Abies lasiocarpa establishment of subalpine meadows in Glacier National Park, Montana
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Abstract
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.
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Keywords
Subalpine meadows, climate, establishment, invasion, seedling regeneration