Mangrove forests dominate the world's tropical and subtropical coastlines. Along coasts with very low topographic relief mangroves often gradually give way to brackish and freshwater marsh vegetation communities. This is characteristic of regions like the Gulf of Carpentaria (northern Australia), the Sunderbans (Bangladesh), and the Florida Everglades. Analysis of historical aerial photographs of the southwest coast of Everglades National Park reveal that the position of the mangrove / marsh ecotone has moved substantially in some locations and hardly at all in others. Several causal factors have been advanced to account for the encroachment of mangroves into the adjacent marshes, including: sea level rise and reduced freshwater inflow from upstream. Additional influences may include drought, fire and freezes.

I have conducted detailed studies along a 300m transect on the Harney River, on the southwest coast of Everglades National Park. The area was chosen specifically because it has shifted inland by >150m since 1940. The transect begins in the tall mangrove forest adjacent to the river and runs southward, ending in a sawgrass plain (Cladium jamaicense). Permanent vegetation monitoring plots and porewater salinity sampling wells were established at five sites along the transect. At each end of the transect hydrological sampling wells have been established to measure surface and ground water elevations and salinities. Site 1 is 15m from the river and consists of tall (25m) mangrove forest with all three species present (Avicennia germinans, Laguncularia racemosa and Rhizophora mangle). Site 2 is 50m further inland. All three mangroves are present and the forest canopy is some 20m high. Site 3 is 70m further inland. Rhizophora and Laguncularia are common, Avicennia is uncommon and Conocarpus erectus is present. Laguncularia is the dominant tree at Site 4, which is another 70m inland. Rhizophora and Conocarpus are present here and sawgrass forms an understory beneath the open canopy. Site 5 is 300m from the Harney River and is dominated by sawgrass. Scattered individuals of Myrica cerifera and Persea borbonia are present.

Sediment porewater salinity has been measured in replicate wells (n=3) from two depths (30 and 60cm) at each site at approximately bi-weekly intervals since April 1997. Two permanent vegetation monitoring plots were established in 1997 at each site (10 total) and have been remeasured annually. Seventy-three plots have been monitored to record mangrove seedling establishment and growth since the fall of 1997. For each plot, individual tree stems were identified, measured for Diameter at Breast Height (DBH), permanently tagged and mapped.

Similar patterns in the variation of sediment porewater salinity have been observed over time at each site, however the magnitude of variation is different among sites. In general salinity is highest nearest the Harney (.15o/oo) and decreases inland. However the lowest salinity was at site 4, with the sawgrass marsh having slightly increased salinities. Porewater salinity tended to be highest at all sites in the late dry season (May-June). This dry season high varied between years. For example, highest salinities at site 1 were 30o/oo in May-June of 1997 and only 20o/oo in May-June 1998. During this late dry season period, salinities were highest at 30cm depth and less at 60cm depths. This difference was almost 10o/oo at site 1 (30 vs. 20o/oo). This depth difference was observed at all sites and in all years. The magnitude of the difference decreased from the river to the sawgrass marsh and the differences also varied among years of observation. With the onset of summer rains, sediment salinities dropped quickly at all sites. Furthermore, surface salinities (30cm) dropped to lower levels than measured for deeper (60cm) sediments.

Mangrove stem density increased moving inland and was greatest at sites 3 and 4. Stem density then dropped to zero at site five. Stem size however was greatest near the Harney River. Sites 3 and 4 were dominated by small very sized (<3cm DBH) Laguncularia racemosa. Various measures of vegetation structure such as stem density, species richness and basal area were found to be uncorrelated with patterns of salinity or salinity variation. At present, vegetation dynamics across this mangrove / sawgrass ecotone appear to be more affected by episodic events or long term forcings. For example, a hard freeze in January 1997, "top-killed" many Laguncularia along the ecotone, moving it towards the river. However, the root stocks of these plants survived and resprouted. Observations at other ecotonal areas in southwest coastal Everglades indicate that fire plays an important role, particularly for mangrove-sawgrass and mangrove-Spartina ecotones. Long-term shifts in the position of these ecotones may be related to sea-level rise and upstream water management practices.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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