J. Morgan Varner

• PhD
• Managing Director at Tall Timbers Research Station

Fire behavior and effects in forests and woodlands are influenced by surface fuels and senesced leaf litter in particular. We have known that species exhibit differential flammability for some time, but isolated efforts have often attributed differences to disparate mechanisms. Recent research has expanded the diversity of species evaluated, clarif...

Fire is integral to the functioning of terrestrial ecosystems of the southeastern USA and is a strong selective force on plant species. Among woody plants, oak species (Quercus spp. L) have diverse life history traits that appear to reflect their evolution in this fire-prone region. Oaks also occur across wide gradients of fire frequency and intens...

Increasing evidence that pervasive warming trends are altering disturbance regimes and their interactions with fire has generated substantial interest and debate over the implications of these changes. Previous work has primarily focused on conditions that promote non-additive interactions of linked and compounded disturbances, but the spectrum of...

The dead foliage of scorched crowns is one of the most conspicuous signatures of wildland fires. Globally, crown scorch from fires in savannas, woodlands and forests causes tree stress and death across diverse taxa. The term crown scorch, however, is inconsistently and ambiguously defined in the literature, causing confusion and conflicting interpr...

Each year wildland fires kill and injure trees on millions of forested hectares globally, affecting plant and animal biodiversity, carbon storage, hydrologic processes, and ecosystem services. The underlying mechanisms of fire-caused tree mortality remain poorly understood, however, limiting the ability to accurately predict mortality and develop r...

Canopy hydrology and forest water inputs are directly linked to the physical properties of tree crowns (e.g., foliar and woody surfaces), which determine a tree’s capacity to intercept and retain incident rainfall. The changing forest structure, notably the decline of oak’s (Quercus) dominance and encroachment of non-oak species in much of the upla...

Wildfires are doing necessary work for western US forests that have evolved with fire. Here we define "good fire" as areas that burn in an ecologically beneficial way, with a severity that is analogous to their historical fire regimes prior to European settlement. We focus on low-moderate severity wildfire as this type of burning can promote regene...

Fire and drought are expected to increase in frequency and severity in temperate forests due to climate change. To evaluate whether drought increases the likelihood of post‐fire tree mortality, we used a large database of tree survival and mortality from 32 years of wildland fires covering four dominant western North American conifers. We used Baye...

Background Forest structural characteristics, the burning environment, and the choice of ignition pattern each influence prescribed fire behaviors and resulting fire effects; however, few studies examine the influences and interactions of these factors. Understanding how interactions among these drivers can influence prescribed fire behavior and ef...

Background: Ecosystem management, community restoration, and managing for climate resilience have become major priorities of land management in recent decades. For woodlands and savannas (i.e., ‘open forests’), this transition has meant moving fire-deprived, closed-canopy forests to structures and compositions characteristic of natural communities...

In the summer of 2023, unprecedented amounts of smoke from Canadian wildfires descended upon the north eastern United States. As a result, millions of people in this fire-infrequent region were exposed to extremely hazardous air quality and grew more aware of wildland fire issues they had previously been largely insulated from. Before this event fa...

Overstory trees serve multiple functions in grassy savannas. Past research has shown that understory species can vary along gradients of canopy cover and basal area in savannas. This variation is frequently associated with light availability but could also be related to other mechanisms, such as heterogeneity in soil and litter depth and fire inten...

Fire ecology is a complex discipline that can only be understood by integrating biological, physical, and social sciences. The science of fire ecology explores wildland fire’s mechanisms and effects across all scales of time and space. However, the lack of defined, organizing concepts in fire ecology dilutes its collective impact on knowledge and m...

Our ecocultural state-of-knowledge report brings together Indigenous Knowledge (IK) and Western Science (WS) to support climate and wildfire adaptation strategies for US public forest landscapes. Our report builds on federal directives to respectfully and intentionally braid IK and WS knowledge systems in a Two-Eyed Seeing approach that informs cli...

Reestablishing appropriate tree species is an important step in converting off‐site monocultures into woodlands. Species conversion is often necessary, as off‐site exotic species rarely function like woodland species. However, when off‐site tree species are native, and functionally redundant to woodland species, conversion may be unnecessary. To ex...

Prescribed fire is an important land conservation tool to meet ecological, cultural, and public safety objectives across terrestrial ecosystems. While estimates of prescribed burning in the U.S.A. exceed 4.5 million hectares annually, tracking the extent of prescribed fire is problematic for several reasons and prevents an understanding of spatial...

Fire is an important process in savanna as well as forest ecosystems, but its effects on herbaceous vegetation across biomes remain unelucidated. In theory, fire- vegetation feedbacks maintain the vegetation structure of both savannas and forests. In savannas, a continuous layer of highly flammable herbaceous vegetation promotes frequent fires, whi...

Disturbances including fire, insect and disease outbreaks, and drought are ubiquitous in forests and rangelands, and many disturbance events are parts of the natural dynamics of forest and rangeland ecosystems. This chapter is a new addition to the Resources Planning Act (RPA) Assessment and summarizes disturbance trends in the recent past and...

Fire-vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest biomes should have differential responses to fire, but fire response data for herbaceous vegetation has yet to be synthesized across sites. Here, we examined herbaceous vegetation responses to experimental fire at 30...

Tree litter is the primary fuel affecting surface fire behavior in most fire-prone forest and woodland ecosystems in northeastern North America. Fire exclusion and land use changes have dramatically altered fire regimes in many of these ecosystems, shifting toward species that reinforce the continued diminution of fire. Comparative litter flammabil...

Peatland drainage may degrade system resilience to high intensity, soil‐consuming fires. Peat soil fires are unique in that they can smolder vertically through the soil column, with a multitude of consequences including large carbon emissions, altered hydrology, and dramatic shifts in vegetation communities. In this work we developed and verified a...

Background Wildland fires are fundamentally landscape phenomena, making it imperative to evaluate wildland fire strategic goals and fuel treatment effectiveness at large spatial and temporal scales. Outside of simulation models, there is limited information on how stand-level fuel treatments collectively contribute to broader landscape-level fuel m...

A key uncertainty of empirical models of post‐fire tree mortality is understanding the drivers of elevated post‐fire mortality several years following fire, known as delayed mortality. Delayed mortality can represent a substantial fraction of mortality, particularly for large trees that are a conservation focus in western US coniferous forests. Cur...

Our understanding of the relationship between northern bobwhite (Colinus virginianus; hereafter, bobwhite) and fire began with Herbert Stoddard’s work in the early 20th century. Research on the topic has continued, but our application of fire is deeply rooted in Stoddard’s work, even as it has become evident that fire regimes must be adapted to var...

Aim Each year, wild and managed fires burn roughly 4 million km ² [~400 million hectares (Mha)] of savanna, forest, grassland and agricultural ecosystems. Land use and climate change have altered fire regimes throughout the world, with a trend toward higher‐severity fires found from Australia, the Americas, Europe and Asia, to the Arctic. In 2020,...

Recent wildfires across western North America have burned with uncharacteristically high severity, representing a substantial departure from natural fire regimes. In mixed‐conifer and pine–oak ecosystems of the southern Cascade Range, widespread shifts in stand structure and composition have led to a diversity of post‐wildfire vegetation responses....

Wildland fires are among the most complicated environmental phenomena to model. Fire behavior models are commonly used to predict the direction and rate of spread of wildland fires based on fire history, fuel, and environmental conditions; however, more sophisticated computational fluid dynamic models are now being developed. This quantitative anal...

Fire is an integral component of ecosystems globally and a tool that humans have harnessed for millennia. Altered fire regimes are a fundamental cause and consequence of global change, impacting people and the biophysical systems on which they depend. As part of the newly emerging Anthropocene, marked by human-caused climate change and radical chan...

Fire is an integral component of ecosystems globally and a tool that humans have harnessed for millennia. Altered fire regimes are a fundamental cause and consequence of global change, impacting people and the biophysical systems on which they depend. As part of the newly emerging Anthropocene, marked by human-caused climate change and radical chan...

Pinus species dominate fire-prone ecosystems throughout the northern hemisphere. Their litter drive fires that control plant community flammability and multiple ecological processes. To better understand the patterns and mechanisms of pine flammability, we measured leaf characteristics (needle length and thickness) and conducted combustion experime...

The reintroduction of fire to long-unburned pine ecosystems is a silvicultural tool to restore their ecological and economic value. However, if prescribed fire is used after long periods of fire exclusion, high amounts of duff consumed during fire can result in tree stress and mortality. Prescribed burning with a focus on managing duff has therefor...

Fire is a strong ecological and evolutionary driver of plant species and terrestrial ecosystem dynamics. Numerous studies have shown that plant species vary widely in their flammability and associated traits that coincide with clear strategies of persistence in fire‐prone ecosystems. Yet, intraspecific variation and phenotypic plasticity in plant f...

Peatland functions (e.g., carbon sequestration and flora diversity) are largely driven by soil moisture dynamics and thus dependent on interactions between hydrologic regimes and organic soil properties. Understanding these interactions is particularly important in drained peatlands, where drier conditions may alter soil properties with feedbacks t...

Reference sites with relatively unaltered plant communities are used regularly to establish targets for ecological restoration, yet few are free from anthropogenic disturbances. In this study, we sought to understand the capacity for native community indicators and other plant species to recover from abandoned woods roads and tilled soils within re...

Significance The coronavirus pandemic, COVID-19, led to strict social-distancing guidelines that severely impacted human livelihood and economic activity. Workplace closures reduced travel, and early in spring 2020, improvements in air and water quality, reduced seismic activity, and reductions in greenhouse gas emissions were observed. COVID-19–re...

Increased prescribed burning is needed to provide a diversity of public benefits, including wildfire hazard reduction, improved forest resilience, and biodiversity conservation. Though rare, escaped burns or significant smoke impacts may result in harm to individuals and property. Liability for potential damages reduces the willingness of fire mana...

The role of fire in the eastern broadleaf and Appalachian forest regions, until recently, was poorly understood or minimally examined, as this region was long overlooked as a flammable landscape and fire was seen primarily as a threat to the timber resource and wildlife. In the past few decades, a significant body of research has enhanced our under...

Southeastern pineland ecosystems fall into two broad groups: ecosystems with a grassy understory that depend on frequent low-intensity fires to maintain structure and biodiversity [longleaf (Pinus palustris), south Florida slash (P. densa), shortleaf (P. echinata)], and ecosystems with a shrubby understory that burn less frequently with higher inte...

Widespread fire exclusion and land-use activities across many southeastern United States forested ecosystems have resulted in altered species composition and structure. These changes in composition and structure have been implicated in positive fire-vegetation feedbacks termed “mesophication” where fire spread and intensity are diminished. In fores...

Interest in prescribed fire science has grown over the past few decades due to the increasing application of prescribed fire by managers to mitigate wildfire hazards, restore biodiversity, and improve ecosystem resilience. Numerous ecological disciplines use prescribed fire experiments to provide land managers with evidence-based information to sup...

Bark is a complex multifunctional structure of woody plants that varies widely among species. Thick bark is a primary trait that can protect trees from heat generated in surface fires. Outer bark on species that allocate resources to thick bark also tends to be rugose, with bark being thickest at the ridges and thinnest in the furrows. Tree diamete...

Efforts to restore longleaf pine (Pinus palustris Mill.) in the southeastern US require substantial artificial regeneration. Once established, important questions remain about when to introduce fire. We investigated the impact of initial planting density on tree branching and how prescribed fire might interact with tree architecture and survival. A...

Globally increasing wildfires have been attributed to anthropogenic climate change. However, providing decision makers with a clear understanding of how future planetary warming could affect fire regimes is complicated by confounding land use factors that influence wildfire and by uncertainty associated with model simulations of climate change. We...

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling...

Global change has contributed to substantial vegetation shifts across many ecosystems through altered fire-vegetation feedbacks, but the direction, strength, and mechanism of these processes vary considerably. Most prior studies have focused on fire-vegetation feedbacks that enhance community flammability with less emphasis on examples of feedbacks...

Pyrophytic oak landscapes across the central and eastern United States are losing dominance as shade-tolerant, fire-sensitive, or opportunistic tree species encroach into these ecosystems in the absence of periodic, low-intensity surface fires. Mesophication, a hypothesized process initiated by intentional fire exclusion by which these encroaching...

Background Predictive models of post-fire tree and stem mortality are vital for management planning and understanding fire effects. Post-fire tree and stem mortality have been traditionally modeled as a simple empirical function of tree defenses ( e.g., bark thickness) and fire injury ( e.g., crown scorch). We used the Fire and Tree Mortality datab...

The iconic American chestnut (Castanea dentata) once spanned a large portion of eastern North America before its functional extinction in the early 20th century due primarily to non-native fungal pathogens. The pronounced loss of this species likely resulted in an abrupt alteration of many ecological processes , including fire. The potential to res...

Global change has shifted fire regimes, but the long-term consequences for ecosystems are uncertain because of variability in environmental conditions, fire types, and plant composition. We tested how fire-frequency manipulations of 16-64 years affect tree communities and traits using 374 plots from 29 sites on four continents. More frequently burn...

Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire a...

Fire is a powerful ecological and evolutionary force that regulates organismal traits, population sizes, species interactions, community composition, carbon and nutrient cycling and ecosystem function. It also presents a rapidly growing societal challenge, due to both increasingly destructive wildfires and fire exclusion in fire‐dependent ecosystem...

Mechanical mastication is a fuels treatment that shreds midstorey trees and shrubs into a compacted woody fuel layer to abate fire hazards in fire-prone ecosystems. Increased surface fuel loading from mastication may, however, lead to undesirable fire intensity, long-duration flaming or smouldering, and undesirable residual tree mortality. Two majo...

Historically open oak and pine savannas and woodlands have transitioned to closed forests comprised of increased numbers of tree species throughout the eastern United States. We reviewed evidence for and against a suite of previously postulated drivers of forest transition focused on (1) change in fire regimes, (2) increased precipitation, (3) incr...

The realm of wildland fire science encompasses both wild and prescribed fires. Most of the research in the broader field has focused on wildfires, however, despite the prevalence of prescribed fires and demonstrated need for science to guide its application. We argue that prescribed fire science requires a fundamentally different approach to connec...

Background Litter is the predominant fuel that drives surface fire behavior in most fire-prone forest and woodland ecosystems. The flammability of litter is driven by fuel characteristics, environmental factors, and the interactive effects of the two. Solar radiation can influence litter flammability through its effect on fuel moisture and temperat...

Aim Functional traits are a crucial link between species distributions and the ecosystem processes that structure those species’ niches. Concurrent increases in the availability of functional trait data and our ability to model species distributions present an opportunity to develop functional trait biogeography (i.e., the mapping of functional tra...

Prescribed fire is commonly used in southeastern US forests and is being more widely applied in fire-prone ecosystems elsewhere. Research on direct effects of burning has focused on aboveground impacts to plants with less attention to belowground effects. We measured soil heating during experimental burns in longleaf pine sandhill and flatwoods eco...

Pacific Northwest USA oak woodlands and savannas are fire-resilient communities dependent on frequent, low-severity fire to maintain their structure and understory species diversity, and to prevent encroachment by fire-sensitive competitors. The re-introduction of fire into degraded ecosystems is viewed as essential to their restoration, yet can be...

Bastin et al .’s estimate (Reports, 5 July 2019, p. 76) that tree planting for climate change mitigation could sequester 205 gigatonnes of carbon is approximately five times too large. Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of s...

Loss of native foundation tree species to introduced pests profoundly alters the structure and function of many forest ecosystems. Recent advances to resurrect or prevent the loss of species by developing resistant hybrids hold promise, but uncertainty remains about the potential impacts of introducing a novel genotype on ecological processes, such...

Prescribed burning is an essential tool for forest and rangeland management that requires specific weather conditions to enable the efficient and safe application of fire. Prescribed burning is often limited by the ability to find suitable burn-days that fit within the identified weather parameters that balance good smoke dispersion and erratic fir...

Smouldering fire vulnerability in organic-rich, wetland soils is regulated by hydrologic regimes over short (by antecedent wetness) and long (through influences on soil properties) timescales. An integrative understanding of these controls is needed to inform fire predictions and hydrologic management to reduce fire vulnerability. The Great Dismal...

Key message Bark thickness relative to stem diameter and radial stemwood and height growth patterns provided a more accurate discrimination of fire resistance than absolute bark thickness alone in juvenile stems. Abstract Thick bark provides thermal protection to cambial tissues and adventitious buds during fires, and thus, is a functional trait a...

Premise of the Study: Aboveground biomass (AGB) of herbaceous vegetation is a primary source of fuel in frequent surface fires that maintain grasslands, savannas, and woodlands. Methods for nondestructively estimating AGB are required to understand the mechanisms by which fuels affect fire behavior and the effects of time since the last burn. We de...

Fire-stimulated or pyrogenic flowering is one of many traits that enable plants in fire-prone ecosystems to persist or dominate following frequent fires. Obligate pyrogenic flowering has long been observed in wiregrass (Aristida beyrichiana), a foundation bunchgrass in frequently burned longleaf pine ecosystems in the southeastern United States. Wi...

This chapter provides a overview of direct and indirect causes of tree mortality from fire, describes resprouting vs. top-kill responses, and applications that predict post-fire tree mortality.

Drainage is a globally common disturbance in forested peatlands that impacts peat soils, forest communities, and associated ecosystem functions, calling for informed hydrologic restoration strategies. The Great Dismal Swamp (GDS), located in Virginia and North Carolina, U.S.A., has been altered since colonial times, particularly by extensive ditch...

Purpose of Review The search for causal mechanisms in fire ecology has been slow to progress for two main reasons. First, many fire ecology investigations often occur after fires, with no detailed information on fire behavior. These fire effects are then used to infer both fire behavior and the subsequent effects themselves. Second, that fire behav...

Oak woodlands are dependent on frequent fire to maintain the low stem density and diverse understories that typify these ecosystems. Without this recurrent disturbance, fire-sensitive conifer competitors encroach on oaks, reducing their vigor, and diminishing habitat quality. In fire-excluded oak woodlands, stand-replacing wildfire can trigger shif...

Many fire‐maintained savannas and woodlands are suffering the effects of fire exclusion and the concomitant invasion of fire‐sensitive trees. In the Pacific West, woodlands dominated by either Oregon white oak (Quercus garryana) or California black oak (Quercus kelloggii) have transitioned from oak‐dominated to conifer‐dominated (primarily by the n...

Fire exclusion over the past century has substantially altered composition, structure, and fuel dynamics in upland oak-hickory (Quercus-Carya) forests in the southeastern U.S. Numerous restoration efforts have been made to re-establish historical disturbance regimes into these altered forests. However, our understanding of the implications of resto...

Much of fire behavior is driven by fine-scale patterns of fuel moisture; however, moisture predictions typically occur over large scales. The source of fine-scale variation in moisture results from a combination of fuelbed properties and overstory forest structure that influences water movement and distribution of solar radiation. Fine-scale moistu...

• Smoke from wildfires is a public health concern. Smoke affected the entire Pacific Northwest region in 2015, and again in 2017. Scientists developed the BlueSky Modeling Framework that forecasts where smoke will travel, allowing public health agencies and communities to prepare for smoke impacts. • Wildfires are here to stay. Scientists predict t...

Wildland fire behavior research has largely focused on the steady-state interactions between fuels and heat fluxes. Contemporary research is revealing new questions outside the bounds of this simplified approach. Here, we explore the complex interactions taking place beyond steady-state assumptions through acknowledging the manufactured separation...

Fire plays an important role in wildland ecosystems, critical to sustaining biodiversity, wildlife habitat and ecosystem health. By area, 70% of US prescribed burns take place in the Southeast, where treatment objectives range widely and accomplishing them depends on finding specific weather conditions for the effective and controlled application o...

Fire exclusion in eastern North American Quercus–Carya woodlands has resulted in overstory compositional changes, linked to altered fuel composition, structure, and ultimately, altered fire regimes. These compositional changes have been implicated in a dampening effect on fire behavior in formerly fire-prone ecosystems, the positive feedback termed...

Despite historically dominating many landscapes of the southeastern United States, old-growth stands of southern pine (Pinus palustris, P. elliottii var. densa, P. elliottii Engelm. var. elliottii, P. echinata, and P. taeda) are extremely rare and have high ecological value, but may be underrepresented and mischaracterized due to small fragment siz...

Of perhaps all forests in North America, the fire regime of coast redwoods (Sequoia sempervirens (D. Don) Endl.) is most enigmatic. Widely considered a temperate rainforest, a large number of fire history studies depict a forest dominated by frequent surface fire regimes. Coast redwood also has a long list of traits that allow it to persist and dom...

Non-native pathogens affect forests throughout North America, resulting in changes in species composition, structure, and ecosystem processes. We studied the effects of the emergent disease sudden oak death (SOD) and the resulting functional extinction of tanoak, a highly susceptible native tree with flammable litter, on future community flammability...

Fires have important ecological and socioeconomic effects in fire-prone regions globally. Human-caused wildfires often pose considerable safety and environmental hazards, and can result in sizeable economic losses. This study analyzed the relative importance of temporal, spatial, and socioeconomic factors on occurrence of 52,532 human-caused wildfi...