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 trees in mountain meadows of the Pacific Northwest. We reconstructed patterns 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 anthropogenic disturbance(sheep grazing). Spatial distributions and age structures of invasive tree populations were analysed with respect to climatic records and grazing history. Patterns of establishment varied considerably 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 versus fluctuating water tables. in upland mesic/dry montane meadows, timing of invasion corresponded with cessation of sheep grazing (early 1940s) and the onset of wetter summers (mid 1940s). In the subalpine zone, climate and aspect interacted to produce contrasting histories of invasion on north- and south-facing slopes. Establishment on north-facing slopes, concentrated in heath-shrub communities, coincided with regional warming (ca. 1920-1945) when snowpacks were lighter and melted earlier. Recruitment of trees onto south-facing slopes occurred later, when conditions were wetter (1945-1985). In many environments, the spatial distribution of recruitment suggests that once trees have established, autogenic factors become increasingly important as individual trees or groups of trees alter the physical or biotic conditions that once inhibited establishment. Knowledge of the factors that influence invasion, and of their varying importance across gradients in environment and vegetation, is critical to predicting future changes in these dynamic systems.