COMPARATIVE TOLERANCES OF RIPARIAN WILLOWS AND COTTONWOODS TO WATER-TABLE DECLINE

Cottonwoods (Populus sp.) and willows (Salix sp.) generally dominate riparian landscapes across western North America. To investigate their relative tolerances to water-table decline, rooted shoot cuttings (saplings) of two willows, Salix exigua and S. drummondiana, and two cottonwoods, Populus angustifolia, and P. balsamifera, were grown in rhizopods, controlled growth devices that allow water-table manipulation. Water-table-decline rates of 0 to 12 cm/d were applied and plant growth and survival were monitored. In a second study, seedlings of S. exigua, S. lutea, P. balsamifera, and P. deltoides were grown in rhizopods and exposed to water-table-decline rates of 0 to 8 cm/d. For saplings and seedlings of both genera, gradual declines of 1 or 2 cm/d promoted root elongation and often promoted shoot growth relative to the constant water-table treatment (0 cm/d). More abrupt declines (> 2 cm/d) reduced growth and survival, and there were some differences in sensitivity within as well as across the two genera. Thus, the willow and cottonwood saplings were similarly affected by abrupt water-table decline, but willow seedlings were slightly more vulnerable than cottonwood seedlings. It is recommended that gradual stream stage recession along regulated rivers of about 2.5 cm/d in the late spring would encourage the recruitment of cottonwood seedlings, but subsequently, more gradual recession of about 1 cm/d through the lower elevational streamside zones would enable recruitment of S. exigua and other willows.