Light interception efficiency explained by two simple variables: a test using a diversity of small- to medium-sized woody plants.

• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. • We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). • The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. • These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

Wood mechanics, allometry, and life-history variation in a tropical rain forest tree community.

Wood density plays a central role in the life-history variation of trees, and has important consequences for mechanical properties of wood, stem and branches, and tree architecture. Wood density, modulus of rupture, modulus of elasticity, and safety factors for buckling and bending were determined for saplings of 30 Bolivian rain forest tree species, and related to two important life-history axes: juvenile light demand and maximum adult stature. Wood density was strongly positively related to wood strength and stiffness. Species safety factor for buckling was positively related to wood density and stiffness, but tree architecture (height : diameter ratio) was the strongest determinant of mechanical safety. Shade-tolerant species had dense and tough wood to enhance survival in the understorey, whereas pioneer species had low-density wood and low safety margins to enhance growth in gaps. Pioneer and shade-tolerant species showed opposite relationships between species traits and adult stature. Light demand and adult stature affect wood properties, tree architecture and plant performance in different ways, contributing to the coexistence of rain forest species.

Leaf traits determine the growth-survival trade-off across rain forest tree species.

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.

Performance of trees in forest canopies: explorations with a bottom-up functional-structural plant growth model.

Here we present a functional-structural plant model that integrates the growth of metamers into a growing, three-dimensional tree structure, and study the effects of different constraints and strategies on tree performance in different canopies. The tree is a three-dimensional system of connected metamers, and growth is defined by the flush probability of metamers. Tree growth was simulated for different canopy light environments. The result suggest that: the constraints result in an exponential, logistic and decay phase; a mono-layered-leaf crown results from self-shading in a closed canopy; a strong apical control results in slender trees like tall stature species; the interaction between weak apical control and light response results in a crown architecture and performance known from short stature species in closed forest; correlated leaf traits explain interspecific differences in growth, survival and adult stature. The model successfully unravels the interaction effects of different constraints and strategies on tree growth in different canopy light environments.