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Sustaining Forests

Understanding the Ecological Roles of Natural Disturbance


The key to sustaining forests is successfully renewing all their elements after disturbance, across landscapes and through time. NRS scientists are working to strengthen our understanding of the relationships of certain elements of forest communities in stands and forests of different ages, species compositions, and disturbance history and patterns. Many forest types result from natural disturbances such as fire, storm blowdown, and native tree pests. Generally these are not devastating disturbances; those usually result from non-native pests.

Research Studies

[photo:] Dense understories formed following co-occurring fire and gaps in the absence of browsing.  These plots are dominated by browse intolerant shrubs and tree species and contain high regenerating tree species richness, but lower herbaceous species richness.Interactions between Fire, Gaps, and Deer Browsing
In eastern deciduous forests, low-intensity understory fires, canopy gaps, and ungulate browsing are generally regarded as the main forces influencing vegetation dynamics.  Over the past 100 years, all three of these historic disturbance and herbivory regimes have been altered to some extent (e.g., fire suppression, smaller, less frequent gap formation in 2nd growth forests, and overabundant deer herds) and have been putatively linked to observed changes in regeneration patterns and losses in understory herbaceous diversity. 


[image:]   Burned area determined from multiple infrared heat release images taken from an aircraft during a prescribed fire in the Ohio Hills.Fuels and Fire Behavior in Eastern Hardwoods
An ability to predict fuel loads and fire behavior are needed to improve prescriptions for prescribed fire and answer questions about smoke emissions and transport and fire effects on flora and fauna. Our fuels and fire behavior research seeks to develop process-based (mechanistic) approaches to predicting fuel characteristics and fire behavior, with particular focus on hardwoods in Appalachian topography.


[photo:] Precipitation manipulation experiment in a piñon pine - juniper ecosystem at the Sevilleta Long Term Ecological Research (LTER) site at the Sevilleta National Wildlife Refuge in central New Mexico, USA.Increasing the reliability of predictions of the landscape effects of climate change
Climate change is producing new environmental conditions that have not existed for study in the scientific age.  It is quite possible that many of the empirically-derived relationships developed under the conditions of the past for predicting forest dynamics will be rendered obsolete by climate change.  For example, forest growth and yield models are based on empirical relationships measured nearly a century ago when CO2 and temperatures were lower.  Recent research suggests that tree growth in the Midwest may be up to 20% higher in the current century because of CO2 fertilization.


[photo:] Moderate-intensity surface fire in mixed-oak forestEcosystem Management Study: Restoration of Mixed-oak Forests with Prescribed Fire
Historically, fire was a frequent disturbance process in the mixed-oak forests of the central hardwoods region. Fire control has altered forest structure and composition. Forests are more dense and the sustainability of oak and hickory dominance is now threatened by an abundance of shade-tolerant and fire sensitive tree species such as red maple, sugar maple, and beech. Prescribed fire has been advocated to promote and sustain open-structured mixed-oak forests and the plants and animals that have adapted to these communities. However, long-term research on fire effects is lacking.


[photo:] Redring milkweed (Asclepias variegata), growing in an Ohio mixed-oak forest after repeated prescribed fires.Plant Diversity in Managed Forests
The great majority of plant diversity in forests is contained in the herbaceous layer, comprised of both herbaceous and woody species.  We seek a better understanding of how forest management activities affect plant diversity.  NRS-2 scientists are investigating the direct and indirect effects of timber harvesting, prescribed burning, herbicide application, and deer browsing (alone and in combination) on plant composition and diversity in mixed oak, Allegheny, and Northern Hardwood forests.


[photo:]  Sugar Maple Decline on the unglaciated Allegheny Plateau in northern PennsylvaniaSite, Stress, Nutrition, and Forest Health Interactions
A range of stressors including defoliating insects, pathogens, droughts, inadequate soil base cations, and changing climate have interacted to affect the health and regeneration of selected northern and central hardwood forest species. In the 1980s and 1990s sugar maple dieback and mortality was extensive across the unglaciated Allegheny Plateau in northern Pennsylvania. 


[photo:]  Red maple stem heating experiment.Fire Effects in Eastern Forests
Understanding fire effects requires consideration of the processes by which the effects occur.  We are applying process-based (mechanistic) approaches to modeling fire effects on endangered Indiana bats and fire-caused tree injury and mortality.  Fires pose risks for bats but also provide opportunities for improving bat roosting habitat, our project considers both sides of the problem. 


[photo:] [photo:] Shortleaf pine commonly grows with hardwoods in the Missouri Ozarks and elsewhere in the Central Hardwood Region.  Management of pine with hardwoods in natural stands is a new line of research to develop silvicultural systems for regenerating pine and sustaining it in forest and woodland systems.The Ecology and Silviculture for Restoration of Shortleaf Pine
There is renewed interest in restoring shortleaf pine throughout its native range in the Ozark Highlands and elsewhere in the Central Hardwood Forest Region and in the southeastern United States. Restoring shortleaf pine on former pine and oak-pine sites is viewed as a long-term strategy for mitigating chronic oak decline. There also is an increasing interest in restoring native oak-pine woodland communities where they once were abundant.


[photo:] [photo:] Shortleaf pine commonly grows with hardwoods in the Missouri Ozarks and elsewhere in the Central Hardwood Region.  Management of pine with hardwoods in natural stands is a new line of research to develop silvicultural systems for regenerating pine and sustaining it in forest and woodland systems.The Role of Fire in Restoration of Woodlands and Savannas
Our research is focused on understanding the role of fire in sustaining woodland and savanna composition and structure. Our goals are to develop fundamental information about how to manage and regenerate oak and oak-pine woodlands and savannas with timber harvests and prescribed fire and to use this information to develop management guidelines to be applied on both public and private land.


[photo:] Researcher lighting a prescribed burn fire on the Vinton Furnace Experimental Forest.Restoration of Mixed Oak Forests in Southern Ohio with Prescribed Fire
In the eastern U.S., fire suppression has been implicated as a primary factor facilitating shifts in forest composition from oak to other species, often maples. The state of Ohio began suppressing fires in 1923. Oaks are considered well-adapted to a regime of frequent low- to moderate-intensity fires because they possess thick bark, the ability sprout repeatedly after being topkilled, and are intermediate in shade tolerance.


[image:]  Map of contigous USA - shows locations of FFS sitesFire and Fire Surrogate Treatments: The Central Appalachian Plateau Site
Current forests in many fire-dependent ecosystems of the United States are denser and more spatially uniform, have many more small trees and fewer large trees than did their presettlement counterparts. Causes include fire suppression, past livestock grazing and timber harvests, and changes in land use. The results include a general deterioration in forest ecosystem integrity and the threat of losing important, widespread forest types. Such conditions are prevalent nationally, especially in forests with historically short-interval, low- to moderate-severity fire regimes, such as the upland oak forests of the central hardwoods region.   


[photo] The Northern Forest Ecosystem Experiment is a large-scale, long-term field experiment in which harvested forests regenerate in atmospheres with enhanced concentrations of carbon dioxide (CO2), ozone (O3) or both gasses combined.  The physiology, genetics, and distribution of ponderosa pine species vary with changes in elevation and environmental conditions
In the desert southwest, significant variations in moisture and temperature occur along steep gradients in elevation.  Notably, the endemic ponderosa pine species vary with changes in elevation and the differences in elevation are repeated throughout the ranges of the species.  The long-term goal of the study is to provide a foundation for future regional studies of species range limitations by water or temperature stress. 


[photo:]  eastern spruce budworm (Choristoneura fumiferana Clemens).Designing Pest-Resistant Forest Landscapes: The Importance of Spatial Pattern
Defoliating insects damage millions of acres of forested land annually in the United States.  The balance of evidence suggests forest insect outbreaks today are more damaging than ever because of changes in forest composition and structure induced by fire suppression and post-harvest proliferation of tree species intolerant to herbivory.  Our central hypothesis is that landscape connectivity of acceptable host types increases defoliator population connectivity, altering the dynamics and spatial structure of defoliator populations, and thus increasing forest susceptibility to insect pest damage.  


[photo:] Gap created following a severe windstorm in July 2003.Catastrophic Wind Disturbance
In eastern forests, severe wind disturbances are common and salvaging occurs after these disturbances.  The Allegheny Plateau region alone receives an average of 11 high wind events and one tornado per year (National Climate Data Center) making wind the predominant natural disturbance event.  Given the ubiquity of storm damage and salvaging on the Allegheny and elsewhere, there is a surprising paucity of experimental work demonstrating the impact of salvage logging on post-disturbance forest regeneration patterns. 


[photo:] TIonesta long term research area 1943 - 1942 Hobblebush and browsed hemlockLong-term Change at Hearts Content and Tionesta Scenic and Research Natural Area
Our ability to understand how unmanaged forests develop over time through various climatic and environmental changes depends on long-term data from unmanaged forests, especially remnants of the forest that predated European settlement of our region. Impacts of acid deposition, deer overabundance, beech bark disease, recovery of vegetation after windthrow, climate change, invasive plants – all are better understood by examining their effects in forests that have not been directly manipulated by humans, as well as those that have been managed.


[photo:] Deer overbrowsing has dramatically reduced the plant diversity of eastern US forests. Hay-scented fern (Dennstaedtia punctilobula) dominates the understory of this forest in Pennsylvania, which would make it difficult for many plant species to recover even if deer populations were eventually brought under control.Native Invasive Species
Recent changes in disturbance and browsing regimes have strongly impacted species composition in forest understories worldwide.  In some cases, these changes have led to large increases in the density and cover of a small number of native understory plant species which may then expand to form interfering vegetation layers. 


[photo:] Photo shows regeneration of multiple speciesRegeneration of Northern and Allegheny Hardwoods
The Allegheny hardwood forest type is a variant of the northern hardwood type consisting primarily of black cherry, red maple, sugar maple and American beech. Associated species include white ash, yellow‑poplar, black birch, yellow birch, cucumber magnolia and hemlock. Black cherry and the maples usually dominate stands in Pennsylvania and southward; white ash and sugar maple tend to be more important, and red maple less important, in the New York portion of the range.


[image:] Status of weather anomalies around the U.S. in early 2002, compiled by the National Weather Service. Adapting Forests to Climate Change
Climate models have projected significant increases in temperature over the next century for the Northeast and Midwest.  Climate change will also affect rainfall patterns, but scientists cannot yet predict how regional rainfall patterns will change.  Growing seasons will lengthen further in both spring and fall.  According to the Intergovernmental Panel on Climate Change, there is very high confidence that the vulnerability of North America depends on the effectiveness and timing of adaptation and the distribution of coping capacity, which vary spatially and among sectors. Climate change will constrain North America’s over-allocated water resources, increasing competition among agricultural, municipal, industrial and ecological uses (very high confidence).


[photo:] Amatina jacksonianaTropical Forest Mycology
The Center for Forest Mycology Research (CFMR, part of the Northern Research Station of the US Forest Service) leads critical research on the biology of tropical fungi native to Hawaii, US territories in the Caribbean (Puerto Rico and the US Virgin Islands) and to other countries in the Caribbean Basin.


[photo:] Tower used in fire research at Silas Little Experimental ForestFire and Fuels Research at the Silas Little Experimental Forest
The Silas Little Experimental Forest was reinstated using National Fire Plan funding in 2003 to conduct multi-disciplinary fire and atmospheric science research to provide fire and forest managers with better tools for predicting fire danger, fire risk, air quality, and ecosystem functioning under changing environmental conditions. 


[image:] Map of predicted losses of nitrogen from forested lands of the Chesapeake Bay watershed under no nitrogen deposition, current nitrogen deposition, and doubled nitrogen deposition (Pan et al. 2004).Mid-Atlantic Forests and the Chesapeake Bay Watershed
Forest landscapes are changing as a consequence of climate and environmental change. These changes affect people and the forest ecosystems they depend on for clean water, clean air and forest products, and recreation. How can we best manage our forest resources to sustain this array of ecosystem services under increasing environmental stress and a changing climate?


[image:] Changes in climate, atmospheric components, land use and disturbance regimes affect forest carbon sequestration and biofuel product. It is important to understand these processes and attribute the effects to different causesImpacts of Disturbances and Climate on Carbon Sequestration and Biofuels
Currently, U.S. forests and forest products offset about 20% of the nation’s fossil fuel emissions. However, recent findings cast doubt on the sustainability of this offset. First, the strength of the U.S. forest carbon offset may be weakening due to forest ageing, climate variability, and increasing natural disturbances. Second, climate change is expected to further increase frequencies of insect outbreaks and wildfire, and alter species composition in forest ecosystems, consequently influencing forest carbon pools in a significant way.  These current and projected forest carbon cycle dynamics need to be considered in strategic forest planning and management decisions in coming decades if the nation’s forests are to provide stable or even increasing ecosystem services.


[photo:]   Lake Baikai, Siberia by Eric GustafsonPredicting global change effects on forest biomass and composition in south-central Siberia
Multiple global changes such as timber harvest of previously unexploited areas and climate change will undoubtedly affect the composition and spatial distribution of boreal forests, which will in turn affect the ability of these forests to sequester carbon.  To reliably predict future states of the boreal forest it is necessary to understand the complex interactions among forest regenerative processes (succession), natural disturbances (e.g., fire, wind and insects) and anthropogenic disturbances (e.g., timber harvest). 


[photo:] Birch tree at Aspen FACE siteEffects of Global Atmospheric Change on Forest Insects
We are studying seasonal and annual changes in forest insect populations at the Aspen FACE experiment site in northern Wisconsin where trees are growing under both elevated CO2 (+200 ppm above ambient) and ozone (+50% above ambient).


[photo:] 2008 Wildfire on Council Grounds, Wisconsin by Richard LaValleyStudying fire mitigation strategies in multi-ownership landscapes: Balancing management of fire-dependent ecosystems and fire risk.
Fire risk mitigation within multi-owner landscapes containing flammable but fire-dependent ecosystems epitomizes the complexities of managing public lands.  The cumulative effects of fire and forest management over the last century have exacerbated fire risk in some regions and threatened fire-dependent systems in many others.  The issue is further complicated by the recent encroachment of human homes into fire prone ecosystems that simultaneously increase fire ignitions and increase demands on fire suppression agencies to protect lives and property.  Consequently, the balance between forest restoration, human rural development, and fire risk remains an issue of major concern to natural resource agencies.


[photo:] Ground estimates of insect defoliation are scaled-up to regional scales using multiple remote sensing platforms, including aerial photography, Hyperion, Landsat ETM+, and MODIS.Effects of Insect Defoliation on Regional Carbon Dynamics of Forests
On an annual basis, insects severely defoliate more than 20 million acres of forested land in the conterminous United States, affecting a larger area and incurring higher economic costs than any other disturbance.  However, the long-term costs and ecosystem consequences of insect outbreaks on forest health and productivity are difficult to quantify at the regional scale because of the variety of pests involved, differences in forest types affected, and varying spatial scale and intensity of the impacts.  In particular, the effect of insect activity on carbon cycling and sequestration at the annual and decadal scale is poorly characterized.  


[image:] WFI Study Site Map by Sue Lietz The Working Forest Initiative: Simulating the cumulative effects of the forest management strategies of multiple landowners on landscape pattern and biodiversity
Sustainable forestry involves the extraction of forest products while maintaining ecosystem integrity to conserve biodiversity and to provide other non-commodity benefits to society.  Population viability is a function of the combined actions of multiple landowners, which create a dynamic mosaic of forest types, stand structures and age distributions.  Consequently, it is necessary to understand how the actions of individual land owners interact with the actions of others to determine the spatial pattern of the landscape mosaic, and therefore its ability to maintain biodiversity. 


Fire in Northern hardwood forest.Modeling silvicultural treatments after natural disturbance
Modeling silviculture after natural disturbance to maintain biodiversity is a popular concept., yet its application remains elusive. We discuss difficulties inherent to this idea, and suggest approaches to facilitate implementation, using longleaf pine (Pinus palustris). In this species, natural disturbances such as lightning, hurricanes, surface fires, and windthrow all lead to similar structures, but at different rates. The outcomes of natural disturbances are inherently different from those of silviculture (for example, harvesting always removes boles) and it is instructive to think of silvicultural disturbances along a gradient in structural outcomes, reflecting degree of disparity with natural disturbance. Interactions of frequency, severity, intensity, seasonality, and spatial pattern define a disturbance regime and these components may not have equal weight in affecting biodiversity and some are easier to emulate with silviculture than are others.


Last Modified: 03/31/2015