Patterns of secondary succession in a mangrove forest of Southern Florida

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<ul><li><p>Oecologia (Berl.) 44, 226-235 (1980) Oecologia 9 by Springer-Verlag 1980 </p><p>Patterns of Secondary Succession in a Mangrove Forest of Southern Florida </p><p>Marilyn C. Ball ~ </p><p>Department of Environmental Engineering, Post, Buckley, Schuh and Jernigan, Inc., 7500 NW 52 St., Miami, Florida, USA </p><p>Summary. Successional patterns were studied in mangrove forests which had developed recently in response to salinization of areas formerly supporting freshwater marshes along Biscayne Bay in North Miami, Florida. The population structures of these Induced Forests were compared with an adjacent Historical Forest which consisted of a nearly pure stand of Rhizophora mangle. A mixed forest of Rhizophora and Laguncularia raeemosa had developed in intertidal areas, while areas above the mean high water elevation supported a scrub community dominated by Laguncularia. Maxi- mum growth of both Rhizophora and Laguncularia occurred in intertidal areas, while both species were stunted and had sparse, poorly formed canopies in drier environments above the mean high water level. Analysis of population structure suggests that Induced Forests in intertidal areas are undergoing succession to a stand of Rhizophora. Laguncularia is unable to compete effec- tively with Rhizophora in these areas and it is suggested that it eventually will be limited to the drier areas, where competition from Rhizophora will be reduced or absent. </p><p>Introduction </p><p>Mangroves are the dominant form of coastal vegetation in South- ern Florida. There are three species: Rhizophora mangle L. (red mangrove), Laguncularia racemosa Gaernt. (white mangrove) and Avicennia germinans L. (black mangrove). 1 Although the composi- tion and structure of mangrove forests may vary considerably with local physical conditions, the mangrove species chaicacteristi- cally are distributed in a banded zonation pattern. Davis (1940) recognized seven distinct zones within this pattern. Some of these zones are dominated by a single species while others consist of a mixture of species. </p><p>The significance of zonation patterns has been a controversial topic in mangrove ecology. It has been argued that zonation pat- terns are equivalent to seral stages in succession to the climatic climax, a tropical forest (Davis, 1940; Chapman, 1944, 1970). In this Clementsian interpretation, the coastal zone is the pioneer </p><p>* formerly Kimball </p><p>a Scientific nomenclature throughout follows: Long, R.W. and Lakela, O. (1971) A Flora of Tropical Florida, University of Miami Press, Coral Gables, Florida Present address: Department of Environmental Biology, Research School of Biological Sciences, Australian National University, Canberra City, ACT 2601, Australia </p><p>stage and more landward zones are progressively later stages in succession, thereby implying seaward motion of the system (Davis, 1940; Chapman, 1944, 1970). This classical view is inconsistent with geological data; the predicted patterns of sediment accumula- tion are not always realized in successive zones (Egler, 1952) and the zonation patterns of some forests have existed in situ for millenia (Thom et al., 1975). Further, studies of the distribution of mangroves in relation to coastal geomorphology have shown that vegetational change occurs in response to changes in the physical characteristics of the environment (Thorn, 1967, 1975; Thom et al., 1975; Cintron et al., 1978). </p><p>Extensive changes in the vegetation of Southern Florida have been caused by alteration of the hydrological conditions by man during the twentieth century (Alexander and Crook, 1974). Hydrological changes leading to salinization of soil and water have induced the landward expansion of mangrove vegetation along Biscayne Bay into areas which formerly were freshwater marshes (Reark, 1975; Teas et al., 1976). The present study reports on species interactions in the development of a portion of this mangrove forest. Changes in the distribution of mangroves and marshes are mapped from aerial photographs and historical data. The population structure of a Historical Forest is compared with Induced Forests of recent origin and successional trends are inferred from these data. The results emphasize the role of compe- tition in the formation of zonation patterns. </p><p>Site History </p><p>Early observations of the vegetation along Biscayne Bay were recorded on survey maps made by William de Brahm in 1770 (Chardon, 1976). A mangrove forest was indicated near Arch Creek and the Oleta River (see Fig. 1); landward vegetation was described as ~ tall grass with scattered trees, subject to inundation" (Chardon, 1976). Bowman (1917) reported a dense forest of Rhizo- phora and other trees, presumably freshwater swamp hardwoods, along the banks of the Miami River, several kilometers to the south. Rhizophora growing along the river were 7-8 m in height, whereas only dwarf forms of Rhizophora were found in adjacent freshwater marshes, dominated by the sawgrass, Cladium ja- maicensis Crantz. These observations are consistent with vegeta- tion patterns in the Southeastern Everglades (Egler, 1952), and with geological studies of the Biscayne Bay area (Wanless, 1976) and probably are representative of conditions in the study area at the same period. </p><p>A reconstruction of the vegetation of the study area based on a 1928 aerial photograph is shown in Fig. 2 and probably </p><p>0029-8549/80]0044/0226/$2.00 </p></li><li><p>[ ] Historical forest,1928 . . . . . Tidal creeks </p><p>TP Tidal pond - - - - - - Forest boundaries 1977 </p><p>Fig. 1. Map of the boundaries of the Historical Forest in 1928 and 1977 and the Induced Forest in 1977. Only the limits of the Induced Forest B study area are shown although the forest encompasses the Oleta River </p><p>approximates closely to the conditions before 1900. The location of study sites with respect to the historical vegetation is shown in Fig. 1. The vegetation consisted principally of a mangrove swamp (the Historical Forest) and marshes. The mangrove swamp was dominated by Rhizophora, and this species appears to have been abundant along the margins of the Oleta River and lesser drainages which are outlined on the photograph by dense growth of trees and shrubs. The cabbage palm (Sabal palmetto Lodd. ex Schultes.) and the buttonwood (Conocarpus erecta L.), both of which are tolerant of brackish conditions, occurred in the man- grove forest. Crowns of the palm are visible in the photograph and trunks of dead individuals of both species were found in the present field studies. Some areas near the mouth of Arch Creek may have supported salt marshes, probably composed of either Juncus or Spartina. Three tidal ponds separated the present mangrove forest from upland freshwater marshes, probably domi- nated by sawgrass (Cladium jamaicensis). Although it is now al- most absent from the area, clumps of living Cladium were found growing on spoil piles from 1935 1936 mosquito ditch excavations. This species reproduces almost entirely by vegetative means (Alex- ander, 1971) and may be a remnant of the original marsh, </p><p>The location of these plant communities before 1900 corre- sponds to the distribution of soils (Fig. 3). The location of the Historical Forest is marked by deep peat deposits which character- istically formed to depths of 1-4 m in the swamps of Biscayne Bay (Wanless, 1976). The landward edge of the peat probably represents the historical limits of tidal influence, as accumulation of red mangrove peat generally is limited to elevations in the upper half of the tidal range (Scholl, 1964). Freshwater marshes occurred on Perrine marl, which is of freshwater origin and typi- cally accumulates in marshes with a moderate growth of sawgrass </p><p>or a sparse covering of spike rush (Eleocharis cellulosa Torr.) (Craighead, 1974). The thin layer of organic materials covering marl in some intertidal areas is probably of recent origin. </p><p>A detailed account of urban development and associated hydrological changes which have caused the expansion of man- grove vegetation along Biscayne Bay has been reported by Teas et al. (1976). The Historical Forest probably developed at a time when the salinity regime ranged from fresh to slightly brackish during the wet and dry seasons, respectively (Teas et al., 1976). The mangroves were limited to this seasonally saline environment by their intolerance of the seasonal droughts and fires typical of freshwater wetlands in Southern Florida (Egler, 1952) and by their apparent inability to compete effectively with freshwater veg- etation (Thorn, 1967). Drainage of the Everglades and diversion of runoff reduced the height of the freshwater table and allowed intrusion of saline water into wetlands along Biscayne Bay (Bu- channon and Klein, 1976). Further, the excavation of mosquito ditches in 1935-1936 reduced impediments to the flow of tidal waters and facilitated the landward advance of the effective mean high water level (Teas et al., 1976). Freshwater marsh communities underwent succession to mangrove communities following hydro- logic changes leading to salinization (Alexander and Crook, 1974; Reark, 1975; Teas et al., 1976). </p><p>Landward expansion of mangroves at the expense of freshwater marshes occurred by 1938 (Fig. 2). Mosquito ditches appear to have promoted the spread of mangroves in western areas of the site, whereas mangroves expanded laterally from the margins of creeks and ponds in other areas. The white mangrove (Laguncula- ria) can be identified from the aerial photograph as the dominant canopy component in much of this new growth. In one area, the species composition of this new mangrove growth is not clear from the aerial photograph and is shown as "mixed mangrove" on the map. Marshes persisted over large areas. </p><p>In 1945 (Fig. 2) the freshwater marshes were reduced to patches encircled by mangrove vegetation. The composition of the Induced Forest at this stage is not clear from the aerial photographs. The canopy probably consisted of a mixture of both Rhizophora and Laguncularia; the former species may have been dominant along water courses but Laguncularia probably was the dominant species in most of the forest. The tidal ponds were reduced in size. Rhizophora colonized the shallow areas, forming scattered "tree islands". </p><p>In 1958 (Fig. 2), mangrove vegetation covered the entire wet- land area. The tidal ponds were reduced further in size and the merger of Rhizophora "tree islands" nearly covered the central pond. </p><p>Present Site Characteristics </p><p>The study was conducted in three Induced Forests (A, B and C) and one Historical Forest along Biscayne Bay in North Miami, Florida (Fig. 4), latitude 25 ~ North and longitude 80 ~ West. An- nual rainfall averages 1.64 m with most rain falling from June through October; mean annual temperature is 23.5~ (U.S. Weather Bureau, 1972). The mean high water elevation is +0.325m with a tidal range of approximately 0.27 m. Details of the hydrology have been presented elsewhere (Kimball et al., 1978). The distribution of marl and peat soils on the site is sum- marized in Fig. 3 (Leighty and Henderson, 1958). </p><p>The present site configuration as shown in Fig. 4 was formed by landfilling operations and construction of NE 151 Street in 1963 and NE 135 Street in 1969. The remaining area of Historical </p><p>227 </p></li><li><p>LEGEND [ ] Rhizophora [ ] Upland, disturbed [ ] Marsh (salt) [ ] Mud flats, tree islands [ ] Open water [ ] taguncularia [ ] Mixed mangrove [ ] Marsh (fresh/ TP Tidal pond </p><p>Fig. 2. Change in vegetation from 1928-1958 based on aerial photographs in the years indicated </p><p>Forest together with a contiguous region of Induced Forest A occupy 85.7 hectares. The Historical Forest and most of Induced Forest A occupy an intertidal zone; a narrow band along the landward limits of Induced Forest A is at ground elevations above the mean high water level. Two 1.5 m culverts were installed be- neath NE 135 Street to permit exchange of tidal waters with North Biscayne Bay in which the salinities approach full strength seawater (i.e. 28-320o). Tidal waters are distributed throughout the site by a network of mosquito ditches. Water movement is vigorous in the southeastern area (Historical Forest) and decreases with distance north and west of a centrally located tidal pond to almost stagnant conditions near the landward limits of the intertidal zone. </p><p>Induced Forest B is an integral part of the remaining Oleta River system. The entire site occurs within an intertidal zone and receives natural overland flow of tidal water from the Oleta River. Although this area receives pulses of freshwaters during periods of storm runoff from urban areas, the surface waters generally are brackish (i.e. 15~ Tidal flushing is vigorous, al- lowing very little leaf litter to accumulate on the forest floor. </p><p>A tidal creek marks the eastern limit of the study area (Fig. 4). Induced Forest C covers an isolated tract of 4.9 ha. It previ- </p><p>ously was drained by a southern continuation of the same creek which flows through Induced Forest B. Mosquito ditches link this forest with North Biscayne Bay; however, the culverts are nearly filled with silt and debris and permit very little exchange of tidal waters. Although the ground elevations of Induced Forest C are less than the mean high water level of +0.325 m, it is effectively an upland area because normal tidal flooding has been prevented by surrounding development. </p><p>Method </p><p>Selection of study sites was-based on preliminary reconnaissance of the hydrologic and floristic characteristics in the general area. Vegetation was surveyed by field observations and false color infrared aerial photographs taken in April, 1974. Study sites were then selected in what were considered to be representative areas of the forests. Their locations are shown in Fig. 4. </p><p>228 </p></li><li><p>Table 1. Characteristics of the mangrove vegetation </p><p>Living </p><p>Height (m) </p><p>D.B.H. (cm) </p><p>Relative density (%) </p><p>Basal area (m 2 ha- l) </p><p>Dominance (% total basal area) </p><p>Dead </p><p>Sapling density (m- 2) </p><p>Tree density (m- 2) </p><p>Tree D.B.H. (cm) </p><p>(a) Areas flooded by daily tides </p><p>(1) Historical Forest </p><p>Station 1 Rhizophora Laguncularia Station 2 Rhizophora Laguncularia Station 3 Rhizophora Laguncularia Station 4 Rhizophora Laguncularia Station 5 Rhizophora Laguncularia </p><p>(2) Induced Forests </p><p>(a) Induced Forest A </p><p>Station 1 Rhizophora Laguncularia Station 2 </p><p>Rhizophora Laguncularia Station 3 Rhizophora Laguncularia Station 4 Rhizophora Laguncularia (b) Induced Forest B </p><p>Station 1 Rhizophora Laguncularia Station 2 Rhizophora Laguncularia Station 3 Rhizophora Laguneularia Station 4 Rhizophora Laguncularia Station 5 Rhizophora Laguncularia Station 6 Rhizophora Laguncularia </p><p>5 </p><p>13 </p><p>20 </p><p>24 </p><p>10 10 </p><p>8 10 </p><p>8 12 </p><p>9 11 </p><p>8 12 </p><p>8 11 </p><p>4.2 8.0 </p><p>4.9 </p><p>7.8 </p><p>11.1 </p><p>13.2 </p><p>5.0 7.0 </p><p>4.5 4.5 </p><p>3.2 3.1 </p><p>3.4 3.7 </p><p>7.5 9.5 </p><p>3.4 7.2 </p><p>3.5 6.7 </p><p>5.9 6.0 </p><p>3.5 6.7 </p><p>3.9 8.9 </p><p>87.5 12.5 </p><p>100.0 0.0 </p><p>100.0 0.0 </p><p>100.0 0.0 </p><p>100.0 0.0 </p><p>90.0 10.0 </p><p>57.1 42.9 </p><p>46.5 53.5 </p><p>43.9 56.1 </p><p>83.3 16.7 </p><p>55.0 45.0 </p><p>75.0 25.0 </p><p>75.0 25.0 </p><p>63.2 36.8 </p><p>58.3 41.7 </p><p>4.2 2.0 </p><p>13.3 </p><p>13.7 </p><p>12.5...</p></li></ul>