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 (P) and leaf nitrogen content (n). These hypotheses were tested experimentally for plants with inherently different architectures and different photosynthetic modes. A garden experiment was carried out with a C monocot [rice, Oryza sativa (L.)], a C dicot [soybean, Glycine max (L.) Merr] a C monocot [sorghum, Sorghum bicolor (L.) Moensch] and a C 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 C and C species was that the C species were found to have a greater slope value of the leaf-level P-n relation. Patterns of N distribution were similar in stands of the C and C 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.

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.

A dominant hypothesis explaining tree species coexistence in tropical forest is that trade-offs in characters allow species to adapt to different light environments, but tests for this hypothesis are scarce. This study is the first that uses a theoretical plant growth model to link leaf trade-offs to whole-plant performances and to differential performances across species in different light environments. Using data of 50 sympatric tree species from a Bolivian rain forest, we observed that specific leaf area and photosynthetic capacity codetermined interspecific height growth variation in a forest gap; that leaf survival rate determined the variation in plant survival rate under a closed canopy; that predicted height growth and plant survival rate matched field observations; and that fast-growing species had low survival rates for both field and predicted values. These results show how leaf trade-offs influence differential tree performance and tree species' coexistence in a heterogeneous light environment.Keywords: carbon, economy, growth-survival trade-off, leaf trade-off, rain forest trees, plant growth model.One way of understanding why plant species can coexist is by assuming that they partition resource gradients in space or time. Plant species from high-resource environments tend to have high resource acquisition rates, high resource turnover rates, and fast growth rates, whereas plant species from low-resource environments have lower acquisition and turnover rates and persist for a longer time. This general observation applies to a variety of plant communities, ranging from grasslands to forests. For example, grass species of eutrophic environments have higher nitrogen uptake and loss rates than grass species of oligotrophic environments (Berendse and Elberse 1989), and forest trees of high-light gaps have higher carbon uptake and loss rates than slow-growing trees that survive in the shade (Veneklaas and Poorter 1998;Walters and Reich 1999).From these observations it has been suggested that in productive environments, plants maximize their growth rates by continuously placing new roots or leaves in favorable patches. The high resource availability allows them to rapidly "pay back" the costs of producing leaves and roots. Conversely, in nonproductive environments, plants pay back the construction costs through slow turnover rates of roots and leaves and long residence times of key resources such as nitrogen and carbon in plant parts. Here, the slow turnover enhances the survival of a plant because otherwise, the species would run out of essential nutrient and carbon resources and die. Such a conservative resource-use strategy, however, comes at the expense of a reduced growth rate (Williams et al. 1989). These interactions between environmental productivity and turnover rates of plant components, carbon, and nutrients can thus explain the trade-off between growth and persistence across species (e

Models have been formulated for monospecific stands in which canopy photosynthesis is determined by the vertical distribution of leaf area, nitrogen and light. In such stands, resident plants can maximize canopy photosynthesis by distributing their nitrogen parallel to the light gradient, with high contents per unit leaf area at the top of the vegetation and low contents at the bottom. Using principles from game theory, we expanded these models by introducing a second species into the vegetation, with the same vertical distribution of biomass and nitrogen as the resident plants but with the ability to adjust its specific leaf area (SLA, leaf area : leaf mass). The rule of the game is that invaders replace the resident plants if they have a higher plant carbon gain than those of the resident plants. We showed that such invaders induce major changes in the vegetation. By increasing their SLA, invading plants could increase their light interception as well as their photosynthetic nitrogen-use efficiency (PNUE, the rate of photosynthesis per unit organic nitrogen). By comparison with stands in which canopy photosynthesis is maximized, those invaded by species of high SLA have the following characteristics : (1) the leaf area index is higher ; (2) the vertical distribution of nitrogen is skewed less ; (3) as a result of the supra-optimal leaf area index and the more uniform distribution of nitrogen, total canopy photosynthesis is lower. Thus, in dense canopies we face a classical tragedy of the commons : plants that have a strategy to maximize canopy carbon gain cannot compete with those that maximize their own carbon gain. However, because of this strategy, individual as well as total canopy carbon gain are eventually lower. We showed that it is an evolutionarily stable strategy to increase SLA up to the point where the PNUE of each leaf is maximized.

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.