California Forest Research Articles

Lithological and bioclimatic impacts on soil phosphatase activities in California temperate forests

Organic phosphorus (Po) biogeochemical cycling is known to be impacted by soil-forming factors, yet little is known about interactions among soil-forming factors on the drivers of Po cycling, soil phosphatases. A comprehensive assessment of lithological and bioclimatic impacts on soil phosphatase activities was conducted using lithological and vegetation-constrained climatic (i.e., bioclimatic) gradients in temperate California forests. Soils (0–5, 5–15 cm depth) from twelve combinations of three lithologies (andesite, basalt, granite) and four bioclimatic zones (blue oak, ponderosa pine, red fir, white fir) encompassing a wide range of MAP (330–1400 mm) and MAT (5.0–17.0 °C) were analyzed for resin P (Presin), Po, microbial biomass P and total P (Pt), activities of three phosphatases, and soil properties (e.g., organic carbon [OC], pedogenic minerals). Across soil depths, lithology influenced phosphomonoesterase (ACP, ALP) activities more than bioclimate, whereas the opposite occurred for phosphodiesterase (PDE). Correlations of phosphatases with edaphic (e.g., pedogenic minerals, clay content) and climatic variables (MAP, MAT) varied by lithology (e.g., ACP and OC, Rbasalt = 0.61, p < 0.01 and Rgranite = 0.20, p = 0.28), indicating lithological dependence of bioclimatic impacts. Within bioclimates, phosphomonoesterase but not PDE activities were correlated with OC much more strongly than with pedogenic minerals, suggesting limited lithological impacts in vegetation- and climate-constrained ecosystems. This study demonstrates that the enzymes that catalyze the two distinct steps of Po mineralization, phosphomonoesterase versus phosphodiesterase, can express differential and even opposite relationships with edaphic variables depending on the combination of lithology and bioclimate. Additionally, OC, Presin, and Po may not necessarily be the best predictors of phosphatase activities as commonly proposed. Lithology type and/or pedogenic minerals that integrate lithology and bioclimate context may enable a more comprehensive assessment of phosphatases in soil P cycling sensitive to bioclimatic variability.

Reduced resilience as an early warning signal of forest mortality

Climate-induced forest mortality is being widely observed across the globe. Predicting forest mortality remains challenging because the physiological mechanisms causing mortality are not fully understood and empirical relations between climatology and mortality are subject to change. Here, we show that the temporal loss of resilience, a phenomenon often detected as a system approaches a tipping point, can be used as an early warning signal (EWS) to predict the likelihood of forest mortality directly from remotely sensed vegetation dynamics. We tested the proposed approach on data from Californian forests and found that the EWS can often be detected before reduced greenness, between 6 to 19 months before mortality. The EWS shows a species-specific relation with mortality, and is able to capture its spatio-temporal variations. These findings highlight the potential for such an EWS to predict forest mortality in the near-term. Predicting mortality in forests is challenging because its underlying causes are spatially varied and not well known. Reduced resilience detected from remotely sensed time series of vegetation dynamics can serve as an effective early warning signal to indicate the potential for forest mortality.

Fire severity and changing composition of forest understory plant communities

AbstractQuestionsGradients of fire severity in dry conifer forests can be associated with variation in understory floristic composition. Recent work in dry conifer forests in California, USA, has suggested that more severely burned stands contain more thermophilic taxa (those associated with warmer and drier conditions), and that forest disturbance may therefore accelerate floristic shifts already underway due to climate change. However, it remains unknown how rapidly thermophilic taxa shifts occur following disturbance, how long such shifts are likely to persist, and how different thermophilic post‐disturbance communities are from pre‐disturbance communities.LocationColorado Front Range, USA.MethodsWe investigated these questions using a unique 15‐year vegetation plot dataset that captures pre‐ and post‐fire understory community composition across a gradient of fire severity in dry conifer forests, classifying taxa using the biogeographic affinity concept.ResultsThermophilization (defined here as a decrease in the ratio of cool‐mesic taxa to warm‐xeric taxa, based on biogeographic affinity of paleobotanical lineages) was observed as early as one year post‐fire for all fire severity classes, but was stronger at sites that burned at higher severity. The ratio of cool‐mesic to warm‐xeric taxa recovered to pre‐fire levels within 10 years in stands that burned at low severity, but not in stands that burned at moderate or high severity. The process of thermophilization after high‐severity fire appears to be driven primarily by the gain of warm‐xeric taxa that were absent before the fire, but losses of cool‐mesic taxa, which did not return during the duration of the study, also played a role.ConclusionsDecreases in canopy cover appear to be a main contributor to understory thermophilization. Fine‐scale heterogeneity in post‐fire forest structure is likely an important driver of floristic diversity, creating the microclimatic variation necessary to maintain floristic refugia for species mal‐adapted to increasingly warm and dry conditions.

Anticoagulant rodenticides in Strix owls indicate widespread exposure in west coast forests

Exposure of nontarget wildlife to anticoagulant rodenticides (AR) is a global conservation concern typically centered around urban or agricultural areas. Recently, however, the illegal use of ARs in remote forests of California, USA, has exposed sensitive predators, including the federally threatened northern spotted owl (Strix occidentalis caurina). We used congeneric barred owls (S. varia) as a sentinel species to investigate whether ARs pose a threat to spotted owls and other old-forest wildlife in northern regions of the Pacific Northwest. We analyzed the liver tissue from 40 barred owls collected in Oregon and Washington and confirmed exposure to ≥1 AR compounds in 48% of the owls examined. Brodifacoum, an extremely toxic second-generation AR, was the most common compound detected (89% of positive cases), followed by bromadiolone (11%), difethialone (11%), and warfarin (5%). Brodifacoum was also detected in one barred owl and one spotted owl opportunistically found dead (liver concentrations were 0.091 and 0.049 μg/g, respectively). We found no evidence that exposure varied with proximity to developed and agricultural areas, or among different study areas, age-classes, and sexes. Rather, exposure was ubiquitous, and the rates we observed in our study (38–64%) were similar to or greater than that reported previously for barred owls in California (40%). Together these studies indicate widespread contamination in forested landscapes used by spotted owls and other wildlife of conservation concern. Owls collected in older forests may have been exposed via illegal use of ARs, highlighting a mounting challenge for land managers and policy makers.

Escape Route Index: A Spatially-Explicit Measure of Wildland Firefighter Egress Capacity

For wildland firefighters, the ability to efficiently evacuate the fireline is limited by terrain, vegetation, and fire conditions. The impacts of terrain and vegetation on evacuation time to a safety zone may not be apparent when considering potential control locations either at the time of a wildfire or during pre-suppression planning. To address the need for a spatially-explicit measure of egress capacity, this paper introduces the Escape Route Index (ERI). Ranging from 0 to 1, ERI is a normalized ratio of the distance traveled within a time frame, accounting for impedance by slope and vegetation, to the optimal distance traveled in the absence of these impediments. An ERI approaching 1 indicates that terrain and vegetation conditions should have little impact on firefighter mobility while an ERI approaching 0 is representative of limited cross-country travel mobility. The directional nature of evacuation allows for the computation of four ERI metrics: (1) ERImean (average ERI in all travel directions); (2) ERImin (ERI in direction of lowest egress); (3) ERImax (ERI in direction of highest egress); and (4) ERIazimuth (azimuth of ERImax direction). We demonstrate the implementation of ERI for three different evacuation time frames (10, 20, and 30 min) on the Angeles National Forest in California, USA. A previously published, crowd-sourced relationship between slope and travel rate was used to account for terrain, while vegetation was accounted for by using land cover to adjust travel rates based on factors from the Wildland Fire Decision Support System (WFDSS). Land cover was found to have a stronger impact on ERI values than slope. We also modeled ERI values for several recent wildland firefighter entrapments to assess the degree to which landscape conditions may have contributed to these events, finding that ERI values were generally low from the crews’ evacuation starting points. We conclude that mapping ERI prior to engaging a fire could help inform overall firefighter risk for a given location and aid in identifying locations with greater egress capacity in which to focus wildland fire suppression, thus potentially reducing risk of entrapment. Continued improvements in accuracy of vegetation density mapping and increased availability of light detection and ranging (lidar) will greatly benefit future implementations of ERI.

Patterns of woodboring beetle activity following fires and bark beetle outbreaks in montane forests of California, USA

BackgroundIncreasingly frequent and severe drought in the western United States has contributed to more frequent and severe wildfires, longer fire seasons, and more frequent bark beetle outbreaks that kill large numbers of trees. Climate change is expected to perpetuate these trends, especially in montane ecosystems, calling for improved strategies for managing Western forests and conserving the wildlife that they support. Woodboring beetles (e.g., Buprestidae and Cerambycidae) colonize dead and weakened trees and speed succession of habitats altered by fire or bark beetles, while serving as prey for some early-seral habitat specialists, including several woodpecker species. To understand how these ecologically important beetles respond to different sources of tree mortality, we sampled woodborers in 16 sites affected by wildfire or bark beetle outbreak in the previous one to eight years. Study sites were located in the Sierra Nevada, Modoc Plateau, Warner Mountains, and southern Cascades of California, USA. We used generalized linear mixed models to evaluate hypotheses concerning the response of woodboring beetles to disturbance type, severity, and timing; forest stand composition and structure; and tree characteristics.ResultsWoodborer activity was often similar in burned and bark beetle outbreak sites, tempered by localized responses to bark beetle activity, burn severity, tree characteristics, and apparent response to ignition date. Larval woodborer activity was inversely related to bark beetle sign within a sampling quadrat, was higher on pines, and—in burned sites—was higher on the south-facing sides of smaller trees. Adults—especially buprestids—were more abundant where burn severity was higher. Fires with intermediate ignition dates during the study period supported higher rates of larval woodborer activity and higher numbers of adult buprestids as well as cerambycids in the genus Monochamus Dejean 1821.ConclusionsWoodboring beetle abundance was related to bark beetle activity, burn severity, tree characteristics, and ignition date. Considering these patterns when managing disturbed forest stands could yield improved outcomes for wildlife, including species that prey on woodboring beetles. We also reported preliminary evidence that the current trend toward more frequent wildfires might not stimulate larger woodboring beetle populations if those fires increasingly occur outside the historical fire season.

Source or Sink? A comparison of Landfire- and FIA-based estimates of change in aboveground live tree carbon in California’s forests

Forests play a central role in addressing climate change, and accurate estimates of forest carbon are critical for the development of actions that reduce emissions from forests and that maximize sequestration by forests. Methodological challenges persist regarding how best to estimate forest carbon stocks and flux at regulatory-relevant scales. Using California, USA as a case study, we compare two approaches to stock-difference forest carbon estimation for aboveground live trees: one based on ground inventories and one on land cover classification of remotely-sensed data. Previous work using ground inventory data from the Forest Inventory and Analysis Program (FIA) showed net aboveground carbon (AGC) sequestration by live trees in California forests, while estimates using land cover classification from the Landscape Fire and Resource Management Planning Tools (Landfire) showed net reductions in live tree AGC over a similar time period. We examined the discrepancy by re-analyzing the FIA inventory data through the lens of a category-change analysis based on Landfire. This analysis showed more than 50% of the live tree AGC in fewer than 4% of Landfire-equivalent categories and that the overwhelming majority (>80%) of forest area did not change height category between measurement periods. Despite the lack of categorical change, the majority of FIA plots increased in both 95th percentile tree height and in live tree AGC. These findings suggest that an approach based on observing categorical changes risks undercounting AGC sequestration resulting from growth and thus overstating the relative importance of AGC reductions that result from disturbances. This would bias AGC flux estimates downward, leading us to validate the conclusion that live trees in California were a net sink of aboveground carbon in the decade ending in 2016. Our findings suggest an inventory-based or hybrid approach is preferable to methods that depend on categorical bins for estimating AGC in disturbance-prone forest ecosystems.

California forest die-off linked to multi-year deep soil drying in 2012–2015 drought

Widespread episodes of recent forest die-off have been tied to the occurrence of anomalously warm droughts, although the underlying mechanisms remain inadequately understood. California’s 2012–2015 drought, with exceptionally low precipitation and warmth, and widespread conifer death, provides an opportunity to explore the chain of events leading to forest die-off. Here, we present the spatial and temporal patterns of die-off and moisture deficit during California’s drought, based on field and remote sensing observations. We found that die-off was closely tied to multi-year deep-rooting-zone drying, and that this relationship provides a framework with which to diagnose and predict mortality. Marked tree death in an intensively studied Sierra Nevada forest followed a four-year moisture overdraft, with cumulative 2012–2015 evapotranspiration exceeding precipitation by ~1,500 mm, and subsurface moisture exhaustion to 5–15-m depth. Observations across the entire Sierra Nevada further linked tree death to deep drying, with die-off and moisture overdraft covarying across latitude and elevation. Unusually dense vegetation and warm temperatures accelerated southern Sierran evapotranspiration in 2012–2015, intensifying overdraft and compounding die-off by an estimated 55%. Climate change is expected to further amplify evapotranspiration and moisture overdraft during drought, potentially increasing Sierran tree death during drought by ~15–20% °C−1. Deep soil drying, caused by high evaporation, can explain California forest die-off in the droughts during 2012–2015, according to analyses of patterns of die-off and moisture deficit.

Within-Patch Structures Influence Small Mammal Abundance in Managed Forests of Northern California, USA

AbstractWe evaluated how forest type, vegetation structure in trapping webs, and proximate forest types influenced localized (~6.35 hectares) abundances for commonly captured small mammals in northern California, USA. We trapped from May to August of 2011–13 in 69 forest patches that represented: (1) clearcuts (3–5 years postharvest), (2) 10–20 year-old conifer plantations, (3) rotation-aged conifer stands, and (4) Watercourse and Lake Protection Zones. We captured 11 species; four in sufficient numbers for regression modeling. Our average abundance estimates for the study were 4.57 (standard error [SE] = 0.43), 0.32 (SE = 0.11), 0.90 (SE = 0.30), and 0.25 (SE = 0.09) individuals per web location (~0.75 hectares) for Peromyscus spp., Neotoma spp., California ground squirrels, and Allen’s chipmunks. We found that web-level ground cover (shrubs and grass), downed wood, and types of forests containing our trapping webs best described small mammal abundances, whereas proximate forest types were not important. Our results indicated that retaining localized structures in the form of understory shrub cover and downed wood positively influences small mammal abundance in intensively managed forests of northern California.

Estimating Biomass Availability and Cost When Implementing Forest Restoration with Tethered Harvest Systems

AbstractUsing an adaptation of Forest Inventory and Analysis’s BioSum framework, which models prospective management of forested landscapes using forest inventory data, we tested several fire-resistance-promoting restoration treatments, implemented with tethered cut-to-length harvest systems, for effectiveness and economic feasibility in the dry national forests of southern Oregon and northern California. Treatments elevated fire resistance on most forested area, primarily via increases in the separation of canopy and surface fuels and among tree crowns, and the most effective treatments could more than cover treatment cost with sales of wood in most stands. If, instead of disposal by burning at the landing, small-diameter wood was delivered to a biochar facility capable of paying US$50 per bone dry ton, this would increase the share of forest area on which treatment could break even from 61 percent to 67 percent, slightly more than the 66 achievable with a treatment subsidy of US$100 ac−1. Potential treatment area appears to be currently constrained by institutional capacity, not treatment effectiveness, economics, opportunity, or need. Even with the currently modest scale of management activity, sufficient biochar feedstock is available in the upper Klamath Basin to supply at least one large-scale biochar facility over the next 20 years.

Phytophthora ramorum and Phytophthora gonapodyides Differently Colonize and Contribute to the Decomposition of Green and Senesced Umbellularia californica Leaves in a Simulated Stream Environment

Plant pathogenic as well as saprotrophic Phytophthora species are now known to inhabit forest streams and other surface waters. How they survive and function in aquatic ecosystems, however, remains largely uninvestigated. Phytophthora ramorum, an invasive pathogen in California forests, regularly occurs in forest streams, where it can colonize green leaves shed in the stream but is quickly and largely succeeded by saprotrophically competent clade 6 Phytophthora species, such as Phytophthora gonapodyides. We investigated, using controlled environment experiments, whether leaf litter quality, based on senescence, affects how P. ramorum and P. gonapodyides compete in leaf colonization and to what extent each species can contribute to leaf decomposition. We found that both Phytophthora species effectively colonized and persisted on green or yellow (senescing) bay leaves, but only P. gonapodyides could also colonize and persist on brown (fully senesced and dried) leaves. Both Phytophthora species similarly accelerated the decomposition of green leaves and yellow leaves compared with non-inoculated controls, but colonization of brown leaves by P. gonapodyides did not affect their decomposition rate.

Transgenerational effects in an ecological context: Conditioning of adult sea urchins to upwelling conditions alters maternal provisioning and progeny phenotype

Transgenerational plasticity occurs when the conditions experienced by the parental generation influence the phenotype of their progeny. This may in turn affect progeny performance and physiological tolerance, providing a means by which organisms cope with rapid environmental change. We conditioned adult purple sea urchins, Strongylocentrotus purpuratus, to combined pCO2 and temperature conditions reflective of in situ conditions of their natural habitat, the benthos in kelp forests of nearshore California, and then assessed the performance of their progeny raised under different pCO2 levels. Adults were conditioned during gametogenesis to treatments that reflected static non-upwelling (~650 μatm pCO2, ~17 °C) and upwelling (~1300 μatm pCO2, ~13 °C) conditions. Following approximately 4 months of conditioning, the adults were spawned and embryos were raised under low pCO2 (~450 μatm pCO2) or high pCO2 (~1050 μatm pCO2) treatments to determine if differential maternal conditioning impacted the progeny response to a single abiotic stressor: pCO2. We examined the size, protein content, and lipid content of eggs from both sets of conditioned female urchins. Offspring were sampled at four stages of early development: hatched blastula, gastrula, prism, and echinopluteus. This resulted in four sets of offspring: (1) progeny from non-upwelling-conditioned mothers raised under low pCO2, (2) progeny from non-upwelling-conditioned mothers raised under high pCO2, (3) progeny from upwelling-conditioned mothers raised under low pCO2, and (4) progeny from upwelling-conditioned mothers raised under high pCO2. We then assessed the effects of maternal conditioning along with the effects of developmental pCO2 levels on body size of the progeny. Our results showed that differential maternal conditioning had no impact on average egg size, although non-upwelling females produced eggs that were more variable in size. Maternal conditioning did not affect protein content but did have a modest impact on egg lipid content. Developing embryos whose mothers were conditioned to simulated upwelling conditions (~1300 μatm pCO2, ~13 °C) were greater in body size, although this effect was no longer evident at the echinopluteus larval stage. Although maternal conditioning affected offspring body size, the pCO2 levels under which the embryos were raised did not. Overall, this laboratory study provides insight into how transgenerational effects may function in nature. The impacts of parental environmental history on progeny phenotype during early development have important implications regarding recruitment success and population-level effects.

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.

High-Resolution Mapping of Redwood (Sequoia sempervirens) Distributions in Three Californian Forests

High-resolution maps of redwood distributions could enable strategic land management to satisfy diverse conservation goals, but the currently-available maps of redwood distributions are low in spatial resolution and biotic detail. Classification of airborne imaging spectroscopy data provides a potential avenue for mapping redwoods over large areas and with high confidence. We used airborne imaging spectroscopy data collected over three redwood forests by the Carnegie Airborne Observatory, in combination with field training data and application of a gradient boosted regression tree (GBRT) machine learning algorithm, to map the distribution of redwoods at 2-m spatial resolution. Training data collected from the three sites showed that redwoods have spectral signatures distinct from the other common tree species found in redwood forests. We optimized a gradient boosted regression model for high performance and computational efficiency, and the resulting model was demonstrably accurate (81–98% true positive rate and 90–98% overall accuracy) in mapping redwoods in each of the study sites. The resulting maps showed marked variation in redwood abundance (0–70%) within a 1 square kilometer aggregation block, which match the spatial resolution of currently-available redwood distribution maps. Our resulting high-resolution mapping approach will facilitate improved research, conservation, and management of redwood trees in California.

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.

Novel disturbance interactions between fire and an emerging disease impact survival and growth of resprouting trees.

Human-altered ecological disturbances may challenge system resilience and disrupt biological legacies maintaining ecosystem recovery. Yet, the extent to which novel regimes challenge these legacies varies. This may be partially explained by differences in the vulnerability of life history strategies to disturbance characteristics. In the fire-prone, resprouter-dominated coast redwood forests of California, the introduced disease sudden oak death (SOD) alters fuel profiles, fire behavior, and aboveground tree mortality; however, this system is dominated by resprouting trees that are well-adapted to aboveground damage, and belowground survival of individuals may represent the principal biological legacy connecting pre- and post-fire communities. Much of the research exploring altered disturbances and forest recovery has focused on legacies determined by seed dispersal and aboveground survival of adults. In this work, we use pre- and post-fire data from a long-term monitoring network to assess the impacts of novel disturbance interactions between wildfire and SOD on the belowground survival and vegetative reproduction of resprouters. We found that increasing accumulation of coarse woody surface fuels from SOD-killed hosts decreased the likelihood of belowground survival for resprouting tanoak trees, but not for redwoods. Tanoaks' belowground survival was negatively related to substrate burn severity, which increased with the volume of surface fuels from hosts, suggesting heat damage as a possible mechanism influencing altered patterns of resprouter mortality. These impacts increased with decreasing tree size. By contrast, redwood and tanoak trees that survived both disturbances resprouted more vigorously, regardless of post-fire infection by P.ramorum, and generated similar recruitment at the stand level. Our results demonstrate that disease-fire interactions can narrow recruitment filters for resprouters, which could impact long-term population and demographic structure; yet, compounded disturbance may also reduce stand density and disease pressure, allowing competitive release of survivors. Resprouters displayed vulnerabilities to altered disturbance, but our research suggests that legacies maintained by resprouting may be more resilient to certain compounded disturbances, compared to seed-obligate species, because of high rates of individual survival under increasingly severe events. These trends have important implications for conservation of declining tree species in SOD-impacted forests, as well as predictions of human impacts in other disturbance-prone systems where resprouters are present.

A pulse of summer precipitation after the dry season triggers changes in ectomycorrhizal formation, diversity, and community composition in a Mediterranean forest in California, USA

Rapid responses of microbial biomass and community composition following a precipitation event have been reported for soil bacteria and fungi, but measurements characterizing ectomycorrhizal fungi remain limited. The response of ectomycorrhizal fungi after a precipitation event is crucial to understanding biogeochemical cycles and plant nutrition. Here, we examined changes in ectomycorrhizal formation, diversity, and community composition at the end of a summer drought and following precipitation events in a conifer–oak mixed forest under a semiarid, Mediterranean-type climate in CA, USA. To study the effects of different amounts of precipitation, a water addition treatment was also undertaken. Ectomycorrhizal fungal diversity and community composition changed within 6 days following precipitation, with increased simultaneous mortality and re-growth. Ectomycorrhizal diversity increased and community composition changed both in the natural rainfall (less than 10 mm) and water addition (50 mm) treatments, but larger decreases in ectomycorrhizal diversity were observed from 9 to 16 days after precipitation in the water addition treatment. The changes were primarily a shift in richness and abundance of Basidiomycota species, indicating higher drought sensitivity of Basidiomycota species compared with Ascomycota species. Our results indicate that ectomycorrhizal formation, diversity, and community composition rapidly respond to both precipitation events and to the amount of precipitation. These changes affect ecosystem functions, such as nutrient cycling, decomposition, and plant nutrient uptake, in semiarid regions.

Quantification of uncertainty in aboveground biomass estimates derived from small-footprint airborne LiDAR

To address uncertainty in biomass estimates across spatial scales, we determined aboveground biomass (AGB) in Californian forests through the use of individual tree detection methods applied to small-footprint airborne LiDAR. We propagated errors originating from a generalized allometric equation, LiDAR measurements, and individual tree detection algorithms to AGB estimates at the tree and plot levels. Larger uncertainties than previously reported at both tree and plot levels were found when AGB was derived from remote sensing. On average, per-tree AGB error was 135% of the estimated AGB, and per-plot error was 214% of the estimated AGB. We found that from tree to plot level, the allometric equation constituted the largest proportion of the total AGB uncertainty. The proportion of the uncertainty associated with remote sensing errors was larger in lower AGB forests, and it decreased as AGB increased. The framework in which we performed the error propagation analysis can be used to address AGB uncertainties in other ecosystems and can be integrated with other analytical techniques.

Vulnerability of Physically Protected Soil Organic Carbon to Loss Under Low Severity Fires

Soil aggregate degradation during medium and high severity fires is often identified as the main mechanism that leads to loss of soil organic matter due to fire. Low severity fires, however, are considered not to cause aggregate degradation assuming that temperatures <250o C, as occurring during low-severity burns, have only limited effects on the stability of the soil organic binding agents. Recent studies suggest that low severity burns may cause soil aggregate degradation due to rapid vaporization of soil pore water that can induce pressure on the soil aggregates beyond their yield stress. Such pressure-driven degradation of soil aggregates may expose physically protected organic carbon to decomposition. Our study investigated the effect of a low-severity fire on soil organic matter, water extractable organic C and N as well as respiration for two initial soil moisture conditions undergoing three heating regimes using aggregates from a California forest and Nevada shrubland soil. We found that initially moist soil aggregates that were rapidly heated up degraded the most, showing increased cumulative carbon mineralization when compared to aggregates that were not heated, aggregates that were dry before being heated , and initially moist soil aggregates that were slowly heated. Our results suggest that exposure of previously physically protected organic carbon within the soil aggregates to oxidative conditions was the most likely cause of increased rates of decomposition of organic matter after low-intensity burns. Additionally, we show that for a shrubland soil, aggregates with relatively low organic carbon content, low severity burns increased cumulative carbon mineralization. We hypothesized that this was due to decomposition of cytoplasmic material from lysed microbes. Our results suggest that low severity burns can accelerate decomposition of soil organic carbon protected in soil aggregates.