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Numerical ranges over a certain family of finite fields were classified in 2016 by a team including our fifth author [5]. Soon afterward Ballico generalized these results to all finite fields and published some new results about the cardinality of the finite field numerical range [1,2]. In this paper we study the geometry of these finite fields using the boundary generating curve, first introduced by Kippenhahn in 1951 [8,9]. We restrict our study to square matrices of dimension 2, with at least one eigenvalue in F q 2 . [ABSTRACT FROM AUTHOR]

Fire exclusion was first implemented across large areas of California and other areas of western North America in the late 19th or early 20th centuries but few studies have investigated how forests in the early decades after this decision were impacted. Jeffrey pine-mixed conifer forests of northern Baja California, Mexico, offer an area where this can be examined since fire suppression did not begin until the early 1970's. We use repeated forest and fuel measurements taken over a 25-year period (1998–2023) and found significant changes in forest structure and fuel loads (increased tree density, fine fuel loads, large fuel loads, snag density, snag basal area) that together clearly demonstrate the cumulative effects of fire removal. Interestingly, Jeffrey pine became more dominant despite the lack of recent fire, which is counter to the shift towards more shade-tolerant tree species observed in many other fire-suppressed, frequent-fire forests in the western US. Although these changes in Baja California forests point towards increased fire hazard, they are still in relatively low hazard conditions compared to long-fire suppressed forests in the western US. Prescribed fire or managed wildfire could easily be applied to counter the increased fuel loads and tree densities detected in this work, without the need for mechanical manipulation. In addition to maintaining resilient forests, using fire will reduce the risk of losing the large astronomical observatory at this site to wildfire. Restoration and stewardship of resilient forest structures similar to those in Baja California is the only way forward to conserve similar forests of the Sierra Nevada, southern California mountains, and elsewhere in the western US. • Wildfires are impacting large areas of the western US forests. • Early fire suppression impacts are modest but are increasing hazards in the Sierra San Pedro Martir. • Prescribed fire can easily be used today in Baja California, Mexico, forests. • Prescribed fire can maintain high forest resiliency and protect the large telescope at this site. [ABSTRACT FROM AUTHOR]

As increasing wildfire activity puts pressure on wildland fire suppression resources both nationally and within the state of California, further development of programs and infrastructure that emphasize preventative fuels treatments, e.g. prescribed burning, is critical for mitigating the impacts of wildfire at large spatial scales. Among many factors that limit the use of prescribed fire, weather and fuel moisture conditions are among the most critical. We analyzed a 2-km gridded hourly surface weather dataset over a 23-yr period to explore the relationship between climatological trends and prescribed fire weather windows. Pairing this dataset with burn prescription parameters provided by experienced regional fire practitioners, we seek to identify the timing and extent of changes in weather-related opportunities for prescribed fire in two distinct geographic regions within California. We found an increasing trend in opportunities for prescribed fire use in Sonoma County, a representative coastal Mediterranean region of CA, and a decreasing trend in Plumas County, a montane region that extends through the Northern Sierra Nevada. Seasonally, we see more nuances—increased winter opportunities in both counties, as well as increased summer opportunities in Sonoma. Most notably, we see great variation spatially in the occurrence of suitable weather windows for prescribed burning. Fire management resource availability and air quality regulations further constrain burn windows. We observed a greater influence of these factors in Sonoma County vs. Plumas. Resource availability is the greatest constraint in the Summer and Fall, during wildfire season, and air quality regulations are a greater constraint in the Winter. Our findings provide information to decision-makers and regulators at the county and other government levels to more effectively support use of prescribed fire to achieve land management and fuels reduction goals. • Prescribed fire weather windows increased in the coastal area for the study period. • Prescribed fire weather windows decreased in the montane region for the study period. • Wintertime weather windows increased for both Plumas and Sonoma counties. • Resource availability and air quality regulations had a greater influence on prescribed burn windows in Sonoma County than in Plumas County. [ABSTRACT FROM AUTHOR]

Highlights • Landscape fuel treatments reduced wildfire severity in a natural experiment. • Treatments promoted post-fire seedling regeneration, particularly for firs. • Fir seedling densities were higher in areas with lower neighborhood fire severity. • The strategic fuel treatment network promoted both fire resistance and recovery. Abstract Strategically placed landscape area treatments (SPLATs) are landscape fuel reduction treatments designed to reduce fire severity across an entire landscape with only a fraction of the landscape treated. Though SPLATs have gained attention in scientific and policy arenas, they have rarely been empirically tested. This study takes advantage of a strategically placed landscape fuel treatment network that was implemented and monitored before being burned by a wildfire. We evaluated treatment efficacy in terms of resistance, defined here as the capacity to withstand disturbance, and recovery, defined here as regeneration following disturbance. We found that the treated landscape experienced lower fire severity than an adjacent control landscape: in the untreated control landscape, 26% of land area was burned with >90% basal area mortality, according to the remote-sensing-derived relative differenced Normalized Burn Ratio (RdNBR), while in the treated landscape only 11% burned at the same severity. This difference was despite greater pre-treatment fire risk in the treatment landscape, as indicated by FARSITE fire behavior modeling. At a more local scale, monitoring plots within the treatments themselves saw greater regeneration of conifer seedlings two years following the fire than plots outside the treatments. Mean seedling densities for all conifer species were 7.8 seedlings m−2 in treated plots and only 1.4 seedlings m−2 in control plots. These results indicate that SPLATs achieved their objective of increasing forest resistance and recovery. [ABSTRACT FROM AUTHOR]

Frequent-fire forests were historically characterized by lower tree density, a higher proportion of pine species, and greater within-stand spatial variability, compared to many contemporary forests where fire has been excluded. As a result, such forests are now increasingly unstable, prone to uncharacteristically severe wildfire or high levels of tree mortality in times of drought stress. While reducing tree density might help to restore resilience, thinning treatments are frequently seen as conflicting with management for other resources such as wildlife habitat, in part because standard thinning prescriptions don’t typically produce the degree of within-stand heterogeneity found in historical forests. In this study, we compare stand structures and heterogeneity produced by two different mechanical thinning treatments and in an unthinned control, all with or without prescribed fire as a follow-up treatment. The “high variability” thinning treatment was designed to produce the spatial variability once found in frequent fire forests and was based on historical data from nearby old-growth stands, while the “low variability” thinning treatment retained a similar number of trees but at a relatively even crown spacing. Stand averages and degree of variation for common forest metrics were calculated and values compared to a historical old-growth reference stand. Both thinning treatments reduced tree density and basal area, and shifted species composition towards historical values. Thinning treatments contained a deficit of trees in both the smallest (<25 cm) and largest (>80 cm) size classes, relative to historical conditions. The high variability thinning treatment increased forest structure variation more than the low variability thinning treatment for most measures and retained a broader distribution of canopy closure values across the treatment units. While prescribed fire also reduced stand density and increased the amount of within-stand heterogeneity (when delayed mortality was included), the magnitude was much less than that produced by thinning. Prescribed fire did not significantly reduce basal area or alter the species composition. Prescribed burning did significantly reduce surface fuel loads, while thinning alone had no effect for most fuel classes. Our results show that high variability thinning coupled with prescribed burning resulted in a forest better aligned with the conditions present in historical frequent-fire forests, which were known to be more resilient to both wildfire and drought. [ABSTRACT FROM AUTHOR]

Stand-replacing fire has profound ecological impacts in conifer forests, yet there is continued uncertainty over how best to describe the scale of stand-replacing effects within individual fires, and how these effects are changing over time. In forests where regeneration following stand-replacing fire depends on seed dispersal from surviving trees, the size and shape of stand-replacing patches are critical metrics that are difficult to describe and often overlooked. We used a novel, recently-developed metric that describes the amount of stand-replacing area within a given distance of a live-tree patch edge, in order to compare fires that may be otherwise similar in fire size or the percentage of stand-replacing effects. Specifically, we analyzed 477 fires in California pine, fir, and mixed-conifer forests between 1984 and 2015 and asked whether this metric, the stand-replacing decay coefficient (SDC), has changed over time, whether it is affected by fire management, and how it responds to extreme weather conditions at the time of the fire. Mean annual SDC became smaller over time (significantly so in the Sierra Nevada region), indicating that stand-replacing patches became larger and more regularly shaped. The decrease in SDC was particularly pronounced in the years since 2011. While SDC is correlated with percent high-severity, it is able to distinguish fires of comparable percent high-severity but different spatial pattern, with fires managed for suppression having smaller SDC than fires managed for resource benefit. Similarly, fires managed by the US Forest Service had smaller SDC than fires managed by the National Park Service. Fire weather also played an important role, with higher maximum temperatures generally associated with smaller SDC values. SDC is useful for comparing fires because it is associated with more conventional metrics such as percent high-severity, but also incorporates a measure of regeneration potential – distance to surviving trees at stand-replacement patch edges – which is a biological legacy that directly affects the resilience of forests to increasingly frequent and severe fire disturbances. We estimate that from 1984 to 2015, over 80,000 ha of forestland burned with stand-replacing effects greater than 120 m in from patch edges, denoting areas vulnerable to extended conifer forest loss due to dispersal limitation. Managing unplanned ignitions under less extreme weather conditions can achieve beneficial “fine-grained” effects of stand-replacing fire where regeneration limitation is less of a concern. Because SDC is a useful single metric to compare fires, we introduce a web application (stevensjt.shinyapps.io/sdc_app) to calculate SDC for any high-severity spatial layer that may be of interest. [ABSTRACT FROM AUTHOR]

• Large shifts in vegetation cover type were detected across the study area. • Landscape proportion identified as forest (dense canopy cover) increased by 12%. • The weighted mean patch size of forest vegetation increased from 59 to 871 ha. • Over 50% of areas identified as woodland and shrub cover experienced conversion. • Historical aerial imagery can be used to quantify landscape vegetation change. The North Bay area of California is a populous and ecologically diverse area that has experienced significant changes in the past century, as well as a series of recent wildfires, after over a century of fire suppression practices. While much research has been conducted quantifying drivers and patterns of vegetation change in conifer-dominated ecosystems, and how such changes have influenced current trends in fire behavior, studies of similar focus and scale are rarer in non-conifer ecosystems, including mixed-hardwood forests or shrub-dominated ecosystems in central and coastal Northern California. This is despite the fact that ecosystems other than conifer forests make up the majority of area burned in California wildfires. As such, expanding research focused on patterns of large-scale vegetation change as a possible driver of this trend in this area is a priority. In this study, we sought to map the overall extent and patch sizes of broad vegetation classes across a 52,000 ha study area based on historical (1948) and contemporary (2014) aerial imagery and to investigate shifts in vegetation patterns, as well as potential pathways and drivers of detected changes. We classified vegetation types through segmenting our imagery into homogenous polygons, and assigning broad vegetation categories using a random forest algorithm. We then analyzed patterns of change using spatial statistics and conditional inference tree analyses. We detected a large increase (12%) in the relative landscape proportion and average patch size of the forest class, characterized by dense tree canopy cover. Woodlands and shrub patches were most susceptible to type change, with the majority (57% and 65%, respectively) of converted areas subsequently identified as denser forest stands. By contrast, herbaceous and forest patches were most persistent. We additionally found that disturbance history, specifically whether an area burned or not, and topographic variables, including elevation and aspect, were important influences on the likelihood of vegetation persistence, while slope and water availability were not. Historical aerial imagery, which provides fine resolution and accurate data over a large spatial scale, is a useful tool for detecting landscape-scale vegetation shifts in ecosystems where widespread vegetation monitoring was not common historically. The marked increase in dense forest we detected, specifically due to the conversion of large areas of shrub and woodland vegetation may correspond to higher surface and ladder fuel load and continuity, and potentially higher wildfire risk. Fuel reduction treatments typically implemented in conifer-dominated forests may also be warranted in these mixed hardwood forests. However, more research is needed to understand drivers of change in non-conifer-dominated ecosystems in California and how such change influences wildfire behavior. [ABSTRACT FROM AUTHOR]

Studies of historical fire and vegetation conditions in dry conifer forests have demonstrated a high degree of heterogeneity across landscapes. However, there is a limit to the amount of inference that can be drawn from historical fire reconstructions. Contemporary “reference” landscapes may be able to provide information that is not available from historical reconstructions. In this study, we characterized variability in vegetation structure and composition across two Sierra Nevada landscapes with long-established fire restoration programs. We used tree, shrub, and surface fuel data from 117 initial plots, 86 of which were re-measured 8–12 years later, to identify the mechanisms driving variability in vegetation and fuel conditions. Our analyses identified nine distinct vegetation groups, with mean live tree basal area and density ranging from 0.3 to 72.7 m 2 ha −1 and 2.5 to 620 trees ha −1 for individual groups. For all plots combined, mean live tree basal area and density was 28.4 m 2 ha −1 and 215 trees ha −1 , but standard deviations (SD) were 29.1 m 2 ha −1 and 182 trees ha −1 , respectively. These ranges and SDs demonstrate considerable variability in vegetation structure, which was partially related to site productivity and previous fire severity. Fine surface fuel loads were generally low (overall mean, 16.1 Mg ha −1 ), but also exhibited high variability (SD, 12.6 Mg ha −1 ). Surprisingly, surface fuel loads based on initial measurement and change between measurements were not related to fire characteristics. The only statistical relationship found was that surface fuel loads were associated with forest structure and composition. These results capture a contemporary ‘natural’ range of variability and can be used to guide landscape-level restoration efforts. More specifically, these results can help identify distinct targets for variable forest structures across landscapes. [ABSTRACT FROM AUTHOR]

Increasingly, objectives for forests with moderate- or mixed-severity fire regimes are to restore successionally diverse landscapes that are resistant and resilient to current and future stressors. Maintaining native species and characteristic processes requires this successional diversity, but methods to achieve it are poorly explained in the literature. In the Inland Pacific US, large, old, early seral trees were a key historical feature of many young and old forest successional patches, especially where fires frequently occurred. Large, old trees are naturally fire-tolerant, but today are often threatened by dense understory cohorts that create fuel ladders that alter likely post-fire successional pathways. Reducing these understories can contribute to resistance by creating conditions where canopy trees will survive disturbances and climatic stressors; these survivors are important seed sources, soil protectors, and critical habitat elements. Historical timber harvesting has skewed tree size and age class distributions, created hard edges, and altered native patch sizes. Manipulating these altered forests to promote development of larger patches of older, larger, and more widely-spaced trees with diverse understories will increase landscape resistance to severe fires, and enhance wildlife habitat for underrepresented conditions. Closed-canopy, multi-layered patches that develop in hot, dry summer environments are vulnerable to droughts, and they increase landscape vulnerability to insect outbreaks and severe wildfires. These same patches provide habitat for species such as the northern spotted owl, which has benefited from increased habitat area. Regional and local planning will be critical for gauging risks, evaluating trade-offs, and restoring dynamics that can support these and other species. The goal will be to manage for heterogeneous landscapes that include variably-sized patches of (1) young, middle-aged, and old, closed-canopy forests growing in upper montane, northerly aspect, and valley bottom settings, (2) a similar diversity of open-canopy, fire-tolerant patches growing on ridgetops, southerly aspects, and lower montane settings, and (3) significant montane chaparral and grassland areas. Tools to achieve this goal include managed wildfire, prescribed burning, and variable density thinning at small to large scales. Specifics on “how much and where?” will vary according to physiographic, topographic and historical templates, and regulatory requirements, and be determined by means of a socio-ecological process. [ABSTRACT FROM AUTHOR]

The balance between ecosystem emissions of carbon to the atmosphere and removals from the atmosphere indicates whether ecosystems are exacerbating or reducing climate change. Forest ecosystems in the State of California, USA, contain carbon that reaches the highest densities (mass per unit area) in the world, but it has been unresolved whether California ecosystems currently comprise a net sink or source of carbon. The California Global Warming Solutions Act of 2006 established greenhouse gas reduction targets for fossil fuel-burning sectors and ecosystems, underscoring the importance of tracking ecosystem carbon. Here, we conduct statewide spatial inventories of the aboveground live carbon stocks of forests and other terrestrial ecosystems of California, excluding agricultural and urban areas. We analyzed biomass data from field measurements of the Forest Inventory and Analysis program, published biomass information and remote sensing data on non-forest vegetation, and spatial distributions of vegetation types, height, and fractional cover derived by the Landfire program from Landsat remote sensing at 30 m spatial resolution. We conducted Monte Carlo analyses of the uncertainty of carbon stock change estimates from errors in tree biomass estimates, remote sensing, and estimates of the carbon fraction of biomass. The carbon stock in aboveground biomass was 850 ± 230 Tg (mean ± 95% confidence interval) in 2010. We found a net aboveground live carbon stock change of −69 ± 15 Tg from 2001 to 2010, a rate of change of −0.8 ± 0.2% y − 1 . Due to slow decay of some dead wood, all of the live carbon stock change does not immediately generate emissions. Wildfires on 6% of the state analysis area produced two-thirds of the live carbon stock loss. This suggests that increased tree densities from a century of fire suppression have allowed the accumulation of fuel for carbon losses in recent wildfires. Remote sensing errors in vegetation classification accounted for most of the uncertainty in the carbon stock change estimates. Improvements are also needed to track spatial patterns of growth and dead wood. Our results establish the beginning of a time series for the state greenhouse gas inventory and provide information on the role of forest conservation and management in California in mitigating global climate change. [ABSTRACT FROM AUTHOR]

The 2013 Rim Fire, originating on Forest Service land, burned into old-growth forests within Yosemite National Park with relatively restored frequent-fire regimes (≥2 predominantly low and moderate severity burns within the last 35 years). Forest structure and fuels data were collected in the field 3-4 years before the fire, providing a rare chance to use pre-existing plot data to analyze fire effects. We used regression tree and random forests analysis to examine the influence of forest structure, fuel, fire history, topographic and weather conditions on observed fire severity in the Rim Fire, as estimated from an initial fire severity assessment based on the relative differenced normalized burn ratio (RdNBR). Plots that burned on days with strong plume activity experienced moderate- to high-severity fire effects regardless of forest conditions, fire history or topography. Fire severity was also highly negatively associated with elevation, with lower severity observed in plots over 1700 m. Burning index (a composite index of fire weather), time since the last fire, and shrub cover had strong positive associations with fire severity. Plots that had experienced fire within the last 14 years burned mainly at low severity in the Rim Fire, while plots that exceeded that time since last fire tended to burn at moderate or high severity. This effect of time since last fire was even more pronounced on days when the burning index was high. Our results suggest that wildfire burning under extreme weather conditions, as is often the case with fires that escape initial attack, can produce large areas of high-severity fire even in fuels-reduced forests with restored fire regimes. [ABSTRACT FROM AUTHOR]

• Most western US frequent-fire forests have been changed by attempted fire exclusion. • Drought and bark beetle tree mortality can exacerbate wildfire behavior and effects. • Dead biomass and tree densities the most important variables predicting fire severity. • Mass fire behavior cannot be predicted by operational fire models. • Forest restoration needed at necessary paces and scales to prevent adverse wildfires. Large, severe wildfires continue to burn in frequent-fire adapted forests but the mechanisms that contribute to them and their predictability are important questions. Using a combination of ground based and remotely sensed data we analyzed the behavior and patterns of the 2020 Creek Fire where drought and bark beetles had previously created substantial levels of tree mortality in the southern Sierra Nevada. We found that dead biomass and live tree densities were the most important variables predicting fire severity; high severity fire encompassed 41% of the area and the largest high severity patch (19,592 ha) comprised 13% of total area burned. Areas with the highest amounts of dead biomass and live tree densities were also positively related to high severity fire patch size indicating that larger, more homogenous conditions of this forest characteristic resulted in adverse, landscape-scale fire effects. The first two days of the Creek Fire were abnormally hot and dry but weather during the days of the greatest fire growth was largely within the normal range of variation for that time of year with one day with lower windspeeds. From September 5 to 8th the fire burned almost 50% of its entire area and fire intensity patterns inferred from remotely sensed brightness-temperature data were typical except on September 6th when heat increased towards the interior of the fire. Not only was the greatest heat concentrated away from the fire perimeter, but a significant amount of heat was still being generated within the fire perimeter from the previous day. This is a classic pattern for a mass fire and the high amount of dead biomass created from the drought and bark beetles along with high live tree densities were critical factors in developing mass fire behavior. Operational fire behavior models were not able to predict this behavior largely because they do not include post-frontal combustion and fire-atmosphere interactions. An important question regarding this mass fire is if the tree mortality event that preceded it could have been avoided or reduced or was it within the natural range of variation for these forests? We found that the mortality episode was outside of historical analogs and was exacerbated by past management decisions. The Creek Fire shows us how vulnerable of our current frequent-fire forest conditions are to suffering high tree mortality and offering fuel conditions capable of generating mass fires from which future forest recovery is questionable because of type conversion and probable reoccurring high severity fire. [ABSTRACT FROM AUTHOR]

Highlights: [•] We model impacts of a completed fuel treatment network cross a 20,000ha landscape. [•] The treatment network substantially reduced in hazardous fire potential. [•] Tree ingrowth levels strongly affected hazardous fire potential over time. [•] Additional treatments may be necessary to mitigate growing fire hazard over time. [Copyright &y& Elsevier]

Highlights: [•] We assess changes in forest spatial structure following fire exclusion and logging. [•] The historical forest had greater structural heterogeneity. [•] The contemporary forest lacked gaps, and was dominated by large, dense tree groups. [•] This homogenization of structure is likely associated with a loss of resilience. [•] Spatial measures are presented that may be applied to forest restoration treatments. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the longevity of fuel treatments in terms of their ability to maintain fire behavior and effects within a desired range is an important question. The objective of this study was to determine how fuels, forest structure, and predicted fire behavior changed 7-years after initial treatments. Three different treatments: mechanical only, mechanical plus fire, and prescribed fire only, as well as untreated control, were each randomly applied to 3 of 12 experimental units. Many aspects of the initial fuel treatments changed in 7 years. The overall hazard of the control units increased significantly indicating continued passive management has further increased already high fire hazards. Mechanical only fire hazard decreased after 7 years and are now similar to the two fire treatments, which both maintained low hazards throughout the study. Tree density declined significantly 7 years after the initial fire only treatments, while basal area in both fire treatments was unchanged relative to immediate post-treatment conditions. Our findings indicating reduced fire hazard over time in mechanical only treatments might provide an opportunity for a staggered treatment schedule that included prescribed fire which could increase overall treatment longevity to approximately 20years. Changes in our mixed conifer forests after fuel treatment were generally larger than those reported from ponderosa pine forests in the Rocky Mountains. [Copyright &y& Elsevier]

Abstract: The cultural practices associated with Euro-American settlement in the United States have altered forest structure and ultimately changed fundamental ecosystem processes. Coarse woody debris (CWD) and canopy cover are recognized as having great importance for many wildlife species and ecological processes. Little information is available from forests on historical levels of canopy cover and CWD before European settlement. A great deal of uncertainty exists concerning the long-term role of fire and the dynamics of CWD, especially in forests that once experienced frequent, low-moderate intensity fire regimes. The objective of this study was to quantify CWD and forest canopy cover in an area where harvesting has never occurred and limited fire suppression began in the 1970s. This study was done in Jeffrey pine-mixed conifer forests in the Sierra San Pedro Martir (SSPM) in northwestern Mexico. Canopy cover, canopy closure, and CWD were sampled on a grid of plots. Average canopy cover was 26.8%, average canopy closure was 40.1%. A total of 102 CWD pieces were measured, and nearly half of the plots (45.7%) had no CWD present. Average CWD density, percent cover, volume, and weight were 108piecesha−1, 1.5%, 47.5m3 ha−1, and 15.7tonnesha−1, respectively. All of the CWD sampled were in the later stages of decay. Less than average values for CWD density, percent cover, volume, and weight were recorded in 57%, 64%, 67%, and 69% of the plots, respectively. CWD dynamics in forests that experience frequent, low-moderate intensity fires are fundamentally different than those having long-interval, high-severity fires. There was a large amount of variability in all CWD and forest canopy cover measurements taken from Jeffrey pine-mixed conifer forests in the SSPM. Spatial heterogeneity in forest structure should be included in the desired conditions of xeric, pine-dominated forests in the United States that once experienced frequent, low-moderate intensity fire regimes. It should be noted that heterogeneity by itself may not lead to sustainable forests unless that heterogeneity includes stand structures that are resistant/resilient to high-severity fire, drought, insects, and disease. [Copyright &y& Elsevier]

Abstract: The widespread attention that has been devoted to wildfires by the public, as well as by state and federal governments, over the last several years in the United States has created a demand for fuel reduction activities aimed at alleviating wildfire hazard. While the appropriateness of these fuel reduction activities has been discussed in detail in previous studies, only a few studies have experimentally examined the effects of fuel reduction on forests. This paper investigates the initial effects of three different fuel reduction strategies on forest structure and understory plant communities using replicated treatments, which are compared to untreated controls. Understory plants are grouped by plant growth form (shrub, forb, graminoid) and by plant origin (native, exotic). The effects of each treatment alternative: mechanical, prescribed fire, mechanical followed by prescribed fire, and untreated control, are reported for each plant group. Each fuel treatment modified forest structure such that growing space increased and allowed for rapid reestablishment of forbs and graminoids, which did not differ in abundance from pre-treatment levels. The mechanical only treatments (thinning from below and rotary mastication) significantly reduced shrub cover relative to the control, however mechanical plus fire and fire only treatments did not. Mechanical plus fire treatments altered forest structure most substantially, which may explain the observed increases in richness and cover of exotic species. However, the magnitude of these differences was small. Both treatments involving fire decreased native species richness significantly, but differences in native species cover were insignificant for any of the active treatments. These results demonstrate a relatively high degree of resilience in these Sierra Nevada mixed conifer understory communities, at least initially, to fuel reduction activities. [Copyright &y& Elsevier]

• With changing climate, resilience has become a management objective but is difficult to quantify. • Minimal competition supports vigorous tree growth which may have been associated with resilience. • Using the stand density index, we show historical forests were largely free of competition. • In contrast, most contemporary forests are in full competition or imminent mortality. • Resilience in frequent-fire forests may require significantly reducing tree densities. With the increasing frequency and severity of altered disturbance regimes in dry, western U.S. forests, treatments promoting resilience have become a management objective but have been difficult to define or operationalize. Many reconstruction studies of these forests when they had active fire regimes have documented very low tree densities before the onset of fire suppression. Building on ecological theory and recent studies, we suggest that this historic forest structure promoted resilience by minimizing competition which in turn supported vigorous tree growth. To assess these historic conditions for management practices, we calculated a widely-used measure of competition, relative stand density index (SDI), for two extensive historical datasets and compared those to contemporary forest conditions. Between 1911 and 2011, tree densities on average increased by six to seven fold while average tree size was reduced by 50%. Relative SDI for historical forests was 23–28% of maximum, in the ranges considered 'free of' (<25%) to 'low' competition (25–34%). In contrast, most (82–95%) contemporary stands were in the range of 'full competition' (35–59%) or 'imminent mortality' (≥60%). Historical relative SDI values suggest that treatments for restoring forest resilience may need to be much more intensive then the current focus on fuels reduction. With the contemporary increase in compounding stresses such as drought, bark beetles, and high-severity wildfire, resilience in frequent-fire forests may hinge on creating stands with significantly lower densities and minimal competition. Current management practices often prescribe conditions that maintain full competition to guide development of desired forest conditions. Creating stands largely free of competition would require a fundamental rethinking of how frequent-fire forests can be managed for resilience. [ABSTRACT FROM AUTHOR]

• Fuel treatments have lasting impacts on forest structure and drought resistance. • Thinning treatments effectively met and maintained structural objectives. • Positive relationship between snag basal area, time, and woody fuel loads. • Tree ring data indicates thinning treatments improve drought resistance. Sierra Nevada mixed-conifer forests have undergone significant changes in structure and composition and are increasingly vulnerable to altered disturbance regimes and climate-related extreme events. Fuel reduction treatments, including thinning and follow-up surface fuel treatments, can reduce this vulnerability by creating forest structural and woody fuel conditions that not only allow forest stands to mitigate wildfire, but also alleviate individual tree stress. However, direct observations that quantify these longer-term effects are lacking. This study compares observed changes in forest structure, tree species composition, and downed woody fuel loads across three distinct time periods: pre-treatment, 1 yr post-treatment, and 10 yr post-treatment. Additionally, using tree ring data, we assessed whether treatments affected individual tree resistance to a severe statewide drought (2012–2015). Thinning treatments were able to effectively reduce tree density and basal area, increase the retention of both larger-sized and shade-intolerant trees, and mitigate tree mortality. Treatments were also associated with significantly lower coarse woody fuel and snag basal area. Snag basal area and time since treatment were related to the accumulation of fine and coarse woody surface fuel loads. Tree ring information indicated that treatments improved drought resistance as well, especially in units with lower residual live basal area. This study complements previous studies on fuel reduction thinning by demonstrating that these treatments have lasting effects on forest structure, which also confers a degree of drought resistance. [ABSTRACT FROM AUTHOR]

• Post-fire forest management paradigm heavily affects mixed-conifer regeneration. • Passive post fire management can cause conversion of conifer forests to shurblands. • Replanting burnt areas is more successful when competing vegetation is controlled. Understanding forest regeneration in the wake of large-scale wildfire events is critically important because these disturbances are expected to occur more frequently given future climate projections. While the impacts of individual management prescriptions on prevention, mitigation, and response to severe fire events have been studied, the influence of property ownership on their implementation and success has received less attention. The objective of this study was to compare how the management practices of two common US forestland owners- public (U.S. Forest Service) and a private forest resource company- influenced forest regeneration following a 26,000 ha wildfire in the northern Sierra Nevada. Spectral unmixing was used to track revegetation for 11 years following a 2007 wildfire. Classified vegetation maps were field validated and generated using remotely sensed imagery for the 2007 (pre-fire) and 2018 timepoints to track landcover transitions. Public ownership within the fire perimeter was the majority at 18,760 ha, while private ownership accounted for 7617 ha. Significant differences in forest regeneration were found with vegetation establishment on publicly owned lands occurring at twice the rate of their privately owned counterpart. However, by 2018 over half (10,062 ha) of publicly owned lands converted from forest (pre-fire) to a shrub-dominated land-cover type while only 2.2% (122 ha) of privately owned lands did so. Additionally, only 1% (249 ha) of publicly owned lands were characterized by young regenerating conifer forests, whereas approximately 70% (3875 ha) of privately owned lands were characterized as such. These results demonstrate a strong contrast in post-fire vegetation regeneration that will likely persist for many decades into the future. The implications of this contrast significantly impact the ecosystem services these forests provide, as well as future disturbance potential. [ABSTRACT FROM AUTHOR]

• Studied first-entry fires following long-term fire suppression, Sierra Nevada, USA. • Examined forest opening & tree clump structure patterns using airborne lidar data. • Compared burns to frequent-fire reference areas and control areas outside fires. • Moderate-severity fires replicated resilient structures of reference areas. • Proportional loss of taller trees with greater burn severity. A century of fire suppression has left fire-dependent forests of the western United States increasingly vulnerable to wildfire, drought, and insects. Forest managers are trying to improve resilience using treatments such as mechanical thinning and prescribed fire; however, operational and resource constraints limit treatments to a fraction of the needed area each year. An alternative is to let wildfires burn under less-than-extreme fire weather where human lives and infrastructure are not at risk. We examined post-fire forest structure using airborne lidar data to determine whether a single wildland fire following an extended fire-free period could produce forest structures resembling fire-resilient historical conditions. We studied forest structures resulting from these "first-entry" fires in a forest with a history of timber management (2008 American River Complex Fires, Tahoe National Forest) and in a wilderness area (2009 Big Meadow Fire, Yosemite National Park). We compared the results of these first-entry fires with nearby reference areas that had experienced 2+ fires that burned predominately at low- and moderate-severity. We identified visible overstory trees from the lidar data and examined their patterns in terms of individuals, tree clumps, and openings. We found that moderate-severity fire effects in these first-entry fires produced similar patterns to the reference areas with area in openings at approximately 40% and trees predominately in small (2 to 4 trees) and medium (5–9 trees) clumps High-severity fire produced mortality likely to lead to large canopy openings that were historically uncharacteristic in these forests. As burn severity increased, the amount of the residual canopy area represented by taller trees (>16 m and especially >32 m) decreased, which could result from fires preferentially killing taller trees or from locations with taller trees more commonly experiencing lower burn severities. Our study suggests that first-entry fires allowed to burn under less-than-extreme conditions can reproduce spatial patterns resembling historical conditions resilient to fires and drought but possibly at the disproportionate expense of larger trees. [ABSTRACT FROM AUTHOR]

Abstract: SAR studies to improve the selectivity and metabolic stability of a class of recently discovered MMP-13 inhibitors are reported. Improved selectivity was achieved by modifying interactions with the S1′ pocket. Metabolic stability was improved through reduction of inhibitor lipophilicity. This translated into lower in vivo clearance for the preferred compound. [ABSTRACT FROM AUTHOR]

Abstract: Potent, highly selective and orally-bioavailable MMP-13 inhibitors have been identified based upon a (pyridin-4-yl)-2H-tetrazole scaffold. Co-crystal structure analysis revealed that the inhibitors bind at the active site pocket and are not ligands for the catalytic zinc atom. Compound 29b demonstrated reduction of cartilage degradation biomarker (TIINE) levels associated with cartilage protection in a preclinical rat osteoarthritis model. [Copyright &y& Elsevier]

Abstract: Discovery and optimization of potency and selectivity of a non-Zn-chelating MMP-13 inhibitor with the aid of protein co-crystal structural information is reported. This inhibitor was observed to have a binding mode distinct from previously published MMP-13 inhibitors. Potency and selectivity were improved by extending the hit structure out from the active site into the S1′ pocket. [Copyright &y& Elsevier]