Spatial and temporal tree colonization in a midstream sediment bar and the mechanisms governing tree mortality during a flood event

Abstract

Reduced disturbances in regulated rivers can result in stable tree colonisation in downstream reaches. We examined spatial and temporal tree colonisation on a midstream sediment bar, focussing on three dominant taxa: Salix spp. (primarily Salix gilgiana), Robinia pseudoacacia and Albizia julibrissin. We developed allometric relationships for tree height, tree age and tree diameter at breast and knee height, as well as relationships between above and below-ground biomass. The spatial profiles for all individual trees of these taxa were obtained before and after a major flood in September 2007. Pre-flood data indicate that R. pseudoacacia colonize the highest elevations, A. julibrissin lower elevations and Salix spp. the lowest elevations of the three taxa. The oldest trees at each elevation were found to have survived 1.8–2.5 m inundation in past floods. Results of investigations subsequent to the flood event in September 2007 suggest that trees can survive flooding as long as their roots remain embedded in the soil. Individuals of Salix spp. experienced higher levels of inundation, but showed the greatest survival. This is probably due to morphological acclimations, particularly deep root systems and high elasticity of shoots. Considering inundation depths, erosion depths and sediment particle sizes in tree habitats, we found that sediment mobilization was the major cause of tree mortality. Thus, the ability of Salix spp. to colonize coarse sediments greatly contributed to its survival, as coarse sediments result in significantly less tractive force. Copyright # 2009 John Wiley & Sons, Ltd.