Climate change is forcing us to find innovative solutions to help managed forests cope with rapidly shifting environmental conditions. One of these tools is assisted forest migration, the deliberate movement of individuals or genetic material from native sources (i.e. provenance) to locations within or beyond their current ranges. This study aims to assess the climate analogue concept as seed sourcing method in an assisted migration field trial. We evaluated the five-year survival and growth of nine species in mixedwood plantings established in 2018 in Quebec, Canada. The factorial experimental design comprised cutting treatments (1.2 ha patch clearcut vs. 40 % uniform shelterwood), cervid exclusion (excluded vs. non-excluded) and competing vegetation (brushcut vs. control) treatments. Seedlings were grown from seeds of locations associated to three climate analogues: current climate, projected climate for mid-century (2041-2070) and end-of-century (2071-2100). Five-year survival averaged 84 %, ranging from 69 % for Carya ovata to 90 % for Quercus rubra. End-of-century analogue performed less well than others for relocations > 500 km. All species grew larger in patch clearcut than in shelterwood, especially Pinus, Picea and Thuja spp. (3-4x diameters, 2-3x heights). To a lesser extent, brushing slightly improved diameter growth of Carya ovata, Quercus rubra and Thuja occidentalis, but only in patch clearcuts for Prunus serotina, Pinus and Picea spp. Impact of cervid was minimal likely due to snowpack protection. We observed limited effects of climatic mismatch on translocated seedlings, which supports the climate analogue approach as seed sourcing method. Longer-term monitoring will be required to confirm trends.

Forest managers recognize that proactive management strategies, such as forest assisted migration (FAM) of tree species, intended to accelerate the pace of forest adaptation, may be necessary to maintain resilient forests and combat the stressors of climate change. However, the impact of interactions between climate change and ungulate browsers of trees, both of which have profound effects on the landscape, on the success of FAM efforts is unknown. We used a forest landscape model (LANDIS‐II) to simulate assisted migration and browsing by ungulate (white‐tailed deer [ Odocoileus virginianus ]) populations on a northern Wisconsin (USA) landscape under alternate climate futures. After accounting for effects of FAM strategy and climate change scenario, simulated ungulate browsing reduced species richness and the proportion of the landscape with tree species preferred by browsers and resulted in more of the landscape developing into novel forest communities that supported fewer ecosystem goods and services. Our results suggest that managers may need to select FAM species less preferred by ungulates or use seedling protection measures to mitigate the negative effects of chronically high ungulate populations in efforts to transition future forests to sustain ecosystem goods and services.

Understanding the complexity of forest community dynamics is essential in forest management planning and stewardship, yet lowland northern white-cedar ( Thuja occidentalis L.) are often managed as homogenous communities. Through this study, we defined lowland white-cedar forest community types in unharvested and harvested forest stands within the State of Michigan and examined community type associations with ecological variables. Data collected in unharvested stands revealed three white-cedar community subtypes: (1) cedar-deciduous, (2) cedar-conifer, and (3) cedar-shrub. These unharvested subtypes were dominated by white-cedar, yet characterized by different soils, hydrology, geochemical gradients, and associated tree species. In harvested stands, six community types were identified: (1) aspen-fir, (2) winterberry-willow, (3) balsam fir, (4) cedar-red maple, (5) cedar-black spruce, and (6) alder-tamarack. These harvested community types were located along ecological gradients, including soil type (organic or mineral) and soil water pH. Using community types in unharvested and harvested stands, and associated ecological gradients, potential pathways of compositional transition were theorized. Findings suggest that cedar community subtype affects the likelihood of cedar regeneration and dictates the alternative species replacing cedar after harvest. These findings and potential pathways are useful to forestry practitioners, as they highlight potential changes in tree species dominance following harvest across a range of lowland white-cedar community types, allowing refinement of silvicultural prescriptions to ensure desired outcomes.

The physical structure of forest canopies regulates numerous ecological processes and functions, and management strategies to promote specific structural features, such as high structural complexity, are increasingly desired. Partial disturbances - including partial harvests - can modify forest canopy structure, but their specific effects may be influenced by differences in characteristics such as intensity and frequency. Understanding the variable effects of silvicultural treatments on canopy structure will be essential to designing silvicultural regimes that can promote canopy structural complexity. We evaluated the effect of partial harvesting regimes on canopy structure (i.e., density, cover, height, arrangement, and heterogeneity) and complexity (as rugosity), quantified using a suite of metrics derived from portable canopy lidar, across three levels of harvesting intensity in three multi-decadal silvicultural experiments. We specifically assessed: 1) how repeated partial harvesting disturbance affected canopy structure and complexity relative to unmanipulated controls, 2) whether effects of partial harvesting differed among harvesting intensities or 3) between old- and second-growth stands, and 4) if canopy light interception differed among stands with different histories of partial harvesting disturbance. Findings indicated that canopy structure development under partial harvesting regimes differed from that of control stands, reducing vertical and canopy structural complexity at a fine (i.e., 1-6 ha treatment unit) scale, but increasing among-unit variability in canopy structure relative to controls. Variation in intensity among treatments produced relatively little distinction in fine-scale canopy structural outcomes, but the higher intensity selection harvesting regime in particular differed significantly from controls across all metrics of canopy structure. Partial harvesting affected the developmental trajectories of both old-growth and maturing, secondary forest stands, where selection-based management seemed to decrease structural features associated with old forests. Our findings suggest management regimes developed for timber goals could accommodate alternative goals that promote multi-scale canopy structural complexity across managed northern hardwood forest landscapes by adapting the timing (e.g., extending cutting intervals) and intensity (e.g., low intensity harvests) of partial harvesting to retain or develop elements of canopy complexity.

IntroductionGlobal climate change and associated stressors threaten forest ecosystems due to the rapid pace of climate change, which could exceed the natural migration rate of some tree species. In response, there is growing interest to research and implement forest assisted migration (FAM). Here, we used a species-independent indicator based on climate analogy, according to the sigma (dis)similarity (σd) index, to match planting sites across the eastern US with (future) climatically-compatible seedlots (CCS).MethodsWe developed CCS for a grid composed of 1 × 1° of latitude and longitude. CCS were based on future climate analogs with ≤2σd analogy to ensure CCS were representative of future climate change. CCS were located for three time periods, 2030's, 2050's, and 2090's and three emissions scenarios (SSP2-4.5, SSP3-7.0, and SSP5-8.5) from the Coupled Model Intercomparison Project phase 6 database, using 12 climate variables.ResultsCCS were identified for the majority of 1 × 1° grids based on the SSP3-7.0 scenario. Approximately 28% of 1 × 1° grid's 2090's projections included future climate novelty. The 2030's, 2050's, and 2090's CCS were located on average 222, 358, and 662 km or 1, 2, and 3 eastern seed zones away from the 1 × 1° grids, respectively. CCS were also located further south-southwest (188–197°). In addition, the average forest cover of CCS was approximately 2%, 5%, and 10% less than that of the 1 × 1° grids.DiscussionOur development and synthesis of CCS emphasized four key results: (i) average distances to 2030's and 2050's CCS were similar to seed-transfer guidelines for some tree species, but 2090's CCS exceeded current recommendations; (ii) south-southwesterly locations of CCS aligned with tree species habitat distribution dynamics; (iii) future climate novelty potentially challenges the conceptual basis of FAM if tree species are not adapted to climate change; and (iv) variation in forest cover among CCS presents potential opportunities and challenges due the presence or absence of forestland to source seed. Ultimately, our goal was to locate and synthesize CCS that could enable FAM decision support.

Currently barrens communities only represent about 1% of their original area in the Great Lakes region. To maintain or restore barrens vegetation, prescribed fire is often applied to limit the regeneration of undesirable species and shrubs. Vegetation community response is a combination of direct fire effects on the vegetation vitality and the indirect effect of soil nitrogen (N) loss that favors nutrient-poor adapted barren communities. In this study, we assessed forest floor and upper mineral soil (0-5 cm) pools of carbon (C), N, and mercury (Hg) before and after prescribed fire of the Moquah Barrens in northwest Wisconsin. Although we took measurements in four distinct cover types, we found no relationship between cover type and soil pools. Across all cover types, prescribed fire led to considerable emissions of C, N, and Hg in the forest floor but only Hg in the upper mineral soils (0-5 cm), presumably because maximum fire temperatures were met for Hg volatilization. We classified fire severity and soil surface temperatures at the quadrat scale, but no discernable relationships with emissions were observed. The lack of detectable relationships is likely the result of a mismatch between the scales of response variables and predictors. As a result, we calculated ecosystem-scale fire emissions based on the total area burned because we could not discern other smaller scale predictors. Overall emissions from dormant, spring season prescribed fires at the Moquah Barrens were approximately 11,000 Mg (5.5 Mg ha-1) for C, 350 Mg for N (0.17 Mg ha-1), and 4,500 g for Hg (2.3 g ha-1). Imperiled barrens communities only represent a small area of their original distribution. As a result of the decrease in barrens habitat, associated fauna and flora are also in decline. Prescribed fire is used to maintain or restore barrens ecosystems. Fire leads to the changes in soil carbon, nitrogen, and mercury pools and leads to associated emissions. Emissions from the Moquah Barrens prescribed fires were minor and did lead to the goal of decreasing soil nitrogen.

Invasive plants can significantly impact the diversity of understory ground flora and forest regeneration in eastern North America. However, managing invasive plants has resulted in positive, negative, or neutral effects on key ecosystem components depending on treatment type, duration, and intensity. Management may also result in short-term control, but legacy effects from prior land use or secondary invasions may hamper desired long-term outcomes. We conducted a systematic review of the invasive plant management literature for eastern North American forests to examine treatment outcomes for invasive and native plants, tree regeneration, and secondary invasions. Our review included 165 articles with few papers published in the 1980s but the number of papers increasing through time thereafter. A variety of control methods were used, including herbicide applications, prescribed burning, torching, girdling, clipping, mastication, soil amendments, flooding, enrichment plantings, and biocontrol, as well as combinations of these treatments. Species included some of the most common forest invaders, such as the privets (Ligustrum spp.), honeysuckles (Lonicera spp.), autumn olive (Elaeagnus umbellata), buckthorns (Rhamnus cathartica and Frangula alnus), garlic mustard (Alliaria petiolata), Japanese stiltgrass (Microstegium vimineum), cogongrass (Imperata cylindrica), tree-of-heaven (Ailanthus altissima), and Chinese tallow (Triadica sebifera). The literature also included recent invaders in eastern North America, such as Callery pear (Pyrus calleryana) and fig buttercup (Ficaria verna). Findings suggest that invasive plant control efficacy is highly variable and context dependent. Information on long-term effects is limited because most studies reported on findings occurring within a few years of treatment. However, long-term success may be limited without additional management (e.g., enrichment plantings, artificial tree regeneration, re-establishing historic fire regimes, reducing herbivore densities) that ameliorates impacts from past land-use, disturbance history, or other factors. We suggest that future studies and the development of control tactics consider comprehensive approaches to building resilience in forest communities where invasive plants are only one aspect of the forest management continuum.

Forest managers are exploring options to proactively facilitate forest adaptation to climate change (resistance or resilience) or to introduce species (transition) that are better suited to future climates. Forest managers must have confidence that implementing an assisted migration (AM) transition strategy will maintain a more reliable stream of goods and services than strategies emphasizing resistance or resilience. The outcome of transition strategies can be evaluated with forest landscape models having direct links to climate and atmospheric drivers. We used the LANDIS-II forest landscape model to conduct a simulation experiment in northern Wisconsin (USA) with climate scenarios and AM strategies as treatment factors, and metrics of ecosystem goods and services as response variables. We found that major forest functional types were maintained under some climate change with AM strategies that selected species having similar silvics and site adaptations as existing species but sourced from different climate regions. We also found that AM alone was increasingly unable to maintain ecosystem goods and services (e.g., productivity, wildlife food) with increasing severity of climate change. For instance, total woody biomass, total harvested biomass, and species and age class richness were largely determined by the climate scenario and not AM strategy. Our results suggest that modest changes in climate are likely to enhance species diversity and increase biomass production through longer growing seasons and CO2 fertilization, but that under extreme climate change even the most aggressive AM strategies fail to mitigate the deleterious effects of moisture stress and increased respiration on overall productivity. Where AM strategies were successful, there were subtle and unintended changes in the extent of available landscape goods and services. Additional research is needed to further refine AM strategies to conserve a complete range of goods and services under changing climate.

Since the development of contemporary stocking techniques a century ago, the combination of climatic, atmospheric, financial, and social factors that determine forest management strategies have changed, altering aspen stand dynamics in the western Great Lakes, USA. Despite this, aspen management is still informed by 1970s management guides that are based on 1920s inventories; hence, a century exists between the data that underlie current management guidelines and current stand conditions. We hypothesized that current aspen stands may support higher stocking and height growth than nearly a century ago at relatively similar age and site indices, due to increased atmospheric CO2 concentrations and fertilization, intensive coppice harvests, and other factors. To explore this question, we compared historic aspen observations with comparable contemporary data from the USDA Forest Service’s Forest Inventory and Analysis program. The results show increased stand stocking levels as well as increased height growth of aspen throughout the region over the historic inventory data. Although other controlled experimental studies support the hypothesis of increased carbon fertilization altering aspen size-density relationships, our study is the first to examine an empirical application to forest management guides. Our results suggest a comprehensive reevaluation of aspen growth dynamics under contemporary environmental conditions is warranted. We highlight the need to assess the value of current stocking standards in an era of increasingly variable environmental conditions and to reimagine a more dynamic, responsive, and predictive approach to guide forest management for future application as global change may accelerate.

Ecosystem-based forest management associated with partial harvesting (PH) is intended to balance ecological and economic values of sustainable forest management. The potential for delayed growth response and elevated mortality of advance regeneration following PH remains a critical concern, and may present a barrier to more widespread implementation of this approach. We used 835 permanent continuous forest inventory plots to examine the rate and time course of species-specific regeneration growth and mortality of eight tree species in the first fifteen years following operational partial harvests in the mixed-species forests of Maine, United States. We aimed to provide a quantitative understanding on how regeneration of different species responded to PH in terms of growth and mortality. In addition, we evaluated how the patterns and magnitudes of growth and mortality responses developed over time, if these responses occur gradually or suddenly, and if the patterns of the responses were persistent. We found that the response magnitude, temporal trajectories of responses, and the length of initial lag-period largely varied across species, PH treatments, and the variables examined. For sapling diameter growth, paper birch (Betula papyrifera Marshall) and red maple (Acer rubrum L.) showed immediate responses to high-intensity PH, while a five-year lag-period was observed in balsam fir (Abies balsamea (L.) Mill.), American beech (Fagus grandifolia Ehrh.), red spruce (Picea rubens Sarg.) and eastern hemlock (Tsuga canadensis (L.) Carrie`re) and a 10-year lag period in northern white-cedar (Thuja occidentalis L.). The initial increase in sapling mortality was observed in balsam fir, American beech, red maple and northern white-cedar, but not in other species. Sapling survival reached a stable state irrespective of species after the initial five-years following harvests. In partially harvested stands, identifying preharvest conditions related to postharvest density, growth, and mortality was complex and interacted with time since harvest. Our results suggest that broad application of PH only results in species-specific gains, losses, and delays in regeneration responses within mixed-species stands. Future research should consider PH in combination with other treatments to initiate immediate re-sponses to a wider range of species.

Global change is reshaping climatic conditions at a tempo that exceeds natural migration rates for most tree species. As climate change amplifies the disparity between species' adaptive capacity and local climates, tree populations risk becoming geographically stranded in increasingly unsuitable conditions. This mismatch may cause catastrophic losses of key forest ecosystem services such as carbon sequestration, habitat provisioning, and forest products. In response, forest managers and researchers are developing a suite of climate-adaptive strategies designed to sustain forest diversity and function. Among these, forest assisted migration (FAM) involves the movement of planting stock from source populations to locations either within or beyond their current ranges. The goal is to establish forests that can survive in today's climate and expected to thrive in future conditions, thereby sustaining ecosystem good and services. Because FAM is still in its infancy, implementation is limited by many uncertainties. Climatically derived seed sourcing is needed to ensure that planting stock possesses the ecophysiological amplitude to withstand both current and future climatic conditions at the destination site. Additionally, more knowledge about the impacts of local herbivores and intraspecific competition are needed because these drivers will co-regulate seedling success along with climate. Practically, these uncertainties must be addressed to instill in managers sufficient confidence that FAM investments will fulfill long-term management and societal goals relative to other silvicultural approaches. The Desired REgeneration through Assisted Migration (DREAM) framework is an international collaboration that uses basic and applied research to reduce these uncertainties and derive climate-informed planting approaches. DREAM is self-reinforcing in that each step in the process informs and strengthens subsequent phases. Namely, it sources seed in a climatically informed manner, experimentally tests this sourced stock to probe for physiological maladaptation under controlled settings, grows the stock in the field under a range of silvicultural scenarios, and finally forecasts long-term outcomes using models parameterized from the controlled- and fieldtests. In this paper, we describe the DREAM framework and illustrate aspects of its implementation drawing from two experimental sites: one in Que & PRIME;bec, Canada and one in Wisconsin, USA. Moreover, we place the DREAM study into the broader FAM context by briefly contrasting it with other operational examples throughout North America. Knowledge gained from this research-management collaboration will expand current reforestation paradigms to include future climate-adaptive ones that aim to use the right seed, planted in the right places, under the right conditions.

Climate change represents an existential threat to many forest ecosystems because tree populations are often adapted to local climate means and variability. If tree populations cannot migrate or adapt, they risk becoming increasingly maladapted with climate change. This emerging mismatch underscores the need for climate adaptive management techniques, such as assisted migration of tree species, to help mitigate climate change impacts on forest ecosystems. Although biotic and abiotic factors are known to constrain tree establishment success, the extent to which they may determine the success of assisted migration plantings is poorly understood. Thus, defining the extent to which trees affect—and in turn are affected by local soil environments and microbial communities (i.e., plant-soil feedbacks; hereafter PSFs) remains important for guiding effective climate-adaptive forest management. Our objectives were to synthesize the current state of knowledge about the direction and magnitude of PSF effects on temperate tree species of eastern North America, and to identify key hypotheses important for guiding future research. To accomplish these goals, we conducted a meta-analysis of 26 peer-reviewed publications that addressed our criteria. Our compiled database included 61 tree species and was composed primarily of short-term greenhouse experiments that studied PSF effects by manipulating the soil biota in three ways: (1) soil was previously inoculated by a conspecific or heterospecific tree species (i.e., home vs. away), (2) soil was live or sterilized, or (3) soil was untreated or treated with fungicide. We found that PSF had significant effects on tree growth, with the direction and magnitude of PSF strongly dependent on tree mycorrhizal guild. Arbuscular mycorrhizal tree species grown in live or home soils grew 13–33% less than those in sterile or away soils, while ectomycorrhizal tree species grew 11–44% more in live or home than sterile or away soils. PSF effects were associated with several plant functional traits, including specific leaf area, tissue nitrogen, and specific root length. We provide suggestions on incorporating PSFs into assisted migration trials and outline key knowledge gaps for future research. Our synthesis of context-dependent effects of PSFs on tree performance will help inform management decisions involving assisted migration.

AbstractWhat are typical values and dynamic status of live‐tree biomass pools in old‐growth, mesic, cool temperate forests? A handful of biomass density estimates in eastern North American temperate forests show large biomass/carbon reserves on a per‐area basis. However, it is less clear whether these ecosystems are, over multi‐decade scales, typically steady‐state or non‐equilibrial carbon pools. Previous studies have suggested both possibilities, but claims are based on inferences from short‐term studies or proxy data sets. An unusually long‐term and extensive data set from repeatedly sampled permanent plots (84 yr, ca. 10 ha sample area, 6–8 measurements), from old‐growth conifer‐hardwood forest in northern Michigan, USA, allows direct estimation of multi‐decade trends in aboveground live‐tree biomass. Results confirm prior suggestions of high‐biomass density for old‐growth temperate forests (averaging >300 Mg/ha), but, despite significant decade‐scale variation, show no overall, long‐term directional change. Study plots typically show multi‐decade trends of gradually increasing biomass density, interrupted by sharp declines attributed to intermediate‐severity disturbances, with recovery of pre‐disturbance biomass density requiring upwards of a half‐century. At the stand scale, biomass dynamics are strongly historically contingent, and short‐term studies may yield biased or misleading results. Disturbance legacies, through demographic and structural effects, can have multi‐decade effects on vulnerability to further disturbance. While this study shows no general trend in aboveground biomass pools, it suggests that changes in disturbance regime may drive important feedbacks in biomass pool dynamics.

Sustainable management of forest ecosystems requires the use of reliable and easy to implement biodiversity and naturalness indicators. Tree-related microhabitats (TreMs) can fulfill these roles as they harbor specialized species that directly or indirectly depend on them, and are generally more abundant and diverse in natural forests or forests unmanaged for several decades. The TreM concept is however still recent, implying the existence of many knowledge gaps that can challenge its robustness and applicability. To evaluate the current state of knowledge on TreMs, we conducted a systematic review followed by a bibliometric analysis of the literature identified. A total of 101 articles constituted the final corpus. Most of the articles (60.3%) were published in 2017 or after. TreM research presented a marked lack of geographical representativity, as the vast majority (68.3%) of the articles studied French, German or Italian forests. The main themes addressed by the literature were the value of TreMs as biodiversity indicators, the impact of forest management on TreMs and the factors at the tree- and stand-scales favoring TreMs occurrence. Old-growth and unmanaged forests played a key role as a “natural” forest reference for these previous themes, as TreMs were often much more abundant and diverse compared to managed forests. Arthropods were the main phylum studied for the theme of TreMs as biodiversity indicators. Other more diverse themes were identified, such as restoration, remote sensing, climate change and economy and there was a lack of research related to the social sciences. Overall, current research on TreMs has focused on assessing its robustness as an indicator of biodiversity and naturalness at the stand scale. The important geographical gap identified underscores the importance of expanding the use of the TreMs in other forest ecosystems of the world. The notable efforts made in recent years to standardize TreM studies are an important step in this direction. The novelty of the TreM concept can partially explain the thematic knowledge gaps. Our results nevertheless stress the high potential of TreMs for multidisciplinary research, and we discuss the benefits of expanding the use of TreMs on a larger spatial scale.