A novel method of measuring leaf epidermis and mesophyll stiffness shows the ubiquitous nature of the sandwich structure of leaf laminas in broad-leaved angiosperm species.

Plant leaves commonly exhibit a thin, flat structure that facilitates a high light interception per unit mass, but may increase risks of mechanical failure when subjected to gravity, wind and herbivory as well as other stresses. Leaf laminas are composed of thin epidermis layers and thicker intervening mesophyll layers, which resemble a composite material, i.e. sandwich structure, used in engineering constructions (e.g. airplane wings) where high bending stiffness with minimum weight is important. Yet, to what extent leaf laminas are mechanically designed and behave as a sandwich structure remains unclear. To resolve this issue, we developed and applied a novel method to estimate stiffness of epidermis- and mesophyll layers without separating the layers. Across a phylogenetically diverse range of 36 angiosperm species, the estimated Young's moduli (a measure of stiffness) of mesophyll layers were much lower than those of the epidermis layers, indicating that leaf laminas behaved similarly to efficient sandwich structures. The stiffness of epidermis layers was higher in evergreen species than in deciduous species, and strongly associated with cuticle thickness. The ubiquitous nature of sandwich structures in leaves across studied species suggests that the sandwich structure has evolutionary advantages as it enables leaves to be simultaneously thin and flat, efficiently capturing light and maintaining mechanical stability under various stresses.

How light competition between plants affects their response to climate change.

How plants respond to climate change is of major concern, as plants will strongly impact future ecosystem functioning, food production and climate. Here, we investigated how vegetation structure and functioning may be influenced by predicted increases in annual temperatures and atmospheric CO2 concentration, and modeled the extent to which local plant-plant interactions may modify these effects. A canopy model was developed, which calculates photosynthesis as a function of light, nitrogen, temperature, CO2 and water availability, and considers different degrees of light competition between neighboring plants through canopy mixing; soybean (Glycine max) was used as a reference system. The model predicts increased net photosynthesis and reduced stomatal conductance and transpiration under atmospheric CO2 increase. When CO2 elevation is combined with warming, photosynthesis is increased more, but transpiration is reduced less. Intriguingly, when competition is considered, the optimal response shifts to producing larger leaf areas, but with lower stomatal conductance and associated vegetation transpiration than when competition is not considered. Furthermore, only when competition is considered are the predicted effects of elevated CO2 on leaf area index (LAI) well within the range of observed effects obtained by Free air CO2 enrichment (FACE) experiments. Together, our results illustrate how competition between plants may modify vegetation responses to climate change.

Functional traits determine trade-offs and niches in a tropical forest community.

How numerous tree species can coexist in diverse forest communities is a key question in community ecology. Whereas neutral theory assumes that species are adapted to common field conditions and coexist by chance, niche theory predicts that species are functionally different and coexist because they are specialized for different niches. We integrated biophysical principles into a mathematical plant model to determine whether and how functional plant traits and trade-offs may cause functional divergence and niche separation of tree species. We used this model to compare the carbon budget of saplings across 13 co-occurring dry-forest tree species along gradients of light and water availability. We found that species ranged in strategy, from acquisitive species with high carbon budgets at highest resource levels to more conservative species with high tolerances for both shade and drought. The crown leaf area index and nitrogen mass per leaf area drove the functional divergence along the simulated light gradient, which was consistent with observed species distributions along light gradients in the forest. Stomatal coordination to avoid low water potentials or hydraulic failure caused functional divergence along the simulated water gradient, but was not correlated to observed species distributions along the water gradient in the forest. The trait-based biophysical model thus explains how functional traits cause functional divergence across species and whether such divergence contributes to niche separation along resource gradients.

Trampling, defoliation and physiological integration affect growth, morphological and mechanical properties of a root-suckering clonal tree.

BACKGROUND AND AIMS: Grazing is a complex process involving the simultaneous occurrence of both trampling and defoliation. Clonal plants are a common feature of heavily grazed ecosystems where large herbivores inflict the simultaneous pressures of trampling and defoliation on the vegetation. We test the hypothesis that physiological integration (resource sharing between interconnected ramets) may help plants to deal with the interactive effects of trampling and defoliation. METHODS: In a field study, small and large ramets of the root-suckering clonal tree Populus simonii were subjected to two levels of trampling and defoliation, while connected or disconnected to other ramets. Plant responses were quantified via survival, growth, morphological and stem mechanical traits. KEY RESULTS: Disconnection and trampling increased mortality, especially in small ramets. Trampling increased stem length, basal diameter, fibrous root mass, stem stiffness and resistance to deflection in connected ramets, but decreased them in disconnected ones. Trampling decreased vertical height more in disconnected than in connected ramets, and reduced stem mass in disconnected ramets but not in connected ramets. Defoliation reduced basal diameter, leaf mass, stem mass and leaf area ratio, but did not interact with trampling or disconnection. CONCLUSIONS: Although clonal integration did not influence defoliation response, it did alleviate the effects of trampling. We suggest that by facilitating resource transport between ramets, clonal integration compensates for trampling-induced damage to fine roots.

A model of botanical collectors' behavior in the field: never the same species twice.

PREMISE OF THE STUDY: Because of their numbers, specimens in natural-history museums cannot be ignored when trying to answer one of the fundamental questions in science: What determines species diversity? The nonrandom nature of collecting does not allow most statistical tests or extrapolations of species estimates, or comparison of richness between areas (which, however, is still done frequently). METHODS: We present a simple simulation model, which starts from the assumption that collectors never collect the same species twice during collecting trips. The model allows the generation of the abundance distribution in a herbarium for any natural species abundance distribution, using a simple set of collecting strategies. KEY RESULTS: We show that, in essence, the strategy of "never collect the same species twice" is enough to generate the relative abundance distribution as found in a herbarium. We illustrate this using real plot and specimen data from two well-collected areas, one in central Guyana and one in Suriname. CONCLUSIONS: Because of the oversampling of rare species, it is perhaps not possible to use museum data to reconstruct the community structure in the field or even estimate a proper diversity number other than the number of species in a region.

The role of wood mass density and mechanical constraints in the economy of tree architecture.

By applying engineering theory, we found that in order to achieve a certain degree of stem mechanical stability, trees with low wood dry-mass density (rho(D)) need to produce thicker stems but invest less mass per unit stem length than those with high rho(D). Mechanical stability was expressed as the ability of the vertical stem to either support a plant's weight (i.e., the buckling safety factor) or resist wind forces without rupture. This contradicts the general notion that trees with low rho(D) are more prone to mechanical failure. Contrary to our results for stems, we predicted that high rho(D) can be more efficient than low rho(D) in terms of the mass needed to produce a branch of given length and resistance to rupture under its own weight. Such branches were also predicted to be more flexible. These predictions were generally in accordance with literature data for tropical tree species. This shows that differences in scaling rules associated with vertical self-loading, resistance to external forces, and the production of stable horizontal branches have important implications for the way in which different crown traits determine the balance between economy of crown design and mechanical stability.

Wind and mechanical stimuli differentially affect leaf traits in Plantago major.

• Analysing plant phenotypic plasticity in response to wind is complicated as this factor entails not only mechanical stress but also affects leaf gas and heat exchange. • We exposed Plantago major plants to brushing (mechanical stress, MS) and wind (MS and air flow) and determined the effects on physiological, morphological and mechanical characteristics of leaf petioles and laminas as well as on growth and biomass allocation at the whole-plant level. • Both MS and wind similarly reduced growth but their effects on morphological and mechanical plant traits were different. MS induced the formation of leaves with more slender petioles, and more elliptic and thinner laminas, while wind tended to evoke the opposite response. These morphological and mechanical changes increased lamina and petiole flexibility in MS plants, thus reducing mechanical stress by reconfiguration of plant structure. Responses to wind, on the other hand, seemed to be more associated with reducing transpiration. • These results show that responses to mechanical stress and wind can be different and even in the opposite direction. Plant responses to wind in the field can therefore be variable depending on overall environmental conditions and plant characteristics.

Effects of light and nutrient availability on leaf mechanical properties of Plantago major: a conceptual approach.

BACKGROUND AND AIMS: Leaf mechanical properties, which are important to protect leaves against physical stresses, are thought to change with light and nutrient availabilities. This study aims to understand phenotypic changes of leaf mechanical properties with respect to dry mass allocation and anatomy. METHODS: Leaf lamina strength (maximum force per unit area to fracture), toughness (work to fracture) and stiffness (resistance against deformation) were measured by punch-and-die tests, and anatomical and physiological traits were determined in Plantago major plants grown at different light and nutrient availabilities. A conceptual approach was developed by which punch strength and related carbon costs can be quantitatively related to the underlying anatomical and morphological traits: leaf thickness, dry-mass allocation to cell walls and cell-wall-specific strength. KEY RESULTS: Leaf lamina strength, toughness and stiffness (all expressed on a punch area basis) increased with light availability. By contrast, nutrient availability did not change strength or toughness, but stiffness was higher in low-nutrient plants. Punch strength (maximum force per unit punch area, F(max)/area) was analysed as the product of leaf mass per area (LMA) and F(max)/leaf mass (= punch strength/LMA, indicating mass-use efficiency for strength). The greater strength of sun leaves was mainly explained by their higher LMA. Shade leaves, by contrast, had a higher F(max)/leaf mass. This greater efficiency in shade leaves was caused by a greater fraction of leaf mass in cell walls and by a greater specific strength of cell walls. These differences are probably because epidermis cells constitute a relatively large fraction of the leaf cross-section in shaded leaves. Although a larger percentage of intercellular spaces were found in shade leaves, this in itself did not reduce 'material' strength (punch strength/thickness); it might, however, be important for increasing distance between upper and lower epidermis per unit mass and thus maintaining flexural stiffness at minimal costs. CONCLUSIONS: The consequences of a reduced LMA for punch strength in shaded leaves was partially compensated for by a mechanically more efficient design, which, it is suggested, contributes importantly to resisting mechanical stress under carbon-limited conditions.

Trait Acclimation Mitigates Mortality Risks of Tropical Canopy Trees under Global Warming.

There is a heated debate about the effect of global change on tropical forests. Many scientists predict large-scale tree mortality while others point to mitigating roles of CO2 fertilization and - the notoriously unknown - physiological trait acclimation of trees. In this opinion article we provided a first quantification of the potential of trait acclimation to mitigate the negative effects of warming on tropical canopy tree growth and survival. We applied a physiological tree growth model that incorporates trait acclimation through an optimization approach. Our model estimated the maximum effect of acclimation when trees optimize traits that are strongly plastic on a week to annual time scale (leaf photosynthetic capacity, total leaf area, stem sapwood area) to maximize carbon gain. We simulated tree carbon gain for temperatures (25-35°C) and ambient CO2 concentrations (390-800 ppm) predicted for the 21st century. Full trait acclimation increased simulated carbon gain by up to 10-20% and the maximum tolerated temperature by up to 2°C, thus reducing risks of tree death under predicted warming. Functional trait acclimation may thus increase the resilience of tropical trees to warming, but cannot prevent tree death during extremely hot and dry years at current CO2 levels. We call for incorporating trait acclimation in field and experimental studies of plant functional traits, and in models that predict responses of tropical forests to climate change.

A game theoretic analysis of research data sharing.

While reusing research data has evident benefits for the scientific community as a whole, decisions to archive and share these data are primarily made by individual researchers. In this paper we analyse, within a game theoretical framework, how sharing and reuse of research data affect individuals who share or do not share their datasets. We construct a model in which there is a cost associated with sharing datasets whereas reusing such sets implies a benefit. In our calculations, conflicting interests appear for researchers. Individual researchers are always better off not sharing and omitting the sharing cost, at the same time both sharing and not sharing researchers are better off if (almost) all researchers share. Namely, the more researchers share, the more benefit can be gained by the reuse of those datasets. We simulated several policy measures to increase benefits for researchers sharing or reusing datasets. Results point out that, although policies should be able to increase the rate of sharing researchers, and increased discoverability and dataset quality could partly compensate for costs, a better measure would be to directly lower the cost for sharing, or even turn it into a (citation-) benefit. Making data available would in that case become the most profitable, and therefore stable, strategy. This means researchers would willingly make their datasets available, and arguably in the best possible way to enable reuse.

Stimulating seedling growth in early stages of secondary forest succession: a modeling approach to guide tree liberation.

Excessive growth of non-woody plants and shrubs on degraded lands can strongly hamper tree growth and thus secondary forest succession. A common method to accelerate succession, called liberation, involves opening up the vegetation canopy around young target trees. This can increase growth of target trees by reducing competition for light with neighboring plants. However, liberation has not always had the desired effect, likely due to differences in light requirement between tree species. Here we present a 3D-model, which calculates photosynthetic rate of individual trees in a vegetation stand. It enables us to examine how stature, crown structure, and physiological traits of target trees and characteristics of the surrounding vegetation together determine effects of light on tree growth. The model was applied to a liberation experiment conducted with three pioneer species in a young secondary forest in Vietnam. Species responded differently to the treatment depending on their height, crown structure and their shade-tolerance level. Model simulations revealed practical thresholds over which the tree growth response is heavily influenced by the height and density of surrounding vegetation and gap radius. There were strong correlations between calculated photosynthetic rates and observed growth: the model was well able to predict growth of trees in young forests and the effects of liberation there upon. Thus, our model serves as a useful tool to analyze light competition between young trees and surrounding vegetation and may help assess the potential effect of tree liberation.

Modelling functional trait acclimation for trees of different height in a forest light gradient: emergent patterns driven by carbon gain maximization.

Forest trees show large changes in functional traits as they develop from a sapling in the shaded understorey to an adult in the light-exposed canopy. The adaptive function of such changes remains poorly understood. The carbon gain hypothesis suggests that these changes should be adaptive (acclimation) and that they serve to maximize net vegetative or reproductive growth. We explore the carbon gain hypothesis using a mechanistic model that combines an above-ground plant structure, a biochemical photosynthesis model and a biophysical stomatal conductance model. Our simulations show how forest trees that maximize their carbon gain increase their total leaf area, sapwood area and leaf photosynthetic capacity with tree height and light intensity. In turn, they show how forest trees increased crown stomatal conductance and transpiration, and how the carbon budget was affected. These responses in functional traits to tree height (and light availability) largely differed from the responses exhibited by exposed trees. Forest and exposed trees nevertheless shared a number of emergent patterns: they showed a similar decrease in the average leaf water potential and intercellular CO(2) concentration with tree height, and kept almost constant values for the ratio of light absorption to electron transport capacity, the ratio of photosynthetic capacity to water supply capacity, and nitrogen partitioning between electron transport and carboxylation. While most of the predicted qualitative responses in individual traits are consistent with field or lab observations, the empirical support for capacity balances is scarce. We conclude that modelling functional trait optimization and carbon gain maximization from underlying physiological processes and trade-offs generates a set of predictions for functional trait acclimation and maintenance of capacity balances of trees of different height in a forest light gradient, but actual tests of the predicted patterns are still scarce.

Height convergence in response to neighbour growth: genotypic differences in the stoloniferous plant Potentilla reptans.

Using a new experimental set up, the way in which height growth of stoloniferous plants is adjusted to that of their neighbours, as well as differences between genotypes in their ability to keep up with neighbour height growth were tested. Five Potentilla reptans genotypes inherently differing in petiole length were subjected to three experimental light gradients, involving light intensity and red : far-red ratio. Each plant was placed in a vertically adjustable cylinder of green foil, and the treatments differed in the speed of cylinder height increase and final height. Total weight of plants decreased from the 'Slow' to the 'Fast' treatment, while petiole length increased. Leaves reaching the top of the cylinder stopped petiole elongation, resulting in similar final heights for all genotypes in the 'Slow' treatment. In the 'Fast' treatment only the fastest-growing genotype maintained its position in the top of the cylinder and genotypes differed strongly in final height within the cylinders. Plants adjust their height growth to that of the surrounding vegetation, leading to height convergence in short light gradients that slowly increase. These adjustments and genotypic differences in ability to keep up with fast-growing neighbours can influence the outcome of competition for light.

The effects of mechanical stress and spectral shading on the growth and allocation of ten genotypes of a stoloniferous plant.

BACKGROUND AND AIMS: Because plants protect each other from wind, stand density affects both the light climate and the amount of mechanical stress experienced by plants. But the potential interactive effects of mechanical stress and canopy shading on plant growth have rarely been investigated and never in stoloniferous plants which, due to their creeping growth form, can be expected to respond differently to these factors than erect plants. METHODS: Plants of ten genotypes of the stoloniferous species Potentilla reptans were subjected to two levels of mechanical stress (0 or 40 daily flexures) and two levels of spectral shading (15 % of daylight with a red:far red ratio of 0.3 vs. 50 % daylight and a red:far red ratio of 1.2). KEY RESULTS: Mechanically stressed plants produced more leaves with shorter more flexible petioles, more roots, and more but less massive stolons. Responses to spectral shading were mostly in the opposite direction to thigmomorphogenesis, including the production of thinner, taller petioles made of more rigid tissue. The degree of thigmomorphogenesis was either independent of light climate or stimulated by spectral shading. At the genotypic level there were no clear correlations between responses to shade and mechanical stress. CONCLUSIONS: These results suggest that in stoloniferous plants mechanical stress results in clones with a more compact, shorter shoot structure and more roots. This response does not appear to be suppressed by canopy shading, which suggests that wind shielding (reduced mechanical stress) by neighbours in dense vegetation serves as a cue that induces shade avoidance responses such as increased petiole elongation.