With increasing temperatures and altered precipitation patterns over the past several decades, montane environments are undergoing rapid anthropogenic changes. Climate controlled treelines are moving up in elevation worldwide, with dramatic consequences for alpine ecosystems, however little is known about the local drivers of variation that influence the rate of tree line change, including rain shadows and small-scale topography. Repeat oblique photography has been used to assess long-term changes in alpine landscapes over decades, but treeline studies using this method are lacking in the Pacific Northwest. We conducted a study to assess the rate of tree encroachment into the subalpine meadows of Mount Rainier National Park in various local landforms (coves, ridges, and slopes) as well as quantify the rate of treeline change over the 20th and early 21st century. Our six oblique photo replication sites allowed us to clearly distinguish changes in tree density across the wet western and drier eastern slopes of Mount Rainier. We did this by aligning historical and repeat oblique photographs in a Geographic Information System and manually delineating tree encroachment in randomly generated points within the subalpine zone. We also verified our results by comparing them to LiDAR canopy and ground models as well as aerial imagery collected from 1970 through 2007.
Results/Conclusions
Tree encroachment has occurred in all sites on Mount Rainier National Park over the last century. Oblique photographs showed an approximate thirty percent increase in tree cover from the 1930s to 2016. West side and east side sites do not have a significant difference in the amount of tree encroachment occurring in the various local topographies. Local topography did have a significant influence on the amount of tree encroachment. Small-scale topographic ridges were predicted to show the greatest amount of tree encroachment. Our results contradict this prediction, showing that sloped sites have the greatest increase in trees, followed by ridges, then topographic depressions (coves). Analyses of aerial photos are ongoing, but are expected to show similar results. Classifications of the aerial images will help us more easily distinguish changes in the various environments at Mount. Rainier, as well as correlate them to environmental drivers like snow duration. This study helps fill an important gap in knowledge about how local topography mediates climate change impacts on tree line, and reveals the relationships between different methodologies for detecting tree line change.