For many conifer species in dry conifer forests of North America, seeds must be present for postfire regeneration to occur, suggesting that seed dispersal from surviving trees plays a critical role in postfire forest recovery. However, the application of tree fecundity and spatial arrangement to postfire conifer recovery predictions have only recently become more common, and is often included at relatively coarse scales (i.e., 30 meters). In this study, we mapped surviving trees using lidar and created a spatially explicit estimate of seed density (seed shadows) with 10 m, 50 m, and 100 m median dispersal distances. We estimated the number of seeds produced by each tree using allometric relationships between tree size and fecundity. Along with the seed shadows, we used a suite of topographic variables as inputs to negative binomial hurdle models to predict conifer seedling abundance in 131 plots following the 2018 Carr Fire in northern California, USA. We compared models using each of the seed shadows to each other as well as to a model using the distance to the nearest surviving tree, which served as a baseline. All model formulations indicated that estimated seed availability was positively associated with conifer regeneration. Despite the importance of seed availability plays in regeneration and the substantial differences in seed availability represented by the different seed shadows in our analysis, we found surprisingly little difference in model performance regardless of which seed shadow was used. However, the models employing seed shadows outperformed the models with distance to the nearest live tree. Although we have demonstrated a modest improvement in predicting postfire conifer regeneration, the uncertainty in our results highlights the importance of tree detection and classification in future studies of this kind. Future studies may find it useful to consider other factors such as predation, site suitability, and seed mortality as potential drivers of discrepancies between total and realized dispersal kernels.

Background Within California’s chaparral ecosystems, fuel reduction treatments are commonly used to reduce the negative impacts of wildfire but the durability of fuel treatment changes to fuels and vegetation when exposed to wildfire is less well understood. This study examined the interactive effects of 15-year-old fuel treatments and an extreme wildfire on burn severity, fuel loading, and vegetation in chaparral and oak vegetation types in Whiskeytown National Recreation Area in northern California, USA. Fuel treatment types included hand thinned, mechanical mastication, mechanical mastication + prescribed burning, and prescribed burning only. Results Vegetation and substrate burn severity was characterized as moderate across the study site and did not differ among treatments. Contrasting with higher pre-fire shrub density in the mastication + burning treatment, 2-year post-fire live shrub density did not differ among treatments. Higher pre-fire fine woody fuel loading in the mastication treatment did not correspond to post-fire fuel loading among treatments, while the hand thinned treatment was the only treatment where fine fuel loading was not significantly reduced post-fire. Total plant species richness increased in all treatment types following wildfire, largely driven by an increase in exotic species. Native cover decreased, and exotic cover increased in oak and chaparral types, but greater exotic species cover in the mastication + burning treatment in chaparral was maintained following wildfire. Conclusions Pre-fire differences in fuel and vegetation responses among treatments largely did not persist or were not detectible 1 to 2 years following wildfire. These findings suggest that the extreme wildfire conditions superseded long-term treatment differences in many fuel and vegetation metrics observed prior to wildfire. Despite subtle treatment differences, the hand thinned treatment resulted in the lowest change in fuel loading relative to all other treatments. Lastly, pre-fire differences in exotic species among fuel treatments were retained following wildfire, suggesting some treatments may have greater potential for exotic species expansion or type conversion to exotic grasslands.

The invasion, or “encroachment,” of native conifers commonly occurs in the absence of frequent fire in deciduous woodlands and grasslands of the Pacific Northwest, USA. To effectively target restoration activities, managers require a better understanding of the outcomes of prescribed fire and the spatial patterns of conifer invasions. We examined the duration of prescribed fire effectiveness for managing conifer invasions, as well as multiple site characteristics (including distance to potential seed trees, prescribed fire history, and topographic variables) that influenced conifer invasions following fire in grassland and oak woodland communities in the Bald Hills of Redwood National Park, California. Prescribed fire substantially reduced counts of small conifers (<0.91 m in height), but reinvasion was rapid for sites ≤75 m from the forest edge, returning to pre‐fire levels by 2 years post‐fire. Following prescribed fires, the presence of conifers was largely determined by the proximity of overstory trees, with more than 95% of conifer seedlings (stems <1.37 m in height) found within 44 m of an overstory conifer. Number of fires and years since the most recent fire were not strongly related to counts of conifer seedlings and density of conifer saplings (stems from 0.1 to 10 cm diameter at breast height, 1.37 m). Our results suggest that in the Bald Hills vulnerability to conifer invasion is principally a function of proximity to seed sources, and the frequent application of prescribed fire or surrogate treatments are needed to prevent conifer seedlings from attaining fire‐resistant sizes.

Aims Fuel reduction treatments are broadly implemented to reduce the risk of extreme wildfire, but research on the long-term effectiveness and impacts of these treatments is lacking. In this study, we examined short- and long-term (two and 15 years) changes in fuels and understorey vegetation after treatment in chaparral and oak-dominated stands. Location Whiskeytown National Recreation Area, California, USA. Methods Treatments, including hand thinning, mastication only, burn only, and mastication + burning, were applied randomly to one or two units within each of 10 blocks across two vegetation types (chaparral and oak-dominated). Vegetation data were initially measured two years after treatment and remeasured 15 years later with additional sampling of dead surface fuels and shrubs. Fuel and vegetation data were analyzed to examine the effect of treatment, time, and vegetation type. Results In chaparral stands, shrub height and cover in the hand-thinning, mastication, and mastication + burning treatments were still lower than in the control after 15 years. However, only the hand-thinning treatment reduced fine woody fuels. Hand thinning and mastication + burning increased native species richness after two years, but this was also associated with an increase in exotic species richness and cover that persisted after 15 years. In oak-dominated stands, treatments had varied and relatively fewer changes to fuels and vegetation. Shrub height was reduced in both the hand-thinning and Burn-only treatments, but only mastication reduced shrub cover. Species richness and vegetative cover were largely unaffected by treatment in oak-dominated stands, except for lower native plant cover in mastication and mastication + burning treatments. Conclusion Treatments varied in their level of effectiveness and most involved trade-offs between their impacts on fuels and vegetation responses that differed by vegetation type and time. Our findings provide insights for managers interested in balancing these trade-offs when making fuel treatment decisions and emphasize the importance of examining longer-term effectiveness of fuels treatments.

The prolonged absence of fire in Oregon white oak (Quercus garryana) woodlands and savannas of the Pacific Northwest has resulted in substantial conifer encroachment over the past century. Use of low intensity prescribed burns often lacks sufficient intensity to kill larger encroached trees, requiring alternative approaches. In the Bald Hills region of Redwood National Park, managers have implemented girdling treatments to kill Douglas fir (Pseudotsuga menziesii) over the past two decades with the objective of recovering remnant oaks and restoring historical woodland area. We surveyed 258 girdled Douglas-fir to examine the effectiveness of girdling treatments to create snags and the impacts of girdling on tree regeneration and fuel recruitment over a 17 year time since girdling chronosequence. Girdling was successful in killing 91% of the treated Douglas-fir independent of the method used (axe or chainsaw). Larger trees with a low girdle width-to-tree diameter ratio tended to survive girdling. Trees with a girdle width-to-tree diameter ratio >= 1 were most effective at killing trees. Snags generated through girdling decayed over time but did not significantly reduce in height over the time period examined. Bird activity was observed on 19% of snags across the chronosequence but 88% of 10 y old snags had signs of bird foraging. Fine woody fuel loading and fuelbed depths were potentially elevated 7 years after treatment but did not persist through the time period examined. Regeneration of Douglas-fir and oaks were highly variable and did not seem to be clearly linked to girdling activity. Our results indicate that girdling is a highly effective treatment to kill Douglas-fir and aid restoration of Oregon oak woodlands with limited negative impacts on surface fuel hazards or regeneration.

Smoldering fires are common in deep forest floor fuels in long-unburned forests. These "duff fires" that burn through lower forest floor horizons (Oe and Oa) are often implicated in postfire stress and mortality of large conifers, long-duration soil heating, and noxious emissions. We present data on duff ignition and consumption patterns from burn studies in long-unburned Pinus palustris forests in northern Florida, USA. In small experimental and large operational-scale burns, ignition probability and duff consumption were greatest near tree bases, despite moisture condition or the presence of ignition vectors (pine cones and woody fuels). Upper duff (Oe) horizons dried throughout the 45 days of small-scale experiments, whereas lower duff (Oa) moisture was more constant. Duff adjacent to trees, however, was deeper and typically drier than beyond duff mounds. Oa horizons were least dense at tree bases, whereas Oe bulk density did not differ with tree proximity. Our results highlight the propensity for duff to ignite at the base of mature longleaf pine trees and significant basal duff consumption to occur at moisture contents <60%. Restoring fire to long-unburned coniferous forests may be a challenge where basal duff has accumulated and will require special attention to forest floor moisture to minimize negative effects.

California deciduous oak woodlands provide many ecological, cultural, and economic benefits, and often represent unique plant communities that harbor native rare and declining species. Oak woodlands have suffered substantial losses in area and ecological integrity in the post-settlement era due to land conversion and widespread fire exclusion. Remnant oak woodlands in many areas are undergoing further conversion to conifer forest as shadetolerant, and often less fire-tolerant species invade and increase in abundance. This process, known as conifer encroachment, has been identified across the Pacific West; efforts to restore these ecosystems have increased in California over the past several decades. The process of conifer encroachment is known to occur in many ecosystems in California, but principally affects oak woodlands dominated or co-dominated by Oregon white oak (Quercus garryana) and California black oak (Quercus kelloggii). Encroachment proceeds through four phases including establishment, piercing, overtopping, and decadent; oak crown recession occurs by the third stage, after which oak mortality becomes abundant. The concomitant increased shading and needlecast from the conifers diminishes plant and animal biodiversity and ecosystem services, and alters fire regimes. We discuss encroached oak woodland structure and dynamics in northern California, identifying conifer species that have the capability and propensity under a fire-suppression management regime to invade and degrade remnant oak woodlands.

Fire behavior and effects in forests and woodlands are influenced by surface fuels and senesced leaf litter in particular. We have known that species exhibit differential flammability for some time, but isolated efforts have often attributed differences to disparate mechanisms. Recent research has expanded the diversity of species evaluated, clarified patterns at the fuelbed level, and provided evidence that the physical and chemical traits of litter or fuelbeds drive flammability. To date, little effort has focused on uniting methods, clarifying the awkward terminology, or, perhaps most importantly, comparing laboratory findings to field observations of fire behavior. Here, we review recent literature and synthesize findings on what we know about the flammability of litter and propose future research directions.

The ignition and combustion of forest floor duff are poorly understood yet have been linked to soil heating and overstory tree mortality in many temperate coniferous forests. Research to date has focused on the characteristics of duff that facilitate ignition and spread, including fuel moisture, mineral content, and depth. Field observations suggest that the presence of pine cones on and within the forest floor might facilitate ignition of intermixed forest floor fuels. We investigated the effect of cone fuel additions on the ignition of underlying forest floor from fuels collected in long-unburned longleaf pine (Pinus palustris Mill.) forests in northern Florida, USA. Fuels were wetted to threshold gravimetric moisture contents to evaluate the relative effect on ignition. In stark contrast to fuel beds without cones, in which duff ignition only occurred in 17% of samples, those with cones added ignited the underlying duff 94% of the time. Flame heights were 40% taller and flaming duration was 47% longer in fuel beds with cones. Where present, pine cones act as vectors of ignition for forest floor fuels, and their role in fires deserves more attention to enhance our understanding of forest floor combustion.

Duff fires have been implicated in overstory mortality and soil heating in long-unburned pine forests. In the South's punctuated climate, duff moisture can change rapidly, falling below moisture thresholds that protect trees or increasing after brief downpours. To date, managers lack an instantaneous measure of duff moisture, a hurdle to the implementation of prescribed burns. Here, we evaluate a low-cost tool, the Campbell Scientific Duff Moisture Meter (DMM) 600 (Campbell Scientific, Logan, UT), to estimate duff moisture content in the field. Comparisons of the DMM 600 outputs with paired oven-dried duff fuel samples revealed statistically significant differences, with DMM 600 moisture output explaining 54% of the variation in oven-dried moisture content. Comparisons with previously published data indicate that large variations in duff moisture calculations may predict a broad range of observed duff consumption and overstory tree mortality levels, limiting its applicability for some management objectives. DMM 600 outputs were only weakly correlated with Keetch-Byram drought index (R-2 = 0.30). In addition, we encountered some operational difficulties in prolonged field use. In spite of a few shortcomings, the DMM 600 provides a low-cost tool to assist in prescribed fires where deep forest floor fuels exist.

Tree encroachment in fire-maintained woodlands and grasslands is a major management concern, yet little information exists regarding the mechanisms of small tree mortality following prescribed burns. We sought to clarify the relative importance of tree size and fire-induced injury in the post-fire mortality of encroaching Douglas-fir trees and to compare results with an existing mortality model for larger Douglas-fir trees. Crown injury to small Douglas-fir trees was a significant explanatory variable in post-fire mortality models, with results suggesting a 20% threshold in crown scorch. Crown injury was strongly related to bole injury, and delayed mortality was important as we documented new mortality 20 months post-burn. Mortality models for large Douglas-fir tend to over-predict small tree mortality, underscoring the need to better understand the mechanisms of fire-caused mortality for small, encroaching trees.

Fire is one of the most important processes driving plant community composition and structure. Fire regimes are largely governed by climate, vegetation structure, and individual plant traits that influence flammability. We assessed the mechanistic drivers of flammability for a diverse group of 18 California Quercus and allied Chrysolepis and Notholithocarpus species, addressing variation in leaf physical traits, growth form (tree or shrub), phylogeny (Quercus subgenera), and fire regime (low, mixed, or high severity). Differences in flammability were not strongly driven by leaf habit, leaf margin type, or surface area to volume ratio; simple measures of leaf size accounted for most of the observed variation. Further, leaf size was tightly linked to fuelbed depth, a known driver of fire behavior. Litter from trees was generally more flammable than litter from shrubs, primarily a function of differences in leaf size. A hierarchical clustering analysis on the flammability data set divided the oaks into three clusters of low, intermediate, and high flammability, corresponding closely to high-, mixed-, and low-severity fire regimes, respectively. The link between plant flammability traits and fire regime provides further evidence that individual species affect ecosystem processes.

The role of fire in the maintenance of oak-dominated ecosystems is widely recognized. Fire exclusion results in structural and compositional shifts that alter fuelbed composition and structure, together influencing fire behavior and effects. To clarify the influence of overstory structure on fuels and fire intensity in oak woodlands and savannas, we examined fuelbeds across a gradient from open grassland to Douglas-fir- (Pseudotsuga menziesii (Mirb.) Franco) invaded Oregon white oak (Quercus garryana Douglas ex Hook.) woodland in the Bald Hills of Redwood National Park, California, USA. Herbaceous mass decreased markedly from a high in grasslands (3.38 Mg ha−1) to a low in invaded woodlands (0.03 Mg ha−1), whereas leaf litter and woody fuel mass increased substantially along this gradient. Mean fire temperatures at 30 cm height ranged from 74.7 °C in invaded woodland up to 207.9 °C in grassland. Highly flammable grassland and savanna communities maintain heavy herbaceous mass, but low woody mass, favoring quick-spreading, relatively high-intensity fires. The encroachment of Douglas-fir into grasslands and oak-dominated communities dampens flammability through changes in fuelbed composition and structure (e.g., the replacement of herbaceous fuels with woody fuels), underscoring the necessity for ecological restoration efforts that focus on fuelbed structure in addition to other common restoration goals.

ranked the inter-specific flammability of four Great Lakes conifers at both oven-dry FMC (1.2 to 2.4%) and across a FMC range (17.3 to 217.6%) by burning collected needle samples under controlled conditions in a combustion chamber. Oven-dry burns used five replicates of 15g needle samples burned over a 35cm by 35cm grid of xylene-soaked cotton strings. Burning characteristics measured included maximum flame height (cm), flaming duration (s), smoldering duration (s), fuel mass consumption (%), and mass loss rate (g s -1 ). The conifers differed for all burning characteristics measured (ANOVA; P< 0.001), with Pinus resinosa showing the highest overall flammability ranking with significantly greater fire sustainability and consumability values, while Tsuga canadensis showed the lowest overall flammability ranking. We evaluated the effect of drying time on FMC and its effect on needle burning characteristics by soaking five replicates of 15g oven-dry needle samples in a water bath for 60 minutes followed by four oven drying treatments at 40 o c: 25, 50, 75 and 100 minutes. Following drying, FMCs on a dry weight basis were recorded and samples burned using the methods outlined above. The effects of the drying treatments were significant (P< 0.001) with the overall highest mean FMC recorded for T. canadensis at 217.6% following the 25 minute treatment, and the lowest for P. resinosa at 17.3% for the 100 minutes treatment. Inter-specific flammability rankings related to combustibility, sustainability and consumability as established from the dry burns were maintained across an FMC range. These findings suggest that while FMC alone plays a critical role in litter flammability, a species' inherent flammability should also be considered important for predicting flammability even across a wide fuel moisture range.