Tree canopy patterns in a temperate forest in Oaxaca, Mexico after bark beetle outbreaks: Implications for ecological resistance and resilience

Abstract

Bark beetles (BB) are an insect that can become a forest pest and that principally affects temperate regions globally, massively killing tree hosts, mainly coniferous species. When tree hosts are the dominant species in the canopy, BB outbreaks may impose important changes in the structure, composition and diversity of forests, from the stand to the regional scale. In Mexico, during the current century, BB pests have become the main biotic threat in forests where Pinus species are dominant. Questions addressed in this study are: What is the structure, composition and diversity of the forest canopy after BB disturbances? and “What are the implications for ecological resistance and resilience of the forest”? The study site was located in the Sierra Norte region in Oaxaca, Mexico, where more a decade ago BB outbreaks affected the forest. In ninety plots of 500 m2 (4.5 ha) all live trees with ≥ 5 cm diameter at breast height (DBH) were located, their DBH was measured, and the species was identified. As well, all stumps resulting from previous sanitation logging and timber extraction (TE) were located and for each condition, basal diameter was estimated and the genera identified. For the post-disturbance canopy, density basal area, species composition and diversity (alpha and beta) were estimated. This data plus the analysis of the stumps was used to estimate the tree canopy structure before the BB outbreaks. Statistical comparisons among BB, TE disturbances and condition without disturbance were conducted. Based on interviews with local experts, a rank of risk to BB among Pinus species was established. A total of 4,053 live trees and 547 stumps were found. The live trees corresponded to 24 species from 8 genera. Pinus genus and, particularly P. patula predominated. Seventy-five percent of the individuals had < 20 cm DBH and a J-inverted diametric distribution resulted. Individuals with DBH ≥ 100 cm were found only in the condition without disturbance. Three of the nine Pinus species, were BB main-host species, and the least preferred host was P. ayacahuite. The three BB main-host species dominated the post-disturbance canopy (including regeneration). In contrast, P. ayacahuite and other non-host tree species dominated in the conditions without disturbance, and the three main-host species showed low density. As well, high species richness occurred in areas affected by BB. Since climate change will likely continue to exacerbate BB outbreaks, it is proposed that to increase ecological resistance and resilience forest management practices should focus on reducing density, promoting a mix of host and non-host species, increasing tree diversity and avoiding stands with monodominance of main-host species.