Hydrological processes as affected by stand composition in semiarid forest in central Mexico

Semiarid forest of Northern Mexico have been affected by diverse land use change types including; species-specific deforestation, firewood and pinyon harvesting, and grazing. These multiple usages have induced shifts in the distribution and proportion of Pinus cembroides and Quercus potosina the dominant tree species. Changes in species dominance and subsequently in the resulting functional attributes of vegetation assemblages may affect ecosystem processes. We examined three different forest assemblages in a semiarid forest located in San Luis Potosí, Mexico in terms of hydrological processes. In this semiarid ecosystem we examined three important hydrological processes of rainwater redistribution including; throughfall, stemflow and superficial runoff as a function of stand composition. We selected 12 plots along a 2 km North facing stand that include three forest assemblages including, Pinus, Quercus, and the mixture of both. Measurements of gross rainfall, throughfall, stemflow and runoff were conducted during summer and fall 2006 and winter and spring 2007. Both throughfall, stemflow were determined by gross rainfall rather than dasometric characteristics of species (stem diameter). Stemflow was influenced by stem diameter only in Quercus but not in Pinus (P<0.05). Throughfall was linearly related to gross rainfall in both species. We observed significant differences among the three assemblages in superficial runoff (P<0.05). In this case, Pine stands observed twice as much runoff as compared to Quercus (14 vs. 7 liters on average) while the mixed stand presented intermediate values. Our results show that only certain hydrological processes are affected by tree stand composition in semiarid forests from Central Mexico.