From: Miller, E. A., and C. B. Halpern. 1998. Effects of environment and grazing disturbance on tree establishment in meadows of the western Cascade Range, Oregon, USA. Journal of Vegetation Science 9:265-282. (.pdf file)

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, U.S.A. sites representing diverse physical environments and vegetation types, and experiencing different histories of recent anthropogenic disturbance (sheep grazing). Spatial distributions and age structures of tree populations were analyzed with respect to climatic histories and grazing records.

Patterns of establishment varied considerably among meadows, reflecting strong differences in site environment and grazing history. In hydric montane meadows, tree establishment was spatially clumped beneath large, old trees and on elevated microsites, however the timing of invasion differed between sites with stable and fluctuating water tables. In upland mesic/dry montane meadows, timing of invasion correlated 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 N-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 S-facing slopes occurred later, when climate was 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.



Transect line spanning a forest - meadow ecotone where
lodgepole pine has invaded Deschampsia meadow.

Spatial and temporal patterns of conifer establishment across a SW-facing
forest - meadow ecotone at 1500 m elevation. Points represent dates of establishment vs. distance from forest edge (vertical dashed line). Bars
represent age structure of trees in the meadow. The period of sheep grazing
is defined by the two-ended arrow.