Variability in the fundamental versus realized niches of North American mangroves

Aim Climate change is leading to large‐scale shifts in species’ range limits. Mangroves, for example, are encroaching into saltmarshes at numerous tropical–temperate transition zones. However, mangrove expansion varies geographically, in large part because mangroves might not be fully occupying their fundamental niches across their range limits. Here we characterize and compare the fundamental versus realized thermal niches of two mangrove species found near their northern range limits on both coasts of North America. Location Atlantic and Pacific coasts of North America. Taxa Red mangrove ( Rhizophora mangle ), black mangrove ( Avicennia germinans ). Methods Red and black mangrove propagules were collected near range limits on the Atlantic and Pacific coasts and experimentally exposed to simulated overnight freezes ranging from −0.5 to −15°C, and grown in water temperatures ranging from 13 to 25°C. We then modelled range‐specific distributions based on threshold survival responses to cold treatments and compared these predictions to current distributions and climate envelopes. Results On the Atlantic coast, laboratory physiological thresholds closely matched realized distributions for both black and red mangroves. The Pacific black mangroves were less tolerant to freezes than the Atlantic populations, but laboratory determined thresholds essentially matched their realized distributions. In contrast, Pacific red mangroves were surprisingly freeze tolerant, and our laboratory threshold‐based model predicted suitable habitat far north of their current range limit. Our cold‐water tolerance experiments indicated that mangroves can tolerate chronically colder water temperatures than are currently experienced at either range limit. Main conclusions On its own, cold water temperature does not seem to be a limiting factor on either coast of North America. On the Atlantic coast, range limits for both mangrove species are set by extreme cold air temperatures and rapidly shifting in response to climate change. On the warmer but more arid Pacific coast, range limits for black mangroves only appear limited by cold air temperatures, but neither species seems to be undergoing climate change‐related migration. This supports the importance of other range‐restricting factors, such as aridity or dispersal limitation. Thus, distribution models need to incorporate species and range‐specific physiological data to predict the effects of climate change on population‐specific range limits.