The expansion is due to decrease in extreme cold events

Adding to the growing evidence of climate change-induced poleward migration and/or expansion of many fish and terrestrial plant species, a paper published recently in the Proceedings of the National Academy of Sciences (PNAS) journal has found a massive expansion of mangroves in Florida. The expansion has been to an extent of more than 3,000 acres over a 28-year period — 1984 to 2011.

By default, mangroves are restricted to the tropical areas as they cannot tolerate the extreme cold events that are so very typical of the temperate zone. Therefore, the massive expansion of mangroves in the temperate zone of Florida during the last three decades is proof of climate change’s role.

“These increases [expansion] correspond to decrease in the frequency of extreme cold events in this region,” notes Kyle Cavanaugh of the Smithsonian Environmental Research Centre, Smithsonian Institute, U.S. Dr. Cavanaugh is the first author of the paper.

Aside from pointing out the poleward expansion, the study has highlighted another important parameter — the “dynamics of mangrove” coverage. The authors have found that the 26.75 degree N latitude plays a critical role.

While there was an increase in the mangrove coverage north of this latitude, there was “little or small decrease” in the area south of this latitude. The increase in mangrove coverage north of 26.7 degree N was about 4,200 acres; the area south of this latitude had reduced by about 1,164 acres. “The 26.75 degree N [is] a breakpoint in the relationship between latitude and change in mangrove area,” they write.

According to the authors, the reason for this strange concomitant expansion/reduction in mangrove coverage boils down to extreme cold events — days colder than -4 degree C. Minus four degree C is the physiological temperature limit of mangrove survival.

The region north of 26.75 degree N latitude had registered an increase in coverage as there was a “significant decrease” in the frequency of extreme cold events during the last three decades, they note. Such extreme cold events are “rare or nonexistent” in the region south of this latitude.

The impact of extreme cold event (days colder than -4 degree C) has been well documented by the hard-free winter of 1989 to 1990 — the cold event decreased the mangrove area. It took about nine years for the mangroves to reach the pre-1989 levels.

The authors rule out the role of temperature increase in bringing about an expansion. It is true that the mean winter temperatures had risen at seven of eight coastal weather stations in the study area. But if overall warming was indeed the factor for the expansion, then an overall increase in coverage should have been observed both north and south of this latitude. According to them, no other factor like rainfall and urban or agricultural land use could account for this strange trend. Only a reduction in the extreme cold events could.

“It is likely that cold thresholds act as a barrier to the poleward extension of mangroves in other parts of the world,” they write. “Relatively small future decrease in the frequency of extreme cold events could lead to further increase in mangrove cover near the current poleward limit of mangrove forests in Florida.”

Unlike terrestrial plants, a change in the type of vegetation cover would be relatively rapid in the case of mangroves as water is far superior to wind and animals in dispersing plants.