Wolfe, Brett

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    Post-fire regeneration in subtropical dry forest of Puerto Rico
    (2009) Wolfe, Brett; Van-Bloem, Skip J.; College of Agricultural Sciences; Chinea, Jesús Danilo; Macchiavelli, Raúl; Department of Crops and Agro-Environmental Sciences; Perdomo, José L.
    Dry forests were widespread in the tropics before being cleared for agricultural and urban use, but they are now considered to be one of the most endangered ecosystem types. Cleared and degraded tropical dry forests commonly form exotic-grassland communities that are maintained by periodic fires and are difficult to reforest. In the dry zone of southwest Puerto Rico, exotic grasses invade disturbed areas and expand into intact forest through successive burning. When fire is prevented in such areas they are colonized by the exotic legume tree Leucaena leucocephala, which dominates the canopy for >80 yr. A better understanding of the factors that drive these vegetative patterns is needed to improve management aimed at forest restoration and protection. This project assessed the fire resistance of common tree species, the seed rain in degraded forest areas, and the performance of saplings in repeatedly burned areas of Guánica Forest. Bark thickness was measured on 474 trees of 26 species and related to stem size. Most species obtained putative fire-resistant bark thickness only in the largest trees or not at all, while two species, Bursera simaruba and Bucida buceras obtained it in relatively small stems. Seed rain was collected with 100 traps of 0.25-m² for one year (beginning July 2007) at four sites that had five focal vegetation types along a gradient of degradation. The seed count totaled 35,610 non-graminoid seeds. Seed rain of native trees and L. leucocephala were similarly low in open grass areas. Exotic-forest areas, dominated by L. leucocephala, had an increased density of animal-dispersed tree seeds compared to open grass areas. Planted saplings were used to compare growth and survival in the forest understory to grass-dominated areas with and without protection from fires. In November 2007, 455 saplings of 13 native tree species and L. leucocephala were planted in three areas dominated exotic grasses and in nearby forest. Survival of the burned saplings was initially high (36.4%), but eight months later few survived. The majority died during a seasonal drought in June - July 2008. In the same period sapling survival in the unburned grasslands was reduced compared to forests. 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 grew 11.7 mm in diameter and 30.0 cm height. These results suggest that native trees are not resistant to fire at sapling or adult sizes, and they are 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.