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.

Research Highlights: Age structure was used to infer fire regimes in the Monarch Butterfly Biosphere Reserve. Uneven-aged structures in stands dominated and co-dominated by pine and fir species, which are distributed according to an altitudinal gradient, indicated a regime of frequent, low-severity, and low-intensity fires. Background and Objectives: Age structure analyses have been used to infer natural and disrupted fire regimes when field-based descriptions of fires are scarce or unavailable. In montane conifer forests, fire regimes typically vary according to an altitudinal gradient, shaping contrasting tree establishment patterns. In the Monarch Butterfly Biosphere Reserve, Mexico, the altitudinal distribution and fire regimes of sacred fir forests (Abies religiosa), smooth-bark Mexican pine forests (Pinus pseudostrobus), and mixed-conifer forests are poorly documented. The objectives of this study were to determine the altitudinal ranges occupied by mono-dominant and co-dominant stands and to reconstruct tree establishment history to infer historical fire regimes. Materials and Methods: Six altitudinal transects were established along the reserve, each one at elevations from 2400 to 3300 m, with sampling sites at every 150 m of elevation. In each site, increment cores were collected from the base of 25 mature trees. A total of 800 increment cores were collected and cross-dated. Results: P. pseudostrobus is dominant in stands between 2400 and 2850 m, A. religiosa between 3150 and 3300 m, and both species co-dominate between 2850 and 3150 m. The establishment pattern for both species has been continuous, represented by uneven-aged structures, suggesting that tree establishment in smooth-bark Mexican pine forests, mixed-conifer forests, and sacred fir forests, is likely to be associated with frequent, low-severity, and low-intensity fires. Conclusions: These fire regimes suggest, by the one hand, the disruption of natural fire regimes by human activities, limiting the occurrence of high-severity fires; on the other hand, a distinctive feature of these tropical montane forests.

El fuego es un agente de disturbio para los bosques, cuyos efectos varían de acuerdo con elrégimen natural de fuegos, bajo el cual las especies que ahí habitan evolucionaron. El objetivo deeste documento es exponer un panorama sobre el papel del fuego en la regeneración de losbosques de coníferas. Primeramente, se hace una revisión sobre el régimen de fuegos y susefectos sobre la regeneración arbórea. Posteriormente, se describen los principales métodos dereconstrucción de los regímenes del fuego y se exploran los regímenes de bosques de coníferastropicales montanos. Por último, se describe el posible efecto del aprovechamiento maderablesobre los regímenes del fuego.Se ha planteado que esta actividad es, actualmente, el agente dedisturbio que ejerce mayor impacto sobre los regímenes de fuego a nivel mundial. En esteartículo de revisión proponemos que el entendimiento del papel del fuego sobre la regeneraciónde los bosques de coníferas es fundamental para prevenir fuegos catastróficos y que el empleo dedicho elemento como herramienta de manejo forestal puede contribuir a la conservación de laintegridad de estos bosques.

Avocado cultivation has reduced the extent of forest ecosystems in central Mexico, even in natural protected areas such as the Monarch Butterfly Biosphere Reserve (MBBR) where information on the extent and expansion dynamics of avocado cover is scant. This study aimed to identify avocado plantations within the MBBR through photo interpretation for the 2006–2018 period. Change rates of the avocado cover extent were calculated for the northern, central, and southern zones of the MBBR, and topographic attributes such as elevation, soil type, slope, and slope aspect were identified. A total extent of 958 ha is covered by avocado plantations within the MBBR. The southern zone hosted the largest area under avocado cultivation (570 ha), but the northern zone had the highest change rate between 2006 and 2018 (422%). Most avocado orchards have been established mainly in Acrisol soils, south-facing slopes, on steep hillsides, and in elevations between 2050 and 2800 m. The conversion from traditional agricultural lands has been the main mechanism for the establishment of avocado orchards. However, 40 ha under avocado cultivation derived from deforestation, mainly in the central zone. The expansion of avocado plantations could trigger environmental impacts, even threatening the overwintering habitat and the migratory phenomenon of the monarch butterflies.

Introduction: Ecological niche modeling is a fundamental tool to know the areas suitable for the establishment of silvopastoral systems, based on the environmental requirements of the species. Objectives: To model the ecological niche of the forage species Chloris gayana Kunth and Eragrostis curvula (Schrad.) Nees, and the conifers Pinus devoniana Lindley, Pinus lawsonii Roezl ex Gordon and Pinus montezumae Lambert and to identify potential areas for silvopastoral systems. Materials and methods: The ecological niche was modeled based on data from georeferenced collection sites in Mexico and 23 environmental variables. Subsequently, areas were identified and the extent, where forage species converge with conifers in both agricultural and forest areas, was estimated. Results and discussion: Potential areas for silvopastoral systems with the five species were found mainly in the Neovolcanic Axis and the Sierra Madre del Sur, with a greater area suitable for agricultural areas compared to forestry areas. Pinus devoniana had the largest potential area to be intercropped with C. gayana and E. curvula. The ecological niche models had high predictive capacity with an area under the curve values greater than 0.93. Altitude and annual temperature range were the most important variables. Conclusions: Mexico has the potential to establish silvopastoral systems with the species understudy in the center and south of the country, which could contribute to decreasing soil degradation, provide forage for livestock and maintain tree cover for agricultural and forest areas.