We investigated the potential impacts of climate change on the life history processes of tree species. Climate is widely assumed to influence physiological and demographic processes in trees, and hence forest composition, biomass, and carbon storage. We used two unique datasets to quantify climate drivers of seed production and growth for common tree species of the Rocky Mountains. A 40 year long record of annual seed output in Engelmann spruce from 13 sites was analyzed to determine spatial and temporal patterns. We also collected increment cores for 5 tree species distributed across broad elevation and edaphic gradients spanning a latitudinal range from New Mexico to the Canadian border. We used regression models in a likelihood framework to quantify climate drivers of reproduction and growth. We did not make any assumptions concerning the shape of the response to climate and therefore used flexible functions in our models that can fit a range of data distributions. Instead of detrending or standardizing tree growth measurements, we used a neighborhood analysis to explicitly quantify the effects of biotic interactions on growth in individual trees.
Annual seed production in spruce was highly variable between years, yet strongly synchronized among sites in a given year. Model results indicate that climate conditions across multiple years cumulatively determine reproductive output. High seed rain is associated with high summer temperatures in the year that seeds are dispersed, and reduced snowfall and longer growing seasons in the preceding two years. Significant positive trends in seed output over the 40 years were identified, driven by increasing warmth and anomalous aridity.
Relationships between tree growth and climate varied by species and location. For all species, growth responded positively to increases in annual precipitation up to a threshold level. Responses to temperature were more nuanced. Modest linear responses to temperature were observed at many sites. However, in extreme environments, growth responses were more pronounced and varied non-linearly.
The presence of neighbors negatively impacted growth in all species. Evidence for facilitative effects were not detected. For some species, neighborhood effects more strongly influenced growth performance than climate. Competitive interactions also mediated growth responses to climate to some degree. These analyses suggest that climate change will have complex, species-specific effects on tree growth in the Rocky Mountains due to non-linear responses to climate, differentiated growth processes that vary by location and complex species interactions that potentially attenuate climate effects.