Feike Schieving

Patterns of light and nitrogen distribution in relation to whole canopy carbon gain in C3 and C4 mono- and dicotyledonous species

An analytical model was used to describe the optimal nitrogen distribution. From this model, it was hypothesized that the non-uniformity of the nitrogen distribution increases with the canopy extinction rate for light and the total amount of free nitrogen in the canopy, and that it is independent of the slope of the relation between light saturated photosynthesis (Pm) and leaf nitrogen content (nL). These hypotheses were tested experimentally for plants with inherently different architectures and different photosynthetic modes. A garden experiment was carried out with a C3 monocot [rice, Oryza sativa (L.)], a C3 dicot [soybean, Glycine max (L.) Merr] a C4 monocot [sorghum, Sorghum bicolor (L.) Moensch] and a C4 dicot [amarantus, Amaranthus cruentus (L.)]. Leaf photosynthetic characteristics as well as light and nitrogen distribution in the canopies of dense stands of these species were measured. The dicot stands were found to have higher extinction coefficients for light than the monocot stands. Dicots also had more non-uniform N distribution patterns. The main difference between the C3 and C4 species was that the C4 species were found to have a greater slope value of the leaf-level Pm—nL relation. Patterns of N distribution were similar in stands of the C3 and C4 species. In general, these experimental results were in accordance with the model predictions, in that the pattern of nitrogen allocation in the canopy is mainly determined by the extinction coefficient for light and the total amount of free nitrogen.

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.

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 (ρD) need to produce thicker stems but invest less mass per unit stem length than those with high ρD. 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 ρD are more prone to mechanical failure. Contrary to our results for stems, we predicted that high ρD can be more efficient than low ρD 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.

3-D GROWTH PATTERNS OF TREES: EFFECTS OF CARBON ECONOMY, MERISTEM ACTIVITY, AND SELECTION

A functional-structural plant growth model was used to explore how selection might influence the ontogenetic patterns in three-dimensional (3-D) growth of trees. The 3-D plant structure is defined by the orientation of metamers. The dynamics in the 3-D plant structure depend on the production of metamers and/or leaf pipes and the loss of such plant components. In the simulations, metamer and leaf-pipe traits were kept constant, so all ontogenetic changes depended on the spatial arrangement of metamers and/or leaf pipes. This study explores the consequences of three new assumptions for ontogenetic changes in 3-D plant structure: (1) meristems are produced at the positions where branches fall, thus enabling a tree to maintain a viable meristem population within the crown; (2) metamers are placed at meristem positions in the 3-D structure where the carbon benefit over the expected life span of a leaf pipe is maximized; (3) the carbon allocation to reproduction maximizes the long-term reproductive output. In combination with the constraints set by the morphology of metamer and leaf pipe, the carbon economy, and light conditions, these assumptions explain how selection may cause a sigmoid expansion phase and a stable steady-state phase; adaptive responses in 3-D structure during ontogeny; limits to tree size (including height); constant allometric scaling during the expansion phase; different scaling for trees in different light environments; and responses in optimal reproductive allocation to forest light environments. These results support the idea that selection for maximizing the net carbon gain determines how trees change in 3-D tree structure during ontogeny and, at the same time, how they acclimate in 3-D structure in response to light gradients.

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.

A model on optimal root–shoot allocation and water transport in clonal plants

Abstract Experimental studies have shown that interconnected ramets of clonal plants may exchange resources and adjust root–shoot allocation patterns when exposed to spatially heterogeneous habitats, thereby enhancing the efficiency of resource extraction from patchy environments. If ramets of a clonal plant are placed in conditions of high light and low water availability, and connected ramets are exposed to a low light and high water supply, they can reciprocally translocate water and assimilates. In addition, ramets in both patch types can adjust their root–shoot allocation in a way to increase the uptake of a locally abundant resource. Resource exchange and morphological specialization in the uptake of above- versus below-ground resources (analogous to a ‘spatial division of labour’ in economic systems) can considerably enhance the performance of clonal plants. A mathematical model is constructed to formalize the primary processes held responsible for the beneficial effects of a spatial division of labour in clonal plants. The model simulates water flow through a two-ramet system as function of environmental conditions, plant-related characteristics and finds the optimal root–shoot allocation for the two ramets. Optimality is defined as the total rate of photosynthesis of the two ramets. Simulations are run in spatially homogeneous and heterogeneous environments. The model predictions suggest that interconnected ramets exposed to complementary patch types can maximise their performance by a functional specialization (i.e. plastic changes in root–shoot allocation) in the uptake of a locally abundant resource and by internally redistributing captured resources. This is in qualitative agreement with the results of experimental studies. The model outcome is largely determined by plant tissue conductivities for water transport, i.e. by root conductivity for water uptake, leaf conductivity for water loss and by internode conductivity for water sharing between ramets. The results of this study suggest a crucial role for the conductivity of stolon internodes in determining the degree of specialization and co-operation in clonal plants.

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.

Sapling performance along resource gradients drives tree species distributions within and across tropical forests

Niche differentiation is a major hypothesized determinant of species distributions, but its practical importance is heavily debated and its underlying mechanisms are poorly understood. Trait-based approaches have been used to infer niche differentiation and predict species distributions. For understanding underlying mechanisms, individual traits should be scaled up to whole-plant performance, which has rarely been done. We measured seven key traits that are important for carbon and water balance for 37 tropical tree species. We used a process-based plant physiological model to simulate the carbon budget of saplings along gradients of light and water availability, and quantified the performance of the species in terms of their light compensation points (a proxy for shade tolerance), water compensation points (proxy for drought tolerance), and maximum carbon gain rates (proxy for potential growth rate). We linked species performances to their observed distributions (the realized niches) at two spatial scale...

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.

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.