Geomorphic variation in riparian tree mortality and stream coarse woody debris recruitment from record flooding in a coastal plain stream

Large floods are an important process controlling the structure and function of stream ecosystems. One of the ways floods affect streams is through the recruitment of coarse woody debris from stream-side forests. Stream valley geomorphology may mediate this interaction by altering flood velocity, depth, and duration. Little research has examined how floods and geomorphic features interact to control debris recruitment from riparian forests. With this in mind, we examined debris recruitment resulting from tree mortality during a record flood in a Georgia (U.S.A.) stream. We quantified debris characteristics as related to riparian geomorphology, and we examined the influence of floods on the structure of stream-side forests. The flood killed, and recruited into the stream debris pool, an average of 22 trees/km. Variation in recruitment was related to geomorphology; mortality was highest in reaches having narrow valleys and high elevations of riparian landforms, while it was lowest in reaches having wide valleys and low landform elevations. Species differed in probability of mortality; three taxa, out of 47, contributed 75% of new debris to the stream. The structure of stream-side forests reflected the influence of floods on tree mortality; forests along constrained reaches lack small individuals. Our results suggest that constrained reaches are the primary sources of debris during large floods, while unconstrained reaches function as debris sinks. Debris characteristics may be linked to floods through tree demography. Specifically, large floods have the potential to limit future recruitment of larger-sized susceptible species into the stream, by limiting the number of small trees that grow into the canopy.