Search

Conservation of old-growth forests has become an increasingly important objective of Forest Service managers over the last three decades. The US Forest Service recently made changes to policies that prohibit cutting of live trees >53 cm (the “21-inch rule”). We review the disturbance ecology of dry and mesic old-growth forests of Oregon and contrast conservation policies for these two forest types. We describe the development of age-based alternatives to the 21-inch rule on the Klamath Reservation and in the Malheur National Forest in eastern Oregon. We conclude by outlining an adaptive management strategy to conserve dry forest old growth that seeks to restore the ecological processes that perpetuate old tree populations over time. We argue that what is good for dry forest ecosystems is good for dry forest old growth, especially in the face of changing climate and disturbance regimes.

Forest-restoration efforts are increasing in the western United States in response to realized and expected changes in climate and disturbance regimes. Managers are challenged to find practical and defensible targets to shift forest composition, structure, and spatial pattern to a more resistant and resilient state. The Northeast Washington Forest Vision 2020 project on the Colville National Forest presented an opportunity to map and use previously uncaptured mesic stand-level historical reference conditions to a large restoration project. We reconstructed historical forest conditions in 12 plots across a range of plant-association groups and mapped five restoration treatment units after implementation. We evaluated treatment effectiveness both in terms of meeting the prescriptions’ stated objectives and by similarity to observed reference conditions using metrics of density, species composition, clump-size patterns, and open-space patterns. We found that dry plant associations were historically dominated by distributed clumps of large shade-intolerant trees, whereas cold mesic plant associations were structured as a gap-matrix spatial pattern. Treatments were effective at meeting prescribed density and species-composition targets, but generally resulted in stands that were overly uniform or clumped compared to historical reference conditions.

Where competition suppresses tree growth, mortality of adjacent trees can release the surviving individuals, leading to a growth increase. However, primary forests are complicated systems, where trees interact in both competitive and facilitative ways mediated by their size, species, and the broad ecological context in which they grow. Thus, the magnitude and even direction of growth responses to the mortality of nearby trees may vary, which has implications for our understanding of community- and ecosystem-level dynamics following mortality events. Unfortunately, although many studies focus on the impacts of light availability and general crowding on tree growth, we know relatively little about the effects of naturally occurring mortality events on the growth of neighboring trees. To address this issue, we used 40 years of data from 15 permanent forest-monitoring plots in Mt Rainier old-growth forests, comparing observed to expected radial growth of individual trees following the death of their nearest neighbor. Although we found evidence of a general growth-release response, this was not universal among all trees, with small trees in particular exhibiting growth suppression (rather than release) following neighboring tree mortality. In addition to small size, growth-suppression was more likely if the dead neighbor was the same species, consistent with facilitative effects as mediated through belowground networks. At the stand level, the average growth release after nearest neighbor mortality was greatest in low-density stands with large trees, with elevation and community composition also playing a role. Decades more monitoring could reveal how long growth release (or suppression) is sustained by individual trees following neighboring mortality events, as well as potential response lags and the role of species identity in determining whether interactions with neighbors are competitive or facilitative. Nonetheless, our results suggest that although mature trees have competitive effects on their larger neighbors, they also have an important role in supporting the ingrowth of small trees. More broadly, we demonstrate that the nature of interactions between individual neighboring trees is highly context dependent., Forest Ecology and Management, 529, ISSN:0378-1127, ISSN:1872-7042

Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here we find from a synthesis of tree species in North America that climate-condition interactions dominate responses through two pathways, i) effects of growth that depend on climate, and ii) effects of climate that depend on tree size. Because tree fecundity first increases and then declines with size, climate change that stimulates growth promotes a shift of small trees to more fecund sizes, but the opposite can be true for large sizes. Change the depresses growth also affects fecundity. We find a biogeographic divide, with these interactions reducing fecundity in the West and increasing it in the East. Continental-scale responses of these forests are thus driven largely by indirect effects, recommending management for climate change that considers multiple demographic rates., Nature Communications, 12, ISSN:2041-1723

Post-fire tree mortality is a spatially structured process driven by interacting factors across multiple scales. However, empirical models of fire-caused tree mortality are generally not spatially explicit, do not differentiate among scales, and do not differentiate immediate from delayed mortality. We aimed to quantify cross-scale linkages between forest structure—including spatial patterns of trees—and the progression of mortality 1–4 years post-fire in terms of rates, causes, and underlying demography. We used data from a long-term study site in the Sierra Nevada, California to build a post-fire tree mortality model predicted by lidar-measured estimates of structure. We calculated structural metrics at scales from individual trees to 90 × 90 m neighborhoods and combined them with metrics for topography, site water balance, and burn weather to predict immediate and delayed post-fire tree mortality. Mortality rates decreased while average diameter of newly killed trees increased each year post-fire. Burn weather predictors as well as interactive terms across scales improved model fit and parsimony. Including landscape-scale information improved finer-scale predictions but not vice versa. The amount of fuel, fuel configuration, and burning conditions predicted total mortality at broader scales while tree group-scale fuel connectivity, tree species fire tolerance, and local stresses predicted the fine-scale distribution, timing, and agents of mortality. Landscape-scale conditions provide the template upon which finer-scale variation in post-fire tree mortality is arranged. Post-fire forest structure is associated with the etiologies of different mortality agents, and so landscape-level heterogeneity is a key part of ecosystem stability and resilience.

Large trees are critically important for structuring ecosystems and providing habitat, and trees with complex crowns provide more of these services than comparably sized trees with simple crowns. Forest managers are increasingly emulating old-growth structure by retaining various densities of aggregated and dispersed trees. This study explores how individual within-crown complexity develops in a wide range of tree densities and interprets them in terms of management goals. Stratified LiDAR data were used to select 36 Picea sitchensis trees 55 to 91 m tall and from the complete range of tree densities in Olympic rainforests. These were climbed, three-dimensionally mapped—including the trunk and every appendage—to quantify tree allometry and habitat-related structures within their crowns. Sampled trees were then aged with crossdated increment cores to construct a chronosequence of crown development. The sample ranged from 70 to 250 cm diameter and 100 to 400 years old. Circular 30 m radius plots were installed around each tree to quantify neighborhood density. An additional 19 trees ranging from 10 to 94 m tall were added to the sample (N = 55) to describe baseline crown development and then compared to the 36-tree sample in which we modeled how this baseline is altered by neighborhood competition. After ∼200 yr, crown mass increases faster than trunk mass due to accumulation and growth of large appendages. The largest neighbors within 25 m compete with Picea, reducing trunk and appendage diameters as well as crown volume by roughly 20% for each 25% increase in density for a given tree age. Reduced growth and higher mortality of lower-crown appendages in dense forests delays development of large (>15 cm diameter) appendages for over 100 yr. Abundance of structures associated with reiterated trunks is unrelated to neighborhood density but increases with crown damage. These findings contribute to an empirically-based conceptual framework for crown development in Picea and interpretation of management guidelines. Picea is then compared to Pseudotsuga menziesii—another well-studied tall conifer of Olympic rainforests. Picea accumulates ecologically important crown structures much faster than Pseudotsuga yet has roughly half the longevity, indicating that these co-occurring species have complementary functions. Because of its shade tolerance and extremely rapid growth in rainforests, Picea can be used to accelerate development of complex crown structures in forests typically planted with Pseudotsuga after logging.

Restoration of fire-dependent forests is often guided by reference conditions from forests with an active fire regime, thought to be resilient to current and future disturbances and stresses. Reference conditions are usually based on historical data or reconstruction, which greatly limits the scale and completeness of data that can be collected. In the Sierra Nevada of California, large areas with reintroduced active fire regimes coupled with extensive lidar data coverage provide the unique opportunity to develop a contemporary regional reference condition dataset across a wide gradient of biophysical conditions. We developed this dataset with a focus on three questions: (1) What is the geographic and environmental distribution of restored active-fire forest areas in the Sierra Nevada mixed-conifer zone? (2) What are the ranges of variation in forest structure and spatial patterns across reference areas? And (3) How do stand density, tree clumping, and canopy opening patterns vary by topography and climate in reference areas? We analyzed fire history and environmental conditions over 10.8 million ha, including 3.9 million ha in the Sierra Nevada mixed-conifer zone, and found 30,377 ha of restored active-fire areas. Although reference areas were distributed throughout the Sierra Nevada they were more abundant on National Park lands (81% of reference areas) than National Forest lands and were associated with higher lightning strike density. Lidar-measured ranges of variation in reference condition structure were broad, with tree densities of 6–320 trees ha−1 (median 107 trees ha−1), basal area of 0.01–113 m2 ha−1 (median 21 m2 ha−1), average size of closely associated clumps of trees from >1 to 207 trees (median 3.1 trees), and average percent of stand area >6 m from the nearest canopy ranging from 0% to 100% (median 5.1%). These ranges correspond well with past studies reporting density and spatial patterns of contemporary and historical active-fire reference stands in the Sierra Nevada, except this study observed greater total variation due to the much greater spatial extent of sampling. Within the montane forest zone, reference areas at middle elevations had lower density (86 vs. 121 trees ha−1), basal area, (13.7 vs. 31 m2 ha−1), and mean clump size (2.7 vs. 4.0 trees) compared to lower- and higher-elevation reference areas, while ridgetops had lower density (101 vs. 115 trees ha−1), basal area (19.6 vs. 24.1 m2 ha−1), and mean clump size (3.0 vs. 3.3 trees) and more open space (7.4% vs. 5.1%) than other landforms. Many of the relationships between physiography and reference structure were context-dependent, suggesting that management practices should create heterogeneous forest structure congruent with local climatic and topographic factors influencing stand conditions.

The chapter explores a series of overlapping discussions from the 1960s to the mid-1970s concerning the need to preserve natural areas for ecological research and observation, as well as the need for ecologists to be advocates for ecological science in the light of modern environmental problems. The U.S. Forest Service was also actively promoting the creation of Research Natural Areas and making them available for ecological research. A report prepared under the auspices of The Institute of Ecology in the mid-1970s drew attention to the desirability of creating a large network of experimental ecological reserves across the U.S. and its territories. This report led to three workshops during which ecologists debated what such a network of research sites might look like. The third workshop’s proposals led directly to the creation of the Long Term Ecological Research (LTER) Program, which began in 1980 with funding from the National Science Foundation (NSF). The chapter follows these conversations with particular emphasis on the participation of Jerry F. Franklin, who took part in early discussions about expanding ecological research infrastructure while serving as Ecosystem Studies Program Officer at NSF during 1973–1975. He subsequently was director of the H.J. Andrews Ecosystem Research Project, which joined the LTER Program in 1980, chaired the LTER Program’s Coordinating Committee from 1982 to 1995, and established and directed LTER’s coordinating office from 1982 to 1996. In 1997 the Network Office shifted to the University of New Mexico, beginning a new phase in the coordination of the developing network.

Diverse structural elements play an important role in sustaining biodiversity in old-growth forests. Therefore, it is important to have thorough knowledge about these habitats and their condition in protected areas. Metsakorralduse Büroo OÜ conducted a large-scale Natura 2000 forest habitat inventory that covered 60% of Lahemaa National Park (LNP) area. Western taiga habitat data (7,191 ha) from this inventory was used for analysis of forest stages. The data was grouped according to stand classes by total area and these classes were compared by deadwood occurrence and relative density. For more precise evaluation of coarse woody debris (CWD) we used 27 sample plots from the Estonian Network of Forest Research Plots (ENFRP) located in LNP. The biggest areas of Western taiga habitat were covered by mature and old stages. CWD occurrence (over 5 m3ha−1 of snags and logs) by habitat representativeness is higher in old and natural forest classes. This logical result is overshadowed by the outcome that the CWD was not substantially present on 32% of the area in the old-growth forest class. This indicates that these areas lack the potential of biological legacies to provide critical niche habitats for different species. The study shows that the CWD quantity in Estonian conditions is similar to that reported in previous studies in this region, but because many of the older forest stage classes do not have any CWD present, these areas should include ecological restoration practices in their conservation planning. By creating snags, logs and gaps in these areas, we are able to create better structural variability and include wood into different decay processes.

Understanding how climate affects tree growth is essential for assessing climate change impacts on forests but can be confounded by effects of competition, which strongly influences tree responses to climate. We characterized the joint influences of tree size, competition, and climate on diameter growth using hierarchical Bayesian methods applied to permanent sample plot data from the montane forests of Mount Rainier National Park, Washington State, USA, which are mostly comprised of Abies amabilis Douglas ex Forbes, Tsuga heterophylla (Raf.) Sarg., Pseudotsuga menziesii (Mirb.) Franco, and Thuja plicata Donn ex D. Don. Individual growth was sensitive to climate under low but not high competition, likely because tree ability to increase growth under more favorable climates (generally greater energy availability) was constrained by competition, with important variation among species. Thus, climate change will likely increase individual growth most in uncrowded stands with lower competition. However, crowded stands have more and (or) larger trees, conferring greater capacity for aggregate absolute growth increases. Due to these contrasting effects, our models predicted that climate change will lead to greater stand-scale growth increases in stands with medium compared with low crowding but similar increases in stands with medium and high crowding. Thus, competition will mediate the impacts of climate change on individual- and stand-scale growth in important but complex ways.

Collaborative approaches to natural resource management are becoming increasingly common on public lands. Negotiating a shared vision for desired conditions is a fundamental task of collaboration and serves as a foundation for developing management objectives and monitoring strategies. We explore the complex socio-ecological processes involved in developing a shared vision for collaborative restoration of fire-adapted forest landscapes. To understand participant perspectives and experiences, we analyzed interviews with 86 respondents from six collaboratives in the western U.S., part of the Collaborative Forest Landscape Restoration Program established to encourage collaborative, science-based restoration on U.S. Forest Service lands. Although forest landscapes and group characteristics vary considerably, collaboratives faced common challenges to developing a shared vision for desired conditions. Three broad categories of challenges emerged: meeting multiple objectives, collaborative capacity and trust, and integrating ecological science and social values in decision-making. Collaborative groups also used common strategies to address these challenges, including some that addressed multiple challenges. These included use of issue-based recommendations, field visits, and landscape-level analysis; obtaining support from local agency leadership, engaging facilitators, and working in smaller groups (sub-groups); and science engagement. Increased understanding of the challenges to, and strategies for, developing a shared vision of desired conditions is critical if other collaboratives are to learn from these efforts.

Ecological memory is central to how ecosystems respond to disturbance and is maintained by two types of legacies – information and material. Species life-history traits represent an adaptive response to disturbance and are an information legacy; in contrast, the abiotic and biotic structures (such as seeds or nutrients) produced by single disturbance events are material legacies. Disturbance characteristics that support or maintain these legacies enhance ecological resilience and maintain a “safe operating space” for ecosystem recovery. However, legacies can be lost or diminished as disturbance regimes and environmental conditions change, generating a “resilience debt” that manifests only after the system is disturbed. Strong effects of ecological memory on post-disturbance dynamics imply that contingencies (effects that cannot be predicted with certainty) of individual disturbances, interactions among disturbances, and climate variability combine to affect ecosystem resilience. We illustrate these concepts and introduce a novel ecosystem resilience framework with examples of forest disturbances, primarily from North America. Identifying legacies that support resilience in a particular ecosystem can help scientists and resource managers anticipate when disturbances may trigger abrupt shifts in forest ecosystems, and when forests are likely to be resilient.

© 2016 Elsevier B.V. Increasingly, objectives for forests with moderate- or mixed-severity fire regimes are to restore successionally diverse landscapes that are resistant and resilient to current and future stressors. Maintaining native species and characteristic processes requires this successional diversity, but methods to achieve it are poorly explained in the literature. In the Inland Pacific US, large, old, early seral trees were a key historical feature of many young and old forest successional patches, especially where fires frequently occurred. Large, old trees are naturally fire-tolerant, but today are often threatened by dense understory cohorts that create fuel ladders that alter likely post-fire successional pathways. Reducing these understories can contribute to resistance by creating conditions where canopy trees will survive disturbances and climatic stressors; these survivors are important seed sources, soil protectors, and critical habitat elements. Historical timber harvesting has skewed tree size and age class distributions, created hard edges, and altered native patch sizes. Manipulating these altered forests to promote development of larger patches of older, larger, and more widely-spaced trees with diverse understories will increase landscape resistance to severe fires, and enhance wildlife habitat for underrepresented conditions.Closed-canopy, multi-layered patches that develop in hot, dry summer environments are vulnerable to droughts, and they increase landscape vulnerability to insect outbreaks and severe wildfires. These same patches provide habitat for species such as the northern spotted owl, which has benefited from increased habitat area. Regional and local planning will be critical for gauging risks, evaluating trade-offs, and restoring dynamics that can support these and other species. The goal will be to manage for heterogeneous landscapes that include variably-sized patches of (1) young, middle-aged, and old, closed-canopy forests growing in upper montane, northerly aspect, and valley bottom settings, (2) a similar diversity of open-canopy, fire-tolerant patches growing on ridgetops, southerly aspects, and lower montane settings, and (3) significant montane chaparral and grassland areas. Tools to achieve this goal include managed wildfire, prescribed burning, and variable density thinning at small to large scales. Specifics on "how much and where?" will vary according to physiographic, topographic and historical templates, and regulatory requirements, and be determined by means of a socio-ecological process.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-22025-2

Aggregated retention, in which patches of trees (aggregates) remain unlogged within larger harvested units, was first applied commercially in 1986. A primary goal was to maintain greater diversity of forest-dependent species through harvest, relative to conventional clearcutting. Despite its global application, the long-term benefits for biodiversity and the comparative responses of disparate taxonomic groups to aggregated retention are largely unknown. A critical knowledge gap relates to the role of ‘forest influence’ – whether and to what extent aggregates affect biodiversity in neighboring harvested areas. We sampled plants, beetles, and spiders/harvestmen in the world’s three oldest aggregated-retention sites (21–26 years old), matched with three recently harvested sites (5–8 years old). For each taxonomic group, we compared species composition between undisturbed aggregates and regenerating forests to assess the ‘lifeboating’ function of aggregates. For each group, we also modeled changes in species composition, and in the numbers of aggregate- and regeneration-affiliated species, with distance from the aggregate edge into the regenerating forest along transects at north-facing edges. For all three taxa, species composition differed between aggregates and regenerating forests in both older and recent sites, confirming the long-term effectiveness of aggregates for lifeboating. The compositional difference between habitats was significantly greater at recent than at older sites for plants, but not for invertebrates. Plants and spiders/harvestmen responded to forest influence, with a marginal response for beetles. Responses for plants and spiders generally manifested as increased numbers of aggregate-affiliated species and decreased numbers of regeneration-affiliated species in regenerating areas closer to edges. Our results indicate that aggregated retention has short- and long-term benefits for biodiversity reflecting both the lifeboating and forest-influence functions of aggregates. However, variation in the responses of plants, beetles and spiders suggests that these benefits cannot be generalized among taxa. We advocate broader application of aggregated retention in forests managed for timber production and encourage managers to incorporate the benefits of forest influence in harvest designs by arranging aggregates to reduce average distances from harvested areas to unlogged habitats.

Few empirical studies exist on structural transitions from mature forests dominated by Douglas-fir to structurally complex older forests. Stand structure (live tree diameter, height distributions, snag and log metrics) and composition of nine early old-growth (200–350 years old) forests were quantified and compared with those of fully developed old-growth (400–600 years old) forests. We investigated Douglas-fir and western hemlock-dominated forests of the Pacific Northwest where Douglas-fir established following a single stand-replacing disturbance. Stand-level attributes were summarized using descriptive statistics, nonlinear regression, and old-growth indices. Variability in individual structural features was large between sites but broadly consistent with models of natural Douglas-fir forest development. Compared to older (>450 years old) forests, diameter distributions exhibited similar reverse J-shapes. Tree height distributions showed that shade tolerant species occupy lower canopy positions. Coarse woody debris was abundant in early old-growth forests for both snags (42–140 m3 ha−1) and logs (172–584 m3 ha−1). Early old-growth stands scored high enough on old-growth indices to qualify as old growth but their scores were significantly lower than older forests (400–600 years-old). Early old-growth structure fits well with the natural conceptual model of development providing a basis for ecologically focused management. Furthermore, structural conditions and variability of early old-growth forests provides a closer temporal target for managers seeking to accelerate the development of structure in younger stands or retain natural elements such as spatial patterning of trees.

Earth observation networks (EONs) are an emerging, surveillance-based approach to environmental monitoring and research that are fundamentally different than traditional question-driven, experimentally designed approaches. There is an urgent need to find an optimal balance between these approaches and to develop new integrated initiatives that take advantage of key features of them both.

Historical forest conditions in frequent-fire forests may be increasingly useful in guiding contemporary forest management given (1) projections for increased drought stress associated with climate change and (2) increases in vertical and horizontal fuel connectivity related to changes in land use over the past 150 years. Records from a 1922–25 timber inventory reveal historical variability at the landscape-level on mixed-conifer habitats on the eastern slopes of the Cascade Range in northern Oregon. Live conifers >15 cm dbh (diameter at breast height) were tallied by species and diameter class in a 20% sample of over 50,000 hectares (ha). Forests were predominantly low density (66 tph, standard deviation = 32, range = 0–289) relative to current conditions (312 ± 245, 0–1643 tph). Historical basal area averaged 14 ± 7 (0–70) m 2 ha −1 . Total stand density, large tree (>53 cm dbh) density, and ponderosa pine density were relatively stable across a wide moisture gradient (42–187 cm annual precipitation). Large trees dominated total basal area (73 ± 16%) and comprised 42 ± 17% of total trees per hectare (tph). Ponderosa pine contributed 62 ± 27% of basal area. Together, ponderosa pine and Douglas-fir constituted 91 ± 15% of basal area. Large ponderosa pine and Douglas-fir were nearly ubiquitous across the landscape in this historical data set, occurring on 94% and 83% of transects respectively. Large grand fir occurred on 20% of transects but contributed only 2 ± 6% to large tree basal area. Higher-density values (>120 tph), although rare, were distributed throughout the mixed-conifer habitat while large (>1.6 ha) treeless (no conifers >15 cm dbh) areas were almost entirely restricted to higher elevation, colder, wetter habitat types. Currently ponderosa pine no longer dominates large tree basal area, large trees no longer dominate total basal area, and Douglas-fir is now the dominant species across the landscape. Current mean tree densities are more than four times greater than values recorded in the historical cruise, and current basal area is approximately two times greater. Currently, large trees dominate basal area on only 29% of area inventoried compared to 91% in 1922–25. This systematic sample of a large landscape provides information about variability in species composition, densities, and structures at multiple spatial scales, which are highly relevant to management activities to restore and conserve desired ecosystem functions. Forest conditions comparable to those in this historical record have demonstrated resilience and resistance to fire and drought-related stressors in other frequent-fire forests.

Approximately 20 years ago, the preeminent goal for management of the federal forests of the Pacific Northwest shifted suddenly and permanently from sustained timber harvest to conservation of biodiversity and ecological processes, following a series of court cases over protection of species in decline that were associated with old forests. While old growth harvest has largely ceased, some key species are still in decline and forest management has been restricted more than intended. Creation of openings, even those based on disturbance processes, has been especially difficult. Some lessons from this experience include the difficulty of adaptive management, the importance of ecological foundations for management, and the need for stakeholder collaboration. In addition, it is essential to provide society with a vision of ecologically-based forestry, including field demonstrations, and to communicate this approach and its scientific foundation in the popular media.

The rate at which trees regenerate following stand-replacing wildfire is an important but poorly understood process in the multi-century development of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) forests. Temporal patterns of Douglas-fir establishment reconstructed from old-growth forests (>450 year) have generated contradictory models of either rapid (100 year) periods of establishment, while patterns of tree establishment in mid-aged (100 to 350 year) forests remains largely unknown. To determine temporal patterns of Douglas-fir establishment following stand-replacing fire, increment cores were obtained from 1455 trees in 18 mature and early old-growth forests in western Washington and northwestern Oregon, USA. Each of the stands showed continuous regeneration of Douglas-fir for many decades following initiating fire. The establishment period averaged 60 years (range: 32–99 years). These results contrast both with the view of rapid (one- to two-decade) regeneration of Douglas-fir promoted in the early forestry literature and with reports of establishment periods exceeding 100 years in older (>400 year) Douglas-fir–western hemlock stands. These results have important implications for management designed to create and promote early-seral forest characteristics.

Millions of acres of dry, frequent-fire woodlands and forests in western North America are the focus of multi-million dollar ecosystem restoration and fuel treatment activities. Societal awareness and engagement with these ecosystems has intensified due to recent mega fires and projections for increased vulnerability of these systems to fire, insects, and drought-related stressors. Also, the importance of goods and services provided by dry forests has expanded to include many values, such as watershed protection, habitat for biodiversity, and recreation as well as timber. Public awareness of fire and other risks associated with current conditions in dry forests generally is high and broad support exists for active management over passive alternatives. Efforts to integrate scientific principles with societal goals in dry forest restoration programs are encouraging but significant social barriers remain related to funding, conflicting goals (e.g., smoke vs. human health and restoration vs. preservation of species habitat), and stakeholder trust. The limited area restored relative to the extensive vulnerable area suggests that the seriousness and complexity of the threats are not fully appreciated or not given sufficient priority for funding, despite stated preferences for restorative management. Hence, challenges remain. Societal choices ultimately determine the goals, extent, and methodology of dry forest restoration programs with science stimulating and informing policy and management decisions.

Most silvicultural methods have been developed with the principal aim of ensuring adequate regeneration of commercial tree species after harvesting. Much less effort has been directed towards developing methods that benefit the re-establishment of all forest biodiversity. The concept of ‘forest influence’ relates the probability of species re-establishment to the distance from mature forest. This idea is central to contemporary retention forestry practices as well as connectivity theory in natural landscape management. Some species from all major forest biodiversity groups respond to forest influence following harvesting, however, the temporal and spatial scales of forest influence are mostly poorly known. This paper reviews global knowledge of mechanisms and scales at which forest influence operates, and shows that these are highly variable. Important general factors and mechanisms that underlie the ability of organisms to re-establish include qualities of retained elements, dispersal capacity, suitability of habitat conditions, and interspecific interactions, all of which may vary with distance from intact mature forest. Forest influence may enable species to persist in harvested areas through buffering of microclimate, and/or assist re-colonisation via proximity to source populations or essential habitat elements. Although foresters have often applied a “rule of thumb” that the extent of forest influence is within one tree height of mature forest, existing scientific literature provides little evidence of a universal relationship between canopy height of retained forest and re-establishment success. One-tree-height-from-retention guidelines can help plan harvest layouts, but only as long as plans allow for variation in re-establishment success among species and groups. The evidence from this review is that variability in harvest layouts will positively benefit biodiversity conservation in managed forest landscapes.

The moist coniferous forests of the Pacific Northwest are notable for the dominance of long-lived evergreen conifers, productivity, and the massiveness of the older forests (Franklin and Dyrness 1988; Franklin and Halpern 2000) (plate 2A). The environment of this region is extremely favorable to forest growth, with its moderate temperatures, high precipitation, and relatively fertile soils. The dominance of evergreen conifers is unusual, however, as most moist temperate forest regions of the world are dominated by hardwoods (angiosperms) (e.g., Kuchler 1946; Askins 2014). The climate allows photosynthesis to occur on essentially a year-round basis, which has been identified as one factor favoring evergreen conifers (Waring and Franklin 1979). Although the high productivity of the forests is important, the massiveness of the older forests is as much the consequence of the dominance of tree species present in these forests: for example, Douglas-fir is capable of continuous growth over many centuries and also produces decay-resistant heartwood, which persists for several more centuries. Hence, older forests have accumulations of organic matter (biologically sequestered carbon) that are among the greatest in the world (plate 1B).

Large old trees are critical organisms and ecological structures in forests, woodlands, savannas, and agricultural and urban environments. They play many essential ecological roles ranging from the storage of large amounts of carbon to the provision of key habitats for wildlife. Some of these roles cannot be replaced by other structures. Large old trees are disproportionately vulnerable to loss in many ecosystems worldwide as a result of accelerated rates of mortality, impaired recruitment, or both. Drivers of loss, such as the combined impacts of fire and browsing by domestic or native herbivores, chemical spray drift in agricultural environments, and postdisturbance salvage logging, are often unique to large old trees but also represent ecosystem-specific threats. Here, we argue that new policies and practices are urgently needed to conserve existing large old trees and restore ecologically effective and viable populations of such trees by managing trees and forests on much longer time scales than is currently practiced, and by protecting places where they are most likely to develop. Without these steps, large old trees will vanish from many ecosystems, and associated biota and ecosystem functions will be severely diminished or lost.

Stand-level spatial pattern influences key aspects of resilience and ecosystem function such as disturbance behavior, regeneration, snow retention, and habitat quality in frequent-fire pine and mixed-conifer forests. Reference sites, from both pre-settlement era reconstructions and contemporary forests with active fire regimes, indicate that frequent-fire forests are complex mosaics of individual trees, tree clumps, and openings. There is a broad scientific consensus that restoration treatments should seek to restore this mosaic pattern in order to restore resilience and maintain ecosystem function. Yet, methods to explicitly incorporate spatial reference information into restoration treatments are not widely used. In addition, targets from reference conditions must be critically evaluated in light of climate change. We used a spatial clump identification algorithm to quantify reference patterns based on a specified intertree distance that defines when trees form clumps. We used climatic water balance parameters, downscaled climate projections, and plant associations to assess our historical reference sites in the context of projected future climate and identify climate analog reference conditions. Spatial reference information was incorporated into a novel approach to prescription development, tree marking, and monitoring based on viewing stand structure and pattern in terms of individuals, clumps, and openings (ICO) in a mixed-conifer forest restoration case study. We compared the results from the ICO approach with simulations of traditional basal area and spacing-based thinning prescriptions in terms of agreement with reference conditions and functional aspects of resilience. The ICO method resulted in a distribution of tree clumps and openings within the range of reference patterns, while the basal area and spacing approaches resulted in uniform patterns inconsistent with known reference conditions. Susceptibility to insect mortality was lower in basal area and spacing prescriptions, but openings and corresponding opportunities for regeneration and in situ climate adaptation were fewer. Operationally, the method struck a balance between providing clear targets for spatial pattern directly linked to reference conditions, sufficient flexibility to achieve other restoration objectives, and implementation efficiency. The need to track pattern targets during implementation and provide immediate feedback to marking crews was a key lesson. The ICO method, especially when used in combination with climate analog reference targets, offers a practical approach to restoring spatial patterns that are likely to enhance resilience and climate adaptation.

We present a detailed case study of conservation and restoration of the Australian arboreal marsupial Leadbeater's Possum (Gymnobelideus leadbeateri) and its Mountain Ash forest habitat to illustrate the important intersection between forest restoration principles and the general principles for forest biodiversity conservation. Mountain Ash forests have been extensively modified through a century of intensive logging, recurrent wildfires and post-fire salvage logging. These disturbances have led to a reduction in old growth forest to 1/30th-1/60th of the extent of historical levels, a rapid collapse followed by a prolonged (≯30-year) shortage of populations of hollow-bearing trees throughout the Mountain Ash forests (which are critical habitat elements for many species of cavity-dependent vertebrates), and an increased risk of re-burning of landscapes dominated by young, regrowth forest. The consequences of the severe decline and consequent ‘temporary extinction’ of large old trees will be the potential g...

Rates and spatial patterns of tree mortality are predicted to change during forest structural development. In young forests, mortality should be primarily density dependent due to competition for light, leading to an increasingly spatially uniform pattern of surviving trees. In contrast, mortality in old-growth forests should be primarily caused by contagious and spatially autocorrelated agents (e.g., insects, wind), causing spatial aggregation of surviving trees to increase through time. We tested these predictions by contrasting a three-decade record of tree mortality from replicated mapped permanent plots located in young (60-year-old) and old-growth (300-year-old) Abies amabilis forests. Trees in young forests died at a rate of 4.42% per year, whereas trees in old-growth forests died at 0.60% per year. Tree mortality in young forests was significantly aggregated, strongly density dependent, and caused live tree patterns to become more uniform through time. Mortality in old-growth forests was spatially aggregated, but was density independent and did not change the spatial pattern of surviving trees. These results extend current theory by demonstrating that density-dependent competitive mortality leading to increasingly uniform tree spacing in young forests ultimately transitions late in succession to a more diverse tree mortality regime that maintains spatial heterogeneity through time.

Timber harvest can adversely affect forest biota. Recent research and application suggest that retention of mature forest elements (retention forestry), including unharvested patches (or aggregates) within larger harvested units, can benefit biodiversity compared to clearcutting. However, it is unclear whether these benefits can be generalized among the diverse taxa and biomes in which retention forestry is practiced. Lack of comparability in methods for sampling and analyzing responses to timber harvest and edge creation presents a challenge to synthesis. We used a consistent methodology (similarly spaced plots or traps along transects) to investigate responses of vascular plants and ground-active beetles to aggregated retention at replicate sites in each of four temperate and boreal forest types on three continents: Douglas-fir forests in Washington, USA; aspen forests in Minnesota, USA; spruce forests in Sweden; and wet eucalypt forests in Tasmania, Australia. We assessed (1) differences in local (plot-scale) species richness and composition between mature (intact) and regenerating (previously harvested) forest; (2) the lifeboating function of aggregates (capacity to retain species of unharvested forest); and whether intact forests and aggregates (3) are susceptible to edge effects and (4) influence the adjacent regenerating forest. Intact and harvested forests differed in composition but not richness of plants and beetles. The magnitude of this difference was generally similar among regions, but there was considerable heterogeneity of composition within and among replicate sites. Aggregates within harvest units were effective at lifeboating for both plant and beetle communities. Edge effects were uncommon even within the aggregates. In contrast, effects of forest influence on adjacent harvested areas were common and as strong for aggregates as for larger blocks of intact forest. Our results provide strong support for the widespread application of aggregated retention in boreal and temperate forests. The consistency of pattern in four very different regions of the world suggests that, for forest plants and beetles, responses to aggregated retention are likely to apply more widely. Our results suggest that through strategic placement of aggregates, it is possible to maintain the natural heterogeneity and biodiversity of mature forests managed for multiple objectives.

We outline elements of a forest restoration strategy designed to produce ecological and economic benefits on federal forests in Oregon and Washington, along with some of their policy and management implications. Implementation of this restoration strategy has begun on 11 projects (at scales from hundreds to thousands of acres) on federal lands. On Moist Forest sites (MF), the strategy calls for reserving older forest stands, thinning plantations to accelerate development of structural complexity, and implementing variable retention harvests in younger forests to help provide diverse early seral ecosystems. On Dry Forest (DF) sites, the strategy calls for silvicultural treatments that retain and release older trees, reduce stand densities, shift composition toward fireand drought-tolerant tree species, and incorporate spatial heterogeneity at multiple spatial scales. Immediate goals of this restoration framework include increased ecological integrity and resilience in DFs, increased diversity and complexity of successional stages in MFs, and provision of wood products to local communities. Over the long run, we believe this program can provide an acceptable pathway to sustained yield on federal forestlands in the Pacific Northwest.

Approximately 85% of the global forest estate is neither formally protected nor in areas dedicated to intensive wood production (e.g., plantations). Given the spatial extent of unprotected forests, finding management approaches that will sustain their multiple environmental, economic, and cultural values and prevent their conversion to other uses is imperative. The major global challenge of native forest management is further demonstrated by ongoing steep declines in forest biodiversity and carbon stocks. Here, we suggest that an essential part of such management—supplementing the protection of large reserves and sensitive areas within forest landscapes (e.g., aquatic features)—is the adoption of the retention approach in forests where logging occurs. This ecological approach to harvesting provides for permanent retention of important selected structures (e.g., trees and decayed logs) to provide for continuity of ecosystem structure, function, and species composition in the postharvest forest. The retention approach supports the integration of environmental, economic, and cultural values and is broadly applicable to tropical, temperate, and boreal forests, adaptable to different management objectives, and appropriate in different societal settings. The widespread adoption of the retention approach would be one of the most significant changes in management practice since the onset of modern high-yield forestry.

Background In forests subject to stand-replacing disturbances, conventional models of succession typically overlook early-seral stages as a simple re-organization/establishment period. These models treat structural development in essentially ‘relay floristic’ terms, with structural complexity (three-dimensional heterogeneity) developing primarily in old-growth stages, only after a closed-canopy ‘self-thinning’ phase and subsequent canopy gap formation. However, is it possible that early-successional forests can sometimes exhibit spatial complexity similar to that in old-growth forests – i.e. akin to an ‘initial floristic’ model of structural development? Hypothesis Based on empirical observations, we present a hypothesis regarding an important alternative pathway in which protracted or sparse forest establishment and interspecific competition thin out tree densities early on – thereby precluding overstorey canopy closure or a traditionally defined self-thinning phase. Although historically viewed as an impediment to stand development, we suggest this process may actually advance certain forms of structural complexity. These young stands can exhibit qualities typically attributed only to old forests, including: (1) canopy gaps associated with clumped and widely spaced tree stems; (2) vertically heterogeneous canopies including under- and mid-stories, albeit lower stature; (3) co-existence of shade-tolerant and intolerant species; and (4) abundant dead wood. Moreover, some of these qualities may persist through succession, meaning that a significant portion of eventual old-growth spatial pattern may already be determined in this early stage. Implications The relative frequency of this open-canopy pathway, and the degree to which precocious complexity supports functional complexity analogous to that of old forests, are largely unknown due to the paucity of naturally regenerating forests in many regions. Nevertheless, recognition of this potential is important for the understanding and management of early-successional forests.

At global and regional scales, tree mortality rates are positively correlated with forest net primary productivity (NPP). Yet causes of the correlation are unknown, in spite of potentially profound implications for our understanding of environmental controls of forest structure and dynamics and, more generally, our understanding of broad-scale environmental controls of population dynamics and ecosystem processes. Here we seek to shed light on the causes of geographic patterns in tree mortality rates, and we consider some implications of the positive correlation between mortality rates and NPP. To reach these ends, we present seven hypotheses potentially explaining the correlation, develop an approach to help distinguish among the hypotheses, and apply the approach in a case study comparing a tropical and temperate forest. Based on our case study and literature synthesis, we conclude that no single mechanism controls geographic patterns of tree mortality rates. At least four different mechanisms may be at play, with the dominant mechanisms depending on whether the underlying productivity gradients are caused by climate or soil fertility. Two of the mechanisms are consequences of environmental selection for certain combinations of life-history traits, reflecting trade-offs between growth and defense (along edaphic productivity gradients) and between reproduction and persistence (as manifested in the adult tree stature continuum along climatic and edaphic gradients). The remaining two mechanisms are consequences of environmental influences on the nature and strength of ecological interactions: competition (along edaphic gradients) and pressure from plant enemies (along climatic gradients). For only one of these four mechanisms, competition, can high mortality rates be considered to be a relatively direct consequence of high NPP. The remaining mechanisms force us to adopt a different view of causality, in which tree growth rates and probability of mortality can vary with at least a degree of independence along productivity gradients. In many cases, rather than being a direct cause of high mortality rates, NPP may remain high in spite of high mortality rates. The independent influence of plant enemies and other factors helps explain why forest biomass can show little correlation, or even negative correlation, with forest NPP.

Forests characterized by mixed-severity fires occupy a broad moisture gradient between lower elevation forests typified by low-severity fires and higher elevation forests in which high-severity, stand replacing fires are the norm. Mixed-severity forest types are poorly documented and little understood but likely occupy significant areas in the western United States. By definition, mixed-severity types have high beta diversity at meso-scales, encompassing patches of both high and low severity and gradients in between. Studies of mixed-severity types reveal complex landscapes in which patch sizes follow a power law distribution with many small and few large patches. Forest types characterized by mixed severity can be classified according to the modal proportion of high to low severity patches, which increases from relatively dry to relatively mesic site conditions. Mixed-severity regimes are produced by interactions between top-down forcing by climate and bottom-up shaping by topography and the flammability of vegetation, although specific effects may vary widely across the region, especially the relation between aspect and fire severity. History is important in shaping fire behavior in mixed-severity landscapes, as patterns laid down by previous fires can play a significant role in shaping future fires. Like low-severity forests in the western United States, many dry mixed-severity types experienced significant increases in stand density during the 20th century, threatening forest health and biodiversity, however not all understory development in mixed-severity forests increases the threat of severe wild fires. In general, current landscapes have been homogenized, reducing beta diversity and increasing the probability of large fires and insect outbreaks. Further loss of old, fire tolerant trees is of particular concern, but understory diversity has been reduced as well. High stand densities on relatively dry sites increase water use and therefore susceptibility to drought and insect outbreaks, exacerbating a trend of increasing regional drying. The need to restore beta diversity while protecting habitat for closed-forest specialists such as the northern spotted owl call for landscape-level approaches to ecological restoration.

Large-scale natural disturbances are commonplace around the world. They can have profound effects on human infrastructure and populations, as well as substantially influencing key ecological processes, shaping landscapes, and affecting many species. Major natural disturbances also have the potential to produce important ecological and resource management insights. Here, we argue that it is essential to initiate research quickly after such disturbances, and provide examples illustrating the ecological and management discoveries that arise from the early initiation of post-disturbance studies. However, there are currently few funding programs available to support this type of rapid research response. Consequently, many ecological and management learning opportunities are lost. We propose ways to address these problems, including: (1) establishing funding programs specifically directed at supporting studies of ecosystem response and recovery after major disturbances, and (2) encouraging scientists to develop...

Aspects of the tree mortality regime were characterized for old-growth conifer forests in Mount Rainier National Park, Washington, USA, using individual tree (stems ≥5 cm diameter at breast height (dbh)) records from a network of permanent forest research plots. Average annual forest-wide mortality rates of trees ≥15 cm dbh never exceeded 1% on a stem-density or basal-area basis; mortality was slightly higher for stems