Distribution and abundance of tree species along climate gradients in the Rocky Mountains

Background/Question/Methods

Today there are numerous models of climate change impacts on tree species, but empirical analyses of the effects of climate on the distribution and abundance of tree species continue to lag the models. Recent empirical study of the eastern deciduous forest has found that the frequency of a tree species' occurrence among stands, rather than relative abundance when present, varies consistently along climate gradients. Here, we expand these analyses and ask if similar patterns occur in western coniferous forests. We used Forest Inventory and Analysis (FIA) data to characterize variation in two components of tree species distribution and abundance – relative abundance when present, and the frequency of sites within a particular climate where a species occurred – along climate gradients for the 25 most common tree species in the Rocky Mountain region of the USA. We were specifically interested in how these two very different metrics of abundance responded to spatial variation in climate, and whether there was evidence of functional groups of species with similar niche dimensions, or whether there was a more individualistic distribution of species along the climate gradients.

Results/Conclusions

Both measures of species abundance – local frequency and relative abundance – showed pronounced patterns along gradients of mean annual temperature and precipitation. Overall, western species display much strong climate patterning than eastern species. Similar to eastern species, frequency (i.e., the percentage of plots in a given climate in which a species occurred) varied strongly for nearly all 25 species as a function of both temperature and precipitation with most western species having a Gaussian distribution along a climate gradient. Unlike in the eastern US, however, relative abundance for most western species when present in a plot also varied markedly across their climatic ranges, and these patterns were highly individualistic (i.e. Gaussian, negative, positive, neutral). Although the range limits for both temperature and precipitation were quite broad for all of the species, the range of climates within which a species was common (i.e., high frequency) was much narrower. Because frequency in sites within a given climate shows a strong sensitivity to climate, this suggests that the processes determining canopy tree recruitment on new sites also vary strongly with climate. In general, our results suggest that processes that influence the presence of adults of a species (i.e., canopy recruitment and local extinction) will be particularly important in governing the shifts in tree species distribution and abundance under climate change.