Forests Of Southern California Research Articles

Comparing individual and collective valuation of ecosystem service tradeoffs: A case study from montane forests in southern California, USA

Accounting for the tradeoffs and importance urban, disadvantaged communities place on ecosystem services has implications for the management of nearby forests. Although stated preference valuation approaches are often used, they are based on an individual’s perspective and rarely account for collective or societal values. Thus, alternative methods are needed to capture this dichotomy from urban communities who may not even be aware of these benefits to themselves or society at-large. We explored individual and collective importance of, and tradeoffs for, ecosystem services (ES) and ecosystem disservices (ED) by urban residents living near montane forests in greater Los Angeles, California, USA. Using an online panel survey, individual (I-rationality) versus collective (We-rationality) scenarios, best-worst scaling (BWS) choice experiments, and latent class analyses, we ranked the importance and tradeoffs among ES-ED attributes tonearby residents based on the frequency of visits to montane forests as well as Hispanic ethnicity. Results show statistically significant tradeoffs and differences in importance rankings between individual versus collective valuation scenarios. Under the individual valuation scenario, non-Hispanics highly ranked the high forest density indicator, which has implications for wildfire EDs to montane forests and communities. Gender and income were more influential sociodemographic factors affecting importance for water and recreation-related ES than was education. Our BWS and econometric methods, attributes, and importance rankings can facilitate participatory processes with diverse urban communities and designing more effective policies and management guidelines. This approach canalso more inclusively, and equitably, account for the tradeoffs and values that nearby urban communities place on ES/ED from Wildland-Urban Interface forests.

San Bernardino Flying Squirrel Use of Residential Areas: A Citizen Science Approach

The San Bernardino flying squirrel (Glaucomys oregonensis californicus) is a California Species of Special Concern restricted to montane forests of southern California. We confirmed the distribution of this species in residential areas of the San Bernardino and San Jacinto Mountains with the assistance of citizen scientist volunteers. Project participants placed motion sensor camera traps near bird feeders on their property and uploaded their results to a project webpage and associated iNaturalist project. Flying squirrels were documented at all sites monitored in the San Bernardino Mountains but were not detected in the San Jacinto Mountains, consistent with survey results in recent decades. Forest structure plots between the 2 ranges differed in tree density, tree height, litter depth, and canopy closure, but it is unclear whether these differences alone can explain the lack of detections in the San Jacinto Mountains. Habitat in residential areas may provide subsidized food and water resources that are attractive to flying squirrels and important to their persistence, especially in seasonally dry forests and areas subject to prolonged drought, such as those in the study area.

Status and trends of fire activity in southern California yellow pine and mixed conifer forests

Frequent, low to moderate severity fire in mixed conifer and yellow pine forests of California played an integral role in maintaining these ecosystems historically. Fire suppression starting in the early 20th century has led to altered fire regimes that affect forest composition, structure and risk of vegetation type conversion following disturbance. Several studies have found evidence of increasingly large proportions and patches of high severity fire in fire-deprived conifer forests of northern California, but few studies have investigated the impacts of fire suppression on the isolated forests of southern California. In this study, spatial data were used to compare the current fire return interval (FRI) in yellow pine and mixed conifer forests of southern California to historical conditions. Remotely sensed burn severity and fire perimeter data were analyzed to assess changes in burn severity and fire size patterns over the last 32–100 years. Half of the yellow pine and mixed conifer forest in this 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 for the 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 turn of the 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 infrequently 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.

Characterizing the Influence of Fire on Hydrology in Southern California

The chaparral-dominated national forests of southern California were in part established to provide water provision services to the surrounding urban populations and irrigation for agriculture. However, water provision in the form of groundwater recharge and surface runoff depends on the climatological conditions of any given year and also landscape-scale disturbances such as fire. Fire is increasing in frequency in southern California and understanding its impacts both immediately postfire and as vegetation recovers, and the interactions between fire and hydrology, are key components to managing federal lands effectively. In this study we focus on nine fires in a study area that encompasses the four southern California national forests (Los Padres, Angeles, San Bernardino, and Cleveland) and use a water balance model to investigate the effects of water provision services post-fire at a regional scale. We found that runoff and recharge increased post-fire, with increases in recharge being greater with recovery times ranging from 2 to 4 y post-fire. Vegetation recovery occurred 2 y post-fire for all basins as indicated by remotely sensed imagery measuring vegetation greenness having returned to or exceeded pre-fire values for the basin. We found that runoff and recharge were more sensitive to the effects of climate than to length of time post-fire. Findings from these modeling tools allow users to anticipate the impact of fire on water provision services in the region and develop management strategies that help reduce the impacts of wildfire.

Climate change impacts on hydrological services in southern California

Water availability is one of the most critical issues facing southern California. Consequently, the role and management of intact watersheds on public lands that supply water are paramount. We undertake the first regional study of climate impacts on hydrological services (runoff, recharge, and climatic water deficit) across the four national forests of southern California—the Angeles, Los Padres, Cleveland and San Bernardino. We assess the exposure, sensitivity, and vulnerability of water resources by comparing current conditions (1981–2010) to mid-century (2040–2069) and end-of-century (2070–2099) using three general circulation models (GCMs) under RCP8.5. Half of the study area is projected to exceed 2015’s drought conditions in 10%–30% of the years between now and end-of-century under the moderate GCM (CCSM4), and one-third of the area is projected to exceed 2015 in 50% of the years under the hotter, drier projection (MIROC-ESM). Under a moderate projection, mean runoff increased by 1.2× by the end-of-century for three of the national forests, while mean recharge decreased by 0.9× across all forests. Projected end-of-century climatic water deficit increased on average 1.1× across the four forests. We assessed the vulnerability of watersheds by comparing the projected mean change between current and future climates with the current inter-annual variability using three categories of vulnerability. Under the moderate projection, one-third of the 385 watersheds were moderately vulnerable to changes in runoff and recharge (+/−0.2 to +/−1× the standard deviation of current inter-annual variability) and ∼12 watersheds were highly vulnerable, suggesting an era of new hydrological conditions by the end-of-century. Half of the Forest Service’s priority management watersheds had moderate or high vulnerability for runoff and recharge. Spatial data on hydrological services and their vulnerability can directly assist in climate-smart planning, allowing tradeoffs to be assessed between proposed management actions and their effect on hydrological services.

Eat whole and less often: ontogenetic shift reveals size specialization on kelp bass by the California moray eel, Gymnothorax mordax

Despite the importance of predation in many ecosystems, gaps remain in our understanding of nocturnal marine predators. Although the kelp forests of Southern California are some of the most well-studied ecosystems, California morays, Gymnothorax mordax, are predominately nocturnal predators that have remained largely unstudied and their predatory effects on the kelp forest ecosystem are unknown. We use a multi-year data set to examine the dietary breadth of G. mordax and to determine the functional role of this predator. We also quantify bite force to examine the potential performance limitations of morays in exploiting prey. Stomach content analyses and linear selectivity index values indicate that G. mordax specializes on kelp bass, Paralabrax clathratus. Average size of kelp bass consumed varies across years, suggesting that morays respond to fluctuations in prey size availability. The scaling relationship of kelp bass standard length and moray head length reveals an ontogenetic shift, where maximum prey size increases with moray size and small prey are dropped from the diet of larger individuals. Moray bite force exhibited strong positive allometry with moray head size, suggesting that larger morays exhibit greater bite forces for their head and body size. However, we found no relationship between prey size and bite force, suggesting that a disproportional increase in bite force does not facilitate the consumption of disproportionately larger prey. Our results indicate that while G. mordax of Catalina Island is a dietary specialist, it is capable of exhibiting functional shifts in prey size and species based on their abundance.

Who’s making all that racket? Seasonal variability in kelp forest soundscapes

Kelp forests off the coast of southern California harbor a myriad of soniferous organisms, creating diverse and complex soundscapes. However, the temporal and spatial variability of kelp forest soundscapes remain poorly documented. Here, the seasonal variability in the soundscapes from two areas, one inside and the other outside a marine protected area off the coast of La Jolla, CA, is described. The din of snapping shrimp snaps was constant in both areas and throughout the seasons, and many distinct, likely fish calls in the 80 Hz—1000 Hz band were heard year-round. Most notably, a late spring/early summer chorus of two putative fish calls (one with most energy at ~90 Hz, the other ~380 Hz) dominated the dusk soundscape of both areas. Many stressors (e.g., pollution run-off, anthropogenic noise, and ocean warming) buffet and degrade the kelp forests of southern California. Documenting how these soundscapes change as the kelp forest conditions change might provide a way to rapidly assess the quality of an area, as well as provide a way to measure the success of conservation techniques and recovery within protected areas.

Updating land cover automatically based on change detection using satellite images: case study of national forests in Southern California

Observing dynamic change patterns and higher-order complexities from remotely sensed images is warranted, but the main challenges include image inconsistency, plant phenological differences, weather variations, and difficulties of incorporating natural conditions into automatic image processing. In this study, we proposed a new algorithm and demonstrated it by producing 2002–2008 and 2010 land-cover maps in heterogeneous Southern California based on an existing 2009 land-cover map. The new algorithm improves the baseline land-cover map quality by discarding potential bad land-cover pixels and dividing each land-cover type into several subclasses. Time series Landsat images were used to detect changed and unchanged areas between baseline year and target year t. Subsequently, for each individual year t, each pixel that was identified as unchanged inherited the baseline classification. Otherwise, each pixel in the changed areas was classified by a similar surrogate majority classifier. The demonstration results in Southern California showed that the land-cover temporal pattern captured the observed successional stages of the ecosystem very well. The accuracy assessment had an overall classification accuracies ranging from 81% to 86% and overall kappa coefficients ranging from 0.79 to 0.83.

A method for estimating transpiration of irrigated urban trees in California

Transpiration of urban forests in southern California is highly uncertain and challenging to quantify because of variability of tree characteristics and stomatal responses among species and locations. However, as California undergoes the most severe drought on record, it is imperative to develop approaches to estimating transpiration of irrigated urban trees (ETrees). We examined the landscape coefficient method recommended by the California Irrigation Management Information System (CIMIS) and widely used to estimate irrigation needs of urban landscapes. The CIMIS method uses reference evapotranspiration (ET0) calculated from the Penman-Monteith equation and a set of species-specific factors to adjust ET0 for particular landscapes. We found a mismatch between CIMIS predictions and actual patterns of urban tree transpiration that we attributed to underrepresentation of tree physiological mechanisms in ET0. As an alternative, we propose an empirical model of ETrees based on in situ measurements on 108 urban trees (14 species) in the Los Angeles region: ETrees=Eref(0.23lnD+0.002I0+0.55). Here D is the vapor pressure deficit of the air, I0 is incoming solar radiation and Eref is species-specific parameter representing ETrees at D=1kPa that may be estimated using mean sapwood area of a tree stand. This model may be used to estimate ETrees for practical applications and to improve representation of irrigated urban forests in hydrologic models.

Object-based Image Mapping of Conifer Tree Mortality in San Diego County based on Multitemporal Aerial Ortho-imagery

Two GEOBIA approaches are compared for their effectiveness in mapping dead trees within island montane forests of Southern California: a spatial contextual approach using an artificial neural network classifier, and a segmentation and multi-pixel classification approach. Both approaches are tested with multitemporal aerial orthoimagery having varying spatial resolutions. Spectral transformation inputs are also tested. An object-based accuracy assessment is conducted. Accuracies range between 30 percent to 90 percent for the dead tree class and are significantly higher for the spatial-contextual approach. Inclusion of spectral transforms increased accuracies by 5 percent for the true object-based approach, up to 13 percent for the spatial contextual approach, and reduced commission error up to 10 percent for both approaches. Masking techniques increased accuracies of the spatial contextual approach by 20 percent. With manual editing, the most accurate maps of individual live and dead trees from the spatial contextual approach are suitable for studying spatio-temporal trends in montane conifer mortality.

Object-based Image Mapping of Conifer Tree Mortality in San Diego County based on Multitemporal Aerial Ortho-imagery

Two GEOBIA approaches are compared for their effectiveness in mapping dead trees within island montane forests of Southern California: a spatial contextual approach using an artificial neural network classifier, and a segmentation and multi-pixel classification approach. Both approaches are tested with multitemporal aerial orthoimagery having varying spatial resolutions. Spectral transformation inputs are also tested. An object-based accuracy assessment is conducted. Accuracies range between 30 percent to 90 percent for the dead tree class and are significantly higher for the spatial-contextual approach. Inclusion of spectral transforms increased accuracies by 5 percent for the true object-based approach, up to 13 percent for the spatial contextual approach, and reduced commission error up to 10 percent for both approaches. Masking techniques increased accuracies of the spatial contextual approach by 20 percent. With manual editing, the most accurate maps of individual live and dead trees from the spatial contextual approach are suitable for studying spatio-temporal trends in montane conifer mortality.

Microbiomes of Muricea californica and M. fruticosa: Comparative Analyses of Two Co-occurring Eastern Pacific Octocorals.

Octocorals are sources of novel but understudied microbial diversity. Conversely, scleractinian or reef-building coral microbiomes have been heavily examined in light of the threats of climate change. Muricea californica and Muricea fruticosa are two co-occurring species of gorgonian octocoral abundantly found in the kelp forests of southern California, and thus provide an excellent basis to determine if octocoral microbiomes are host specific. Using Illumina MiSeq amplicon sequencing and replicate samples, we evaluated the microbiomes collected from multiple colonies of both species of Muricea to measure both inter- and intra-colony microbiome variabilities. In addition, microbiomes from overlying sea water and nearby zoanthids (another benthic invertebrate) were also included in the analysis to evaluate whether bacterial taxa specifically associate with octocorals. This is also the first report of microbiomes from these species of Muricea. We show that microbiomes isolated from each sample type are distinct, and specifically, that octocoral species type had the greatest effect on predicting the composition of the Muricea microbiome. Bacterial taxa contributing to compositional differences include distinct strains of Mycoplasma associated with either M. californica or M. fruticosa, an abundance of Spirochaetes observed on M. californica, and a greater diversity of γ-Proteobacteria associated with M. fruticosa. Many of the bacterial taxa contributing to these differences are known for their presence in photosymbiont-containing invertebrate microbiomes.

Estimating biomass of benthic kelp forest invertebrates from body size and percent cover data

The inability to compare different measures of species abundance (such as density and percent cover) or different metrics of species biomass (such as wet mass and ash-free dry mass) hampers quantitative studies of community dynamics, trophic interactions, energy flow and biodiversity. This has been especially problematic for the dynamic and highly productive communities inhabiting shallow reefs in temperate seas where varied metrics are commonly used to characterize the abundance and biomass of different suites of species. To facilitate the conversion of abundance data into common metrics of biomass, we developed quantitative relationships between wet mass and length and wet mass and percent cover, and conversion factors for transforming wet mass into dry mass, shell-free and decalcified dry mass, and ash-free dry mass for 84 species of benthic macroinvertebrates common to giant kelp forests in southern California. Regressions for all 84 species were highly significant, and regression fits were very good for most species. Interspecific differences between regression slopes and in the ratios used to convert one metric of mass into another varied by as much as an order of magnitude among species within the same taxonomic group indicating that caution should be used when attempting to estimate biomass using generic relationships or ratios that were developed for other species, even if those species are closely related.

Copper tolerance and distribution of epibiotic bacteria associated with giant kelp Macrocystis pyrifera in southern California.

Kelp forests in southern California are important ecosystems that provide habitat and nutrition to a multitude of species. Macrocystis pyrifera and other brown algae that dominate kelp forests, produce negatively charged polysaccharides on the cell surface, which have the ability to accumulate transition metals such as copper. Kelp forests near areas with high levels of boating and other industrial activities are exposed to increased amounts of these metals, leading to increased concentrations on the algal surface. The increased concentration of transition metals creates a harsh environment for colonizing microbes altering community structure. The impact of altered bacterial populations in the kelp forest have unknown consequences that could be harmful to the health of the ecosystem. In this study we describe the community of microorganisms associated with M. pyrifera, using a culture based approach, and their increasing tolerance to the transition metal, copper, across a gradient of human activity in southern California. The results support the hypothesis that M. pyrifera forms a distinct marine microhabitat and selects for species of bacteria that are rarer in the water column, and that copper-resistant isolates are selected for in locations with elevated exposure to transition metals associated with human activity.

Effects of land use on riparian birds in a semiarid region

We investigated the influence of landscape characteristics on avian species occupancy in riparian forests embedded in a matrix of urban and agricultural land use in a semiarid region of the Southwestern US. We conducted bird and vegetation (local-scale characteristics) surveys within 196 50-m radius sample points in 10 riparian forests in southern California. We quantified landscape composition within a 500 m-radius surrounding each point. For each species we developed 8 single-season occupancy models using principal components summarizing local- and landscape-scale characteristics and a spatial autocovariate as covariates. Of 21 species analyzed, occupancy by 11 was associated with landscape characteristics, by 6 with local vegetation characteristics, by 3 with both local and landscape characteristics, and by 1 with none. Five species positively responded to surrounding urban development (2 negative), whereas 4 negatively responded to agricultural land (1 positive). The amount of riparian forests had a strong positive effect on the occurrence of riparian obligates. Our results emphasize the importance of landscape characteristics on species occupancy patterns in riparian systems although relationships were also species-specific. Our results imply a positive effect of urbanization compared to agricultural land uses in this region, most likely due to enhanced vegetation development.

Disturbance and riparian tree establishment in the Sespe Wilderness, California, USA

The riparian forests of southern California are subject to disturbance by both fire and flood. These agents are capable of causing pulses of mortality and recruitment, but it remains unclear how they interact to determine patterns of stand development. We use dendrochronology to identify establishment dates for stems of major riparian tree species in the Sespe Creek watershed, in order to examine their relationship to regional flooding and fire history. Our 11 study sites were burned by major fires in 1932 and 2002, with a smaller 1975 fire affecting only two sites; major floods were concentrated within the second half of the 1933–2009 streamflow record, with the largest floods occurring in 1969, 1978, and 1983. Three periods of stand development are evident: (1) the oldest alder (Alnus), cottonwood (Populus), and oak (Quercus) stems became established soon after the 1932 Matilija Fire, (2) minimal stem establishment between the 1940s and mid-1960s, and (3) continued, although irregular, recruitment of alder and cottonwood since the late 1960s. These patterns show episodes both of regeneration following a catastrophic site-clearing event (Matilija Fire) and of more localized stem replacement during the recent period of increased flood magnitude, with implications for changes in the composition of these forests.

Elk Kelp, Pelagophycus porra, distribution limited due to susceptibility of microscopic stages to high light

Elk Kelp, Pelagophycus porra (Leman) Setchell, is commonly observed in deep (20–50m) water along the outer edge of Giant Kelp, Macrocystis pyrifera (C. Agardh), forests in southern California, USA and northern Baja California, Mexico, but rarely occurs in shallower water or within the adjacent M. pyrifera beds. To investigate the factors that limit P. porra from establishing populations within these shallower habitats, juvenile P. porra sporophytes were transplanted from a deep (20m) water P. porra zone to shallower (15 and 8m) sites within and inside an adjacent M. pyrifera beds. Transplanted P. porra exhibited no differences in survival or growth among depths, although reproductive maturity was observed only at the two shallower sites. When P. porra propagules were experimentally introduced to areas under dense M. pyrifera canopies and areas cleared of the M. pyrifera canopies within the shallower M. pyrifera bed, P. porra still failed to recruit even though this procedure resulted in a four-fold increase in recruitment in the natural P. porra zone. Laboratory-based culturing of P. porra microscopic stages revealed that they grew and survived better under low light conditions characteristic of the natural P. porra zone (2–4μmol photonsm−2s−1) than under higher light conditions characteristic of the M. pyrifera beds(18–20μmol photonsm−2s−1). Pulse-amplitude modulated (PAM) fluorometry indicated that while naturally-occurring P. porra's adult sporophytes were able to photoacclimate to increasing irradiance as they grew from the benthos towards the surface, P. porra's microscopic stages were unable to photoacclimate to increased irradiances, and subsequently exhibited 100% mortality under these higher light conditions. Altogether, our study suggests that vulnerability of P. porra microscopic stages to higher irradiances appears to be the primary factor inhibiting P. porra from establishing populations in shallower water and stresses the importance of a multiple life-history approach when investigating species distributions.

Loss of predators and the collapse of southern California kelp forests (?): Alternatives, explanations and generalizations

It is increasingly argued that human-induced alterations to food webs have resulted in the degradation of coastal ecosystems and even their “collapse.” We examined the evidence for this argument for Macrocystis pyrifera (giant kelp) forests in southern California. Others have concluded that forests in this region collapsed between 1950 and 1970 as a result of sea urchin grazing driven by overfishing of sea urchin predators (sheephead wrasse and spiny lobsters) and competitors (abalone), and that the kelp forests recovered but are currently sustained as a result of a commercial sea urchin fishery that began in the early 1970s. Our examination of the historical record, primary publications, and previously unpublished data showed that there was no widespread decline in the region between 1950 and 1970, but there were localised declines in mainland kelp forests near the rapidly growing cities of Los Angeles and San Diego. The preponderance of evidence indicates that kelp losses were caused primarily by large increases in contaminated sewage discharged into coastal waters, sedimentation from coastal development, and the 1957–1959 El Niño. Increases in active sea urchin foraging were most likely a secondary effect following dwindling food resources. The forests recovered when sewage treatment improved and sewage outfalls were relocated. The effects of fisheries were explored by correlation analysis between kelp canopy cover and commercial sea urchin landings, and among fisheries landings for sea urchins, abalone, sheephead and lobster. These correlations were generally insignificant, but were often confounded by differences in the spatial scale over which the data were collected as well as factors other than simple abundance that affect the fisheries. However, where area-specific data were available, the landings of sea urchins generally tracked kelp abundance, most likely because roe (gonad) development is directly related to food availability. A literature review showed that although sheephead and lobsters may control sea urchin abundance at small spatial scales within some sites, there is little evidence they do so over large areas. That abalone and sea urchins compete, such that sea urchins increased as a result of abalone harvesting, is largely conjecture based on their similar habitat and food utilization. This study shows that kelp forests in southern California did not collapse, and that declines in some coastal sites were caused primarily by degradation of water quality, increased sedimentation and contamination, and unfavorable oceanographic conditions. We conclude that management by species' protection or reserves will not be effective if poor habitat quality impacts the ability of giant kelp to survive and thrive.

Research Article: Opinions of Latino Outdoor Recreation Visitors at Four Urban National Forests

It is important to evaluate use of urban-proximate outdoor recreation sites by diverse groups and obtain visitor viewpoints about those sites. Of particular importance are day-use sites, which receive a large amount of use but little research emphasis. Managers of urban-proximate day-use sites can better manage with detailed specific information about participation patterns, site preferences, and visitor perceptions. Results are offered from visitor contact surveys conducted at day-use sites on four urban national forests in Southern California between 2001 and 2004, with a focus on areas where Latinos recreate. These data indicate many similarities among the Latino visitors to specific sites in four Southern California forests. There were commonalities in participation in outdoor recreation activities, the relative importance of site attributes, and perceptions reported about their recreation experiences. The results suggest that management decisions about serving these groups consider the range of activity options identified, that there is a consistent desire for facilities and amenities, and that the Latino visitors are likely to continue to recreate in these places and will tell others about it, probably leading to increased use by these respondent groups in the future.

Impacts of Wildfire on the Composition and Structure of Riparian Forests in Southern California

In southern California, wildfire is a ubiquitous agent shaping plant communities. Although fire impacts have been widely studied in chaparral-covered uplands, few data are available regarding fire and riparian vegetation. This study provides an example of the impact of a severe fire on riparian habitat. Plant species found in southern California gallery forests are typically adapted to maintaining populations following flood disturbances; we seek to determine whether structural and compositional changes following fire here demonstrate a similar quasi-equilibrium response. We sampled 65 quadrats on 11 transects along two streams in the Los Padres National Forest to characterize tree species size–class distributions before and after the 2002 Wolf Fire. We tested whether species exhibited differential patterns of survivorship and regeneration following the fire, and also tested for spatial variability in mortality within the floodplain. Alnus rhombifolia dominated the pre-fire forest, but experienced severe mortality in the fire and showed very limited resprouting after 3 years. Other prominent taxa (Populus, Salix, Quercus spp.) also lost considerable standing basal area, but had substantially greater rates of resprouting, resulting in a dramatically altered post-fire vegetation composition and structure. Fire impacts did not vary with landform position, leading to a distinctive homogenizing disturbance that contrasts with the spatially zoned and relatively stabilizing compositional influence that flood events have in this same riparian setting.