Patricia Negreros-Castillo

Effects of partial overstory removal on the natural regeneration of a tropical forest in Quintana Roo, Mexico

Abstract In a tropical, semi-evergreen forest in Quintana Roo, Mexico, a study was conducted to observe the effects of partial overstory removal (creation of multiple gaps) on natural regeneration, with special attention to commercially valuable tree species. Five 0.5-ha plots were subjected to different levels of overstory removal of 0%, 8%, 28%, 45% and 55%. The regeneration population 3 years after overstory removal was similar in density and composition to the population before overstory removal except for an unidentified liana that was not found in 1986 but was moderately abundant in 1989. Regeneration of commercial tree species (tolerant and intolerant), noncommercial tree species, and nontree species was compared with the residual basal area and percentage of basal area removed. Frequency of intolerant commercial species increased as residual basal area decreased. Frequency of tolerant commercial species was not affected by residual basal area or percentage of basal area reduced.

Stand and Species Growth of a Tropical Forest in Quintana Roo, Mexico

Abstract A study of the effect of different levels of basal area removal on stand and tree growth was established in a second growth forest in Quintana Roo, Mexico. In five 0.5 ha plots (subdivided into 10 × 10 m2 plots), data were collected periodically for 15 years and used to estimate stand and diameter growth of 30 tropical forest tree species. Net basal area per hectare growth was almost 0, -0.1% yr-1. For surviving trees only, basal area growth was 1.6% per year. On the 10 × 10 m2 plots basal area growth was strongly correlated to initial basal area when only surviving trees were considered. Maximum average diameter growth rate was 0.31 cmy-1 (Swietenia macrophylla King). Most species grew relatively slowly, probably due to low quality of the site resulting from calcareous soils and low precipitation rates and possibly due to being in a relatively old second growth forest. Mortality by species during the 15 years of the study ranged from 0% to 40%. The results are contrary to information used to develop management plans for the forests of the area and should be used to modify these plans to avoid overcutting.

SURVIE DES SEMENCES DE SWIETENIA MACROPHYLLA SEMÉES SUR BRÛLIS DANS L’ÉTAT DE QUINTANA ROO AU MEXIQUE

In tropical America, regeneration of big- leaf mahogany, Swietenia macrophylla King, the most important commercial tree species, is problematic because of its fruiting and dispersal characteristics, very low tolerance of shade and response to logging. In Quintana Roo, Mexico, abandoned slash-and-burn cropfields are excellent candidates for seedling esta- blishment, but an efficient way to esta- blish mahogany seedlings has not been identified. This study assessed the proba- bility of predation on directly sown maho- gany seeds, examining the effectsofthree treatments (four types of seed protection, two sowing methods, two times of day for sowing). The mean percentage of seeds either partially consumed or removed during a 12-hour period varied from 1.1 to 7.7% among the four seed protection treatments and did not differ between the two sowing methods. Precipitation had an important positive effect on survi- val, and minimum daily temperature had a marginal negative effect on survival. The Julian calendar date had a positive effect on seed survival. Seed predation did not differ among the seed protec- tion treatments that allowed access only to rodents, only to insects, and to both types of seed predators, and predation was greater with these three treatments than with the no-access treatment. Seed predation was not influenced by the time of day of sowing. Direct sowing offers a good option for regenerating mahogany, especially if the mahogany seeds are soaked for at least 2 weeks before sowing to reduce predation risks and increase prospects for seed survival and seedling establishment.