EFFECTS OF ESTABLISHED WILLOWS ON PRIMARY SUCCESSION ON LYMAN GLACIERFOREFRONT, NORTH CASCADE RANGE, WASHINGTON, USA - EVIDENCE FOR SIMULTANEOUS CANOPY INHIBITION AND SOIL FACILITATION

The effect of established shrub willows (Salix commutata and S. phylic ifolia) was tested in a primary successional ecosystem at Lyman Glacie r forefront in the North Cascade Range (Washington, U.S.A.). To examin e the hypothesis that early successional plant individuals form center s of establishment for subsequent vascular plant colonizers, two exper iments were conducted to assay the effect of shrub willows on the esta blishment 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 e ffects of willow canopies and associated soils on germinant emergence and survival of an indigenous taxon, Pinus contorta, were evaluated (e xperiment 2). Both experiments indicated that the shrub willows do not serve as nuclei that facilitate the establishment of new, emerging pl ant individuals. In experiment 1, the willow canopy had no effect on t he 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 c anopies 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 developi ng beneath the willow can actually be a positive factor towards plant establishment.