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Impacts of Global Climate Change
In 1993, the Sustainable Biosphere Initiative Project Office and the Environmental Protection Agency Office of Research and Development formed a collaborative project to establish a dialogue between a committee of prominent researchers (Fuad 1994) and EPA sponsored principal investigators. These investigators were with three projects that used models to study the effects of global climate change on habitat sensitivity. The three projects were spatially distant; North and South Dakota, the Florida mangrove system, and the Savannah River Site in South Carolina. At the 1995 Annual Meeting of the Ecological Society of America, committee members, principal investigators, and moderators participated with an audience in a discussion of the results. Three broad questions were addressed: 1) What are the comparative vulnerabilities of species and ecosystems to changes in climate? 2) Given that each project found changes in hydrology to exert strong effects on wetland functions, which functions are particularly sensitive to changes in climate? 3) How do land use changes compare or relate to changes in climate?
The project, in the glaciated prairie region of North and South Dakota, led by W. Carter Johnson of South Dakota State University and co-led by Karen Poiani of Cornell University, Lester Flake of South Dakota State University, and Thomas Loveland of the EROS Data Center, focused on the vulnerability of glaciated prairie wetlands to altered hydrology associated with climate change. In the glaciated prairie region, long-term wetland data are available to test and evaluate climate-driven simulation models of wetland hydrology and vegetation. The investigators expanded a previously tested and published site-scale model of a semi-permanent wetland to represent wetlands of other permanence classes (temporary, seasonal types) and for longer time periods.
Sam Snedaker and Mark Harwell of the University of Miami and Robert Twilley
of the University of Southwestern Louisiana led the project on the mangrove
systems of South Florida, utilizing unit models developed at the University
of Southwestern Louisiana to describe the hydrology and forest structure of
mangrove ecosystems. The hydrology model of a basin mangrove forest was developed
and used to simulate the sensitivity of mangroves under different tidal regimes
to changes in evapotranspiration and precipitation expected with climate scenarios
for southern Florida. The forest development model used growth and mortality
functions for each of three mangrove species (Rhizophora mangle, Avicennia genninans,
and Laguncularia racemosa) to project the biomass and forest structure under
different scenarios of climate change. Different climate change scenarios were
created in the model by fluctuating salinity, hydroperiod, nutrient conditions,
frost-induced mortality, and hurricane damage.
The Savannah River project was led by Ron Pulliam of the University of Georgia
(now with the National Biological Service). Other investigators included John
Dunning of Purdue University, Brent Danielson of Iowa State University, and
Justin Congdon, Barbara Taylor, John Pinder, and Phil Dixon of the Savannah
Research Ecology Laboratory. The study was a combination of field investigations
of patterns of landscape change and simulation modeling of responses by different
vertebrate species. Terrestrial taxa under study included birds and mammals,
especially those found in early successional habitats, while aquatic studies
concentrated on amphibians of isolated wetlands. Field work focused on population
dynamics and dispersal of the various taxa, and provided crucial life history
data to feed into the landscape models.
What are the comparative vulnerabilities of species and ecosystems to changes in climate?
In the glaciated prairie wetland system, the amount and distribution of emergent vegetation is a keystone process, affecting the ratio of open water to vegetation, which, in turn, affects the number and density of birds, invertebrates, and the rate of herbivory by muskrats. In the models, changes in hydrology, resulting from changes in temperature, precipitation, and/or land use, had a dramatic impact on the amount and distribution of emergent vegetation. Vegetation dynamics were particularly sensitive to the frequency and duration of drought periods, as these periods greatly impacted emergent germination and establishment. Changes in moderate and high water levels had less impact on vegetation dynamics, though deep, prolonged flooding eliminated emergent cover in some cases.
In semi-permanent wetlands, summer drought led to extensive germination the following spring by the emergent Scirpus species and the cattails Typha latifolia, T angustifolia, and the hybrid, T. glauca. Emergent species in the temporary and seasonal wetlands include Carex, Scolochloa, and Phragmites spp. Following germination, the proportions of emergent cover to open water were affected by the wet/dry cycles driven by climate. The dynamics of this cover ratio were highly vulnerable to increases in temperature. In simulations of semi-permanent wetlands with +2'C and +4'C and variations in precipitation level, 9 out of 10 cases moved from 50% open water, where the community reached maximum population density and diversity, to complete and sustained vegetative cover. A 20% increase in precipitation was required to compensate for increased evapotranspiration losses with a 2' increase in temperature.
According to Snedaker, Harwell, and Twilley, using the Mangroves project models, slight changes in hydrology due to climate change separated mangrove communities and strongly induced fragmentation. The response to fragmentation may be specific to community types based on differences in the adaptations of the individual species to high salinities, frost, turbidity, hydroperiod, hurricanes, and nutrients. Shifts in salinity or turbidity of mangrove systems may change the balance of plant-animal associations in certain coastal settings. For example, studies on sesarmid crabs in Australia have demonstrated that crabs can have a significant impact on zonation, biogeochemistry, and productivity of mangroves. If mangrove productivity is strongly related to nutrient remineralization, which is influenced by crabs, then changes in hydroperiod that modify crab life cycles might indirectly affect mangrove structure and production. These interactions suggest that crabs may have a keystone position in Old World mangroves.
Other examples of plant-animal associations in mangroves include gap dynamics that may result not only from physical disturbance (lightning strikes), but from biological disturbance as well. Insect herbivory on mangroves is potentially important to forest structure. Also, mutualistic mechanisms of mangrove-sponge communities that influence the structure, productivity, and nutrient dynamics of mangroves have recently been described. Trophic interactions, therefore, may have important implications to both the demographic and functional properties of mangrove ecosystems, and changes in climate parameters that affect animal "partners" may ultimately change mangrove distribution.
The Savannah River Site is a mostly forested region managed for biodiversity and timber production. In the project, suitable habitat for early successional bird and mammal species consisted of isolated patches created through U.S. Forest Service logging. Locations of suitable sites were variable in time and space, as timber harvest changed patches unsuitable to early successional bird and mammal species to a suitable condition, and then tree growth made the patches unsuitable again. The period of time in which a site was suitable for a given species was termed its time window of suitability. Keystone species can be considered those species that had a significant effect on the time windows of other species.
For example, the two principal tree species planted by the Forest Service in clearcut areas are longleaf pine (Pinus palustris) and loblolly pine (Pinus taeda). Longleaf pine grows relatively slowly, about 1 m/year, leaving a time window of about 10 years post-planting during which the planted stand is suitable for early successional species. In contrast, loblolly pine grows quickly, resulting in a time window of only 3-4 years before all open space is filled. Changes in climate may differentially affect the growth rates of pine species, which in turn could affect the time windows of suitability for other species. The species that are most vulnerable to changes in climate are those with the shortest time windows.
In the aquatic realm, several amphibian species are dependent on the isolated, temporary wetlands of Carolina Bays. In the models, the length of time these wetlands held water exerted a strong effect on amphibian survival and reproduction. Rather than being strongly affected by a keystone species, species diversity and abundance in the wetlands was positively correlated with a keystone ecosystem process - the hydroperiod. However, during years in which the water level in bays remained high, predatory fish entered bays and dramatically decreased the number and diversity of amphibians. Thus fish may be considered keystone in some years.
Which wetland functions are particularly sensitive to changes in climate?
Using hypothetical climate scenarios, Johnson, Poiani, Flake, and Loveland found that wetland hydrology in the Dakotas is extremely sensitive to changes in climate. Specifically, variation in spring precipitation (±10%) and subsequent runoff in the models had a dramatic effect on hydrology. Similarly, fluctuations in summer temperature (±2'C, ±4'C) yielded substantial variability in rates of evapotranspiration and had a strong effect on wetland hydrology. If changes in climate lead to a warmer, drier spring and summer, temporary and seasonal wetlands, critical to waterfowl, may be lost. In contrast, a warmer, wetter climate could increase the value of the wetlands to waterfowl. Vegetation, though strongly, affected by extreme changes in temperature and precipitation, exhibited lower sensitivity to changes in climate than hydrology did in the models.
Unless there is extreme climate change, no total loss of wetland functions
or types is expected. However, shifts toward a warmer, drier climate could shift
wetlands away from the permanent class and more toward semi-permanent and temporary
classes, resulting in a transition of waterfowl guilds from divers to dabblers.
The Florida Mangroves are forested intertidal wetlands that inhabit the upper
range of tolerance to frost conditions for subtropical plants. Thus the wetland
functions most sensitive to climate change were those influenced by the hydrology
and temperature of forested ecosystems, as, well as frequency of hurricanes
in this region. Although Florida has experienced a, slight (approximately I'C)
warming trend over the past several decades, severe freezing events during the
same period have caused extensive mortality at the north range limit for mangroves,
particularly along the eastern coast. The species most affected is the red mangrove
(Rhizophora mangle), whose northern range limit is being pushed southward. Wetland
functions such as primary productivity, biodiversity, decomposition, export
of organic matter, and biogeochemical cycling (serving as nutrient and carbon
sinks) were also sensitive to potential changes in climate.
Where changes in hydrology result in no salt inundation in the models, the high intertidal dominant, R. mangle, was replaced by upland forest species. Increasing hydroperiod and depth of inundation physically limited the establishment of mangroves and changed the biogeochemical characteristics of the wetlands, such as sulfide concentrations in porewater. The black mangrove (Avicennia germinans) has a shallow horizontal root system, and was the species likely to be most affected by a rise in sea level. In contrast, the peat producing red mangrove, which has a vertical root structure, was likely to be less affected so long as precipitation and/or freshwater runoff was sufficient to prevent both hypersaline conditions and the anaerobic decomposition of peat by sulfate-reducing microorganisms. Since a rise in sea level may not reach inland forests, saline wetlands may be more sensitive to climate change than freshwater wetlands. Because mangrove forest productivity and distribution were strongly affected by changes in hydroperiod, it is expected that the proportion of fringe, basin, and riverine forest in a particular area may also change.
The regional effect of changes in mangrove distribution on habitat quality for estuarine-dependent species, as well as habitat for migratory birds, is one of the most vulnerable functions of mangroves to climate change. Since access to these habitats is largely controlled by flooding frequency, changes in hydroperiod could influence temporal availability. Finally, changes in mangrove distribution may lead to loss of the physical stability and nutrient abatement provided by mangroves as a shoreline buffer.
In the Savannah River Site project, Dixon and Taylor developed models for Carolina Bays, which are relatively isolated wetlands of the southeastern coastal plain. They found that models of bay hydrology can be produced from simple evapotranspiration measurements. Therefore the impact of climate change on hydrology can be explored in the models by adjustments to weather factors that affect evapotranspiration. Decreases in hydroperiod led to loss of amphibian populations as ponds dried up before young metamorphs changed to terrestrial adults. If a bay filled and dried quickly, many metamorphs were stranded. While amphibians can withstand extended periods of dryness through the protection afforded by mud banks, <60 days of standing water resulted in failure of a year class. With standing water >150 days, there was a major increase in the amphibian population. Yet a decrease in precipitation of 15% from current levels, coupled with a 30% increase in temperature, still left some room for amphibian survival. Thus climate change scenarios that changed hydroperiod had the greatest impact on wetland functions.
How do land use changes compare or relate to changes in climate?
Land use has a dramatic impact on wetlands in the eastern Dakotas, since virtually all available space is farmed. The principal effects of land use changes have not been on climate, but rather on sedimentation, runoff, and nesting cover for waterfowl. For example, when cover in surrounding uplands was changed from grassland to corn, the water budget was greatly affected. While changes in land use in the Dakotas may not significantly alter the climate, climate change may lead to changes in agricultural practices. These changes, in turn, may affect wetland habitats, resulting in an indirect effect of climate change on wetlands. In response to these changes, converting cultivated land to grassland may lead to higher water levels in wetlands, possibly counterbalancing the undesirable impacts of a drier future climate.
In the South Florida mangroves project, mangrove forests were affected by urban development, road construction, dams and drainage systems, impoundments, reclamation, and construction of shrimp ponds. In addition to restriction of the intertidal zone by human-imposed barriers, such land use changed the freshwater input, water quality, nutrient loading, and sediment input to wetlands, and modified natural changes in sea level and temperature caused by global climate change. In changing the hydrological balance, large changes of forested wetland areas to agricultural systems modified precipitation patterns (due to change in cloudiness) at the regional scale. Road construction and impoundments strongly affected hydrology and water quality, and therefore had an indirect effect on the survival of plant and animal species that depend on the hydroperiod found in these intertidal systems. Total loss of mangrove habitat is a direct effect that resulted from urban development or construction of shrimp ponds.
Due to the timber harvest program at the Savannah River Site, the direct impact of land use changes on communities greatly overshadowed the indirect impact of land use changes via climate change. While the time scale within which human land use changes directly impact communities is 1-5 years, the effects of land use changes on climate may operate on a scale of 100-500 years. As such, predictions for climate change may be most effectively incorporated into a model that is based on land use patterns. Future patterns of landscape change can be predicted using a 50year management plan put forth by the U.S. Forest Service, emphasizing increased forest age and increased biodiversity. By reducing clearcuts and employing its new management plan, the Forest Service expects to move from the existing dominant age class of 30-40 year old pines to an even distribution of age classes. Even though land use changes have dramatic and direct impact on communities, climate change effects could still be discerned through impacts on keystone processes such as the growth rates of pines.
Conclusion
Results from the research projects and the discussion indicate that key vulnerabilities of wetland and forested ecosystems to changes in climate can be discerned. Vegetation dynamics, as influenced by hydrological balance, proved pivotal in prairie pothole communities, while specific animal-plant interactions and variation in time windows of suitability played significant roles in the Florida mangrove and Savannah River Site communities, respectively. Also, comparison between sites allows for linkages in terms of the relative importance of changes to hydrology/wetlands as a function of geography and local climate conditions. Lastly, results from the three projects suggest that land use changes can act directly or synergistically on the same variables as climate change. For example, in the prairie pothole system, cultivation of nearby land exacerbated the dry summer conditions associated with a warmer, drier climate. The model results from each of the three projects featured in this discussion suggest that wetland functions are highly dependent on hydrological balance. As such, changes in hydrology resulting directly from changes in land use and/or changes in temperature or precipitation, as well as natural disasters, can have dramatic effects on the health and functioning of ecosystems.
Literature cited
Fuad, T.D. 1994. SBI-EPA Habitat Project Update. Bulletin of the Ecological Society of America 75: 137-138.
Jeremy 0. F. Eddy and
Tara D. Fuad
Sustainable Biosphere Initiative
2010 Massachusetts Avenue, NW
Washington, D.C 20036
(202) 833-8748
From: Eddy, J.O.F. and T.D. Fuad. 1996. Global Climate Change Impacts on Habitats: Assessing Ecological Implications of Changes in Climate. Bulletin of the Ecological Society of America 77: 109-112.
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