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Landscape Characteristics of Rhizophora Mangle Forests and Propagule Deposition in Coastal Environments of Florida (USA)

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Landscape Characteristics of Rhizophora Mangle Forests and Propagule Deposition in Coastal Environments of Florida (USA) Landscape Ecology (2005) 20: 63–72 Ó Springer 2005 DOI 10.1007/s10980-004-0468-8 0 Research article Landscape characteristics of Rhizophora mangle forests and propagule deposition in coastal environments of Florida (USA) Raja Sengupta1,2, Beth Middleton3,4,*, Chen Yan1, Michelle Zuro1 and Heidi Hartman4 1Department of Geography, Southern Illinois University at Carbondale, Carbondale, IL 62901-4514, USA 2Department of Geography, 805 Sherbrooke Street W., McGill University, Montreal, Canada QC H3A 3Department of Plant Biology, Southern Illinois University at Carbondale, Carbondale, IL 62901-4501 USA 4National Wetlands Research Center, U.S. Geological Survey, 700 Cajundome Boulevard, Lafayette LA 70506, USA; *Author for correspondence ([email protected]) Received 18 December 2003; Accepted in revised form 27 May 2004 Key words: Coastal wetlands, Dispersal, Fragmentation, GIS, Landscape connectivity, Modeling, Recruitment limitation, Remote sensing, Restoration ecology Abstract Field dispersal studies are seldom conducted at regional scales even though reliable information on mid-range dispersal distance is essential for models of colonization. The purpose of this study was to examine the potential distance of dispersal of Rhizophora mangle propagules by comparing deposition density with landscape charac- teristics of mangrove forests. Propagule density was estimated at various distances to mangrove sources (R. mangle) on beaches in southwestern Florida in both high-and low-energy environments, either facing open gulf waters vs. sheltered, respectively. Remote sensing and Geographic Information Systems were used to identify source forests and to determine their landscape characteristics (forest size and distance to deposition area) for the regression analyses. Our results indicated that increasing density of propagules stranded on beaches was related negatively to the distance of the deposition sites from the nearest stands of R. mangle and that deposition was greatly diminished 2 km or more from the source. Measures of fragmentation such as the area of the R. mangle forests were related to propagule deposition but only in low-energy environments. Our results suggest that geographic models involving the colonization of coastal mangrove systems should include dispersal dynamics at mid-range scales, i.e., for our purposes here, beyond the local scale of the forest and up to 5 km distant. Studies of mangrove propagule deposition at various spatial scales are key to understanding regeneration limitations in natural gaps and restoration areas. Therefore, our study of mid-range propagule dispersal has broad application to plant ecology, restoration, and modeling. Introduction fragmented forests to deposition sites if landscape characteristics associated with fragment size and Mangrove forests are becoming more isolated in many connectivity are altered (With and Crist 1995; Gu locations as coastal regions of the tropics develop. et al. 2002). Smaller and more distant fragments of This isolation could lead to regeneration limitation mangrove forests may disperse fewer propagules than that could reduce recruitment into safe sites (sensu- larger, less distant fragments (sensu island biogeog- Harper 1977). Specifically, too few mangrove propa- raphy theory as stated in Middleton 1999). Therefore, gules or other disseminules may be dispersed from the fragmentation characteristics of these forest 64 sources of propagules ultimately may dictate the po- long-distance dispersal in high-energy oceanic envi- tential for regeneration within landscapes (Gustafson ronments, where ocean currents were thought to carry and Gardner 1996). Even though deposition charac- Rhizophora mangle propagules at least 100 km (Davis teristics of propagules relative to landscape fragmen- 1940; Murray 1986). However, more recent studies tation have been studied little in the field and mostly that focused on propagule dispersal at local scales on localized levels within or directly adjacent to for- indicate that dispersal is more limited for most species ests (Pither and Taylor 1998; Middleton 2000), many (Nathan 2001). For example, the majority of propa- models rely on basic information concerning dispersal gules of R. mucronata in Malaysia dispersed less than dynamics (Doak and Mills 1994; Gu et al. 2002; 20 m from the parent tree, and only a few propagules Tischendorf and Fahrig 2000). In biogeographical dispersed more than 65 m (Chan and Husin 1985). models, dispersal dynamics are recognized as a key Most propagules of Avicennia marina dispersed near factor limiting worldwide mangrove distribution their parent trees (Clarke and Myerscough 1991), (Drexler 2001; de Lange and de Lange 1994; Higgins although a few propagules can disperse up to 10 km et al. 2003). Field studies that provide information for (Clarke 1993). Similarly, most propagules of Ceriops dispersal deposition traits in modeling are necessary tagal dispersed less than 3 m from their sources but lacking, and would be particularly helpful to make (McGuinness 1997). The argument that propagule predictions about regional landscape function in set- dispersal is very local (de Lange and de Lange 1994) tings of anthropogenic fragmentation. Nonetheless, is further bolstered by findings that suggest a limited post-dispersal factors such as herbivory and shade duration of propagule bouyancy (e.g., A. marina is ultimately may be more important in determining the bouyant for only 4 days; Steinke 1975). Some species zonation patterns of mangrove species than dispersal of propagules may have a greater capacity for long- (Clarke et al. 2001; Clarke and Kerrigan 2002; Clarke distance dispersal than A. marina, and these dispersal 2004). However, dispersal undoubtedly is important characteristics could ultimately affect supply. Rhizo- from the perspective of supply. phora mangle propagules remain bouyant for 20–100 Past engineering projects in riverine and tidal wet- days, and are viable for up to a year or more (Rabi- lands that fragment forests may limit the available nowitz 1978), although certain other species will sink propagules if forest characteristics (e.g., size, distance in as little as 15 days (Clarke et al. 2001). Objects that and interconnectivity; Middleton 1999, 2003) are al- float for long periods of time have a great potential for tered on a regional scale. At local scales within for- long-distance migration as a result of transport by ests, foresters have recognized the importance of ocean currents; for example, Nike shoes from five having adequate amounts of seed for the regeneration containers that were swept off a ship in the mid- of forests in logged sites (Clark et al. 1999), but little Pacific and spilled onto the ocean traveled 2800 km in thought has been given to the process at a regional 174–211 days, and subsequently were stranded on scale (directly adjacent to and up to 5 km distant from several beaches in Washington and Oregon (Ebbes- the source forest). The relationship of regional dis- meyer and Ingraham 1992). Undoubtedly, long-dis- persal to regeneration limitation may be very direct, so tance dispersal is relevant to this discussion. We test in that an understanding of the relationship of propagule this paper that shorter-range dispersal is primarily deposition to mangrove source characteristics is responsible for regeneration limitation in mangrove essential both to the restoration of coastal mangroves forests, and that propagules deposition may be related and the recolonization of gaps after disturbances such primarily to local fragmentation characteristics (i.e., as hurricanes, wind-throw, or insect damage (sensu- forest distance and size). Duke 2001; Sousa et al. 2003). Studies of mangrove The objectives of this research, therefore, were to dispersal also are relevant because some restoration document deposition patterns of Rhizophora mangle projects have relied solely on naturally dispersed propagules on beaches with regard to the distance and propagules; however, the landscape characteristics of degree of the fragmentation of regional source forest in these settings are not known (Buchanan 1989; Lewis both high- and low-energy coastal settings, i.e., facing 1990; Middleton 1999). open gulf waters vs. sheltered, respectively. Our For the purposes of modeling, field studies of the objectives were related to dispersal potential and not to deposition characteristics of mangrove propagules in a regenerative potential since propagules do not regen- regional context are warranted. Anecdotal research on erate on beaches. Necessarily, we observed the rela- the dispersal of mangrove propagules emphasized tionship of dispersal and fragmentation to deposition 65 Figure 1. Location of study beaches along the coast of southwestern Florida (USGS Terra Server. 2003). patterns in non-regenerating sites, since there is the beaches that were farther away, and, (2) adjacent to potential for recording larger scale deposition patterns source forests of larger, rather than smaller areal on beaches rather than on mangrove forests. extent. The study area was located in southwestern Florida (Fort Myers to Ten Thousand Islands) Study area (Figure 1, Figure 2) and consisted of mangrove forests naturally isolated by physical geography and/ The study area was located in southwest Florida from or anthropogenic development. In both low- and Fort Myers to Ten Thousand Islands (Figure 1; 25.85° high-energy environments, we tested whether the N to 26.4° N latitude, and 81.88 to 81.54° W longi- density of mangrove
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