Ontogenetic differences in the distribution of conifer tree species in the Interior West, USA

Background/Question/Methods

Climate-induced shifts in tree species distributions are underway and tree establishment is crucial in driving shifting range limits. Even though the U. S. Interior West has already experienced widespread, climate-related mortality events and reproductive failures, the region has received surprisingly little attention from species distribution modelers. Furthermore, few studies have compared the biogeography of different life stages of trees to test hypotheses related to climate change. Using data from the U. S. Forest Service Forest Inventory and Analysis program and PRISM Climate Group, we developed species distribution models of six western conifer tree species for tree seedlings, saplings, and adults. We quantified the influence of spatial variation in contemporary climate on the probability of species occurrence. By comparing the degree of overlap in climate envelopes of differing life-stages, we examined ontogenetic differences in contemporary species ranges in response to summer and winter temperature and precipitation patterns that might contribute to tree species persistence or migration.

Results/Conclusions

Our results indicated that immature (saplings and seedlings) and adult trees differ in their distributional responses to climate, leading to differences in the positions of range boundaries. Sapling and seedling climate envelopes were narrower and shifted in comparison with adult trees. Ontogenetic differences in climatic range boundaries tended to be greater for montane compared to subalpine species, but the magnitude and direction of responses varied by species. The predicted probabilities of occurrences for seedlings or saplings were always less than for adult trees, and this difference was greatest for montane species. The greatest ontogenetic differences in the predicted probabilities of occurrence were observed along range boundaries, especially in the southern portion of species’ ranges. Low probabilities of sapling or seedling occurrence compared to trees may result from various sources, including episodic regeneration or sampling design, but regional variation in ontogenetic differences could be indicative of changing regeneration potentials at regional scales driven by climate. By examining ontogenetic differences in climatic niche, this research adds to our understanding of climatic influences on the distributions of different tree species and life-stages and highlights potential risks of regeneration failure at regional scales.