13C labelling reveals different contributions of photoassimilates from infructescences for fruiting in two temperate forest tree species

Radial stem growth dynamics and phenology of a multi-stemmed species (Corylus avellana L.) across orchards in the Northern and Southern hemispheres

Climate change and the global economy impose new challenges in the management of food-producing trees and require studying how to model plant physiological responses, namely growth dynamics and phenology. Hazelnut (Corylus avellana L.) is a multi-stemmed forest species domesticated for nut production and now widely spread across different continents. However, information on stem growth and its synchronization with leaf and reproductive phenology is extremely limited. This study aimed at (i) defining the sequencing of radial growth phases in hazelnut (onset, maximum growth and cessation) and the specific temperature triggering stem growth; and (ii) combining the stem growth phases with leaf and fruit phenology. Point dendrometers were installed on 20 hazelnut trees across eight orchards distributed in the Northern and Southern hemisphere during a period of three growing seasons between 2015 and 2018. The radial growth variations and climatic parameters were averaged and recorded every 15 min. Leaf and reproductive phenology were collected weekly at each site. Results showed that stem radial growth started from day of year 84 to 134 in relation to site and year but within a relatively narrow range of temperature (from 13 to 16.5 °C). However, we observed a temperature-related acclimation in the cultivar Tonda di Giffoni. Maximum growth always occurred well before the summer solstice (on average 35 days) and before the maximum annual air temperatures. Xylogenesis developed rapidly since the time interval between onset and maximum growth rate was about 3 weeks. Importantly, the species showed an evident delay of stem growth onset with respect to leaf emergence (on average 4-6 weeks) rarely observed in tree species. These findings represent the first global analysis of radial growth dynamics in hazelnut, which is an essential step for developing models on orchard functioning and management on different continents.

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

BACKGROUND AND AIMS

In plants, high costs of reproduction during some years can induce trade-offs in resource allocation with other functions such as growth, survival and resistance against herbivores or extreme abiotic conditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortage at particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocation to reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was to test how allocation to reproduction was impacted by resource limitation.

METHODS

We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercus ilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal on fruit development (survival, growth and germination potential) and on the production of vegetative and reproductive organs the year following defoliation.

KEY RESULTS

We found that defoliation had little impact on fruit development. Fruit survival was not affected by the intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explained by an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germination success. However, in the following spring defoliated branches produced fewer shoots and compensated for leaf loss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocation to reproduction the following season but did not affect sex ratio.

CONCLUSIONS

Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale. Defoliation had larger legacy effects than immediate effects.

Growth rate reduction causes a decline in the annual incremental trunk growth in masting Fagus crenata trees

Tree trunk annual increments are markedly reduced in mast years. There are two hypotheses that could explain the mechanism for this phenomenon: (1) a reduction in the duration of growth due to switching the resource allocation from somatic growth to seed production; (2) reduction of growth rate due to resources being shared between somatic growth and reproduction simultaneously. In this study, we aimed to test these hypotheses in Fagus crenata  Blume from the point of view of resource allocation. The radial growth patterns in F. crenata during a year without reproduction (2014) and a masting year (2015) were monitored using a digital dendrometer. At the same time, shoot growth patterns were monitored by sampling branches from the top of the canopy. Data obtained using the digital dendrometer were fitted to a sigmoidal function, and the parameters of the function were evaluated with a hierarchal Bayesian approach; estimated parameters were used to represent the properties of trunk growth phenology. Trunk growth started synchronously just after leaf unfurling in both mass-fruiting (F15) and limited-fruiting (NF15) trees in 2014 and 2015. Reproduction reduced the growth rate in 2015. This was due to the resources being allocated for the development of cupules and for formation of relatively thick branches, both of which occurred simultaneously with trunk growth. There was no clear difference in the duration of radial growth between F15 and NF15 trees in the 2 years, although seed maturation started after trunk growth ceased. As a result, the annual trunk radius increment was reduced in the F15 trees in 2015. These results suggested that reduction of radial growth rate (Hypothesis 2) caused the reduction in annual trunk increment of reproducing trees of this species.

Mechanisms of mast seeding: resources, weather, cues, and selection

546 I. 546 II. 547 III. 548 IV. 552 V. 554 VI. 556 VII. 558 VIII. 558 IX. 559 559 References 559 SUMMARY: Mast seeding is a widespread and widely studied phenomenon. However, the physiological mechanisms that mediate masting events and link them to weather and plant resources are still debated. Here, we explore how masting is affected by plant resource budgets, fruit maturation success, and hormonal coordination of cues including weather and resources. There is little empirical support for the commonly stated hypothesis that plants store carbohydrates over several years to expend in a high-seed year. Plants can switch carbohydrates away from growth in high-seed years, and seed crops are more probably limited by nitrogen or phosphorus. Resources are clearly involved in the proximate mechanisms driving masting, but resource budget (RB) models cannot create masting in the absence of selection because some underlying selective benefit is required to set the level of a 'full' seed crop at greater than the annual resource increment. Economies of scale (EOSs) provide the ultimate factor selecting for masting, but EOSs probably always interact with resources, which modify the relationship between weather cues and reproduction. Thus, RB and EOS models are not alternative explanations for masting - both are required. Experiments manipulating processes that affect mast seeding will help clarify the physiological mechanisms that underlie mast seeding.

Pulse-labelling trees to study carbon allocation dynamics: a review of methods, current knowledge and future prospects

Pulse-labelling of trees with stable or radioactive carbon (C) isotopes offers the unique opportunity to trace the fate of labelled CO(2) into the tree and its release to the soil and the atmosphere. Thus, pulse-labelling enables the quantification of C partitioning in forests and the assessment of the role of partitioning in tree growth, resource acquisition and C sequestration. However, this is associated with challenges as regards the choice of a tracer, the methods of tracing labelled C in tree and soil compartments and the quantitative analysis of C dynamics. Based on data from 47 studies, the rate of transfer differs between broadleaved and coniferous species and decreases as temperature and soil water content decrease. Labelled C is rapidly transferred belowground-within a few days or less-and this transfer is slowed down by drought. Half-lives of labelled C in phloem sap (transfer pool) and in mature leaves (source organs) are short, while those of sink organs (growing tissues, seasonal storage) are longer. (13)C measurements in respiratory efflux at high temporal resolution provide the best estimate of the mean residence times of C in respiratory substrate pools, and the best basis for compartmental modelling. Seasonal C dynamics and allocation patterns indicate that sink strength variations are important drivers for C fluxes. We propose a conceptual model for temperate and boreal trees, which considers the use of recently assimilated C versus stored C. We recommend best practices for designing and analysing pulse-labelling experiments, and identify several topics which we consider of prime importance for future research on C allocation in trees: (i) whole-tree C source-sink relations, (ii) C allocation to secondary metabolism, (iii) responses to environmental change, (iv) effects of seasonality versus phenology in and across biomes, and (v) carbon-nitrogen interactions. Substantial progress is expected from emerging technologies, but the largest challenge remains to carry out in situ whole-tree labelling experiments on mature trees to improve our understanding of the environmental and physiological controls on C allocation.

Leaf traits, shoot growth and seed production in mature Fagus sylvatica trees after 8 years of CO2 enrichment

BACKGROUND AND AIMS

Masting, i.e. synchronous but highly variable interannual seed production, is a strong sink for carbon and nutrients. It may, therefore, compete with vegetative growth. It is currently unknown whether increased atmospheric CO(2) concentrations will affect the carbon balance (or that of other nutrients) between reproduction and vegetative growth of forest species. In this study, reproduction and vegetative growth of shoots of mature beech (Fagus sylvatica) trees grown at ambient and elevated atmospheric CO(2) concentrations were quantified. It was hypothesized that within a shoot, fruiting has a negative effect on vegetative growth, and that this effect is ameliorated at increased CO(2) concentrations.

METHODS

Reproduction and its competition with leaf and shoot production were examined during two masting events (in 2007 and 2009) in F. sylvatica trees that had been exposed to either ambient or elevated CO(2) concentrations (530 µmol mol(-1)) for eight consecutive years, between 2000 and 2008.

KEY RESULTS

The number of leaves per shoot and the length of terminal shoots was smaller or shorter in the two masting years compared with the one non-masting year (2008) investigated, but they were unaffected by elevated CO(2) concentrations. The dry mass of terminal shoots was approx. 2-fold lower in the masting year (2007) than in the non-masting year in trees growing at ambient CO(2) concentrations, but this decline was not observed in trees exposed to elevated CO(2) concentrations. In both the CO(2) treatments, fruiting significantly decreased nitrogen concentration by 25 % in leaves and xylem tissue of 1- to 3-year-old branches in 2009.

CONCLUSIONS

Our findings indicate that there is competition for resources between reproduction and shoot growth. Elevated CO(2) concentrations reduced this competition, indicating effects on the balance of resource allocation between reproduction and vegetative growth in shoots with rising atmospheric CO(2) concentrations.

The influence of climate and fructification on the inter-annual variability of stem growth and net primary productivity in an old-growth, mixed beech forest

The periodic production of large seed crops by trees (masting) and its interaction with stem growth has long been the objective of tree physiology research. However, very little is known about the effects of masting on stem growth and total net primary productivity (NPP) at the stand scale. This study was conducted in an old-growth, mixed deciduous forest dominated by Fagus sylvatica (L.) and covers the period from 2003 to 2007, which comprised wet, dry and regular years as well as two masts of Fagus and one mast of the co-dominant tree species Fraxinus excelsior (L.) and Acer pseudoplatanus (L.). We combined analyses of weather conditions and stem growth at the tree level (inter- and intra-annual) with fruit, stem and leaf production, and estimates of total NPP at the stand level. Finally, we compared the annual demand of carbon for biomass production with net canopy assimilation (NCA), derived from eddy covariance flux measurements, chamber measurements and modelling. Annual stem growth of Fagus was most favoured by warm periods in spring and that of Fraxinus by high precipitation in June. For stem growth of Acer and for fruit production, no significant relationships with mean weather conditions were found. Intra-annual stem growth of all species was strongly reduced when the relative plant-available water in soil dropped below a threshold of about 60% between May and July. The inter-annual variations of NCA, total NPP and leaf NPP at the stand level were low (mean values 1313, 662 and 168 g C m(-2) year(-1), respectively), while wood and fruit production varied more and contrarily (wood: 169-241 g C m(-2) year(-1); fruits: 21-142 g C m(-2) year(-1)). In all years, an annual surplus of newly assimilated carbon was calculated (on average 100 g C m(-2) year(-1)). The results suggest that stem growth is generally not limited by insufficient carbon resources; only in mast years a short-term carbon shortage may occur in spring. In contrast to common assumption, stem growth alone is not a sufficient proxy for total biomass production or the control of carbon sequestration by weather extremes.

Timing of canopy closure influences carbon translocation and seed production of an understorey herb, Trillium apetalon (Trilliaceae)

BACKGROUND AND AIMS

The light availability on a temperate, deciduous-forest floor varies greatly, reflecting the seasonal leaf dynamics of the canopy trees. The growth and/or reproductive activity of understorey plants should be influenced by the length of the high-irradiance period from snowmelt to canopy closure. The aim of the present study was to clarify how spring-blooming species regulate the translocation of photosynthetic products to current reproduction and storage organs during a growing season in accordance with the changing light conditions.

METHODS

Growth pattern, net photosynthetic rate, seed production, and shoot and flower production in the next year of Trillium apetalon were compared between natural and experimentally shaded conditions. Furthermore, translocation of current photosynthetic products within plants was assessed by a labelled carbon-chase experiment.

KEY RESULTS

During the high-irradiance period, plants showed high photosynthetic ability, in which current products were initially used for shoot growth, then reserved in the rhizome. Carbon translocation to developing fruit occurred after canopy closure, but this was very small due to low photosynthetic rates under the darker conditions. The shading treatment in the early season advanced the time of carbon translocation to fruit, but reduced seed production in the current year and flower production of the next year.

CONCLUSIONS

Carbon translocation to the storage organ had priority over seed production under high-irradiance conditions. A shortened bright period due to early canopy closure effectively restricts carbon assimilation, which greatly reduces subsequent reproductive output owing to low photosynthetic products for fruit development and small carbon storage for future reproduction. As populations of this species are maintained by seedling recruitment, acceleration of canopy closure timing may influence the maintenance and dynamics of populations.