Search

In southwestern US forests, the combined impact of climate change and increased fuel loads due to more than a century of human‐caused fire exclusion is leading to larger and more severe wildfires. Restoring frequent fire to dry conifer forests can mitigate high‐severity fire risk, but the effects of these treatments on the vegetation composition and structure under projected climate change remain uncertain.We used a forest landscape model to assess the impact of thinning and prescribed burns in dry conifer forests across an elevation gradient, encompassing low‐elevation pinyon‐juniper woodlands, mid‐elevation ponderosa pine and high‐elevation mixed‐conifer forests.Our results demonstrated that the treatments decreased the probability of high‐severity fires by 42% in the study area. At low elevation, the treatments did not prevent loss in forest cover and biomass with decreases in Pinus edulis and Juniperus monosperma abundances. At mid‐elevation, changes in fire effects maintained a greater diversity of tree species by favouring the maintenance of cohorts of old trees, in particular Pinus ponderosa which accumulated 5.41 Mg ha−1 more above‐ground biomass than without treatments by late‐century. Treatments in dry conifer forests modified fire effects beyond the treated area, resulting in increased cover and biomass of old Picea englemannii and Abies lasiocarpa cohorts.Synthesis and applications: Our findings indicate that thinning and prescribed burning can enhance tree species diversity in dry conifer forests by protecting old cohorts from stand‐replacing fires. Moreover, our results suggest that treatments mainly implemented in dry pine forests with high risk of high‐severity fires can be beneficial for subalpine species conservation by reducing the chance that high‐severity fire at mid‐elevation is transmitted into high‐elevation forest. [ABSTRACT FROM AUTHOR]

Carbon sequestration in the forests of the eastern United States is an important offset to the country's CO2 emissions. Much of the eastern forestland is the product of reforestation of abandoned agricultural land or recovery following clear‐cutting over a century ago. This has led to concerns that eastern forests are even‐aged and that rates of carbon sequestration will decline as forests increase in carbon. Our objective was to examine the successional dynamics of forest carbon sequestration—using live tree carbon stocks as a proxy for successional status—for the six broadly defined forest types present in the region. We used datasets from the National Forest Inventory (NFI) for the 31 US states from Minnesota south to Louisiana and eastward and analyzed live tree net carbon increment for 2007–2021, the period for which NFI plot remeasurement data were available for all 31 states. Spruce–fir and southern pines were the only forest types for which carbon increment declined even modestly over a significant fraction of the range of live tree carbon observed in the region, and southern pine–hardwood forests were the only forests in which predicted sequestration in live tree carbon declined to zero within the range of carbon stocks observed in the region. Northern hardwood–conifer forests, oak–hickory forests, and lowland forests experienced either no decline or a slight increase in sequestration in live tree carbon across the range of successional status observed in the region. Thus, the average stocks of live tree carbon per unit area increased steadily over the study period. At some point in succession, rates of mortality are expected to increase and balance gross growth, leading to zero net sequestration in live tree carbon. Mature and old‐growth stands, however, are rare in all six forest types, and mortality as a fraction of live tree carbon for all six forest types declined across the range of successional status present in the region. Our results thus provide no support for the hypothesis that the successional dynamics of forests in this region can be expected to lead to near‐term declines in net carbon sequestration. [ABSTRACT FROM AUTHOR]

Spatially explicit reconstruction of historical land cover change is a prerequisite for a more comprehensive understanding of environmental changes. Anthropogenic activities have dramatically altered the land cover of the conterminous United States (CONUS), encroaching heavily on the primary vegetation. However, few datasets exist that depict the historical trajectory of forest and grassland cover changes in CONUS over the last millennium, and previous efforts have only focused on reconstructions for the last four centuries. By integrating remote sensing-derived land use/cover change (LUCC) data and potential vegetation data, we determined the potential extent of natural forest (PENF) and grassland (PENG) in CONUS. Based on a qualitative analysis of the trends and driving forces of forest and grassland changes, we devised a method of subtracting reconstructed historical cropland (1000–2000 AD) and built-up land (1850–2000 AD) from PENG and PENF to reconstruct a 5 min × 5 min grid dataset of forest and grassland cover at 13 time-points over the past millennium. The results showed that forest and grassland cover in CONUS underwent a slow decline (1000–1600 AD), an accelerated decline (1600–1800 AD), a dramatic decline (1800–1950 AD), and finally, a recovery (1950–2000 AD) over the study period. The modelled forest fraction decreased from 49% in 1000 AD to 33% in 2000 AD, representing a 32% area reduction, whereas the modelled grassland fraction decreased from 37% to 22%, representing a 42% area reduction. The reduction occurred primarily in the last 200 years, with forest and grassland reductions accounting for 86% and 97% of the total reduction over the millennium, respectively. Spatially, more than 80% of the land was originally covered by forests and grasslands, and the loss occurred mainly in the eastern CONUS and Great Plains over the past millennium. After the 1930s, farmland abandonment began in central and eastern CONUS, simultaneously with environmental protection laws. Federal government regeneration programs for forest and grassland resources and the Shelterbelt Project all contributed to a slowdown in forest and grassland decline and recovery in cover. [ABSTRACT FROM AUTHOR]

Aim: Anticipating when and where changes in species' demographic rates will lead to range shifts in response to changing climate remains a major challenge. Despite evidence of increasing mortality in dry forests across the globe in response to drought and warming temperatures, the overall impacts on the distribution of dry forests are largely unknown because we lack comparable large‐scale data on tree recruitment rates. Here, our aim was to develop range‐wide population models for dry forest tree species (pinyon pine and juniper), quantifying both mortality and recruitment, to better understand where and under what conditions species range contractions are occurring. Location: Western United States. Major taxa studied: Two pinyon pine (Pinus spp.) and three juniper (Juniperus spp.) species. Methods: We developed range‐wide demographic models for five species using forest inventory data from across the western United States and estimated population trends and climate vulnerability. Results: We find that four of the five species are declining in parts of their range, with Pinus edulis having the largest proportion of populations declining (24%). Population vulnerability increases with aridity and temperature, with up to ~50% of populations declining in the warmest and driest conditions. Mortality and recruitment were both essential to explaining where populations are declining. Main conclusions Our results suggest that dry forest species are undergoing an active range shift driven by both changing recruitment and mortality, and that increasing temperatures and drought threaten the long‐term viability of many of these species in their current range. While four of the five species examined were experiencing some declines, P. edulis is currently most vulnerable. Management actions such as reducing tree density may be able to mitigate some of these impacts. The framework we present to estimate range‐wide demographic rates can be applied to other species to determine where range contractions are most likely. [ABSTRACT FROM AUTHOR]

Here, we use 16‐year satellite and reanalysis data in combination with a multivariate regression model to investigate how aerosols affect cloud fraction (CF) over the East Coast of the United States. Cloud droplet number concentrations (Nd), cloud geometrical thickness, lower tropospheric stability, and relative humidity at 950 hPa (RH950) are identified as major cloud controlling parameters that explain 97% of the variability in CF. Nd is shown to play an important role in regulating the dependence of CF on RH950. The observed annual‐mean CF shows no significant trend due to the cancellation from the opposite trends in Nd and RH950. The multivariate regression model revealed that the decline in Nd alone would lead to about a 20% relative decline in CF. Our results indicate the significant aerosol effects on CF and suggest the need to account for pollution‐induced cloud changes in quantifying cloud feedback based on long‐term observations. Plain Language Summary: Understanding how aerosols affect cloud cover is critical for reducing the large uncertainty of the radiative forcing associated with aerosol‐cloud interactions for future climate projections. By choosing a region (East Coast of the United States) that has experienced a significant decline in cloud droplet number concentrations due to pollution control in recent decades, we quantify aerosol effects on cloud amount based on long‐term satellite observation and reanalysis data in combination with a statistical regression model that isolates aerosol effects from meteorology influences. Our results reveal significant effects of aerosols on cloud amount and indicate the need to account for changes in cloud microphysics for constraining and quantifying cloud feedback based on long‐term observations. Key Points: Nd plays an important role in regulating the dependence of cloud fraction (CF) on RH950The observed annual‐mean CF shows no significant trend due to the cancellation from the opposite trends in Nd and RH950When fixing the meteorology, we found significant aerosol effects on cloud amount based on long‐term satellite observations [ABSTRACT FROM AUTHOR]

Increasing aridity is a challenge for forest managers and reducing stand density to minimize competition is a recognized strategy to mitigate drought impacts on growth. In many dry forests, the most widespread and common forest management programs currently being implemented focus on restoration of historical stand structures, primarily to minimize fire risk and enhance watershed function. The implications of these restoration projects for drought vulnerability are not well understood. Here, we examined how planned restoration treatments in the Four Forests Restoration Initiative, the largest forest restoration project in the United States, would alter landscape‐scale patterns of forest growth and drought vulnerability throughout the 21st century. Using drought–growth relationships developed within the landscape, we considered a suite of climate and treatment scenarios and estimated average forest growth and the proportion of years with extremely low growth as a measure of vulnerability to long‐term decline. Climatic shifts projected for this landscape include higher temperatures and shifting seasonal precipitation that promotes lower soil moisture availability in the early growing season and greater hot‐dry stress, conditions negatively associated with tree growth. However, drought severity and the magnitude of future growth declines were moderated by the thinning treatments. Compared to historical conditions, proportional growth in mid‐century declines by ~40% if thinning ceases or continues at the status quo pace. By comparison, proportional growth declines by only 20% if the Four Forest Restoration Initiative treatments are fully implemented, and <10% if stands are thinned even more intensively than currently planned. Furthermore, restoration treatments resulted in dramatically fewer years with extremely low growth in the future, a recognized precursor to forest decline and eventual tree mortality. Benefits from density reduction for mitigating drought‐induced growth declines are more apparent in mid‐century and under RCP4.5 than under RCP8.5 at the end of the century. Future climate is inherently uncertain, and our results only reflect the climate projections from the representative suite of models examined. Nevertheless, these results indicate that forest restoration projects designed for other objectives also have substantial benefits for minimizing future drought vulnerability in dry forests and provide additional incentive to accelerate the pace of restoration. [ABSTRACT FROM AUTHOR]

Changing forest disturbance regimes and climate are driving accelerated tree mortality across temperate forests. However, it remains unknown if elevated mortality has induced decline of tree populations and the ecological, economic, and social benefits they provide. Here, we develop a standardized forest demographic index and use it to quantify trends in tree population dynamics over the last two decades in the western United States. The rate and pattern of change we observe across species and tree size-distributions is alarming and often undesirable. We observe significant population decline in a majority of species examined, show decline was particularly severe, albeit size-dependent, among subalpine tree species, and provide evidence of widespread shifts in the size-structure of montane forests. Our findings offer a stark warning of changing forest composition and structure across the western US, and suggest that sustained anthropogenic and natural stress will likely result in broad-scale transformation of temperate forests globally. The nature of forest disturbances are changing, yet consequences for forest dynamics remain uncertain. Using a new index, Stanke et al. show the populations of over half of the most abundant tree species in the western US have declined in the last two decades, with grim implications for how temperate forests globally will respond to sustained anthropogenic and natural stress. [ABSTRACT FROM AUTHOR]

Warming climate and resulting declines in seasonal snowpack have been associated with drought stress and tree mortality in seasonally snow‐covered watersheds worldwide. Meanwhile, increasing forest density has further exacerbated drought stress due to intensified tree–tree competition. Using a uniquely detailed data set of population‐level forest growth (n = 2,495 sampled trees), we examined how inter‐annual variability in growth relates to snow volume across a range of forest densities (e.g., competitive environments) in sites spanning a broad aridity gradient across the United States. Forest growth was positively related to snowpack in water‐limited forests located at low latitude, and this relationship was intensified by forest density. However, forest growth was negatively related to snowpack in a higher latitude more energy‐limited forest, and this relationship did not interact with forest density. Future reductions in snowpack may have contrasting consequences, as growth may respond positively in energy‐limited forests and negatively in water‐limited forests; however, these declines may be mitigated by reducing stand density through forest thinning. [ABSTRACT FROM AUTHOR]

The rural United States is sometimes viewed as a paragon of stability, but demographic change has been a constant. Rural and small-town America is diverse, dynamic, and relational to (rather than separate from) urban places. Dominant demographic trends in the rural U.S. over recent decades include depopulation (driven by population aging and youth out-migration), increasing ethnoracial diversity (especially as linked to Hispanic population growth), and in-migration to select areas associated with metropolitan adjacency (i.e., exurban growth), retirement destinations, and natural amenities. This special issue of Population Research and Policy Review assembled a group of papers focused squarely on the changing demography of rural and small-town America in the early twenty-first century that address issues of broad interest to demographers: population growth and decline, fertility, mortality, migration, ethnoracial composition, and economic inequality. The prospect is for a rural America marked by ever more diversity and inequality within and between places. Our hope is that these articles, and the broader spectrum of scholarship on rural demography they represent, will inspire the next generation of research in this area. [ABSTRACT FROM AUTHOR]

The 2016 National Land Cover Database (NLCD) product suite (available on www.mrlc.gov), includes Landsat-based, 30 m resolution products over the conterminous (CONUS) United States (U.S.) for land cover, urban imperviousness, and tree, shrub, herbaceous and bare ground fractional percentages. The release of NLCD 2016 provides important new information on land change patterns across CONUS from 2001 to 2016. For land cover, seven epochs were concurrently generated for years 2001, 2004, 2006, 2008, 2011, 2013, and 2016. Products reveal that land cover change is significant across most land cover classes and time periods. The land cover product was validated using existing reference data from the legacy NLCD 2011 accuracy assessment, applied to the 2011 epoch of the NLCD 2016 product line. The legacy and new NLCD 2011 overall accuracies were 82% and 83%, respectively, (standard error (SE) was 0.5%), demonstrating a small but significant increase in overall accuracy. Between 2001 and 2016, the CONUS landscape experienced significant change, with almost 8% of the landscape having experienced a land cover change at least once during this period. Nearly 50% of that change involves forest, driven by change agents of harvest, fire, disease and pests that resulted in an overall forest decline, including increasing fragmentation and loss of interior forest. Agricultural change represented 15.9% of the change, with total agricultural spatial extent showing only a slight increase of 4778 km2, however there was a substantial decline (7.94%) in pasture/hay during this time, transitioning mostly to cultivated crop. Water and wetland change comprised 15.2% of change and represent highly dynamic land cover classes from epoch to epoch, heavily influenced by precipitation. Grass and shrub change comprise 14.5% of the total change, with most change resulting from fire. Developed change was the most persistent and permanent land change increase adding almost 29,000 km2 over 15 years (5.6% of total CONUS change), with southern states exhibiting expansion much faster than most of the northern states. Temporal rates of developed change increased in 2001–2006 at twice the rate of 2011–2016, reflecting a slowdown in CONUS economic activity. Future NLCD plans include increasing monitoring frequency, reducing latency time between satellite imaging and product delivery, improving accuracy and expanding the variety of products available in an integrated database. [ABSTRACT FROM AUTHOR]

Insects and pathogens are widely recognized as contributing to increased tree vulnerability to the projected future increasing frequency of hot and dry conditions, but the role of parasitic plants is poorly understood even though they are common throughout temperate coniferous forests in the western United States. We investigated the influence of western hemlock dwarf mistletoe (Arceuthobium tsugense) on large (≥45.7 cm diameter) western hemlock (Tsuga heterophylla) growth and mortality in a 500 year old coniferous forest at the Wind River Experimental Forest, Washington State, United States. We used five repeated measurements from a long‐term tree record for 1,395 T. heterophylla individuals. Data were collected across a time gradient (1991–2014) capturing temperature increases and precipitation decreases. The dwarf mistletoe rating (DMR), a measure of infection intensity, varied among individuals. Our results indicated that warmer and drier conditions amplified dwarf mistletoe effects on T. heterophylla tree growth and mortality. We found that heavy infection (i.e., high DMR) resulted in reduced growth during all four measurement intervals, but during warm and dry intervals (a) growth declined across the entire population regardless of DMR level, and (b) both moderate and heavy infections resulted in greater growth declines compared to light infection levels. Mortality rates increased from cooler‐wetter to warmer‐drier measurement intervals, in part reflecting increasing mortality with decreasing tree growth. Mortality rates were positively related to DMR, but only during the warm and dry measurement intervals. These results imply that parasitic plants like dwarf mistletoe can amplify the impact of climatic stressors of trees, contributing to the vulnerability of forest landscapes to climate‐induced productivity losses and mortality events. [ABSTRACT FROM AUTHOR]

Forest carbon sequestration via forest preservation can be a viable climate change mitigation strategy. Here, we identify forests in the western conterminous United States with high potential carbon sequestration and low vulnerability to future drought and fire, as simulated using the Community Land Model and two high carbon emission scenario (RCP 8.5) climate models. High‐productivity, low‐vulnerability forests have the potential to sequester up to 5,450 Tg CO2 equivalent (1,485 Tg C) by 2099, which is up to 20% of the global mitigation potential previously identified for all temperate and boreal forests, or up to ~6 yr of current regional fossil fuel emissions. Additionally, these forests currently have high above‐ and belowground carbon density, high tree species richness, and a high proportion of critical habitat for endangered vertebrate species, indicating a strong potential to support biodiversity into the future and promote ecosystem resilience to climate change. We stress that some forest lands have low carbon sequestration potential but high biodiversity, underscoring the need to consider multiple criteria when designing a land preservation portfolio. Our work demonstrates how process models and ecological criteria can be used to prioritize landscape preservation for mitigating greenhouse gas emissions and preserving biodiversity in a rapidly changing climate. [ABSTRACT FROM AUTHOR]

Oaks (Quercus spp.) are keystone species in many ecosystems and are ecologically as well as economically valuable. The objective of this study was to gather and evaluate information from a diverse group of oak experts on current and future biotic and abiotic threats to oaks in the eastern United States. Using a Delphi survey method with three iterative surveys, we found that oak decline and climate change were identified as critical current and future threats, respectively, in this region. Focusing on climate change, experts were asked a series of questions to assess its potential future temporal and spatial impact on oaks. With respect to climate change, the majority of experts surveyed believe climate change will manifest gradually, although they were generally uncertain about the geographical distribution of climate change in the future, i.e., the areas where oaks are likely to be impacted by climate change in the future. New/emerging pests and pathogens were seen as the most critical future threat by the third survey round. Results from this study can be used to better inform management practices and research priorities for ensuring resilient oak resources for the future. [ABSTRACT FROM AUTHOR]

Plantation silviculture is increasing globally and is particularly intensive in temperate coniferous forests, where densely planting trees requires practices common to non‐conifer systems that can alter forest floor microhabitat, and potentially threaten amphibian persistence. Most declining amphibian species depend on specific forest microhabitats as terrestrial refugia, but amphibian extirpation associated with tree harvest alone appears unlikely, suggesting that impacts of planting forests on groundcover might better predict recent declines in amphibian occupancy.We repeatedly sampled larval presence or absence of 10 amphibian species native to temperate coniferous forest in the Southeastern United States for one year at 62 isolated wetlands, located in either naturally regenerating or planted forest (plantation). We assessed three direct ways that planted forests might reduce amphibian breeding site occupancy by: (a) increasing conifer densities, (b) decreasing groundcover, and (c) an indirect pathway, whereby increased tree densities at plantations might reduce groundcover and thus amphibian site occupancy.After controlling for wetland traits and accounting for differences in detection, breeding site occupancy for 8/10 amphibian species was dependent upon whether forests were planted surrounding wetlands (within 300 m). Herbaceous groundcover, not canopy, most commonly influenced occupancy and increased occupancy for declining surface active or fossorial amphibians.Path analyses showed that, by directly and indirectly reducing groundcover (via conifer densities), plantations had significantly lower occupancy of two declining surface active or fossorial frog species, whereas two common aquatic frog species were tolerant to planting conifers. Among declining species, salamanders showed a greater reduction in occupancy than anurans, likely because of greater vulnerability to the drier forest floor conditions of plantation than naturally regenerating forests.Synthesis and applications. Direct negative impacts of coniferous plantation on amphibians can be addressed by limiting groundcover and soil impacts, including switching from high intensity practices, such as mechanical chopping vegetation or bedding soil, to lower intensity site preparation treatments that are less likely to significantly disturb groundcover. Indirect negative effects of dense canopy cover at planted forests could be lowered by periodically thinning canopies prior to final harvest, thus increasing intact forest groundcover and the conservation of both common and declining amphibians. [ABSTRACT FROM AUTHOR]

• Mature and old-growth forests are defined by ecology, policy, and values. • Advanced tree age and relative lack of human impact are common determinants. • However, humans have influenced old forest structure and process for millennia. • Forest structure metrics are practical and useful criteria. • Mapping older forests across a landscape accurately is not currently possible. Mature and old-growth forests are valued for biodiversity, carbon sequestration, habitat, hydrologic function, aesthetics, and spirituality, as well as Tribal and Indigenous histories, cultures, and practices. Over the last 500 years, land use change and industrialization have resulted in global declines in the area of older forests (however defined). The goal of this study was to identify concepts and indicators to define mature and old-growth forests across the vegetation types of the United States in order to quantify their abundance and distribution. Defining old growth has been described as a "wicked problem" that involves values, science, and management; requires multiple disciplines; and can be expressed from many contradictory approaches. The most common approach to defining mature and old-growth forests is to place them in a successional continuum of increases in tree size, biodiversity, habitat niches, and structural diversity with forest age. Time since severe disturbance, including human impact, is often a consideration, although humans have influenced the development of many forests for millennia. The successional framework is less useful in low-productivity or frequently-disturbed forests, or where current structural diversity under fire suppression may not reflect historic or desired future conditions. In order to classify forests into "old" and "not old", existing structure-based approaches apply minima of one or more structural or compositional criteria. Site productivity and/or plant association is an element of many definitions. Once defined, estimating the area of mature and old-growth forest presents challenges. The only comprehensive, consistent field data of US forests is the Forest Inventory and Analysis (FIA) network of >140,000 forested plots. While the 0.067 ha sample area of FIA plots limits the number of structural metrics that might be useful and the plot density cannot capture fine-scale spatial heterogeneity, measurements enable a granular application of multiple structural and compositional criteria by vegetation type at broad spatial extents, and the ability to track change consistently over time. Spatial models integrate field and remotely-sensed data to predict the distribution of structural classes at finer spatial grain, but with substantial error in high-resolution estimates. There does not seem to be a readily-available method to map mature and old-growth stands across a landscape with a high degree of accuracy. Identifying mature and old growth forests in a stand management context will likely require additional measurements, adjustments to criteria at local scales, and incorporation of social and traditional knowledge within a consistent definition framework. [ABSTRACT FROM AUTHOR]

• Ponderosa pine is experiencing natural regeneration failure from seed. • We found regeneration failure in 29% of forests, in absence of severe disturbance. • The climate conditions leading to regeneration failure will intensify in the future. • There is high potential for regeneration failure to expand across latitudes. • Regeneration failure is a widespread and active threat to forest persistence. As climate changes in coming decades, ponderosa pine forest persistence may be increasingly dictated by their regeneration. Sustained regeneration failure has been predicted for forests of the southwestern US (SWUS) even in absence of stand-replacing wildfire, but regeneration in undisturbed and lightly disturbed forests has been studied infrequently and at a limited number of locations. We characterized 77 ponderosa pine sites in 7 SWUS locations, documented regeneration occurring over the past ∼ 20 years, and utilized gridded meteorological estimates and water balance modeling to determine the climate and environmental conditions associated with regeneration failure (R0). Of these sites, 29 % were R0, illuminating that regeneration failure in these forests is widespread. R0 sites were distinguished by high above- and belowground heat loading, loss of cool-season climate, and high soil moisture variation. Explanatory variables had high accuracy in identifying R0 sites, and illustrate the climate-driven pathway by which regeneration failure has occurred in the SWUS. Regeneration failure has high potential to increase in a warmer, more hydrologically variable climate, and expand regionally from lower to higher latitudes. Yet, we also found that human management interventions were associated with environmental conditions that avoided regeneration failure. To counteract regeneration-associated forest declines, interventions will need to influence climate-driven environmental change by adjusting forest characteristics at local scales. Regeneration failures are a major threat to ponderosa pine forest persistence, and they have potential to intensify and expand in a changing climate. [ABSTRACT FROM AUTHOR]

The black turpentine beetle, Dendroctonus terebrans Olivier is the largest pine-infesting bark beetle native to the southern and eastern United States. It generally reproduces in fresh stumps and bases of trees weakened or killed by other biotic or abiotic agents, although it can also infest and sometimes kills apparently healthy trees. Its numbers can build when large amounts of host material become available (typically through a disturbance), and black turpentine beetle-caused mortality at a local scale can become considerable. Here, we provide a complete review of the literature on this species, including its taxonomy, host, life history, chemical ecology, arthropod and microbial associates, and management options. We also provide original data on numbers of instars, acoustic signals, and pheromone chirality in this species. Our survey of the existing literature revealed that key biological characteristics of black turpentine beetles are known, but interactions with closely associated organisms, economic and ecological impacts, and improvements to monitoring and management practices have been only partially investigated. [ABSTRACT FROM AUTHOR]

Climate change is altering the conditions for tree recruitment, growth, and survival, and impacting forest community composition. Across southeast Alaska, USA, and British Columbia, Canada, Callitropsis nootkatensis (Alaska yellow‐cedar) is experiencing extensive climate change‐induced canopy mortality due to fine‐root death during soil freezing events following warmer winters and the loss of insulating snowpack. Here, we examine the effects of ongoing, climate‐driven canopy mortality on forest community composition and identify potential shifts in stand trajectories due to the loss of a single canopy species. We sampled canopy and regenerating forest communities across the extent of C. nootkatensis decline in southeast Alaska to quantify the effects of climate, community, and stand‐level drivers on C. nootkatensis canopy mortality and regeneration as well as postdecline regenerating community composition. Across the plot network, C. nootkatensis exhibited significantly higher mortality than co‐occurring conifers across all size classes and locations. Regenerating community composition was highly variable but closely related to the severity of C. nootkatensis mortality. Callitropsis nootkatensis canopy mortality was correlated with winter temperatures and precipitation as well as local soil drainage, with regenerating community composition and C. nootkatensis regeneration abundances best explained by available seed source. In areas of high C. nootkatensis mortality, C. nootkatensis regeneration was low and replaced by Tsuga. Our study suggests that climate‐induced forest mortality is driving alternate successional pathways in forests where C. nootkatensis was once a major component. These pathways are likely to lead to long‐term shifts in forest community composition and stand dynamics. Our analysis fills a critical knowledge gap on forest ecosystem response and rearrangement following the climate‐driven decline of a single species, providing new insight into stand dynamics in a changing climate. As tree species across the globe are increasingly stressed by climate change‐induced alteration of suitable habitat, identifying the autecological factors contributing to successful regeneration, or lack thereof, will provide key insight into forest resilience and persistence on the landscape. [ABSTRACT FROM AUTHOR]

In 1961 families across the United States witnessed the sudden growth of one of the most remarkable consumer products of the Cold War: the home fallout shelter. This article charts the rise of domestic sales for home fallout shelters between 1961 and 1963, the growth in the number of shelter salesmen, the public backlash against their sales techniques, and the eventual decline of the home shelter market. The story of the family fallout shelter exposes the limitations of consumer capitalism in mobilizing and sustaining popular support for national security policy. Questioning the validity of the product being sold and the trustworthiness of the person pitching it, homeowners challenged the citizen-consumer ideal that supposedly went hand-in-hand with the state sanctioned vision of privatized survival. [ABSTRACT FROM AUTHOR]

Municipalities are embracing greening initiatives as a key strategy for improving urban sustainability and combatting the environmental impacts of expansive urbanization. Many greening initiatives include goals to increase urban canopy cover through tree planting, however, our understanding of street tree ecosystem dynamics is limited and our understanding of vegetation structure and function based on intact, rural forests does not apply well to urban ecosystems. In this study, we estimate size-specific growth, mortality, and planting rates in trees under municipal control, use a box model to forecast short-term changes in street tree aboveground carbon pools under several planting and management scenarios, and compare our findings to rural, forested systems. We find accelerated rates of carbon cycling in street trees with mean diameter growth rates nearly four times faster in Boston, MA, USA (0.78 ± 0.02 cm yr-1) than in rural forest stands of MA (0.21 ± 0.02 cm yr-1) and mean mortality rates more than double rural forested rates (3.06 ± 0.25% yr-1 in street trees; 1.41 ± 0.04% yr-1 in rural trees). Despite the enhanced growth of urban trees, high mortality losses result in a net loss of street tree carbon storage over time (-0.15 ± 0.09 Mg C ha-1 yr-1). Planting initiatives alone may not be sufficient to maintain or enhance canopy cover and biomass due to the unique demographics of urban ecosystems. Initiatives to aid in the establishment and preservation of tree health are central for increasing street tree canopy cover and maintaining/increasing carbon storage in vegetation. Strategic combinations of planting and maintenance will maximize the viability of greening initiatives as an effective climate mitigation tool. [ABSTRACT FROM AUTHOR]

Eastern hemlock (Tsuga canadensis) is undergoing widespread decline throughout the eastern United States due to the introduction of the nonnative hemlock woolly adelgid (Adelges tsugae). In light of hemlock's unique significance for avian communities, we examined the long-term response of the breeding bird community to the decline and die-off of this foundational forest species. We conducted variable-radius point counts to survey bird communities in both hemlock and hardwood stands in 2000, during the early stages of adelgid infestation, and again in 2015 and 2016, following significant hemlock decline. We also measured the severity of hemlock decline and associated vegetation variables in the same hemlock stands in which avian surveys were completed. We used multispecies occupancy models to examine species-specific and species group responses to hemlock decline. Results showed that, across hemlock stands, hemlock basal area declined from a mean of 56% of the total basal area to 46%, and that hemlock decline was correlated with current vegetation structure, including an increased deciduous understory. Species richness of hemlock-associated birds declined by an average of ~1 species per survey location. The Black-throated Green Warbler (Setophaga virens) was the most strongly affected, declining in occupancy by 30%. All other species groups responded positively, with the strongest responses by species associated with the shrub layer, forest edge, and mature deciduous habitat. The species composition in hemlock and hardwood stands became more similar over time as the unique species assemblages in hemlock stands gave way to the avian community of the surrounding hardwood landscape, highlighting a trend toward homogenization of the avian community. Where hemlock-associated bird species persisted, their presence was correlated with a combined effect of greater hemlock basal area and better hemlock condition. Our results demonstrate that the decline of hemlocks has restructured vegetation communities and their associated avian communities over time, with strong implications for forest-specialist birds. [ABSTRACT FROM AUTHOR]

• Native biological disturbance agents (BDAs) are diverse natural influences on fuels and fire in the western U.S., but most fire research has focused on bark beetles especially the mountain pine beetle. • Collectively, BDAs influence fuels and fire across a gradient of space and time, from episodic outbreaks that change with time-since-event, to chronic mortality, and from individual tree to regional forest scales. • BDA effects on fuels depend on specific fuelbed characteristics; elevated surface fuels could increase fire spread rate and more readily transfer surface fire to crown fire, while less canopy fuels may reduce crown fire potential. • BDAs have both positive or negative impacts on fire risk and fire behavior attributes. Forest biological disturbance agents (BDAs) are insects, pathogens, and parasitic plants that affect tree decline, mortality, and forest ecosystems processes. BDAs are commonly thought to increase the likelihood and severity of fire by converting live standing trees to more flammable, dead and downed fuel. However, recent research indicates that BDAs do not necessarily increase, and can reduce, the likelihood or severity of fire. This has led to confusion regarding the role of BDAs in influencing fuels and fire in fire-prone western United States forests. Here, we review the existing literature on BDAs and their effects on fuels and fire in the western US and develop a conceptual framework to better understand the complex relationships between BDAs, fuels and fire. We ask: 1) What are the major BDA groups in western US forests that affect fuels? and 2) How do BDA-affected fuels influence fire risk and outcomes? The conceptual framework is rooted in the spatiotemporal aspects of BDA life histories, which drive forest impacts, fuel characteristics and if ignited, fire outcomes. Life histories vary among BDAs from episodic, landscape-scale outbreaks (bark beetles, defoliators), to chronic, localized disturbance effects (dwarf mistletoes, root rots). Generally, BDAs convert aboveground live biomass to dead biomass, decreasing canopy fuels and increasing surface fuels. However, the rate of conversion varies with time-since-event and among BDAs and forest types, resulting in a wide range of effects on the amount of dead fuels at any given time and place, which interacts with the structure and composition of the stand before and subsequent to BDA events. A major influence on fuels may be that BDAs have emerged as dominant agents of forest heterogeneity creation. Because BDAs play complex roles in fuels and fire heterogeneity across the western US which are further complicated by interactions with climate change, drought, and forest management (fire suppression), their impacts on fuels, fire and ecological consequences cannot be categorized simply as positive or negative but need to be evaluated within the context of BDA life histories and ecosystem dynamics. [ABSTRACT FROM AUTHOR]

We synthesize insights from current understanding of drought impacts at stand-to-biogeographic scales, including management options, and we identify challenges to be addressed with new research. Large stand-level shifts underway in western forests already are showing the importance of interactions involving drought, insects, and fire. Diebacks, changes in composition and structure, and shifting range limits are widely observed. In the eastern US, the effects of increasing drought are becoming better understood at the level of individual trees, but this knowledge cannot yet be confidently translated to predictions of changing structure and diversity of forest stands. While eastern forests have not experienced the types of changes seen in western forests in recent decades, they too are vulnerable to drought and could experience significant changes with increased severity, frequency, or duration in drought. Throughout the continental United States, the combination of projected large climate-induced shifts in suitable habitat from modeling studies and limited potential for the rapid migration of tree populations suggests that changing tree and forest biogeography could substantially lag habitat shifts already underway. Forest management practices can partially ameliorate drought impacts through reductions in stand density, selection of drought-tolerant species and genotypes, artificial regeneration, and the development of multistructured stands. However, silvicultural treatments also could exacerbate drought impacts unless implemented with careful attention to site and stand characteristics. Gaps in our understanding should motivate new research on the effects of interactions involving climate and other species at the stand scale and how interactions and multiple responses are represented in models. This assessment indicates that, without a stronger empirical basis for drought impacts at the stand scale, more complex models may provide limited guidance. [ABSTRACT FROM AUTHOR]

Evidence of shifting dominance among major forest disturbance agent classes regionally to globally has been emerging in the literature. For example, climate-related stress and secondary stressors on forests (e.g., insect and disease, fire) have dramatically increased since the turn of the century globally, while harvest rates in the western US and elsewhere have declined. For shifts to be quantified, accurate historical forest disturbance estimates are required as a baseline for examining current trends. We report annual disturbance rates (with uncertainties) in the aggregate and by major change causal agent class for the conterminous US and five geographic subregions between 1985 and 2012. Results are based on human interpretations of Landsat time series from a probability sample of 7200 plots (30 m) distributed throughout the study area. Forest disturbance information was recorded with a Landsat time series visualization and data collection tool that incorporates ancillary high-resolution data. National rates of disturbance varied between 1.5% and 4.5% of forest area per year, with trends being strongly affected by shifting dominance among specific disturbance agent influences at the regional scale. Throughout the time series, national harvest disturbance rates varied between one and two percent, and were largely a function of harvest in the more heavily forested regions of the US (Mountain West, Northeast, and Southeast). During the first part of the time series, national disturbance rates largely reflected trends in harvest disturbance. Beginning in the mid-90s, forest decline-related disturbances associated with diminishing forest health (e.g., physiological stress leading to tree canopy cover loss, increases in tree mortality above background levels), especially in the Mountain West and Lowland West regions of the US, increased dramatically. Consequently, national disturbance rates greatly increased by 2000, and remained high for much of the decade. Decline-related disturbance rates reached as high as 8% per year in the western regions during the early-2000s. Although low compared to harvest and decline, fire disturbance rates also increased in the early- to mid-2000s. We segmented annual decline-related disturbance rates to distinguish between newly impacted areas and areas undergoing gradual but consistent decline over multiple years. We also translated Landsat reflectance change into tree canopy cover change information for greater relevance to ecosystem modelers and forest managers, who can derive better understanding of forest-climate interactions and better adapt management strategies to changing climate regimes. Similar studies could be carried out for other countries where there are sufficient Landsat data and historic temporal snapshots of high-resolution imagery. [ABSTRACT FROM AUTHOR]

Human modifications of the landscape often cause changes in regional surface and subsurface water flow that in turn can result in alterations in the natural cycle of flooding and drawdown in wetland ecosystems. In the present study, excessive flooding resulting from altered land-use activities increased mortality in wetland tree communities. We investigated tree decline status, tree size, species composition, and capacity for clonal propagation along an elevation/hydrologic gradient to understand community structure and composition dynamics resulting from excessive flooding.Prior to disturbance, the density of small stems (2 cm≤DBH<10 cm) doubled with a decrease of <1 m elevation along the transect while density of other tree sizes (DBH≥10 cm) remained relatively constant at all elevations. This increase in small stems corresponded with an increase in the number of individuals growing clonally. Tree communities at the highest elevations were dominated by less flood-tolerant species, such as Quercus laurifolia and Liquidambar styraciflua, whereas tree communities at lower elevations were dominated by highly flood-tolerant species such as Nyssa sylvatica var. biflora and Fraxinus caroliniana.Prolonged flooding was causing tree communities at low elevations to shift towards a more shrubby structure and composition dominated by more flood-tolerant species that could regenerate clonally. Among most species, trees greater than 10 cm DBH were generally more sensitive to flooding than smaller trees at high levels of flood disturbance and the ability to reproduce clonally was found to be an important mechanism allowing species such as F. caroliniana and Acer rubrum to persist under near permanently flooded conditions. The mechanism of clonal propagation for surviving severe flooding events may be important for wetland forest communities throughout the southeastern US. [Copyright &y& Elsevier]

Koa (Acacia koa) is an endemic Hawaiian tree that serves as a keystone species in the upper elevation forests of all the main islands. In the Mauna Loa Strip area of Hawaii Volcanoes National Park, mature koa stands are suffering from an unexplained dieback that has increased in severity since it was first noticed approximately 25 years ago. The dieback is often evident in patches, and generally spreads within stands in a radial fashion from a localized infection center. Entire crowns of affected trees become wilted, with foliage gradually progressing from an apparent healthy to a completely chlorotic condition. Although most trees die soon after the onset of symptoms, some trees are able to survive crown death by producing epicormic shoots on the lower portions of the trunk. Previously published studies reported that a vascular wilt fungus (Fusarium oxysporum f. sp. koae) was associated with koa seeds and the rhizosphere of healthy and dieback-affected koa stands. The purpose of this study was to characterize the stand structure, soil conditions, and physiological condition of dieback-affected trees, and to assess the possible role of F. oxysporum f. sp. koae in the current dieback stands. This fungus was isolated from branches of symptomatic koa in dieback-affected stands and roots from healthy and dieback-affected stands. Possible differences in the pathogenicity and virulence of F. oxysporum f. sp. koae isolates obtained from the roots of healthy koa in unaffected stands and those from branches of dieback-affected koa were determined by greenhouse inoculations of koa seedlings. Healthy koa saplings in stands unaffected by dieback were also inoculated to determine if disease symptoms could be induced by inoculation of injured roots in the field. Both branch and root isolates were pathogenic; with the percent mortality of inoculated seedlings ranging from 30 to 60% for all isolates. Disease severity between branch and root isolates was not significantly different. Wilt symptoms were not observed in field inoculations of koa saplings after 8 months. Investigation of the dieback stand structure determined that the larger size classes (i.e., DBH>3 cm) were all affected equally, with the smallest size class (i.e., 0–3 cm DBH) having the lowest percentage of dieback at each site. Soil from dieback stands had higher water content and was more acidic than that from neighboring unaffected stands. Phyllodes on epicormic shoots of dieback trees had a reduced area, mass, and mass to area ratio. Phyllodes on dieback trees also had reduced stomatal conductance and higher (less negative) leaf water potential. Results from the greenhouse inoculations demonstrated that F. oxysporum f. sp. koae is pathogenic to koa, but the pathogen’s activity in the field may be influenced by predisposing factors such as temperature fluctuations, water availability, soil type, and interactions with other soil organisms. F. oxysporum f. sp. koae appears to play an important role in regulating koa stand dynamics, growth, and physiology under environmental and edaphic conditions favorable for disease development. [Copyright &y& Elsevier]

The fluctuation of streamflow in snowmelt-dominated watersheds may be an indicator of climate change. However, the relationship between the start of growing season (SOS) and the streamflow in snowmelt-dominated watersheds is not clear. In this study, we update the Coupled Hydro-Ecological Simulation System (CHESS) model by incorporating the Growing Season Index (GSI) module to estimate the start of the growing season. The updated CHESS model is then used to calculate the streamflow in the Cleve Creek, Incline Creek and Twin River watersheds located in Nevada in the United States from 1981 to 2017. This updated CHESS can be applied in any regions that are suitable for deciduous vegetation. The streamflow in the static and dynamic scheme in the three watersheds have been simulated between 1981 and 2017 with the NS of 0.52 and 0.80 in the Cleve Creek, 0.46 and 0.75 in the Incline Creek, and 0.42 and 0.70 in the Twin River watersheds, respectively. The results illustrate that the SOS have come around 3–5 weeks earlier during the last 37 years. The results illustrate a high correlation between the temperature and the timing of the SOS. Early SOS leads to a substantial increase in total annual transpiration. An increase in annual transpiration can reduce aquifer recharge and increase cumulative growing season soil moisture deficit. Comparing to the streamflow without vegetation, the streamflow with vegetation is smaller due to transpiration. As the SOS comes earlier, the peaks of the streamflow with vegetation also come earlier. If the shifts in SOS continue, the effects on annual rates of transpiration can be significant, which may reduce the risk of flooding during snowmelt. On the other hand, earlier SOS may cause soil moisture to decline during summer, which would increase the drought stress in trees and the risk of wildfires and insect infestation. [ABSTRACT FROM AUTHOR]

The death of many mature red spruce (Picea rubens Sarg.) trees in the northeastern United States has sparked controversy over the roles natural factors and air pollutants play. The goal of this work was to investigate the effects of long-term ozone exposures on development of injury and, in particular, the expression of winter injury in red spruce. Red spruce seedlings were exposed to elevated levels of ozone during two growing seasons, starting just after budbreak, and continuing into the period of early frosts. Trees spent both winter periods under ambient conditions of light and temperature. No visible lesions on foliage that could be attributed to ozone developed during the growing seasons. Yellow and brown needles appeared each winter, with much more injury in the second year. However, in the spring after two seasons of ozone exposure, there was no significant effect of ozone on the overall appearance of the entire population of seedlings. Samples were taken for histology and ultrastructure approximately monthly. Ozone treatments increased mesophyll cell disruption after frosts in early winter in the first year, but not in the second. There was no correlation between histological injury in the autumn and visible symptoms in the spring, indicating that there may be different aspects of the winter injury syndrome. In the second summer from 20 to 40°, of all mesophyll cells were damaged. This damage was not related to ozone treatment, and probably resulted from high summer temperatures. [ABSTRACT FROM AUTHOR]

Yellow cedar is a valuable tree species that is experiencing an extensive forest decline on over 200,000 hectare of unmanaged forest in southeast Alaska. Biotic factors appear secondary and some abiotic factor is probably the primary cause of this naturally occurring decline. A warming climate, which coincided with the onset of extensive tree mortality about 100 years ago, may have triggered one of the possible abiotic causes such as freezing damage and/or soil toxicity.

Climate variability and change acting at broad scales can lead to divergent changes in plant production at local scales. Quantifying how production responds to variation in climate at local scales is essential to understand underlying ecological processes and inform land management decision-making, but has historically been limited in spatiotemporal scale based on the use of discrete ground-based measurements or coarse resolution satellite observations. With the advent of cloud-based computing through Google Earth Engine (GEE), production responses to climate can be evaluated across broad landscapes though time at a resolution useful for ecological and land management applications. Here, GEE was employed to synthesize a multi-platform Landsat time series (1988–2014) and evaluate relationships between the soil-adjusted vegetation index (a proxy for plant production) and climate across deserts and plant communities of the southwestern U.S. A "climate pivot point" approach was adopted in GEE to assess the trade-off between production responses to increasing wetness and resistances to drought at 30-m resolution. Consistent with a long-term seasonal climate gradient, production was most related to climate variance during the cool-season in the western deserts, during the warm-season in the eastern deserts, and equally related to both seasons within several desert areas. Communities dominated by grasses and deciduous trees displayed large production responses to an increase in wetness and low resistances to water deficit, while shrublands and evergreen woodlands had variable responses and high drought resistances. Production in plant communities that spanned multiple deserts responded differently to seasonal climate variability in each desert. Defining these plant production sensitivities to climate at 30-m resolution in GEE advances forecasts of how long-term climate trajectories may affect carbon storage, wildlife habitat, and the vulnerability of water-limited ecosystems. • GEE allows high resolution understanding of climate-plant production relationships. • This knowledge is essential for ecological and land management applications. • Production closely tracks seasonality of water availability across deserts. • Production responses to water availability diverge within and among plant communities. • This divergence has implications for carbon storage and ecosystem transitions. [ABSTRACT FROM AUTHOR]

The sequestration of atmospheric carbon (C) in forests has partially offset C emissions in the United States (US) and might reduce overall costs of achieving emission targets, especially while transportation and energy sectors are transitioning to lower-carbon technologies. Using detailed forest inventory data for the conterminous US, we estimate forests' current net sequestration of atmospheric C to be 173 Tg yr−1, offsetting 9.7% of C emissions from transportation and energy sources. Accounting for multiple driving variables, we project a gradual decline in the forest C emission sink over the next 25 years (to 112 Tg yr−1) with regional differences. Sequestration in eastern regions declines gradually while sequestration in the Rocky Mountain region declines rapidly and could become a source of atmospheric C due to disturbances such as fire and insect epidemics. C sequestration in the Pacific Coast region stabilizes as forests harvested in previous decades regrow. Scenarios simulating climate-induced productivity enhancement and afforestation policies increase sequestration rates, but would not fully offset declines from aging and forest disturbances. Separating C transfers associated with land use changes from sequestration clarifies forests' role in reducing net emissions and demonstrates that retention of forest land is crucial for protecting or enhancing sink strength. [ABSTRACT FROM AUTHOR]