Background/Question/Methods Dry forests were widespread in the tropics before being cleared for agricultural and urban use in pre-Columbian through present times. They are now considered to be one of the most endangered ecosystem types. Cleared and degraded tropical dry forests commonly form stable exotic-grassland communities that are maintained by periodic fires and are difficult to reforest. In the dry zone of southwestern Puerto Rico, the 4000-ha Guánica Forest and Biosphere Reserve has been invaded by African forage grasses that dominate roadside areas and expand into intact forest through successive burning. A better understanding of the factors that inhibit forest regeneration in these areas is needed to improve management aimed at restoration and protection of the remaining forest. We planted saplings to compare growth and survival in the forest understory to grass-dominated areas with and without protection from fire. In November 2007, 455 saplings of 13 native tree species and the naturalized tree Leucaena leucocephala (Lam.) deWit were planted in three areas dominated by the exotic grass Pennisetum ciliare (L.) Link and in nearby forest. In February 2008, prescribed fires were applied to a subset of the saplings in the grasslands.
Results/Conclusions The prescribed fires were relatively intense, yet sapling survival was initially high; 67 of 184 (36%) saplings resprouted after burning. Eight months later the survival rate of burned saplings was reduced to 5%. The majority of these saplings died during a seasonal drought from May-June, 2008. During the same period sapling survival in the unburned grasslands and forest understories was reduced to 49% and 70% respectively. Growth during the first year was minimal and dieback was common on shoots of surviving saplings. An exception was L. leucocephala in the unburned grasslands, which on average doubled in basal diameter and increased 40% in height. We attribute the low sapling survival in the grassland areas to the stresses of water deficit combined with high temperature and light levels. The slow growth, high mortality, and continued fire susceptibility in larger size-classes of native trees renders them ineffective for reforestation of burned sites in Guánica Forest. The use of L. leucocephala as an intermediary between exotic grassland and native forest is suggested.