Expediting reforestation in tropical grasslands: Distance and isolation from seed sources in plantations

Where tropical forests have been cleared and abandoned, forest regrowth is often slow. Because of the increasing extent of such degraded lands and desire for these areas to fulfill conservation and ecosystem functions, it has become important to assess mechanisms that accelerate reforestation. In situations where site conditions, limit tree establishment, one option to facilitate reforestation is to establish plantations of fast-growing noninvasive trees that ameliorate site conditions and promote the growth of indigenous trees. To quantify the effect of distance and isolation from seed sources on the regeneration of indigenous trees, we evaluated tree species richness and stem density in four pine plantations surrounded by natural forest within Kibale National Park, Uganda, and one isolated pine plantation surrounded by agriculture near Kibale. Tree regeneration in these plantations was compared to that in natural forest and to an anthropogenic grassland similar to those upon which the plantations were established. Tree species richness and stem density were negatively correlated with distance to the plantation edge. The grassland had lower tree species richness and stem density than the isolated plantation, which had lower species richness and stem density than the plantations within Kibale. Leguminous species dominated tree regeneration in the isolated plantation and grassland, whereas small-seeded animal-dispersed species dominated regeneration in plantations adjacent to forest. We suggest that these differences are due to decreased seed dispersal across large distances or unfavorable habitats (e.g., grasslands) and to greater disturbance (e.g., fire) in the isolated plantation and grassland than in plantations within the National Park. This study demonstrates that forest reestablishment can be faster if plantations are established than if grasslands are left to recover without management. However, differences within and among plantations can lead to very different communities with respect to species composition, species richness, and stem density.