ALBERT

Description: Key message Mechanical properties of wood constrain most conifers to an excurrent form and limit the width of tree crowns. Development of support tissue alters allometric relations during ontogeny. Abstract Biomechanical constraints on tree architecture are explored. Torque on a tree branch is a multiplicative function of mass and moment arm. As such, the need for support rises faster than branch length, which leads to increased taper as branch size increases. This violates assumptions of models, such as the pipe-model theory, for large trees and causes changing allometry with tree size or exposure. Thus, assumptions about optimal design for light capture, self-similarity, or optimal hydraulic architecture need to be modified to account for mechanical constraints and costs. In particular, it is argued that mechanical limitations of compression wood in conifers prevent members of this taxon from developing large branches. With decurrent form ruled out (for larger species), only a conical or excurrent form can develop. Wind is shown to be a major mortality risk for trees. Adaptations for wind include dynamic responses of wood properties and height. It is argued that an adaptation to wind could be the development of an open crown in larger trees to let the wind penetrate, thereby reducing wind-throw risk. It is thus argued that crown shape and branching may result not just from optimal light capture considerations but also from adaptation to and response to wind as well as from mechanical constraints. Results have implications for allometric theory, life history theory, and simulations of tree architecture.

Description: Key message Biomass allocation in Castanea sativa varies according to the environmental conditions. Specifically, leaf-to-sapwood area ratio is higher on sites with good water supplies and lower in water-stressed conditions. Abstract Ecological plasticity allows organisms to adapt and to cope with environmental conditions. This is a key trait for species with long live span, which will probably be more vulnerable to changing climate because of their lower adaptation potential via natural selection. We studied the case of the sweet chestnut tree, a naturalized forest species in many European mountain areas, where it grows on very different climates and sites. This raises the question of its adaptation capacity to very different environmental conditions. To test this hypothesis, we applied the pipe model approach for analysing the variation in the leaf-to-sapwood area ratio ( A L : A S ) in 82 chestnut trees growing in very different site conditions (e.g., water-stressed convex vs. water-rich concave sites). We used linear regression analyses to model the A L : A S relationship to environmental and dendrometric parameters. Results confirm that A L : A S is significantly higher when trees grow on good nutrient- and water-supplied concave sites with respect to water-stressed, convex sites. Chestnut trees are thus able to vary their biomass allocation between sapwood and leaves to adapt their hydraulic characteristics to the site conditions. Trees seem to react to water-stressed conditions by allocating more biomass in the sapwood in respect to the leaves. A L : A S may thus represent a useful indicator of tree species plasticity and their adaptation potential to different environmental and climate conditions.

Description: Key message The endangered status and narrow distribution in Syringa pinnatifolia , a sub-alpine shrub species, were primarily caused by its poor drought adaptation related to its lower degree of physiological adjustment despite its greater degree of morphological plasticity. Abstract Although deep rooting is usually considered a drought-tolerant trait, we found that Syringa pinnatifolia , a deep rooting and hydrotropic shrub, has a limited distribution in arid areas. To elucidate the mechanisms for its narrow distribution, we conducted two experiments to examine the physiological and morphological responses to water availability and heterogeneity in S. pinnatifolia and a widespread congeneric species, S. oblata . We measured gas exchange, water use efficiency, and plasticity index in plants of these two species grown at different levels of soil water regimes and in containers with patched water distribution. Our results showed that high photosynthetic capacity in the narrowly distributed S. pinnatifolia was an important factor enabling its survival in the harsh sub-alpine environment. High photosynthetic capacity in S. pinnatifolia , however, was obtained at the expense of high transpiratory water loss, resulting in lower integrative water use efficiency. Biomass allocation to roots in S. pinnatifolia increased by 73 % when soil water increased from 75 to 95 % field capacity, suggesting that S. pinnatifolia could be less competitive for above-ground resources under favorable water regimes. The horizontal root hydrotropism and vertical root hydrotropism of S. pinnatifolia in soil with patched water patterns were likely related to compensation for leaf water loss at low soil water level, indicating a limited capacity for homeostasis within the plant for water conservation and lower level of inherent drought-tolerance. In summary, greater degree of morphological plasticity but lower degree of physiological adjustment may be the main causes for the hydrotropism and narrow distribution of S. pinnatifolia in the sub-alpine habitats.

Description: Key message The Acacia phyllode leaf form is hypothesised to be an adaptation to drought. However, in this experiment, the timing of phyllode development was not related to a low water treatment. Abstract Acacia species have markedly different leaf forms known as compound leaves, transitional leaves, and phyllodes, also known as heteroblastic development. The different leaf types are thought to confer an advantage under varying moisture regimes, with phyllodes favoured in drier conditions. The hypothesis that phyllodes develop earlier under low water treatment was tested in this experiment. Three watering level treatments (100, 50, and 25 %) were imposed on seedlings of A. implexa to assess developmental traits (leaf emergence, initial onset of transitional leaves, and phyllodes), biomass allocation patterns (root, stem, compound leaf area/mass, transitional leaf area/mass, and phyllode area/mass), and leaf anatomy traits (epidermis, palisade and spongy mesophyll, and stomatal density). Across watering treatments, there was no difference in the developmental onset of transitional leaves or phyllodes (produced at the 6th and 9th nodes, respectively). Under low watering treatment, there was a decrease in stem height per unit stem diameter, shorter internodes, and greater allocation of biomass to roots. There was no significant difference in leaf anatomy traits. Under the low watering treatment, there was less compound leaf area and mass due to leaf shedding. In this experiment, the low watering treatment did not favour phyllode development at the expense of compound leaf development. Rather, it was found that A. implexa responds to a low water treatment similarly to many other plant species: increased allocation to roots, increased stem area per unit stem height, decrease in leaf area through senescence of older leaves, and lower relative growth rates.

Description: Key message The total uptake of 15 NO 3 -N was twofold higher than that of 15 NH 4 -N when supplied with ammonium and/or nitrate in different seasons; the seedlings fertilized with NO 3 -N had good growth with high photosynthetic rate and total biomass. Abstract Appropriate fertilization is crucial for maximum plant growth and improving nitrogen use efficiency. Poplar is an important fast-growing tree species for biomass production, however, little is known about fertilizer management of poplar plantations growing on barren soil in different seasons. To understand nitrogen uptake and allocation of Populus simonii supplied with different forms of nitrogen in different seasons, we determined nitrogen uptake and allocation of P. simonii potted seedlings after a 4-day supply of 15 NH 4 -N, 15 NO 3 -N, 15 NH 4 NO 3 , and NH 4 15 NO 3 in May, July, and September. The total 15 N uptake was twofold higher when supplied with sole 15 NO 3 -N compared to sole 15 NH 4 -N in all the investigated seasons. In the presence of ammonium nitrate ( 15 NH 4 NO 3 and NH 4 15 NO 3 ), the total 15 N uptake was two times higher when supplied with NH 4 15 NO 3 compared to 15 NH 4 NO 3 . Per unit biomass, the 15 N-uptake ability of fine roots was higher in May and July compared to that in September. 15 N was present mainly in leaves in May and July, and was mainly stored in roots and stems in autumn. The effect of nitrogen on the growth of P. simonii seedlings was studied by fertilizing with NH 4 -N, NO 3 -N, and NH 4 NO 3 for 8 weeks. The seedlings fertilized with NO 3 -N had good growth with high photosynthetic rate and total biomass indicating that NO 3 -N is crucial for P. simonii growth. These data contribute to understand the nitrogen uptake in different seasons in trees supplied with different forms of nitrogen. This provides important theoretical bases for fertilizer management of poplar plantations.

Description: Key message A hypergeometric model is proposed explicitly instead of two previous stochastic models (the Poisson model and Neyman-A model) to describe the topological relationship of trees and the influence of the exclusion distance on gap fraction and clumping index of forest plantation canopies. Abstract Gap fraction (GF) and clumping index (CI) play key roles in plant light interception, and therefore they have strong impacts on plant growth and canopy radiative transfer processes. Trees are usually assumed to be randomly distributed in natural forests in many previous studies. However, few studies have shown how trees are distributed in forest plantations and how these distribution patterns affect GF and CI in these forests. In this paper, a simple and general distance factor defined as relative allowable shortest distance between centers of two adjacent crowns divided by the mean diameter of the crowns (RASD) is proposed to describe quantitatively the degree of mutual exclusion among trees in forest plantations of various tree distribution patterns. A hypergeometric model is proposed instead of two previous stochastic tree distribution models (the Poisson model and Neyman-A model) to describe the topological relationship of trees and the influences of the exclusion distance on the GF and CI of the forest plantation canopies. The results show that: (1) the hypergeometric model is more suitable than the Poisson model and Neyman-A model for describing the topological relationship of trees in forest plantations; (2) the exclusion distance has strong impacts on GF and CI: there are significant differences between the results of the hypergeometric model and the Poisson model. Larger RASD causes lower GF and larger CI. The simulations are verified by field measurements in four forest plantation stands. Similarly, impacts of RASD on GF and CI are also found for other two crown shapes (prolate and oblate ellipsoids).

Description: Key message Congeneric species showed similar stem and crown allometry, but differed in crown dimensions indicating that crown size is adaptive and variable despite mechanical restrictions. Abstract Morphological adaptations favor differential use of the space in tropical trees, but the variability in stem and crown allometry can be constrained by phylogenetic and mechanical factors. In addition, dioecious species show marked differences in their energy requirements related to reproduction, but little information is available about the role of shape and allometry on differential acquisition of energy between the sexes. We studied the stem and crown dimensions of congeneric dioecious trees to determine if there are: (i) differences in the allometry between the sexes, (ii) different average sizes among sympatric species, and (iii) differences in stem and crown allometry between sympatric and allopatric species. Two pairs of sympatric Virola (Myristicaceae) in Brazil and Costa Rica were studied. SMA regression models were used to investigate allometric relationships between diameter at breast height (DBH) and tree height, and between DBH and crown volume (CV). No sexual dimorphism in stem and crown morphology was observed in this study, indicating that differences in resource allocation for reproduction between the sexes do not impact the stem and crown structure in these species. Overall, low variability among the species was observed. Only one species differed in stem allometry and none differed in crown allometry. CV differed between sympatric species. Stem and crown allometry are related to structural stability and our results support similar mechanical restriction for these species. The ecological significance of differences in CV among canopy species remains to be explored.

Description: Key message Across five biogeographic areas, DBH-CA allometry was characterized by inter-site homogeneity and intra-site heterogeneity, whereas the reverse was observed for DBH-H allometry. Abstract Tree crowns play a central role in stand dynamics. Remotely sensed canopy images have been shown to allow inferring stand structure and biomass which suggests that allometric scaling between stems and crowns may be tight, although insufficiently investigated to date. Here, we report the first broad-scale assessment of stem vs. crown scaling exponents using measurements of bole diameter (DBH), total height ( H ), and crown area (CA) made on 4148 trees belonging to 538 species in five biogeographic areas across the wet tropics. Allometries were fitted with power functions using ordinary least-squares regressions on log-transformed data. The inter-site variability and intra-site (sub-canopy vs. canopy trees) variability of the allometries were evaluated by comparing the scaling exponents. Our results indicated that, in contrast to both DBH- H and H -CA allometries, DBH-CA allometry shows no significant inter-site variation. This fairly invariant scaling calls for increased effort in documenting crown sizes as part of tree morphology. Stability in DBH-CA allometry, indeed, suggests that some universal constraints are sufficiently pervasive to restrict the exponent variation to a narrow range. In addition, our results point to inverse changes in the scaling exponent of the DBH-CA vs. DBH- H allometries when shifting from sub-canopy to canopy trees, suggesting a change in carbon allocation when a tree reaches direct light. These results pave the way for further advances in our understanding of niche partitioning in tree species, tropical forest dynamics, and to estimate AGB in tropical forests from remotely sensed images.

Description: Key message After release, balsam fir has greater ability than red spruce to rapidly capture available light and growing space in a wide range of canopy opening. Abstract A large proportion of yellow birch ( Betula alleghaniensis Britt.)—conifer stands feature abundant advance growth of shade-tolerant balsam fir ( Abies balsamea [L.] Mill.) and red spruce ( Picea rubens Sarg.), on which ecosystem-based regeneration strategies should capitalize. However, the morphological response of advanced regeneration to higher light levels following harvesting has yet to be described. Preferential carbon allocation to apical vs. lateral meristems has a significant implication on post-cut forest dynamics. We assessed the morphological response of balsam fir and red spruce saplings (1.3–4.0 m in height) to different levels of canopy opening (removal of 0, 40, 50, 60 and 100 % of the basal area) in two sites located in the Québec City region, Canada. Five years after cutting, the height growth (HG), lateral shoot growth (LG) and HG/LG ratio of the two conifer species were highest in the 100 % cut, intermediate in the three partial cuts, and lowest in the control. However, balsam fir had a 40 % higher HG and a 60 % higher LG, resulting in an HG/LG ratio 30 % lower than red spruce. At that time, both species increased their LG in the upper part of the crown in cutting treatments, although this shift was less pronounced for spruce in partial cuts. The number of internodal branches increased following canopy opening, while the number of branches per whorl was less impacted. Relationships between light availability and morphological variables were strong (pseudo- R 2  = 0.54–0.71) and comparable between the two conifer species. However, balsam fir had higher HG and LG, and produced more internodal branches at all light levels. Fir even responded to higher light levels (60–70 % of full light) than spruce (〈60 %). Therefore, results from this study clearly demonstrate that balsam fir has greater ability than red spruce to capture available light and growing space in the short term and in a wide range of canopy opening.

Description: Key message In drought-stressed apple, leaf area decrease and stem growth cessation increase are key determinants of drought avoidance. Both the genotype and its possible phenotypic plasticity contribute to the variability of responses. Abstract Under soil water restriction, plant growth is impaired by the indirect negative impact on plant carbon balance of stomata closure and hydraulic failure. The relative contributions of these two phenomena have been scarcely explored in trees under different drought-stress intensities, and even less work is published which accounts for genetic variability and phenotypic plasticity. Working on 21 apple genotypes from the same progeny, we assessed the effects of two consecutive periods of soil drought, moderate and severe, on growth and functional patterns of leaf and stem. Leaf area decreased while temporary stem growth cessation increased under drought with strong variations depending on the genotype. These results suggested that both reduction of transpiring leaf area and leaf organogenesis are key determinants for drought avoidance in the apple. Results also confirmed the pivotal role of stomatal conductance ( g s ) in maintaining percent loss of conductivity of the stem xylem (PLC) under values for runaway embolism (ca. 14 % under severe drought). The sorting of genotypes according to their morphological response to drought showed that genotypes with high reduction of growth were characterized by similar g s but lower PLC than genotypes with medium and low reduction of growth. This suggests that for a given level of stomatal closure the drought-related reduction of leaf area could also limit the progression of cavitation in stem xylem. As a whole, the variability of morphological responses of apple genotypes to contrasted drought conditions indicated that both genetic variability and phenotypic plasticity are involved in the range of iso-anisohydry documented for this species.