Challenges and approaches to projecting changes in forest distributions in complex mountain landscape

Abstract:

The extent and density of forest trees in mountain landscapes is a first order control on watershed function, affecting patterns of snow accumulation, timing of snowmelt, and amount and quality of run-off, through alterations of surface energy and water fluxes and wind. Climate change is increasingly affecting the density and distribution of mature forests through changes to disturbance regimes, increases in physiological stress and increases in mortality due to warmer temperatures. In addition, climate change is likely altering patterns of regeneration and driving establishment of trees in high elevation meadows and alpine tundra. Though hard to detect in current forestry datasets, changes in tree establishment are critical to the future of forests. Experimental approaches, such as our climate warming experiment in the Colorado Front Range, can provide valuable data regarding seedling sensitivity to climate variability and change across important landscape positions. We’ve found that warming enhances negative effects of water stress across forest, treeline and alpine sites, reducing recruitment in the absence of additional summer moisture. At the lowest elevation, reductions with warming have reduced Engelmann spruce recruitment to zero. Species differ in their responses to warming in the alpine, but together confirm the importance of seed dispersal to upward forest shifts. The presence of trees or other vegetation can facilitate tree establishment by modifying microclimates, especially at and above treeline. Ultimately, these ecological and demographic processes govern the timescales of tree and forest responses to climate variability and change. For the long-lived species that dominate high elevation watersheds, these processes can take decades or centuries to play out, meaning many tree populations are and will continue to be out of equilibrium with a rapidly changing climate. Projecting changes in tree distributions and abundances across mountain landscapes requires integration of changes in hydroclimatic conditions across diverse topoclimatic settings; the sensitivity of recruitment, growth and mortality to climate; and feedbacks between trees and microclimate into modeling tools that represent time-explicit ecological and demographic processes.