Trees have overlapping potential niches that extend beyond their realized niches

Tree species appear to prefer distinct climatic conditions, but the true nature of these preferences is obscured by species interactions and dispersal, which limit species’ ranges. We quantified realized and potential thermal niches of 188 North American tree species to conduct the first continental-scale test of the architecture of niches. We found strong and consistent evidence that species occurring at thermal extremes occupy less than three-quarters of their potential niches and species’ thermal niches overlap at a mean annual temperature of approximately 12 °C. These results clarify the breadth of thermal tolerances of temperate tree species and support the centrifugal organization of thermal niches. Accounting for the non-realized components of ecological niches will advance theory and prediction in global change ecology.

Methods
We compiled occurrence data for 188 North American tree species. We submitted a data request to Botanic Gardens Conservation International (BGCI) for lists of arboreta in which 298 North American trees were growing and surviving. We received information from 447 arboreta around the world. After taxonomic matching, we analyzed the 188 species that had a minimum of 20 occurrences in arboreta and for which natural occurrence data was available.

Realized niches for each species were quantified as the range of climate conditions across their native ranges in North America. We downloaded native occurrence data for these species from the Botanical Information and Ecology Network (BIEN 4.1). We removed cultivated records from the BIEN data to ensure these were native occurrence records and limited all occurrences to within North America. We supplemented occurrence records in BIEN with point samples from Little’s range maps to ensure full sampling of species ranges into Canada and Mexico. We used CHELSA V2.1 climate normals (1980-2010) to quantify mean annual temperature, maximum temperature of the warmest month, and minimum temperature of the coldest month for each occurrence record.

Potential niches for each species were quantified as the range of climate conditions across their native ranges in North America in addition to the range of climate conditions across the globally-distributed arboretums (i.e., native occurrences plus arboreta occurrences), because realized niches are subsets of potential niches. We use the term ‘potential niche’ to represent the conditions that permit survival but not necessarily reproduction. Given that natural occurrences outnumbered the arboretum occurrences by several orders of magnitude and would overwhelm the estimates of niche ranges, we randomly sampled occurrences from BIEN and Little’s range maps at 10 times the number of occurrences in the arboretums for a total of 165,315 occurrences.