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

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