EFFECTS OF ENVIRONMENT AND GRAZING DISTURBANCE ON TREE ESTABLISHMENT IN MEADOWS OF THE CENTRAL CASCADE RANGE, OREGON, USA

Within the last century there has been widespread establishment of tre es in mountain meadows of the Pacific Northwest. We reconstructed patt erns of tree invasion at 17 meadow sites in the central Cascade Range of Oregon, USA - sites representing diverse physical environments and vegetation types and experiencing different histories of recent anthro pogenic disturbance(sheep grazing). Spatial distributions and age stru ctures of invasive tree populations were analysed with respect to clim atic records and grazing history. Patterns of establishment varied con siderably among meadows, reflecting strong differences in environment and grazing history. In montane hydric meadows, tree establishment was spatially clumped beneath large old trees and on elevated microsites; however the timing of invasion differed between sites with stable ver sus fluctuating water tables. in upland mesic/dry montane meadows, tim ing of invasion corresponded with cessation of sheep grazing (early 19 40s) and the onset of wetter summers (mid 1940s). In the subalpine zon e, climate and aspect interacted to produce contrasting histories of i nvasion on north- and south-facing slopes. Establishment on north-faci ng slopes, concentrated in heath-shrub communities, coincided with reg ional warming (ca. 1920-1945) when snowpacks were lighter and melted e arlier. Recruitment of trees onto south-facing slopes occurred later, when conditions were wetter (1945-1985). In many environments, the spa tial distribution of recruitment suggests that once trees have establi shed, autogenic factors become increasingly important as individual tr ees or groups of trees alter the physical or biotic conditions that on ce inhibited establishment. Knowledge of the factors that influence in vasion, and of their varying importance across gradients in environmen t and vegetation, is critical to predicting future changes in these dy namic systems.