Forest Ecology and Management 441 (2019) 20–31 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco Status and trends of fire activity in southern California yellow pine and T mixed conifer forests ⁎ Katherine Nigroa,b, , Nicole Molinaric a University of California Santa Barbara, Santa Barbara, CA 93106, United States b Colorado State University, Forest and Rangeland Stewardship, 200 W. Lake St, 1472 Campus Delivery, Fort Collins, CO 80523-1472, United States c USDA Forest Service, Pacific Southwest Region, Los Padres National Forest, 6750 Navigator Way, Suite 150, Goleta, CA 93117, UnitedStates ARTICLE INFO ABSTRACT Keywords: Frequent, low to moderate severity fire in mixed conifer and yellow pine forests of California played anintegral Southern California role in maintaining these ecosystems historically. Fire suppression starting in the early 20th century has led to Fire return interval altered fire regimes that affect forest composition, structure and risk of vegetation type conversion following Burn severity disturbance. Several studies have found evidence of increasingly large proportions and patches of high severity Fire size fire in fire-deprived conifer forests of northern California, but few studies have investigated the impactsoffire Natural range of variation suppression on the isolated forests of southern California. In this study, spatial data were used to compare the Yellow pine Mixed conifer current fire return interval (FRI) in yellow pine and mixed conifer forests of southern California tohistorical conditions. Remotely sensed burn severity and fire perimeter data were analyzed to assess changes inburn severity and fire size patterns over the last 32–100 years. Half of the yellow pine and mixed conifer forestinthis study has missed multiple burn opportunities and has not experienced fire in the 109-year fire record. The average proportion of conifer forests burned in high severity fire (> 90% tree basal area loss), average high severity patch size, and maximum high severity patch size all increased significantly from 1984 to 2016. The average fire proportion burned at high severity from 2000 to 2016 is 1.5 times higher than predicted forthe natural range of variation (NRV). Additionally, the years after 2000 had high severity patches larger than 25 ha, whereas no fires before 2000 had patches this large, indicating a deviation from NRV since the turnof century. Fire size in conifer forests significantly increased from 1910 to 2016, owed to a substantial increase in the occurrence of large fires (larger than the natural range of variation) after 2000. This analysis indicates that southern California conifer forests are like their northern counterparts in that they have burned very infre- quently since the early 1900s, resulting in large and homogenous areas of stand replacing burns. This is likely exacerbated even further by recent fire-conducive weather conditions and extended periods of drought. In southern California, recovery from large, high severity burns is likely to be impeded by the small and disparate nature of mixed conifer forests and limited seed dispersal capabilities of remaining trees. Therefore, preemptive forest treatments and careful fire management is needed to return natural structure and function to these forests. 1. Introduction per year) (Stephens et al., 2007). The frequency and severity with which these fires burned helped shape the numerous vegetation types Fire has played an integral part in defining and shaping vegetation that exist in California today. However, effective national fire sup- patterns and composition in California for millions of years, both via pression practices beginning in the early 20th century have severely natural, and more recently, human-caused ignitions (van Wagtendonk reduced the annual area burned in California, which averaged only et al., 2018). Before Native American colonization, lightning was the 102,000 ha per year from 1950 to 1999 (Stephens et al., 2007; Safford main source of ignition for fire (Minnich, 1988) and fire frequencies in and Stevens, 2017). some parts of the state increased following Native American occupation The fire regime in mixed conifer and yellow pine forests hashis- (Anderson, 2006). Millions of hectares of vegetation burned annually in torically been defined by one of the most frequent fire return intervals California, consuming 4.5–12% of the state’s area (1.8–4.8 million ha (FRI) of any vegetation type in the state (Van de Water and Safford, ⁎ Corresponding author at: Colorado State University, Forest and Rangeland Stewardship, 200 W. Lake St, 1472 Campus Delivery, Fort Collins, CO 80523-1472, United States. E-mail address: [email protected] (K. Nigro). https://doi.org/10.1016/j.foreco.2019.01.020 Received 11 September 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 21 March 2019 0378-1127/ © 2019 Elsevier B.V. All rights reserved. K. Nigro and N. Molinari Forest Ecology and Management 441 (2019) 20–31 2011), partially due to the high concentration of lightning strikes where scrub, desert scrub) (Minnich and Everett, 2001). These conifer forests these forests exist (Safford and Stevens, 2017). An extensive review of exist at only the highest elevations of the Transverse and Peninsular the literature on fire regime prior to 1850 revealed that yellow pine and Ranges above the chaparral belt and often occur in disparate patches mixed conifer forests burned an average of every 11–16 years in Cali- throughout the region (Minnich and Everett, 2001). Therefore, conifer fornia (Van de Water and Safford, 2011). Analog systems that have forests are a unique resource for recreation and biodiversity in southern experienced little fire suppression, such as the Sierra de San Pedro California (Stephenson and Calcarone, 1999; Minnich, 2007). To better Mártir (SSPM) (pre-1970s) in Baja California, Mexico and parts of Yo- understand the susceptibility and resilience of these systems to dis- semite and Sequoia and Kings Canyon national parks managed with a turbance, we set out to characterize the status and trends for fire ac- policy of wildland fire use, have maintained recent fire frequencies si- tivity across southern California conifer forests. milar to historical estimates (Caprio and Graber, 2000; Stephens et al., In this study, we specifically aim to (1) characterize the current FRI 2003; Collins, 2007). in conifer forests across southern California and compare it to pre-Euro- Most mixed conifer and yellow pine forests in California have been American settlement estimates of fire frequency, (2) analyze changes in managed under a strict policy of fire suppression for the last century burn severity patterns in southern California conifer forests over the last and, as a result, 75% of these forests have not experienced fire since 32 years, with respect to proportion of area burned at high severity and 1908 (Steel et al., 2015). The effects of this altered fire frequency have high severity patch size, and (3) evaluate trends in fire size since the been well studied in the Sierra Nevada of northern California, where early 1900s. longer intervals between fires are contributing to stand densification and increased fire severity. In the beginning of the 20th century, before 2. Material and methods significant fire suppression began, mixed conifer forests in the Sierra Nevada were dominated by trees that were large (average DBH > 2.1. Study area 90 cm), old (250–350 years on average), and sparsely distributed (McKelvey and Johnston, 1992; Collins et al., 2011; Knapp et al., 2013; Our analysis focuses on the four southern California national forests Safford and Stevens, 2017). However, resampling efforts in the last (Cleveland (CNF), San Bernardino (SBNF), Angeles (ANF), and Los 35 years have revealed that tree density and canopy cover have in- Padres (LPNF)) (Fig. 1), which contain 70% of the conifer forest in creased significantly, owing to a substantial increase in the number of southern California (defined as Monterey County, and all counties south small-diameter trees in these forests (McKelvey and Johnston, 1992; and inclusive of San Luis Obispo and Kern). This analysis was restricted Collins et al., 2011; Knapp et al., 2013). While the loss of large diameter to U.S. Forest Service land as it contains most of the conifer forest in trees in the Sierra Nevada mixed conifer forests has been attributed to southern California and this ensured that land management practices past logging practices, the increase in density of smaller diameter trees were similar across the study area. In addition, the spatial data required is most likely due to fire suppression (Knapp et al., 2013). Even Baja for the analyses (described below) were generated using the same California’s recent 30 years of fire suppression are starting to reveal methods across these lands. The LPNF was split into two assessment signs of densification, which were measured as a 237% increase in areas – the Monterey District (LPN) in the north, which extends through seedling density, a 120% increase in live tree density, and a 13% in- Big Sur in Monterey County, and the lower Los Padres (LPS), which is crease in tree basal area since 1998 (Dunbar-Irwin and Safford, 2016). mostly contained in San Luis Obispo, Santa Barbara, and Ventura Stand densification and increased ladder fuels (small and inter- counties. Since these are geographically and environmentally distinct