Browsing by Autor "H. ARNOLD VAN GELDER"

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Mechanical branch constraints contribute to life‐history variation across tree species in a Bolivian forest
(Wiley, 2006) Frank J. Sterck; H. ARNOLD VAN GELDER; Lourens Poorter
Summary Trade‐offs among plant traits may contribute to specialization for different environments and coexistence of plant species. This may be the first study that shows how trade‐offs among branch traits contribute to variation in crown size, light requirements and maximum height across multiple sympatric tree species in a tropical rain forest. Ten saplings were selected for each of 30 tree species in a Bolivian rain forest. Sapling height and crown dimensions were measured and branch and stem samples were harvested. Fresh density, dry density, modulus of rupture, centre of mass, biomass and diameters were determined for those samples. For each species, cantilever theory predicted the mass needed to produce a stable 1‐m long horizontal branch. Generally, shade‐tolerant species had denser and stronger branches, and produced a stable horizontal branch at lower resource costs. These species had branches with a higher resistance against mechanical failure, and a wide crown that favours effective light acquisition. Less shade‐tolerant species had low density and weak branches, short branches, high resource costs per unit branch length, and low resource costs per unit stem length. These traits seem advantageous under conditions of prolonged exposure to direct sunlight, where such species grow rapidly to reproductive size, while mechanical risks are low and light levels are favourable. Branch (wood and bark) traits are good predictors for performance differences across tree species in heterogeneous forest light environments. Physical trade‐offs among branch traits contribute to the specialization of tree species for different light habitats and to tree species coexistence in tropical rain forests, even within classical functional groups such as pioneers and shade tolerants.
Wood mechanics, allometry, and life‐history variation in a tropical rain forest tree community
(Wiley, 2006) H. ARNOLD VAN GELDER; Lourens Poorter; Frank J. Sterck
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.