Survival and growth of tree seedlings in anthropogenically disturbed Mexican montane rain forests

The montane rain forest in Chiapas, Mexico has been subject to continuous human disturbance, mainly by wood extraction, cattle grazing and fire. To address the effect of this kind of disturbance regime on regeneration, survival and growth of naturally established tree seedlings were evaluated during a 3-yr period in three forest stands which varied in frequency and intensity of human disturbance. Frequency of cattle browsing on transplanted seedlings of Cornus disciflora, Liquidambar styraciflua and Oreopanax xalapensis planted on different facing slopes was evaluated over an 18-mo period, their survival and growth was also determined. The density of seedlings, number of species and survival of naturally recruited tree seedlings decreased with increased intensity of disturbance. Only seedlings of Pinus and Quercus species were recorded in heavily disturbed conditions. Seedlings grew more vigorously under moderately disturbed than in undisturbed or heavily disturbed conditions. More extreme microclimatic variation (air temperature, relative humidity and solar radiation) was recorded in heavily vs moderately disturbed forest. Survival and growth of transplanted tree seedlings differed with species. Grazing damage varied among seedlings located on different facing slopes. Plants located on NE facing slopes survived better than those on other slope positions. The mean proportion of surviving individuals of L. styraciflua (80%%) and C. disciflora (78%%) was higher than those for O. xalapensis (60%%). A higher number of C. disciflora and O. xalapensis seedlings were grazed than L. styraciflua in any slope orientation treatment. The results suggest that human disturbance negatively affects natural recruitment, reduces richness of tree seedlings and favours a small number of tree species.Abbreviations: PF == Pinus forest; PPFD == Photosynthetic photon flux density; PQLF == Pinus-Quercus-Liquidambar forest; QPF == Quercus-Podocarpus forest; RDGR == Relative basal diameter growth rate; RHGR == Relative height growth rate; TMRF == Tropical montane rain forest.Nomenclature: Breedlove (1981).