Patterns of litter production across a salinity gradient in a Pterocarpus officinalis tropical wetland

Abstract

Historically, Pterocarpus officinalisJacq. (Leguminoseae) dominated freshwater wetlands in the coastal plains of Puerto Rico, but deforestation has reduced its distribution to small patches adjacent to mangrove forests in areas of higher salinity. The objective of this study was to determine how a gradient in soil salinity affected litter, flower, and fruit production in a Pterocarpus officinalis.Three 100 m2 plots were established in each of three sites along a salinity gradient: pasture/Pterocarpus edge (low salinity, mean salinity at 60 cm–9.7 g Kg−1), Pterocarpus forest (intermediate salinity, 11.5 g Kg−1) and a Pterocarpus/mangrove ecotone (high salinity, 15.0 g Kg−1). Across this gradient, P. officinalis accounted for 100% of the relative basal area in the low and intermediate sites and 43% in the high salinity site which was domimated by Laguncularia racemosa. The basal area of P. officinalis decreased along the gradient from 73.5 m2 ha−1 in the low salinity site to 42.0 m2 ha−1 in the high salinity site. Litterfall was sampled on average every 23 days in 45 0.25 m2 traps (5 traps per plot) for two years. Annual litterfall for the forest was 11.9 Mg  ha−1 yr−1. Peaks in litterfall were associated with high precipitation in May 1995 and tropical storms in September 1995. Leaf fall of P. officinalis was significantly higher in the low salinity site (4.8 Mg ha−1 yr−1) than the high salinity site (1.8 Mg ha−1 yr−1), but total stand litterfall was greatest in the area of high salinity due to the greater contribution of L. racemosa. Pterocarpus flower and fruit production was approximately 10 times greater in low and intermediate salinity sites in comparison with the high salinity site. An increase in global temperature, will lead to higher sea level and higher soil salinity in costal wetlands. To conserve this wetland forest type it is critical to expand the distribution into areas of lower salinity where this species occurred historically.