Creation of Alternate Stable States in Tropical Dry Forests as a Consequence of Human Disturbance

Abstract:

Anthropogenic disturbances in Caribbean subtropical dry forests of the Caribbean lead to either novel species assemblages in post-disturbance forest, or stable degraded grassland states. Among these disturbances are fire and clearing. Short-term responses to both disturbances include sharp increases in nutrient fluxes, shifts in soil moisture availability, and the spread of invasive grass species if seed sources are nearby. In Guánica Forest, Puerto Rico, our studies in sites that were frequently burned and a chronosequence of once-burned sites beginning 30 years ago showed that fire is associated with invasion by exotic grass species and that once invaded, grass dominated sites do not revert to forest. After 30 years, single burn sites were still ≥65% lower in basal area and species richness than native forest. Frequently burned sites had 60-75% lower soil N, 50-65% lower S, and 56-68% lower C, as well as elevated Fe and pH. A single burn in a previously unburned forest elevated NO₃ flux ~6-fold for at least the next year. Once dominated by grass, tree seed rain is extremely low (7 seeds/m²/y). Low dispersal rates combined with low germination rates makes forest regeneration unlikely. In contrast, sites that cleared or plowed without grass invasion regain structural characteristics of native forests in approximately 40 years. However, these sites are typically dominated by Leucaena leucocephala, a naturalized tree introduced to the island in the 1700s. The proportion of native-species saplings in the understory of Leucaena-dominated forests increased by 0.85%/yr in stands >35 years old until reaching ~95% in 76 yr old stands, suggesting that Leucaena facilitated the reestablishment of native species. We conclude that when anthropogenic disturbance creates grass-dominated sites, these areas will remain stable degraded states. Leucaena can serve as an alternate pathway to forest restoration, facilitating the return of forest cover with a novel combination of species.