Cottonwood understory zonation and its relation to floodplain stratigraphy

Species composition in floodplain vegetation is often related to initial floristics, succession, and disturbances such as flooding and herbivory. However, on a braided, cobble-bedded river with contrasting sediment facies, much of the composition of understory vegetation within a cottonwood forest was related to edaphic factors controlled by stream-deposit stratigraphy. Large shifts in understory species composition were more related to soil texture and water availability than deposit age or hydroperiod. Finer-textured and thicker overbank deposits had greater water availability and supported mesic species, while sites with coarse sediments to the surface supported only xeric species. A strong gradient of site moisture corresponded to a gradient in species composition. Soil water potential (Psi(s)) indicated by pre-dawn xylem water potential (XWPPD) in a shallow-rooted grass reached -4.2 MPa on xeric-species sites in mid-summer; on mesic-species sites, XWPPD was always greater than -1.5 MPa for the same grass species. Deep-rooted plants had a narrower range of XWPPD across all sites, and XWPPD was typically between 0.0 and -1.0 MPa. Psi(s) derived from actual soil water content followed the same pattern indicated by species composition, soil texture, and XWPPD.