Forest restoration and fuels reduction work: Different pathways for achieving success in the Sierra Nevada.

Fire suppression and past selective logging of large trees have fundamentally changed frequent-fire-adapted forests in California. The culmination of these changes produced forests that are vulnerable to catastrophic change by wildfire, drought, and bark beetles, with climate change exacerbating this vulnerability. Management options available to address this problem include mechanical treatments (Mech), prescribed fire (Fire), or combinations of these treatments (Mech + Fire). We quantify changes in forest structure and composition, fuel accumulation, modeled fire behavior, intertree competition, and economics from a 20-year forest restoration study in the northern Sierra Nevada. All three active treatments (Fire, Mech, Mech + Fire) produced forest conditions that were much more resistant to wildfire than the untreated control. The treatments that included prescribed fire (Fire, Mech + Fire) produced the lowest surface and duff fuel loads and the lowest modeled wildfire hazards. Mech produced low fire hazards beginning 7 years after the initial treatment and Mech + Fire had lower tree growth than controls. The only treatment that produced intertree competition somewhat similar to historical California mixed-conifer forests was Mech + Fire, indicating that stands under this treatment would likely be more resilient to enhanced forest stressors. While Fire reduced modeled wildfire hazard and reintroduced a fundamental ecosystem process, it was done at a net cost to the landowner. Using Mech that included mastication and restoration thinning resulted in positive revenues and was also relatively strong as an investment in reducing modeled wildfire hazard. The Mech + Fire treatment represents a compromise between the desire to sustain financial feasibility and the desire to reintroduce fire. One key component to long-term forest conservation will be continued treatments to maintain or improve the conditions from forest restoration. Many Indigenous people speak of "active stewardship" as one of the key principles in land management and this aligns well with the need for increased restoration in western US forests. If we do not use the knowledge from 20+ years of forest research and the much longer tradition of Indigenous cultural practices and knowledge, frequent-fire forests will continue to be degraded and lost.

Empirically validated drought vulnerability mapping in the mixed conifer forests of the Sierra Nevada.

Severe droughts are predicted to become more frequent in the future, and the consequences of such droughts on forests can be dramatic, resulting in massive tree mortality, rapid change in forest structure and composition, and substantially increased risk of catastrophic fire. Forest managers have tools at their disposal to try to mitigate these effects but are often faced with limited resources, forcing them to make choices about which parts of the landscape to target for treatment. Such planning can greatly benefit from landscape vulnerability assessments, but many existing vulnerability analyses are unvalidated and not grounded in robust empirical datasets. We combined robust sets of ground-based plot and remote sensing data, collected during the 2012-2016 California drought, to develop rigorously validated tools for assessing forest vulnerability to drought-related canopy tree mortality for the mixed conifer forests of the Sequoia and Kings Canyon national parks and potentially for mixed conifer forests in the Sierra Nevada as a whole. Validation was carried out using a large external dataset. The best models included normalized difference vegetation index (NDVI), elevation, and species identity. Models indicated that tree survival probability decreased with greenness (as measured by NDVI) and elevation, particularly if trees were growing slowly. Overall, models showed good calibration and validation, especially for Abies concolor, which comprise a large majority of the trees in many mixed conifer forests in the Sierra Nevada. Our models tended to overestimate mortality risk for Calocedrus decurrens and underestimate risk for pine species, in the latter case probably due to pine bark beetle outbreak dynamics. Validation results indicated dangers of overfitting, as well as showing that the inclusion of trees already under attack by bark beetles at the time of sampling can give false confidence in model strength, while also biasing predictions. These vulnerability tools should be useful to forest managers trying to assess which parts of their landscape were vulnerable during the 2012-2016 drought, and, with additional validation, may prove useful for ongoing assessments and predictions of future forest vulnerability.

Simulated treatment effects on bird communities inform landscape-scale dry conifer forest management.

Human land use and climate change have increased forest density and wildfire risk in dry conifer forests of western North America, threatening various ecosystem services, including habitat for wildlife. Government policy supports active management to restore historical structure and ecological function. Information on potential contributions of restoration to wildlife habitat can allow assessment of tradeoffs with other ecological benefits when prioritizing treatments. We predicted avian responses to simulated treatments representing alternative scenarios to inform landscape-scale forest management planning along the Colorado Front Range. We used data from the Integrated Monitoring in Bird Conservation Regions program to inform a hierarchical multispecies occupancy model relating species occupancy and richness with canopy cover at two spatial scales. We then simulated changes in canopy cover (remotely sensed in 2018) under three alternative scenarios, (1) a "fuels reduction" scenario representing landscape-wide 30% reduction in canopy cover, (2) a "restoration" scenario representing more nuanced, spatially variable treatments targeting historical conditions, and (3) a reference, no-change scenario. Model predictions showed areas of potential gains and losses for species richness, richness of ponderosa pine forest habitat specialists, and the ratio of specialists to generalists at two (1 km2 and 250 m2 ) spatial scales. Under both fuels reduction and restoration scenarios, we projected greater gains than losses for species richness. Surprisingly, despite restoration more explicitly targeting ecologically relevant historical conditions, fuels reduction benefited bird species richness over a greater spatial extent than restoration, particularly in the lower montane life zone. These benefits reflected generally positive species associations with moderate canopy cover promoted more consistently under the fuels reduction scenario. In practice, contemporary forest management is likely to lie somewhere between the fuels reduction and restoration scenarios represented here. Therefore, our results inform where and how active forest management can best support avian diversity. Although our study raises questions regarding the value of including landscape-scale heterogeneity as a management objective, we do not question the value of targeting finer scale heterogeneity (i.e., stand and treatment level). Rather, our results combined with those from previous work clarify the scale at which targeting structural heterogeneity and historical reference conditions can promote particular ecosystem services.

Recent bark beetle outbreaks influence wildfire severity in mixed-conifer forests of the Sierra Nevada, California, USA.

In temperate forests, elevated frequency of drought related disturbances will likely increase the incidence of interactions between disturbances such as bark beetle epidemics and wildfires. Our understanding of the influence of recent drought and insect-induced tree mortality on wildfire severity has largely lacked information from forests adapted to frequent fire. A recent unprecedented tree mortality event in California's Sierra Nevada provides an opportunity to examine this disturbance interaction in historically frequent-fire forests. Using field data collected within areas of recent tree mortality that subsequently burned in wildfire, we examined whether and under what conditions wildfire severity relates to severity of prefire tree mortality in Sierra Nevada mixed-conifer forests. We collected data on 180 plots within the 2015 Rough Fire and 2016 Cedar Fire footprints (California, USA). Our analyses identified prefire tree mortality as influential on all measures of wildfire severity (basal area killed by fire, RdNBR, and canopy torch) on the Cedar Fire, although it was less influential than fire weather (relative humidity). Prefire tree mortality was influential on two of three fire-severity measures on the Rough Fire, and was the most important predictor of basal area killed by fire; topographic position was influential on two metrics. On the Cedar Fire, the influence of prefire mortality on basal area killed by fire was greater under milder weather conditions. All measures of fire severity increased as prefire mortality increased up to prefire mortality levels of approximately 30-40%; further increases did not result in greater fire severity. The interacting disturbances shifted a pine-dominated system (Rough Fire) to a cedar-pine-fir system, while the pre-disturbance fir-cedar system (Cedar Fire) saw its dominant species unchanged. Managers of historically frequent-fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems. This study contributes to a growing body of evidence that the influence of prefire tree mortality on wildfire severity in temperate coniferous forests may depend on other conditions capable of driving extreme wildfire behavior, such as weather.

Wildfire severity influences offspring sex ratio in a native solitary bee.

Although ecological disturbances can have a strong influence on pollinators through changes in habitat, virtually no studies have quantified how characteristics of wildfire influence the demography of essential pollinators. Nevertheless, evaluating this topic is critical for understanding how wildfire is linked to pollinator population dynamics, particularly given recent changes in wildfire frequency and severity in many regions of the world. In this study, we measured the demographic response of the blue orchard bee (Osmia lignaria) across a natural gradient of wildfire severity to assess how variation in wildfire characteristics influenced reproductive output, offspring sex ratio, and offspring mass. We placed nest blocks with a standardized number and sex ratio of pre-emergent adult bees across the wildfire gradient, finding some evidence for a positive but highly variable relationship between reproductive output and fire severity surrounding the nest site at both local (100 m) and landscape (750 m) scales. In addition, the production of female offspring was > 10% greater at nest sites experiencing the greatest landscape-scale fire severity relative to the lowest-severity areas. The finding that blue orchard bees biased offspring production towards the more expensive offspring sex with increasing fire severity shows a functional response to changes in habitat quality through increased density of flowering plants. Our findings indicate that burned mixed-conifer forest provides forage for the blue orchard bee across a severity gradient, and that the increase in floral resources that follows high-severity fire leads females to shift resource allocation to the more costly sex when nesting.

Forest recovery following extreme drought in California, USA: natural patterns and effects of pre-drought management.

Rising temperatures and more frequent and severe droughts are driving increases in tree mortality in forests around the globe. However, in many cases, the likely trajectories of forest recovery following drought-related mortality are poorly understood. In many fire-suppressed western U.S. forests, management is applied to reverse densification and restore natural forest structure and species composition, but it is unclear how such management affects post-mortality recovery. We addressed these uncertainties by examining forest stands that experienced mortality during the severe drought of 2012-2016 in California, USA. We surveyed post-drought vegetation along a gradient of overstory mortality severity in paired treated (mechanically thinned or prescribed-burned) and untreated areas in the Sierra Nevada. Treatment substantially reduced tree density, particularly in smaller tree size classes, and these effects persisted through severe drought-related overstory mortality. However, even in treated areas with severe mortality (>67% basal area mortality), the combined density of residual (surviving) trees (mean 44 trees/ha) and saplings (mean 189 saplings/ha) frequently (86% of plots) fell within or exceeded the natural range of variation (NRV) of tree density, suggesting little need for reforestation intervention to increase density. Residual tree densities in untreated high-mortality plots were significantly higher (mean 192 trees/ha and 506 saplings/ha), and 96% of these plots met or exceeded the NRV. Treatment disproportionately removed shade-tolerant conifer species, while mortality in the drought event was concentrated in pines (Pinus ponderosa and P. lambertiana); as a consequence, the residual trees, saplings, and seedlings in treated areas, particularly those that had experienced moderate or high drought-related mortality, were more heavily dominated by broadleaf ("hardwood") trees (particularly Quercus kelloggii and Q. chrysolepis). In contrast, residual trees and regeneration in untreated stands were heavily dominated by shade-tolerant conifer species (Abies concolor and Calocedrus decurrens), suggesting a need for future treatment. Because increased dominance of hardwoods brings benefits for plant and animal diversity and stand resilience, the ecological advantages of mechanical thinning and prescribed fire treatments may, depending on the management perspective, extend even to stands that ultimately experience high drought-related mortality following treatment.

Dry conifer forest restoration benefits Colorado Front Range avian communities.

Fire suppression has increased stand density and risk of severe, stand-replacing wildfire in lower elevation dry conifer forests of western North America, threatening ecological function. The U.S. Forest Service's Collaborative Forest Landscape Restoration Program (CFLRP) aims to mitigate impacts to ecological function, while mandating effectiveness monitoring to verify restoration success. Expected benefits include improved conditions for biodiversity, but relatively few empirical studies evaluate restoration effects on biodiversity. We applied the Integrated Monitoring in Bird Conservation Regions program to survey birds in relation to CFLRP treatments along the Colorado Front Range in 2015-2017. We employed hierarchical models to analyze species occupancy and richness at 1972 points nested within 141 1-km2 grid cells. Our objectives were to investigate (1) species occupancy relationships with treatments at local (point) and landscape (grid) spatial scales, (2) potential mechanisms for treatment relationships considering species and treatment relationships with forest structure and composition (i.e., habitat relationships), and (3) treatment and habitat relationships with species richness. The data supported positive and negative point-level treatment relationships, suggesting uneven species distributions between treated and untreated points. At the grid scale, however, we only found positive species relationships with percent area treated, and accordingly, grid-level species richness increased with treatment extent. Potential mechanisms for treatment relationships included treatments generating foraging opportunities for aerial insectivores by opening the canopy, improving conditions for ground-associated species by increasing herbaceous growth, and limiting opportunities for shrub-nesting species by reducing shrub cover. Landscape-scale patterns suggest CFLRP treatments can benefit avian communities by generating habitat for open-forest species without necessarily eliminating habitat for closed-forest species. Our results provide evidence for a commonly expected but rarely verified pattern of increased species richness with forest heterogeneity. We suggest restoration treatments will most benefit forest bird diversity by reducing canopy cover, encouraging herbaceous ground cover, limiting ladder fuel species, and encouraging shrub diversity in canopy openings, while maintaining some dense forest stands on the landscape.

Cumulative effects of wildfires on forest dynamics in the eastern Cascade Mountains, USA.

Wildfires pose a unique challenge to conservation in fire-prone regions, yet few studies quantify the cumulative effects of wildfires on forest dynamics (i.e., changes in structural conditions) across landscape and regional scales. We assessed the contribution of wildfire to forest dynamics in the eastern Cascade Mountains, USA from 1985 to 2010 using imputed maps of forest structure (i.e., tree size and canopy cover) and remotely sensed burn severity maps. We addressed three questions: (1) How do dynamics differ between the region as a whole and the unburned portion of the region? (2) How do dynamics vary among vegetation zones differing in biophysical setting and historical fire frequency? (3) How have forest structural conditions changed in a network of late successional reserves (LSRs)? Wildfires affected 10% of forests in the region, but the cumulative effects at this scale were primarily slight losses of closed-canopy conditions and slight gains in open-canopy conditions. In the unburned portion of the region (the remaining 90%), closed-canopy conditions primarily increased despite other concurrent disturbances (e.g., harvest, insects). Although the effects of fire were largely dampened at the regional scale, landscape scale dynamics were far more variable. The warm ponderosa pine and cool mixed conifer zones experienced less fire than the region as a whole despite experiencing the most frequent fire historically. Open-canopy conditions increased slightly in the mixed conifer zone, but declined across the ponderosa pine zone even with wildfires. Wildfires burned 30% of the cold subalpine zone, which experienced the greatest increase in open-canopy conditions and losses of closed-canopy conditions. LSRs were more prone to wildfire than the region as a whole, and experienced slight declines in late seral conditions. Despite losses of late seral conditions, wildfires contributed to some conservation objectives by creating open habitats (e.g., sparse early seral and woodland conditions) that otherwise generally decreased in unburned landscapes despite management efforts to increase landscape diversity. This study demonstrates the potential for wildfires to contribute to regional scale conservation objectives, but implications for management and biodiversity at landscape scales vary geographically among biophysical settings, and are contingent upon historical dynamics and individual species habitat preferences.

Impacts of different land management histories on forest change.

Many western North American forest types have experienced considerable changes in ecosystem structure, composition, and function as a result of both fire exclusion and timber harvesting. These two influences co-occurred over a large portion of dry forests, making it difficult to know the strength of either one on its own or the potential for an interaction between the two. In this study, we used contemporary remeasurements of a systematic historical forest inventory to investigate forest change in the Sierra Nevada. The historical data opportunistically spanned a significant land management agency boundary, which protected part of the inventory area from timber harvesting. This allowed for a robust comparison of forest change between logged and unlogged areas. In addition, we assessed the effects of recent management activities aimed at forest restoration relative to the same areas historically, and to other areas without recent management. Based on analyses of 22,007 trees (historical, 9,573; contemporary, 12,434), live basal area and tree density significantly increased from 1911 to the early 2000s in both logged and unlogged areas. Both shrub cover and the proportion of live basal area occupied by pine species declined from 1911 to the early 2000s in both areas, but statistical significance was inconsistent. The most notable difference between logged and unlogged areas was in the density of large trees, which declined significantly in logged areas, but was unchanged in unlogged areas. Recent management activities had a varied impact on the forest structure and composition variables analyzed. In general, areas with no recent management activities experienced the greatest change from 1911 to the early 2000s. If approximating historical forest conditions is a land management goal the documented changes in forest structure and composition from 1911 to the early 2000s indicate that active restoration, including fire use and mechanical thinning, is needed in many areas.

Disturbance and productivity interactions mediate stability of forest composition and structure.

Fire is returning to many conifer-dominated forests where species composition and structure have been altered by fire exclusion. Ecological effects of these fires are influenced strongly by the degree of forest change during the fire-free period. Response of fire-adapted species assemblages to extended fire-free intervals is highly variable, even in communities with similar historical fire regimes. This variability in plant community response to fire exclusion is not well understood; however, ecological mechanisms such as individual species' adaptations to disturbance or competition and underlying site characteristics that facilitate or impede establishment and growth have been proposed as potential drivers of assemblage response. We used spatially explicit dendrochronological reconstruction of tree population dynamics and fire regimes to examine the influence of historical disturbance frequency (a proxy for adaptation to disturbance or competition), and potential site productivity (a proxy for underlying site characteristics) on the stability of forest composition and structure along a continuous ecological gradient of pine, dry mixed-conifer, mesic mixed-conifer, and spruce-fir forests following fire exclusion. While average structural density increased in all forests, species composition was relatively stable in the lowest productivity pine-dominated and highest productivity spruce-fir-dominated sites immediately following fire exclusion and for the next 100 years, suggesting site productivity as a primary control on species composition and structure in forests with very different historical fire regimes. Species composition was least stable on intermediate productivity sites dominated by mixed-conifer forests, shifting from primarily fire-adapted species to competition-adapted, fire-sensitive species within 20 years of fire exclusion. Rapid changes to species composition and stand densities have been interpreted by some as evidence of high-severity fire. We demonstrate that the very different ecological process of fire exclusion can produce similar changes by shifting selective pressures from disturbance-mediated to productivity-mediated controls. Restoring disturbance-adapted species composition and structure to intermediate productivity forests may help to buffer them against projected increasing temperatures, lengthening fire seasons, and more frequent and prolonged moisture stress. Fewer management options are available to promote adaptation in forest assemblages historically constrained by underlying site productivity.

Evidence of fuels management and fire weather influencing fire severity in an extreme fire event.

Following changes in vegetation structure and pattern, along with a changing climate, large wildfire incidence has increased in forests throughout the western United States. Given this increase, there is great interest in whether fuels treatments and previous wildfire can alter fire severity patterns in large wildfires. We assessed the relative influence of previous fuels treatments (including wildfire), fire weather, vegetation, and water balance on fire-severity in the Rim Fire of 2013. We did this at three different spatial scales to investigate whether the influences on fire severity changed across scales. Both fuels treatments and previous low to moderate-severity wildfire reduced the prevalence of high-severity fire. In general, areas without recent fuels treatments and areas that previously burned at high severity tended to have a greater proportion of high-severity fire in the Rim Fire. Areas treated with prescribed fire, especially when combined with thinning, had the lowest proportions of high severity. The proportion of the landscape burned at high severity was most strongly influenced by fire weather and proportional area previously treated for fuels or burned by low to moderate severity wildfire. The proportion treated needed to effectively reduce the amount of high severity fire varied by spatial scale of analysis, with smaller spatial scales requiring a greater proportion treated to see an effect on fire severity. When moderate and high-severity fire encountered a previously treated area, fire severity was significantly reduced in the treated area relative to the adjacent untreated area. Our results show that fuels treatments and low to moderate-severity wildfire can reduce fire severity in a subsequent wildfire, even when burning under fire growth conditions. These results serve as further evidence that both fuels treatments and lower severity wildfire can increase forest resilience.

Repeated wildfires alter forest recovery of mixed-conifer ecosystems.

Most models project warmer and drier climates that will contribute to larger and more frequent wildfires. However, it remains unknown how repeated wildfires alter post-fire successional patterns and forest structure. Here, we test the hypothesis that the number of wildfires, as well as the order and severity of wildfire events interact to alter forest structure and vegetation recovery and implications for vegetation management. In 2014, we examined forest structure, composition, and tree regeneration in stands that burned 1-18 yr before a subsequent 2007 wildfire. Three important findings emerged: (1) Repeatedly burned forests had 15% less woody surface fuels and 31% lower tree seedling densities compared with forests that only experienced one recent wildfire. These repeatedly burned areas are recovering differently than sites burned once, which may lead to alternative ecosystem structure. (2) Order of burn severity (high followed by low severity compared with low followed by high severity) did influence forest characteristics. When low burn severity followed high, forests had 60% lower canopy closure and total basal area with 92% fewer tree seedlings than when high burn severity followed low. (3) Time between fires had no effect on most variables measured following the second fire except large woody fuels, canopy closure and tree seedling density. We conclude that repeatedly burned areas meet many vegetation management objectives of reduced fuel loads and moderate tree seedling densities. These differences in forest structure, composition, and tree regeneration have implications not only for the trajectories of these forests, but may reduce fire intensity and burn severity of subsequent wildfires and may be used in conjunction with future fire suppression tactics.

Incorporating Resource Protection Constraints in an Analysis of Landscape Fuel-Treatment Effectiveness in the Northern Sierra Nevada, CA, USA.

Finding novel ways to plan and implement landscape-level forest treatments that protect sensitive wildlife and other key ecosystem components, while also reducing the risk of large-scale, high-severity fires, can prove to be difficult. We examined alternative approaches to landscape-scale fuel-treatment design for the same landscape. These approaches included two different treatment scenarios generated from an optimization algorithm that reduces modeled fire spread across the landscape, one with resource-protection constrains and one without the same. We also included a treatment scenario that was the actual fuel-treatment network implemented, as well as a no-treatment scenario. For all the four scenarios, we modeled hazardous fire potential based on conditional burn probabilities, and projected fire emissions. Results demonstrate that in all the three active treatment scenarios, hazardous fire potential, fire area, and emissions were reduced by approximately 50 % relative to the untreated condition. Results depict that incorporation of constraints is more effective at reducing modeled fire outputs, possibly due to the greater aggregation of treatments, creating greater continuity of fuel-treatment blocks across the landscape. The implementation of fuel-treatment networks using different planning techniques that incorporate real-world constraints can reduce the risk of large problematic fires, allow for landscape-level heterogeneity that can provide necessary ecosystem services, create mixed forest stand structures on a landscape, and promote resilience in the uncertain future of climate change.

Comparing growth phenology of co-occurring deciduous and evergreen conifers exposed to drought.

Plant phenological events are influenced by climate factors such as temperature and rainfall. To evaluate phenological responses to water availability in a Spring Heath-Pine wood (Erico-Pinetum typicum), the focus of this study was to determine intra-annual dynamics of apical and lateral growth of co-occurring early successional Larix decidua and Pinus sylvestris and late successional Picea abies exposed to drought. The effect of reduced plant water availability on growth phenology was investigated by conducting a rainfall exclusion experiment. Timing of key phenological dates (onset, maximum rate, end, duration) of growth processes were compared among species at the rain-sheltered and control plot during 2011 and 2012. Shoot and needle elongation were monitored on lateral branches in the canopy at c. 16 m height and radial growth was recorded by automatic dendrometers at c. 1.3 m height of > 120 yr old trees. Different sequences in aboveground growth phenology were detected among the three species under the same growing conditions. While onset of radial growth in April through early May was considerably preceded by onset of needle growth in Larix decidua (5 - 6 weeks) and shoot growth in Pinus sylvestris (c. 3 weeks), it occurred quite simultaneously with onset of shoot growth in Picea abies. Low water availability had a minor impact on onset of aboveground growth, which is related to utilization of stored water, but caused premature cessation of aboveground growth. At the control plot mean growing season length was 130 days in Pinus sylvestris, 95 days in Larix decidua and 73 days in Picea abies supporting the hypothesis that early successional species are resource expenders, while late successional species are more efficient in utilizing resources and develop safer life strategies. High synchronicity found in culmination of radial growth in late spring (mid-May through early June) prior to occurrence of more favourable environmental conditions in summer might indicate sink competition for carbohydrates to belowground organs. This is supported by completion of apical growth in mid June in all species, except for needle growth of Pinus sylvestris, which lasted until early August. Phenological observations of conifers exposed to drought revealed that tree water status early during the growing season determines total annual aboveground growth and besides temperature, species-specific endogenous and/or environmental factors (most likely photoperiod and/or different threshold temperatures) are involved in controlling apical and lateral growth resumption after winter dormancy.

Wildfire loss of forest soil C and N: Do pre-fire treatments make a difference?

Losses of C and N from the forest floor and top 20-cm of soil were estimated following separate severe wildfires at two Long-Term Soil Productivity sites in the Sierra Nevada of California, USA. Experimental treatments applied 20 years prior to the wildfires included factorial combinations of 1) organic matter (OM) removal following clear-cut harvesting (SO, stem only harvest, WTH, whole-tree harvest, and WTH + FF, WTH plus the forest floor removal), 2) soil compaction (three levels of intensity), and 3) with and without understory vegetation control. Wildfires caused complete losses of the forest floor in all treatments and also oxidized varying portions of OM in the topsoil. As such, pre-fire forest floor measures were used as an estimate of forest floor C and N loss, and post-fire soil measures of C and N were compared to pre-fire soil data to estimate of mineral soil losses. Averaged over all treatments, the less-productive site that also had lesser accumulations of detritus (Wallace) lost 35.1 Mg C ha-1, or 25 % of its original C stores, while the more-productive site with greater detritus (Rogers) lost 18.4 Mg C ha-1, or 20 % of its original. The SO treatments that left harvest residue on site ended up with much greater losses of C: 36 % versus 15 and 17 % for WTH and WTH + FF, respectively. The SO also yielded the largest losses (25-30 %) of C in the top 10-cm of soil. The other treatments had smaller or inconsistent effects (understory vegetation control) or no effect (soil compaction). Our results suggest that potential benefits from SO by leaving residue on site to soil C and N accumulation can also be readily eliminated by wildfire which commonly occurs at these fire-prone forest ecosystems.

Tree resistance and recovery from drought mediated by multiple abiotic and biotic processes across a large geographic gradient.

Worldwide, increasing severity of droughts threatens to change forest ecosystem functioning and community structure. Understanding how forest resilience is determined by its two underlying components, resistance and recovery, will help elucidate the mechanisms of drought responses and help inform management strategies. However, drought responses are shaped by complex processes across different scales, including species-specific drought strategies, tree size, competition, local environmental conditions, and the intensity of the drought event. Here, we quantified the reduction in tree growth during drought (an inverse measure of drought resistance) and post-drought recovery for three montane conifers (Abies concolor, Pinus jeffreyi, and Pinus lambertiana) in California. We used tree ring analysis to quantify responses to drought events of varying intensity between 1895 and 2018 across a geographic climatic gradient, to examine the roles of tree size (DBH) and competition (tree density) in mediating drought responses. We found that years of more intense drought corresponded with larger growth reductions and recovery rates were lower following drought years where trees suffered larger reductions. We found little variation among species in their growth reductions during drought events, but significant differences among species in their recovery post-drought. Across the geographic gradient, trees in the driest locations were susceptible to large growth reductions, signaling either strong sensitivity to drought intensity or exposure to the most extreme drought conditions. These growth reductions were not always compensated for by higher recovery rates. We also found that larger trees were more susceptible to drought due to a steeper negative relationship between recovery rates and the intensity of growth reduction during the drought. Contrary to expectations, recovery rates following the most detrimental drought years were higher in denser forests. Our results demonstrate the importance of considering how factors at various spatial and temporal scales affect the different components of drought responses.

The Fire and Smoke Model Evaluation Experiment-A Plan for Integrated, Large Fire-Atmosphere Field Campaigns.

The Fire and Smoke Model Evaluation Experiment (FASMEE) is designed to collect integrated observations from large wildland fires and provide evaluation datasets for new models and operational systems. Wildland fire, smoke dispersion, and atmospheric chemistry models have become more sophisticated, and next-generation operational models will require evaluation datasets that are coordinated and comprehensive for their evaluation and advancement. Integrated measurements are required, including ground-based observations of fuels and fire behavior, estimates of fire-emitted heat and emissions fluxes, and observations of near-source micrometeorology, plume properties, smoke dispersion, and atmospheric chemistry. To address these requirements the FASMEE campaign design includes a study plan to guide the suite of required measurements in forested sites representative of many prescribed burning programs in the southeastern United States and increasingly common high-intensity fires in the western United States. Here we provide an overview of the proposed experiment and recommendations for key measurements. The FASMEE study provides a template for additional large-scale experimental campaigns to advance fire science and operational fire and smoke models.

A montane Mediterranean climate supports year-round photosynthesis and high forest biomass.

The mid-elevation forest of California's Sierra Nevada poses a bioclimatic paradox. Mid-elevation trees experience a montane Mediterranean climate, with near-freezing winter days and rain-free summers. The asynchrony between warmth and water input suggests low primary production, limited by photosynthetic dormancy in winter cold, and again in summer and early autumn with drought, yet this forest is characterized by tall trees and high biomass. We used eddy covariance in a mid-elevation Sierra stand to understand how winter cold and summer drought limit canopy photosynthesis and production. The trees exhibited canopy photosynthesis year-round. Trees avoided winter dormancy, and daytime CO2uptake continued despite a deep snowpack and near-freezing temperatures. Photosynthesis on sunny days continued at half of maximum rates when air temperature was 0 °C. Likewise, the vegetation avoided summer drought dormancy, and high rates of daytime CO2uptake and transpiration continued despite a 5-month period with only negligible water input. We attribute this drought avoidance to deep rooting and availability of deep soil water. Year-round photosynthesis helps explain the large biomass observed in the Sierra Nevada, and implies adaptive strategies that may contribute to the resiliency or vulnerability of Sierran vegetation to climate change.