Forest range shifts under climate change: microenvironment impacts to tree recruitment at a climatic ecotone
Date
2019
Authors
Foster, Alison Connolly, author
Redmond, Miranda, advisor
Martin, Patrick, advisor
Battaglia, Michael, committee member
Rocca, Monique, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Warming across the western United States is projected to cause dramatic shifts in tree species recruitment, with the most pronounced changes in composition at range edges where species are at their thresholds of reproductive tolerance. Yet microenvironments experienced by juvenile trees can be decoupled from regional climate due to variations in canopy cover, microtopography, and organic matter. As a result, tree recruitment may be strongly controlled by microenvironments and not follow species range projections based on regional climate, even at ecotone boundaries in which species at their upper range distributions are predicted to increase in density. This is likely especially pronounced in undisturbed forests with dense canopy cover in which microclimate is more strongly de-coupled from regional climate. To address these hypotheses of tree recruitment at species range margins we conducted a field experiment and observational study at the upper montane – subalpine ecotone on the Colorado Front Range. We characterized site microenvironment and observed germination and survival of six common conifer species, Douglas-fir, ponderosa pine, lodgepole pine, subalpine fir, Engelmann spruce, and limber pine. To quantify water availability and wildlife limitations, we sowed seeds from five study species and applied treatments of simulated precipitation and wildlife exclusion. Strong recruitment limitations were observed for nearly all species in experimental and observational studies, with strong negative effects of low soil moisture and maximum temperature. Notably, only subalpine fir exhibited increased seedling dominance, likely due to the limited light availability and cooler temperatures associated with shaded microenvironments. Recruitment success was unrelated to range position and do not match established migration predictions for these species. This research further illustrates the complexity of recruitment dynamics and the need to study regeneration at multiple scales.