Linking old‐growth forest composition, structure, fire history, climate and land‐use in the mountains of northern México

Abstract

Old‐growth forests are biologically and ecologically valuable systems that are disappearing worldwide at a rapid rate. México still holds large areas covered by temperate forests in the mountains of the Sierra Madre Occidental, but few of these retain old‐growth characteristics. We studied four sites with remnant old‐growth forests in Mesa de las Guacamayas, a site in the Sierra Madre Occidental in northwestern Chihuahua, to assess their composition, structure, and age characteristics. Overstory tree densities and basal areas at each site were based on measurements of all trees >1.3 m tall. The overstory was dominated by large Pinus durangensis, P. strobiformis, and Pseudotsuga menziesii (270–335 trees ha−1, basal area 24–42 m2 ha−1), with a subcanopy formed mostly of oaks. This species composition, combined with the lack of vertical structural development, and thus of fuel ladders, suggests that these forests are relatively resistant to severe wildfire. We evaluated forest attributes in the context of local fire regimes and regional climatic patterns, and found that frequent disturbance by surface fires has been part of the study sites' histories for at least 250 years. While climate was a driver of fire regimes historically in this mountain range, humans appear to have played a role in interruptions of the fire regime in the second half of the 20th century. Age distributions showed recruitment to the canopy over ∼250 years, while fires in the four sites recurred every 6–12 years. Temporary interruption of the fire regime in the mid‐20th century at three sites was associated with increased tree establishment, especially by broadleaved species. One site had an uninterrupted fire regime and showed continuous tree establishment, consistent with the self‐reinforcing role of frequent fire in regulating live and dead fuel loads. Remnant old‐growth forests such as those we sampled are becoming increasingly rare in the Sierra Madre Occidental. The biodiversity and ecological processes that they support are highly threatened and their conservation must be made a priority in the U.S.‐México borderlands.

Climatic influence on fire regime (1700 to 2008) in the Nazas watershed, Durango, Mexico

“…This has increased the risk and susceptibility of these forests to maintain forest dynamics, hydrological functions, forest productivity, and other ecosystem services. This change in fire frequency is not exclusive to the study area; studies in other locations of the Sierra Madre Occidental, in the states of Durango and Chihuahua, have reported similar changes in fire frequency since the 1950s Covington 1997, 1999;Heyerdahl and Alvarado 2003;Cortes-Montaño et al 2012). In other sites, the greater amplitude in fire frequency intervals has resulted in an increase in quantity and continuity of fuel loads (White and Vankat 1993;Skinner et al 2008).…”

Fire history and moisture influences on historical forest age structure in the sky islands of southern Arizona,USA

“…This highlights the significance of fire as a thinning agent and the important relationship between fire‐free periods and age peaks. Similar increases in pine establishment during periods of decreased fire activity have also been reported in Mexican pine forests with frequent fire regimes (Fulé & Covington, ; Cortez‐Montaño et al ., ).…”

Characterizing Spatial Neighborhoods of Refugia Following Large Fires in Northern New Mexico USA

“…Additionally, surveys of old growth forest structure [88] and species requiring specialized habitats protected from severe fire (e.g., Jemez Mountain salamander [89]) can also be taken as indicators of refugia on disturbed or undisturbed landscapes. We concur with recommended conservation measures considering the potential impacts of climate change that include maintenance of the greatest possible number of local refugia as habitat networks [25].…”