Dendrochronological reconstruction of fire history in coniferous forests in the Monarch Butterfly Biosphere Reserve, Mexico

Abstract

Background: Some have proposed that fire return intervals lengthen with elevation in montane tropical coniferous forests, such as those found in central Mexico. This would generate patterns of synchronous tree establishment in higher elevation forests dominated by Abies sp. Mill., and patterns of continuous tree establishment in lower elevation forests dominated by Pinus sp. L. However, it is common to find codominant stands of both genera at intermediate elevations. The Monarch Butterfly Biosphere Reserve (MBBR) is located in this region, and is covered by Abies religiosa (Kunth) Schltdl. & Cham-dominant forest, A. religiosa-Pinus pseudostrobus Lindley-codominant forest, and P. pseudostrobus-dominant forest. Despite the ecological importance of the MBBR's coniferous forests, little is known about their fire return intervals, including their relationship with climate, and their effects on tree establishment patterns according to the elevational range proposed above. In this study, using dendrochronological techniques, we reconstructed the historical fire return intervals for each forest type, evaluated their relationship with droughts and with the El Niño Southern Oscillation (ENSO), and assessed the effect of fire return intervals on tree establishment patterns. A total of 110 cross-sections were obtained, from 23 sites across the MBBR. Results: The fire chronology covered the period between 1925 and 2015. Fire return intervals averaged two years in stands dominated by A. religiosa, by P. pseudostrobus, and codominated by both species, regardless of elevation. No fire-free periods were detected, even after the establishment of the reserve in 2000. Fire frequency was not associated with periods of drought and ENSO. There was no relationship between fire return intervals and patterns of tree establishment, which was continuous in all three forest types. Conclusions: Our results suggest that coniferous forests of the MBBR have historically experienced frequent fires that are unrelated to climate and tree establishment. This may be evidence that natural fire regimes in these coniferous forests have been drastically altered by human activities. Although these frequent low-severity fires could adversely affect tree regeneration, and eventually decrease the integrity of the overwintering habitat of the Monarch Butterfly, the continuous tree regeneration observed suggests that these effects have not yet taken place.

Fire History (1896–2013) in an Abies religiosa Forest in the Sierra Norte of Puebla, Mexico

“…Fire history studies conducted in central Mexico have documented uninterrupted fire frequencies up until the early part of the 21st century [26,48,49,56], while others have documented fire exclusion after 1970 [49]. Our results suggest the frequency of widespread fires changed after 1983 (Figure 3C).…”

“…These results address the first research question and are similar to results from other mixed conifer studies that include Abies species [51] in the southwestern United States and northern Mexico, which generally found that fire occurred once per decade before the current fire suppression period. Relatively short mean fire intervals of five years were also found in mixed conifer stands that included Abies in the states of Chihuahua [25], Durango [24], Michoacán and Mexico [26]. In addition, many of the fire years recorded within A. religiosa forests were synchronized with fire years in adjacent P. hartwegii forests located less than 5 km away [47].…”

“…Historical fire regimes based on dendrochronological techniques have been studied but only in mixed-conifer forests that include Pinus-Abies-Pseudotsuga in northern Mexico [24,25]. In central Mexico, where the largest Abies populations are concentrated, studies have mainly focused on the Monarch Butterfly Biosphere Reserve [26]. Therefore, this is the first study focused on fire regimes within pure Abies dominated forests in central Mexico.…”

Fire scar characteristics in two tropical montane conifer species from central Mexico

“…Schlecht et Cham) and smooth-bark Mexican pine (Pinus pseudostrobus Lindley) are two conifer species widely distributed in temperate forests in central Mexico. Field-based observations and two recent fire chronologies confirm the possibility of reconstructing fire regimes by analyzing fire scars in trees of both species (Sáenz-Ceja and Pérez-Salicrup 2019b;Cerano-Paredes et al 2021). However, little is known about the characteristics of fire scars in samples obtained from the two species, information that could be useful when conducting field sampling in dendroecological research.…”

“…These are relict forests from the last glaciation period and host high pine species diversity (Farjon 1996). Most of the reconstructed fire regimes have focused on pine species (Yocom and Fulé 2012;Cerano-Paredes et al 2015;Sáenz-Ceja and Pérez-Salicrup 2019b). In contrast, studies on other conifers, such as the Abies species, are very few in these ecosystems because it has been assumed that these experience standreplacement fires (Rodríguez-Trejo 2008), and forest legacies decay quickly owing to moist conditions (Moreno-Valdez et al 2018).…”