Dynamics of tropical dry forests in Saint Lucia, West Indies.

Abstract

Tropical dry forests are the most endangered and least studied of all tropical forest ecosystems. On the Eastern Caribbean island of St. Lucia, as well as elsewhere in the tropics, dry forests are heavily utilized by humans and highly disturbed. I studied the ecological importance of anthropogenic disturbance and edaphic properties on forest composition and structure in the tropical dry forest life zone of St. Lucia. I reviewed the tropical dry forest literature, providing (1) a new synthesis on how disturbance is often the dormant influence on forest succession, structure, composition, and (2) a review of nutrient cycling in tropical dry forests, highlighting the role of soil properties and processes. This was essential for understanding the ecology of St. Lucia's dry forest zone and how it relates to other tropical dry forests. Canopy heights in tropical dry forests range from 3 to 35 m with most less than 20 m. Compared to tropical moist forests, dry forests produce less litter with slower turnover rates. Soil organic C for tropical dry forests ranged from 2 to 11%. St. Lucia's tropical dry forest life zone is covered primarily by a scrub forest, short in stature and dominated by Guettarda scabra, and Croton spp. Stem density (stems $\ge$ 1.0 cm) is 39,407 stems ha$\sp{-1}$, and average DBH is 1.75 cm. The average canopy height is 4 m, and average basal area is 15 m$\sp2$ ha$\sp{-1}$. Soil organic C is 3%, potential net N mineralization is 1.49 ug N g$\sp{-1}$d$\sp{-1}$ nitrification is 1.61 ug NO$\sb{3\sp-}$-N g$\sp{-1}$d$\sp{-1}$. In a relatlvely undisturbed stand of dry forest in St. Lucia, soil problems exhibited spatial autocorrelation at a distance of 24 m or less. Geostatistical maps of soil properties suggest that spatial patterns of soil N transformations, organic C and available P are likely driven by variation in organic matter inputs from plant litter production, and not by physical soil properties like texture. Ranges for soil values in St. Lucia's scrub forest are nearly the same as those of the relatively undisturbed forest. Thus, the short stature and small diameters that characterize St. Lucia's dry forest zone are likely due to disturbance, rather than soil nutrient status. As such, this region has the potential to support forests of larger stature.