0042-B1

Old-growth Forests in Canada - A Science Perspective

A. Mosseler^{[1] [2],}, I. Thompson^[3] and B. Pendrel^[4]

Abstract

In response to public concern, the Canadian Forest Service organized a national symposium in 2001 to discuss the old-growth issue from a science perspective. The objectives were: (i) to bring together Canadian expertise on old-growth forests, (ii) to define old growth within the main forest regions of Canada, (iii) to understand its biological complexities and ecological roles, and (iv) to discuss management and restoration experiences and options. Some forest regions of Canada still contain significant old-growth forest (e.g. some boreal forest regions), although other regions contain very little primary, relatively undisturbed, older forest (e.g., eastern temperate zone forest regions). One of the difficulties in managing and conserving old-growth forests is defining them in a scientifically meaningful, yet in an operational and policy-relevant manner. This difficulty may be overcome by developing an index of "old-growthness" related to specific forest regions or forest types. Such an old-growth index would allow for the inclusion of specific attributes, composition, functions and processes seen as relevant to different ecological regions or specific forest types, and could serve as a basis for prioritizing local or regional conservation and management activities. Thus, such an index approach has worldwide applicability. Traditionally, old-growth forests have been valued primarily as habitat for forest-dependent, specifically old-growth-dependent, wildlife. Recent results from research on old-growth forests in eastern Canada suggest that as tree populations age, they tend to increase in genetic diversity and reproductive fitness, suggesting that old-growth forests may serve as natural reservoirs of genetic diversity and reproductive fitness for the constituent tree species. This has important implications for the dispersal and adaptation of trees across increasingly fragmented forest landscapes subject to the anticipated rapid climatic changes and the introduction of new pest and disease problems. Old-growth conservation goes well beyond the more traditional areas of watershed (including water quality) and habitat protection and includes emerging issues such as the conservation of genetic resources and carbon sequestration. It is very much a cross-sectoral issue with many interdisciplinary linkages. Therefore, conservation and protection of old-growth forests should be of wide general interest to the forest sector.

A science perspective

Maintaining old-growth forests is an important policy and ecological issue in Canada. The public is concerned with the apparent decline in the amount and distribution of older forests on the landscape, and they have expressed a desire to maintain old-growth forests, especially those with the large, old trees, that once dominated many areas of the pre-European settlement landscape. Canada is often singled out at the international level for not preserving sufficient old forests, even though more than 40% of our primary forests remain largely intact. The scientific and forest management communities are concerned with maintaining old forests in Canada in the context of sustainable forest management. Canada has pledged, in the Convention on Biological Diversity, to develop its forests sustainably; this requires close attention to old growth as a distinct component of those forests. An important question is how much old forest within a given landscape is needed to sustain species and to satisfy public concerns from a social, biological, or ecological perspective. Beyond the more traditional roles of watershed and habitat protection, the roles of old-growth forest in efficiently sequestering carbon over long periods of several hundred years, and in the conservation of genetic resources, are among several important emerging issues with respect to old-growth conservation. These are areas of active research in Canada.

The Canadian Forest Service (CFS) organized a symposium, entitled Old-growth Forests in Canada - a Science Perspective, which was held in October 2001. Our objectives were: (i) to bring together Canadian scientific expertise on old-growth forests: (ii) to help define old growth for the major forest regions of Canada, (iii) to understand its biological complexities and ecological roles, and (iv) to discuss management and restoration experiences and options. The principal role of the CFS was to facilitate a national consensus among scientists and land managers by building a science-based foundation for developing public policy to address concerns about the decline of old-growth forests. This paper provides a preliminary summary of the results achieved towards a national consensus, and presents a forest-sector perspective on old growth, using examples from the Acadian Forest Region (Rowe 1972) of the Maritime provinces, one of the temperate forest regions of Canada, to illustrate some of the ideas that were presented at the symposium. We do not pretend that any definitive work on Canada's old-growth forests exists. Defining old-growth forests, understanding their ecological roles and values within a broad landscape context, and developing management and restoration protocols are all subjects of current scientific investigations.

Defining old growth in a scientifically meaningful, yet policy-relevant, manner presents some basic difficulties, especially if a simple, unambiguous, and rigorous scientific definition is sought. However, such difficulties are common among biological sciences and often cannot be avoided. For instance, science is still struggling with a comprehensive definition for the concept of "species," especially in light of recent advances in genetics and bio-engineering. Nevertheless, the absence of a fixed definition has not prevented progress in the biological sciences. We have several good working definitions of what constitutes a species. The same arguments can be made about the concept of "life" itself.

Participants at the conference agreed that "old-growth forest" was a valid scientific concept, and that the actual features that describe the old-growth stage would likely differ in character and degree depending on the forest region, although many common metrics were suggested (e.g., large trees, snags, accumulated woody debris, etc.). Participants also widely agreed that old-growth forests need not necessarily be primary forests - that is, those where no commercial or major anthropogenic disturbances has ever occurred. Instead, they suggested that secondary forest following harvesting could, in time become old-growth forest. Among the major forest regions (Rowe 1972) in Canada, it was recognized that old growth is most easily defined for the temperate-zone forests such as Acadian Forest, Deciduous Forest, Great Lakes-St. Lawrence Forest, and the Coastal Forest regions of British Columbia, where fire rotation periods are exceptionally long. In all the other regions, a clear decision on when to designate a given stand "old growth" or not, was less clear to the delegates because of disturbances and successional patterns in the absence of disturbance. For example, in the absence of fire, a mixedwood forest in an eastern boreal type might succeed from deciduous forest, to a mixedwood forest, to any of several conifer-dominated types, and perhaps ultimately to balsam fir (Abies balsamea) forest, over a thousand years or more. Yet, at each stage there are large old trees, many snags, and multiple structures. However, there was general concensus that in all forest regions multiple states of old growth were possible at the same site, as various forest types succeeded into old age.

The lack of an all-encompassing, consensual, and uniform definition of an old-growth forest should not constrain our research, nor can it be used as an excuse for either disregarding old growth as a conservation issue, or avoiding the development of appropriate public policy. Perhaps our best attempt at defining old growth will come from combining the structural attributes, species composition, ecological functions and processes, to provide an index of "old-growthness" for a particular forest region (Mosseler et al. 2002a). Such an index can be used to compare forest stands within and among regions as a measure of their development towards some ideal state. This index concept would recognize the dynamic state of old-growth forests and could (e.g., Table 1) be used to evaluate stands and to establish priorities for conservation and management within a regional or national context. The index concept has worldwide applicability.

Table 1. Some attributes of late-successional, temperate-zone, old-growth forest types for consideration in developing an index of "old-growthness" and for defining old-growth forests

An index of "old-growthness" would include a number of the attributes and processes associated with old-growth forest, considered most important from a biological and ecological perspective, for a given forest type or forest region. The index would be derived by assigning a value and/or weighting attributes, functions, and processes for important components of the forest ecosystem, so that a proportion or percentage of "old-growthness" could be established. Such an index would require considerable research and understanding of a given forest type to determine reasonable valuation for each of the variables included within the index. One possible use of the index would be to rank forests according to some priority and for their potential biological/ecological significance for management or conservation objectives. An index of "old-growthness" could include animal associations. In fact, our primary biological concerns with old growth have to date revolved around their importance as habitat for forest-dependent, or old-growth-dependent, wildlife. The physical attributes of old growth have special relevance for certain animal species because they provide optimal habitat for breeding. For example, old-growth forests provide structures, including temporal stability of such structures, and many microenvironments that may be important to an individual species or guild and, perhaps in some cases, critical to the survival of a whole range of organisms from lichens (Selva 1994) to certain wide-ranging mammals such as caribou, Rangifer tarandus (Schaeffer and Pruitt 1991, Chubbs et al. 1993). Specific physical and biomass attributes that have a functional relevance would figure prominently in an old-growth forest index that might be used to evaluate or establish priorities for conservation and management.

In temperate forests, processes such as natural succession in the absence of catastrophic disturbances (e.g., fire) and tree species composition will have special significance in determining old-growth conditions. Some have argued that the only "true" old-growth forests result from continuous natural processes, uninterrupted by humans. Others contend that such a definition is overly restrictive, especially given human-induced climate changes. In the Acadian Forest Region, forests dominated by long-lived, shade-tolerant, late-successional trees, adapted to high atmospheric moisture, such as red spruce (Picea rubens), eastern hemlock (Tsuga canadensis), sugar maple (Acer saccharum), American beech (Fagus grandifolia), and yellow birch (Betula alleganiensis), are capable of forming naturally regenerating, self-perpetuating forests in the absence of catastrophic stand disturbances. White pine (Pinus strobus) could also be included on this list because of its widespread association with later-successional forest types, despite being generally considered as intermediate for shade-tolerance and longevity. In the absence of major disturbances, late-successional, temperate region forests can maintain and perpetuate themselves over thousands of years (Rowe 1972, Lorimer 1977). In developing old-growth indices for the Acadian Forest Region, species composition would be considered an important factor determining "old-growthness." Thus, the development of regional, or forest-type-specific, old-growth indices would be a rather simple first step in defining and understanding old-growth forests, and in developing forest policies aimed at old-growth conservation and management. Next steps would be to add values to variables such as presence and density of various indicator species, important structures such as snags and woody debris, percentage of forest canopy in gaps, abundance of certain lichens (Selva 1994) and mosses, and appropriate density of large trees. Regional differences in actual values would obviously be expected.

Understanding the biological complexities and ecological functions of old growth is much more demanding and requires scientific rigor. As already noted, our main concern as forest managers is generally with protecting habitat for forest-dependent species, and more specifically for old-growth-dependent species such as caribou, marten (Martes americana) (Thompson and Harestad 1994), black-backed woodpeckers (Picoides arcticus) (Imbeau et al. 1999, Thompson et al. 1999), and certain lichens (Selva 1994). The roles of old-growth forests in carbon sequestration, nutrient cycling, water and soil protection, and ecological stability of dependent species are also of concern within regional and national contexts and are areas of ongoing research in Canada and elsewhere.

Although wildlife habitat has been the most prominent focus of concern in protecting old-growth forests, some recent research in eastern Canada has focused on the potential role of old-growth forests as reservoirs of genetic diversity and reproductive fitness of the component tree species (Mosseler et al. 2002a, b). This aspect of old growth and its ecological value has received minimal attention to date. Recent results suggest that, as populations age, they tend to increase in genetic diversity and reproductive fitness. Therefore, older forests may serve as important gene pools for the dispersal and maintenance of tree species across a landscape that is often becoming increasingly fragmented, particularly in southern Canada, and where natural mature forests, particularly late-successional forest types, have been extensively converted to earlier-successional forest types and more homogeneous forest plantations. These landscape alterations and processes have implications for forest biodiversity conservation (e.g., Andren 1994).

Research focusing on techniques to restore late-successional, old-growth forest types is also being conducted in order to recover certain forest types that were decimated following European settlement. One approach to restoration may be to use an old-growth index to identify potential forest types, regardless of age, in suitable stands that would make good candidates for active restoration management. Alternatively, established forest plantations can be used as nurse crops to provide the required environment for development of forests of shade-tolerant, long-lived species, adapted to high atmospheric moisture contents, capable of perpetuating themselves in the absence of catastrophic stand disturbances. For instance, older plantations of red pine (Pinus resinosa) that were originally established as part of extensive land reclamation programs in central Canada are now functioning as nurse crops for the rehabilitation of late-successional forest types following more than 75 years of successional change. Expertise developed in plantation forestry provides opportunities for longer-term restoration of forest types ultimately capable of perpetuating themselves as old-growth forests. For example, the CFS is currently experimenting with modifying existing red pine plantations as a means to re-establish typical old-growth species such as red spruce following its post-settlement demise across large areas of eastern and central Canada.

Elsewhere in Canada, particularly in boreal systems, the best strategy to maintain old growth likely requires explicit planning within the context of forest management units. In other words, once harvested and successfully regenerated, a portion of the landscape should be allowed to develop through to the old-growth stage, based on plant and animal species population considerations. A further component to this strategy is to leave large areas as protected areas, untouched except by natural processes, to serve as benchmarks and reserves for study.

What should be the management response to the maintenance of old-growth forests? Judging from participation at the national symposium on old-growth forests in Canada, several of the larger forest companies, primarily those operating on the west coast of Canada, have made substantial efforts to accommodate old-growth values in their areas of operation, including renouncing clear-cutting as a means of harvesting in temperate coastal forest types capable of self-perpetuation in the absence of stand-replacement types of disturbance. Nevertheless, there remain many legitimate concerns and questions posed by industry when they are confronted with demands to protect old-growth forest. Not least among these questions are those aimed at determining how much is needed to preserve what values in terms of (i) ecological integrity, and (ii) maintenance of viable wild communities in space and time.

One approach to building sound arguments in favor of old-growth conservation, has been to develop an understanding of historical conditions and pre-settlement amounts of old-growth forests within defined forest regions or political jurisdictions. Several efforts have been made to characterize the condition of the forest before European settlement based on historical (e.g., land survey) records, palynological analyses (Green 1987), and modelling. Combined with an ecological approach based on understanding natural succession trajectories under the prevalent natural disturbance regimes and the geological/palynological evidence, we may arrive at some reasonable estimates of pre-settlement forest conditions. Estimates of the amount of relatively undisturbed forest cover necessary to accommodate some of the larger mammals are also available for many areas of Canada. Large mammals, especially carnivores and omnivores such as bears (Ursus spp.), wolves (Canis lupus), and wolverines (Gulo gulo), require large home ranges and hence large intact forest areas to maintain viable populations. Some of the deer species, such as black-tailed deer (Odocoileus hemionus columbianus), Roosevelt elk (Cervus elaphus roosevelti), and woodland caribou, require old forests, especially in winter to escape deep snow and predators. Species such as these may, in part, "drive" management requirements for maintaining old-forest ecosystems on the landscape (e.g., Nyberg 1990, Thompson and Harestad 1994). Taken together, these kinds of information can be used to estimate how much old growth may be needed and must be built into the local or regional index of "old-growthness."

The concept of ecological representation can also guide us in setting conservation priorities. Given our estimates of what existed in pre-settlement times, a reasonable suggestion is to maintain a certain portion of the dominant old-growth forest types within each ecological region to preserve the tree species gene pools that constitute those forest types. Wildlife objectives will then need to be superimposed on those basic requirements. Through this process, we may achieve, firstly, some consensus on what are the basic ecological requirements for maintaining old-growth forest and, then, some idea of the areas involved.

For the Acadian Forest Region, a region with very little intact, old-growth forest, we have some idea of what the pre-settlement forest looked like based on our ecological understanding of the interactions of successional trajectories and the characteristic disturbance regime (Mosseler et al. 2002a). We also have some estimates of how much of this original, relatively undisturbed forest remains, at least for some important components of it (e.g., old-growth, late-successional conifer forest). After more than 300 years of forest harvesting in southern Canada's temperate zone forests, it is not surprising that there is little original forest left in these areas. Nevertheless, much of this region remains forested, albeit in a younger state, and in many areas these original forest types are being restored. Therefore, with proper direction provided by science and policy, forest managers will be able to re-establish and maintain old-growth forests across much of this area. For example, old-growth objectives have been established for the three Model Forests located in southeastern Canada. Also, the Province of New Brunswick uses protection of old and mature forests as a management tool. The "Old-Growth Symposium" has provided us with a strong impetus for future management and research within these forests and has clarified the strong concern that exists within the forest sector to meet national and international objectives pertaining to old-growth forests.

Concerns over old-growth conservation go well beyond the more traditional areas of watershed (including water quality) and habitat protection, and includes emerging issues such as the conservation of genetic resources and carbon sequestration. Conservation of old growth is very much a cross-sectoral issue with many interdisciplinary linkages. It is important to dispel the notion that concerns about the disappearance of old-growth forests from our landscape are simply the preoccupation of environmentalists. It is an important issue with implications for ecological science, the long-term health of our forest economy, and our quality-of-life. It is time for the wider forestry community - the forest sector as a whole - to embrace this issue in a more serious way and to take up the cause of old-growth conservation.

Literature cited

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