Ecophysiological traits associated with drought in Mediterranean tree seedlings: individual responses versus interspecific trends in eleven species

Temperate woody species across the angiosperm phylogeny acquire tolerance to water deficit stress during the growing season

Understanding the capacity of temperate trees to acclimate to limited soil water has become essential in the face of increasing drought risk due to climate change. We documented seasonal - or phenological - patterns in acclimation to water deficit stress in stems and leaves of tree species spanning the angiosperm phylogeny. Over 3 yr of field observations carried out in two US arboreta, we measured stem vulnerability to embolism (36 individuals of 7 Species) and turgor loss point (119 individuals of 27 species) over the growing season. We also conducted a growth chamber experiment on 20 individuals of one species to assess the mechanistic relationship between soil water restriction and acclimation. In three-quarters of species measured, plants became less vulnerable to embolism and/or loss of turgor over the growing season. We were able to stimulate this acclimatory effect by withholding water in the growth chamber experiment. Temperate angiosperms are capable of acclimation to soil water deficit stress, showing maximum vulnerability to soil water deficits following budbreak and becoming more resilient to damage over the course of the growing season or in response to simulated drought. The species-specific tempo and extent of this acclimatory potential constitutes preadaptive climate change resilience.

Functional response and resistance to drought in seedlings of six shrub species with contrasting leaf traits from the Mediterranean Basin and California

Extreme drought events during post-fire regeneration are becoming increasingly frequent in Mediterranean-type ecosystems. Understanding how plants with different traits and origins respond to such conditions during early life stages is therefore critical for assessing the effect of climate change. Here, seedlings of three Cistus (semi-deciduous malacophylls from the Mediterranean Basin) and three Ceanothus (evergreen sclerophylls from California) species, two post-fire seeder genera with contrasting leaf traits, were subjected to complete water deprivation for 3 months in a common garden experiment. The leaf and plant structure and plant tissue water relations were characterized before the drought, and the functional responses (water availability, gas exchange and fluorescence) were monitored during the drought. Both genera exhibited contrasting leaf structure and tissue water relations traits, with higher leaf area and specific leaf area as well as higher osmotic potential at maximum turgor and turgor loss point in Cistus than Ceanothus. During drought, Ceanothus showed a more conservative use of water than Cistus, with a water potential less sensitive to decreasing soil moisture and a strong decline in photosynthesis and stomatal conductance in response to water deficit, but also a level of fluorescence more responsive to drought than Cistus. However, we could not find a different degree of drought resistance between the genera. This was particularly clear between Cistus ladanifer L. and Ceanothus pauciflorus DC., the two most functionally contrasting species, but at the same time, the two most drought-resistant. Our findings demonstrate that species with different leaf traits and functional responses to water stress may not differ in their degree of drought resistance, at least during the seedling stage. This underlines the need to take general categorizations by genus or functional types with caution and to deepen our knowledge about the Mediterranean-type species ecophysiology, especially during early life stages, in order to anticipate their vulnerability to climate change.

Arbuscular mycorrhizae reduce the response of important plant functional traits to drought and salinity. A meta-analysis study

We aimed at exploring the plant functional traits whose responses to drought or salinity are altered by the presence of arbuscular mycorrhiza (AM). We performed a meta-analysis across 114 articles spanning 110 plant species or cultivars. We quantified the size effect of AM symbiosis on the stress response of several functional traits, using linear mixed model analysis (LMM). Correlation analysis between functional traits and total biomass responses to stresses were also performed through LMM. The literature search and further selection yielded seven functional traits, extracted from 114 laboratory studies, including 888 observations and 110 plant species/cultivars. Evidence for significant effects of predictor variables (type of stress, AM symbiosis and/or their interaction) on functional trait response were found for leaf area ratio (LAR), root mass fraction (RMF) and root-shoot (R:S) ratio. Our results provided evidence to accept the hypothesis that AM fungal inoculation may reduce the stress response of these plant functional traits by decreasing its magnitude. We also found a weak correlation between stress responses of these traits and total biomass variation. Although our literature search and data collection were intensive and our results robust, the scope of our conclusions is limited by the agronomical bias of plant species targeted by the meta-analysis. Further knowledge on non-cultivable plant species and better understanding of the mechanisms ruling resources allocation in plants would allow more generalised conclusions.

Limited Acclimation in Leaf Morphology and Anatomy to Experimental Drought in Temperate Forest Species

Simple Summary

Climate change shown to have a significant impact on the forest ecosystem due to increased and more frequent occurrence of extreme drought. However, in order to successfully adjust to the xeric environments, plants can usually adopt a variety of adaptation strategies. Here, we investigated the morpho-anatomical traits and biomass allocation patterns as acclimation mechanisms in drought conditions. We found that the interrelation between leaf morphological and anatomical traits were equally affected by drought conditions across all species. This suggests that there is no convincing evidence to classify taxa based on drought resistance vs. drought tolerance. However, based on the biomass allocation pattern, we found that P. koraiensis and F. mandshurica had the higher RMF and total PB, but lower LFM, suggesting higher drought tolerance than those of the other species. Therefore, our dataset revealed some easily measurable traits, such as LMF, RMF, and PB, which demonstrated the seedling’s ability to cope with drought and which could be utilized to choose drought-tolerant species for reforestation in the temperate forest.

Abstract

Drought is a critical and increasingly common abiotic factor that has impacts on plant structures and functioning and is a challenge for the successful management of forest ecosystems. Here, we test the shifts in leaf morpho-anatomical or hydraulic traits and plant growth above ground caused by drought. A factorial experiment was conducted with two gymnosperms (Larix gmelinii and Pinus koraiensis) and two angiosperms (Fraxinus mandshurica and Tilia amurensis), tree species grown under three varying drought intensities in NE China. Considering all the species studied, the plant height (PH), root collar diameter (RCD), and plant biomass (PB) were significantly decreased by drought. The leaf thickness (LT) increased, while the leaf area (LA) decreased with drought intensity. In the gymnosperms, the mesophyll thickness (MT) increased, and the resin duct decreased, while in the angiosperms the palisade mesophyll thickness (PMT), the spongy mesophyll thickness (SMT), and the abaxial (ABE) and adaxial epidermis (ADE) thickness were increased by drought. The correlation analysis revealed that P. koraiensis and F. mandshurica had the higher RMF and total plant biomass, but the least LMF, suggesting drought tolerance. In contrast, the L. gmelinii had the least RMF and higher LMF, suggesting vulnerability to drought. Similarly, T. amurensis had the higher leaf size, which increased the evaporative demand and depleted the soil water quickly relative to the other species. The interrelation among the morpho-anatomical leaf traits was equally affected by drought across all the studied species, suggesting that there is no clear evidence to differentiate the taxa based on drought resistance vs. drought tolerance. Thus, we have identified some easily measurable traits (i.e., LMF, RMF, and PB) which evidenced the seedling’s ability to cope with drought and which therefore could be used as proxies in the selection of drought tolerant species for reforestation in the temperate forest.

The role of nutritional impairment in carbon-water balance of silver fir drought-induced dieback

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ¹⁸ O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower δ¹⁸ O than ND trees. The uncoupling between iWUE and δ¹⁸ O, together with the positive correlations between P and K leaf concentrations and δ¹⁸ O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.

Multi-trait genetic variation in resource-use strategies and phenotypic plasticity correlates with local climate across the range of a Mediterranean oak (Quercus faginea)

Resource-use strategies are hypothesized to evolve along climatic gradients. However, our understanding of the environmental factors driving divergent evolution of resource-use strategies and the relationship between trait genetic variation and phenotypic plasticity is far from complete. Using the Mediterranean tree Quercus faginea as study system, we tested the hypothesis that a conservative resource-use strategy with increased drought tolerance and reduced phenotypic plasticity has evolved in areas with longer and more severe dry seasons. We conducted a glasshouse experiment in which we measured leaf morphological, physiological, growth and allocation traits in seedlings from 10 range-wide climatically contrasting populations, grown under two different watering treatments. Both univariate and multivariate analyses revealed a genetic gradient of resource-use strategies and phenotypic plasticity associated with provenance climate. In particular, populations from harsher (drier and colder) environments had more sclerophyllous leaves, lower growth rates, better physiological performance under dry conditions and reduced multi-trait phenotypic plasticity compared to populations from more mesic and milder environments. Our results suggest that contrasting precipitation and temperature regimes play an important role in the adaptive intraspecific evolution of multivariate phenotypes and their plasticity, resulting in coordinated morphology, physiology, growth and allometry according to alternative resource-use strategies.

Photosynthetic and Growth Responses in a Pioneer Tree (Japanese White Birch) and Competitive Perennial Weeds (Eupatorium sp.) Grown Under Different Regimes With Limited Water Supply to Waterlogging

For a successful natural regeneration of Japanese white birch (Betula platyphylla var. japonica), competitive vegetation should be managed. Here, we clarified how soil water condition modifies the competitiveness of Japanese white birch against perennial weeds, Eupatorium species, based on an ecophysiological approach combining a glasshouse experiment and a field survey. We investigated photosynthetic and growth responses to various water regimes from water deficit to waterlogging (two times-a-week irrigation, three times-a-week irrigation, half waterlogging, and full waterlogging) in pot-grown seedlings of Japanese white birch and the competitive weed Eupatorium makinoi. The ratio of seedling height of Japanese white birch to seedling height of E. makinoi showed a decreasing trend from two times-a-week irrigation to full waterlogging, which suggests a lower competitiveness for light resource in Japanese white birch with increasing soil wetness. The maximum rate of Rubisco carboxylation (Vc,max) based on unit N was lower in waterlogging treatments than in two times- and three times-a-week irrigation in Japanese white birch, whereas E. makinoi showed the opposite response. This suggests that N partitioning into Rubisco and/or Rubisco activation might be suppressed in Japanese white birch but enhanced in E. makinoi under waterlogging. The maximum photochemical efficiency of photosystem II (Fv/Fm) was also lower in seedlings of Japanese white birch grown under waterlogging treatments. We further conducted a field survey on the relationship between Fv/Fm and topographic wetness index (TWI) in seedlings of Japanese white birch and E. glehnii (closely related to E. makinoi) naturally grown in a study site 5 years after canopy tree cutting. Lower Fv/Fm was observed in seedlings of Japanese white birch with increasing TWI, whereas no significant trend was observed in E. glehnii, in agreement with the glasshouse experiment. Thus, keeping soils not always humid might be favorable to photosynthetic performance and growth competitive ability of Japanese white birch against Eupatorium species.

Excessive nitrogen application under moderate soil water deficit decreases photosynthesis, respiration, carbon gain and water use efficiency of maize

The impact of water stress and nitrogen (N) nutrition on leaf respiration (R), carbon balance and water use efficiency (WUE) remains largely elusive. Therefore, the objective of the present study was to investigate the effect of soil water and N stresses on growth, physiological responses, leaf structure, carbon gain and WUE of maize. The plants were subjected to different soil water and N regimes to maturity. The results showed that the photosynthesis (A_n) and stomatal conductance (G_s) decreased significantly under the water stressed treatments across the N treatments mainly ascribed to the decreased plant water status. The moderate water stress reduced the photosynthetic capacity and activity and also caused damage to the structure of leaves, resulting in the significant reduction of A_n, and thus decreased WUE_i. The dark respiration (R_d) was significantly decreased due to the damage of mitochondria, however, the R_d/A_n increased significantly and the carbon gain was seriously compromised, eventually inhibiting biomass growth under the moderately water stressed treatment. Increasing N dose further aggravated the severity of water deficit, decreased A_n, G_s and WUE_i, damaged the structure and reduced the number of mitochondria of leaves, while increased R_d/A_n considerably under moderate water stress. Consequently, the biomass accumulation, carbon gain and plant level WUE_p in the moderately water stressed treatment decreased markedly under the high N supply. Therefore, excessive N application should be avoided when plants suffer soil water stress in maize production.

Physiological, biochemical, and anatomical responses of Araucaria araucana seedlings to controlled water restriction

Water stress triggers acclimation responses and can damage plants, which varies by species and stress levels. Ongoing climate change is projected to result in longer and more intense water stress conditions leading to an alarming increase in drought-induced forest decline. The aim of this study was to evaluate the physiological responses of leaves and stem wood anatomy from Araucaria araucana pot-grown three-year old seedlings, a conifer tree from northwestern Patagonia. Plants were subjected to moderate and severe water restriction regimes and compared to well-watered controls. Severe water stress reduced relative leaf water content and triggered an accumulation of free proline in leaves, regardless of age. Epicuticular wax extrusions increased in apical leaf stomata while photosynthetic pigments decreased, resulting in differential oxidative damage. The concentration of phenolic compounds was not affected by water restrictions. Plants exposed to restricted water regimes showed diminished middle leaf biomass and expansion (~60% of total leaves), increased stem wood density, and experienced 7% and 30% mortality rates under moderate and severe water stress, respectively. Our findings suggest that under moderate water stress, analogous to short-term droughts, A. araucana seedlings activate physiological mechanisms that allow them to withstand short periods of drought, while more severe water stress and longer droughts can be severely harmful.

Previous drought exposure leads to greater drought resistance in eucalypts through changes in morphology rather than physiology

Over their lifetime, trees are repeatedly exposed to droughts. It is therefore important to understand whether repeated drought exposure makes trees more or less drought tolerant. Here, we investigated the effect of repeated droughts on functional trait expression and tree function in Eucalyptus obliqua. Further, we tested whether previous drought exposure enabled trees to avoid leaf death for longer under a subsequent severe drought. Trees were subjected for 1 year to 2 drought-rewatering cycles (drought treatment) or well-watered conditions, before imposing a severe drought. Trees in the drought treatment reduced their overall leaf area and biomass, whereas leaf-level anatomical, morphological and physiological traits remained mostly unaffected. There were no differences in water potential at the turgor loss point, leaf xylem vulnerability to embolism, leaf size, maximum xylem vessel diameter or cell wall thickness between treatments after the conditioning period. When exposed to a subsequent severe drought, trees previously exposed to drought were more drought tolerant due to a lower water potential at leaf death and tree-level morphological rather than physiological adjustments. Trees previously exposed to drought were smaller and used less water, which delayed leaf death for 39 days compared with 22 days for the well-watered trees. Our study indicates that previous drought exposure can facilitate tree-level morphological adjustment, which potentially enhances survival of E. obliqua trees during subsequent drought events.

Leaf traits and performance vary with plant age and water availability in Artemisia californica

BACKGROUND AND AIMS

Leaf functional traits are strongly tied to growth strategies and ecological processes across species, but few efforts have linked intraspecific trait variation to performance across ontogenetic and environmental gradients. Plants are believed to shift towards more resource-conservative traits in stressful environments and as they age. However, uncertainty as to how intraspecific trait variation aligns with plant age and performance in the context of environmental variation may limit our ability to use traits to infer ecological processes at larger scales.

METHODS

We measured leaf physiological and morphological traits, canopy volume and flowering effort for Artemisia californica (California sagebrush), a dominant shrub species in the coastal sage scrub community, under conditions of 50, 100 and 150 % ambient precipitation for 3 years.

KEY RESULTS

Plant age was a stronger driver of variation in traits and performance than water availability. Older plants demonstrated trait values consistent with a more conservative resource-use strategy, and trait values were less sensitive to drought. Several trait correlations were consistent across years and treatments; for example, plants with high photosynthetic rates tended to have high stomatal conductance, leaf nitrogen concentration and light-use efficiency. However, the trade-off between leaf construction and leaf nitrogen evident in older plants was absent for first-year plants. While few traits correlated with plant growth and flowering effort, we observed a positive correlation between leaf mass per area and performance in some groups of older plants.

CONCLUSIONS

Overall, our results suggest that trait sensitivity to the environment is most visible during earlier stages of development, after which intraspecific trait variation and relationships may stabilize. While plant age plays a major role in intraspecific trait variation and sensitivity (and thus trait-based inferences), the direct influence of environment on growth and fecundity is just as critical to predicting plant performance in a changing environment.

Drought and Pathogen Effects on Survival, Leaf Physiology, Oxidative Damage, and Defense in Two Middle Eastern Oak Species

The charcoal disease agents, Biscogniauxia mediterranea and Obolarina persica are two latent, ascomycetous oak pathogens in the Middle Eastern Zagros forests, where they have devastating effects, particularly during drought. Under greenhouse conditions, we investigated the effects of the two charcoal disease agents individually and in combination with drought on survival, growth, foliar gas-exchange, pigment content, oxidative stress and the antioxidant response of Quercus infectoria and Q. libani, two of the dominant tree species in this region. Commonly, the strongest negative effects emerged in the drought–pathogen interaction treatments. Q. infectoria showed less severe lesions, higher survival, more growth, and less leaf loss than Q. libani under combined biotic and abiotic stress. In both oak species, the combination of pathogen infection and drought resulted in more than 50% reduction in foliar gas-exchange parameters with partial recovery over time in Q. infectoria suggesting a superior defense system. Indeed, enhanced foliar anthocyanin, total soluble protein and glutathione concentrations imply an upregulation of the antioxidant defense system in Q. infectoria under stress while none of these parameters showed a significant treatment response in Q. libani. Consequently, Q. infectoria foliage showed no significant increase in superoxide, lower lipoxygenase activity, and less electrolyte leakage compared to the highly elevated levels seen in Q. libani indicating oxidative damage. Our findings indicate greater drought tolerance and pathogen resilience in Q. infectoria compared to Q. libani. Under future climate scenarios, we therefore expect changes in forest community structure driven by a decline in Q. libani and closely associated organisms.

Plasticity of the Root System Architecture and Leaf Gas Exchange Parameters Are Important for Maintaining Bottle Gourd Responses under Water Deficit

The evaluation of root system architecture (RSA) development and the physiological responses of crop plants grown under water-limited conditions are of great importance. The purpose of this study was to examine the short-term variation of the morphological and physiological plasticity of Lagenaria siceraria genotypes under water deficit, evaluating the changes in the relationship between the root system architecture and leaf physiological responses. Bottle gourd genotypes were grown in rhizoboxes under well-watered and water deficit conditions. Significant genotype-water regime interactions were observed for several RSA traits and physiological parameters. Biplot analyses confirmed that the drought-tolerant genotypes (BG-48 and GC) showed a high net CO₂ assimilation rate, stomatal conductance, transpiration rates with a smaller length, and a reduced root length density of second-order lateral roots, whereas the genotypes BG-67 and Osorno were identified as drought-sensitive and showed greater values for average root length and the density of second-order lateral roots. Consequently, a reduced length and density of lateral roots in bottle gourd should constitute a response to water deficit. The root traits studied here can be used to evaluate bottle gourd performance under novel water management strategies and as criteria for breeding selection.

Frameworks on Patterns of Grasslands’ Sensitivity to Forecast Extreme Drought

Climate models have predicted the future occurrence of extreme drought (ED). The management, conservation, or restoration of grasslands following ED requires a robust prior knowledge of the patterns and mechanisms of sensitivity—declining rate of ecosystem functions due to ED. Yet, the global-scale pattern of grasslands’ sensitivity to any ED event remains unresolved. Here, frameworks were built to predict the sensitivity patterns of above-ground net primary productivity (ANPP) spanning the global precipitation gradient under ED. The frameworks particularly present three sensitivity patterns that could manipulate (weaken, strengthen, or erode) the orthodox positive precipitation–productivity relationship which exists under non-drought (ambient) condition. First, the slope of the relationship could become steeper via higher sensitivity at xeric sites than mesic and hydric ones. Second, if the sensitivity emerges highest in hydric, followed by mesic, then xeric, a weakened slope, flat line, or negative slope would emerge. Lastly, if the sensitivity emerges unexpectedly similar across the precipitation gradient, the slope of the relationship would remain similar to that of the ambient condition. Overall, the frameworks provide background knowledge on possible differences or similarities in responses of grasslands to forecast ED, and could stimulate increase in conduct of experiments to unravel the impacts of ED on grasslands. More importantly, the frameworks indicate the need for reconciliation of conflicting hypotheses of grasslands’ sensitivity to ED through global-scale experiments.

Practical Implications of Different Phenotypic and Molecular Responses of Evergreen Conifer and Broadleaf Deciduous Forest Tree Species to Regulated Water Deficit in a Container Nursery

Recent climatic changes have resulted in an increased frequency and prolonged periods of drought and strained water resources affecting plant production. We explored the possibility of reducing irrigation in a container nursery and studied the growth responses of seedlings of four economically important forest trees: broadleaf deciduous angiosperms Fagus sylvatica L., Quercus petraea (Matt.) Liebl., and evergreen conifers Abies alba Mill. and Pinus sylvestris L. We also studied markers of water stress including modifications of biomass allocation, leaf anatomy, proline accumulation, and expression of selected genes. Growth of the broadleaved deciduous species was more sensitive to the reduced water supply than that of conifers. Remarkably, growth of the shade tolerant Abies was not affected. Adjustment of biomass allocations was strongest in P. sylvestris, with a remarkable increase in allocation to roots. In response to water deficit both deciduous species accumulated proline in leaves and produced leaves with shorter palisade cells, reduced vascular tissues, and smaller conduit diameters. These responses did not occur in conifers. Relative transcript abundance of a gene encoding the Zn-finger protein in Q. petraea and a gene encoding the pore calcium channel protein 1 in A. alba increased as water deficit increased. Our study shows major differences between functional groups in response to irrigation, with seedlings of evergreen conifers having higher tolerance than the deciduous species. This suggests that major water savings could be achieved by adjusting irrigation regime to functional group or species requirements.

Mechanisms of severe dieback and mortality in a classically drought-tolerant shrubland species (Arctostaphylos glauca)

PREMISE

Mortality events involving drought and pathogens in natural plant systems are on the rise due to global climate change. In Santa Barbara, California, United States, big berry manzanita (Arctostaphylos glauca) has experienced canopy dieback related to a multi-year drought and infection from fungal pathogens in the Botryosphaeriaceae family. A greenhouse experiment was conducted using Neofusicoccum australe to test the specific influences of drought and fungal infection on A. glauca.

METHODS

A full factorial design was used to compare four treatment groups (drought + inoculation; drought - inoculation; watering + inoculation; and control: watering - inoculation). Data were collected for 10 weeks on stress symptoms, changes in leaf fluorescence and photosynthesis, and mortality.

RESULTS

Results indicated significant effects of watering and inoculation treatments on net photosynthesis, dark-adapted fluorescence, and disease symptom severity (P < 0.05), and a strong correlation was found between physiological decline and visible stress (P < 0.0001). Mortality differed between treatments, with all groups except for the control experiencing mortality (43% mortality in drought - inoculation, 83% in watering - inoculation, and 100% in drought + inoculation). A Kaplan-Meier survival analysis showed drought + inoculation to have the least estimated survivorship compared to all other treatment groups.

CONCLUSIONS

In addition to a possible synergistic interaction between drought and fungal infection in disease onset and mortality rates in A. glauca, these results indicate that young, non-drought-stressed plants are susceptible to mortality from N. australe infection, with important implications for the future of wildland shrub communities.

Hydropeaking affects germination and establishment of riverbank vegetation

Hydropeaking, defined as frequent and rapid variation in flow in regulated rivers with hydropower plants over a short period of time, usually sub-daily to weekly, alters hydraulic parameters such as water levels or flow velocity and exerts strong impacts on fluvial ecosystems. We evaluated the effects of hydropeaking on riverbank vegetation, specifically assessing the germination and establishment of seedlings and cuttings of plant species representing a variation in traits. We used seeds and seedlings and cuttings varying in size as phytometers, and transplanted them to riverbanks both above and below dams used for hydropower production in northern Sweden, selected to represent a gradient in hydropeaking intensity, and along a free-flowing reach. We also analyzed sub-daily water-level variables modified by hydropeaking to identify variables key in explaining the observed vegetation patterns. We found that plant responses to hydropeaking varied with species, with flood-intolerant species being the most strongly affected, as early as the germination stage. In contrast, seeds of flood-tolerant species managed to germinate and survive the early establishment phase, although strong erosive processes triggered by hydropeaking eventually caused most of them to fail. The fate of flood-intolerant species identifies germination as the most critical life-history stage. The depth and frequency of the inundation were the leading variables explaining plant responses, while the duration of shallow inundation explained little of the variation. The rise and fall rates of water levels were key in explaining variation in germination success. Based on the results, we propose restoration measures to enhance establishment of riparian plant communities while minimizing the impact on hydropower electricity production. Given the strong decrease in the germination of species intolerant to prolonged flooding with hydropeaking, planting of seedlings, preferably of large sizes, together with restrictions in the operation of the power plant during the establishment phase to enhance survival would be the best restoration option. Given the high probability of plant uprooting with hydropeaking, bank protection measures have the potential to increase riparian plant survival of all species, including flooding-tolerant species.

Seasonal and daily variations in primary and secondary metabolism of three maquis shrubs unveil different adaptive responses to Mediterranean climate

Maquis species play a central role in the maintenance of coastal ecosystems thanks to anatomical, physiological and biochemical features evolved to cope with severe stress conditions. Because the seasonal and daily dynamics of physiological and biochemical traits of maquis species are not fully addressed, we performed a field study on three coexisting Mediterranean shrubs (Pistacia lentiscus L. and Phillyrea latifolia L., evergreen schlerophylls, and Cistus incanus L., semi-deciduous) aiming at detecting the main adaptive differences, on a seasonal and daily basis, in primary and secondary metabolism along with the principal climatic determinants. These species differed in their physiological and biochemical responses especially on a seasonal level. In P. latifolia, a great investment in antioxidant phenylpropanoids contributed to maintain high photosynthetic rates throughout the whole growing season. In C. incanus, high carotenoid content associated with chlorophyll (Chl) regulation alleviated oxidative damage during the hot and dry summers and help recover photosynthesis in autumn. In P. lentiscus, high abscisic acid levels allowed a strict control of stomata, while fine Chla/Chlb regulation concurred to avoid photoinhibition in summer. Temperature resulted the most important climatic factor controlling the physiological and biochemical status of these coexisting shrubs and, thus, in determining plant performances in this Mediterranean coastal habitat.

Soil fertility and rainfall during specific phenological phases affect seed trait variation in a widely distributed Neotropical tree, Copaifera langsdorffii

PREMISE

Knowledge of intra-specific variation in seed traits and its environmental determinants is important for predicting plant responses to environmental changes. Here, we tested the hypothesis that differences in soil fertility and rainfall during specific phenological phases drive variation in seed traits in a widely distributed tree, Copaifera langsdorffii. We also tested the hypothesis that climatic heterogeneity increases within-plant variation in seed traits.

METHODS

Inter- and intra-population and within-plant variation in seed mass, number, and seed size/seed number were evaluated for 50 individuals from five populations distributed along a rainfall gradient and occurring on varying soil types. Using multivariate approaches, we tested the effects of soil fertility characteristics and rainfall in five reproductive phenological phases on seed traits.

RESULTS

The seed traits varied greatly both among populations and within plants. Inter-population variation in seed mass was driven by total rainfall during fruit development, and variation in seed number was influenced by total rainfall during the dry season before the reproductive phase. Phosphorus levels and potential acidity of the soil also explained the variations in seed mass and seed mass/seed number, respectively. A positive association between intra-annual variation in rainfall and within-plant variation in seed mass and seed number was found.

CONCLUSION

Both rainfall during specific reproductive phases and soil conditions shape the variation in the seed mass and number of C. langsdorffii. Environment-driven seed trait variation may contribute to this species' broad niche breadth, which in turn may determine the species' persistence under future climatic conditions.

Drought Impacts and Compounding Mortality on Forest Trees in the Southern Sierra Nevada

The increase in compounding disturbances, such as “hotter droughts” coupled with insect outbreaks, has significant impacts on the integrity of forested ecosystems and their subsequent management for important ecosystem services and multiple-use objectives. In the Southern Sierra Nevada, years of severe drought have resulted in unprecedented tree mortality across this mountainous landscape. Additionally, past land management practices, including fire suppression, have led to overly stocked, homogenous forest stand structures, dominated by small diameter, shade-tolerant and fire-intolerant tree species. Thus, the current condition of the landscape has further increased the susceptibility of forest trees to multiple stressors. We sought to determine the effects of extreme drought and insect outbreaks on tree mortality and their influence on forest stand structure and composition. To characterize mortality patterns, we monitored the condition of mature forest trees (>25.4 cm diameter at breast height) across 255 monitoring plots with four repeated measurements from 2015 through 2017. Tree mortality varied by species and through time. Reductions in pine species (Pinus lambertiana Douglas and P. ponderosa Lawson & C. Lawson) occurred earlier in the study period than Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr. or Calocedrus decurrens (Torr.) Florin. Across species, larger tree size, most often associated with tree height, was consistently related to increased survival in mature, overstory trees. As expected, sites with greater pine stocking and subsequently more bark beetle (Curculionidae: Scolytinae) host availability had increased pine mortality, especially for P. ponderosa. For Abies concolor, lower overstory basal area increased tree survival for this species. This study highlights the importance of effective forest monitoring, especially during a period of unprecedented ecological change as the compounding disturbance had a disproportional effect on pine species in smaller diameter classes. Proactive forest management may be necessary to maintain and promote these ecologically important species in heterogeneous mixtures across the landscape.

Differences in the Response to Acute Drought and Phytophthora cinnamomi Rands Infection in Quercus ilex L. Seedlings

The sustainability of “dehesas” is threatened by the Holm oak decline. It is thought that the effects of root rot on plant physiology vary depending on external stress factors. Plant growth and biomass allocation are useful tools to characterize differences in the response to drought and infection. The study of physiological responses together with growth patterns will clarify how and to what extent root rot is able to damage the plant. A fully factorial experiment, including drought and Phytophtora cinnamomi Rands infection as factors, was carried out with Quercus ilex L. seedlings. Photosynthesis, biomass allocation and root traits were assessed. Photosynthetic variables responded differently to drought and infection over time. The root mass fraction showed a significant reduction due to infection. P. cinnamomi root rot altered the growth patterns. Plants could not recover from the physiological effects of infection only when the root rot coincided with water stress. Without additional stressors, the strategy of our seedlings in the face of root rot was to reduce the biomass increment and reallocate resources. Underlying mechanisms involved in plant-pathogen interactions should be considered in the study of holm oak decline, beyond the consideration of water stress as the primary cause of tree mortality.

Relationships between plant drought response, traits, and climate of origin for green roof plant selection

The ideal species for green or vegetated roofs should have high water use after rainfall to maximize stormwater retention but also survive periods with low water availability in dry substrates. Shrubs have great potential for green roofs because they have higher rates of water use, and many species are also drought tolerant. However, not all shrub species will be suitable and there may be a trade-off between water use and drought tolerance. We conducted a glasshouse experiment to determine the possible trade-offs between shrub water use for stormwater management and their response to drought conditions. We selected 20 shrubs from a wide range of climates of origin, represented by heat moisture index (HMI) and mean annual precipitation (MAP). Under well-watered (WW) and water-deficit (WD) conditions, we assessed morphological responses to water availability; evapotranspiration rate (ET) and midday water potential (Ψ_MD ) were used to evaluate species water use and drought response. In response to WD, all 20 shrubs adjusted their morphology and physiology. However, there were no species that simultaneously achieved high rates of water use (high ET) under WW and high drought tolerance (low Ψ_MD ) under WD conditions. Although some species which had high water use under WW conditions could avoid drought stress (high Ψ_MD ). Water use was strongly related to plant biomass, total leaf area, and leaf traits (specific leaf area [SLA] and leaf area ratio [LAR]). Conversely, drought response (Ψ_MD ) was not related to morphological traits. Species' climate of origin was not related to drought response or water use. Drought-avoiding shrubs (high Ψ_MD ) could optimize rainfall reduction on green roofs. Water use was related to biomass, leaf area, and leaf traits; thus, these traits could be used to assist the selection of shrubs for stormwater mitigation on green roofs. The natural distribution of species was not related to their water use or drought response, which suggests that shrubs from less arid climates may be suitable for use on green roofs. Selecting species based on traits and not climate of origin could both improve green roof performance and biodiversity outcomes by expanding the current plant palette.

Warming increases the sensitivity of seedling growth capacity to rainfall in six temperate deciduous tree species

Predicting the effects of climate change on tree species and communities is critical for understanding the future state of our forested ecosystems. We used a fully factorial precipitation (three levels; ambient, -50 % ambient, +50 % ambient) by warming (four levels; up to +4 °C) experiment in an old-field ecosystem in the northeastern USA to study the climatic sensitivity of seedlings of six native tree species. We measured whole plant-level responses: survival, total leaf area (TLA), seedling insect herbivory damage, as well as leaf-level responses: specific leaf area (SLA), leaf-level water content (LWC), foliar nitrogen (N) concentration, foliar carbon (C) concentration and C:N ratio of each of these deciduous species in each treatment across a single growing season. We found that canopy warming dramatically increased the sensitivity of plant growth (measured as TLA) to rainfall across all species. Warm, dry conditions consistently reduced TLA and also reduced leaf C:N in four species (Acer rubrum, Betula lenta, Prunus serotina, Ulmus americana), primarily as a result of reduced foliar C, not increased foliar N. Interestingly, these conditions also harmed the other two species in different ways, increasing either mortality (Populus grandidentata) or herbivory (Quercus rubra). Specific leaf area and LWC varied across species, but did not show strong treatment responses. Our results indicate that, in the northeastern USA, dry years in a future warmer environment could have damaging effects on the growth capacity of these early secondary successional forests, through species-specific effects on leaf production (total leaves and leaf C), herbivory and mortality.

Drought responses, phenotypic plasticity and survival of Mediterranean species in two different microclimatic sites

Climate models predict a further drying of the Mediterranean summer. One way for plant species to persist during such climate changes is through acclimation. Here, we determine the extent to which trait plasticity in response to drought differs between species and between sites, and address the question whether there is a trade-off between drought survival and phenotypic plasticity. Throughout the summer we measured physiological traits (photosynthesis - A_max , stomatal conductance - g_s , transpiration - E, leaf water potential - ψl) and structural traits (specific leaf area - SLA, leaf density - LD, leaf dry matter content - LDMC, leaf relative water content - LRWC) of leaves of eight woody species in two sites with slightly different microclimate (north- versus south-facing slopes) in southern Spain. Plant recovery and survival was estimated after the summer drought period. We found high trait variability between species. In most variables, phenotypic plasticity was lower in the drier site. Phenotypic plasticity of SLA and LDMC correlated negatively with drought survival, which suggests a trade-off between them. On the other hand, high phenotypic plasticity of SLA and LDMC was positively related to traits associated with rapid recovery and growth after the drought period. Although phenotypic plasticity is generally seen as favourable during stress conditions, here it seemed beneficial for favourable conditions. We propose that in environments with fluctuating drought periods there can be a trade-off between drought survival and growth during favourable conditions. When climate become drier, species with high drought survival but low phenotypic plasticity might be selected for.

Tree mortality across biomes is promoted by drought intensity, lower wood density and higher specific leaf area

Drought events are increasing globally, and reports of consequent forest mortality are widespread. However, due to a lack of a quantitative global synthesis, it is still not clear whether drought-induced mortality rates differ among global biomes and whether functional traits influence the risk of drought-induced mortality. To address these uncertainties, we performed a global meta-analysis of 58 studies of drought-induced forest mortality. Mortality rates were modelled as a function of drought, temperature, biomes, phylogenetic and functional groups and functional traits. We identified a consistent global-scale response, where mortality increased with drought severity [log mortality (trees trees^-1  year^-1 ) increased 0.46 (95% CI = 0.2-0.7) with one SPEI unit drought intensity]. We found no significant differences in the magnitude of the response depending on forest biomes or between angiosperms and gymnosperms or evergreen and deciduous tree species. Functional traits explained some of the variation in drought responses between species (i.e. increased from 30 to 37% when wood density and specific leaf area were included). Tree species with denser wood and lower specific leaf area showed lower mortality responses. Our results illustrate the value of functional traits for understanding patterns of drought-induced tree mortality and suggest that mortality could become increasingly widespread in the future.

Taxonomic and ecological relevance of the chlorophyll a fluorescence signature of tree species in mixed European forests

The variability of chlorophyll a fluorescence (ChlF) parameters of forest tree species was investigated in 209 stands belonging to six European forests, from Mediterranean to boreal regions. The modifying role of environmental factors, forest structure and tree diversity (species richness and composition) on ChlF signature was analysed. At the European level, conifers showed higher potential performance than broadleaf species. Forests in central Europe performed better than those in Mediterranean and boreal regions. At the site level, homogeneous clusters of tree species were identified by means of a principal component analysis (PCA) of ChlF parameters. The discrimination of the clusters of species was influenced by their taxonomic position and ecological characteristics. The species richness influenced the tree ChlF properties in different ways depending on tree species and site. Tree species and site also affected the relationships between ChlF parameters and other plant functional traits (specific leaf area, leaf nitrogen content, light-saturated photosynthesis, wood density, leaf carbon isotope composition). The assessment of the photosynthetic properties of tree species, by means of ChlF parameters, in relation to their functional traits, is a relevant issue for studies in forest ecology. The connections of data from field surveys with remotely assessed parameters must be carefully explored.

Chaparral Shrub Hydraulic Traits, Size, and Life History Types Relate to Species Mortality during California's Historic Drought of 2014

Chaparral is the most abundant vegetation type in California and current climate change models predict more frequent and severe droughts that could impact plant community structure. Understanding the factors related to species-specific drought mortality is essential to predict such changes. We predicted that life history type, hydraulic traits, and plant size would be related to the ability of species to survive drought. We evaluated the impact of these factors in a mature chaparral stand during the drought of 2014, which has been reported as the most severe in California in the last 1,200 years. We measured tissue water potential, native xylem specific conductivity, leaf specific conductivity, percentage loss in conductivity, and chlorophyll fluorescence for 11 species in February 2014, which was exceptionally dry following protracted drought. Mortality among the 11 dominant species ranged from 0 to 93%. Total stand density was reduced 63.4% and relative dominance of species shifted after the drought. Mortality was negatively correlated with water potential, native xylem specific conductivity, and chlorophyll fluorescence, but not with percent loss in hydraulic conductivity and leaf specific conductivity. The model that best explained mortality included species and plant size as main factors and indicated that larger plants had greater survival for 2 of the species. In general, species with greater resistance to water-stress induced cavitation showed greater mortality levels. Despite adult resprouters typically being more vulnerable to cavitation, results suggest that their more extensive root systems enable them to better access soil moisture and avoid harmful levels of dehydration. These results are consistent with the hypothesis that short-term high intensity droughts have the strongest effect on mature plants of shallow-rooted dehydration tolerant species, whereas deep-rooted dehydration avoiding species fare better in the short-term. Severe droughts can drive changes in chaparral structure as a result of the differential mortality among species.

Above-Ground Dimensions and Acclimation Explain Variation in Drought Mortality of Scots Pine Seedlings from Various Provenances

Seedling establishment is a critical part of the life cycle, thus seedling survival might be even more important for forest persistence under recent and future climate change. Scots pine forests have been disproportionally more affected by climate change triggered forest-dieback. Nevertheless, some Scots pine provenances might prove resilient to future drought events because of the species' large distributional range, genetic diversity, and adaptation potential. However, there is a lack of knowledge on provenance-specific survival under severe drought events and on how acclimation alters survival rates in Scots pine seedlings. We therefore conducted two drought-induced mortality experiments with potted Scots pine seedlings in a greenhouse. In the first experiment, 760 three-year-old seedlings from 12 different provenances of the south-western distribution range were subjected to the same treatment followed by the mortality experiment in 2014. In the second experiment, we addressed the question of whether acclimation to re-occurring drought stress events and to elevated temperature might decrease mortality rates. Thus, 139 four-year-old seedlings from France, Germany, and Poland were subjected to different temperature regimes (2012-2014) and drought treatments (2013-2014) before the mortality experiment in 2015. Provenances clearly differed in their hazard of drought-induced mortality, which was only partly related to the climate of their origin. Drought acclimation decreased the hazard of drought-induced mortality. Above-ground dry weight and height were the main determinants for the hazard of mortality, i.e., heavier and taller seedlings were more prone to mortality. Consequently, Scots pine seedlings exhibit a considerable provenance-specific acclimation potential against drought mortality and the selection of suitable provenances might thus facilitate seedling establishment and the persistence of Scots pine forest.

Above-Ground Dimensions and Acclimation Explain Variation in Drought Mortality of Scots Pine Seedlings from Various Provenances

Seedling establishment is a critical part of the life cycle, thus seedling survival might be even more important for forest persistence under recent and future climate change. Scots pine forests have been disproportionally more affected by climate change triggered forest-dieback. Nevertheless, some Scots pine provenances might prove resilient to future drought events because of the species' large distributional range, genetic diversity, and adaptation potential. However, there is a lack of knowledge on provenance-specific survival under severe drought events and on how acclimation alters survival rates in Scots pine seedlings. We therefore conducted two drought-induced mortality experiments with potted Scots pine seedlings in a greenhouse. In the first experiment, 760 three-year-old seedlings from 12 different provenances of the south-western distribution range were subjected to the same treatment followed by the mortality experiment in 2014. In the second experiment, we addressed the question of whether acclimation to re-occurring drought stress events and to elevated temperature might decrease mortality rates. Thus, 139 four-year-old seedlings from France, Germany, and Poland were subjected to different temperature regimes (2012-2014) and drought treatments (2013-2014) before the mortality experiment in 2015. Provenances clearly differed in their hazard of drought-induced mortality, which was only partly related to the climate of their origin. Drought acclimation decreased the hazard of drought-induced mortality. Above-ground dry weight and height were the main determinants for the hazard of mortality, i.e., heavier and taller seedlings were more prone to mortality. Consequently, Scots pine seedlings exhibit a considerable provenance-specific acclimation potential against drought mortality and the selection of suitable provenances might thus facilitate seedling establishment and the persistence of Scots pine forest.

Species-specific adaptations explain resilience of herbaceous understorey to increased precipitation variability in a Mediterranean oak woodland

To date, the implications of the predicted greater intra-annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on leaf-level physiological responses of five species covering the functional groups grasses, forbs, and legumes in the understorey of a Mediterranean oak woodland, with increasing precipitation variability, without altering total annual precipitation inputs. Although extending the dry period between precipitation events from 3 to 6 weeks led to increased soil moisture deficit, overall treatment effects on photosynthetic performance were not observed in the studied species. This resilience to prolonged water stress was explained by different physiological and morphological strategies to withstand periods below the wilting point, that is, isohydric behavior in Agrostis, Rumex, and Tuberaria, leaf succulence in Rumex, and taproots in Tolpis. In addition, quick recovery upon irrigation events and species-specific adaptations of water-use efficiency with longer dry periods and larger precipitation events contributed to the observed resilience in productivity of the annual plant community. Although none of the species exhibited a change in cover with increasing precipitation variability, leaf physiology of the legume Ornithopus exhibited signs of sensitivity to moisture deficit, which may have implications for the agricultural practice of seeding legume-rich mixtures in Mediterranean grassland-type systems. This highlights the need for long-term precipitation manipulation experiments to capture possible directional changes in species composition and seed bank development, which can subsequently affect ecosystem state and functioning.

Quantitative Analysis of Adventitious Root Growth Phenotypes in Carnation Stem Cuttings

Carnation is one of the most important species on the worldwide market of cut flowers. Commercial carnation cultivars are vegetatively propagated from terminal stem cuttings that undergo a rooting and acclimation process. For some of the new cultivars that are being developed by ornamental breeders, poor adventitious root (AR) formation limits its commercial scaling-up, due to a significant increase in the production costs. We have initiated a genetical-genomics approach to determine the molecular basis of the differences found between carnation cultivars during adventitious rooting. The detailed characterization of AR formation in several carnation cultivars differing in their rooting losses has been performed (i) during commercial production at a breeders’ rooting station and (ii) on a defined media in a controlled environment. Our study reveals the phenotypic signatures that distinguishes the bad-rooting cultivars and provides the appropriate set-up for the molecular identification of the genes involved in AR development in this species.

Species assemblage patterns around a dominant emergent tree are associated with drought resistance

Water availability has long been recognized as an important driver of species distribution patterns in forests. The conifer Agathis australis (D. Don) Lindl. (kauri; Araucariaceae) grows in the species-rich forests of northern New Zealand. It is accompanied by distinctive species assemblages, and during summer the soil beneath A. australis is often significantly drier than soils beneath surrounding broadleaved angiosperm canopy species. We used a shade house dry-down experiment to determine whether species that grow close to A. australis differed in drought tolerance physiology compared with species that rarely grow close to A. australis. Stomatal conductance (g(s)) was plotted against leaf water potential (ψ) to identify drought tolerance strategies. Seedlings of species that occur in close spatial association with A. australis (including A. australis seedlings) were most resistant to drought stress, and all displayed a drought avoidance strategy of either declining gs to maintain ψ or simultaneous declines in g(s) and ψ. The species not commonly occurring beneath A. australis, but abundant in the surrounding forest, were the most drought-sensitive species and succumbed relatively quickly to drought-induced mortality with rapidly declining gs and ψ values. These results were confirmed with diurnal measurements of g(s) and assimilation rates throughout the day, and leaf wilting analysis. We conclude that the varied abilities of the species to survive periods of drought stress as seedlings shapes the composition of the plant communities beneath A. australis trees. Furthermore, forest diversity may be impacted by climate change as the predicted intensification of droughts in northern New Zealand is likely to select for drought-tolerant species over drought-intolerant species.

Adaptive phenotypic plasticity of Siberian elm in response to drought stress: increased stomatal pore depth

Leaf stomatal characteristics of Siberian elm (Ulmus pumila) were investigated by electron microscopy and white light scanning interferometry. On the basis of average annual precipitations, two types of tree specimens were collected from Korea, China, and Mongolia: (1) trees under normal environmental conditions and (2) trees under arid conditions. Field emission scanning electron microscopy revealed oval-shaped stomata on the lower surface, and they were ca. 20 μm in width. In-lens secondary electron imaging showed differences in electron density and stomatal pore depth between the two types. According to the line profile analysis by white light scanning interferometry, stomata under arid conditions appeared to have higher levels of the stomatal pore depth than ones under normal conditions. Focused ion beam-field emission electron microscopy supported the increased stomatal pore depth with the increasing drought stress gradient. These results suggest that complementary microscopy can be employed to unravel the adaptive phenotypic plasticity of Siberian elm in response to drought stress.

Measured and modelled leaf and stand-scale productivity across a soil moisture gradient and a severe drought

Environmental controls on carbon dynamics operate at a range of interacting scales from the leaf to landscape. The key questions of this study addressed the influence of water and nitrogen (N) availability on Pinus palustris (Mill.) physiology and primary productivity across leaf and canopy scales, linking the soil-plant-atmosphere (SPA) model to leaf and stand-scale flux and leaf trait/canopy data. We present previously unreported ecophysiological parameters (e.g. V(cmax) and J(max)) for P. palustris and the first modelled estimates of its annual gross primary productivity (GPP) across xeric and mesic sites and under extreme drought. Annual mesic site P. palustris GPP was ∼23% greater than at the xeric site. However, at the leaf level, xeric trees had higher net photosynthetic rates, and water and light use efficiency. At the canopy scale, GPP was limited by light interception (canopy level), but co-limited by nitrogen and water at the leaf level. Contrary to expectations, the impacts of an intense growing season drought were greater at the mesic site. Modelling indicated a 10% greater decrease in mesic GPP compared with the xeric site. Xeric P. palustris trees exhibited drought-tolerant behaviour that contrasted with mesic trees' drought-avoidance behaviour.

Foliage response of young central European oaks to air warming, drought and soil type

Three Central European oak species, with four provenances each, were experimentally tested in 16 large model ecosystem chambers for their response to passive air warming (AW, ambient +1-2 °C), drought (D, -43 to -60% irrigation) and their combination (AWD) for 3 years on two forest soil types of pH 4 or 7. Throughout the entire experiment, the influence of the different ambient and experimental climates on the oak trees was strong. The morphological traits of the Quercus species were affected in opposing ways in AW and D treatments, with a neutral effect in the AWD treatment. Biochemical parameters and LMA showed low relative plasticity compared to the morphological and growth parameters. The high plasticity in physiologically important parameters of the three species, such as number of intercalary veins or leaf size, indicated good drought acclimation properties. The soil type influenced leaf chlorophyll concentration, C/N and area more than drought, whereas foliage mass was more dependent on drought than on soil type. Through comparison of visible symptom development with the water deficits, a drought tolerance threshold of -1.3 MPa was determined. Although Q. pubescens had xeromorphic leaf characteristics (small leaf size, lower leaf water content, high LMA, pilosity, more chlorophyll, higher C/N) and less response to the treatments than Q. petraea and Q. robur, it suffered more leaf drought injury and shedding of leaves than Q. petraea. However, if foliage mass were used as the criterion for sustainable performance under a future climate, Q. robur would be the most appropriate species.

Interspecific variation in compensatory regrowth to herbivory associated with soil nutrients in three Ficus (Moraceae) saplings

Plant compensatory regrowth is an induced process that enhances plant tolerance to herbivory. Plant behavior against herbivores differs between species and depends on resource availability, thus making general predictions related to plant compensatory regrowth difficult. To understand how soil nutrients determine the degree of compensatory regrowth for different plant species, we selected saplings of three Ficus species and treated with herbivore insects and artificial injury in both glasshouse conditions and in the field at two soil nutrient levels. Compensatory regrowth was calculated by biomass, relative growth rate and photosynthetic characteristics. A similar pattern was found in both the glasshouse and in the field for species F. hispida, where overcompensatory regrowth was triggered only under fertile conditions, and full compensatory regrowth occurred under infertile conditions. For F. auriculata, overcompensatory regrowth was stimulated only under infertile conditions and full compensatory regrowth occurred under fertile conditions. Ficus racemosa displayed full compensatory regrowth in both soil nutrient levels, but without overcompensatory regrowth following any of the treatments. The three Ficus species differed in biomass allocation following herbivore damage and artificial injury. The root/shoot ratio of F. hispida decreased largely following herbivore damage and artificial injury, while the root/shoot ratio for F. auriculata increased against damage treatments. The increase of shoot and root size for F. hispida and F. auriculata, respectively, appeared to be caused by a significant increase in photosynthesis. The results indicated that shifts in biomass allocation and increased photosynthesis are two of the mechanisms underlying compensatory regrowth. Contrasting patterns among the three Ficus species suggest that further theoretical and empirical work is necessary to better understand the complexity of the plant responses to herbivore damage.

Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture

BACKGROUND AND AIMS

Plasticity in structural and functional traits related to water balance may determine plant performance and survival in ecosystems characterized by water limitation or high levels of rainfall variability, particularly in perennial herbaceous species with long generation cycles. This paper addresses whether and the extent to which several such seasonal to long-term traits respond to changes in moisture availability.

METHODS

Using a novel approach that integrates ecology, physiology and anatomy, a comparison was made of lifetime functional traits in the root xylem of a long-lived perennial herb (Potentilla diversifolia, Rosaceae) growing in dry habitats with those of nearby individuals growing where soil moisture had been supplemented for 14 years. Traditional parameters such as specific leaf area (SLA) and above-ground growth were also assessed.

KEY RESULTS

Individuals from the site receiving supplemental moisture consistently showed significant responses in all considered traits related to water balance: SLA was greater by 24 %; roots developed 19 % less starch storing tissue, an indicator for drought-stress tolerance; and vessel size distributions shifted towards wider elements that collectively conducted water 54 % more efficiently - but only during the years for which moisture was supplemented. In contrast, above-ground growth parameters showed insignificant or inconsistent responses.

CONCLUSIONS

The phenotypic changes documented represent consistent, dynamic responses to increased moisture availability that should increase plant competitive ability. The functional plasticity of xylem anatomy quantified in this study constitutes a mechanistic basis for anticipating the differential success of plant species in response to climate variability and change, particularly where water limitation occurs.