Forest Restoration Treatments Research Articles

Restoration treatments enhance tree growth and alter climatic constraints during extreme drought.

The frequency and severity of drought events are predicted to increase due to anthropogenic climate change, with cascading effects across forested ecosystems. Management activities such as forest thinning and prescribed burning, which are often intended to mitigate fire hazard and restore ecosystem processes, may also help promote tree resistance to drought. However, it is unclear whether these treatments remain effective during the most severe drought conditions or whether their impacts differ across environmental gradients. We used tree-ring data from a system of replicated, long-term (>20 years) experiments in the southwestern United States to evaluate the effects of forest restoration treatments (i.e., evidence-based thinning and burning) on annual growth rates (i.e., basal area increment; BAI) of ponderosa pine (Pinus ponderosa), a broadly distributed and heavily managed species in western North America. The study sites were established at the onset of the most extreme drought event in at least 1200 years and span much of the climatic niche of Rocky Mountain ponderosa pine. Across sites, tree-level BAI increased due to treatment, where trees in treated units grew 133.1% faster than trees in paired, untreated units. Likewise, trees in treated units grew an average of 85.6% faster than their pre-treatment baseline levels (1985 to ca. 2000), despite warm, dry conditions in the post-treatment period (ca. 2000-2018). Variation in the local competitive environment promoted variation in BAI, and larger trees were the fastest-growing individuals, irrespective of treatment. Tree thinning and prescribed fire altered the climatic constraints on growth, decreasing the effects of belowground moisture availability and increasing the effects of atmospheric evaporative demand over multi-year timescales. Our results illustrate that restoration treatments can enhance tree-level growth across sites spanning ponderosa pine's climatic niche, even during recent, extreme drought events. However, shifting climatic constraints, combined with predicted increases in evaporative demand in the southwestern United States, suggest that the beneficial effects of such treatments on tree growth may wane over the upcoming decades.

When do contemporary wildfires restore forest structures in the Sierra Nevada?

BackgroundFollowing a century of fire suppression in western North America, managers use forest restoration treatments to reduce fuel loads and reintroduce key processes like fire. However, annual area burned by wildfire frequently outpaces the application of restoration treatments. As this trend continues under climate change, it is essential that we understand the effects of contemporary wildfires on forest ecosystems and the extent to which post-fire structures are meeting common forest restoration objectives. In this study, we used airborne lidar to evaluate fire effects across yellow pine and mixed conifer (YPMC) forests of California’s Sierra Nevada. We quantified the degree to which forest structures in first-entry burned areas (previously unburned since ~ 1900s) and unburned controls aligned with restoration targets derived from contemporary reference sites. We also identified environmental conditions that contributed to more restorative fire effects.ResultsRelative to unburned controls, structural patterns in first-entry burned areas aligned more closely with reference sites. Yet, across all burn severities, first-entry wildfires were only moderately successful at meeting targets for canopy cover (48% total area) and ladder fuels (54% total area), and achieving these targets while also producing tree clump and opening patterns aligning with reference sites was less common (16% total area). Moderate-severity patches had the highest proportion of restorative fire effects (55–64% total area), while low- and high-severity patches were either too dense or too open, respectively. Our models (and publicly-available mapped predictions) indicated a higher probability of restorative effects within 1 km of previous fires, within the mid-upper climate range of the YPMC zone, and under moderate fire intensities (~ 1–2 m flame lengths).ConclusionsFirst-entry wildfires can sometimes restore structural conditions by reducing canopy cover and ladder fuels and increasing structural heterogeneity, especially within moderate-severity patches. However, these initial fires represent just one step toward restoring dry forest ecosystems. Post-fire landscapes will require additional low- to moderate-intensity fires and/or strategic management interventions to fully restore structural conditions. In yet unburned forests, managers could prioritize mechanical treatments at lower elevations, early-season burning at mid to high elevations, and resource objective wildfires in landscapes with mosaics of past wildfires.

Forest Restoration Treatments Indirectly Diversify Pollination Networks Via Floral‐ and Temperature‐Mediated Effects

Forest Restoration Treatments Indirectly Diversify Pollination Networks Via Floral‐ and Temperature‐Mediated Effects

Canopy-mediated microclimate refugia do not match narrow regeneration niches in a managed dry conifer forest

Structurally heterogeneous forest conditions provide diverse microclimates which can support juvenile tree regeneration, survival, and growth. However, the extent to which juvenile trees depend on moderated microclimates depends on their physical maturity and species-specific tolerances for environmental stresses related to variables like soil moisture, heat, and shade. Linking juvenile responses to microclimates for different life-stages and species facilitates more nuanced understanding of regeneration niches and is especially important in dry conifer forests where structurally heterogeneous conditions are a frequent objective of forest restoration treatments. In this study, we sampled microclimate conditions and planted seeds and greenhouse-grown seedlings across a range of overstory structure and microclimate conditions in a heterogeneous forest restoration treatment in Colorado, USA. We assessed how fine-scale (∼1–20 m) spatial variation in overstory structure and topography related to variability in microclimate conditions and, collectively, how these biophysical conditions influenced survival and growth of ponderosa pine (Pinus ponderosa var. scopulorum) and Douglas-fir (Pseudotsuga menziesii var. glauca) over 3 years. We found evidence of strong canopy buffering of mean daily vapor pressure deficit (VPD), but buffering of daily maximum VPD was moderate, and buffering of soil moisture was weak except in the hottest and driest month of our 3-year study. Although canopy cover provided refuge from hot and dry conditions, survival and growth of juvenile trees across life-stages and species were greatest in above-average canopy openness, with warm and dry microclimate conditions in May. The spatial patterns of seedling establishment relative to canopy cover, driven by early-season microclimate conditions, did not match canopy-mediated, moderate microclimates later in growing seasons when weather conditions were most limiting. Importantly, our results reflect a specific regeneration trajectory initiated by favorable early-growing season conditions in the first year of study, which promoted survival and growth resilience through subsequent limiting conditions, most notably a hot and dry second year of study. The patterns observed in this study are consistent with drivers of landscape-scale regeneration previously identified for dry conifer forests and show that overstory structure and microclimate variation can mediate these patterns at very fine spatial scales. The narrow spatial and temporal ranges of conditions which supported survival and growth in this study highlight opportunities for restoration treatment planning in similar forests, but also underscore the susceptibility of these dominant dry forest species to potential future changes in the timing and intensity of abiotic conditions which limit regeneration success.

Resource objective wildfire leveraged to restore old growth forest structure while stabilizing carbon stocks in the southwestern United States

Wildfire futures and aboveground carbon (C) dynamics associated with forest restoration programs that integrate resource objective wildfire as part of a larger treatment strategy are not well understood. Using simulation modeling, we examined alternative forest and fuel management strategies on a 237,218-ha study area within a 778,000-ha landscape that is a high priority target for federal restoration programs. We simulated two wildfire management scenarios combined with three levels of conventional forest restoration treatments over 64 years using a detailed landscape disturbance and succession model developed in prior work. We found accelerated forest restoration used in concert with resource objective wildfire was the most effective at returning old growth forest structure, while stabilizing aboveground C stocks and restoring the fire return interval to its historic range of variation. In scenarios without forest restoration, the continued practice of resource objective wildfires during shoulder fire seasons reduced summer emissions in a negative feedback loop. In the short term, scenarios without forest restoration increased live tree C, but also increased the likelihood of C loss during wildfire activity driven by extreme fire weather. We found scenarios most effective at restoring fire-excluded pine forests to their historical old growth conditions came at a short-term cost of lost C, but with the long-term benefit of substantially increasing fire-resistant live tree C. Our results inform how local decision making can best balance competing goals of sequestering C, and stabilizing C stocks in frequent-fire pine forests using the principles of local fire ecology to restore and maintain old growth forest structure.

Forest restoration treatments indirectly diversify pollination networks via floral- and temperature-mediated effects.

In North American conifer forests, a variety of federally initiated thinning programs are implemented to restore pre-European settlement forest structures, but these changes may impact ecosystem function via impacts on sensitive biotic communities. Across the wildland-urban interface of the Front Range region of Colorado, agencies associated with the Collaborative Forest Landscape Restoration Program (CFLRP) have implemented thinning treatments across thousands of hectares of ponderosa pine forest; here we leverage these treatments as an experimental framework to examine thinning effects on a pollinator community. We measured variation in forest structure and sampled bee community assemblages using multiple methods (trapping and netting) to compare bee biodiversity and patterns of floral visitation by bees (bee-flower networks) between mechanically thinned stands that were 3-10 years after treatment and nonthinned stands. Three key findings emerged: (1) Native bee abundance, richness, and diversity were 120%, 53%, and 37% greater, respectively, in thinned stands. In addition, nestedness, richness, and abundance of bee-flower interactions were all substantially higher in thinned stands, and there was increased functional redundancy in bee assemblages after thinning. (2) Structural equation modeling indicated that variation in temperature and floral abundance were mediated by canopy openness and correlated with bee richness and abundance, thereby indirectly driving variation in bee-flower interactions. (3) Four floral species (Penstemon virens, Cerastium arvense, Erysimum capitatum, and Geranium caespitosum) were identified as key connectors in bee-flower interaction networks, though these were not necessarily the most abundant flowering plants. Our analyses indicate that native bee α-diversity and bee-flower interactions positively responded to thinning treatments, and these effects were indirectly driven by canopy removal. We conclude that CFLRP treatments have conservation value for native bee communities. Further monitoring is warranted to evaluate the longevity of these effects.

Mechanical forest restoration treatments stimulate understory plants in the Colorado Front Range

Restoration efforts are underway in dry conifer forests across the western United States to increase their resilience to wildfire and other disturbances. Because such treatments typically decrease overstory density and homogeneity, they can also drive changes in the understory plant community. Past studies of post-treatment changes in understories have found variable results over short time frames and across regions, highlighting the need to study longer-term, region-specific responses. We investigated whether mechanical restoration treatments benefited understory plants in dry conifer forests of the Colorado Front Range, and what biotic and abiotic variables modified understory plant responses in treated areas. We analyzed data collected 1–2 years pre-treatment, 1–2 years post-treatment, and 4–6 years post-treatment in 168 plots, which were distributed across 8 sites and 16 pairs of treated and nearby untreated areas. Treatments were implemented by removing trees with heavy machinery or hand tools (i.e., thinning). By the time treatments were 4–6 years old, native understory plant cover was 1.7 times higher in treated compared to untreated plots, and native richness was 1.1 times higher. Heightened cover and richness values in treated plots were not driven by a single native plant functional group, but by a large portion of the native community; long-lived, graminoid, vegetatively spreading, and non-vegetatively spreading plants all had higher cover in treated plots, while short-lived, long-lived, forb, graminoid, and non-vegetatively spreading plants had higher richness in treated plots. Non-native plants showed 3.1 times higher cover and 4.4 times higher richness in treated compared to untreated plots at 4–6 years post-treatment, but were present at very low levels (e.g., ≤ 0.5% mean cover in either treatment). Greater native plant cover and richness at 4–6 years post-treatment were associated with lower overstory basal areas that resembled 19th-century forest structural conditions for the landscape. Contrary to expectations, a long-term measure of moisture availability (i.e., 30-year average climatic water deficit) was not a strong predictor of native cover or richness in treated plots 4–6 years post-treatment; rather, they were better predicted by moisture availability during the spring months prior to sampling. Overall, the consistent and enduring stimulation of cover and richness of native understory plants after mechanical treatments, with only limited invasion from non-native species, illustrates the important benefits of ongoing restoration activities in dry conifer forests of the Colorado Front Range.

The prediction of uneven snowpack response to forest thinning informs forest restoration in the central Sierra Nevada

AbstractThe Sierra Nevada has experienced unprecedented wildfires and reduced snowmelt runoff in recent decades, due partially to anthropogenic climate change and over a century of fire suppression. To address these challenges, public land agencies are planning forest restoration treatments, which have the potential to both increase water availability and reduce the likelihood of uncontrollable wildfires. However, the impact of forest restoration on snowpack is site specific and not well understood across gradients of climate and topography. To improve our understanding of how forest restoration might impact snowpack across diverse conditions in the central Sierra Nevada, we run the high‐resolution (1 m) energy and mass balance Snow Physics and Lidar Mapping (SnowPALM) model across five 23–75 km2 subdomains in the region where forest thinning is planned or recently completed. We conduct two virtual thinning experiments by removing all trees shorter than 10 or 20 m tall and rerunning SnowPALM to calculate the change in meltwater input. Our results indicate heterogeneous responses to thinning due to differences in climate and wind across our five central Sierra Nevada subdomains. We also predict the largest increases in snow retention when thinning forests with tall (7–20 m) and dense (40–70% canopy cover) trees, highlighting the importance of pre‐thinning vegetation structure. We develop a decision support tool using a random forests model to determine which regions would most benefit from thinning. In many locations, we expect major forest restoration to increase snow accumulation, while other areas with short and sparse canopies, as well as sunny and windy climates, are more likely to see decreased snowpack following thinning. Our decision support tool provides stand‐scale (30 m) information to land managers across the central Sierra Nevada region to best take advantage of climate and existing forest structure to obtain the greatest snowpack benefits from forest restoration.

Estimating proximity effects to wildfire fuels treatments on house prices in Cibola National Forest, New Mexico, USA

Forested landscapes in the Western United States are subject to growing size and severity of wildfires, in part due to historical management strategies focusing on wildfire suppression. Forest restoration treatments and fuels reductions, including thinning and prescribed burning, can reduce the frequency and intensity of wildfires. Extensive restoration and fuels treatment efforts are underway across many areas in the Southwestern United States, including New Mexico. The tradeoff between amenity values provided by forested landscapes and the wildfire risk associated with forested landscapes is becoming increasingly important to understand as development in the wildland-urban interface increases. Understanding how house proximity, relative to forest restoration or fuels treatments, is capitalized into home sale prices can provide useful information about how individuals value forested landscapes that have been altered to reduce wildfire risk or severity. We use a Hedonic Property Model to estimate the average treatment effect of proximity to forest restoration or fuel treatments in New Mexico, United States. We use matching methods to estimate the average treatment effect of proximity to forest restoration. We find that proximity to the forest has a positive amenity value; however, proximity to recent forest restoration or fuel treatments results in a decrease in house sale prices. We combine the results of our two models and calculate that homes not within one kilometer of a treated forest and within one kilometer of Cibola National Forest sell for an average $73,626 premium. The average premium drops to $22,996 for homes within one kilometer of a forest that has been recently treated and within one kilometer of Cibola National Forest.

Adaptive governance strategies to address wildfire and watershed resilience in New Mexico's upper Rio Grande watershed

Global climate models project that New Mexico's Upper Rio Grande watershed is expected to become more arid and experience greater climatic and hydrological extremes in the next 50 years. The resulting transitions will have dramatic implications for downstream water users. The Upper Rio Grande and its tributaries provide water to about half of New Mexico's population, including the downstream communities of Albuquerque and Santa Fe, and surrounding agricultural areas. In the absence of formal climate adaptation strategies, informal governance arrangements are emerging to facilitate watershed climate adaptation strategies, including fuel treatments and stream remediation. One example is the Rio Grande Water Fund (RGWF), a collaborative effort coordinating work to protect storage, delivery, and quality of Rio Grande water through landscape-scale forest restoration treatments in tributary forested watersheds. This article examines the RGWF as one example of an emerging adaptation strategy that is working within—and beyond—existing legal and policy frameworks to accomplish more collaborative efforts across jurisdictional lines and administrative barriers. We identified ten (10) key characteristics of adaptive governance from the relevant literature and then applied them to the RGWF's experience in the watershed to date. Key findings include: (1) the RGWF's approach as a collaborative network created the right level of formality while also keeping flexibility in its design, (2) a scalar fit to the environmental challenge built social capital and investment in its work, (3) leadership from key stakeholders leveraged opportunities in the watershed to create and maintain stability, and (4) use of adaptive management and peer review processes built capacity by creating the feedback loops necessary to inform future work.

Forest restoration treatments increased growth and did not change survival of ponderosa pines in severe drought, Arizona.

We report on survival and growth of ponderosa pines (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) 2 decades after forest restoration treatments in the G. A. Pearson Natural Area, northern Arizona. Despite protection from harvest that conserved old trees, a dense forest susceptible to uncharacteristically severe disturbance had developed during more than a century of exclusion of the previous frequent surface-fire regime that ceased upon Euro-American settlement in approximately 1876. Trees were thinned in 1993 to emulate prefire-exclusion forest conditions, accumulated forest floor was removed, and surface fire was re-introduced at 4-years intervals (full restoration). There was also a partial restoration treatment consisting of thinning alone. Compared with untreated controls, mortality of old trees (mean age 243 years, maximum 462 years) differed by <1 tree ha-1 and old-tree survival was statistically indistinguishable between treatments (90.5% control, 92.3% full, 82.6% partial). Post-treatment growth as measured by basal area increment of both old (pre-1876) and young (post-1876) pines was significantly higher in both treatments than counterpart control trees for more than 2 decades following thinning. Drought meeting the definition of megadrought affected the region almost all the time since the onset of the experiment, including 3 years that were severely dry. Growth of all trees declined in the driest 3 years, but old and young treated trees had significantly less decline. Association of tree growth with temperature (negative correlation) and precipitation (positive correlation) was much weaker in treated trees, indicating that they may experience less growth decline from warmer, drier conditions predicted in future decades. Overall, tree responses after the first 2 decades following treatment suggest that forest restoration treatments have led to substantial, sustained improvement in the growth of old and young ponderosa pines without affecting old-tree survival, thereby improving resilience to a warming climate.

Using wildfire as a management strategy to restore resiliency to ponderosa pine forests in the southwestern United States

AbstractThe long‐term outcome from accelerated forest restoration using resource objective wildfire in combination with fuel management on fire‐excluded landscapes is not well studied. We used simulation modeling to examine long‐term trade‐offs and synergies of alternative land management strategies by combining two wildfire management alternatives with three levels of contemporary forest restoration treatments on a 778,000‐ha landscape over 56 years. We found that managing wildfires for resource objectives diminished the likelihood of irregular fire events over time by making wildfire activity more predictable. Overall, adding resource objective wildfire reduced the proportion of high‐severity fire in relation to total area burned, but increased total area burned and the area of high‐severity fire. We also found resource objective wildfire changed the distribution of high‐severity burn patches by increasing their total number and range, their likelihood of containing disjunct core areas, and their edge complexity. The results suggested that alongside the current pace of active forest management, expanding the fire footprint to achieve low‐cost restoration carries the potential for increased high‐severity fire and associated impacts to ecological values including old forest structure and wildlife habitat. Concurrently, adding resource objective wildfire expanded the footprint of conventional restoration treatments by fivefold, and restoration objectives were achieved in 25 years when managing resource objective wildfires alongside restoration treatments five times the current pace. This study demonstrates the first fire suppression model to replicate local decision making by fire managers during simulated fire events to manage risk by considering both fire proximity to values at risk, and daily weather conditions. The study paves the way for further investigations of the synergies between wildfires and conventional forest restoration to improve resiliency in fire‐excluded pine forests.

Abundance of mites, but not of collembolans or nematodes, is reduced by restoration of a Pinus ponderosa forest with thinning, mastication, and prescribed fire

Thinning, mastication, and prescribed fire are restoration treatments frequently employed in unnaturally dense second-growth Pinus ponderosa forests of the Western United States. Although a goal of these treatments is to restore ecosystem structure and function, little information is available regarding treatment effects on soil micro- and mesofauna, which comprise the overwhelming majority of metazoan forest inhabitants and occupy key positions in soil food webs. We quantified nematodes, mites, and collembolans in soil and litter habitats within untreated control, thinned (comprising thinning and masticating wood), and burned (comprising thinning and masticating, followed by broadcast burn) in P. ponderosa forest management units at Valles Caldera National Preserve in New Mexico, USA. We linked patterns in animal abundance to resource and habitat characteristics, hypothesizing that resources and available habitat for many taxa would increase with thinning and decrease with burning. Two years after thinning, densities of collembolans and nematodes in the thinned unit were higher than in the untreated control unit, but one year post-fire, their densities in the burned unit were similar to those of the untreated control unit. Mite abundance, however, was not elevated in the thinned unit and was lower in the burned unit. Although faunal communities were highly heterogeneous, a significant proportion of the variance in faunal abundances was explained by easily and inexpensively measured habitat and resource characteristics: bulk density, soil organic matter (SOM), pH, grass cover, and litter cover and depth. These findings demonstrate the abiotic and biotic factors that structure faunal habitats so that forest managers have a more complete understanding of the impacts of forest restoration treatments.

Integrating airborne and mobile lidar data with UAV photogrammetry for rapid assessment of changing forest snow depth and cover

Forest structure and topography can influence the ecohydrologic function and resiliency to drought and changing climate. It is, therefore, important to understand how forest restoration treatments alter snowpack distribution and design the treatments accordingly. We use a combination of aerial lidar, multi-temporal terrestrial mobile lidar, and UAV photogrammetry to estimate rapidly changing snow depth and cover in northern Arizona, USA. We then examine the impact of forest structure and topography on snow depth and snow cover persistence to inform forest restoration treatments. Our results show that mobile lidar data can be used to estimate snow depth with standard errors of 8 cm when differenced with snow-off airborne lidar data. UAV-based Structure-from-Motion data can be used to estimate snow cover persistence with 92–97% overall accuracies in forested ecosystems. Random forest models indicate spatially varying importance of forest structural and topographic variables in predicting snow depth and cover persistence, when summarized at different spatial scales (from 5 m to 250 m) and with variable directional location offsets. Forest snow depth was best explained (R2 ≈ 0.46) by canopy height metrics at summary scales of >75 m, while canopy cover was most important at summary scales of <40 m (R2 ≈ 0.3). Snow cover persistence was best explained at very local scales by canopy cover (R2 ≈ 0.38) and less so at larger scales (>75 m) by topographic and forest patch characteristics (R2 ≈ 0.34). Our results demonstrate that 3-dimensional datasets are critical in rapidly characterizing changing snowpack to better understand the impacts of forest structure and topography to inform forest restoration treatment designs. The relationships observed in our study can inform currently ongoing regional-scale forest restoration in the southwest to improve forest health and resiliency.

Similarity in seed removal patterns of four tree species in active and passive restoration treatments of the tropical seasonal forest after 23 years

Seed removal has been considered an important ecological filter in plant community assembly during the first years of restoration. However, little is known about the influence of the restored ecosystems attributes on seed consumption patterns by predators and, consequently, on the composition and structure of the regenerating community. We tested whether seed removal differs between natural and restored ecosystems and whether it depends on the seed consuming agent (insects, rodents and birds). We evaluated seed removal in three forest restoration treatments (two active and one passive) established over 20 years ago. We used a fragment of old-growth native seasonal semideciduous tropical forest close to the experimental area, as a reference ecosystem. We installed exclusion stations for different consumer groups (insects, rodents and birds) of seeds belonging to four tree species from different successional groups and dispersal syndromes (Tabernaemontana hystrix Steud, Schinus terebinthifolius Raddi, Pterogyne nitens Tul and Anadenanthera colubrina var. cebil (Griseb.) Altschul. Insects were the main agent removing seeds from the species tested as a whole, regardless of the restoration treatment. Seed removal proportions were generally low and differed between active and passive restoration treatments, depending on the tree species. However, they were similar to the native forest, regardless of the restoration treatment. We found an effect of the interaction of restoration treatments and consuming agents only for one of the zoochoric species tested (Tabernaemontana hystrix). Seed removal no longer seems to represent a biotic filter limiting plant recruitment for T. hystrix, S. terebinthifolius, P. nitens and A. colubrina recruitment in the restoration treatments, regardless of the restoration technique.

Surface water runoff response to forest management: Low-intensity forest restoration does not increase surface water yields

Ecohydrology of the Western United States faces two interconnected, issues: a decline in snowpack reducing surface water availability and timing, and an increase in the frequency and severity of forest fires. Prior research generally agrees that large severe fires and clearcut timber harvest increase surface water runoff due to reductions in interception, infiltration, and evapotranspiration. It has thus been postulated that forest restoration with ecologically guided stand density reduction and prescribed underburning in dry forests to reduce fire risk, will also increasing surface water runoff. This study evaluates this proposition in two subbasins of the Ashland Creek watershed in southwest Oregon where forest restoration and fuel mitigation treatments, including commercial density management, surface and ladder fuel reduction, and prescribed burning, were implemented from 2012 to 2019. Despite cumulatively treating roughly 15% and 20% of the area in two basins, canopy cover at the watershed scale was decreased just 3% and 4%,. We found that in the post treatment period, the West and East basins experienced an average annual water yield decline of 26% and 24% respectively with 66% (West) and 72% (East) of the changes in water yield attributed to annual variations in precipitation. Our results demonstrate that climatic drivers may overwhelm anticipated increased surface water runoff from forest restoration treatments at this scale and intensity.

Linking Federal Forest Restoration with Wood Utilization: Modeling Biomass Prices and Analyzing Forest Restoration Costs in the Northern Sierra Nevada

Over half of California’s forestland is managed by the US Forest Service, and the agency has identified a need to scale up forest restoration treatments in the state to one million acres per year by 2025. However, the high costs of mechanical fuel reduction and lack of markets for biomass pose significant barriers to accomplishing this target. The objectives of this case study were: (1) to identify costs of forest restoration treatments on federally-managed land in the Northern Sierra under a variety of harvesting scenarios and haul distances to biomass facilities, and (2) to understand what market prices for biomass must be offered to support such efforts. We modeled silvicultural prescription and harvesting options, machine productivity and costs, and transportation costs to assess economic thresholds. Biomass harvest, chip, and haul costs ranged from $55/bone dry ton to $118/bone dry ton, depending on the harvesting system scenario and distance from the biomass disposal site. Results suggest that the cost of forest restoration far exceeds current market prices for biomass, and additional investment is needed to adequately pay for federal forest restoration in California. Additional takeaways include that biomass outlets closer to supply sources can reduce both haul costs and production costs, and local wood utilization campuses can play a key role in supporting forest restoration.

Ecological restoration guided by historical reference conditions can increase resilience to climate change of southwestern U.S. Ponderosa pine forests

Broad-scale forest restoration projects are implemented across the western United States to restore seasonally dry, frequent-fire-adapted ecosystems to improve ecological function and enhance resilience by increasing resistance to crown fire and climatic stressors. Despite the widespread use of restoration treatments that center on tree thinning and application of prescribed fire, the longevity of beneficial effects and the robustness of outcomes under future climate change predictions remains unclear. In this study, we remeasured a set of experimental areas established a minimum of 20 years ago that comprise a network of ponderosa pine (Pinus ponderosa) forest restoration study sites in northern Arizona. We analyzed ecological resiliency by evaluating forest conditions in terms of resistance to climatic stressors and potential crown fire in units that were thinned following evidence-based restoration guidelines (ERG), then burned with prescribed fire at multiple intervals, compared against paired untreated controls. Resilience indicators included forest structure, tree mortality, tree growth, regeneration, canopy fuels, and crowning index. We also simulated future forest conditions under a warming climate scenario (RCP 4.5) with a range of prescribed fire return intervals. Results indicated that experimental areas where restoration treatments were implemented remained more resilient to climate stressors compared to controls after 20 years. Treated areas had significantly lower tree mortality and greater average diameter growth compared to controls. Furthermore, forest structure generally remained similar to historical reference conditions in treated units with the exception of increases in ingrowth of sprouting species at the drier sites. Canopy fuel load and crown fire hazard in treated units remained significantly lower than controls, indicating that treatments remained effective in reducing crown fire potential over the 20-year study period without the need for additional tree thinning. Modeling basal area, crowning index, and the proportion of basal area in large trees under a future warming scenario suggested that the treated units underwent less changed than untreated areas. Under climate change, management of fire regimes even at longer-than-historical intervals (historical ≈ 5 yr, tested 5, 10, and 20 yr) would maintain basal area within our historical range of variability and maintain fire resistant forest over the next several decades. However, decline by the end of the century is concerning. Our results suggest that forest restoration treatments, guided by historical reference conditions, promote ecological resilience in the long-term and continued maintenance burning into the future is likely warranted even with continued drought and warming.

Regional‐scale forest restoration effects on ecosystem resiliency to drought: a synthesis of vegetation and moisture trends on Google Earth Engine

AbstractLarge‐scale changes in forest structure and ecological function throughout western North America have led to increased frequency, size, and severity of wildfires. The US Forest Service is implementing state‐wide forest restoration initiatives to reduce wildfire hazards and improve forest health. We provide a synthesis of pre‐ and post‐treatment forest vegetation and ecosystem moisture trends between 1990 and 2017 in Arizona, the first US state where the initiative has implemented a variety of thinning and burning methods in over 1,200 polygon areas across 3.5 million ha. Using 4,426 Landsat satellite images on Google Earth Engine, we calculated normalized difference moisture index (NDMI), normalized difference water index (NDWI), and normalized difference vegetation index (NDVI) to create dense time‐series datasets. The indices and 1990‐2017 annual total precipitation dataset were then examined using a Mann–Kendall tau test to identify statistically significant upward and downward trends for each pixel. Our results indicate that much of the study region were experiencing drought conditions prior to restoration treatments and NDVI values were significantly decreasing, especially during the dry spring season. However, both NDMI and NDWI trends indicate that the forest restoration treatments have contributed to increased total ecosystem moisture, while precipitation in the post‐treatment period exhibit stable trends. Forest restoration treatments appear to have improved the overall forest health and resiliency to drought, especially during the dry spring season, when forests are most vulnerable to water stress and wildfire risks. Our results of the spatial patterns and long‐term trends in these variables can inform the currently ongoing and future restoration treatments to better target the treatment strategy across the southwestern USA. Google Earth Engine enabled our synthesis of these long‐term trends over the large region and will enhance our continued monitoring in the coming decade.

Native and non-native understory vegetation responses to restoration treatments in a dry conifer forest over 23 years

Restoration of fire-prone forests is a common practice intended to increase resilience to wildfire, drought, and bark beetles. However, the long-term effects of restoration treatments on understory species, particularly non-native species, are poorly understood. We investigated long-term (23 years) effects of restoration treatments on native and non-native understory vegetation at the Lick Creek Demonstration-Research Forest, a ponderosa pine/Douglas-fir forest in western Montana, USA. Restoration treatments included shelterwood-with-reserves harvesting implemented alone or in combination with prescribed burning under either wet or dry conditions, plus a no-action control. Changes in understory vegetation cover and species richness (measured pre-treatment, 2, 3, 4, 5, 15, and 23 years post-treatment) were analyzed by growth form (forb, graminoid, and shrub) and origin (native vs. non-native) to test how understory dynamics respond to disturbance intensity. We also accounted for basal area of trees to consider how differential overstory conditions affected final treatment responses. Native vegetation cover responses followed predictable successional patterns: after initial declines by grasses and shrubs immediately after disturbance (43% and 40% reduction, respectively), all groups increased, peaked about five years after disturbance (74% greater than pre-treatment on average) and then generally declined to levels similar to pre-treatment. The magnitude of these changes was related to the disturbance intensity (i.e., cut-and-dry-burn > cut-and-wet-burn > cut-and-no-burn > control). For all native groups, variation in tree basal area at year 23 explained remaining differences among treatments. Non-native grasses and forbs followed similar overall trajectories, but cover of these groups increased immediately after treatment and rose more steeply to their peak five years post-treatment (12 times greater than pre-treatment on average), with differences persisting through year 23. While non-native cover responses in year 23 were negatively correlated with tree basal area, as seen for native taxa, overstory conditions alone did not fully account for differences between burned and unburned treatments. This suggests that effects of burning unrelated to tree regeneration favored non-natives. Nonetheless, in the last sampling year treatment-induced increases in non-native cover were modest, suggesting that after 23 years understory responses in this forest type were fairly resilient to restoration treatments. However, because forest structure and composition were reverting to pre-treatment conditions by year 23, retreatment may be necessary at intervals of <23-years in our system, which could exacerbate non-native responses. Results of our long-term experiment provide insight into how different forest restoration treatments and their timing may affect understory susceptibility to non-native plant invasion.