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live oak (Quercus virginiana) and other trees with which it competes, and can itself grow as a |
tree, a shrub, or a woody vine (Spector and Putz 2006), it crowds out native tree species in coastal |
hammocks up to about the same latitude as where mangroves stop. |
Rising sea levels have driven the inland and uphill migration of Big Bend ecosystems since |
the end of the last glacial period some 14,000 years ago as they did during previous interglacials. |
The current rate of rise is by no means unprecedented; sea levels were rising more than twice as |
fast when paleoindians occupied the area. What is different now is that the uphill and inland |
migration is often impeded by the infrastructure of humans who are less willing to move than |
68 • MICHAEL I. VOLK ET AL. |
our coastal predecessors. Given the low human population density and sparsity of development |
along the Big Bend Coast, the financial consequences of sea level rise are modest in the aggregate |
while devastating for the people who do live in the region. |
Changes in Mangrove Distribution within the Florida Peninsula |
Mangrove forests consisting of black mangrove, red mangrove (Rhizophora mangle), and white |
mangrove (Laguncularia racemose) are a common coastal community on both the low energy |
Gulf and Atlantic shorelines in Florida. Along with tidal marshes and other coastal ecosystems, |
they provide a number of important services including carbon storage, shoreline protection and |
sediment accretion, water quality improvement, habitat for a number of important fish and |
wildlife species, as well as recreational opportunities (Osland et al. 2013). |
The northern extent of each of the three mangrove species endemic to Florida varies due to |
differing resilience to freeze events. Precise range boundaries are difficult to determine, but over |
the past century, black and white mangroves have been found as far north as the Guana Tolomato |
Matanzas National Estuarine Research Reserve on the East Coast (Wunderlin and Hansen 2008; |
Zomlefer et al. 2006) and as far north as Cedar Key on the West Coast of Florida. Typically red |
mangroves are found further south than the other species due to a greater sensitivity to cold |
temperatures. |
However, these ranges are not static, and as already noted change continues to occur. For |
example, in the Ten Thousand Islands, between 1927 and 2005 mangrove encroachment occurred |
upstream into salt and brackish marshes resulting in a roughly 35% increase in mangrove |
coverage (Krauss et al. 2011). Within the Tampa Bay region, Raabe et al. (2012) have |
documented conversion of marsh to mangrove habitat by comparing digitized nineteenth century |
topographic and public land surveys with 2005 digital land cover. Though specific conversion |
rates varied in different locations, the average ratio of non-mangrove to mangrove habitat over a |
125-year period reversed from 86:14 to 25:75 across the four sites that they examined. |
Depending on location, there are varying and interrelated reasons for these shifts that have |
been cited, including construction of waterways and interruption/reduction of freshwater flows |
(Krauss et al. 2011; Raabe et al. 2012), sea level rise (Krauss et al. 2011; Raabe et al. 2012), and |
changes in temperature resulting from climate change (Raabe et al. 2012; Williams et al. 2014) |
Storm disturbances have been a historic driver of change in forest structure (Doyle et al. 1995), |
and future changes may also be driven by precipitation (Ward et al. 2016), |
South to north shifts in mangrove ranges seem particularly telling of the influence of climate |
change because, at least in Florida, the northern distribution of mangroves is limited by |
temperature. When mangroves begin to migrate further north, it is an indication that freeze events |
are no longer limiting colonization of mangroves in places where they have not recently existed. |
Northern migration of mangroves along the Atlantic Coast is now being documented and |
attributed to climate change. The frontline of this change is the Guana Tolomato Matanzas |
National Estuarine Research Reserve. In a 2013 study, Williams et al. (2014) surveyed the |
FLORIDA LAND USE AND LAND COVER CHANGE IN THE PAST 100 YEARS • 69 |
northernmost locations of black, red, and white mangroves, and compared those locations with |
historical data identifying the northern extent of these species. In the case of black mangroves, |
they found an occurrence 27 km north of the prior most northerly occurrence documented in |
2007 (Wunderlin and Hansen 2008). They found a red mangrove 26 km north of the previous |
outlier documented in 2006 (Zomlefer et al. 2006) and a white mangrove occurrence |
approximately 67 km north of historic observations. |
The overall future trend may be a gradual intrusion and northern expansion of mangroves |
into areas that have historically been dominated by saltmarshes or other types of coastal habitat. |
Whether this trend will continue, at what rate, and with what effect remains to be seen. At the |
very least, the changes that have occurred to date underscore the importance of minimizing |
human influences on these systems, including alteration of hydrology and freshwater flows. The |
ultimate impacts from climate change on coastal ecosystems is uncertain, but minimizing human |
impacts will help natural systems remain as resilient as possible to the changes that will occur. |
Land Cover Changes in the Florida Keys |
The Florida Keys are one of the most sensitive and at-risk regions in Florida with regard to |
climate change, and especially sea level rise, and changes to land cover to date are already being |
documented. One example is the loss of South Florida slash pine forest (Pinus elliottii var densa) |
as described by Alexander (1976) and Ross et al. (1993) in the Lower Keys. In a study on Key |
Largo, Alexander (1976) proposed that sea level rise was the cause of this loss, where flooded |
low-lying freshwater dependent pine communities had been replaced by more salt tolerant |
mangroves. A second and later study by Ross et al. (1993) reached the same conclusion through |
an examination of aerial photos and field evidence to compare historic and current distribution |
of pines on Sugarloaf Key. Ross et al. (1993) estimated the historical extent of pines on Sugarloaf |
Key to be approximately 217 acres prior to 1935. At the time of their study in 1991, it had been |
reduced to approximately 74 acres, with the earliest mortalities in areas with the lowest |
elevations. The areas of early pine mortality had been populated by new salt tolerant species. |
They also found that groundwater and soil water salinity were higher in areas of rapid pine forest |
reduction, and that pines in those areas exhibited higher physiological stress. At the time of their |
study, local sea levels had increased by 15 cm over the past 70 years, with the implication that |
further sea level rise would only increase the loss of upland pine communities. Ultimately, the |
entire Florida Keys as an upland ecosystem is endangered by projected sea level rise in the next |
century, which will necessitate consideration of various conservation strategies including |
potentially translocation of the many endangered species and subspecies found here (Noss et al. |
2014). |
The Impacts of Land Cover and Land Use Change on Florida’s Climate |
It is important to understand that the land cover and land use changes that have occurred in |
Florida have affected the climate — certainly in their contribution to the greater phenomenon of |
70 • MICHAEL I. VOLK ET AL. |
global climate change, but also most likely at a regional and local level. These changes in turn |
affect land cover and land use in the future. As described elsewhere in this chapter, the pre-1900 |
landscape of Florida has been significantly altered by agriculture and urbanization. One impact |
of dense urbanization can be the “heat island effect,” where urban areas actually cause an increase |
in local temperatures due to the absorption and re-radiation of solar heat by buildings and paved |
surfaces. Within an urban or suburban environment, local temperatures can vary based on the |
amount of tree cover and density of buildings and paved surfaces. For example, a study conducted |
by Sonne et al. (2000) in Melbourne, Florida showed average summer temperatures to be as |
much as 1.3 degrees cooler in an undeveloped, forested site when compared to an adjacent |
residential site with 4.6 houses/hectare and significant tree canopy. Average temperatures were |
up to 2.9 degrees cooler when compared to a residential site without trees and 10.1 |
houses/hectare. |
At the peninsular scale, Marshall et al. (2004) conducted a series of simulations that found |
urbanization and agricultural conversion during the 20th century has contributed to a regional |
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