Hydrologic Factors Affecting Initial Willow Seedling Establishment along a Subalpine Stream, Colorado, U.S.A.

Abstract

Willows (Salix spp.) are important riparian plants along high elevation streams of western North America, but the hydrologic conditions needed for initial recruitment are not well defined. A replicated plot experiment was used to determine the effects of (1) elevation above the stream (high, middle, or low), (2) soil texture (sand or gravel), (3) watering to simulate additional rainfall, and (4) an upstream water diversion on soil water content and first-year willow seedling survival along a subalpine reach of the Colorado River.Soil water content varied with elevation (F = 163.8, p < 0.001) and soil texture (F = 387.3, p < 0.001), while the effect of the watering treatment was not statistically significant. Soil water content controlled first-year willow seedling survival (r 2 = 0.257, p = 0.002), and no seedlings were found in plots with a mean soil water content of less than 15%. Sandy sites near the river elevation provide the optimal conditions for initial establishment of willows along this and other subalpine streams in the semiarid western United States. Potential establishment sites occur along abandoned channels and in drained beaver ponds, and creation of these sites depends on the occurrence of high flows. Therefore, as at lower elevation, cottonwood-dominated systems, maintenance of a flow regime that includes periodic high flows is essential for sustaining willow establishment along subalpine streams.The upstream diversion reduced the stream stage by up to 13 cm during the growing season, resulting in a similar decline in the water table level and a reduction of up to 15% in plot soil water content. Since seedling survival depends on the availability of soil water, water diversions may adversely affect willow establishment along subalpine streams.