SUSTAINABLE HARVESTING OF TROPICAL TREES - DEMOGRAPHY AND MATRIX MODELS OF TWO PALM SPECIES IN MEXICO

摘要

Thrinax radiata and Coccothrinax readii, two palm species of the dry tropical forest in Quintana Roo, Mexico, had been used in earlier times by the Mayans and adult palms are used extensively today. The overexploitation of these species in the Sian Ka'an Biosphere Reserve has made management and conservation measures necessary. To assist in the preparation of a management plan for sustainable use, we obtained the patterns of survivorship, growth, and reproduction from four populations of Thrinax radiata and one population of Coccothrinax readii. Thrinax were more abundant (less than or equal to 2977 individuals/ha) than Coccothrinax (less than or equal to 760 individuals/ha), and abundance and population structures varied depending on habitat conditions and previous harvesting. Flowering, fruiting, and leaf production varied in both species, apparently as a function of rainfall and site. Seedling survival is high in comparison to other tree species (67-91% in Thrinax). Thrinax attains a height of 3 m at an age of 31-55 yr and Coccothrinax at an age of 63 yr. Both species start reproducing when 4 m tall. Maximum life spans were estimated at 100-145 yr for Thrinax and at >145 yr for Coccothrinax. We estimated transition probabilities for each life cycle category of both species and obtained Lefkovitch (1965) matrices to estimate the populations' finite growth rates (lambda = 1.05 for Coccothrinax and lambda = 1.09-1.15 for different populations of Thrinax), to generate sensitivity and elasticity matrices under studied conditions, and to simulate various harvesting regimes. Growth rates are very sensitive to changes in adult survival and growth. The long recuperation times in both species (47-84 yr for the population to recover to 400 adult Thrinax palms per hectare and >100 yr for Coccothrinax) emphasize the importance of being extremely cautious before harvesting additional adult palms. Simulations suggest that only up to 40 Thrinax and no Coccothrinax adult palms per hectare may be removed each year if the population is to be sustained. If palms are harvested on a rotational basis every 2, 3, or 4 yr, the number of palms that may be sustainably harvested increases up to 40-80 palms/ha in the Thrinax populations and up to 8 palms/ha in the Coccothrinax population. Our simulation results should be taken cautiously because they do net consider year-to-year variation. [References: 40]