Integrating physiological threshold experiments with distribution modeling to understand the controls on mangrove range limits
Understanding the processes that limit the geographic ranges of species is one of the central goals of ecology and biogeography. This issue has become particularly important given that climate change, habitat loss, and biological invasions are altering the range and abundance of species worldwide. In coastal wetlands around the world, there have been observations of mangroves expanding into salt marshes near the current poleward range limits of mangroves. However, there is still uncertainty regarding the factors that control the poleward range limits of mangroves in different parts of the world and the mechanisms behind some of these mangrove expansions. Here we used a combination of controlled laboratory experiments, satellite imagery analysis, and distribution modeling to better understand the factors that set the poleward range limits of mangroves in North America and other regions of the world.
We found that all three species of mangroves found in North America (Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle) exhibited a threshold response to extreme cold, but freeze tolerance thresholds varied among species. From these laboratory experiments we developed a climate metric, freeze degree days (FDD), which incorporates both the intensity and frequency of freezes. When included in distribution models, FDD accurately predicted mangrove presence/absence. Using over 27 years of satellite imagery, we linked FDD to past changes in mangrove abundance in Florida, further supporting the relevance of FDD. While the frequency and intensity of cold events appear to set the poleward range limit of mangroves in southeastern North America, distribution modeling and satellite analysis suggest that other factors such as sea-surface temperature, precipitation, and dispersal limitation play primary roles in other parts of the world.