Abies lasiocarpa establishment of subalpine meadows in Glacier National Park, Montana

Studies on subalpine meadow invasions by Abies lasiocarpa (subalpine fir) in the Sierra Nevada and the Rocky Mountain Front Range are abundant, yet little is understood about this important process in the subalpine zone of Glacier National Park (GNP), Montana. This study evaluates spatiotemporal influences of climate on conifer invasions into subalpine meadows. Seedling establishment of A. lasiocarpa show both time and site dependent relationships to interannual variation in climate. Annual and seasonal climate models were constructed for temperature data, and these data were plotted against establishment. Regression analyses between climate data and conifer establishment were performed, and residual statistics show strong positive relationship between fall temperatures, Pacific Decadal Oscillation, and establishment. Correlations between climate and establishment showed significant positive trend between mean maximum temperature in fall, mean minimum temperature in fall and mean temperature in fall, and forest establishment. Both forest and meadow categories showed significant inverse trends in Pacific Decadal Oscillation and establishment. These data indicate warmer fall temperatures and alteration in snowpack lengthen the growing season and provide more moisture for meadows, a limiting resource, over the course of the growing season. The spatial pattern of tree invasion age and size structure were examined by drawing age class maps based on mapped and aged trees and by drawing size class maps based on mapped tree diameter. A multi-distance spatial analysis was used at tree scale to describe and understand these patterns. The tree age and size structure of A. lasiocarpa invasion showed differences over distance across meadow-forest boundaries attributed to strategies in competition and facilitation and variations in soil depth and topography. One of the small meadows in the study area was distributed in random patterns of tree spatial associations over the extent of the neighborhood. All other meadows showed clumped spatial associations for seedling establishment over the extent of the neighborhood. These meadows showed clustered spatial patterns of tree establishment, with larger trees and seedlings having strong spatial associations over range of the neighborhood at different scales indicating contagious dispersion. Observed spatial differences of conifer invasion in subalpine meadows shows instability in meadow/forest boundaries, and this instability is pronounced along the elevation gradient in erosional and depositional meadows. These results indicate a vegetation dynamic which may result in increased expansion of forests into meadows over time in periods of favorable climate.