Impact of 25 years of grazing on the forest structure of Pinus cembroides in northeast Mexico

Increases or decreases in grazing cause significant disturbances in natural and managed forests. Indeed, grazing impacts can be on par with habitat destruction, exotic species invasion and fire. Grazing can produce changes in the diversity of native and exotic species as well as in functional plant groups at community and ecosystem levels. Unfortunately, overgrazing is a common occurrence in many plant communities, increasing erosion risks, land degradation and plant invasion. We analyze and quantify the effects of grazing by goats as well as cattle and horses on forest structure and spatial distribution of a dominant tree species (Pinus cembroides) in Coahuila, Mexico, an area affected by overgrazing and with problems of desertification. We also assess the influence of other forest variables that may be affected by cattle grazing, such as infestation by Tillandsia recurvata (an atmospheric epiphytic bromeliad) on P. cembroides. The study site is a protected area of 45,000 ha in the Sierra of Zapaliname (Coahuila, Mexico). In this area, grazing is considered a traditional and essential activity as the local economy depends on it. Eight plots (900 m(2)) 100 m apart were set up over a control site that had been grazed by cattle and equines for at least the last 100 years, while another eight plots (also 100 m apart) were established in an area that had excluded grazing for 25 years. Tree species composition and forest structure were analyzed in the plots, as well as other relevant variables, like T. recurvata infestation. The number of tree species (DBH>2.5 cm) differed significantly in control (6) vs. exclusion areas (11), with higher values in the latter. In addition, infestation of T. recurvata was found to be significatively higher in plots where grazing occurred. Spatial analysis of the tree distribution did not reveal any specific patterns related to either grazed or exclusion areas, and neither were there significant differences in species composition. For saplings (>50 cm high and <2.5 cm DBH), five species were present in exclusion plots, while there were only three in control ones. Regarding seedlings (<50 cm high), three species were present in both exclusion and control plots. Our findings suggest that grazing may have modified species richness and made the tree population more vulnerable to parasitic plants, as shown by the higher infestation levels. Also, tree regeneration has been negatively affected, reducing the number of species. We suggest a reduction in grazing pressure and more restrictive grazing management for the forest site as a way of maintaining landscape use, cultural values, and biodiversity.