Resilience and the potential for species distribution shifts under the effects of climate change is a pressing concern for forests. The effects of increasing temperature and changing precipitation patterns are compounded by the legacy of disturbance and altered fire regimes, which gives us the forest present on the landscape today and the likelihood of future changes.
We considered how climate variation influences tree ring growth in a mixed conifer forest in northern New Mexico. We studied tree growth responses along a biophysical gradient in a watershed that spans from near the lower to upper elevation range for two species, ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii). We combined this response with stand dynamics and regeneration patterns of the six major tree species found in the study area to predict species shifts in dominance within the watershed and the potential for future climate-driven mortality. Tree-rings integrate climate and limited resources into a time series of growth that can be related to many factors influencing forest dynamics. To analyze growth we split 1040 trees from 58 sites into groups using CART analysis driven by elevation and aspect. We examined whether species-specific mean tree-ring series by group were better modeled by PRISM-derived temperature or precipitation metrics using AIC model selection, and applied the findings using GIS to show the areas across the watershed that fit each climate-growth model.
Results/Conclusions
We found changes in species distribution between mature trees and seedlings across the elevation-aspect groups, notably increases in pinyon pine (Pinus edulis) and white fir (Abies concolor) while ponderosa pine made up a smaller percentage of seedlings relative to mature trees. New tree establishment was concentrated in mid-elevation southwest sites, reflecting a combination of microclimatic optima and available resources.
Considering past tree growth, we found differences in models selected across species and elevation-aspect groups; for example, on southwest slopes at lower elevation Douglas-fir ring growth responded to precipitation while ponderosa pine in the same group responded primarily to maximum temperature. Ring growth in both species was modeled best by temperature at northeast sites at low elevation and southwest sites at mid elevation, and by precipitation at northeast sites at mid elevations and at high elevations. Differences in modeled climatic drivers indicate species-specific stressors on tree growth at the lower elevational limits of species distribution, which has implications for species-level resilience with differential establishment and survival across a given range, under future conditions.