Modification of Fire Regimes Inferred from the Age Structure of Two Conifer Species in a Tropical Montane Forest, Mexico

Abstract

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.

Reciprocal Common Garden Altitudinal Transplants Reveal Potential Negative Impacts of Climate Change on Abies religiosa Populations in the Monarch Butterfly Biosphere Reserve Overwintering Sites

“…The altitudes of the MBBR range from 2200 to 3640 m a.s.l. and the annual average temperatures range from 8 to 22 • C [37]. The vegetation is mainly comprised by forests of fir (A. religiosa), pine (Pinus spp.…”

Effect of Fire Severity on the Species Diversity and Structure of a Temperate Forest in Northern Mexico

“…The above is due to the fact that severe and infrequent fires trigger the replacement of the stand; later, suitable weather conditions favor a synchronous or pulsed establishment of trees with a uniform age structure [45]. In contrast, uneven-aged forests display a pattern of continuous regeneration that may be associated with frequent and slight disturbances such as shallow or low-intensity fires [46,47].…”

Avocado Cover Expansion in the Monarch Butterfly Biosphere Reserve, Central Mexico

“…Mean annual temperature ranges, according to the zone, between 8 and 22 • C, and mean annual precipitation between 700 and 1250 mm, with rains from July to October [27]. Conifer forests are the most representative vegetation type and are dominated by sacred fir (Abies religiosa Schlecht et Cham) forests in elevations between 3150 and 3300 m, mixed-conifer forests co-dominated by A. religiosa and Pinus pseudostrobus Lindley (smooth-bark Mexican pine) between 2850 and 3150 m, and P. pseudostrobus forests below 2850 m [28]. Other vegetation types with less extent are oak forests, tropical montane cloud forests, and montane grasslands [29].…”