Role of frost tolerance versus climate change in mangrove range expansion

Background/Question/Methods

Mangroves are tropical and subtropical tree species that have recently begun moving into temperate zones. Mangrove distributions are thought to be limited by freezing temperatures, suggesting that poleward migrations result from either a warming climate or selection for cold-tolerant individuals at the edge of the range. To distinguish between these two mechanisms, we examined variation in cold-tolerance among three mangrove species that are common to tropical zones worldwide (black mangrove Avicennia germinans; white mangrove Laguncularia racemosa; and red mangrove Rhizophora mangle). We sampled individuals from along a 200 km stretch of mangrove ecotone on Florida’s eastern coastline, ranging from southern populations where frost events are exceedingly rare to northern populations where frosts are relatively common. In the laboratory, adult branches and propagules were subjected to experimental frosts, after which we recorded loss of leaf photosynthetic function, and monitored the growth and survival of the propagules. We also measured a suite of ecophysiological leaf traits to mechanistically explain these results. 

Results/Conclusions

We found strong differences in frost tolerance among species across both adults and offspring, with black mangroves usually showing the greatest tolerance and white mangroves the least. Species also differed in their cold acclimation. These differences are correlated with species’ latitudinal distributions, suggesting that differential range limits across species are a function of frost tolerance. Despite finding latitudinal differences in multiple leaf traits, there was no latitudinal gradient in adult frost tolerance within species. Leaf traits of offspring and adults were generally non-correlated, suggesting low heritability of frost tolerance. Thus, we found no evidence that latitudinal shifts in mangrove distributions are being driven by selection for cold-tolerant individuals at the edge of the ranges. Instead, our results suggest that mangrove expansion into temperate latitudes is more likely due to a warming climate.