- Lentile et aI. Burn severity and- post-fire vegetation response 1 Post-fire Burn Severity and Vegetation Response Following Eight Large Wildfires Across 2 the Western US 3 4 Submitted to the Journal ofFire Ecology, in review 5 6 Leigh B. Lentilei*, Penelope Morgani, Andrew T. Hudak2, Michael J. Bobbitti, Sarah A. Lewis2. 7 Alistair M.S. Smithi, and Peter R. Robichaud2 8 9 iUniversity ofIdaho, Department ofForest Resources, Moscow, ID 83844-1133 10 2US Department ofAgriculture, Forest Service, Rocky Mountain Research Station, Moscow, ID 11 83843 12 13 ABSTRACT 14 Vegetation response and burn severity were examined following eight large wildfires that 15 burned in 2003 and 2004: two wildfires in California chaparral, two each in dry and moist 16 mixed-conifer forests in Montana, and two in boreal forests in interior Alaska. Our research 17 objectives were 1) to characterize one-year post-fire vegetation species richness and percent 18 canopy cover, and 2) to use remotely-sensed measures ofburn severity to describe landscape­ 19 level fire effects. We correlated one-year post-fire plant species richness and percent canopy 20 cover to bum severity and to soil surface cover immediately after the fires. For all eight 21 wildfires, plant canopy cover and species richness were low and highly variable one year post­ 22 burn. We found a greater number offorbs when compared to other plant life forms, independent 23 ofbum severity. Plant cover was dominated by grasses in chaparral systems, by forbs in mixed­ 24 conifer forests, and by shrubs in boreal forests. Variation among sites, fine-scale variability in 25 post-fire effects on soils, and diversity ofprefire vegetation likely explain the high variation 26 observed in post-fire vegetation responses across sites and burn severities. On most low and 27 moderate bum severity sites, >30% ofthe soil surface was covered with organic material 28 immediately post-fire, and one year later, the canopy cover ofunderstory vegetation averaged 29 10% or more, suggesting low risk to post-fire erosion. In CA, MT-NW and MT-W, 5% or less 30 burned with high severity, while in AK, 58% was mapped as high burn severity. All fires had a 31 mosaic ofdifferent bum severities (as indicated by delta Normalized Burn Ratio, dNBR) with 32 highly variable patch size (mean 1.3 to 14.4 ha, range from <1 to over 100,000 ha). 33 34 KEYWORDS: Fire effects, delta Normalized Bum Ratio, dNBR, species richness, species 35 diversity 36 37 *Corresponding author contact information: [email protected](email); 931-598-3219 38 (phone); 931-598-3331 (fax) 39 40 INTRODUCTION 41 Ecologists have long recognized the enormous variability in fire effects and vegetation 42 response that results from large wildfires. Burn severity (Lentile et al. 2006, Jain et al. 2004, 43 Ryan and Noste 1985) classifications are used to infer fire effects on soil and vegetation, 44 potential successional trajectories, and rates ofecosystem recovery. Because large fires are 45 heterogeneous in their effects across the landscape, such events provide ideal opportunities for 1 Lentile et al. Bum severity andpost-fire vegetation response 1 characterizing initial fire effects and vegetation recovery across the range ofbum severities. It is 2 the inherent variation following disturbance that challenges fire ecologists to identify unifying 3 trends in burn severity and post-fire soil and vegetation response, without losing the fine-scale 4 and localized information that land managers need to support post-fire decisions. 5 This study was part ofa rapid response research (Lentile et al. in press) project designed to 6 address how fire effects on soil and vegetation differ with burn severity, and to identify measures 7 ofimmediate post-fire effects that indicate the degree offire effects on soil and the vegetation 8 response over time. Our research team sampled eight large wildfires, all selected for extended 9 burning over several days to weeks across a diversity ofvegetation, soils, and topographic 10 conditions and where satellite images were available for the burned landscape. With the help of 11 the fire incident management teams, we identified locations where we could safely establish 12 research plots to sample within a few weeks post-fire (also see Hudak et al. this issue, Lewis et 13 al. this issue), and we sampled the same sites again one year later. 14 The specific objectives ofthe research reported here were 1) to characterize one year post- 15 fire vegetation recovery, and 2) to use remotely-sensed measures ofbum severity to describe 16 landscape-level fire effects. We sought to characterize understory plant vegetation response in 17 areas ofdifferent bum severity and to determine which immediate post-fire effects on the soil 18 surface serve as good indicators ofone year post-fire soil and vegetation response (and thus, 19 ultimately longer-term post-fire ecosystem recovery). Many fire ecology studies must rely on 20 retrospective reconstruction ofimmediate post-fire condition. This study is unique for examining 21 vegetation response relative to bum severity using consistent methods across eight large 22 wildfires. 23 24 BACKGROUND 25 Burn severity influences injury and mortality ofplants and the rate ofreestablishment of 26 resprouting species (Lyon and Stickney 1976; Ryan and Noste 1985; Morgan and 27 Neuenschwander 1988; DeBano et al. 1998). Whether the removal of some vegetation and 28 altered soil surface conditions is favorable to vegetation depends upon the characteristics ofthe 29 plant species on the site, their susceptibility to fire, and the means by which they recover after 30 fire (Mutch 1970; Lyon and Stickney 1976; Anderson and Romme 1991; Turner et al. 1998). 31 Alterations to light and nutrient regimes following fires may have major implications for 32 understory plant and seedling recovery in burned stands. We expected greater reductions in 33 understory plant species richness and cover in areas burned more severely, and resprouting to 34 dominate one-year vegetation response. 35 Plant regeneration may occur from on-site seeds, from off-site seed sources, or from 36 resprouting from deeply buried root and stem structures (Lyon and Stickney 1976). Seed 37 production, and therefore, understory plant community composition and abundance are 38 temporally and spatially variable, and likely influenced by site conditions, the pre-fire plant 39 community, and the post-fire climate (Whelan 1995). Relative to high bum severity, we expected 40 quicker recovery ofplant cover in low and moderately burned areas due to on-site sources for 41 plant regrowth (sprouts) and establishment (seeds) after fire. Post-fire forest floor conditions are 42 an important determinant ofpost-fire vegetation recovery, as this determines the amount ofbare 43 soil. Litter accumulation may be higher in areas ofgreatest crown scorch, but lowest where 44 needles were consumed. Scorched needles help to reduce post-fire erosion rates when they 45 blanket the soil surface (Pannkuk and Robichaud 2003). Similarly, the recovery ofvegetation is 2 - Lentile et al. Burn severity and- post-fire vegetation response 1 likely to differ by plant functional groups (e.g., moss, grass, forb, or shrubs) (Rowe and Scotter 2 1973). Lower plant species richness was correlated with significant duff consumption in recent 3 wildfires in ponderosa pine forests (Laughlin et al. 2004). Thus, we expected plant species 4 richness and abundance to be lowest where litter and duff were consumed. 5 Identification ofindicators ofburn severity, and thus potential ecosystem recovery, could 6 prove useful to post-fire planners tasked with strategically rehabilitating areas likely to recover 7 slowly or in undesirable ways. Remotely-sensed data provide one means by which managers can 8 quickly and consistently evaluate burned areas and identify areas in need ofrehabilitation 9 treatment to prevent erosion and weed establishment (Lentile et al. 2006). In general, remotely 10 sensed data more accurately depict post-fire changes in overstory tree canopy than understory or 11 forest floor changes (Hudak et al. this issue). 12 13 METHODS 14 Study Sites 15 We sampled two wildfires in each of four different geographic areas. For a map of site 16 locations, see Hudak et al. (this issue). 17 Our California (CA) sites were dominated by chaparral vegetation and included the Simi (340 18 16' 56" N, 118 0 49' 56" W, centroid; elevation 46 to 1118 m) and Old fires (340 11' 37" N, 1170 19 15' 17" W, centroid; elevation 396 to 2030 m) that burned in southern California during the fall 20 of2003. The Simi Fire began on 25 October 2003 and burned 43,800 ha in Ventura and Los 21 Angeles Counties, while the Old Fire began on 28 October 2003 and burned 23,300 ha north of 22 San Bernadino, California. The Simi Fire burned in a mix ofvegetation types including 23 chaparral, coastal sage scrub, and annual grasslands across a diversity oftopographic conditions 24 including rolling hills and very steep, rocky terrain, where annual precipitation is variable, but 25 generally less than 50 cm. The Old Fire burned in chaparral mixed with interior woodlands, also 26 on rough terrain. 27 Chaparral is a shrubby, sclerophyllous vegetation type that is common in middle elevations 28 throughout much of California (Barro and Conard 1991). Chaparral vegetation is highly adapted 29 to stand-replacing fires that were historically common in this ecosystem (Hanes 1977; Keeley 30 2006b). Common chaparral trees and shrub genera include Adenostoma, Arctostaphylos, 31 Ceanothus, Cercocarpus, Prunus, Quercus, and Rhamnus. Ground cover is relatively sparse 32 when shrubs are dominant, and forb (e.g., Phacelia, Penstomen, Mimulus species) and grass 33 species are common in these ecosystems following fire. The presence ofnative forbs and grasses 34 tend to be ephemeral «2 years), although non-native post-fire invaders, including Bromus 35 diandrus, Bromus tectorum, Centaurea solstitialis, Erodium species, and Trifolium hirtum, may 36 persist longer (Keeley 2006b).