Californian mixed‐conifer forests under unmanaged fire regimes in the Sierra San Pedro Mártir, Baja California, Mexico

Abstract

Aim This study appraises historical fire regimes for Californian mixed‐conifer forests of the Sierra San Pedro Mártir (SSPM). The SSPM represents the last remaining mixed‐conifer forest along the Pacific coast still subject to uncontrolled, periodic ground fire. Location The SSPM is a north–south trending fault bound range, centred on 31°N latitude, 100 km SE of Ensenada, Baja California. Methods We surveyed forests for composition, population structure, and historical dynamics both spatially and temporally over the past 65 years using repeat aerial photographs and ground sampling. Fire perimeter history was reconstructed based on time‐series aerial photographs dating from 1942 to 1991 and interpretable back to 1925. A total of 256 1‐ha sites randomly selected from aerial photographs were examined along a chronosequence for density and cover of canopy trees, density of snags and downed logs, and cover of non‐conifer trees and shrubs. Twenty‐four stands were sampled on‐the‐ground by a point‐centred quarter method which yielded data on tree density, basal area, frequency, importance value, and shrub and herb cover. Results Forests experience moderately intense understory fires that range in size to 6400 ha, as well as numerous smaller, low intensity burns with low cumulative spatial extent. SSPM forests average 25–45% cover and 65–145 trees per ha. Sapling densities were two to three times that of overstory trees. Size‐age distributions of trees ≥ 4 cm dbh indicate multi‐age stands with steady‐state dynamics. Stands are similar to Californian mixed conifer forests prior to the imposition of fire suppression policy. Livestock grazing does not appear to be suppressing conifer regeneration. Main conclusions Our spatially‐based reconstruction shows the open forest structure in SSPM to be a product of infrequent, intense surface fires with fire rotation periods of 52 years, rather than frequent, low intensity fires at intervals of 4–20 years proposed from California fire‐scar dendrochronology (FSD) studies. Ground fires in SSPM were intense enough to kill pole‐size trees and a significant number of overstory trees. We attribute long fire intervals to the gradual build‐up of subcontinuous shrub cover, conifer recruitment and litter accumulation. Differences from photo interpretation and FSD estimates are due to assumptions made with respect to site‐based (point) sampling of fire, and nonfractal fire intensities along fire size frequency distributions. Fire return intervals determined by FSD give undue importance to local burns which collectively use up little fuel, cover little area, and have little demographic impact on forests.

Post-fire epicormic branching in Sierra Nevada Abies concolor (white fir)

“…Ponderosa pine and mixed-conifer forests in the Sierra Nevada, however, are believed to have had a frequent low-severity fire regime, though patches of higher-severity fire may have been important for shaping forest gap patterns and facilitating regeneration of some species such as giant sequoia. Historically, high-severity fire would have occurred in Sierra Nevada mixed-conifer forests particularly during extreme weather events; however, how often and extensive these fires were is still being debated (Minnich et al 2000;Stephens et al 2003). Traumatic reiteration in conifers following fire is rare and we are only aware of two conifer species in which it has been previously documented, Pinus rigida (Stone and Stone 1943) and Sequoiadendron giganteum (Stephens and Finney 2002).…”

Vegetation patch dynamics and tree diversity in a diverse conifer and oak forest in central Mexico

“…Conifer and oak forests were largely converted to agriculture since the beginning of Spanish colonization (1521) on the plateaus and montane valleys of central Mexico (Challenger, 2003). Where such forests are still dominant, their structure, composition and area remain affected by natural and human disturbances, particularly cattle grazing, and forest management (Minnich et al, 2000;Mas et al, 2004, Calderon-Aguilera et al, 2012. In this study we analyze patch dynamics in a conifer and oak forest in Aguascalientes State in Central Mexico, and evaluate the implications for these dynamics in terms of tree diversity preservation.…”

Aspen mortality in the Colorado and southern Wyoming Rocky Mountains: Extent, severity, and causal factors

“…Following a disturbance event (e.g., a stand-replacing fire), aspen colonize the area, either by seed or through sprouting from existing roots (Mueggler, 1985). Numerous studies have established that changes in fire regimes from the early twentieth century onward have clearly altered forest succession (Beaty, et al, 2008;Kaye, et al, 2003;Kurzel, et al, 2007), and favor coniferdominated landscapes, especially among current aspen/mixed conifer stands (Smith and Smith, 2005;Minnich, et al, 2000).…”