FLUVIAL PROCESS AND THE ESTABLISHMENT OF BOTTOMLAND TREES

The effects of river regulation on bottomland tree communities in west ern North America have generated substantial concern because of the im portant habitat and aesthetic values of these communities. Considerati on of such effects in water management decisions has been hampered by the apparent variability of responses of bottomland tree communities t o flow alteration. When the relation between streamflow and tree estab lishment is placed in a geomorphic context, however, much of that vari ability is explained, and prediction of changes in the tree community is improved. The relation between streamflow and establishment of bott omland trees is conditioned by the dominant fluvial process or process es acting along a stream. For successful establishment, cottonwoods, p oplars, and willows require bare, moist surfaces protected from distur bance. Channel narrowing, channel meandering, and flood deposition pro mote different spatial and temporal patterns of establishment. During channel narrowing, the site requirements are met on portions of the be d abandoned by the stream, and establishment is associated with a peri od of low flow lasting one to several years. During channel meandering , the requirements are met on point bars following moderate or higher peak flows. Following flood deposition, the requirements are met on fl ood deposits high above the channel bed. Flood deposition can occur al ong most streams, but where a channel is constrained by a narrow valle y, this process may be the only mechanism that can produce a bare, moi st surface high enough to be safe from future disturbance. Because of differences in local bedrock, tributary influence, or geologic history , two nearby reaches of the same stream may be dominated by different fluvial processes and have different spatial and temporal patterns of trees. We illustrate this phenomenon with examples from forests of pla ins cottonwood (Populus deltoides ssp.,monilifera) along meandering an d constrained reaches of the Missouri River in Montana.