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

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 post1800 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. Résumé : Les forêts de conifères du Nord-Ouest du Mexique sont uniques dans l’Ouest de l’Amérique du Nord n’ayant pas été exploitées ni protégées contre les feux. Le régime des feux dans les forêts mixtes résineuses de Pinus jeffreyi Grev. & Balf. situées dans la Sierra San Pedro Martir, dans la Basse-Californie au Mexique, a été établi en déterminant les dates de 105 feux à partir de 1034 cicatrices de brûlure présentes sur 105 spécimens. Des feux ont été identifiés de 1521 à 1980 et l’intervalle médian entre les feux était inférieur à 15 ans à toutes les échelles de composition. Des différences significatives dans les intervalles moyens entre les feux ont été observées de 1700 à 1800, de 1800 à 1900 et de 1900 à 1997, le plus souvent à une échelle de composition spatiale intermédiaire, et la proportion des arbres avec des cicatrices de brûlure au cours des années 1700 était significativement différente de celles des années 1800, 1900 ou de l’ensemble de la période postérieure à 1800. Une analyse des époques superposées indique que des feux de moyenne et grande envergure spatiale sont survenus lors d’années particulièrement sèches pendant toute la période étudiée. Cependant, avant 1800 ces feux sont survenus après une année marquée par une précipitation significativement plus importante. La prépondérance des feux qui affectent les jeunes forêts dans la Sierra San Pedro Martir est comparable à la saisonnalité observée dans le Sud-Ouest des États-Unis et différente du régime qu’on observe sur les versants ouest de la Sierra Nevada et des Monts Klamath en Californie. [Traduit par la Rédaction] 1101 Stephens et al. Introduction To describe reference conditions, managers increasingly rely on photographic evidence, written historical accounts, and studies that reconstruct pre-fire-suppression conditions primarily from living and dead woody material (Fritts and Swetnam 1989; Swetnam et al. 1999). In doing so, it is often assumed that prehistorical conditions would prevail today in the absence of fire suppression. However, the interaction of climatic variation and fire is complex (Swetnam 1993; Swetnam and Betancourt 1992, 1998; Grissino-Mayer and Swetnam 2000; Veblen et al. 2000; Flannigan et al. 2000), making it uncertain if similar prehistorical conditions would have occurred. There has been considerable variation in climate over the last centuries (Swetnam and Betancourt 1998; Millar and Woolfenden 1999) that may have led to important changes in fire regimes (fire frequency, season, extent, etc.) and resulting forest conditions in the absence of fire suppression (Swetnam 1993; Stine 1996; Chang 1999). As a result, after nearly a century of fire suppression, there are few, if any, forests in western North America that could serve as models or “controls” for forests functioning under the continuing influence of climate variation and frequent fires (Skinner and Chang 1996; Minnich et al. 2000). Conifer forests in isolated ranges of Mexico have not experienced systematic fire suppression or widespread logging (Minnich 1987; Fule and Covington 1999; Minnich et al. Can. J. For. Res. 33: 1090–1101 (2003) doi: 10.1139/X03-031 © 2003 NRC Canada 1090 Received 20 May 2002. Accepted 14 January 2003. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 13 May 2003. S.L. Stephens.1 Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720-3114, U.S.A. C.N. Skinner. United States Forest Service, Pacific Southwest Research Station, Redding, CA 96001, U.S.A. S.J. Gill. Natural Resources Management and Bioresource and Agricultural Engineering Departments, California Polytechnic State University, San Luis Obispo, CA 93407, U.S.A. 1Corresponding author (e-mail: [email protected]). I:\cjfr\cjfr3306\X03-031.vp May 12, 2003 4:59:56 PM Color profile: Generic CMYK printer profile Composite Default screen