Consistent Coupling of Canopy Structure with Stem Diameter Distributions and Demography Reveals Organizing Principles of Amazonian Forest Architecture

The spatial structure of leaf area and light in forest canopies (canopy structure) critically influences ecosystem production and forest dynamics. Increasing light limitation (H1) with greater depth in the canopy may influence growth, mortality and the fluxes of individuals over size classes, which in turn determine the distribution of individuals over tree size (diameter distribution). On the other hand, physiological adaptations may allow trees to grow and survive in low light such that competition for canopy space (H2) regulates demographic fluxes and the diameter distribution. While there is acute need for theory that connects canopy structure with forest dynamics to predict the consequences of global increases in tree mortality, these hypotheses remain little explored and have never been evaluated in a model framework that quantitatively links (i) observations of canopy structure including light and leaf area, (ii) tree demography and (iii) diameter distributions. Building from metabolic scaling theory of tree architecture, we provide such a framework and show that diameter distributions from two sites with contrasting dynamics in the central Amazon can be predicted from remote observations of canopy structure, but only by incorporating crown plasticity to enhance light interception (H3). Uniquely, we derived light absorption estimates for size classes from this model and compared these estimates with plot-based observations of demographic transitions. We found similar relationships between absorption and demographic flux rates in sites, in spite of large differences in patterns of growth and mortality over tree size. This suggested that competition for canopy space may coordinate demographic rates and influence diameter distributions, but with an additional consistent influence of light limitation. Site differences in light absorption over tree size, furthermore, partially explained site differences in diameter distributions. The rapid remote detection of diameter distributions and canopy structure offers a powerful new approach for studying the mechanisms connecting canopy function with size-structured forest dynamics, and may thus critically improve theory of the biosphere.