FLOODING ALTERS APPARENT POSITION OF FLOODPLAIN SAPLINGS ON A LIGHT GRADIENT

Abstract

It has been hypothesized that flooding, interacting with light availability, can determine spatial patterns of tree regeneration by causing differential germination or survival among seedlings in river floodplains. We conducted a canonical correspondence analysis (CCA, a form of direct gradient analysis) of sapling and environmental data collected from 1980 to 1990 to investigate this hypothesis. The CCA recovered two significant axes, and of the eight environmental variables included, only elevation and percentage sky explained significant portions of the variability in species location. Axis 1 was correlated with elevation, while Axis 2 was correlated with percentage sky. Comparison of first axis rankings with species flood tolerance rankings obtained from the literature showed substantial agreement, indicating that elevation was a surrogate for flood tolerance. Comparison of second axis rankings with species shade tolerance rankings obtained from the literature showed little correspondence between position on the light gradient and species shade tolerance rankings; several of the most shade-tolerant species were placed on the high-light end of the light gradient. These results suggest that sapling distributions are responding primarily to flooding and light. The failure of the species to assort along the light gradient in accordance with their shade tolerance suggests that flooding interacts with life history traits of individual species, especially shade tolerance, resulting in complex species-specific responses. When we attempt to tease apart the interaction between light and flooding, we find two components: flood tolerance may allow persistence of some species under lower light conditions than normal, and flood intolerance may limit some normally shade-tolerant species to high-light conditions, where growth is fast enough to allow escape from the flooding hazard.