Post-fire dynamics of growth and structure in mixed conifer forest of northern México.

Abstract

Fire is one of the most important disturbances for fire-dependent ecosystems and drives the patterns of vegetation structure and composition. The objective of this study was to characterize the structural dynamics of a mixed conifer forests of the Sierra Madre Occidental (SMO) affected by fires. Vegetation was sampled at three fire severity levels: high, moderate, and low, and the unburned level was included as a control. Species richness (S), Shannon index (H), and the Evenness index (E) were calculated. The diameter-size class and height class of the trees were determined by applying the indices H, and E indexes and the coefficient of variation (CV). Differences in the indices calculated across fire severity levels were determined by analysis of variance (ANOVA) and Tukey's multiple comparison tests. Results showed no significant differences (p ≤ 0.05) in species diversity indices among fire severity levels; however, diameter and height classes were lower in areas affected by high-severity fire. Increment cores and crosssections of fire-scarred trees were also collected and analyzed using dendrochronological techniques to generate tree ages and descriptive statistics involved in fire history studies. Synchrony between fire history and tree establishment was determined, and climatic data values were correlated with the number of trees established per year. Forty-one fire events were reconstructed over the period 1855-2019. Overall, the mean fire interval (MFI) was 2.28 years and 12.17 years for large fires. The number of trees established per year was influenced by the prevailing dry conditions in September and October of the previous year and the wet conditions that occurred in December of the same year. These results constitute an example of forest response to fire severity and its historical behavior and may support further studies related to the influence of fire on other forest communities present in the SMO.