Can cattle grazing substitute fire for maintaining appreciated pine savannas at the frontier of a montane forest biosphere-reserve?

Abstract

Human induced savannas in subtropical regions are often favored by small-holder farmers for livestock production and extraction of wood or non-wood products. Frequent burning and grazing are required to maintain the savanna vegetation structure. However, in conservation areas, fire suppression is promoted to avoid wildfires; whereas domestic livestock grazing is considered a strong interfering factor for tree establishment, due to trampling and browsing. In tropical forests which were converted to savannas, competitive exotic grasses have often replaced the native grasses. Where exotic grasses are present, aboveground biomass accumulation and thus man-induced fire risk are high and potentially undermine tree recruitment. On the long-term, the savanna state may shift into a grass-dominated state with little tree cover, generating unfavorable conditions from a livelihood perspective. We examined this problem in a human-induced pine savanna in the La Sepultura Biosphere Reserve in Chiapas, Mexico. Smallholder farmers highly valued this savanna for both livestock production and resin extraction from the fire resistant pine Pinus oocarpa. However, fire suppression and the presence of exotic grasses are reducing the tree recruitment. The main research question was to what degree can cattle grazing replace fire in its role of biomass removal and thereby stimulate pine recruitment and maintain the desired savanna state. We determined current savanna extension in the region and interviewed farmers to reconstruct past savannazation processes and expansion of exotic grasses. We related adult species-specific tree density to the herbaceous-grass cover, and pine and oak seedling and sapling densities to understory vegetation cover, canopy closure, and cattle grazing history. Finally, a field experiment was conducted to examine the effects of livestock grazing on survival and growth of planted pine saplings. The savanna currently covers 20% of the study site; it is the result of past slash-and-burn agriculture and selective logging, which have favored the expansion of several exotic grass species. In savannas where exotic grasses are abundant, sapling density was lower compared to sites with a native grass cover. While livestock grazing seemed to increase pine seedling density likely as a consequence of reduced grass cover, pine sapling survival however, was significantly reduced by livestock trampling. By seeking a balance between the livestock’s benefits and adverse effects on pine recruitment, farmers may develop an integrated management system adapted to their specific biotic rangeland conditions. It should allow forage production, while controlling the negative effects of exotic grasses on pine recruitment, thus maintaining a productive pine savanna system.