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

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.