Paul V. McCormick and Eric S. Crawford
U.S. Geological Survey, Leetown Science Center, Kearneysville, WV, USA

The interior of the Arthur R. Marshall Loxahatchee National Wildlife Refuge retains a soft-water (i.e., mineral-poor) chemistry that likely prevailed across much of the predrainage Everglades. Slough-wet prairie (SWP) habitats are a dominant landscape feature and contain several species indicative of low mineral conditions. Intrusion of mineralized canal water into the Refuge is associated with the loss of these species and a shift in the broader landscape towards increased dominance by sawgrass (Cladium jamaicense) stands and reduced SWP cover. These changes are correlated not only with increases in soil and water mineral content, but also with hydrologic changes, increased phosphorus loads, and historic vegetation gradients. Thus, it is difficult to determine the causes of vegetation patterns from field observations alone.

Here we report the initial findings of laboratory experiments designed to assess the sensitivity of plant populations and landscape patterns to increased mineral levels caused by canal-water intrusion. We focused initial experiments on Xyris ambigua (yellow-eyed grass), a SWP species that is restricted to the interior of the Refuge, and on sawgrass, which is increasingly dominant near the Refuge periphery. Seedlings of Xyris were collected in the field and grown in interior SWP soil watered with different concentrations of a solution with a mineral composition similar to that of canal water. Additional seedlings were grown in soil from a SWP habitat with elevated mineral levels near the periphery of the Refuge. Sawgrass plants were obtained from seed and grown in interior SWP and sawgrass soils amended with different mineral levels and in soil from the same peripheral SWP site as for Xyris. Plant growth was measured as increases in height and biomass over several months.

The growth of Xyris declined significantly when exposed to a mineral solution equivalent in strength to canal water, while plants exposed to half-strength water exhibited visible signs of physiological stress (e.g., browning of leaf tips). Similarly, plants in soil from the peripheral SWP site grew significantly slower than those in interior SWP soil. Sawgrass seedlings grew significantly faster in soil from an interior sawgrass stand than in soil from an adjacent SWP habitat. Increasing the mineral content of these soils had a statistically significant, but less pronounced effect on plant growth. Sawgrass growth was significantly faster in peripheral as compared to interior SWP soil but did not exceed that in interior sawgrass soil that received no mineral amendment. Our initial results indicate that elevated mineral concentrations can limit the distribution of certain SWP species in the Refuge but may be no more than a secondary factor affecting the relative dominance of sawgrass and SWP habitats.

Contact Information: Paul V. McCormick, USGS Leetown Science Center, 11649 Leetown Road, Kearneysville, WV 25430 USA, Phone: 304-724-4478, Fax: 304-724-4465, Email: pmccormick@usgs.gov

(This abstract is from the 2006 Greater Everglades Ecosystem Restoration Conference.)

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