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USGS1998

Understory vegetation in old and young Douglas-fir forests of western Oregon

We studied understory composition in thinned and unthinned Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco)/western hemlock ( Tsuga heterophylla (Raf.) Sarg.) stands on 28 sites in western Oregon. These stands had regenerated naturally after timber harvest, 40–70 years before thinning. Commercial thinning had occurred 10–24 years previously, with 8–60% of the volume removed from below with the intent to homogenize spacing among trees. Undisturbed old-growth Douglas-fir stands were present for comparison on 18 of these sites. Total herbaceous cover was greater in thinned (25% cover) stands than in unthinned (13% cover) or old-growth (15% cover) stands. Species richness was also greater in thinned (137) than in unthinned (114) and old-growth (91) stands ( P =0.05). Part of the increased richness was caused by the presence of exotic species in thinned stands, but there were also more native grass and nitrogen-fixing species in thinned stands than in unthinned or old-growth stands. Groups of species differed among stand-types. For example, the frequency of tall cordate-leaved species was greater in old-growth stands ( P =0.009), but their relative cover was different only between old-growth and unthinned stands ( P =0.08). Both the cover and frequency of grasses and sedges in thinned stands were greater than in unthinned or old-growth stands ( P ≤0.002). Ordination of shrub cover showed differences among old-growth and unthinned stands compared to thinned stands, mainly because of the amount of Gaultheria shallon Pursh and Polystichum munitum (Kaulf.) Presl in heavily thinned stands. Ordination of herbaceous community data showed that there were much stronger differences among sites than among stand-types. The lack of difference among stand-types demonstrates the resiliency of herbaceous communities to disturbance associated with past and current forest management.

Oregon↗

USGS2021

Regeneration trends along climate gradients in Taxodium distichum forests of the southeastern United States

The development of relict vegetation at the edges of some ecosystems has taken place particularly in environments where the regeneration of foundational species is declining. As an important stage of regeneration in the Taxodium distichum , this study explored the relationship of cone volume and seed number across environmental gradients in the Mississippi River Alluvial Valley (MRAV) and northern Gulf of Mexico Coast (GOM) in a long-term network of forested wetlands (North American Baldcypress Swamp Network (NABCSN)) from 2007 to 2019. Resembling spheroids, the volumes of Taxodium distichum cones were based on the measured dimensions of the cones collected in swamps across southeastern environmental gradients. Cones with larger volumes also had larger numbers of seeds (r 2 = 0.423, F = 113.9, p < 0.0001; Linear regression equation: Seed number per cone = 9.8925223 + 0.8854056* Cone volume cm 3 ). Mean cone volumes were related to water availability with highest volumes in locations with moderate amounts of total annual precipitation (e.g., White River National Wildlife Refuge (NWR) Arkansas, Tensas NWR Louisiana, and Morgan Brake NWR Mississippi), and longer periods of annual percent time of site drawdown. Cone volume was high in 2018 following the 2017 mega-flood in the Mississippi River Alluvial Valley (MRAV) generated by Hurricane Harvey. Mean annual air temperature was not related to cone volume. Along the Gulf Coast, mean cone volume increased from east to west from Florida to Texas. Especially near the edge of the range of T. distichum forests, smaller cones may be related to regeneration failure and lower seed numbers to support regeneration. A better understanding of regeneration constraints can inform managers of the emergence of relict status in these forests.

Arkansas, Florida, Illinois, Kentucky, Louisiana, ↗

USGS2012

Pattern and process of prescribed fires influence effectiveness at reducing wildfire severity in dry coniferous forests

We examined the effects of three early season (spring) prescribed fires on burn severity patterns of summer wildfires that occurred 1–3 years post-treatment in a mixed conifer forest in central Idaho. Wildfire and prescribed fire burn severities were estimated as the difference in normalized burn ratio (dNBR) using Landsat imagery. We used GIS derived vegetation, topography, and treatment variables to generate models predicting the wildfire burn severity of 1286–5500 30-m pixels within and around treated areas. We found that wildfire severity was significantly lower in treated areas than in untreated areas and significantly lower than the potential wildfire severity of the treated areas had treatments not been implemented. At the pixel level, wildfire severity was best predicted by an interaction between prescribed fire severity, topographic moisture, heat load, and pre-fire vegetation volume. Prescribed fire severity and vegetation volume were the most influential predictors. Prescribed fire severity, and its influence on wildfire severity, was highest in relatively warm and dry locations, which were able to burn under spring conditions. In contrast, wildfire severity peaked in cooler, more mesic locations that dried later in the summer and supported greater vegetation volume. We found considerable evidence that prescribed fires have landscape-level influences within treatment boundaries; most notable was an interaction between distance from the prescribed fire perimeter and distance from treated patch edges, which explained up to 66% of the variation in wildfire severity. Early season prescribed fires may not directly target the locations most at risk of high severity wildfire, but proximity of these areas to treated patches and the discontinuity of fuels following treatment may influence wildfire severity and explain how even low severity treatments can be effective management tools in fire-prone landscapes.

Idaho↗

USGS2016

Oriental bittersweet ( Celastrus orbiculatus ): Spreading by fire

In many forest ecosystems, fire is critical in maintaining indigenous plant communities, but can either promote or arrest the spread of invasive species depending on their regeneration niche and resprouting ability. We examined the effects of cutting and burning treatments on the vegetative response (cover, stem density) and root resources of Oriental bittersweet ( Celastrus orbiculatus ), a liana invasive to North America that was introduced from East Asia. Treatments were control, spring cut, spring burn, spring cut & burn, summer cut, fall cut, fall burn, fall cut & burn, and fall herbicide. Cover was reduced the greatest by herbicide and summer cutting treatments, but increased more in the second year on moraine soils than on sandy soils. Burning and cutting & burning combined resulted in a resprout density four times greater than stem density prior to treatment for stems <2.5 mm diameter than cutting alone. For stems, across all diameter classes, there was a more than 100% increase in stem density with burning and almost a 300% increase in stem density with cutting & burning in the spring. Density of resprouts and root-suckers, and survival increased with increasing stem size. While cutting of C. orbiculatus during the growing season (summer) reduced total nonstructural carbohydrates by 50% below early growing season levels and 75% below dormant season levels, burning did not significantly reduce total nonstructural carbohydrates. Thus, Oriental bittersweet is quite responsive to burning as a disturbance and resprouting and root-suckering creates additional opportunities for growth and attainment of the forest canopy. The positive response of Oriental bittersweet to burning has important implications for management of invasive lianas in fire-dependent forest landscapes.

Indiana↗

USGS2022

What is a stand? Assessing the variability of composition and structure in floodplain forest ecosystems across spatial scales in the Upper Mississippi River

The forest stand typically represents relatively homogenous forest conditions; the forest stand is generally the unit at which forest attributes are assessed, summarized, and subsequently managed. However, some ecosystems, such as the floodplain forests of the Upper Mississippi River (UMR), can exhibit high variability at fine spatial scales that can confound prescription development, implementation, and ultimate success of stand-level management actions. Here we assess how forest composition and structure vary within and across stand management units on the UMR and test at what spatial scale environmental variables relate to forest characteristics. We found that plot-level measures of composition, structure, and diversity were not well represented by site-level averages of these values. When basal area of all overstory species was combined, this variable was more closely related to “site” than to any of the environmental variables, but when analyzed by species, within-plot topographic variation (“microtopography”) was a significant positive predictor for both importance values of an individual species (swamp white oak ( Quercus bicolor Willd.)) as well as importance value of a specific group of species (oaks (Quercus spp.), bitternut hickory ( Carya cordiformis (Wangenh.) K. Koch), hackberry ( Celtis occidentalis L.), and American basswood ( Tilia americana L.)). This work highlights the challenges of using average stand conditions to summarize complex or heterogeneous systems and the need for flexibility and relaxed assumptions in defining management units in these forests.

Iowa, Minnesota, Wisconsin↗

USGS2026

Fostering landscape resilience and species conservation in frequent fire landscapes

Restoration of dry forest landscapes of the Pacific Northwest often involves the removal of some closed-canopy forest that has expanded due to fire exclusion, but this can conflict with species conservation when species require closed-canopy forest as part of their habitat. Fire refugia—areas within fire perimeters that burn less severely or frequently—might provide a solution to this conflict. We used a species-specific approach to understanding fire refugia for northern spotted owls to model the post-fire persistence of nesting and roosting (NR) forest in the frequent fire landscapes of the eastern Washington Cascades and the Klamath province and parts of the southern Cascades. Persistent NR forest occurred in topographically sheltered locations and lower slope positions whereas less persistent NR forest occurred in topographically exposed locations. These patterns of persistence reflect historical vegetation patterns when complex interactions among topography, vegetation and weather maintained resilient landscapes. These vegetation patterns may also resemble higher quality heterogenous habitat for spotted owls. Our models can help managers achieve the dual objectives of maintaining viable populations of native species, including those that rely on closed-canopy forests, while also restoring resilient landscapes. In an era of rapid ecological change, we suggest that northern spotted owl conservation and fire-adapted landscape restoration are not opposing goals. By identifying the biophysical settings that promote persistent NR forest and acknowledging the role of heterogeneous landscapes for maintaining resilient ecosystems and high quality spotted owl habitat, the dual objectives of supporting long-term resilience for native species and ecosystems can be achieved.

California, Oregon, Washington↗

USGS2016

Wood decay in desert riverine environments

Floodplain forests and the woody debris they produce are major components of riverine ecosystems in many arid and semiarid regions (drylands). We monitored breakdown and nitrogen dynamics in wood and bark from a native riparian tree, Fremont cottonwood ( Populus deltoides subsp. wislizeni ), along four North American desert streams. We placed locally-obtained, fresh, coarse material [disks or cylinders (&sim;500–2000 cm 3 )] along two cold-desert and two warm-desert rivers in the Colorado River Basin. Material was placed in both floodplain and aquatic environments, and left in situ for up to 12 years. We tested the hypothesis that breakdown would be fastest in relatively warm and moist aerobic environments by comparing the time required for 50% loss of initial ash-free dry matter ( T 50 ) calculated using exponential decay models incorporating a lag term. In cold-desert sites (Green and Yampa rivers, Colorado), disks of wood with bark attached exposed for up to 12 years in locations rarely inundated lost mass at a slower rate ( T 50 = 34 yr) than in locations inundated during most spring floods ( T 50 = 12 yr). At the latter locations, bark alone loss mass at a rate initially similar to whole disks ( T 50 = 13 yr), but which subsequently slowed. In warm-desert sites monitored for 3 years, cylinders of wood with bark removed lost mass very slowly ( T 50 = 60 yr) at a location never inundated (Bill Williams River, Arizona), whereas decay rate varied among aquatic locations ( T 50 = 20 yr in Bill Williams River; T 50 = 3 yr in Las Vegas Wash, an effluent-dominated stream warmed by treated wastewater inflows). Invertebrates had a minor role in wood breakdown except at in-stream locations in Las Vegas Wash. The presence and form of change in nitrogen content during exposure varied among riverine environments. Our results suggest woody debris breakdown in desert riverine ecosystems is primarily a microbial process with rates determined by landscape position, local weather, and especially the regional climate through its effect on the flow regime. The increased warmth and aridity expected to accompany climate change in the North American southwest will likely retard the already slow wood decay process on naturally functioning desert river floodplains. Our results have implications for designing environmental flows to manage floodplain forest wood budgets, carbon storage, and nutrient cycling along regulated dryland rivers.

Arizona, Colorado, Nevada↗

USGS2020

Post-fire aspen (Populus tremuloides) regeneration varies in response to winter precipitation across a regional climate gradient

Altered climate and changing fire regimes are synergistically impacting forest communities globally, resulting in deviations from historical norms and creation of novel successional dynamics. These changes are particularly important when considering the stability of a keystone species such as quaking aspen ( Populus tremuloides Michx.), which contributes critical ecosystem services across its broad North American range. As a relatively drought intolerant species, projected changes of altered precipitation timing, amount, and type (e.g. snow or rain) may influence aspen response to fire, especially in moisture-limited and winter precipitation-dominated portions of its range. Aspen is generally considered an early-seral species that benefits from fire, but increases in fire activity across much of the western United States could affect the species in unpredictable ways. This study examined post-fire aspen stands across a regional climate gradient spanning from the north-central Great Basin to the northeastern portion of the Greater Yellowstone Ecosystem (USA). We investigated the influence of seasonal precipitation and temperature variables, snowpack, and site conditions (e.g. browsing levels, topography) on density of post-fire aspen regeneration (i.e. all small trees ha −1 ) and recruitment (i.e. small trees ≥2 m tall ha −1 ) across 15 fires that occurred between 2000 and 2009. The range of post-fire regeneration (2500–71,600 small trees ha −1 ) and recruitment (0–32,500 small trees ≥2 m ha −1 ) densities varied widely across plots. Linear mixed effects models demonstrated that both response variables increased primarily with early winter (Oct-Dec) precipitation during the ‘fire-regen period’ (i.e., fire year and five years after fire) relative to the 30-year mean. The 30-year mean of early winter precipitation and fire-regen period snowpack were also positively related to recruitment densities. Both response variables decreased with higher shrub cover, highlighting the importance of considering shrub competition in post-fire environments. Regeneration and recruitment densities were negatively related to proportion browsed aspen leaders and animal pellet densities (no./m 2 ), respectively, indicating the influence of ungulate browsing even at the relatively low levels observed across sites. A post-hoc exploratory analysis suggests that deviation in early winter precipitation during the fire-regen period (relative to 30-year means) varied among sites along directional gradients, emphasizing the need to consider multiple spatiotemporal scales when investigating climate effects on post-fire successional dynamics. We discuss our findings in terms of dynamic management and conservation strategies in light of changing fire regimes and climate conditions.

Idaho, Montana, Nevada, Utah↗

USGS2026

Quantifying post-fire live tree presence and spatial variation using Sentinel-2 time series

Accurate mapping of post-fire surviving trees is important for tracking forest recovery and prioritizing land management decisions. Satellite-based remote sensing is an effective method to assess post-fire forest conditions. Traditionally, differenced satellite-derived burn severity indices are computed by differencing one year pre- and post-fire spectral reflectance values. Differenced burn severity indices are useful for quantifying and mapping the magnitude of ecological change, but their application to detecting and mapping post-fire live trees may not be as appropriate, particularly for delayed tree mortality. Delayed tree mortality (“delayed mortality”) is a phenomenon where trees that initially survive fire then die over an extended period (between one and five years), and it can be challenging to measure and predict. In this study, we demonstrate the potential of mapping delayed mortality using readily available remotely sensed imagery alone. We used random forest models to detect post-fire live trees using 10-m resolution Sentinel-2 data at one-, three-, and five-years post-fire for four fires in the southern Sierra Nevada, California, USA. Using imagery from the National Agriculture Imagery Program (NAIP; 60-cm resolution), we manually classified live tree presence in 6000 Sentinel-2 pixels (500 pixels for each fire-year combination) to calibrate and validate models. Sentinel-2 based model accuracies ranged from 65 % to 86 % with F-scores ranging from 0.52 to 0.86, and their predictions of live pixel area were on average 44 % lower than inferred from more traditional indices such as relative differenced normalized burn ratio (RdNBR). This work represents a promising first step in using freely available post-fire spectral reflectance imagery to detect live trees over an extended period to support post-fire management.

California↗

USGS2011

Enhanced transpiration by riparian buffer trees in response to advection in a humid temperate agricultural landscape

Riparian buffers are designed as management practices to increase infiltration and reduce surface runoff and transport of sediment and nonpoint source pollutants from crop fields to adjacent streams. Achieving these ecosystem service goals depends, in part, on their ability to remove water from the soil via transpiration. In these systems, edges between crop fields and trees of the buffer systems can create advection processes, which could influence water use by trees. We conducted a field study in a riparian buffer system established in 1994 under a humid temperate climate, located in the Corn Belt region of the Midwestern U.S. (Iowa). The goals were to estimate stand level transpiration by the riparian buffer, quantify the controls on water use by the buffer system, and determine to what extent advective energy and tree position within the buffer system influence individual tree transpiration rates. We primarily focused on the water use response (determined with the Heat Ratio Method) of one of the dominant species ( Acer saccharinum ) and a subdominant ( Juglans nigra ). A few individuals of three additional species ( Quercus bicolor, Betula nigra, Platanus occidentalis ) were monitored over a shorter time period to assess the generality of responses. Meteorological stations were installed along a transect across the riparian buffer to determine the microclimate conditions. The differences found among individuals were attributed to differences in species sap velocities and sapwood depths, location relative to the forest edge and prevailing winds and canopy exposure and dominance. Sapflow rates for A. saccharinum trees growing at the SE edge (prevailing winds) were 39% greater than SE interior trees and 30% and 69% greater than NW interior and edge trees, respectively. No transpiration enhancement due to edge effect was detected in the subdominant J. nigra . The results were interpreted as indicative of advection effects from the surrounding crops. Further, significant differences were document in sapflow rates between the five study species, suggesting that selection of species is important for enhancing specific riparian buffer functions. However, more information is needed on water use patterns among diverse species growing under different climatic and biophysical conditions to assist policy and management decisions regarding effective buffer design.

Forest Ecology and Management↗

USGS2026

Root biomass and productivity in coastal swamp forests of Louisiana (USA)

Coastal swamp forests (CSF) occupy approximately 225,000 ha in Southeastern Louisiana (USA), and across a large percentage of this area, CSF are subjected to sediment starvation, salinity intrusion, and chronic submergence. We assessed root biomass and productivity in nine CSF in Louisiana, and found that root biomass ranged from 420.8 g m −2 to 1712.2 g m −2 to a soil depth of 30 cm. Root biomass generally increased with salinity up to a mean and maximum of 1.8 and 4.3 psu, respectively, deviating from how aboveground biomass typically patterns. Submergence is a likely explanation for this deviation given that fresher sites had long hydroperiods with deeper water than salinized sites. In contrast, root productivity over a single year ranged from 112.1 g m −2 y −1 to 844.1 g m −2 y −1 , but did not relate strongly to salinity. On average, 16–87% of live roots were replaced each year among sites, leading to root longevities of 1.5–6.3 years. Also, root turnover was generally lower at more saline sites and higher from freshwater sites under greater submergence. Periodic water drawdown as a result of management intervention to reduce long-hydroperiod, chronic flooding by re-connecting river or tide to degraded CSF might stimulate root productivity and biomass accrual. Sediment amendment, strategic breaching of levees and gapping, and river water re-introductions are possible management actions.

Louisiana↗

USGS2013

Landscape-scale effects of fire severity on mixed-conifer and red fir forest structure in Yosemite National Park

While fire shapes the structure of forests and acts as a keystone process, the details of how fire modifies forest structure have been difficult to evaluate because of the complexity of interactions between fires and forests. We studied this relationship across 69.2 km2 of Yosemite National Park, USA, that was subject to 32 fires ⩾40 ha between 1984 and 2010. Forests types included ponderosa pine ( Pinus ponderosa ), white fir-sugar pine ( Abies concolor/Pinus lambertiana ), and red fir ( Abies magnifica ). We estimated and stratified burned area by fire severity using the Landsat-derived Relativized differenced Normalized Burn Ratio (RdNBR). Airborne LiDAR data, acquired in July 2010, measured the vertical and horizontal structure of canopy material and landscape patterning of canopy patches and gaps. Increasing fire severity changed structure at the scale of fire severity patches, the arrangement of canopy patches and gaps within fire severity patches, and vertically within tree clumps. Each forest type showed an individual trajectory of structural change with increasing fire severity. As a result, the relationship between estimates of fire severity such as RdNBR and actual changes appears to vary among forest types. We found three arrangements of canopy patches and gaps associated with different fire severities: canopy-gap arrangements in which gaps were enclosed in otherwise continuous canopy (typically unburned and low fire severities); patch-gap arrangements in which tree clumps and gaps alternated and neither dominated (typically moderate fire severity); and open-patch arrangements in which trees were scattered across open areas (typically high fire severity). Compared to stands outside fire perimeters, increasing fire severity generally resulted first in loss of canopy cover in lower height strata and increased number and size of gaps, then in loss of canopy cover in higher height strata, and eventually the transition to open areas with few or no trees. However, the estimated fire severities at which these transitions occurred differed for each forest type. Our work suggests that low severity fire in red fir forests and moderate severity fire in ponderosa pine and white fir-sugar pine forests would restore vertical and horizontal canopy structures believed to have been common prior to the start of widespread fire suppression in the early 1900s. The fusion of LiDAR and Landsat data identified post-fire structural conditions that would not be identified by Landsat alone, suggesting a broad applicability of combining Landsat and LiDAR data for landscape-scale structural analysis for fire management.

California↗

USGS2000

Estimating forest crown area removed by selection cutting: a linked regression-GIS approach based on stump diameters

The purpose of this research was to develop a model that could be used to provide a spatial representation of uneven-aged silvicultural treatments on forest crown area. We began by developing species-specific linear regression equations relating tree DBH to crown area for eight bottomland tree species at White River National Wildlife Refuge, Arkansas, USA. The relationships were highly significant for all species, with coefficients of determination (r(2)) ranging from 0.37 for Ulmus crassifolia to nearly 0.80 for Quercus nuttalliii and Taxodium distichum. We next located and measured the diameters of more than 4000 stumps from a single tree-group selection timber harvest. Stump locations were recorded with respect to an established gl id point system and entered into a Geographic Information System (ARC/INFO). The area occupied by the crown of each logged individual was then estimated by using the stump dimensions (adjusted to DBHs) and the regression equations relating tree DBH to crown area. Our model projected that the selection cuts removed roughly 300 m(2) of basal area from the logged sites resulting in the loss of approximate to 55 000 m(2) of crown area. The model developed in this research represents a tool that can be used in conjunction with remote sensing applications to assist in forest inventory and management, as well as to estimate the impacts of selective timber harvest on wildlife.

Forest Ecology and Management↗

USGS2015

Woodland salamander responses to a shelterwood harvest-prescribed burn silvicultural treatment within Appalachian mixed-oak forests

Forest management practices that mimic natural canopy disturbances, including prescribed fire and timber harvests, may reduce competition and facilitate establishment of favorable vegetative species within various ecosystems. Fire suppression in the central Appalachian region for almost a century has contributed to a transition from oak-dominated to more mesophytic, fire-intolerant forest communities. Prescribed fire coupled with timber removal is currently implemented to aid in oak regeneration and establishment but responses of woodland salamanders to this complex silvicultural system is poorly documented. The purpose of our research was to determine how woodland salamanders respond to shelterwood harvests following successive burns in a central Appalachian mixed-oak forest. Woodland salamanders were surveyed using coverboard arrays in May, July, and August–September 2011 and 2012. Surveys were conducted within fenced shelterwood-burn (prescribed fires, shelterwood harvest, and fencing to prevent white-tailed deer [Odocoileus virginianus] herbivory), shelterwood-burn (prescribed fires and shelterwood harvest), and control plots. Relative abundance was modeled in relation to habitat variables measured within treatments for mountain dusky salamanders (Desmognathus ochrophaeus), slimy salamanders (Plethodon glutinosus), and eastern red-backed salamanders (Plethodon cinereus). Mountain dusky salamander relative abundance was positively associated with canopy cover and there were significantly more individuals within controls than either shelterwood-burn or fenced shelterwood-burn treatments. Conversely, habitat variables associated with slimy salamanders and eastern red-backed salamanders did not differ among treatments. Salamander age-class structure within controls did not differ from shelterwood-burn or fenced shelterwood-burn treatments for any species. Overall, the woodland salamander assemblage remained relatively intact throughout the shelterwoodburn silvicultural treatment compared to previous research within the same study area that examined pre-harvest fire effects. However, because of the multi-faceted complexities of this specific silvicultural system, continued research is warranted that evaluates long-term, additive impacts on woodland salamanders within managed central Appalachian deciduous forests.

West Virginia↗

USGS2016

Spatial and temporal trends of drought effects in a heterogeneous semi-arid forest ecosystem

Drought has long been recognized as a driving mechanism in the forests of western North America and drought-induced mortality has been documented across genera in recent years. Given the frequency of these events are expected to increase in the future, understanding patterns of mortality and plant response to severe drought is important to resource managers. Drought can affect the functional, physiological, structural, and demographic properties of forest ecosystems. Remote sensing studies have documented changes in forest properties due to direct and indirect effects of drought; however, few studies have addressed this at local scales needed to characterize highly heterogeneous ecosystems in the forest-shrubland ecotone. We analyzed a 22-year Landsat time series (1985–2012) to determine changes in forest in an area that experienced a relatively dry decade punctuated by two years of extreme drought. We assessed the relationship between several vegetation indices and field measured characteristics (e.g. plant area index and canopy gap fraction) and applied these indices to trend analysis to uncover the location, direction and timing of change. Finally, we assessed the interaction of climate and topography by forest functional type. The Normalized Difference Moisture Index (NDMI), a measure of canopy water content, had the strongest correlation with short-term field measures of plant area index ( R 2 = 0.64) and canopy gap fraction ( R 2 = 0.65). Over the entire time period, 25% of the forested area experienced a significant ( p -value < 0.05) negative trend in NDMI, compared to less than 10% in a positive trend. Coniferous forests were more likely to be associated with a negative NDMI trend than deciduous forest. Forests on southern aspects were least likely to exhibit a negative trend while north aspects were most prevalent. Field plots with a negative trend had a lower live density, and higher amounts of standing dead and down trees compared to plots with no trend. Our analysis identifies spatially explicit patterns of long-term trends anchored with ground based evidence to highlight areas of forest that are resistant, persistent or vulnerable to severe drought. The results provide a long-term perspective for the resource management of this area and can be applied to similar ecosystems throughout western North America.

Colorado, Utah, Wyoming↗

USGS2017

Characterizing interactions between fire and other disturbances and their impacts on tree mortality in western U.S. Forests

Increasing evidence that pervasive warming trends are altering disturbance regimes and their interactions with fire has generated substantial interest and debate over the implications of these changes. Previous work has primarily focused on conditions that promote non-additive interactions of linked and compounded disturbances, but the spectrum of potential interaction patterns has not been fully considered. Here we develop and define terminology, expand on the existing conceptual framework and review the patterns and mechanisms of disturbance interactions with a focus on interactions between fire and other forest disturbances and a specific emphasis on resulting tree mortality. The types of interactions reflect the positive, negative, or neutral responses to the incidence, intensity, and effects of the interaction. These types of interactions are not always mutually exclusive, but can be distinct. The collective effect of the interactions will determine the longer-term ecosystem response that can result in a resistant, resilient, or compounded interaction. Our review indicates that the interactions of drought, bark beetles , or pathogens with fire often result in neutral or maintained interactions that do not negatively or positively influence the incidence or intensity following fire. The effect of these disturbance interactions on tree mortality ranged from antagonistic (reduced mortality compared to individual disturbances) to synergistic (greater mortality compared to individual disturbances) within and among disturbance interaction types but often resulted in additive effects (mortality is consistent with the summation of the two disturbances). Synergistic effects on tree mortality have been observed when the severity of the initial disturbance is moderate to high and time between disturbances is relatively short. When the sequence of disturbance interaction is reversed (e.g., fire precedes other disturbances) the conditions can generally promote impeded interactions (lower incidence of interaction), reduced interactions (lower intensity of interaction), and antagonistic interactions (lower tree mortality). While recent research on fire-disturbance interactions has increased over the last decade and provided important insights, more research that identifies the specific thresholds of incidence, intensity, and effects of interaction by region and forest type are needed to better assist management solutions that promote desired outcomes in rapidly changing ecosystems.

Forest Ecology and Management↗

USGS2012

Recent advances in applying decision science to managing national forests

Management of federal public forests to meet sustainability goals and multiple use regulations is an immense challenge. To succeed, we suggest use of formal decision science procedures and tools in the context of structured decision making (SDM). SDM entails four stages: problem structuring (framing the problem and defining objectives and evaluation criteria), problem analysis (defining alternatives, evaluating likely consequences, identifying key uncertainties, and analyzing tradeoffs), decision point (identifying the preferred alternative), and implementation and monitoring the preferred alternative with adaptive management feedbacks. We list a wide array of models, techniques, and tools available for each stage, and provide three case studies of their selected use in National Forest land management and project plans. Successful use of SDM involves participation by decision-makers, analysts, scientists, and stakeholders. We suggest specific areas for training and instituting SDM to foster transparency, rigor, clarity, and inclusiveness in formal decision processes regarding management of national forests.

Forest Ecology and Management↗

USGS2018

Thinning, tree-growth, and resistance to multi-year drought in a mixed-conifer forest of northern California

Drought is an important stressor in forest ecosystems that can influence tree vigor and survival. In the U.S., forest managers use two primary management techniques to promote resistance and resilience to drought: prescribed fire and mechanical thinning. Generally applied to reduce fuels and fire hazard, treatments may also reduce competition for resources that may improve tree-growth and reduce mortality during drought. A recent severe and prolonged drought in California provided a natural experiment to investigate tree-growth responses to fuel treatments and climatic stress. We assessed tree-growth from 299 ponderosa pine ( Pinus ponderosa ) and Douglas-fir ( Pseudotsuga menziesii ) in treated and untreated stands during severe drought from 2012 to 2015 in the mixed-conifer forests of Whiskeytown National Recreation Area (WNRA) in northern California. The treatment implemented at WNRA removed 34% of live basal area through mechanical thinning with a subsequent pile burning of residual fuels. Tree-growth was positively associated with crown ratio and negatively associated with competition and a 1-year lag of climate water deficit, an index of drought. Douglas-fir generally had higher annual growth than ponderosa pine, although factors affecting growth were the same for both species. Drought resistance, expressed as the ratio between mean growth during drought and mean growth pre-drought, was higher in treated stands compared to untreated stands during both years of severe drought (2014 and 2015) for ponderosa pine but only one year (2014) for Douglas-fir. Thinning improved drought resistance, but tree size, competition and species influenced this response. On-going thinning treatments focused on fuels and fire hazard reduction are likely to be effective at promoting growth and greater drought resistance in dry mixed-conifer forests. Given the likelihood of future droughts, land managers may choose to implement similar treatments to reduce potential impacts.

Forest Ecology and Management↗