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William H. Orem
Tree islands are considered key indicators of the health of the Everglades ecosystem because of their sensitivity to both flooding and drought conditions. Tree islands also act as a sink for nutrients in the ecosystem and may play an important role in regulating nutrient dynamics. Although management strategies to restore and even create tree islands are being formulated, the published data on their age, developmental history, geochemistry, and response to hydrologic alterations is limited. To address these issues, this project integrates floral and geochemical data with geologic and vegetational mapping activities to establish the timing of tree-island formation and impacts of both flooding and droughts on tree islands throughout the Everglades.
Assistance with the previous phase of the project was provided by: Lorraine Heisler, Tim Towles: Florida Game and Fresh-Water Fish Commission. Scientific collaboration and logistic support and elevation and vegetation data from tree islands and guidance in tree-island selection.
Fred Sklar, Susan Newman, Tom Fontaine, Ken Rutchey, Yegang Wu, Steve Krupa, Cindy Bevier, Jeff Giddings: South Florida Water Management District. Scientific collaboration and logistic support, elevation and vegetation data from tree islands and guidance in tree-island selection, and data on ground-water/surface water interactions and cores with lithologic description of underlying bedrock
Laura Brandt: Loxahatchee National Wildlife Refuge. Scientific collaboration and guidance on tree-island selection, vegetation data.
Tom Armentano, David Jones: Everglades National Park. Scientific collaboration on tropical hammocks in Everglades National Park.
David L. Dilcher: Florida Museum of Natural History. Scientific collaboration and vegetational reconstruction and evaluation of Late Holocene CO2 concentrations.
Weimer, Lisa M.
Bernhardt, Christopher E.; Weimer, Lisa (deceased); Gamez, Desire; Cooper, Sherri R.; Jensen, Jennifer
Pielke, Roger A., Sr.; Steyaert, Louis T.; Willard, Debra A.
Willard, D. A.; Lerch, H. E.; Bates, A. L.; Boylan, A.; Corum, M.
Sklar, Fred H. and van der Valk, A. (editors)
Holmes, Charles W.
Weimer, Lisa M.; Riegel, W. L.
Holmes, C. W.; Korvela, M. S.; Mason, D.; Murray, J. B.; Orem, W. H.; Towles, T.
Sklar, F. H. and van der Valk A. (editors)
Orem, W. H.
Willard, D. A.; Marot, M.; Holmes, C. W.
Holmes, C. W.; Weimer, L. M.
Webb, III, T.; Prentice, I. C.
Willard, D. A.
Cronin, T. M.
Bernhardt, C. E.; Holmes, C. W.; Landacre, B.; Marot, M.
Reimer, P. J.
Vogel, J. C.
Fuls, A.; Visser, E,; Becker, B.
Column A (the sample ID) refers to the sites listing. Line 1 lists taxa identified in the samples. For lines 2-13, original raw data on total pollen grains were unavailable, so back-calculation from percentages to original counts was not possible. The numbers provided in these lines are percent abundances. For all other samples, the number of grains counted for each taxon was determined by back-calculation from percentages and total grains counted, as provided by Riegel, 1965.
Pollen from surface sample collected in 1995 and 1998:
Column A (sample ID) refers to the sites listing. Line 1 identifies taxa identified in the samples. Numbers provided for all taxa represent raw counts. Numbers of exotics counted refers to number of Lycopodium grains counted during each pollen count. For each sample, one tablet of Lycopodium grains was added before processing. Each tablet contains 12, 542 +/- 416 Lycopodium grains. Sample weight refers to the dry weight of sample processed. Pollen concentration (pollen grains/gram dry sediment) was calculated using the formula
pollen grains/gram dry sediment= [(pollen counted/exotics counted) x 12542]/sediment weight
We collected sediment cores using a piston corer with a 10 cm diameter core barrel. The core barrel was pushed through the sediments until it contacted the underlying limestone at all sites except in Loxahatchee NWR, where peat thicknesses in excess of 2 meters would require alternative coring strategies. After core collection, we extruded sediment from the core barrel and sampled it at 1 cm increments for the upper 20 cm and at 2 cm increments at greater depths. We described sediment lithology as samples were extruded. We dried samples in a 50 deg C oven and subsampled sediments at the base of each core and at 20 cm increments above the base for radiocarbon dating. Bulk peats were dated using conventional radiocarbon datin
Age models for the last century of deposition are based on 210Pb (lead-210) and, where applicable, first occurrences of pollen of the exotic plant Casuarina which was introduced to south Florida in the late 19th century (Langeland, 1990). Lead-210 (210Pb) activity was measured by alpha spectroscopy using the method outlined in Flynn (1968) in which 210Pb and its progeny, polonium-210 (210Po), are assumed to be in secular equilibrium. Supported 210Pb activity was determined by continuing measurements until activity became constant with depth. Excess 210Pb activity was calculated by subtracting the supported 210Pb activity from the total 210Pb activity. Accumulation rates were calculated by fitting an exponential decay curve to the measured data using least squares optimization and making the assumptions of a constant initial excess lead-210 concentration (the CIC [constant initial concentration] model) . In pre-20th century sediments, models are derived from linear interpolation between radiocarbon data points obtained on bulk sediment samples, which were calibrated to calendar years using the Pretoria Calibration Procedure (Stuiver et al., 1993; Talma and Vogel, 1993; Vogel et al., 1993). The shorter hydroperiods and shallower water depths on tree-island heads result in seasonal drying and oxidation of sediments. We have noted that radiocarbon dates from tree-island head sediments appear to be artificially old relative to those in the tail and adjacent marsh. Cores collected in the near tail, directly downstream from the head, have radiocarbon dates and vegetational trends that are consistent both internally and with adjacent wetlands. Therefore, we used cores from the near tail as our representative sites to detect vegetation changes on teardrop-shaped tree islands and for comparison with patterns documented in the adjacent marsh.
Approximately 0.5-1.0 gram of dry sediment was used for palynological analysis. Pollen and spores were isolated from these samples using standard palynological techniques (Traverse, 1988; Willard et al, 2001a,b). After drying and weighing samples, Lycopodium marker tablets with known concentrations of Lycopodium spores were added to approximately 0.5 g of sediment for calculation of absolute pollen concentrations (Stockmarr, 1971). The samples were first acetolyzed (9 parts acetic anhydride : 1 part sulfuric acid) in a hot-water bath (100 deg C) for 10 minutes, then neutralized, and treated with 10% KOH in a hot-water bath for 15 minutes. Neutralized samples were sieved with 10 µm and 200 µm sieves, and the 10-200 µm fraction was stained with Bismarck Brown, mixed with warm glycerin jelly, and mounted on microscope slides. Raw data for pollen samples are reposited in the North American Pollen Database (NAPD) at the World Data Center for Paleoclimatology in Boulder, CO (<http://www.ngdc.noaa.gov/paleo/pollen.html>) and at the US Geological Survey South Florida Information Access (SOFIA) site (<https://sofia.usgs.gov>).
Pollen and spore identification (minimally 300 grains per sample) was based on reference collections of the United States Geological Survey (Reston, VA) and Willard et al. (2004). Absolute pollen concentrations were calculated using the marker-grain method described by Benninghoff (1962). Marker tablets of Lycopodium spores were the source of the exotic grains, and the quantity of Lycopodium spores in the marker tablets was determined by the manufacturer with a Coulter Counter following the procedures of Stockmarr (1973). Absolute pollen concentration was calculated using the formula (Maher, 1981): Pconc=RM/V,
Where: Pconc = pollen per gram dry sediment; R=pollen grains counted/marker grains counted; M=marker grains added; V=dry weight of sediment
Our interpretations of past plant communities are based on the quantitative method of modern analogs (Overpeck et al., 1985). We calculated squared chord distance (SCD) between down-core pollen assemblages and a suite of 197 surface samples collected throughout southern Florida in the early 1960s and 1995-2002 (Willard et al., 2001b and this research) to define the similarity between each fossil and modern pollen assemblage. Internal comparison among surface samples from ten vegetation types indicates that samples with SCD values < 0.15 may be considered close analogs (Willard et al., 2001b). If analogs were present for a fossil assemblage, we identified the source vegetation for the fossil assemblage as one of the twelve types represented in the modern database. We divided cores into pollen zones based on a combination of visual inspection, objective zonation using CONISS (Grimm, 1992), and modern analogs.
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