Tree islands are vital elements of the Greater Everglades Ecosystem (GEE) landscape, providing relatively dry habitat for flora and fauna and functioning as biogeochemical hotspots within the oligotrophic marsh. Despite their importance, the number, size, and distribution of Everglades tree islands has dramatically declined since the 1950s due to hydrologic modifications of the GEE. The ability of tree islands to be maintained as topographic highs in a flooded landscape may largely be due to the balance between soil accretion and decomposition, both of which are effected by variations in inundation. Changes in tree island litterfall, litter decomposition, and soil accretion rates relative to surface water levels and periods of inundation (hydroperiod) were determined over several years. Sites were arranged based on two relative elevations at head highs and head lows (nominally 90 cm and 45 cm above slough soil surface, respectively) on constructed tree islands in the Loxahatchee Impoundment Landscape Assessment (LILA; ARM Loxahatchee National Wildlife Refuge, Boynton Beach, Florida, USA). We hypothesized that litterfall and decomposition rates would be greater at higher tree island elevations because of shallower water depth and shorter inundation time and the net balance in these factors would determine overall soil accretion.
Results/Conclusions
Litter production varied between 263 and 732 g m-2 yr-1 and was greater at high elevation where trees were maximally productive. Decomposition rates were negatively correlated with inundation time, indicating higher decomposition at high elevations (2.37 and 2.19 % loss mo-1, respectively) due to more favorable aerobic conditions. Measurements of respiration via soil surface efflux of CO2 where inversely related to water depth. Soil accretion rates, using feldspar markers, averaged 0.70 cm yr-1 and maximized at high elevations. Newly accreted surface soils (0-3 cm) exhibited greater total phosphorus (TP, 374 μg g-1 dw), total nitrogen (TN, 14.4 mg g-1 dw), total carbon (TC, 190 mg g-1 dw), and organic matter (OM, 0.36 g g-1 dw) compared to 3-10 cm (older) soils (TP, 216 μg g-1 dw; TN, 10.2 mg g-1 dw; TC, 132 mg g-1 dw; OM, 0.25 g g-1 dw). Our findings indicate that plant growth and litter production influence tree island nutrient dynamics by increasing soil development. Although rates of production (as mass litterfall) and decomposition where both greater at higher elevations, hydrology (reduced inundation) exerted a greater influence on increasing production rather than reducing decomposition.