Facets of climate change, in particular increases in temperature, are impacting ecosystems. To predict and prepare for these impacts, we need to understand the responses of species to environmental change. In many coastal wetlands near tropical-temperate transition zones, mangroves are expanding into salt marshes. Previous work has identified freezes as the primary factor setting mangrove range limits on the Atlantic coast of North America. However, on the Pacific coast, the mangrove range limit occurs in Baja California (Mexico), an area that does not experience freezes. Our goal is to determine whether mangroves in Baja and Florida are controlled by the same processes and environmental thresholds.
Mangroves were collected from populations near both range limits and were grown under controlled laboratory conditions. We conducted two sets of experiments to characterize the tolerance of mangroves to cold air and water. Seedlings were grown in coolers with water temperatures ranging from 30°c to 13°c. We also simulated overnight freezes in freeze chambers, with minimum temperatures ranging from -0.5°c to -15°c. After each treatment, survival and growth were monitored for six months.
We then developed correlative distribution models in order to see if the thresholds identified from these experiments matched the realized niche of mangroves in these regions.
Results/Conclusions
Temperature thresholds associated with mortality varied for the three species collected on the Atlantic coast of Florida (-4°c for white mangrove, -7°c for red mangrove, and -9°c for black mangrove). This matches their realized niches, as black mangroves are found furthest north, followed by red, then white mangroves. This result suggests that these populations are indeed controlled by the occurrence and intensity of freeze events.
The same three mangroves species are found on the Pacific coast, yet very different results were obtained. All species did not survive any of the freeze treatments. However, we did find very clear thresholds associated with cold water tolerance: 15°c for black mangrove, 14°c for red mangrove, and 13°c for white mangrove.
These thresholds do not match the realized niche of mangroves in this region, as the poleward range limit is characterized by waters warmer than 15°c. This suggests that the current environmental conditions at the range may not be at the tolerance limits of mangroves in Baja, implying that mangroves range limits in Baja California could be controlled by other factors such as species interactions or dispersal limitation.