Tropical Dry Forests of St. Lucia, West Indies: Vegetation and Soil Properties

Abstract

The tropical dry forest life zone of St. Lucia is largely covered by scrub forest characterized by a short canopy and a large number of small-diameter trees. However, it is unclear whether these short-stature forests result from edaphic properties that limit the development of taller-stature dry forest or result from the repeated harvest of wood. To investigate the influence of edaphic properties on forest composition and structure, we sampled 21 stands within the dry forest life zone of St. Lucia. Mean canopy height in the scrub forest was 4 m and stands contained an average of 39,407 stems ha-'; 96 percent of those stems were ?5 cm in diameter. Mean basal area in the scrub forest (stems ?1.0 cm DBH) was 15 m2 ha-1, 62 percent of which consisted of stems ?5 cm in diameter. Guettarda scabra (Rubiaceae) and Croton spp. (Euphorbiaceae) were the most common species. Soil organic C averaged 3.2 percent, potential net N mineralization was 1.49 pug N g-1 d-1, and potential net nitrification was 1.61 pug NO3--N g-I d-l. Concentrations of available P ranged from 1 to 3 mg kg-'. We found no relationship between vegetation and soil properties in the scrub forest of St. Lucia. Moreover, most soil properties in the short-stature scrub forest did not significantly differ from those of relatively undisturbed tropical dry forest in our study area, even though overstory composition and structure differed dramatically. Rates of net N mineralization in the scrub forest were 57 percent of those in a relatively undisturbed dry forest; that difference was significant. The similarity of soil properties between short-stature scrub forest and relatively undisturbed dry forest suggest that repeated disturbance, rather than edaphic properties, is a major influence on forest composition and structure within the dry forest zone of St. Lucia.