0789-B1
W. Jan A. Volney[1] and John R. Spence
The forestry sector in Canada makes a significant contribution to the wealth of the nation. Of the forested area, 82% is occupied by the boreal forest, much of which remains uncut. To manage this forestry resource sustainably we must consider the consequences of insect population outbreaks because they and forest diseases cause annual depletions as high as 42% of annual growth. While management techniques can mitigate these effects in intensively managed forests, dead wood generated largely as a consequence of outbreaks represents an important resource for organisms that are critical for the integrity, fertility and proper functioning of native stands. Thus there is a need to better understand these organisms and to conserve them and the processes that generate the stand elements on which they depend. This can be achieved, without adversely affecting the current economic activity in forestry, by designating protected areas to be offset by a corresponding investment in afforestation and intensive forest management.
Early in the 20th century the Canadian boreal forest started to become a source of industrial wood products. This coincided with the decline of wood used in ship building and the opening of pulp mills in the boreal forests of Newfoundland, Ontario, and Manitoba. The next surge in industrial development occurred in the post-World War II years in British Columbia and Saskatchewan. This culminated in the northern expansion of industrial forestry in Alberta in the 1990s. One thousand years after the first European contact, Canada's forest industries shipped $ 69.6 billion worth of products. This accounted for $20.8 billion in balance of trade (Natural Resources Canada 2001). Although considerable wealth is generated by products derived from this forest, there are concerns that the rapid development of the boreal forest is not sustainable (Natural Resources Canada 2001). The challenge to Canadian forestry is to manage forests for sustained productivity without impairing the ecological integrity of the resource. Insect outbreaks are a major concern because of their effects on productivity.
This paper describes the nature of the Canadian boreal forest and the role that insect-caused disturbances play in the ecological integrity of this vast vegetation system. A solution to the special challenges that these disturbances present in managing forests sustainably at the stand, landscape and national levels is discussed.
The North American transcontinental boreal forest is dominated by 4 coniferous tree genera (Picea, Pinus, Abies, and Larix) and two broadleaved species (Betula and Populus). Some form of this forest is represented in all provinces of Canada except Prince Edward Island and Nova Scotia (Rowe 1972). Approximately 83% of the forested land in Canada is occupied by the boreal forest and of this just over half could sustain closed canopied stands. The northern ecotone between the forest and the tundra occupies the area between the 100 and 140 day mean annual growing season isopleths. The southern margin is roughly bounded by the 160 day isopleth (Rowe 1972). To the south, the boreal forest is bounded by the Great Lakes-St. Lawrence forest region in Quebec and Ontario, the forest-grassland ecotone in the Prairie Provinces, and the subalpine region in western Alberta and northeastern British Columbia. White and black spruces (Picea glauca (Moench) Voss and P. mariana (Mill.) BSP) are characteristic of this forest, but other important species include tamarak (Larix laricina (DuRoi) K. Koch), which occurs throughout except in most of the Yukon Territory, balsam fir (Abies balsamea (L.)) and Jack pine (Pinus banksiana Lamb.) in the eastern Canada to be replaced by their ecological analogues subalpine fir (A. lasiocarpa (Hook.) Nutt.) and lodgepole pine (P. contortaI Dougl. Ex Loud. var. latifolia Engelm.). Complementing these conifer species are white birch (Betula papyrifera Marsh.), trembling aspen (Populus tremuloides Michx.) and balsam poplar (P. balsamifera L.). The two poplar species are important components of the central portions of this forest where they become prominent in mixedwood stands (Rowe 1972). Other floristic elements encroach from the southern margins of the boreal forests but generally do not extend into the boreal forests of the central plains and boreal shield region where the prairie forms the vegetation type of the southern margin.
Stand replacing fire is a major disturbance initiating secondary succession (Heinselman 1981). However, secondary succession following stand-replacing fires is largely modulated by insect outbreaks throughout the boreal region. The successional sequence in its most elemental form is found in the central plains of Canada. Here, pioneer species such as aspen become established in extensive, dense stands from sprouts on mesic sites. This community is eventually invaded by shade tolerant white spruce to form a mixed wood community that ultimately develops into conifer-dominated stands. Left unburned, these communities are self-perpetuating. Similarly, jack pine stands originating from seed banks of serotinous cones dominate dry sites initially to be invaded later on by black spruce forming mixed communities. Change in these communities is punctuated by insect outbreaks that cause catastrophic tree mortality, contributing to stand renewal, or at lower intensities, contribute to gap-dynamic processes that generate natural stand structure.
No forest tree tissue is immune to feeding by insects. Forest insects specialize on particular host tissues on a particular tree species or a group of closely related species (see Ives and Wong 1988). This damage is not usually catastrophic unless trees are stressed by some other agent. Few insects known to feed on trees cause sufficient damage to warrant concerns in boreal forests. However, four species present a challenge to forest managers wherever their hosts occur (Volney and Mallett 1998). These are the spruce budworm (SBW) (Choristoneura fumiferana (Clem.)), feeding on balsam fir and spruces; the jack pine budworm (JPBW)(C. pinus Free.), feeding on jack pine; the forest tent caterpillar (FTC) (Malacosoma distria Hbn.) and the large aspen tortrix (LAT) (Choristoneura conflictana(Walker)). The latter two defoliate trembling aspen. All four species feed on developing buds and shoots in early spring.
Outbreaks of these species vary in intensity and duration depending on the host species involved (Cerezke and Volney 1995). Outbreaks of the three species feeding on pioneer tree species usually last for two or three years. However, FTC outbreaks may persist in a region for up to 12 years. Complete defoliation of the current year's foliage is often the result of outbreaks. By contrast, SBW outbreaks last for several years, often extending for as many as 20 years in the same stand. Defoliation varies considerably from year to year, ranging from light to severe. The spruces are most able to tolerate extended bouts of defoliation in mixed coniferous stands. Thus where balsam fir is a stand component, its mortality risk is higher than that of spruce during outbreaks. Its low tolerance of fire, its susceptibility to diseases, and its high moisture requirements make balsam fir especially vulnerable to SBW outbreaks and may explain its low abundance and localized occurrence in boreal forests west of Lake Winnipeg where outbreaks are prolonged.
Periodicities and geographical extent of outbreaks also vary. In eastern Canada there is a well-documented 35-year cycle based on observations and dendrochronological work that extends the record back to the 18th century. Paleoecological evidence suggest that outbreaks have occurred as early as 1190 years BP (Simard et al. 2002). These episodes may have occurred ever since forests came to occupy these lands following glaciation. In western Canada, SBW outbreaks do not appear to occur as regularly as those eastern Canada (Volney and Fleming 2000). Although the insect can be found throughout its host range, SBW outbreaks seem confined to southern forest margins. Similarly, JPBW defoliation fluctuates with a 10-year period in the northwest but cycles with half this period are found at the southern margin of its host's distribution. Outbreaks of FTC populations are far less predictable but seem to return at 12-year intervals on large spatial scales. Large aspen tortrix outbreaks seem aperiodic. Forest tent caterpillar outbreaks occur in the grassland/forest ecotone and adjacent forests. Large aspen tortrix outbreaks tend to occur immediately north of areas defoliated by FTC. For all species outbreaks persist longer at the southern margins of western boreal forests. It is thus attractive to speculate that these insects collectively could be instrumental in driving vegetation changes at the southern boreal forest margin in a warming climate.
The impact of outbreaks on stand structure is best documented for jack pine stands. Smaller trees are clearly most damaged during outbreaks. Nevertheless, larger trees that were previously damaged by other organisms or that did not recover fully from previous outbreaks also are at a higher risk of succumbing from defoliation in the most recent outbreak experienced by jack pine stands (Volney 1999). The result is that the largest and most vigorous trees make up the surviving stand. Gaps produced by mortality of larger trees alter both the horizontal and vertical structure of the resulting stand. This dead wood becomes ecological legacy elements (Franklin et al. 2002). Similar effects account for the observed structure of old spruce stands that have escaped fire for 250 years (Spence and Volney unpublished observations). The situation is not as clear with aspen defoliators but aspen stems are short-lived, prone to stem rots, and thus unlikely to develop into multi-layered stands. However, FTC damage combined with drought and extreme weather in spring have had documented adverse effects on aspen stand development through growth reduction and stem mortality (Hogg et al. 2002). The net result of this damage is that significant ecological resources are created by these outbreaks. Such elements include dead trees that eventually become snags, course woody material in branches and stems that fall to the ground to become habitat elements for vertebrates and invertebrates, and finer materials that are more or less immediately incorporated in nutrient cycles. This dead wood, which might comprise as much as 20% of the standing stems in mature white spruce stands, could also contribute significantly to the fuel loads in these stands and so are responsible for elevated risks of both wildfire occurrence and intensity.
The generation of dead wood by insect outbreaks has manifold consequences for forest management. Fully 30 % to 42% of the estimated annual growth in Canadian forests was lost due to catastrophic insect and disease outbreaks in 1987 to 1992 (Hall and Moody 1994). These losses have consequences at the stand, regional and national level, on the sustainable annual allowable cut (AAC). However, it is neither intended nor desirable to suppress all of these losses because the whole forest estate is not dedicated to timber production, and there are sound ecological reasons for retaining these processes in areas where values other than timber are of prime concern. Even on a landbase extensively managed for timber production, deadwood and its constituent biota may be critical to long-term site productivity.
If the objective is timber production, the impact is viewed as damage and efforts have traditionally been made to mitigate the effects of outbreaks. Management policies vary across the boreal forest. In eastern Canada the intent is to protect trees from SBW defoliation through aerial application of larvicides to protect foliage in the worst damaged stands but this strategy had little impact in altering the course of outbreaks (Kettela 1995). In western Canada, the strategy to control the spruce budworm on white spruce has evolved to attempt population suppression by targeting late instars. This direct control strategy also does little to alter stand conditions under which outbreaks develop (Volney and Mallett 1998).
More recently, silvicutltural approaches, aimed at modifying stand structure through thinnings with different retention levels and cutting patterns, have been attempted to alter conditions within stands where outbreaks occur. These treatments are thought to influence trophic relations among natural enemies of the pest by changing the understory vegetation harboring alternate hosts insects and their parasitoids that would attack the pest population. The prescription also alters tree condition by reducing intra-specific competition and nutrition regimes within treated stands. The dynamics of the insect population can also be altered through changes in mortality rates and dispersal in the new stand environments. Population densities five years post-treatment in one such experiment (Volney et al. 1999) now seem to be showing differences among treatments. Treatments that increase the amount of edge in thinned stands with 25% stem removals harbored lower spruce budworm feeding populations, lower egg mass densities, and females reared from this treatment were least productive in terms of surviving offspring produced. These blocks also sustained the lowest defoliation levels. Research is continuing to determine the mechanism responsible for this result so that it could be generalized to other systems. These results also suggest that small openings within forests may be sufficient to lower mean defoliator population densities and provide long-term protection from defoliation. This is not unlike the gap dynamic process (McCarty 2001) that would contribute to the stability of white spruce stands and allow old stands to develop the complex community and multi-storied structure evident in 200+ year-old stands (Nienstaedt and Zasada 1990)
The creation and retention of dead wood is immensely important in retaining and conserving other arthropod populations. Most significant among these is the loss of species in the saproxylic arthropod community through the removal of coarse woody material in intensive forest management (Heliövaara and Väisänen 1984; Siitonen and Martikainen 1994). Together with fungal associates these organisms are active in nutrient cycling and ultimately determine the fertility of forest sites (Siitonen 2001). Were outbreaks of insects controlled on the entire forest estate and the productivity represented by dead wood normally produced in native stands pre-empted through thinnings in intensive forest management, there is an expectation that saproxylic insects would be extirpated with a consequent loss of site productivity. The management implication here is clearly to preserve stand elements where these insects thrive in protected areas. Much of Canada's boreal forest remains uncut and thus permits us to conserve our natural heritage in a primeval condition. At present 12% of the forest estate is in protected areas. These may not represent all ecosystems that need protection nor is the protected area distribution known to be appropriate. The withdrawal of these forests from timber production could be managed without influencing the AAC nationally by judiciously investing in afforestation and intensive forest management in areas closer to labor pools, manufacturing centers, and transportation hubs. This policy is demonstrably sustainable because it preserves areas where primeval forests are protected, and it conserves species and processes known to be significant in maintaining the integrity of forested ecosystems. Furthermore, these withdrawals can be achieved without damaging the economic heritage that has made Canada a forestry nation.
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Email: [email protected]
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[1] Natural Resources Canada,
Canadian Forest Service, Northern Forestry Centre, 5320 – 122 Street,
Edmonton, Alberta, Canada T6H 3S5. Tel: 1 780 435 7329; Fax: 1 780 435 7359;
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