Populus tremuloides seedling establishment: An underexplored vector for forest type conversion after multiple disturbances

Ecosystem resilience to climate change is contingent on post-disturbance plant regeneration. Sparse gymnosperm regeneration has been documented in subalpine forests following recent wildfires and compounded disturbances, both of which are increasing. In the US Intermountain West, this may cause a shift to non-forest in some areas, but other forests may demonstrate adaptive resilience through increased quaking aspen (Populus tremuloides Michx.) dominance. However, this potential depends on ill-defined constraints of aspen sexual regeneration under current climate. We created an ensemble of species distribution models for aspen seedling distribution following severe wildfire to define constraints on establishment. We recorded P. tremuloides seedling locations across a post-fire, post-blowdown landscape. We used 3 algorithms (Mahalanobis Typicalities,Multilayer Perceptron Artificial Neural Network, and MaxEnt) to create spatial distribution models for aspen seedlings and to define constraints. Each model performed with high accuracy and was incorporated into an ensemble model, which performed with the highest overall accuracy of all the models. Populus tremuloides seedling distribution is constrained primarily by proximity to unburned aspen forest and annual temperature ranges, and secondarily by light availability, summer precipitation, and fire severity. Based on model predictions and validation data, P. tremuloides seedling regeneration is viable throughout 54% of the post-fire landscape, 97% of which was previously conifer-dominated. Aspen are less susceptible to many climatically-sensitive disturbances (e.g. fire, beetle outbreak, wind disturbance), thus, aspen expansion represents an important adaptation to climate change. Continued aspen expansion into post-disturbance landscapes through sexual reproduction at the level suggested by these results would represent an important adaptation to climate change and would confer adaptive forest resilience by maintaining forest cover, but would also alter future disturbance regimes, biodiversity, and ecosystem services.Ye

Mapping Burn Severity using the Composite Burn Index in an Oak Savannah in Central Massachusetts

The goal of this paper is to characterize the spatial variability in burn severity of a wildfire in an oak savannah within the wildland-urban interface in Worcester, Massachusetts using the Composite Burn Index (CBI). This work compares two interpolated surfaces of burn severity values collected in a March 2012 burn area and examines the statistical relationship between burn severity (CBI), fuel type and topography. The wildfire burned at low-to-medium severity (0.1 – 2.0 CBI). Burn severity was highest in mixed grass and shrub fuels beneath oak canopy cover (mean 1.11 CBI), followed by open-canopy shrub fuels (mean 1.09 CBI) and then open-canopy grass fuels (mean 1.06 CBI). Burn severity was negatively correlated with elevation (r2 = -0.22) and slope (r2 = -0.04). These results, coupled with recently acquired historical fire records, presents the first comprehensive fire analysis of the site and indicates the need for a burn and fuel reduction progra

Stem exclusion and mortality in unmanaged subalpine forests of the Swiss Alps

Understanding the causes and consequences of spatiotemporal structural development in forest ecosystems is an important goal of basic and applied ecological research. Most existing knowledge about the sequence and timing of distinct structural stages following stand origin in unmanaged (not actively managed in >50years) forests has been derived from forests in North America, which are characterized by particular topographic, climatic, biotic and other environmental factors. Thus, the effects on structural development remain poorly understood for many other forest systems, such as the dense, unmanaged, subalpine Norway spruce forests of the Swiss Alps. Over the past century, land abandonment and reductions in active forest management have led to a substantial increase in the density of these forests types. Consequently, many stands are entering the stem exclusion stage and are currently characterized by associated self-thinning mortality. However, the environmental influences on the rate of this structural development as well as this structural stage itself have not yet been examined. We studied stem exclusion processes based on forest inventory data (National Swiss Forest Inventory; NFI) over three survey periods (1983-1985, 1993-1995 and 2004-2006) using repeated measures statistics. To complement these analyses, we also collected and analysed 3,700 increment cores from 20 field plots within dense subalpine Norway spruce forests dispersed across the Swiss Alps. Over the past decades, basal area (BA) has generally increased, particularly on N-facing and steeper slopes, and within 300m of potential treeline. The number of dead trees was higher on N-facing compared with S-facing slopes, but the BA of dead wood was higher on S-facing slopes. Tree ring analysis confirmed important differences in growth patterns between N- and S-facing slopes and verified the results of the NFI analysis. This study provides a detailed example of how environmental heterogeneity and management history can influence the spatiotemporal structural development of forest ecosystem

Not only climate: Interacting drivers of treeline change in Europe

Treelines have long been recognized as important ecotones and likely harbingers of climate change. However, over the last century many treelines have been affected not only by global warming, but also by the interactions of climate, forest disturbance and the consequences of abrupt demographic and economic changes. Recent research has increasingly stressed how multiple ecological, biophysical, and human factors interact to shape ecological dynamics. Here we highlight the need to consider interactions among multiple drivers to more completely understand and predict treeline dynamics in Europe

Effects of bark beetle outbreaks on forest landscape pattern in the southern rocky mountains, U.S.A.

Since the late 1990s, extensive outbreaks of native bark beetles (Curculionidae: Scolytinae) have affected coniferous forests throughout Europe and North America, driving changes in carbon storage, wildlife habitat, nutrient cycling, and water resource provisioning. Remote sensing is a cru-cial tool for quantifying the effects of these disturbances across broad landscapes. In particular, Landsat time series (LTS) are increasingly used to characterize outbreak dynamics, including the presence and severity of bark beetle-caused tree mortality, though broad-scale LTS-based maps are rarely informed by detailed field validation. Here we used spatial and temporal information from LTS products, in combination with extensive field data and Random Forest (RF) models, to develop 30-m maps of the presence (i.e., any occurrence) and severity (i.e., cumulative percent basal area mortality) of beetle-caused tree mortality 1997–2019 in subalpine forests throughout the Southern Rocky Mountains, USA. Using resultant maps, we also quantified spatial patterns of cumulative tree mortality throughout the region, an important yet poorly understood concept in beetle-affected forests. RF models using LTS products to predict presence and severity performed well, with 80.3% correctly classified (Kappa = 0.61) and R2 = 0.68 (RMSE = 17.3), respectively. We found that ≥10,256 km2 of subalpine forest area (39.5% of the study area) was affected by bark beetles and 19.3% of the study area experienced ≥70% tree mortality over the twenty-three year period. Variograms indi-cated that severity was autocorrelated at scales \u3c 250 km. Interestingly, cumulative patch-size dis-tributions showed that areas with a near-total loss of the overstory canopy (i.e., ≥90% mortality) were relatively small (\u3c0.24 km2) and isolated throughout the study area. Our findings help to in-form an understanding of the variable effects of bark beetle outbreaks across complex forested regions and provide insight into patterns of disturbance legacies, landscape connectivity, and susceptibility to future disturbance

A global view of aspen : Conservation science for widespread keystone systems

Across the northern hemisphere, six species of aspen (Populus spp.) play a disproportionately important role in promoting biodiversity, sequestering carbon, limiting forest disturbances, and providing other ecosystem services. These species are illustrative of efforts to move beyond single-species conservation because they facilitate hundreds of plants and animals worldwide. This review is intended to place aspen in a global conservation context by focusing on the many scientific advances taking place in such biologically diverse systems. In this manner, aspen may serve as a model for other widespread keystone systems where science-based practice may have world implications for biodiversity conservation. In many regions, aspen can maintain canopy dominance for decades to centuries as the sole major broadleaf trees in forested landscapes otherwise dominated by conifers. Aspen ecosystems are valued for many reasons, but here we highlight their potential as key contributors to regional and global biodiversity. We present global trends in research priorities, strengths, and weaknesses based on, 1) a qualitative survey, 2) a systematic literature analysis, and 3) regional syntheses of leading research topics. These regional syntheses explore important aspen uses, threats, and research priorities with the ultimate intent of research sharing focused on sound conservation practice. In all regions, we found that aspen enhance biodiversity, facilitate rapid (re)colonization in natural and damaged settings (e.g., abandoned mines), and provide adaptability in changing environments. Common threats to aspen ecosystems in many, but not all, regions include effects of herbivory, land clearing, logging practices favoring conifer species, and projected climate warming. We also highlight regional research gaps that emerged from the three survey approaches above. We believe multi-scale research is needed that examines disturbance processes in the context of dynamic climates where ecological, physiological, and genetic variability will ultimately determine widespread aspen sustainability. Based on this global review of aspen research, we argue for the advancement of the “mega-conservation” strategy, centered on the idea of sustaining a set of common keystone communities (aspen) that support wide arrays of obligate species. This approach contrasts with conventional preservation which focuses limited resources on individual species residing in narrow niches.Peer reviewe

Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration. © 2023 the Author(s)

The consequences of changing disturbance regimes for quaking aspen in the western U.S.

Forest ecosystems are being affected by both the indirect and direct results of climate change. Indirect drivers include increasing extent, magnitude and/or frequency of various forest disturbances such as wildfires and insect outbreaks. Direct drivers include droughts and altered temperature and precipitation regimes. Together these drivers are likely to affect the composition of Rocky Mountain forests, including the dominance and extent of quaking aspen (Populus tremuloides). Here I review recent work on these topics and propose possible future consequences for quaking aspen. Warm and dry conditions generally result in increased wildfires and bark beetle outbreaks, particularly in coniferous forests. Both of these disturbances have the potential to increase aspen dominance due to aspen’s ability to regenerate in and dominate post-disturbance environments. Furthermore, compounded disturbances (i.e. two or more disturbances occurring in short succession) also appear to favor regeneration of aspen over conifers and could further increase aspen dominance if compounded disturbances increase with projected shifts in climate regimes. However, aspen demography is contingent on favorable climatic conditions. If the same warm and dry conditions that bring about disturbance regimes potentially favorable to aspen dominance also characterize post-disturbance environments, these climatic conditions may actually inhibit the ability of aspen to regenerate, grow, and survive. If aspen is able to increase dominance due to changing disturbance regimes and a changing climate, this will likely affect forest susceptibility to subsequent disturbances. As aspen stands are generally more mesic than adjacent conifer stands, the former are less likely to burn. Aspen stands are also less susceptible to bark beetle outbreaks that affect conifers and to wind disturbances. Thus any change in the amount of aspen in the landscape has the potential to feedback to the overall disturbance regime at broad scales. The consequences of changing disturbance regimes for quaking aspen in the western U.S. are likely to be complex and contingent on post-disturbance climatic conditions as well as on feedbacks among climate, disturbances, and forest species composition

The influences of climate on aspen dieback

Understanding the links between climatic variability and tree mortality is an important goal of current ecological research, but this relationship remains poorly understood for some widespread species such as quaking aspen (. Populus tremuloides Michx.). Recent observations indicate a sudden onset and rapid progression of quaking aspen dieback in the western United States, which may be climatically driven. We used dendroecological methods to test how climatic variability influences aspen grown and mortality in northwestern Colorado and southern Wyoming. Recent growth and mortality of quaking aspen is strongly associated with climatic variation in this region. In stands undergoing recent aspen dieback (1) the growth of aspen was inhibited by warm temperatures, except at the highest elevations (2) the growth of aspen that died was more sensitive to temperature variation prior to mortality than the growth of living aspen, (3) mortality of aspen was preceded by multiple years of reduced growth, and (4) the frequency of mortality was associated with multiple years of drought as indicated by negative PDSI values. The results of this study suggest that the recent widespread mortality of aspen is strongly associated with recent climatic conditions. © 2012 Elsevier B.V.

Natural forest disturbances and the design of REDD+ initiatives

Basing ecosystem management and conservation on the best available science is essential to meeting intended goals and minimizing surprises. To design effective, efficient, and equitable policies for the REDD+ initiatives, requires that drivers of deforestation and forest degradation are correctly identified, and that the ecological context of those drivers is correctly understood. Contemporary forest ecology and management are based on the recognition that forest ecosystems are dynamic, and that those dynamics are often driven by both anthropogenic- and naturally induced disturbances. Here we examine the degree to which the dynamic view of ecosystems is incorporated into the design of REDD+ initiatives. We conducted content analysis of the World Bank\u27s Forest Carbon Partnership Facility\u27s 36 REDD+ participating countries\u27 Readiness Plan Idea Notes and/or Readiness Preparation Proposals. Across the 36 countries, drivers of deforestation and forest degradation could be grouped into categories of institutional policies, political-economic contexts and social settings. The result of our content analysis indicates that there is a lack of discussion of the dynamic character of ecosystems and of the potential influence of natural disturbances on the identified drivers of deforestation and forest degradation. We argue that REDD+ initiatives must take into account knowledge of natural disturbance regimes (including the size, frequency and severity of key disturbances) in their framing of the drivers of deforestation and forest degradation in order to better understand the ecological stage on which these projects will be implemented after the piloting phase. This paper proposes four approaches to integrate understanding of natural disturbances with the socio-political and economic drivers of deforestation and forest degradation within REDD+ participating countries. © 2013 Elsevier Ltd