Mangrove reemergence in the northernmost range limit of eastern Florida.

Cavanaugh et al. (1) report conclusive evidence of poleward expansion of mangroves at the northern limit of their historic range on the east coast of Florida and associate the expansion to a reduction in the frequency of extreme cold events. We agree with their assertion that mangrove expansion has occurred in eastern Florida, resulting from a lack of extreme freeze perturbations since 1984 (2, 3). We differ, however, with their conclusion that mangrove expansion is evident beyond the northern boundary of their historical range in eastern Florida. We contend that the 2011 mangrove area likely did not exceed the mangrove extent before their study period (1984–2011). The use of 28 y of Landsat satellite data spanning from 1984 to 2011 seems impressive, but in terms of an ecological time scale to study the mangrove’s geographic range expansion, this observation period is inadequate. For such studies, more than 100 y of data may be needed. For example, if the authors would have included one additional year of data (i.e., 1983) in their analysis, the results would likely have been substantially different (Fig. 1). The assumption of 1984 as a baseline historic range of mangroves in eastern Florida is not supported by the data or the literature. Also, no reliable information was cited to show the exact extent of the historical range of mangroves in eastern Florida. Further research is needed to determine the historic range of the mangrove forest using remotely sensed and/or paleoecological data to determine changes in distributional ranges caused by climate change. In a similar study, Giri et al. (2) mapped the mangrove distribution of Louisiana for 1983 and 1984 and every 2 y from 1984 to 2010. The severe freeze in late December of 1983 reduced Louisiana’s mangrove extent by ∼90% (Fig. 2) and devastated mangroves in Texas (4). This dramatic reduction in the mangrove area occurred between 29°N and 30°N, the equivalent latitudinal range where the authors proclaimed mangrove poleward expansion occurred in eastern Florida. Similar to Florida’s mangrove areas, Louisiana’s mangrove area started to increase after the extreme freeze; however, it has not reached the 1983 extent. The forests were also impacted by freezes in 1989 and 2009. Although major change statistics are strongly correlated with winter freeze in Florida and other areas throughout the Gulf Coast, specific causes of the mangrove forest cover change spatially and temporally vary. Besides the winter freeze, other factors such as erosion, land subsidence, and aggradation also affect mangrove extent. Finally, Landsat-based monitoring is capable of capturing the dynamic nature of mangrove forests on the Gulf Coast; however, higher-resolution (<5-m spatial resolution) remotely sensed data are required to capture newly colonized individual stands or relatively small patches of mangrove stands that cannot be captured with 30-m Landsat imagery (3).