Paper presented at 2nd Interface Between Ecology and Land Development in California conference, 18-19 April 1997, Occidental College, Los Angeles, CA; Jon E. Keeley, coordinator. Season of Burn Effects in Southern California Chaparral Jan L. Beyers and Carla D. Wakeman Pacific Southwest Research Station, USDA Forest Service, Riverside, CA 92507 Tel. (909) 680-1527; Fax (909) 680-1501 E-mail: [email protected] Abstract. Prescribed burning for fuel reduction is most safely implemented during the wet season or early summer in southern California chaparral. Previous studies suggest that wet season burns result in poor germination of fire-dependent species and that growing season burns may reduce sprouting vigor of shrubs. We examined vegetation recovery after March, June, and November prescribed fires at different sites, and after spring and fall wildfires in a single area. At all sites, good germination of obligate-seeding Ceanothus was observed, and herbaceous species with both heat- and charate-stimulated germination were found. Shrub sprouting percentage tended to be lower after the late spring burn than the fall burns. Fire intensity and postfire weather patterns may also affect chaparral regeneration response. Keywords: Adenostoma fasciculatum, Ceanothus, chamise, fire effects, fire-followers, fire intensity, prescribed fire Introduction Chaparral is the fire-prone, shrub-dominated vegetation type abundant in the foothills adjacent to many southern California cities. Shrubs are typically 1 to 4 m (3 to 12 ft) tall with evergreen, sclerophyllous leaves (Cooper 1922, Keeley and Keeley 1988). California’s Mediterranean-type climate is characterized by mild, wet winters and hot, dry summers (Barbour and Major 1988). By late summer and early fall chaparral shrubs have developed low tissue water content and conditions are ideal for stand-replacement fires (Green 1981). The natural fire return interval in chaparral is estimated to be 20 to 70 years (Keeley et al. 1989, Minnich 1995). Under extreme conditions of hot, dry “Santa Ana” winds, chaparral fires may rapidly burn thousands of hectares. As a result of the extensive wildland-urban interface that has developed as more people have moved into chaparral areas seeking a semi-rural lifestyle, homes may be consumed by flames as well. To protect human communities intermixed in chaparral and reduce the likelihood of very large fires, agencies such as the USDA Forest Service and the California Department of Forestry and Fire Protection use prescribed fire to convert old stands of chaparral to less flammable new growth and break up fuel continuity (Riggan et al. 1986). Prescribed burning is usually conducted in the cooler seasons, between November and June, when soil moisture and live fuel moisture are relatively high and fires are easier to control (Green 1981). The conditions in which prescribed fires burn, and the resultant fire intensity and fire effects, may be very different from those experienced by chaparral plant and animal communities Beyers, J.L. and Wakeman, C.D. -- 2 Table 1. Chaparral species with seeds that are stimulated to germinate by heat (Keeley 1991). Shrubs and Sub-shrubs Herbaceous Annuals and Perennials Ceanothus (many species) (California lilac) Apiastrum angustifolium (wild celery) Fremontodendron californicum (flannel bush) Calyptridium monandrum (pussypaws) Helianthemum scoparium (rockrose) Calystegia macrostegia (wild morning glory) Lotus scoparius (deerweed) Camissonia hirtella (suncup) Malacothamnus fasciculatus (bush mallow) Lotus salsuginosus (coastal lotus) Malosma laurina (laurel sumac) Lotus strigosus (strigose lotus) Rhus integrifolia (lemonadeberry) Lupinus excubitus Rhus ovata (sugarbush) Salvia apiana (white sage) Beyers, J.L. and Wakeman, C.D. -- 3 with a natural summer or fall fire (Parker 1990). There is concern that cool season burning may adversely affect postfire plant community recovery (Parker 1987). The literature on chaparral growth and fire response is extensive, but few studies have examined the impacts of season of burn directly. This paper will review what is known or can be hypothesized about burn season effects in California chaparral, then examine data on vegetation recovery after burns from two case studies in southern California. Literature Review In a recent review on chaparral fire effects, Borchert and Odion (1995) define “fire intensity” as the relative strength of the fire, which is usually based on observations of fire behavior. This is related to but distinct from “fireline intensity,” which is the measured rate of heat release per unit ground surface area (proportional to flame height and rate of spread) (Wells et al. 1979). The term “fire severity” has been used to describe the effects of fire on the biophysical environment, particularly soil charring and fuel consumption, and DeBano (1990) has described the general relationship between fire intensity and fire severity in chaparral. The terms “intensity” and “severity” are often used interchangeably in the chaparral literature. Chaparral wildfires typically occur during the dry seasons: summer (June - August) and fall (September - November). Summer fires may be started by lightning or by human activity. In southern California, virtually all fall fires are anthropogenic (both accidental and intentional), although historically they may have originated from smoldering of summer fires (Minnich 1987, 1989). During these seasons, soils are dry and fuel moisture is typically low (more so in fall). The combination of high daytime air temperatures and low relative humidity usually produce high fire intensity (Green 1981). Combustion of plant material is fairly complete, and upper soil layers can reach high temperatures (DeBano et al. 1979). The most intense fires tend to be fall fires started under conditions of hot, dry "Santa Ana" winds, and these fires produce the most severe fire effects in terms of soil heating and fuel and litter consumption. By late summer, most chaparral shrubs have completed annual growth and have low tissue water potentials (Oechel et al. 1981, Poole et al. 1981). Winter and spring burning conditions are quite different. Soil moisture may be high at the time of the fire. Particularly during spring, fuel moisture is fairly high and air temperature moderate, so fires generally burn with lower intensity. The effects of fire are less severe: soil heating will be minimized, and fuel consumption may not be complete. Chaparral shrubs typically begin growth after initial winter rains and are physiologically active at the time of the fire, which may affect regeneration potential (Radosevich and Conard 1980). Many species bloom in winter and spring as well (Mooney and Miller 1985). Chaparral species are well adapted to regenerate after fire. Keeley (1977) identified three major modes of postfire reproduction among chaparral shrubs: obligate seeders, in which mature plants are killed by fire and populations regenerate from seedlings that germinate the following winter or spring; sprouters, in which shrubs are top-killed by fire but resprout vigorously from underground storage organs, and seedlings are seldom found; and sprouter/seeders, in which both resprouts and seedlings are produced after fire. Reproductive modes among herbaceous plants include fire followers, which are annual and perennial herbaceous species that are abundant during the first year or two after fire and decline as shrub cover increases (Horton and Kraebel 1955, Sweeney 1956, Keeley and Keeley 1988), and opportunistic annuals, which appear most abundantly the second year after fire and are also Beyers, J.L. and Wakeman, C.D. -- 4 Table 2. Chaparral species with seeds that are stimulated to germinate by charate (leachate from charred wood) (Keeley 1991). Shrubs and Sub-shrubs Herbaceous Annuals and Perennials Adenostoma fasciculatum (chamise) Antirrhinum coulterianum (white snapdragon) Arctostaphylos spp. (manzanita) A. kelloggii (twining snapdragon) Artemisia californica (coastal sagebrush) Camissonia californica (false mustard) Fremontodendron californicum (flannel bush) Chaenactis artemisiaefolia (white pincushion) Garrya flavescens (silk tassel) Clarkia (several species) Rhus trilobata (squawbush) Cryptantha intermedia (popcorn flower) Romneya coulteri (matilija poppy) Emmenanthe penduliflora (whispering bells) Salvia mellifera (black sage) Gilia australis (southern gilia) Toxicodendron diversifolia (poison oak) Gnaphalium californicum (California everlasting) Papaver californicum (wind poppy) Phacelia (several species) Rafinesquia californica (chicory) Stephanomeria virgata (wand chicory) many others Beyers, J.L. and Wakeman, C.D. -- 5 found under canopy gaps between fires (Keeley et al. 1985, Keeley 1991). Plants in these functional groups may respond differently to changes in fire seasonality. Obligate seeders and fire followers -- species that regenerate after fire only from seed -- frequently have refractory seeds, which require either heat or leachate from charred wood (charate) to stimulate germination (Sampson 1944, Sweeney 1956, Keeley 1984, Keeley 1987, Keeley and Keeley 1987). Seeds remain dormant during the long period between fires. Although most species with refractory seeds produce some that are capable of germination without exposure to heat or charate, the most successful regeneration of these species occurs after fire (Keeley 1991). Environmental conditions during wet season and dry season burns will affect the two predominant groups of plants with refractory seeds