Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events

Abstract

Regional warming associated with climate change is linked with altered range and abundance of species and ecosystems worldwide. However, the ecological impacts of changes in the frequency of extreme events have not been as well documented, especially for coastal and marine environments. We used 28 y of satellite imagery to demonstrate that the area of mangrove forests has doubled at the northern end of their historic range on the east coast of Florida. This expansion is associated with a reduction in the frequency of "extreme" cold events (days colder than -4 °C), but uncorrelated with changes in mean annual temperature, mean annual precipitation, and land use. Our analyses provide evidence for a threshold response, with declining frequency of severe cold winter events allowing for poleward expansion of mangroves. Future warming may result in increases in mangrove cover beyond current latitudinal limits of mangrove forests, thereby altering the structure and function of these important coastal ecosystems.

Keywords: Landsat; coastal wetlands; ecological threshold; frost tolerance.

Fig. 1.

(A) Map of study area showing the long-term increase (black) or decrease (red) in mangrove area for each 0.25° latitudinal band from 25.25° to 29.75°N. Long-term change was defined as the relative change in 5-y mean mangrove area from 1984–1988 to 2007–2011. (B) Relationship between latitude and relative change in mangrove area. Solid line represents a piecewise regression and the vertical dotted line gives the breakpoint (26.75°N) of the piecewise regression.

Fig. 2.

A 28-y time series of mangrove area separated into 0.25° latitudinal bands. Mangrove area is displayed relative to the first year in the time series (1984). (Right) A value of 1 indicates no change (100% of 1984 area), 0 indicates total loss of mangroves, and 2 indicates a doubling of mangrove area. Data were calculated from Landsat imagery collected each year between April and July. White areas indicate that no cloud-free Landsat data were available.

Fig. 3.

The correlation between change in mangrove cover and the change in the total number of days with minimum temperature lower than the stated threshold on the x axis. The x axis gives the definition of an extreme cold event: daily minimum temperature must be below this value.

Fig. 4.

Relationship between percent change in mangrove area between successive summers and the minimum temperature during the intervening winter. Solid line, polynomial model; dotted lines, 95% CIs; vertical dashed line, minimum winter temperature associated with no change in mangrove area.