Dendrochronology-based fire history of Jeffrey pine - mixed conifer forests in the Sierra San Pedro Martir, Mexico

Abstract

Conifer forests in northwestern Mexico have not experienced systematic fire suppression or logging, making them unique in western North America. Fire regimes of Pinus jeffreyi Grev. & Balf. mixed conifer forests in the Sierra San Pedro Martir, Baja California, Mexico, were determined by identifying 105 fire dates from 1034 fire scars in 105 specimens. Fires were recorded between 1521 and 1980 and median fire return intervals were less than 15 years at all compositing scales. Significant differences in mean fire return intervals were detected from 1700 to 1800, 1800 to 1900, and 1900 to 1997, most often at intermediate spatial compositing scales, and the proportion of trees scarred in the fires of the 1700s was significantly different from the fires of either the 1800s, the 1900s, or the combined post-1800 period. Superposed epoch analysis determined that moderate and large spatial scale fires occurred on significantly dry years during the length of the record, but before 1800, these fires were preceded by significantly higher precipitation 1 year before the fire. The dominance of earlywood fires in the Sierra San Pedro Martir is similar to the seasonality found in the southwest United States and is different from the western slope of the Sierra Nevada and Klamath Mountains of California.

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).…”

Socioecological transitions trigger fire regime shifts and modulate fire–climate interactions in the Sierra Nevada, USA, 1600–2015 CE

“…Thus, Sierra Nevada changes in the SES are implicated as the trigger for the 1866 fire-regime shift to lower fire activity via fuel fragmentation caused by livestock grazing. Livestock grazing has been implicated in similar fire-regime shifts in other montane pine forests in the American West (39), Mexico (40), and Mongolia (41). In contrast, the effect of logging activity on fuels or ignitions in lower montane forests during this period was spatially limited to population centers and transportation corridors (38).…”

“…In Colorado, fire activity did not change until the mid-to late 19th century with widespread Euro-American settlement (22), and it increased rather than decreased, as in the Sierra Nevada. In the early to mid-17th century, fire extent increased when Spanish missions were established in the American Southwest (52) and in Northwestern Mexico in the late 18th century when missions were abandoned (40). As in the Sierra Nevada, reduced Native American burning and increased fuels are implicated in the fireregime shifts.…”

Topography affected landscape fire history patterns in southern Arizona, USA

“…Such regional drought/fire years often corresponded with La Niñ a events that reduced winter precipitation (Swetnam and Betancourt, 1990) and typically followed wet 'El Niñ o' years (Baisan and Swetnam, 1990;Westerling et al, 2003;Crimmins and Comrie, 2004). Periods of reduced fire frequencies occurred during some decades (e.g., early 1800s) and in some areas (e.g., Stephens et al, 2003;Brown and Wu, 2005), due possibly to reduced El Niñ o and La Niñ a amplitudes (Swetnam and Betancourt, 1998;Kitzberger et al, 2001).…”

“…Sample depth differences between RC and BP were addressed using the SSIZ program. SSIZ is a re-sampling (bootstrap) program used to estimate confidence intervals of mean fire frequencies at different sample sizes (Holmes, 1995;Swetnam and Baisan, 2003;Stephens et al, 2003). Relative area burned for each widespread fire year was estimated based on the proportion of recording plots scarred.…”