Duff Depth Research Articles

Assessing silvopasture management as a strategy to reduce fuel loads and mitigate wildfire risk

Managing private forests for wildfire resilience is challenging due to conflicting social, economic, and ecological decisions that may result in an increase of surface fuel loads leading to greater fire risk. Due to fire suppression and a changing climate, land managers in fire-prone regions face an increasing threat of high severity fires. Thus, land managers need fuel treatment options that match their forest types and management objectives. One potential option for producers that graze livestock is silvopasture management, where livestock, forages, and overstory vegetation are carefully managed for co-benefits on the same unit of land. This study compared forest composition and structure, fuel types, and vegetative biomass between silvopasture and non-grazed managed forests in Washington, U.S. We show that silvopasture management results in reductions in grass biomass, litter, and duff depth when compared to non-grazed managed forest. These findings point to the integrated nature of silvopasture, where management of overstory composition and structure, understory vegetation, and grazing can reduce fuel loads and potential wildfire risk.

Mastication Fuels Did Not Alter Fire Severity or Stand Structure in an Upland Oak Woodland

Abstract In the eastern deciduous forest region, open oak woodlands once occupied significant areas that are now closed-canopy forests, negatively affecting wildlife habitat and biodiversity. We superimposed midstory mastication and prescribed fire treatments onto sites with ice storm damage, subsequently subjected to sanitation thinning for management restoration. Mastication reduced stem density and basal area, created a variable cover of masticated material, and increased cover of forbs, graminoids, and tree regeneration. Prescribed fire was implemented two years after mastication treatment. We examined fuel changes and whether masticated fuels altered fire severity. Masticated duff depth decreased significantly two years after treatment; no change occurred on nonmasticated treatments. Masticated 1-hour fuels decreased 80% compared to 35% in nonmasticated treatments and masticated 10 h fuels decreased 45% compared to 9.6% in treatments without mastication. Prescribed fire reduced 1, 10, and 100 h fuels on the burn only treatment, and 10 h fuels on the mastication/burn treatment. Burn severity, measured by composite burn index, did not differ between treatments, nor did we measure significant effects of mastication on fire temperature or char height. Fire had no significant effect on stand structure but should be reexamined in three to five years. Repeated burning at three to five y intervals may also be beneficial.

Anthropogenic Edges and Effective Preserve Size in the Coast Redwood (<i>Sequoia sempervirens</i> (D.Don) Endl.) Forest

Ecological edges created through human activities influence both biotic and abiotic factors within forest communities. The extent of this influence within a preserve is informed by the arrangement, location, and abruptness of edges, as well as the nature of the disturbances that created them. The purpose of this study was to evaluate the impacts of anthropogenic edges on soil and vegetation in the Sequoia sempervirens (D.Don) Endl. (coast redwood) forest, and to compare two methods for estimating the effective size of forest preserves. We used a combination of field data and remote image analysis collected in six forest preserves in the Santa Cruz Mountains of California. Analysis of field data collected on randomly distributed transects indicated an average depth of influence of 200 meters based on correlations between abiotic and biotic metrics and distance from the forest edge. Abiotic factors including soil temperature and compaction were negatively correlated to distance from the edge in the direction of the forest interior, while soil pH, moisture, and duff depth exhibited positive relationships. Positive correlations were also found for biotic variables including tree canopy cover, the dominance of coast redwood and Notholithocarpus densiflorus (Hook. and Arn.) Manos, C.H. Cannon, and S. Oh (tanoak), total understory plant cover, and the cover of coast redwood forest associated plant species. In contrast, the cover and richness of non-native species were highest for samples closest to the edge. To assess the effective size of preserves, high resolution digital areal images were accessed on an ArcGIS platform. Analysis indicated variation in abruptness between types of edges, with the greatest abruptness found on edges associated with urban development and roads and the lowest abruptness associated with agricultural, grazing, and commercial timber uses. Little variation was exhibited in sinuosity between land use types or in the relative influence of edges for exurban versus urban parks. The cumulative result of edges in the parks studied, based on the depth of influence assessed from field analysis, was a substantial reduction in the operative size of the preserves. Comparison of two methods of estimating this influence indicated a mean affected area of >26% using the “perimeter” method and >64% of the preserved area affected when using an “aggregated” method. These results suggest that including internal anthropogenic edges created by roads, developments, and management activities in estimates of effective preserve size is more accurate and significantly reduces the estimated size of the core area. With an increasing level of active management occurring within coast redwood preserves, it is important to consider the cumulative impact of internal anthropogenic edges and the subsequent potential decline in the effective size of preserves.

Environmental Influences on Density and Height Growth of Natural Ponderosa Pine Regeneration following Wildfires

Over the past century the size and severity of wildfires, as well as post-fire recovery processes (e.g., seedling establishment), have been altered from historical levels due to management policies and changing climate. Tree seedling establishment and growth drive future overstory tree dynamics after wildfire. Post-fire tree regeneration can be highly variable depending on burn severity, pre-fire forest condition, tree regeneration strategies, and climate; however, few studies have examined how different abiotic and biotic factors impact seedling density and growth and the interactions among those factors. We measured seedling density and height growth in the period 2015–2016 on three wildfires that burned in ponderosa pine (Pinus ponderosa) forests in the period 2000–2007 across broad environmental and burn severity gradients. Using a non-parametric multiplicative regression model, we found that downed woody fuel load, duff depth, and fall precipitation best explained variation in seedling density, while the distance to nearest seed tree, a soil productivity index, duff depth, and spring precipitation as snow best explained seedling height growth. Overall, results highlight the importance of burn severity and post-fire climate in tree regeneration, although the primary factors influencing seedling density and height growth vary. Drier conditions and changes to precipitation seasonality have the potential to influence tree establishment, survival, and growth in post-fire environments, which could lead to significant impacts for long-term forest recovery.

Long-term impacts of silvicultural treatments on wildland fuels and modeled fire behavior in the Ridge and Valley Province, Virginia (USA)

Active forest management operations, such as regeneration harvests, can reduce hazardous fuel loads and alter fuel structure, potentially minimizing extreme wildfire conditions while maintaining ecosystem services, such as wildlife habitat and water quality. Regeneration harvests of differing intensities (clearcut, high-retention shelterwood, and low-retention shelterwood) were first applied between 1995 and 1996 to three sites on the George Washington-Jefferson National Forest in the Ridge and Valley Province of Virginia, USA. Over two decades after the clearcut was conducted and 11–12 years after the overwood was removed in the shelterwood stands, woody debris, litter, and duff masses and depths were quantified. One-hour fuel loads were greater in clearcut units than in high-retention shelterwood, low-retention shelterwood, or control units. Ten-hour fuel loads were greater in clearcut and low-retention shelterwood units than in high-retention shelterwood and control units. No significant differences in 100-hour fuels were observed between treatments. Control units contained more rotten and total 1000-hour fuels than all other treatments. The total woody debris load was less in the clearcut and high-retention shelterwood than in the low-retention shelterwood and control. High-retention shelterwood woody fuel depth was greater than clearcut woody fuel depth. Litter and duff loads were less in treated units than in the control units. Total fuel load (woody fuel load + litter load + duff load) was greater in the control than the silvicultural treatments. Litter depth did not differ between treatments, while duff depth was greater in the control than in the treated units. Using the computer modeling software, BehavePlus 6.0.0, these alterations to fuel loads and depths led to increased values in the control units for six fire behavior parameters. Predicted surface flame length in the low-retention shelterwood was the only modeled value that was not less than control values. Overall, these results indicated that harvest intensity and timing may have long-term effects on down and dead woody fuels, forest floor depth, and potential fire behavior. Clearcutting reduced fire behavior most, followed by the high-retention shelterwood system. The potential differences in slash and debris generated by varying shelterwood systems may impact long-term fuel and fire dynamics.

Fuel load, stand structure, and understory species composition following prescribed fire in an old-growth coast redwood (Sequoia sempervirens) forest

BackgroundWith the prevalence of catastrophic wildfire increasing in response to widespread fire suppression and climate change, land managers have sought methods to increase the resiliency of landscapes to fire. The application of prescribed burning in ecosystems adapted to fire can reduce fuel load and fire potential while minimizing impacts to the ecosystem as a whole. Coast redwood forests have historically experienced fire from both natural and anthropogenic sources, and are likely to respond favorably to its reintroduction.ResultsRandom sampling was conducted in three burned sites and in three unburned sites, in an old-growth coast redwood (Sequoia sempervirens [D. Don] Endl.) forest. Data were collected on fuel, forest structure, and understory species composition and compared between treatments. Downed woody fuel, duff depth, litter depth, and density of live woody fuels were found to be significantly lower on sites treated with fire compared to unburned sites. Density of the dominant overstory canopy species, coast redwood and Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco), remained consistent between treatments, and the abundance of herbaceous understory plant species was not significantly altered by burning. In addition, both downed woody fuel and live fuel measures were positively correlated with time since last burn, with the lowest measures on the most recently burned sites.ConclusionsOur results indicated that the use of prescribed burning in old-growth redwood forests can provide beneficial reductions in live and dead surface fuels with minimal impacts to overstory trees and understory herbaceous species.

Wiregrass (Aristida beyrichiana) survival and reproduction after fire in a long-unburned pine savanna.

Restoring fire regimes is a major goal of biodiversity conservation efforts in fire-prone ecosystems from which fire has been excluded. In the southeastern U.S.A., nearly a century of fire exclusion in pine savannas has led to significant biodiversity declines in one of the most species-rich ecosystems of North America. In these savannas, frequent fires that support biodiversity are driven by vegetation-fire feedbacks. Understory grasses are key components of these feedbacks, fueling the spread of fires that keep tree density low and maintain a high-light environment. When fire is reintroduced to long-unburned sites, however, remnant populations of bunchgrasses might experience high mortality from fuel accumulation during periods of fire exclusion. Our objective was to quantify fire effects on wiregrass (Aristida beyrichiana), a key component of vegetation-fire feedbacks, following 16 years without fire in a dry pine savanna typically considered to burn every 1–3 years. We examined how wiregrass size and fuel (duff depth and presence of pinecones) affected post-fire survival, inflorescence and seed production, and seed germination. Wiregrass exhibited high survival regardless of size or fuels. Probability of flowering and inflorescence number per plant were unaffected by fuel treatments but increased significantly with plant size (p = 0.016). Germination of filled seeds was consistent (29–43%) regardless of fuels, although plants in low duff produced the greatest proportion of filled seeds. The ability of bunchgrasses to persist and reproduce following fire exclusion could jumpstart efforts to reinstate frequent-fire regimes and facilitate biodiversity restoration where remnant bunchgrass populations remain.

Spatial and temporal fuels changes in whitebark pine (Pinus albicaulis) from mountain pine beetle (Dendroctonus ponderosae)

Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) causes extensive tree mortality in whitebark pine (Pinus albicaulis Engelm) forests. Previous studies conducted in lodgepole pine (Pinus contorta Douglas), Douglas-fir (Pseudotsuga menziesii (mirb.) Franco), and Engelmann spruce (Picea engelmanni Parry ex. Engelm) have shown that litter, duff, and 1-h (<0.64 cm) and 10-h (0.64–2.54 cm) time lag fuels are altered significantly from MPB outbreaks, while coarse woody fuels are affected over a longer time frame. MPB activity in conifer stands also alters foliar fuel moisture content over the course of the bark beetle rotation. This study evaluated changes to fine surface fuels and foliar fuel moisture in and under whitebark pine trees infested by MPB at two sites in Montana and Wyoming, USA. Fuel loads and foliar moisture were measured for crown condition classes of green trees (healthy), red trees (within two years since initial MPB attack with at least 50% of needles remaining), and gray trees (more than two years since attack with approximately 15% to 45% needles remaining). Tree locations and condition class were mapped using 2011, 2013, and 2015 NAIP imagery, and spatial point patterns were identified. Duff depths were significantly shallower beneath green trees (10.7–11.9 mm) than red (16.7–17.7 mm) and gray (13.9–16.1 mm) trees. Foliar fuel moisture content was altered dramatically across crown condition classes. Red needled trees had the lowest fuel moisture content, which was less than 18%. Point pattern hot spot analysis revealed increased (hot-spots) fuel hazard in trees 2–3 years post MPB attack, and decreased fuel hazard (cold-spots) in trees more than 3 years after MPB attack because foliage fell from trees and left larger diameter aerial fuels which were less likely to ignite. MPB-attacked trees were often clustered, and thus fuels hazard was not uniform across the landscape following MPB attack.

Modelling post-fire tree mortality: Can random forest improve discrimination of imbalanced data?

Predicting post-fire tree mortality is a major area of research in fire-prone forests, woodlands, and savannas worldwide. Past research has relied overwhelmingly on logistic regression analysis (LR) that predicts post-fire tree status as a binary outcome (i.e. living or dead). One of the most problematic issues for LR (or any classification problem) occurs when there is a class imbalance in the training data. In these instances, predictions will be biased toward the majority class. Using a historical prescribed fire data set of longleaf pines (Pinus palustris) from northern Florida, USA, we compare results from standard LR and the machine-learning algorithm, random forest (RF). First, we demonstrate the class imbalance problem using simulated data. We then show how a balanced RF model can be used to alleviate the bias in the model and improve mortality prediction results. In the simulated example, LR model sensitivity and specificity was clearly biased based on the degree of imbalance between the classes. The balanced RF models had consistent sensitivity and specificity throughout the simulated data sets. Re-analyzing the original longleaf pine data set with a balanced RF model showed that although both LR and RF models had similar areas under the receiver operating curve (AUC), the RF model had better discrimination for predicting new observations of dead trees. Both LR and RF models identified duff consumption and percent crown scorch as important predictors of tree mortality, however the RF model also suggested pre-fire duff depth as an important predictor. Our analysis highlights LR limitations when data are imbalanced and supports using RF to develop post-fire tree mortality models. We suggest how RF can be incorporated into future tree mortality studies, as well as possible implementation in future decision-support tools.

Boreal forest vegetation and fuel conditions 12 years after the 2004 Taylor Complex fires in Alaska, USA

BackgroundFire has historically been a primary control on succession and vegetation dynamics in boreal systems, although modern changing climate is potentially increasing fire size and frequency. Large, often remote fires necessitate large-scale estimates of fire effects and consequences, often using Landsat satellite-derived dNBR (differenced Normalized Burn Ratio) to estimate burn severity. However, few studies have examined long-term field measures of ecosystem condition in relation to dNBR severity classes in boreal Alaska, USA. The goals of this study were: 1) assess changes in dominant vegetation at plots resampled one and 12 years post fire; 2) use dNBR classes to characterize vegetation and downed woody fuels 12 years post fire; and 3) characterize the relationship between biophysical, topographic, and remotely sensed characteristics (e.g., moss and duff depth, canopy cover, elevation, aspect, dNBR) and understory species assemblages 12 years post fire.ResultsUnderstory species richness doubled (from 39 to 73) between 2005 and 2016; some common species increased in cover over time (e.g., Ledum groenlandicum Oeder) while others decreased (e.g., Hylocomium splendens [Hedw.] Schimp.). In 2016, live and dead tree densities, tall shrub cover, and 1- and 100-h woody fuels were significantly different among dNBR classes; moss and duff depth, canopy cover, and spruce seedling density were not. Elevation and aspect significantly influenced tall shrub cover, hardwood sapling density, and downed woody fuel loads. Understory plant communities differed between unburned and all burn classes, as well as between low and high dNBR severity. Ordination analysis showed that overstory (e.g., live tree density), understory (e.g., moss depth, woody fuel loading), and site (elevation, aspect, dNBR) significantly influences understory species assemblages.ConclusionRemeasured sites (sampled one and 12 years post fire) showed recruitment of new understory species and differing, diverse responses to burning by several common plant species. In 2016, low-severity burned sites had generally the highest woody fuel loading, which may increase risk of repeated surface burning, although the reduction in live tree density would still result in decreased fire risk and behavior. Understory community composition correlated with multiple biotic and abiotic factors, including moss depth, canopy cover, elevation, aspect, and dNBR. Overall, our findings can improve landscape-level predictions of ecosystem condition following fire based on dNBR.

Wildfire and Prescribed Fire Effects on Forest Floor Properties and Erosion Potential in the Central Appalachian Region, USA

Short- and long-term impacts of wildland fires on forest floor properties and erosion potential were examined at three locations in the Central Appalachian region, U.S.A. In 2018, two wildfires were investigated within six months of burning on the George Washington–Jefferson National Forest (GWJNF) in Bland County, Virginia and the Monongahela National Forest (MNF) in Grant County, West Virginia. An additional wildfire was studied eight years post-fire on the Fishburn Forest (FF) in Montgomery County, Virginia. A 2018 prescribed fire was also studied within six months of burning on the MNF in Pendleton County, West Virginia. Litter and duff consumption were examined to evaluate fire severity and char heights were measured to better understand fire intensity. The Universal Soil Loss Equation for forestlands (USLE-Forest) was utilized to estimate potential erosion values. For the 2018 comparisons, litter depth was least as a result of the wildfires on both the MNF and GWJNF (p &lt; 0.001). Wildfire burned duff depths in 2018 did not differ from unburned duff depths on either the MNF or GWJNF. Eight years after the FF wildfire, post-fire litter depth was less than that of an adjacent non-burned forest (p = 0.29) and duff depth was greater than that of an adjacent non-burned forest (p = 0.76). Mean GWJNF wildfire char heights were greatest of all disturbance regimes at 10.0 m, indicating high fire intensity, followed by the MNF wildfire and then the MNF prescribed fire. USLE-Forest potential erosion estimates were greatest on the MNF wildfire at 21.6 Mg soil ha−1 year−1 due to slope steepness. The next largest USLE-Forest value was 6.9 Mg soil ha−1 year−1 on the GWJNF wildfire. Both the prescribed fire and the 2010 wildfire USLE-Forest values were approximately 0.00 Mg soil ha−1 year−1. Implications for potential long-term soil erosion resulting from similar wildfires in Central Appalachian forests appeared to be minimal given the 2010 wildfire results.

Characterizing Forest Floor Fuels Surrounding Large Sugar Pine (Pinus lambertiana) in the Klamath Mountains, California

Fire exclusion has contributed to substantial basal accumulations of forest floor fuels in many historically fire-prone forests in western North America. These accumulations can contribute to undesired impacts (e.g., elevated tree mortality, more smoke production). Thus, managers interested in limiting some of these impacts require a better understanding of the spatial factors contributing to forest floor fuel variation in long-unburned forests. To address this need, we measured the forest floor mound size and depth of each layer at four distances from the tree (base, half, edge, and beyond) in each cardinal direction from the tree, and a subset of density measures for each layer and position around 25 large sugar pine (Pinus lambertiana) in the Klamath Mountains of California. Based on our study we found: 1) duff depths were greater than litter depths and both decreased with distance from tree; 2) duff densities were typically greater than litter but were highly variable with distance to tree; 3) litter and duff depths were positively correlated with loading; 4) use of a previously developed depth-to-loading equation performed well, but tended to underestimate litter loading and overestimate duff loading; and 5) tree size was the most strongly related to basal accumulations of forest floor fuels. Our results indicate there are consistent tree characteristics that can help predict the accumulation of forest floor fuels. This research can help provide managers with site-specific equations to inform fuel and fire treatments in areas with substantial basal accumulation in forests with large sugar pine.

Alterations to the fuel bed after single and repeated prescribed fires in an Appalachian hardwood forest

To manage upland oak forests across the central and southern Appalachian mountain and plateau regions, prescribed fire is applied more frequently and across larger areas than in the recent past. The often stated objective is to reduce fuels, but there is a paucity of information on the impacts of repeated burning on fuels, including woody materials and the soil organic layer. These are complex components of forest ecosystems with significant impacts on nutrient and carbon cycling, forest successional dynamics, and soil protection from loss via erosion. Thus, understanding fuel bed response to prescribed fire is essential for predicting future forest function. Using study sites distributed across a range of landscape positions in the Daniel Boone National Forest on the Cumberland Plateau of eastern Kentucky, we examined changes to the fuel bed over six years in response to a single fire (burned once in six years), repeated fire (burned four times in six years), and fire-excluded treatments to determine prescribed fire impacts on fuel loads and mineral soil exposure. Prior to burning, fuel loads were generally similar among landscape positions, although the duff layer was lowest on sub-mesic and greatest on sub-xeric positions. A single fire reduced duff depth by 50%, whereas repeated burning led to depth reductions of >60%. Repeated burning also significantly increased mineral soil exposure (25%) compared to single burn and fire-excluded (2–4%) treatments, with the greatest effects on sub-mesic and intermediate landscape positions. Repeated burning significantly reduced fine woody (1-h) fuels, but only after three burns, whereas fine fuel mass on sites burned once was similar to those where fire was excluded. There were no statistically significant effects of burning on large woody fuels (100- and 1000-h fuels). Overall, the primary impact of prescribed fire on the fuel bed was to consume the organic horizon and expose mineral soil, which has the potential to reduce fuel continuity for subsequent burns. Fire behavior in this region is driven primarily by fine fuels (litter and duff) and fuel continuity, both of which recover in relatively short periods of one to several years. Reduction of woody fuels is more intractable under a prescribed fire regime.

Surface fuels in recent Phytophthora ramorum created gaps and adjacent intact Quercus agrifolia forests, East Bay Regional Parks, California, USA

Phytophthora ramorum, cause of “sudden oak death” or SOD, has had significant impacts on composition and structure in coastal forests of central and northern coastal California and southwestern Oregon. Despite the proximity of susceptible coast live oak (Quercus agrifolia) forests to densely populated urban areas, the impacts of SOD on their fuels have not been studied. We sampled surface fuels and vegetation structure in 16 plots in both SOD-caused gaps and intact stands (32 plots total) across two parks in the East Bay Regional Park District, east of San Francisco Bay. Plots were selected from a set of randomly placed pre-existing locations used in determining the disease distribution and intensity across the park system. Among the vegetation characteristics examined, only coast live oak basal area and live and dead tree density, canopy cover and maximum forb height differed between SOD created gap plots and adjacent intact forest plots. However, surface fuels such as vegetation cover, litter cover, wood cover, duff depth, fuels height, ladder fuels abundance, 1h, 10h, 100h, and 1000h fuels were greater in gap plots than intact forest plots. Although no fire behavior models were run, surface fuels suggest SOD created gaps may facilitate passive crown fire due to increased ladder and other fuels. This study represents a spatially explicit (gap focused) point-in-time estimate of surface fuels that will continue to change through time as disease progresses through these stands causing vegetation changes as fuels accumulate and decompose.

Using one year post-fire fire severity assessments to estimate longer-term effects of fire in conifer forests of northern and eastern California, USA

Fire severity maps derived from immediate or one year post-fire satellite images are commonly used by US federal land managers for making post-fire management decisions and provide fundamental insights into broad scale fire ecology questions. How these maps relate to longer term post-fire conditions in the absence of management actions is an unanswered question, and a key issue for post-fire restoration planning. We characterized field-measured fire effects in forests 5–7yearsafter fire in severity categories as they were mapped with one year post-fire satellite data. The severity maps we used were produced from a relativized differenced normalized burn ratio (RdNBR) derived from Landsat images and calibrated to three severity metrics: composite burn index (CBI), percent change in basal area (BA), and percent change in tree canopy cover (CC). Relationships of field-measured values with severity generally followed expected patterns. BA of dead trees and shrub cover increased with increasing fire severity. BA of live trees, total tree CC, conifer CC, litter cover, litter depth and duff depth all decreased with increasing severity. When forest type (yellow pine, dry mixed conifer, moist mixed conifer, fir) was added as an interaction term there were differences in the relationships of field-measured values to severity, some of which can be attributed to differences between pine and fir dominated forests. We did not find any difference in live BA between unburned, unchanged, low and moderate severity plots for yellow pine forests; the only significant decrease in live BA was in high severity plots. In contrast, both dry and moist mixed conifer forests showed more or less steadily decreasing live BA with increasing fire severity. For yellow pine dead BA there were no significant differences between unchanged, low and moderate categories, while there was a difference between unchanged and moderate for all other forest types. In the high-severity categories ⩾91% of plots had <10% conifer CC, and ⩾82% of plots had <10% total tree CC. Comparing our 5–7yearpost-fire dataset to a separate dataset measured one year post-fire demonstrates that delayed tree mortality occurs at all levels of severity, and notable snag fall (especially pines) is already underway a half-decade after fire. The effects we measured provide useful information for managers interested in planning for post-fire activities up to 5–7yearsafter fire, including soil erosion abatement, habitat restoration, fuel and snag management, and tree regeneration management.

Duff Distribution Influences Fire Severity and Post-Fire Vegetation Recovery in Sagebrush Steppe

Woody plant expansion is a global phenomenon that alters the spatial distribution of nutrients, biomass, and fuels in affected ecosystems. Altered fuel patterns across the landscape influences ecological processes including fire behavior, fire effects, and can impact post-fire plant germination and establishment. The purpose of this study was to determine how accumulations of ground fuels beneath western juniper (Juniperus occidentalis ssp. occidentalis) canopies, composed of litter and duff, affect post-fire species response in sagebrush steppe and to quantify fuel loading patterns. Field sampling and analysis was conducted across environmental gradients following the 2007 Tongue-Crutcher Wildfire in southwestern Idaho to determine conditions that were most influential in post-fire vegetation recovery patterns. Duff depth and fire severity were determined to be the most influential factors affecting post-fire vegetation response. Decreasing species richness and native perennial grass cover was represented along the increasing duff depth gradient. Species response grouped by fire severity revealed significant presence of cheatgrass (Bromus tectorum) in low severity sites and a dominance of snowbrush ceanothus (Ceanothus velutinus) in higher severity sites. Determining sub-crown surface fuel characteristics offers the potential to predict future patterns and processes as they relate to burn severity and vegetation recovery components in developing woodlands.

The effect of sudden aspen decline on understory microclimate and vegetation in southwestern Colorado

Sudden aspen decline (SAD), present in many parts of North America, is the sudden dieback of branches, crown loss, and rapid mortality of aspen (Populus tremuloides Michx.). We surveyed 21 plots in southwestern Colorado and categorized each plot by the mean percentage of recent crown loss (RCL) into three SAD levels: low SAD (0%–25% RCL), moderate SAD (25.1%–50% RCL), and high SAD (50.1%–100% RCL). Our research quantified the effects of SAD on microclimate and understory vegetation at the individual species and community level. Mean day surface, day subsurface, and night subsurface temperatures were warmer in high SAD stands than in low ones. High SAD stands had lower soil moisture, lower litter and duff depth, higher bare soil cover, higher photosynthetically active radiation, higher arbuscular mycorrhizal propagule densities, and higher grass biomass. Indicator plant species were uniquely associated with low and high SAD. Our study illustrates that SAD has multiple ecological effects on aspen understories, including a potential positive feedback in which warmer temperatures and decreased soil moisture, consequences of SAD, may lead to increased branch dieback and tree mortality, which would alter microclimate-making conditions more favorable to SAD and escalate the effects of SAD on understory vegetation.

Fire frequency, agricultural history and the multivariate control of pine savanna understorey plant diversity

AbstractQuestionHuman‐altered disturbance regimes and agricultural land uses are broadly associated with reduced plant species diversity in terrestrial ecosystems. In this study, we seek to understand how fire frequency and agricultural land‐use history influence savanna understorey plant diversity through complex relationships (i.e. indirect effects) among multiple biophysical variables.LocationFort Bragg, NC, US, Savannah River Site, SC, US and Fort Stewart, GA, US.MethodsWe use structural equation modelling (SEM) to evaluate the relationships among six groups of predictor variables and their influence on local‐scale species richness in pine savannas at 256 sites from three locations in the southeastern USA. In the model, fire frequency and agricultural history are hypothesized to control richness through a combination of direct effects, and indirect effects mediated by resource availability, tree abundance, understorey plant abundance and the O horizon (litter and duff depth).ResultsFrequent fires promote richness by limiting tree abundance, which increases understorey abundance and reduces the O horizon. Frequent fires also limit the O horizon independent of tree abundance. Of the total positive effect of fire on richness, 70% is attributable to reductions in the O horizon and 30% to reduced tree abundance. Agricultural history has a negative effect on richness through a positive correlation with tree abundance, which decreases understorey abundance and increases the O horizon. Agricultural history has a modest negative effect on richness by reducing resource availability as well as a strong direct negative effect (38% of the total effect) that is unrelated to other modelled variables.ConclusionsThrough a multivariate framework and large‐scale data set, this study unites and tests our understanding of the factors that control plant species diversity in a fire‐dependent ecosystem. We show that the effects of fire frequency and agricultural history on richness are largely mediated through other ecosystem attributes, including vegetation structure (i.e. tree and understorey abundance), resource availability and the O horizon. Persistent, negative effects of agricultural history demonstrate the slow rates of savanna plant community recovery on post‐agricultural land and highlight the conservation value of frequently burned savanna remnants.

Salvage harvest effects on advance tree regeneration, soil nitrogen, and fuels following mountain pine beetle outbreak in lodgepole pine

The extent and severity of recent native bark beetle (Dendroctonae) outbreaks in western North America have created a pressing need for forest managers to understand potential consequences of post-disturbance management. For example, post-outbreak timber harvest (i.e., salvage harvest) could alter future forest development, productivity and susceptibility to subsequent disturbance. To assess the potential for such consequences, we measured first-year effects of post-outbreak timber harvest on tree regeneration, soil nitrogen (N) availability and fuels by using a paired and replicated before–after-control-impact (BACI) experimental design with eight pairs of 0.25-ha plots in beetle-killed lodgepole pine (Pinus contorta var. latifolia) in Greater Yellowstone (Wyoming, USA). Post-outbreak timber harvest reduced total (live +dead) lodgepole pine basal area by 90%. Total sapling density (advance regeneration) declined by about 50% following harvest, with tall (30–140cm) saplings declining most, but mean post-harvest sapling density still exceeded 1600 stems ha−1. Relative species density was unaffected and remained dominated by lodgepole pine. Soil temperature at the litter–soil interface was warmer during summer in harvested stands, and soil NO3- concentration increased with harvest relative to untreated plots. Soil NH4+ concentration and resin bag N accumulation increased through time in all beetle-killed plots and were not affected by harvest. Following harvest, dead woody surface fuels in all size categories doubled, and canopy fuel load and canopy bulk density both were reduced; dead fuel depth, duff depth, and canopy base height did not differ between untreated and harvested plots. Harvest did reduce canopy fuels, but the natural progression of needle shedding after beetle-kill accounted for 25–40% of this total canopy fuel reduction. Salvage harvest seems unlikely to alter post-outbreak successional trajectories in these lodgepole pine forests. However, the altered fuel complex (immediate increase in dead woody surface fuels and expected long-term reduction in large-diameter fuels) in harvested plots could cause subsequent fire behavior and effects to differ between harvested and untreated stands.

Duff mound consumption and cambium injury for centuries-old western larch from prescribed burning in western Montana

Western larch is one of the most fire-adapted conifers in western North America. Its historical perpetuation depended upon regular fire disturbances, which creates open stand conditions and mineral seedbeds. A stand of 200- to 500-year-old larch in western Montana with deep duff mounds resulting from an unusually long 150-year fire-free period was mechanically thinned and prescribed burned to reduce the probability of high intensity wildfire near a community and increase opportunities for larch regeneration. Little documentation is available regarding basal damage to larch from lengthy duff mound burning; therefore this study was established to assess: duff consumption from prescribed burning and resulting cambial damage and tree vitality. Ninety trees averaging 91-cm diameter at breast height were selected, half with duff mounds measured and burned in autumn and half with mounds removed. Duff depths nearest the bole averaged 20 cm and mound consumption approached 100% including large amounts of the basal bark with smouldering combustion lasting 18–24 h. Cambial mortality ranged from 0 to 36% of the basal circumference but no trees had died after 7 years. The cambium mortality was likely due to the spatially infrequent coincident of deep duff and thinner bark. Under similar site and environmental conditions removal of the potential duff consumption injury hazard appears unwarranted.