Ea. Miller et Cb. Halpern, EFFECTS OF ENVIRONMENT AND GRAZING DISTURBANCE ON TREE ESTABLISHMENT IN MEADOWS OF THE CENTRAL CASCADE RANGE, OREGON, USA, Journal of vegetation science, 9(2), 1998, pp. 265-282
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.