SELVA-MANGRO: Integrated Landscape and Stand-level Model of Mangrove Forest Response to Sea-level Rise and Hydrologic Restoration of the Everglades
Thomas W. Doyle and Kenneth W. Krauss
U.S. Geological Survey, National Wetlands Research Center, Lafayette, LA,USA
Marcus Melder and Jason Sullivan
Johnson Control World Services Inc., USGS-NWRC, Lafayette, LA,USA
The near sealevel elevation and flat slope of the protected Everglades ecosystem accounts for one of the largest contiguous tracts of mangrove forests found anywhere in the world and punctuates their potential vulnerability to rising sea level and changes in freshwater runoff. These forests are subject to coastal and inland processes of hydrology largely controlled by regional climate, disturbance regimes, and water management decisions. Mangroves are highly productive ecosystems and provide valued habitat for fisheries and shorebirds. Mangrove forests are universally composed of relatively few tree species and a single overstory strata. Three species of true mangroves are common to intertidal zones of the coastal margin of the Everglades, namely black mangrove, Avicennia germinans (L.) Stearn, white mangrove, Laguncularia racemosa (L.) Gaertn.f., and red mangrove, Rhizophora mangle L.
Mangroves are halophytes and can, therefore, tolerate the added stress of waterlogging and salinity conditions that prevail in lowlying coastal environments influenced by tides. Global warming has been projected to increase sea water temperatures and expansion that may accelerate sea level rise and further compound ecosystem stress in mangrove dominated systems. While early researchers attributed zonation patterns to salinity gradients, more recent field and experimental studies indicate that mangroves have wide tolerances to salinity and other soil factors and may be influenced to a greater degree by local hydrology and episodic disturbance events. Increases in relative sea level will eventually raise saturation and salinity conditions at ecotonal boundaries where mangroves are likely to advance or encroach upslope into freshwater marsh/swamp habitats. Changes in freshwater runoff as a result of climatic variability and water management controls may alter the health and migration of mangrove communities in conjunction with potential sea-level rise.
A landscape simulation model, SELVAMANGRO, was developed for mangrove forests of south Florida to investigate the potential impacts of climate change and freshwater flow on the quality and distribution of future mangrove habitat. The SELVAMANGRO model represents a hierarchically integrated landscape model that manages the exchange of system parameters up, down, and across scale between linked simulation models SELVA and MANGRO. SELVA is a Spatially Explicit Landscape Vegetation Analysis model that tracks predicted changes in the biotic and abiotic conditions of each land unit (1 sq ha) on an annual basis for the entire simulated landscape. The SELVA model administrates the spatial articulation of the landscape composed of land units composed of habitat classifications (forest, marsh, aquatic) and any forcing functions that predict changes in hydrology and disturbance. Intertidal forest units are then simulated with the MANGRO model based on unique sets of environmental factors and forest history. MANGRO is a spatially explicit stand simulation model constructed for mangrove forests of the neotropics. MANGRO is an individualbased model composed of a set of speciesbased functions predicting the growth, establishment, and death of individual trees. MANGRO predicts the tree and gap replacement process of natural forest succession as influenced by stand structure and environmental conditions.
Model applications were conducted to forecast mangrove migration under projected climate change scenarios of sealevel rise and saltwater intrusion for the Everglades coastal margin without hydrologic restoration. A high resolution model of surface topography was needed to predict the rate and fate of coastal inundation from sealevel rise over the next century. Tidal inundation and circulation are key factors controlling mangrove distribution in this coastal environment. The ability to predict landward transgression of mangrove caused by sealevel rise depends on the relationship between landward slope and elevation in relation to tide range and extent plus an understanding of relative sealevel rise. A historic topographic and drainage map circa 1940-50 with 1 ft contour intervals across the south Florida Everglades was rectified and digitized into a geographic information systems application. Boundary zones of major habitat classes were also digitized from the natural vegetation map of Florida produced by Davis (1943) to delineate the lower and upper elevations of the intertidal zone as defined by mangrove extent. The coastline was assigned an elevation of mean sea level while the upper transition zone of mangrove extent was approximated at mean high water for available tide datums along the southwest coast of Florida. These combined data sources and proxy contours served as baseline elevation values for constructing a detailed digital elevation model of south Florida for SELVA-MANGRO application.
Sealevel rise was modeled as a function of historic sealevel conditions at Key West, Florida based on mean annual tide records (1940 to present) projected into the 21st century with the addition of curvilinear rates of eustatic sea level expected from climate change. The data record was extended into the next 100 years for sealevel rise scenarios of 15 cm to 1.1 m by year 2100 based on low, mid, and high projections obtained from global climate change models. Model results show that species and forest cover will change over space and time with increasing tidal inundation across the simulated landscape for all sealevel rise scenarios. The greater the rate of sealevel rise the faster or more extensive the encroachment of mangroves onto the Everglades slope. The model shows that freshwater marsh/swamp habitats will be displaced as the tidal prism increases over time as it moves upslope without Everglades hydrologic restoration. Under these modeling assumptions, mangrove habitat will increase over the next century under climate change and conversely, freshwater marsh/swamp is expected to decrease.
Modeling upgrades are under development for the SELVA-MANGRO model to assess the impact of increased freshwater runoff under various Everglades restoration alternatives. Empirical data of riverine and basin mangrove forests show that precipitation and runoff events effect short-term hydrology and salinity conditions that are relatively minor in relation to coastal influences of daily and seasonal tidal forcing. Everglades restoration alternatives will increase runoff conditions above current normal patterns and may abate any influence of sea-level rise in the near future. Model trials indicate that proposed freshwater flow rates may need to exceed current engineering design to affect stage and salinity at the coastal margin to affect potential mangrove migration and expansion into freshwater habitats.
Contact: Thomas W. Doyle, U.S. Geological Survey, National Wetlands Research Center, 700 Cajundome Blvd., Lafayette, LA, 70506, Phone: 3372668647, Fax: 3372668592, Email: , Oral, Ecology and Ecological Modeling