Moisture availability limits subalpine tree establishment

Authors: Thomas Veblen, Dept. of Geography - University of Colorado Boulder, Kyle Rodman, Dept. of Geography - University of Colorado Boulder, Robert Andrus*, Dept. of Geography - University of Colorado Boulder, Brian Harvey, School of Environmental and Forest Sciences - University of Washington, Sarah Hart, School of the Environment - Washington State University
Topics: Biogeography
Keywords: Engelmann spruce, subalpine fir, subalpine forest, Colorado Front Range, climate change, southern Rocky Mountains
Session Type: Paper
Day: 4/10/2018
Start / End Time: 12:40 PM / 2:20 PM
Room: Zulu, Sheraton, 8th Floor
Presentation File: No File Uploaded

Tree seedlings must establish (germinate and survive) in the forest understory to sustain future forests, particularly in the context of increasing tree mortality across western North America. In the absence of broad-scale disturbance, many temperate coniferous forests experience successful seedling establishment when abundant seed production coincides with favorable climate. Identifying the climate conditions favorable for seedling establishment is therefore essential to understanding how climate warming will affect the frequency of future tree establishment events. In subalpine forests in the southern Rocky Mountains, moisture deficits are projected to increase with warming temperatures, creating a potential limitation to establishment. We compared interannual climate variability over the past 70 years to >950 germination dates for two abundant subalpine tree species [subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii)] collected across a complex topographic-moisture gradient in the Colorado Front Range, CO, U.S.A. We found that broad-scale establishment events occurred in germination years characterized by high soil moisture availability from above-average snowpack and/or cool and wet summers. A decrease in the number of fir and spruce establishment events across the core range of subalpine tree species over the past 35 years coincided with declining snowpack and a multi-decadal trend of rising summer temperature and increasing moisture deficits. Counter to recently observed establishment increases in maritime subalpine forests with climate warming, our results show that recruitment declines will likely occur across the core of subalpine tree ranges in more interior, drier subalpine forests. Shifts in establishment patterns have important implications for the sustainability of future forests.

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