Beyond Old Growth: Chapter 5
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Key ecological and societal factors and issues
The five regional workshops identified a number of key ecological and economic/ social/political factors and issues that must be considered in developing an effective conservation and management strategy for older forests.
Contents
Land-use history and status
One of the most important determinants of the current state of older forests is the way that humans have used the land over past decades and centuries. Each of the five regions has a distinctive land-use history. Examining and comparing these differences can tell us a great deal about the influences that shaped today’s older forests in each region as well as revealing some qualities that they share.
The history of present forest ecosystems in the Northeast, Great Lakes, and Northwest regions began at the end of the last ice age about 12,000 years ago. These three regions had no forests for thousands of years before that time. The Northeast and Great Lakes regions were covered by the southernmost part of the great North American ice sheet, and much of the Northwest had glaciers at higher elevations and barren tundra or semi-desert in the lowlands. When the glaciers retreated northward toward the Arctic and upward to higher elevations, forests gradually reclaimed the ground that they had lost to cold and aridity.
By contrast, forest ecosystems in the Southeast and Southwest are much more ancient, although at the time of the glacial retreat they were dominated by species that are now found in more northerly regions.
Human land use in all five regions began with Native Americans. Although their impacts on the natural environment were minor compared to those of later European settlers, they altered the landscape by using fire for numerous purposes, including clearing land for agriculture.
European settlement brought dramatic changes to the forests of the Northeast. Grazing and farming nearly deforested the region between 1750 and 1850, although most of northern Maine wasn’t cleared for agriculture because of its poor soils and lingering territorial disputes with the French and British. Logging has been the primary land use in northern New England since the late 1600s, and the Adirondacks and slopes of the central Appalachian Mountains also were heavily logged.
Deforestation and agriculture peaked around 1850, then grazing and farming gradually disappeared from New England as the western U.S. frontier opened up. Much cleared land in New England reverted to forest cover in the 20th century, and the Northeast is more heavily forested today than it was in 1850.
Lakes] forests are still recovering from the lumber era, a period of extremely rapid and intensive logging in the late 19th and early 20th centuries that largely denuded regional forest cover and degraded the quality of watersheds and fisheries. So little old growth remained after the cutover that recovery and restoration, rather than preservation, are the central challenges for Great Lakes older-forest management.
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The Great Lakes region is now more heavily forested than at any time since the lumber era, but the forests are very different places than they once were. Less than 1% of pre-cutover old growth remains, with particularly sharp declines in hemlock, yellow birch, and white pine.
The forests of the Southeast have a long history of interaction with humans. Thousands of years of human activities shaped them, and they strongly influenced human societies. These biologically rich older forests provided food, shelter, and clothing for relatively large populations of Native Americans who used fire and simple tools to produce crops in the rich bottomlands for thousands of years. Europeans brought more powerful tools and used them to harvest high-quality timber and to extract naval stores such as pitch and tar from pine resin.
After the railroads came, the forest was almost completely cleared. Much of this cleared land was farmed or grazed. The first forests were harvested with little thought of re-growing them, and the long history of agricultural clearing has nearly eliminated older forests throughout the Southeast. While older forests are extremely rare—less than one half of one percent of the original range— isolated remnants can be found that resemble the forests that existed before European settlement. Given the rapidly changing ownership of industrial forests and the steady influx of people into the region, the southeastern states—some more than others—present opportunities for conservation through active management, preservation, and restoration of older forests.
Pacific Northwest forests have changed continually in composition and structure since the end of the last ice age. People have influenced forests over most or all of that time. For example, Native Americans set fires in the Willamette Valley in Oregon, keeping much of the valley as prairie or savanna ecosystems. Lightning fires also occurred.
Extensive fires didn’t occur at an even pace. In western Oregon and Washington, extensive fire episodes occurred 8,000 to 10,000 years ago, 1,000 years ago, and 500 years ago. Many of today’s old-growth Douglas-fir trees appeared after large fires burned during a period of warm, dry climate around 1400 to 1650.
In the 1880s European-American settlers cleared many old-growth forests or set wildfires that destroyed them. Scientists estimate that about half of the old-growth forests that existed in the western part of the Pacific Northwest at the beginning of the 20th century have since been logged. Most remaining Pacific coastal old-growth forests are on federal lands.
Native Americans in the Southwest occupied and interacted with ponderosa pine ecosystems for centuries before Europeans arrived. They used ponderosa pine for building material, survival food, medicine, and ceremonies and used other plants and animals of the ecosystem in a typically sustainable manner. They also used fire to work with various aspects of the ponderosa pine system. While the extent of their burning practices remains controversial, it’s clear that once Native Americans were forced onto reservations, the low-intensity fires on which the ecosystem depends decreased dramatically.
Low-intensity fires, whether ignited by lightning or by Native Americans, historically occurred at intervals ranging from 2 to 35 years in southwestern ponderosa pine ecosystems. Stand-replacing fires were rare or nonexistent, though they occurred periodically in dry forests in other parts of the Southwest. These fires limited the scope and severity of insect outbreaks and kept dwarf mistletoe (a parasitic plant that can cause major damage to ponderosa pines) in check by maintaining an open stand structure.
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Threats
A major threat to older forests today is the view that the main task is simply to preserve those that now exist. All older forests began as young forests. Healthy forests contain groups of trees of many age classes over large areas. Thus effective protection of older forests must begin with the recognition that they are part of a long cycle and that all parts of that cycle must be protected or restored. In this section, individual categories of threats to older forests are considered, but effective older-forest protection and enhancement efforts must integrate those categories to produce and maintain older forests at a steady or increasing rate. For example, management regimes that include only short rotations will never produce older forests.
Invasive species, pests, and pathogens
Non-native invasive species are species whose introduction threatens or harms natural ecosystems, human health, economic values, or all three. They are a serious threat to older forests in all five regions.
Forest ecosystems can adapt to the slow, natural movement of species. Over the last 200 years, however, human travel between continents has increased dramatically, and people have carried plants, animals, and pathogens (microscopic organisms that cause disease) to places they would never have reached by natural dispersal. Once there, away from parasites and other ecological controls that limited their environmental and socioeconomic impacts on their home continents, some of these non-native species have become invasive.
Over the past century, invasives in the United States have impacted forest biodiversity, health, and productivity; water and soil quality; and socioeconomic values. The loss is more than $2 billion annually just in terms of forest products.
In the mid-1900s chestnut blight eliminated American chestnut as a co-dominant species in oak-hickory forests throughout the Northeast. Beech bark disease, introduced in the 1930s, has spread throughout the region, killing mature trees but not affecting regeneration, leaving some northern hardwood stands dominated by a beech understory. The hemlock woolly adelgid, an insect introduced from Asia in 1924, is killing eastern hemlocks throughout the eastern United States. Eastern hemlock is a common long-lived species in older forests in the region. Thus three keystone old-growth tree species are vulnerable to these human-caused stresses. The overabundance of deer that browse on seedlings and saplings prevents regeneration in older forests in New York and Pennsylvania, challenging efforts to maintain existing older forest. Thus a diversity of stressors is affecting older forest conservation, and they vary in different parts of the region
Exotic diseases and insects also threaten older forests in the Lakes] region. They have the potential to impact all forests significantly and could seriously damage surviving and restored older forests. A multi-state approach involving regulations, research, and control may be the only way to prevent or reduce the impact of these species on native ecosystems.
A potential influx of exotic insects and diseases is one of an array of stresses on forest ecosystems in the Great Lakes region, including high deer populations, drier soil conditions due to European earthworms, and warming temperatures. These stresses could remove many tree species from the forests. Although scientists don’t know how to predict the consequences of such multiple stresses, they are concerned that, even in the absence of climate warming, older forests could be devastated within the next 20-50 years. Just two pests— hemlock woolly adelgid and Asian long-horned beetle—could virtually destroy old-growth hemlock-hardwood forests in wilderness areas, creating an abundance of fuel and coarse woody debris. What will be left? In the short term, red maple, white spruce, balsam fir, and northern white cedar might persist if no new invasives take over. Some species may persist as seedlings and saplings (as chestnut has done in the East), but some of the more devastating invasive pests kill large saplings as well as older trees. They threaten the ecological structure and function of all regional forests, not just old growth.
Tree species in the Lakes] region are at special risk because nearly all of them have close relatives (with associated pests and pathogens) in Asia and western Europe. Yet not all species present the same risk, and control efforts should focus on pests and pathogens that have the potential to devastate the forests. Although agencies are trying to shift their focus, much of the current control effort goes to species that don’t present the greatest threats. Social acceptance plays a large role—for example, the public hates gypsy moths and wants them controlled, and their elected representatives in the legislature can overrule the foresters. Such social constraints affect what can be done.
Invasive species threaten to change the character and dynamics of native older forests in the Southeast by altering patterns of herbivory (grazing on plants), predation, habitat, competition, and disease. They also threaten the preservation and restoration of old-growth ponderosa pine in the Southwest.
Fragmentation and parcelization
Fragmentation occurs when large expanses of forestland are broken up into smaller forest tracts surrounded by other [[land use]s]. It often results from residential or commercial development. Parcelization refers to changes in ownership that divide large forested tracts into smaller parcels that may or may not remain contiguous forest. It often occurs when a large tract of family forest is passed on from a single owner to multiple heirs. Older forests in all five regions face both of these threats.
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Development that converts forestland to non-forest uses is the primary threat to older forests in the part of the Northeast region outside the northernmost commercial forests. In many ways the restoration of abandoned agricultural land in the Northeast to older forest condition is a race against urban sprawl and development. Many old fields that would become older forest in the next couple of decades will probably succumb to development before that time. In fact, post-agricultural reforestation in the eastern United States appears to be coming to an end. Development pressure is reducing forest cover, reversing the forest-recovery trend of the last 100 years. This threat is compounded by increased parcelization as land ownership passes from one generation to the next. Thus, although the small amount of existing true old growth is protected both in the northern and southern sections of the region, the amount of older forest will decline.
Development is a major threat to older forests in the Lakes] region, especially second-home development that fragments forestland into small parcels. Recreational demands are changing land use in the north, with human populations increasing in recent decades after nearly a century of decline. This is particularly serious on the valuable lakeshores, removing habitat and affecting water quality and lake ecosystems.
Urban development is a serious threat to older forest in the Southeast, as this region is rapidly changing from a largely rural society to a highly urbanized one. Urban growth in the Southeast has generally been unconstrained by zoning or planning, resulting in highly fragmented landscapes molded by short-term economic gain rather than long-term collective value to the region. Older forests rarely win the struggle with this type of growth. Urbanization also makes forest management more difficult, bringing restrictions and regulations (on smoke from burning, for example) and elevated land prices that preclude economic forest management.
Logging has fragmented many areas of old growth in the Pacific Northwest in the 20th century. Recent policies have addressed this problem, but it will be many decades before that influence diminishes. For example, most of the old growth in the reserves is fragmented by 20- to 50-year-old forest plantations that cover as much as 40% of the reserve area.
Fire suppression and catastrophic wildfires
Fire (Fire in America) has always been an important element of North American forest ecology. As noted earlier, Native Americans cleared forestland with fire in all of the regions examined by the NCSSF-sponsored workshops, and natural wildfires also played a significant role in the ecological evolution of forests.
Gifford Pinchot, the founder of the United States Forest Service, led a fire suppression movement that began in the late 1890s. The science of ecology (Ecology (collection)) was in its infancy, and this movement was based on the simplistic rationale that fire destroys forests and keeping fire out of forests conserves them just as keeping fire out of cornfields conserves corn. Many foresters and timber, pulp, and paper companies supported the movement. The Forest Service adopted fire control as its principle job, and fire suppression became an important mission for other federal land-management agencies.
But after several decades, unwelcome changes started to appear in forests and grasslands where fire had been systematically suppressed. Dangerous levels of fuel for wildfires accumulated in forests where prescribed burning had been discontinued, and the threat of catastrophic wildfires increased. Undesirable plant species began to dominate some areas. It became increasingly clear to many forest managers and policy-makers that fire is an important element of natural forest ecosystems and that its exclusion threatens their integrity.
Fire exclusion has reduced the pine and oak components of forests in the Lakes] region. Fire is needed to maintain pine and oak and increase their resilience to change, but managers face serious constraints on the use of fire. Some resistance is due to lack of ecological and silvicultural understanding about the roles of fire in various forest types. Systematic experiments comparing the effects of fire on various sites could help resolve these questions. But the largest limits on fire as a tool are social and economic.
Fire suppression and high severity wildfire may be a serious threat to old growth in dry provinces of the Pacific Northwest. Fire suppression has transformed open stands of old-growth species such as ponderosa pine to dense stands with understories of small-diameter conifers. These dense forests in dry landscapes are susceptible to high severity wildfire that can kill old-growth pines and Douglas-firs that may have survived lower intensity fires in the past. Insect and disease outbreaks that can kill old trees may be more common in these dense stands.
Fire suppression has threatened the preservation and restoration of old-growth ponderosa pine in the Southwest by allowing shade- and fire-intolerant species to encroach on the ponderosa pine ecosystem, probably causing the death of old trees. Invasion by shade-tolerant species occurs at higher elevations where ponderosa pine is in transition to the mixed conifer type.
Climate change
Climate change is a threat to older forests in all five regions. Two of its most serious potential impacts may be pressure for species to migrate and an increase in the frequency and severity of large-scale weather disturbances, particularly severe storms, droughts and fires.
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As the global climate warms over the next century, the environment for most existing tree species will shift northward. In the Northeast and Lakes] regions, for example, most forest vertebrates (animals with backbones) probably will be able to move as their habitats move north because they readily disperse and only require mature (60-80 years old) or younger forest. However, species that depend on old growth, such as some lichens, mosses, fungi, and invertebrates, disperse slowly and may be at risk. One strategy for conserving biodiversity in the face of climate change could be a system of “stepping stones” and corridors of older forest that permit such slow-moving species to keep up. This is an important reason for identifying existing old growth in each region and planning to develop future old growth in strategic locations.
Forests of the Southeast have been molded by hurricanes and fire. Some scientists believe that rising ocean temperatures will result in more storms or more intense storm activity. The small footprint of older forests on the southeastern landscape makes this threat more serious. Several hundred years ago, a large storm could affect only a small segment of older forest, but now a stand of older trees may be completely exposed to catastrophic storm damage.
The preservation and restoration of old-growth ponderosa pine in the Southwest are threatened by climate change that probably will continue to alter fire regimes, affecting the types, size, and severity of fire in ways that will bring undesirable consequences for old growth and potential old growth in many ponderosa pine forests.
Socioeconomic values
Unless enough people want older forests and have the political will to foster actions that will maintain and increase them, they will continue to decline in most regions. Thus public apathy toward older forests may constitute the most serious threat to their continuation. Data are scarce on the feelings most people have about older forests, and contact between the average citizen and older forests is decreasing with increasing urbanization. It will be important to understand people’s attitudes toward older forests and to make a factual and compelling case for retaining and restoring them.
More than 70 million people live within a day’s drive of the 26-million-acre Northern Forest of the Northeast region. However, the Northern Forest itself has a paradoxically low human population density, given its proximity to the densely populated Boston-New York City corridor. Surveys indicate that the general public in the region values forests and wilderness areas and wants them maintained; “baby boomers” born between 1946 and 1964 expressed the strongest support for wilderness. However, it’s not clear how much the public wants older forest and wilderness areas and how it wants them distributed across the landscape.
In the Lakes] region, the transformation from sustained-yield management of a few commercial tree species to the protection and management of entire ecosystems is changing many basic premises of forest management. Forest managers often fail to understand why local communities don’t trust them. Many local residents fear that foresters are going to reduce access to forest resources, while urban environmental advocates believe that foresters will do just the opposite. When different groups face conflicts over resources, it can be useful for each group to see themselves as part of the same community, at odds on certain issues, but drawing from a common history and headed toward common goals. Human and ecological histories intertwine in the Great Lakes region, and both shape future management possibilities. Ecological history can be helpful for restoration, but it’s not a template that tells us what to do; consensual goals must emerge from broader social conversations about the values of older forests.
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Older forests represent significant cultural and economic values for the Southeast. Older trees produce high quality wood, and the market value of timber from these forests is two to ten times that of younger trees. This economic value has resulted in extensive timber harvesting in both public and private older forests in the region.
As southeastern older forests decline, appreciation of their cultural and esthetic values is increasing. In the past, the rural nature of the Southeast linked people to forests in fundamental ways. Today, the aesthetics of these old “cathedral” stands of woods often connect people with the cultural heritage of the forests.
Older forests of the Pacific Northwest have many social values, ranging from timber to recreation and aesthetics. The large volume of wood in an old-growth stand makes it quite valuable. For example, one acre of old-growth timber could be worth $25,000 (assuming 50,000 board feet per acre and a market value of $500 per thousand board feet), and a single 50-acre clearcut could be worth $1.2 million. At one time, large-diameter logs commanded a premium at the mill, but now large logs are actually less in demand than small ones. Today, few mills can process logs more than 20 inches in diameter, and new technologies allow large beams to be constructed from small-diameter logs.
The social values of older forests remain high in the Pacific Northwest. Old growth has become an icon for the environmental movement in the that region. For many people, it symbolizes wild and awe-inspiring places that are rich in diverse life forms. It may also be perceived as an environment unaltered by humans, where nature is in a perpetual state of perfect balance. This perception isn’t consistent with our increased understanding of the role of natural disturbances in old growth, especially in fire-dependent ecosystems.
Urban development in forested ecosystems of the Southwest has increased risks related to fire. Many residents of the Southwest understand the ecological role of fire in frequent-fire forests but remain uncomfortable with allowing wildfires to burn.
Scenic and recreation values and expectations are often in conflict with activities such as mining and timber management.
Residents of the Southwest are solidly opposed to logging old growth, although they are not consistently opposed to removing some larger trees during thinning operations. They generally support mechanical thinning to reduce forest fuels and restore forest structure.
Ownership
There are striking differences in the ratio of private and public forest ownership in the three eastern regions and the two western ones. Because different owners have different attitudes toward older forests and different capacities to conserve them, it is important to know more about how owners see the role of older forests in the landscape. In general, older forests are more prevalent and tolerated on public land than on private holdings. Many private owners are motivated primarily by economic considerations and use short timber production cycles to optimize profit, reducing the potential development of older forests. However, some private owners manage forests on long rotations, and some private land harbors significant amounts of older forest.
Compared to the western United States, the Northeast has little public land, and private forest ownership predominates. All northeastern states have less than 15% in public ownership. Maine has the lowest percentage of public ownership (only about 6%), but the highest percentage of large (more than 5000 acres) private forests managed for timber (about 50% of the state).
Private, non-industrial owners dominate tim¬berland ownership in the Lakes] region. In 2000, according to the USDA Forest Service, approximately 69% of the 72.8 million acres of timberland in the Minnesota, Wisconsin, Michigan, Ohio, Illinois, Indiana, and Iowa was privately owned, with 82.6% of this in private non-industrial ownership. Although most large blocks of remaining old growth are on public lands, many smaller fragments of old growth and older forest are in private hands, often with little or no protection.
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The landscape of the Southeast is largely a matrix of predominately private land with scattered islands of public land. A large proportion of older forest is on military reservations where forest conservation has a lower priority than military missions. Thus the Southeast must consider how both public and private land strategies can work with some synergy.
Most remaining old growth in the Pacific Northwest is on federal and state lands. There is relatively little older forest on private lands, where a good economic return on investment typically is a primary management goal. Management goals for most public lands include biological diversity and recreation, making them the easiest places to develop old-growth conservation strategies. Since public lands cover more than 50% of the region, the opportunities for conserving old-growth in the region are generally good by national and global standards.
Although ponderosa pine of the Southwest exists in parts of southern Colorado, southern Utah, southeastern Nevada, western Texas, and northern Mexico, the best available data on ownership come from two states—New Mexico and Arizona. The USDA Forest Service oversees 1.8 million acres or 64% of all the ponderosa pine in New Mexico. Other public agencies manage 172,000 acres, and non-industrial private owners (including Native American tribes) handle 798,000 acres. The majority of this acreage, 2.4 million acres, is in non-reserved status, which means that it has not been legally removed from management for wood production.
Arizona has slightly more than 3 million acres of ponderosa pine, with slightly more than 2 million of those acres managed by the USDA Forest Service. Other public agencies oversee 122,000 acres, and private owners (including Native American tribes) manage 851,000 acres. Only 6% of the total acreage of ponderosa pine in Arizona is protected from harvesting for commercial wood production.
Climate change and the dynamics of ecosystems
Climate change has always affected the amount of older forest through its influence on fire frequency, winter hardiness, and drought tolerance. These factors have shaped the forest cycle and existing and potential older forest in each region. As human-caused climate change accelerates changes in temperature and weather, it will be particularly important to understand the effects on older forests and older forest potential. The best, though imperfect, route to this understanding is through knowledge of past climate changes and their effects.
In the Northeast during the last ice age, the present-day New England states were covered by ice that extended into Pennsylvania. As a consequence, forest cover in this region is relatively young geologically. Current tree species have only existed there for about 2,000 to 3,000 years, and natural forest communities have only occupied particular sites for 1,000 to 2,000 years.
Four major forest types dominate the region. They are:
- spruce-fir in northern New England and on mountaintops south along the spine of the Appalachian Mountains
- northern hardwoods (beech, yellow birch, and maple) in central New England, northern New York and upper elevations extending to West Virginia
- oak-pine in southern New England
- oak-hickory south of New England.
Despite the geological youth of northeastern forests, the dominant tree species have changed frequently since the last ice age, and the abundances and distributions of various species have fluctuated in response to natural climate change and European colonization.
Disturbance processes at various scales control ecosystem dynamics and diversity in the Lakes] region. The glaciers of the last ice age shaped the physical geography of the soils that still serve as key templates for today’s forests. The glacial history of the region means that forest communities are relatively young, having appeared only after the glaciers retreated.
Various species with very different ecological characteristics moved in one by one at different rates from different locations and directions. Whole, complete communities didn’t move together; individual species migrated at rates that depended on their effectiveness in dispersing seeds, their ability to thrive in the changing post-glacial climate, and their competitive ability.
As the climate moderated about 6,000 years ago, pines and oaks moved into the region. About 3,000 years ago pine and oak began to decline on the better, moister loamy soils. Hemlock thrived in these areas and came to dominate the landscape along with yellow birch and sugar maple. Gradual climate changes probably occurred at this time, helping hemlock invade and perhaps lowering the frequency of fires, favoring hemlock over the more fire-dependent pine.
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Nearly all of northern Wisconsin was dominated by pine for several thousand years before hemlock arrived. From about 3,000 to 1,000 years ago, white pine and hemlock co-existed, but hemlock became dominant on loamy soils while pine, aspen, and birch remained on sandier soils. In recent history, northern Wisconsin was dominated by extensive pine forests on sandy outwash soils and hemlock and maple on heavier soils.
Growing evidence from pollen and charcoal studies shows that this general distribution of forest types in the Lakes] region, with hemlock and hardwoods on the better soils and mixtures of pines with some aspen and oak on sands, generally persisted from 3,000 years ago to the period of logging and settlement of the 1800s. This makes the northern Wisconsin landscape of the mid-1800s the best available benchmark of natural processes with a fairly constant climate.
This pattern from pollen and charcoal data is corroborated by historical data describing the forest and trees. Does this provide a template for restoring the landscape? Much has changed since then. But the relative stability of the period leading up to that historical time does provide a good benchmark, indicating the general natural dynamics and ecosystems that existed prior to the industrial period and providing important information about regional potentials. Ultimately, choices of activities and management approaches for this landscape will be based on social decisions. Historical and ecological knowledge will be part of this decision-making process.
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Older forests of the Southeast are among the most biologically diverse temperate-zone forests. In addition to their richness of plant species and reptiles and amphibians, these forests are home to many rare native species. The most commonly recognized are two woodpeckers that emphatically illustrate the ecological importance of these forests. The declining population of red-cockaded woodpeckers has long been connected with the demise of old upland pine forests, and the nearly complete liquidation of the bottomland hardwood forest was thought to be the primary factor that drove the ivory-billed woodpecker to extinction. However, recent sightings of this species reported in Louisiana and Florida provide both hope for the species and a sense of urgency for conservation.
The diversity of forests in the Southeast results mainly from disturbance regimes that prevailed as they evolved. Upland forests flourished on coarse-textured, low-fertility soils that encouraged development of fire-driven ecosystems. In fact, southeastern pine savannas have the most frequent fire return interval of any natural fire regime on earth. The full potential diversity of southeastern forests exists only in native older forests where a frequent-fire regime has been maintained. Hurricanes interacted with fire to create the complex forest structure of the pre-European settlement forest and the variation in bottomland forest that allowed renewal and regeneration.
Older forests have developed in a wide range of forest types and disturbance dynamics in the Pacific Northwest. For example, old-growth ponderosa pine on dry sites is characterized by relatively open understories that were maintained historically by relatively frequent low to moderate intensity fires at intervals of less than 20 years. Old growth in wetter forest types, such as western hemlock and Sitka spruce near the coast, typically has large accumulations of live and dead wood in the understories and experiences stand-replacing disturbances at intervals of one to several centuries. Forests between these extreme are subject to “mixed severity” fire regimes, with fires that range from low to high severity. Fire suppression can alter the dynamics of some of these forest types, and moderate to high severity fire may be needed to maintain a patchwork of older and younger forest stages at landscape levels. The mixed severity regimes are particularly challenging to forest managers seeking to conserve old growth. High severity fire was a natural component of these landscapes and isn’t readily replaced by prescribed burning, which normally occurs at low intensity.
Ponderosa pine ecosystems in the Southwest merge into pinyon-juniper, chaparral mountain shrubland, or grasslands at lower elevations and into mixed conifers at higher elevations. For example, ponderosa pine is found in southern Colorado between lower-elevation grassland or pinyon-juniper and higher-elevation Douglas fir, while in southern Utah ponderosa borders shrubland or Colorado pinyon-Utah juniper woodland.
The dynamics of healthy ponderosa pine ecosystems are inextricably tied to low-intensity, patch-size fires that occur fairly regularly. There are also occasional mixed-severity fires and rare stand-replacement fires. The regular, low-intensity fires historically produced open ponderosa pine stands dominated by clumps of old, fire-resistant trees and a highly diverse understory on fine-textured, basalt-clay soils. On sites with coarse-textured soils, the forest was denser and less clumpy, although nowhere near the density of today’s ponderosa pine forests in those same areas.
As described elsewhere, during the last century the ponderosa pine forests of the Southwest have undergone significant changes in structure, composition, fire frequency, fire intensity and severity, and overall landscape patterns. These changes have resulted in a new ecosystem dynamic in which severe fires threaten to destroy, rather than renew, the ponderosa pine ecosystem.
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