Effects of Established Willows on Primary Succession on Lyman Glacier Forefront, North Cascade Range, Washington, U.S.A.: Evidence for Simultaneous Canopy Inhibition and Soil Facilitation

Abstract

The effect of established shrub willows (Salix commutata and S. phylicifolia) was tested in a primary successional ecosystem at Lyman Glacier forefront in the North Cascade Range (Washington, U.S.A.). To examine the hypothesis that early successional plant individuals form centers of establishment for subsequent vascular plant colonizers, two experiments were conducted to assay the effect of shrub willows on the establishment and survival of indigenous plants. First, the occurrence of indigenous plant species under willow canopies was compared with their occurrence beyond the canopies (experiment 1). Second, the separate effects of willow canopies and associated soils on germinant emergence and survival of an indigenous taxon, Pinus contorta, were evaluated (experiment 2). Both experiments indicated that the shrub willows do not serve as nuclei that facilitate the establishment of new, emerging plant individuals. In experiment 1, the willow canopy had no effect on the observed frequency of most indigenous taxa. Five species, however, were negatively associated with the willow canopies. In experiment 2, willow canopies inhibited the germinant emergence of P. contorta. The greatest emergence occurred in soils transferred from beneath willow canopies to areas beyond the canopies. Results from the two experiments suggest that while the willow canopy is either neutral or inhibitory in its effect on establishment of indigenous plants, the soil developing beneath the willow can actually be a positive factor towards plant establishment.