We studied the influence of anthropogenic disturbance on forest structure and composition in the highly populated Montane Rain Forests of northern Chiapas, Mexico. We evaluated species richness, basal area and stem density on 81 circular plots (0.1 ha each) along a categorical disturbance gradient due to forest extraction, livestock grazing, and fires. A total of 116 tree species (>5 cm DBH) were recorded in three major forest types recognized by TWlNSPAN. The three forest types were: Quercus-Podocarpus Forest (QPF), Pinus-Quercus-Liquidambar Forest (PQLF), and Pinus ForeSt (PF). The number of canopy and understory trees species, absolute density, and basal area decreased with disturbance intensity. Mean basal area of Pinus spp. was high at intermediate and severe disturbed sites (27 and 19 m2 ha-1, respectively), and low (0.2 m2 ha-1) in Well preserved old-growth stands. Distribution of life forms was heterogeneous among forest types, with a high number of understory trees species in QPF, and an impoverished composition in PF. A first axis obtained by factor analysis, represented a combination of anthropogenic disturbance along with environmental and structural variables. Scores of the first factor explained almost 50% of variation, and was positively correlated with livestock grazing, firewood extraction, basal area of Pinus spp. and soil pH, and negatively associated with elevation, plant cover and basal area of Quercus spp. A second factor explained an additional 12% of variation and was associated with forest fires and timber extraction. Distribution Of size classes in the QPF was significantly different (p < 0.05) than in the other two forest types, including the largest individuals in all inventories. Our results suggest that small scale, but frequent anthropogenic disturbance, increases the dominance of Pinus and drastically decreases floristic richness, mostly understory trees. This points to the need of developing restoration practices aimed to attain highly diverse mixed forests from induced depauperate pinelands. On the other hand, the remnant MRF stands are currently under risk of deforestation in a highly populated Mayan territory, and their conservation Under Criteria of sustainable use may require finding alternative high value uses not included in conventional commercial forestry. ® 2001 Elsevier Science B.V. All rights reserved.
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