Ontogenetic niche shifts in eastern U.S. trees

Background/Question/Methods

Biogeographic responses of plant species to climate change are determined by the requirements of juveniles, which can limit spread to new environments.  By contrast, most models of climate response are calibrated to adults, which may not reflect the climate that determined their original establishment. Despite a large literature on ontogenetic niche shifts in animals, there is little direct evidence for plants that could provide insight on its consequences for migration potential. Using USDA Forest Service’s Forest Inventory and Analysis (FIA) data, we built a species distribution model to jointly quantify juvenile and adult trees’ abundance distributions in relationship with the two most commonly used climatological variables, temperature and precipitation.  To accommodate the mismatch between fine scale biological processes and coarse scale climate variables we introduced an aggregation approach to climate calibration. The fitted model allowed us to compare differences between juveniles and adults in climate relationships.

Results/Conclusions

All species show similar climate optima for juveniles and adults.  However, some species showed broader climate calibrations for adults, whereas others showed broader climate calibration for juveniles.  The differences could be partly a consequence of ontogenetic niche differences and partly due to other factors that impact climate response.  Ontogenetic climate expansion would be suggested when adult niches are broader than juvenile niches. This could be expected if adults integrate over fluctuating windows for juvenile establishment in the past, while juveniles more narrowly reflect the current climate conditions. Seedlings can have narrower niche requirements owing to limited root systems, low carbon reserves, and reduced photosynthetic capacity. On the contrary, niche contraction could be suggested by adult niches that appear narrower than those of juveniles. This could occur if seeds can establish in sink populations, where they ultimately do not replace themselves due to competition or herbivory.  We demonstrate for a large number of species across eastern North America how climate calibrations of both adults and juveniles are impacted by climate change and the emerging role of new competitive environments and natural enemies.