The dynamics of forest-meadow ecotones in the Three Sisters Wilderness, Oregon: variation across environmental gradients

Many factors limit the establishment of trees into meadows in Pacific Northwestmountain ranges, including temperature, precipitation, duration of snowpack, depth ofwater table and grazing of livestock. Not all factors operate equally or simultaneouslyin all meadows. I inferred the scope of the mechanisms controlling tree establishmentinto meadows with respect to gradients in environment by examining the patterns ofinvasion into 17 of 21 permanently marked forest-meadow ecotone transectsestablished in the Three Sisters Wilderness, Oregon. Four meadow groups wereidentified using multivariate analysis of transect vegetation, and from observation andmeasures of site physical features: hydric, montane mesic, subalpine mesic andsubalpine mesoxeric meadows. I examined the timing and magnitude of treeestablishment over the last ca. 100 years by developing age structures for eachtransect. Levels of tree establishment were correlated to variation in weather andhydrology, and compared to records of livestock grazing. Invasion into hydricmeadows was correlated with periods of warm, dry weather but may be related towater table depth. Trees established in north-sloping, subalpine mesic meadowsduring periods of moderately early snowmelt and high summer precipitation aboutmid-century. South-facing, subalpine mesoxeric meadows were invaded during cool,wet climate ca. 1945-1985. Thus establishment of trees in subalpine meadows appearsto occur when site environment is ameliorated by weather. The timing ofestablishment into montane mesic meadows was similar to subalpine mesoxericmeadows but also coincided with cessation of sheep grazing ca. 1942-1947; invasion
was limited to short-statured communities on moderately deep and dry soils on mid- toupper-slopes. In general, variation in climate best explains invasion into higherelevation meadows whereas the effects of climate and disturbance are difficult toseparate at lower elevations. Patterns of establishment also vary with soil moisture.Ecotones in one hydric meadow with a high and constant water table have beenrelatively stable. Establishment into a drier hydric meadow peaked during the regionaldrought 1920-1940, and again during recent warm and dry climate. Invasion intosubalpine mesic meadows peaked ca. 1945 during relatively normal weather, andmontane mesic and subalpine mesoxeric meadows were invaded during cool, wetweather ca. 1945-1985. Across all meadow groups, tree establishment was rapid anddense at sites that were overgrazed by livestock, and gradual and less rapid at lessdisturbed sites. Within a single meadow, establishment of trees may respond tomultiple controls, or may respond differently to the same control. Allogenic factors(e.g., climate change, cessation of grazing) appear to become less important thanautogenic factors (e.g., shading, reduced snow scour by trees, local seed sources) asthe density of invading trees increases.