees. This, together with the rapid resumption of growth in the ethylene-insensitive trees after transfer from non-permissive to permissive conditions suggests that ethylene facilitates the SD-induced terminal bud formation, as well as growth arrest. In addition, apical buds of the ethylene-insensitive birch did not accumulate abscisic acid (ABA) under SD, suggesting interaction between ethylene and ABA signalling in the bud. Alterations in SAM functioning were further exemplified by reduced apical dominance and early flowering in ethylene-insensitive birches. Gene expression analysis of shoot apices revealed that the ethylene-insensitive birch lacked the marked increase in expression of a beta-xylosidase gene typical to the SD-exposed wild-type. The ethylene-dependent beta-xylosidase gene expression is hypothesized to relate to modification of cell walls in terminal buds during SD-induced growth cessation. Our results suggest that ethylene is involved in terminal bud formation and in the timely suppression of SAM activity, not only in the shoot apex, but also in axillary and reproductive meristems.

Wound-induced oxidative responses in mountain birch leaves

AIMS

The aim of the study was to examine oxidative responses in subarctic mountain birch, Betula pubescens subsp. czerepanovii, induced by herbivory and manual wounding.

METHODS

Herbivory-induced changes in polyphenoloxidase, peroxidase and catalase activities in birch leaves were determined. A cytochemical dye, 3,3-diaminobenzidine, was used for the in situ and in vivo detection of H2O2 accumulation as a response to herbivory and wounding. To localize peroxidase activity in leaves, 10 mm H2O2 was applied to the dye reagent.

KEY RESULTS

Feeding by autumnal moth, Epirrita autumnata, larvae caused an induction in polyphenoloxidase and peroxidase activities within 24 h, and a concomitant decrease in the activity of antioxidative catalases in wounded leaves. Wounding also induced H2O2 accumulation, which may have both direct and indirect defensive properties against herbivores. Wound sites and guard cells showed a high level of peroxidase activity, which may efficiently restrict invasion by micro-organisms.

CONCLUSION

Birch oxidases together with their substrates may form an important front line in defence against herbivores and pathogens.

Physiological, morphological and allocational plasticity in understory deciduous trees: importance of plant size and light availability

In a 4-year study, we investigated changes in leaf physiology, crown morphology and whole-tree biomass allocation in seedlings and saplings of shade-tolerant sugar maple (Acer saccharum Marsh.) and intermediate shade-tolerant yellow birch (Betula alleghaniensis Britt.) growing in natural understory light (0.5 to 35% of full sunlight) or in understory light reduced by 50% with shade nets to simulate the effect of gap closure. Leaf physiological parameters were mainly influenced by the light gradient, whereas crown morphological and whole-tree allocational parameters were mainly influenced by tree size. No single physiological, morphological or allocational trait was identified that could explain the difference in shade tolerance between the species. Yellow birch had higher growth rates, biomass allocation to branches and leaf physiological plasticity and lower crown morphological plasticity in unmodified understory light than sugar maple. Sugar maple did not display significant physiological plasticity, but showed variation with tree size in both crown morphology and whole-tree biomass allocation. When sugar maple was small, a greater proportion of whole-tree biomass was allocated to roots. However, physiological differences between the species decreased with decreasing light and most morphological and allocational differences tended to disappear with increasing tree size, suggesting that many species differences in shade-tolerance are expressed mainly during the seedling stage. Understory trees of both species survived for 4 years under shade nets, possibly because of higher plasticity when small and the use of stored reserves when taller.

Phenology and photosynthetic traits of short shoots and long shoots in Betula grossa

Leaf phenology, growth irradiance (i.e., photosynthetic photon flux (PPF) at the leaf surface) and photosynthetic capacity (A(area); measured at a PPF of 1000 micro mol m(-2) s(-1) and expressed on a leaf area basis) were investigated in early leaves (ELs) and late leaves (LLs) of Betula grossa Siebold & Zucc. trees. Early leaves first appeared on morphologically distinct long shoots and short shoots. The appearance of ELs, which was restricted to the bud break period, was followed by the successive appearance of LLs on long shoots only. Late leaves appeared successively until the middle of the growing season. Late leaves started to abscise around the middle of the growing season, whereas ELs on both long and short shoots did not abscise until near the end of the growing season. Solar irradiance was higher at the surface of LLs of late appearance than at the surface of either LLs of early appearance or ELs. Solar irradiance at the surface of ELs decreased after LLs appeared. In both long and short shoots, A(area) of ELs increased and then remained stable for 65-80 days before starting to decrease. Although A(area) was higher in LLs than in ELs for a short time in August, it started to decrease earlier in LLs than in ELs. Area-based nitrogen concentration (N(area)) was higher in LLs than in ELs after August. Although N(area) decreased slowly in ELs after August, it did not decrease in LLs. In both ELs and LLs, A(area)/N(area) decreased with time. The crown was thus characterized by a rapidly growing surface with young LLs having high A(area) and by shaded inner parts with ELs having stable low A(area).

Testing the allometric scaling laws

Allometric scaling laws have received increasing attention due to the recent theoretical advancements. However, existing evidence suggests that the scaling relationships may vary a lot without much consistency, which poses a challenge to the applicability of general theories. In this report, I demonstrate that much of the discrepancy may be an artefact caused by the limited use of methods for estimating the parameters in the allometric scaling equations. I suggest alternative procedures that can be utilized to avoid biased interpretations. The comments are largely applicable to any research that involves parameterization of equations.

Size structure of current-year shoots in mature crowns

Characteristics of current-year shoot populations were examined for three mature trees of each of three deciduous broad-leaved species. For first-order branches (branches emerging from the vertical trunk) of the trees examined, lengths or diameters of all current-year shoots were measured. Total leaf mass and total current-year stem mass of first-order branches were estimated using an allometric relationship between leaf or stem mass and length or diameter of current-year stems. For each tree, the number of current-year shoots on a first-order branch was proportional to the basal stem cross-sectional area of the branch. On the other hand, first-order branches had shoot populations with size structures similar to each other. As a result, the leaf mass of a first-order branch was proportional to the basal stem cross-sectional area of the branch, being compatible with the pipe-model relationship. All current-year shoot populations had positively skewed size structures. Because small shoots have a larger ratio of leaf mass to stem mass than large shoots, first-order branches had an extremely large ratio of leaf mass to current-year stem mass. This biased mass allocation will reduce costs for current stem production, respiration and future radial growth, and is beneficial to mature trees with a huge accumulation of non- photosynthetic organs. The allometric relationships between leaf mass and basal stem diameter and that between leaf mass and current-year stem mass of first-order branches were each similar across the trees examined. Characteristics of shoot populations tended to offset inter-species diversity of shoot allometry so that branch allometry shows inter-species convergence.

Silver birch (Betula pendula) plants with aux and rol genes show consistent changes in morphology, xylem structure and chemistry

The effects of Agrobacterium pRiA4 rol and aux genes, controlled by their endogenous promoters, on tree growth and wood anatomy and chemistry were studied in 5- and 7-year-old silver birch (Betula pendula Roth) plants. Southern hybridization confirmed the following rol and aux gene combinations: control plants (no genes transferred); plants with rolC and rolD genes; plants with rolA, rolB, rolC and rolD genes; and plants with rolA, rolB, rolC, rolD, aux1 and aux2 genes. Transgene mRNA was most abundant in phloem/cambium samples and in the developing xylem, whereas no expression was detected in leaves. Plants with rolC and rolD genes or with all the rol genes were significantly shorter and had smaller leaves and a more bushy growth habit than control plants or plants with both aux and rol genes. Morphological observations and wood chemistry analyses revealed that plants with rol genes produced less xylem and broke bud later than control plants or plants with both aux and rol genes. Tension wood was detected in both control and transgenic plants irrespective of their gene combination, probably as a result of greenhouse cultivation. Xylem fibers were shorter in transgenic plants than in control plants, and plants with all the rol genes were characterized by shorter vessels compared with the control plants and a smaller proportional area of vessels compared with the other groups. In addition, silver birch plants with all the rol genes had approximately a 3.3% lower concentration of total acid soluble carbohydrates than control plants. We conclude that the rolC and rolD genes induced the typical "rol-phenotype," and that this was emphasized by concomitant expression of the rolA and rolB genes and alleviated by the presence of aux1 and aux2 genes. We observed consistent phenotypic effects of rol and aux genes on the morphology, anatomy and cell wall chemistry of the plants.

Comparative analysis of leaf trichome structure and composition of epicuticular flavonoids in Finnish birch species

The morphology, ultrastructure, density and distribution of trichomes on leaves of Betula pendula, B. pubescens ssp. pubescens, B. pubescens ssp. czerepanovii and B. nana were examined by means of light, scanning and transmission electron microscopy. The composition of flavonoids in ethanolic leaf surface extracts was analysed by high pressure liquid chromatography. All taxa examined contained both glandular and non-glandular trichomes (short and/or long hairs) but differed from each other in trichome ultrastructure, density and location on the leaf. Leaves of B. pubescens were more hairy than those of B. pendula, but the latter species had a higher density of glandular trichomes. Of the two subspecies of B. pubescens, leaves of ssp. pubescens had more short hairs on the leaf surface and four times the density of glandular trichomes of leaves of ssp. czerepanovii, whereas, in the latter subspecies, short hairs occurred largely on leaf veins, as in B. nana. The glandular trichomes were peltate glands, consisting of medullar and cortical cells, which differed structurally. Cortical cells possessed numerous small, poorly developed plastids and small vacuoles, whereas medullar cells had several large plastids with well-developed thylakoid systems and fewer vacuoles. In B. pubescens subspecies, vacuoles of the glandular cells contained osmiophilic deposits, which were probably phenolic, whereas in B. pendula, vacuoles of glandular trichomes were characterized by the presence of numerous myelin-like membranes. The composition of epicuticular flavonoids also differed among species. The two subspecies of B. pubescens and B. nana shared the same 12 compounds, but five of these occurred only in trace amounts in B. nana. Leaf surface extracts of B. pendula contained just six flavonoids, three of which occurred only in this species. In summary, the structure, density and distribution of leaf trichomes and the composition of epicuticular flavonoids represent good taxonomic markers for Finnish birch species.

Influence of elevated CO2 and O3 on Betula pendula Roth crown structure

Elevated CO(2) and ozone effects were studied singly and in combination on the crown structure of two Betula pendula clones. Measurements were made at the end of the second fumigation period in an open-top-chamber experiment with 9-year-old trees. Shoot ramification (number of long and short daughter shoots), shoot length, and number of metamers, leaves and buds were measured at four positions in every tree. As a result of increased temperature, trees in chambers had longer shoots and more frequent shoot ramification than control trees not enclosed in chambers. Ozone treatment decreased shoot ramification significantly. Additionally, ozone treatment resulted in an increased number of metamers in one clone. There was no statistically significant interaction between ozone effect and crown position; however, there was a slight tendency for the lower crown to be more affected by ozone. Elevated CO(2) caused a significant increase in the number of long-shoot metamers. Therefore, 2x ambient CO(2) concentration partly ameliorated the negative effect of ozone because the increased number of leaves per shoot counteracted the decreased branching. Although the main effects of elevated ozone and CO(2) were similar in the two clones, slight, statistically insignificant, differences appeared in their responses when interactions with crown position were considered.

[Cytogenetic and anatomic features of the weeping birch in the 30-km zone surrounding the Novovoronezh nuclear power plant]

We studied the development of the vegetative and generative spheres in the weeping birch from the 30-km zone surrounding the Novovoronezh nuclear power plant. Being the most sensitive ontogenetic stages, the mitotic and meiotic processes are capable of responding to the effects of various factors, including unfavorable environmental factors. Studies were carried out from 1998 to 2000 in Novovoronezh under conditions of potential hazard for normal development (a possible joint effect of chronic ionizing irradiation at low doses from the Novovoronezh nuclear power plant and elevated concentrations of some chemical substances as side products of the plant activity) and in the ecologically pure territory of the Biological Station Venevitinovo of the Voronezh State University. Significant deviations from the normal course of mitotic and meiotic processes have been recorded in the experimental variant and specific types of disturbances have been described. A suggestion has been put forward concerning some mechanisms underlying the weeping birch adaptation to unfavorable environmental conditions. Specifically, it was shown that the trees studied were of a mixoploid nature. No such studies have ever been carried out in the region of this nuclear power plant, one of the oldest in Russia.

Ammonium and nitrate acquisition by plants in response to elevated CO2 concentration: the roles of root physiology and architecture

We examined changes in root system architecture and physiology and whole-plant patterns of nitrate reductase (NR) activity in response to atmospheric CO2 enrichment and N source to determine how changes in the form of N supplied to plants interact with rising CO2 concentration ([CO2]). Seedlings of Betula alleghaniensis Britt. and Pinus strobus L., which differ in growth rate, root architecture, and the partitioning of NR activity between leaves (Betula) and roots (Pinus), were grown in ambient (400 microl l(-1)) and elevated (800 microl l(-1)) [CO2] and supplied with either nitrate (NO3-) or ammonium (NH4+) as their sole N source. After 15 weeks of growth, plants were harvested and root system architecture, N uptake kinetics, and NR activity measured. Betula alleghaniensis responded to elevated [CO2] with significant increases in growth, regardless of the source of N. Pinus strobus showed no significant response in biomass production or allocation to elevated [CO2]. Both species exhibited significantly greater growth with NH4+ than with NO3-, along with lower root:shoot biomass ratios. Betula showed significant increases in total root length in response to elevated [CO2]. However, root N uptake rates in Betula (for both NO3- and NH4+) were either reduced or unchanged by elevated [CO2]. Pinus showed the opposite response to elevated [CO2], with no change in root architecture, but an increase in maximal uptake rates in response to elevated [CO2]. Nitrate reductase activity (on a mass basis) was reduced in leaves of Betula in elevated [CO2], but did not change in other tissues. Nitrate reductase activity was unaffected by elevated [CO2] in Pinus. Scaling this response to the whole-plant, NR activity was reduced in elevated [CO2] in Betula but not in Pinus. However, because Betula plants were larger in elevated [CO2], total whole-plant NR activity was unaffected.