The Treasure of the Sierra Madre: Ecology of old-growth forests in Chihuahua, Mexico

Old-growth forests are valuable but declining worldwide. México still holds large areas covered by temperate forests in the mountains of the Sierra Madre Occidental, but few of these retain old-growth characteristics. These forests provide habitat for Thick-billed Parrots (Rhynchopsitta pachyrhyncha: Psittacidae; “guacamaya”), a CITES-listed endangered species. We studied four old-growth remnants in Mesa de las Guacamayas, a site in the Sierra Madre Occidental in Chihuahua, México, to assess the composition, structure, and age characteristics of the overstory, relating it to fire histories and continental and regional climatic data. We linked our findings to the habitat needs of Thick-billed Parrots reported by the literature, and we studied the β-diversity of the understory plant communities at these sites and related them to the composition, cover, density and fire regimes of the overstory.
We found that frequent disturbance by surface fires appears to have contributed to maintaining open, diverse, and productive forests for at least 250 years. While climate was a historical driver of the fire regimes in this mountain range, humans appear to have played a role in the fire regime interruptions of the second half of the 20th century. We found large live trees (>60 cm DBH) in the four sampling sites. We also found densities of five or more large snags per ha-1 in two of the sampling sites, which are considered good nesting habitat conditions for Thick-billed Parrots. Pinus strobiformis, an important food source for the parrots, was common in three of the four sites. We also detected close interactions between understory, overstory and fire regimes in the sampled old-growth forests. We did not encounter non-native plant species, which suggests that β-diversity of the plant communities and maintenance of the ecological process of fire could play a “shielding” role in preventing invasions.
We also collected data about overstory age and structure, and understory cover and composition in temperate pine-oak forests inside Parque Nacional Cascada de Basaseachi, and outside the park, in a logged forest. Both of these sites are located in central Chihuahua, in the Sierra Madre Occidental range. We used these datasets to evaluate the effectiveness of the park at conserving plant species richness and diversity. We concluded that the forest plant communities inside the park are more species rich and diverse than outside the park. We proposed a source-sink model in which regional biodiversity conservation goals could be achieved or magnified by combining alternative approaches, such as community-based management with traditional models, such as protected areas.
Autora: Citlali Cortés Montaño