Understanding the processes that limit the geographic ranges of species is one of the central goals of ecology and biogeography. This issue is particularly relevant for coastal wetlands given that climate change is expected to lead to a ‘tropicalization’ of temperate coastal and marine ecosystems. In coastal wetlands around the world, there have already been observations of mangroves expanding into salt marshes near the current poleward range limits of mangroves. However, there is still uncertainty regarding regional variability in the factors that control mangrove range limits.
In this study, we examined trends and variability in the satellite derived Enhanced Vegetation Index (EVI) of mangrove forests near their poleward range limits in North and South America. We compared variability in EVI at these range limits to variability near range centers and assessed the sensitivity of interannual variability in EVI to climate variables thought to control the distribution of mangroves. We hypothesized that (1) recent poleward mangrove expansion would lead to greening trends in EVI at mangrove range limits, (2) variability in EVI would increase towards poleward range limits and (3) that this variability would be correlated with air temperature and/or precipitation.
We found that the trends and variability in EVI and the drivers of that variability varied a great deal across range limits in North and South America. EVI of mangroves near their poleward range limits in eastern North America was highly variable and this variability was closely associated with air temperature and precipitation. There has been a strong linear greening trend in these regions over the past 30 years, which we interpret as expansion of mangroves due to decreases in the frequency of extreme cold events. This suggests that range limits in eastern North America are strongly controlled by climate factors. In contrast, EVI of mangrove range limits in western North America, eastern South America, and western South America was less variable and not as sensitive to climatic fluctuations. Other processes such as dispersal limitation may be interacting with climatic factors to control these range limits. Collectively, our analyses provide insight into the factors that limit the distribution of mangroves and how the relative importance of those factors varies regionally.