Relationships between frequency of ground exposure and forest cover in a mangrove island ecosystem.

Abstract

In mangrove forests, as in all intertidal systems the hydroperiod is of great importance to the vitality of mangrove forests. Despite the importance of inundation and drying periods in tidal ecosystems, spatially-explicit measurements of frequency of ground exposure to air (the drying period) and its effect on mangrove forest cover have rarely been quantified. An eight-month long study was conducted of the tidal regime along a main channel in an overwashed island mangrove forest off the coast of Belize, Central America. Cumulative frequency distribution of tidal data were integrated within a geographic information system to model the spatial extent of ground exposure, and forest cover was estimated using the normalized difference vegetation index (NDVI). Empirical relationships between forest cover and exposure were quantified along seven cross sections, and along 23 transects. On average, exposure accounted for 43% of the variance in NDVI along cross sections. Composite data results for 23 transects showed a positive linear response of NDVI to exposure along both right (R2 = 0.31) and left (R2 = 0.14) banks, with no response in the upper channel area. On average, exposure accounted for 33% of the variance in NDVI on the right bank, and for 13% of the variance on the left bank. At a local scale, NDVI highest responses in the right bank ranged from R2 = 0.44 to R2 = 0.71. Landscape variability in NDVI vs. ground exposure relationships may be due to confounding effects from interactions with other biological and physical variables such as: microbial mats, wind, sun shadows, spatial heterogeneity in topography, rainfall, canopy structure, root density and distribution.