Competition, exogenous disturbances and senescence shape tree size distribution in tropical forest: Evidence from tree mode of death in Central Amazonia

Questions: What processes shape the size-density distribution of tropical trees? Are these processes affected by topographic position? Location: Reserva Ducke, Central Amazonia, Manaus, Brazil. Methods: We identified tree mode of death in a 5-yr census interval (2003-2008) in 72 1-ha permanent plots distributed over 64 km2, and calculated the proportion of standing and fallen modes of deaths across size classes and by topographic position. Lowess regression was used to identify the dominance of standing or fallen modes across tree sizes. We used this information to separate the size-density distribution of trees into two parts hypothetically shaped by different processes. We fitted several models and tested the metabolic theory's prediction that size-density distribution scales with stem diameter to the -2 power. Results: Most dead trees <22 cm DBH and ≥48 cm DBH died standing, while trees between 22 and 48 cm DBH uprooted or snapped. Mode of death varied among topographic positions. On plateaux, standing death predominated across all tree size classes, while on slopes standing death dominated for small trees (<14 cm DBH), while fallen deaths dominated for intermediate to large trees (14 ≤ DBH <46 cm). In valleys, fallen modes of death dominated across all size classes. Scaling and negative exponential functions with a quadratic term fitted better the size-density distribution of all trees. The scaling function fitted better the distribution of small to intermediate-sized trees while the negative exponential function and functions with a quadratic term showed the best fit to distribution of intermediate to large trees. Conclusions: Competition is the major source of mortality for small- to intermediate-sized trees, while exogenous disturbances and senescence predominate for intermediate to large trees. Modes of death, and presumed sources of mortality, change according to topographic position, but the shape of size-density distributions is not affected, demonstrating that unknown mechanisms with different processes not assumed in the metabolic theory can produce similar distributions for tropical trees. Understanding the mechanisms that determine tropical forest structure will allow us to predict forest dynamics under future climate change scenarios. © 2012 International Association for Vegetation Science