Ponderosa Pine-dominated Forests Research Articles

Spatial patterns of vigor by stand density across species groups and its drivers in a pre-harvest ponderosa pine-dominated landscape in northern California

Information on spatial patterns of forest health and stand vigor in historical forests is quite limited, particularly for forests of North America, which have been subsequently harvested. We used forest inventory data from 1934, which pre-dated early 20th century timber harvest, to reconstruct patterns of vigor in overstory trees. Across 4,000 ha of ponderosa pine-dominated forests in the Blacks Mountain Experimental Forest (BMEF), we described the spatial patterns of pre-harvest (1934) forests using a stand density index (SDI) and accounting for different species and vigor classes. We then identified topo-edaphic variables associated with pre-harvest SDI. To do so, we first calculated SDI for two species groups: 1) ponderosa pine (Pinus ponderosa Laws.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.), and 2) incense-cedar (Calocedrus decurrens (Torr.) Florin) and white fir (Abies concolor (Grod. and Glend.)) as well as for seven vigor classes within these two groups. Second, using Moran’s I, we found four spatially aggregated clusters (Moran’s I = 0.35): 1) ponderosa pine-high vigor, 2) ponderosa pine-low vigor, 3) mixed species-high vigor, and 4) mixed species-low vigor. Most of the pre-harvest landscape consisted of high vigor ponderosa pine clusters with low SDI (relative density (RD) < 0.25). High elevation sections consisted of mixed species clusters of both high and low vigor with high SDI (RD > 0.35). Using classification and regression tree analysis, we identified elevation (m), surface-soil depth (cm), and sub-soil depth (cm) were related to the clusters. A multinominal logistic regression model (kappa = 0.44; classification accuracy = 64.4 %) identified surface stoniness, available water capacity at 150 cm depth and its interaction with sub-soil depth as important variables that were related to clusters. Within ponderosa pine-dominated forests, managers consider dense stands of low vigor trees to be at-risk of insect, disease, and fire. Thus, our results can be used by managers as representative baselines for characterizing (pre-harvest) stand conditions to guide management treatments to support high vigor and low density forest conditions.

Application of unmanned aerial system structure from motion point cloud detected tree heights and stem diameters to model missing stem diameters

Monitoring of tree spatial arrangement is increasingly essential for restoration of dry conifer forests. The presented method was developed for high-density point clouds, like those from unmanned aerial system imagery, to extract and model individual tree location, height, and diameter at breast height (DBH). Extraction of tree locations and heights uses a variable window function searching point cloud-derived canopy height models. Tree DBH is extracted for a subset of point cloud trees using a slice at 1.32-1.42 m and a least-squares circle fitting algorithm. Extracted heights and DBHs are spatially matched and filtered against each tree's expected DBH predicted using a regional National Forest Inventory height to DBH relationship. Values remaining after filtering are used to create a site-specific height to DBH relationship for predicting missing DBH values. Applying the method in a ponderosa pine-dominated forest found that extracted height values exceeded the precision of field height measurement approaches, while the accuracy of extracted and modeled DBH values had a mean error of 0.79 cm.•Leveraging National Forest Inventory to filter DBH values eliminates the need for in situ observations.•Produces tree list for all extractable stems in the point cloud.•Transferable to high-density point clouds in open-canopy forests.

Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event.

Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought-related mortality derived from measurements of tree-ring growth (ring width index; RWI) and carbon isotope discrimination (∆13 C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012-2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆13 C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆13 C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine-dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management-particularly in drier regions-may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.

Effects of collaborative monitoring and adaptive management on restoration outcomes in dry conifer forests

In response to large, severe wildfires across the western US, federal initiatives have been enacted to increase the pace, scale, and quality of ecological restoration in fire dependent forests. To address uncertainty and controversy in agreement among specific restoration prescriptions on national forest land, several initiatives adopt a collaborative adaptive management (CAM) strategy wherein monitoring data can inform stakeholder input into future management actions. It is unclear, however, how such approaches may change restoration outcomes. Here we assess the extent to which CAM strategies impact restoration outcomes that were implemented as part of the Collaborative Forest Landscape Restoration Program (CFLRP) in ponderosa pine-dominated forests of the Colorado Front Range. We assessed stand-level desired conditions across 24 projects over a 7-year period to determine how restoration treatments contribute to desired conditions, and compared treatment outcomes over implementation time to assess whether the CAM processes contribute towards treatments better approximating restoration. We found that restoration treatments improve aspects of forest structure related to stand density. However, meeting objectives related to forest composition and horizontal structural complexity goals were not met. Additionally, CAM processes were effective at improving outcomes related to forest density over implementation time, but novel tools and approaches may be required so that outcomes related to forest composition and horizontal structural complexity are more congruent with restoration objectives. Evaluating the success and challenges of CAM provides insight to improve collaborative and large-scale restoration.

Fuel Treatment Longevity in Ponderosa Pine-Dominated Forest 24 Years After Cutting and Prescribed Burning

Fuels reduction treatments to mitigate fire behavior are common in ponderosa pine ecosystems of the western United States. While initial impacts of fuel treatments have been reported, less is known about treatment longevity as live and dead fuels change with time. We analyzed fuel dynamics in ponderosa pine–Douglas-fir forests 21-23 years following experimental fuel reduction designed as two independent studies of cutting combined with burning: one tested a commercial thinning strategy, while a second tested a retention shelterwood strategy to reduce fuels while also restoring ponderosa pine forests. Treated units were harvested in 1992 and half of the units were prescribed burned one-to-two years later. After 22 to 23 years post-treatment, few differences in fuel load persist and all treatments have increased ladder fuels in the form of live saplings and seedlings. Canopy fuel loads were lower in treated units compared to untreated control units; however, no other canopy fuel metric differed between treatments. The only persistent difference in surface fuels was in the retention shelterwood, where 1-hour fuels were lower in the treated units compared to control units. Crown fire hazard varied greatly, but means were similar between treatments. The increased hazard was driven by increases in live surface fuels from seedlings and saplings in the retention shelterwood, which increased canopy bulk density and reduced canopy base height. The overstory was still dominated by ponderosa pine 21-23 years later for all treatments, but the smaller size classes were primarily Douglas-fir, suggesting that without future disturbance, dominance will shift from pine to Douglas-fir dominated forests. The exception to this was the cut+fall burn treatment in the commercial thinning, where ponderosa pine outnumbered Douglas-fir trees across all size classes. The treatments that included a broadcast prescribed burn killed many existing seedlings and saplings. Our findings support other studies showing fuel reduction and restoration treatments are most successful with a combination of cutting and burning strategies, but also show that fuel treatments in low-elevation dry forests will likely not remain effective for much longer than historical mean fire return intervals.

Still standing: Recent patterns of post-fire conifer refugia in ponderosa pine-dominated forests of the Colorado Front Range.

Forested fire refugia (trees that survive fires) are important disturbance legacies that provide seed sources for post-fire regeneration. Conifer regeneration has been limited following some recent western fires, particularly in ponderosa pine (Pinus ponderosa) forests. However, the extent, characteristics, and predictability of ponderosa pine fire refugia are largely unknown. Within 23 fires in ponderosa pine-dominated forests of the Colorado Front Range (1996–2013), we evaluated the spatial characteristics and predictability of refugia: first using Monitoring Trends in Burn Severity (MTBS) burn severity metrics, then using landscape variables (topography, weather, anthropogenic factors, and pre-fire forest cover). Using 1-m resolution aerial imagery, we created a binary variable of post-fire conifer presence (‘Conifer Refugia’) and absence (‘Conifer Absence’) within 30-m grid cells. We found that maximum patch size of Conifer Absence was positively correlated with fire size, and 38% of the burned area was ≥ 50m from a conifer seed source, revealing a management challenge as fire sizes increase with warming further limiting conifer recovery. In predicting Conifer Refugia with two MTBS-produced databases, thematic burn severity classes (TBSC) and continuous Relative differenced Normalized Burn Ratio (RdNBR) values, Conifer Absence was high in previously forested areas of Low and Moderate burn severity classes in TBSC. RdNBR more accurately identified post-fire conifer survivorship. In predicting Conifer Refugia with landscape variables, Conifer Refugia were less likely during burn days with high maximum temperatures: while Conifer Refugia were more likely on moister soils and closer to higher order streams, homes, and roads; and on less rugged, valley topography. Importantly, pre-fire forest canopy cover was not strongly associated with Conifer Refugia. This study further informs forest management by mapping post-fire patches lacking conifer seed sources, validating the use of RdNBR for fire refugia, and detecting abiotic and topographic variables that may promote conifer refugia.

Toward a better understanding of climate and human impacts on late Holocene fire regimes in the Pacific Northwest, USA

In order to fully appreciate the role that fire, both natural and anthropogenic, had in shaping pre-Euro-American settlement landscapes in the Pacific Northwest (PNW), it is necessary to develop a more robust method of evaluating paleofire reconstructions. Here we demonstrate an approach that includes the identification of charcoal morphotypes (i.e. visually distinct charcoal particles), and incorporates both paleoecological and archaeological data sets, to more specifically determine both the nature of past fire regimes (i.e. fuel type and fire severity) and the likely ignition source of those fires. We demonstrate the usefulness of this approach by reconstructing the late Holocene fire and vegetation histories of Lake Oswego (Clackamas County), Oregon, and Fish Lake (Okanogan County), Washington, using macroscopic charcoal and pollen analysis of sediment cores. The histories were compared with climatic records from the PNW as well as archaeological, ethnographic, and historical records from the Lower Columbia River Valley and Southern Columbia Plateau cultural regions. Our results indicate that while centennial-to-millennial-scale climate change had limited influence on the fire regimes at the study sites during the past ∼3800 years, the use of fire by Native Americans for a variety of reasons, particularly after ca. 1200 calendar years before present (AD 750), had a far greater impact. Charcoal morphotype ratios also indicate that fires in the two watersheds were fundamentally different in their severity and impact, and led to major shifts in the forests and woodlands surrounding Lake Oswego, but helped maintain the ponderosa pine-dominated forest at Fish Lake. The elimination of fire from the two study sites during the past 100–300 years is likely the combined result of Euro-American contact and the arrival of disease in the PNW, as well as 20th-century fire suppression and grazing effects on fuel continuity, which has implications for future forest management and restoration efforts in the PNW.

Fuel mass and stand structure 13 years after logging of a severely burned ponderosa pine forest in northeastern Oregon, U.S.A

Stand structure and fuel mass were measured in 2011, 13 years after logging of a seasonally dry, ponderosa pine-dominated forest that had burned severely in the 1996 Summit Wildfire, Malheur National Forest, northeastern Oregon, U.S.A. Data are compared to those taken one year after post-fire logging (1999), and analyzed in the context of a second fire (Sunshine Fire) that burned through one of the four treatment blocks in 2008. Three treatments were evaluated in a randomized block experiment: unlogged control, commercial harvest (most dead merchantable trees removed), and fuel reduction harvest (most dead merchantable trees removed plus most dead trees >10 cm diameter). Density and basal area of live trees trended downward between 1999 and 2011, but this delayed mortality was limited to smaller diameter stems. Dead tree basal area (snags) in unlogged stands declined over 12 years by 74%, compared to an 84% decline in logged stands over the same period. Higher snag fall rate in logged stands over time, combined with snag removal during logging in 1998, resulted in snag densities that were 22% of unlogged stands by 2011. In unlogged mixed-species stands, Douglas-fir snags fell at the same rate as did ponderosa pine snags, but fir snags fell at a faster rate in logged stands. Slash fuel tripled over twelve years in unlogged stands, such that mean slash mass levels became equal for all treatments. Unlogged stands accumulated nearly double the mass of coarse woody debris as logged stands, with most of the difference observed in the sound component of coarse woody fuel. By 13 years after logging (2011), density of regeneration averaged higher overall for unlogged stands, but was highly variable and patchy across all study units. Finally, the Sunshine Fire burned through nearly one complete block of the experiment in 2008, and erased most of the effects of the 1998 logging operation: snags were reduced to <0.5 ha−1 for all treatments, forest floor and slash fuel mass declined to near zero, log mass declined to about 25% that observed in stands unburned by the Sunshine Fire, and shrubs, seedlings, and saplings were nearly eliminated. While the intermediate-term effects of the 1998 logging were still evident in 2011, the 1996 Summit Fire remained the most dominant recent disturbance evident on the landscape.

Collaborative restoration effects on forest structure in ponderosa pine-dominated forests of Colorado

In response to large, severe wildfires in historically fire-adapted forests in the western US, policy initiatives, such as the USDA Forest Service’s Collaborative Forest Landscape Restoration Program (CFLRP), seek to increase the pace and scale of ecological restoration. One required component of this program is collaborative adaptive management, in which monitoring data are used to iteratively evaluate and improve future management actions. Here, we assess the success of seven CFLRP treatments, implemented on 2,300 ha during the first three years of the Colorado Front Range Landscape Restoration Initiative (LRI) at achieving desired forest structure by comparing pre- and post-treatment conditions. We also compare post-treatment conditions with reconstructions of historical (ca. 1860) forest conditions to contextualize the magnitude of treatment effects. Restoration projects moved stands toward desired conditions by reducing basal area, tree density, and canopy cover and increasing average tree diameter, large gap cover, and abundance of small- to medium-sized tree groups. Post-treatment stands were similar to historical stands with respect to basal area of ponderosa pine; however, they had higher total tree density and fewer gaps than historical reference conditions, suggesting that restoration prescriptions may be improved with increased flexibility for density reduction of Douglas-fir and increased gap creation. This examination of early CFLRP treatment outcomes as they relate to desired conditions informs potential areas of adjustments to future treatments and provides baseline data to evaluate the evolution of treatments over the program’s lifespan. We also identify and discuss several scientific, social, and logistical constraints to large-scale restoration success and make several recommendations to improve restoration outcomes.

Changes in forest structure since 1860 in ponderosa pine dominated forests in the Colorado and Wyoming Front Range, USA

Management practices since the late 19th century, including fire exclusion and harvesting, have altered the structure of ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) dominated forests across the western United States. These structural changes have the potential to contribute to uncharacteristic wildfire behavior and effects. Locally-relevant information on historical forest structure can improve efforts to restore more fire adapted conditions. We used a dendrochronological approach to reconstruct pre-settlement era (ca. 1860) structure for 170, 0.5-ha plots in montane ponderosa pine-dominated forests of the Colorado and Wyoming Front Range. Historical reconstructions were quantitatively compared with current conditions to highlight key departures. In lower montane forests, historical basal area averaged 6.3 m2 ha−1, density averaged 97 trees ha−1, and quadratic mean diameter (QMD) averaged 26.5 cm, while current basal area averaged 17.6 m2 ha−1, density averaged 438 trees ha−1, and QMD averaged 24.3 cm. Similar trends were observed in upper montane forests, where historical basal area averaged 9.5 m2 ha−1, historical density averaged 163 trees ha−1, and historical QMD averaged 29.4 cm, while current basal area averaged 17.2 m2 ha−1, current density averaged 389 trees ha−1, and current QMD averaged 25.2 cm. Most differences between historical and current conditions were significant. Across the montane zone, ponderosa pine dominated historical (88% and 83% of basal area in the lower and upper montane, respectively) and current forests (80% and 74% of basal area, respectively), but pine dominance decreased primarily due to infilling of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Much of this establishment occurred around the period of settlement (1861–1920) and continued throughout the 20th century. Results from this study help inform ecological restoration efforts that seek to integrate elements of historical forest structure and aim to increase the resilience of Front Range ponderosa pine forests to future wildfires and a warmer climate.

Predicting forest floor and woody fuel consumption from prescribed burns in southern and western pine ecosystems of the United States

Abstract Reliable estimates of pre-burn biomass and fuel consumption are important to estimate wildland fire emissions and assist in prescribed burn planning. We present empirical models for predicting fuel consumption in natural fuels from 60 prescribed fires in ponderosa pine-dominated forests in the western US and 60 prescribed fires in long-needle pine forests in the southeastern US. There was high variability across sites, but total surface fuel biomass was generally much lower on southern sites (23.0 ± 11.6 Mg ha −1 ) than western sites (61.5 ± 35.8 Mg ha −1 ). Differences in surface fuel composition, pre-burn loading and fuel consumption between the southern and western pine consumption datasets justify the development of regional models for predicting fuel consumption. Southern pine models of herb, shrub and 1-h consumption have close model fit with narrow prediction intervals across the range of sampled values. Relationships between 10-h and 100-h pre-burn loading and consumption produced models with reasonable fit but with no significant correlation with fuel moisture. Model fit of litter and duff consumption models was relatively poor compared to the other southern fuel categories. Western models were developed for 1-h, 10-h and 100-h fine wood, sound coarse wood, rotten coarse wood, litter and duff. All western models had high coefficients of variability, and model residuals indicate higher uncertainty with increasing pre-burn biomass. Although empirical models are widely used, they have limitations in that they are constrained by burning conditions and ranges of predictor variables.

Fortifying the forest: thinning and burning increase resistance to a bark beetle outbreak and promote forest resilience.

Fire frequency in low-elevation coniferous forests in western North America has greatly declined since the late 1800s. In many areas, this has increased tree density and the proportion of shade-tolerant species, reduced resource availability, and increased forest susceptibility to forest insect pests and high-severity wildfire. In response, treatments are often implemented with the goal of increasing ecosystem resilience by increasing resistance to disturbance. We capitalized on an existing replicated study of fire and stand density treatments in a ponderosa pine (Pinus ponderosa)-Douglas-fir (Pseudotsuga menziesii) forest in western Montana, USA, that experienced a naturally occurring mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak 5yr after implementation of fuels treatments. We explored whether treatment effects on tree-level defense and stand structure affected resistance to MPB. Mortality from MPB was highest in the denser, untreated control and burn-only treatments, with approximately 50% and 39%, respectively, of ponderosa pine killed during the outbreak, compared to almost no mortality in the thin-only and thin-burn treatments. Thinning treatments, with or without fire, dramatically increased tree growth and resin ducts relative to control and burn-only treatments. Prescribed burning did not increase resin ducts but did cause changes in resin chemistry that may have affected MPB communication and lowered attack success. While ponderosa pine remained dominant in the thin and thin-burn treatments after the outbreak, the high pine mortality in the control and burn-only treatment caused a shift in species dominance to Douglas-fir. The high Douglas-fir component in the control and burn-only treatments due to 20th century fire exclusion, coupled with high pine mortality from MPB, has likely reduced resilience of this forest beyond the ability to return to a ponderosa pine-dominated system in the absence of further fire or mechanical treatment. Our results show treatments designed to increase resistance to high-severity fire in ponderosa pine-dominated forests in the Northern Rockies can also increase resistance to MPB, even during an outbreak. This study suggests that fuel and restoration treatments in fire-dependent ponderosa pine forests that reduce tree density increase ecosystem resilience in the short term, while the reintroduction of fire is important for long-term resilience.

Overview of the Manitou Experimental Forest Observatory: site description and selected science results from 2008 to 2013

Abstract. The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics &amp;amp; Nitrogen (BEACHON) project seeks to understand the feedbacks and inter-relationships between hydrology, biogenic emissions, carbon assimilation, aerosol properties, clouds and associated feedbacks within water-limited ecosystems. The Manitou Experimental Forest Observatory (MEFO) was established in 2008 by the National Center for Atmospheric Research to address many of the BEACHON research objectives, and it now provides a fixed field site with significant infrastructure. MEFO is a mountainous, semi-arid ponderosa pine-dominated forest site that is normally dominated by clean continental air but is periodically influenced by anthropogenic sources from Colorado Front Range cities. This article summarizes the past and ongoing research activities at the site, and highlights some of the significant findings that have resulted from these measurements. These activities include - soil property measurements; - hydrological studies; - measurements of high-frequency turbulence parameters; - eddy covariance flux measurements of water, energy, aerosols and carbon dioxide through the canopy; - determination of biogenic and anthropogenic volatile organic compound emissions and their influence on regional atmospheric chemistry; - aerosol number and mass distributions; - chemical speciation of aerosol particles; - characterization of ice and cloud condensation nuclei; - trace gas measurements; and - model simulations using coupled chemistry and meteorology. In addition to various long-term continuous measurements, three focused measurement campaigns with state-of-the-art instrumentation have taken place since the site was established, and two of these studies are the subjects of this special issue: BEACHON-ROCS (Rocky Mountain Organic Carbon Study, 2010) and BEACHON-RoMBAS (Rocky Mountain Biogenic Aerosol Study, 2011).

Spatial application of a predictive wildlife occurrence model to assess alternative forest management scenarios in northern Arizona

Wildlife species of conservation concern can present forest managers with a particular challenge when habitat needs appear to be in contrast with other management objectives, particularly fuel reduction to reduce wildfire risk. Proposed actions can be opposed by stakeholders, delaying management activities until a resolution is met. In the southwestern USA, the primary goal of forest management is to reduce the risk of severe wildfire through forest restoration treatments. The USDA Forest Service has designated the northern goshawk (Accipiter gentilis) a management indicator species in this region. However, it has been difficult to achieve a common understanding of goshawk habitat needs among forest stakeholders. We combined two separate and complementary modeling approaches – a statistically based occurrence model and alternative forest treatment models – to yield predictions about forest management effects on the goshawk in ponderosa pine-dominated forests (Pinus ponderosa) on the Kaibab Plateau, Arizona. Forest treatment models were developed based on USDA Forest Service recommendations for goshawk habitat and post-treatment data from ecological restoration experiments, both of which were also components of forest treatment guidance from the Kaibab Forest Health Focus collaborative planning effort. All treatment alternatives resulted in a 22–26% reduction in estimated goshawk occurrence, but the declines were not uniform across the study area, varied by forest type, and were not as large as the effects of recent and severe wildfire (44% reduction in occurrence). Considering the controversial history of forest management with respect to the goshawk, it is prudent to interpret results from this study in the context of tradeoffs between wildfire risk reduction and wildlife habitat quality that can be effectively evaluated through science-based collaborative assessment and planning. While developed for a specific, high-profile species in the southwestern USA, the approach is applicable to many other species whose occurrence has been monitored over multiple years.

Ten years after wildfires: How does varying tree mortality impact fire hazard and forest resiliency?

Severe wildfires across the western US have lead to concerns about heavy surface fuel loading and the potential for high-intensity reburning. Ponderosa pine (Pinus ponderosa) forests, often overly dense from a century of fire suppression, are increasingly susceptible to large and severe wildfires especially given warmer and drier climate projections for the future. However, the majority of research on fuel dynamics after wildfires has focused on high-severity burned areas in more productive forest types. We sampled fuel loadings in 2009 and 2010 across a range of tree mortality on two high-severity wildfires that occurred in 2000: the Pumpkin Fire in Arizona, and the Jasper Fire in South Dakota. We established 60 plots per fire, 10 in each of five post-fire mortality classes: 0–20%, 21–40%, 41–60%, 61–80%, and 81–100% mortality, based on percentage of trees killed, plus unburned control areas. We measured height, diameter, status (alive or dead) and crown base height of each tree, plus fuel loading by size class, litter and duff depth, and herbaceous biomass. Ten years after wildfire, low mortality (0–40%) plots resembled unburned plots in almost every fuels attribute. Basal area in low-mortality plots exceeded reconstructed historical ranges and fire hazard reduction targets by up to 130%. However, coarse woody debris (CWD; woody material >7.62cm) loadings fell below a recommended “optimum” range and herbaceous fuels were sparse. Mid-mortality (41–80%) plots were characterized by more open stands and increased surface fuel loadings, basal area was close to target ranges and CWD loadings were within the recommended range. High mortality (81–100%) plots had few trees but CWD loadings exceeded recommended levels by up to 28%, and herbaceous fuels were adequate to carry a surface fire. These findings suggest that post-fire management should be targeted to the level of mortality. Low mortality and unburned areas should be targeted for reducing stand densities and promoting understory growth, to minimize crown fire hazard and increase site potential. Burned areas with >80% tree mortality have the lowest crown fire hazard, but may benefit from fuel reduction efforts such as low intensity prescribed burning to reduce both fine fuels and some of the CWD. Stand structures and surface fuel loads in mid-mortality plots most resembled historical targets and met numerous restoration objectives for ponderosa pine-dominated forests, especially given predicted climate changes. These areas can be maintained through low-intensity prescribed burning to prevent them from becoming overly dense and thus enhance their longer-term resiliency to future disturbances.

Fuel mass and stand structure after post-fire logging of a severely burned ponderosa pine forest in northeastern Oregon

Stand structure and fuel mass were measured before and after a post-fire logging operation conducted 2 years after the 1996 Summit Wildfire (Malheur National Forest), in a ponderosa pine-dominated forest in northeastern Oregon. Variables were measured both pre- and post-logging in four replicate units for each of three treatments [un-logged control, commercial harvest (most dead merchantable trees removed), fuel reduction harvest (most dead merchantable trees removed plus most dead trees >10 cm diameter)]. Post-fire logging resulted in a significant decrease in mean basal area, down to 46% pre-treatment level in commercial units, and down to 25% in fuel reduction units. Logging significantly reduced tree density, especially for the smallest (<22 cm diameter) and intermediate (23–41 cm) diameter classes. Fuel reduction units also had significantly fewer snags (dead trees >30 cm diameter—4 ha −1), compared to both commercial (23 ha −1) units and to un-logged controls (64 ha −1) in the year following timber harvest. Logging did not change ladder height or tree species composition (% ponderosa pine, Douglas-fir and grand fir). Total woody fuel mass increased significantly in fuel reduction units when compared to controls, with the greatest difference among treatments occurring in the slash fuel (<7.6 cm diameter) component (mean of 6.2 Mg/ha for fuel reduction stands versus 1.3 Mg/ha for un-logged stands). Logging activity caused no change in the mass of the forest floor (litter or duff). Model projections of the fuel bed using the fire and fuels extension of the forest vegetation simulator (FVS–FFE) indicate that the disparity in slash fuel mass between fuel reduction and un-logged units would be sustained until about 15 years post-logging, but a re-burn of moderate intensity occurring during this time would likely kill all young trees, even in un-logged units, because of the influence of other components of the fuel bed, such as grasses and shrubs. Model projections of 1000-h fuels (woody fuels >7.6 cm diameter) indicate that standing structure in all stands would collapse quickly, with the result that un-logged stands would contain two- or three-fold greater masses at 25 and 50 years post-logging, leading to much higher consumption rates of fuel in the event of a re-burn in the same place. Variation in dead tree fall and decay rates did not change the relationship among treatments in 1000-h fuel loads, but changed the time at which treatment differences were projected to disappear. Despite treatment differences in heavy fuel accumulations over time however, FVS–FFE predicts no differences among treatments in mortality of young trees due to either moderate or high intensity fire occurring in the same place at 25, 50, or 100 years post-fire logging. The lack of a re-burn effect is in part due to the reliance on flame length as the primary mechanism leading to tree death in the fire effect models used by FVS–FFE. If tree death turns out to be caused more by root burning or cambial heating, the observed variations in 1000-h fuel loadings among treatments could be significant in the event of a future re-burn.

Advanced Exploratory Data Analysis for Mapping Regional Canopy Cover

USGS Digital Orthophoto Quadrangles (DOQs) are a form of inexpensive, high spatial resolution (1 m ground resolution) imagery available for most regions of the United States. Typically, DOQs have been used in the construction of “basemaps” or as training datasets. In this paper we present a technical approach that uses DOQs as the primary data source to map regional forest canopy cover. This approach, a form of advanced exploratory data analysis (AEDA), can separate areas of crown, shadow, and non-crown vegetation using a single value threshold and a “value range” threshold obtained by analyzing the statistical plots built on a multifractal model. By applying AEDA, we can distinguish the crown, crown boundary zone, shadow areas, and non-crown areas within a DOQ mosaic by their distinctive multifractal properties. Over a period of two years, we mapped canopy cover of 20,000 km 2 of ponderosa pine-dominated forest across northern Arizona using this technique. We used two hundred ground plot measurements from an independent source to assess the accuracy of the canopy cover map across an 8,000 km 2 region.

Post-fire vegetative dynamics as drivers of microbial community structure and function in forest soils

Soil microorganisms have numerous functional roles in forest ecosystems, including: serving as sources and sinks of key nutrients and catalysts of nutrient transformations; acting as engineers and maintainers of soil structure; and forming mutualistic relationships with roots that improve plant fitness. Although both prescribed and wildland fires are common in temperate forests of North America, few studies have addressed the long-term influence of such disturbances on the soil microflora in these ecosystems. Fire alters the soil microbial community structure in the short-term primarily through heat-induced microbial mortality. Over the long-term, fire may modify soil communities by altering plant community composition via plant-induced changes in the soil environment. In this review, we summarize and synthesize the various studies that have assessed the effects of fire on forest soil microorganisms, emphasizing the mechanisms by which fire impacts these vital ecosystem engineers. The examples used in this paper are derived primarily from studies of ponderosa pine-dominated forests of the Inland West of the USA; these forests have some of the shortest historical fire-return intervals of any forest type, and thus the evolutionary role of fire in shaping these forests is likely the strongest. We argue that the short-term effects of fire on soil microflora and the processes they catalyze are transient, and suggest that more research be devoted to linking long-term plant community responses with those of the mutually dependent soil microflora.

Annual and decadal climate forcing of historical fire regimes in the interior Pacific Northwest, USA

Anticipating the consequences of climatic change for fire requires understanding of the causes of variation in historical fire regimes. We assessed the influence of annual and decadal variation in climate on fire regimes of ponderosa pine-dominated forests in eastern Oregon and Washington using existing, annually dated tree-ring reconstructions (1687–1994). In four watersheds, we compared the extent of low-severity fires (total area burned each year) to precipitation and the Southern Oscillation Index, a measure of variation in El Niño-Southern Oscillation (ENSO), which affects weather in this region. At the annual scale, large fires burned during dry years and El Niño years (low SOI) in all watersheds while small fires burned regardless of variation in these climate parameters. Large fires also burned during relatively wet years and La Niña years (high SOI) in one watershed, indicating that local factors can override regional climate controls in some locations. Climate from previous years did not influence current year's fire extent. The influence of ENSO on fire regimes in this region has not previously been demonstrated at these multicentury, regional scales. At the decadal scale, fire extent varied with precipitation, perhaps in response to variation in such climate features as the Pacific Decadal Oscillation. Several decades of low fire extent in the watersheds during the early 1800s was synchronous with a lack of fire at other sites in North and South America, probably in response to a change in the global climate that included a lessening in the frequency and/or intensity of ENSO events.