Influence of elevation and site productivity on conifer distributions across Alaskan temperate rainforests

Abstract

We investigated the influence of landscape factors on the distribution and life stage stability of coastal tree species near the northern limit of their ranges. Using data from 1465 forest inventory plots, we estimated probability of occurrence and basal area of six common conifer species across three broad latitudinal regions of coastal Alaska. By also comparing models across life stages of each species (seedlings, saplings, mature trees, and dead trees), we explored trends in population stability at this leading edge of climate change. Elevation had a stronger influence on the probability of tree species occurrence than on basal area; site productivity impacted both estimated odds of presence and estimated basal area for most species in at least some regions. Interestingly, there were fairly dramatic differences across species in the degree to which the modeled probability of occurrence differed across the four life stages. Western redcedar (Thuja plicata Donn ex D. Don), for example, showed relatively stable distributions but other species appear to be in flux, e.g., yellow-cedar (Callitropsis nootkatensis (D. Don) D.P. Little), which has experienced widespread mortality at low elevations. Differential effects of elevation on live versus dead basal area suggest that mountain hemlock (Tsuga mertensiana (Bong.) Carrière) and yellow-cedar are shifting upslope in some regions and that Sitka spruce (Picea sitchensis (Bong.) Carrière) is shifting downslope in the Northwest region.