René I. Alfaro, Angus Shand, Vince Nealis, Jan Volney and Richard Fleming 1
Conifer-feeding budworms in the genus Choristoneura are eruptive species that periodically defoliate conifer forests in North America, causing growth loss and, ultimately, tree mortality. These impacts create the need for management interventions. Emerging trends in forestry require a holistic approach that considers the implications of budworm management within an ecosystem context. To minimize the negative impacts of forest practices and preserve ecosystems in perpetuity, we now seek to develop forest-management systems that emulate natural disturbances such as budworm outbreaks. This approach requires a detailed understanding of the ecosystem processes that cause the rise and fall of budworm populations, and of the changes that disturbances bring about. In this paper we review the data needs and present a system to manage budworm populations within the framework of ecosystem management.
Conifer-feeding budworms in the genus Choristoneura (Lepidoptera: Tortricidae) are found throughout the conifer forests of North America. Recurrent outbreaks of these native insects cause defoliation to pine (Pinus spp.), spruce (Picea spp.), true fir (Abies spp.), and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests, resulting in substantial loss of merchantable timber and other forest values (Bergeron et al. 1995; Volney 1998; Alfaro et al. 2001). In this paper we describe the characteristics of budworm outbreaks and resulting disturbances, and we present a system to manage budworms at the landscape level within the context of ecosystem management.
Although the genus Choristoneura contains 17 species, only a few cause economic damage to conifers in Canada. The spruce budworm, C. fumiferana (Clem.), is the most widely distributed defoliator of balsam fir and spruce forests in North America. It inhabits boreal forest, extending from the Atlantic coast westward to Alaska and north to the Mackenzie River delta in the Northwest Territories. Other important budworms include the jack pine budworm, C. pinus pinus Freeman, on jack pine; the western spruce budworm, C. occidentalis Freeman, on Douglas-fir and true firs; and the two-year cycle budworm, C. biennis Freeman, on subalpine fir and spruce.
All budworms share a similar life cycle (Nealis and Lomic 1994). Eggs are laid in masses on host foliage in mid-summer. Neonate larvae move to sheltered locations on the branches where they establish a hibernaculum, enter obligatory diapause, and pass the winter. Larvae become active the following spring, feed first on pollen in male strobili or mine needles, but soon migrate to the new expanding shoots and form feeding shelters. For most species, feeding is completed in about six weeks, usually by early July. Mature caterpillars feeding on the current year's growth cause most damage. Outbreaks can last as long as 15 years and may be synchronized over a large geographic area. Feeding by budworm larvae results in severed and partly consumed needles which, when dry, become red and give the trees a scorched look that is easily recognized in aerial defoliation assessment (Figure 1). The impact of defoliation ranges from reduced annual growth to top kill and tree mortality, depending on the number of years and severity of defoliation.
Historically, foresters have regarded budworms as a threat to projected timber yield. Outbreaks were treated with wide-scale, aerial application of insecticides to suppress populations and reduce timberlosses. Today, budworm management is recognized as an integral part of overall forest-management planning and is therefore an explicit element of the new, holistic view of natural-resource management that is currently emerging in North America (Bergeron et al. 1999; Galindo-Leal and Bunnell 1995; Kohm and Franklin 1997). Consequently, whereas past strategies for managing forest pests concentrated on developing efficient methods of pest control to maximize forest productivity at the stand level, the new paradigm recognizes the inevitable and complex role of budworms and other disturbances at the landscape level. Forest management in Canada now requires the determination of the ecological interaction between forests and budworms for the purpose of developing practices consistent with the expected forest transformations caused by natural disturbances. Specifically, we are required to develop an understanding of the ecological conditions in the forest, the physical environment, and the climatic characteristics that lead to outbreaks. Similarly, we need to understand the reactions of defoliated ecosystems so that we can measure the effects of budworms on all forest attributes and values before we prescribe interventions.
As focus shifts from local to landscape-level management, forest-management planning requires scientific information about several parameters that are common to all disturbances. Fortunately, the recorded history of defoliation by budworms, their impact on the landscape, and research about the ecology of budworms in Canada provides a reasonable information base from which to develop decision-support tools for practicing adaptive management.
An understanding of the epidemiology and population dynamics of each budworm species is necessary for assignment of susceptibility and vulnerability of forests based on attributes (e.g., species composition, age, density, etc.), severity and duration of the outbreak, and the links among budworm populations that are mediated by common climatic conditions, natural enemies, and dispersal.
Most information about population dynamics of budworms is associated with outbreaks of the spruce budworm. The resulting theories are a useful starting point for a general discussion of dynamic processes, as available discussions for other budworm species generally reference their discussions to the spruce budworm (Campbell 1993; Regniere and Lysyk 1995).
Recurrence rates can vary from a few years to hundreds of years. However, the observational database of population densities extends over part of a single 35-year cycle (Royama 1992) in the case of the spruce budworm. Only in the case of the jack pine budworm have there been observational studies that span several outbreaks. These outbreaks recur, on average, once per decade in the prairie provinces of Canada (Volney 1988).
Dendro-ecological studies offer the best-available data to reconstruct the history of previous outbreaks (Swetnam and Lynch 1993). Blais (1983), Morin et al. (1993), and Burleigh et al. (2002) described the long-term history of spruce budworm outbreaks in Canada and found that outbreak recurrence rates varied from 19 to 93 years.
Disturbance intensity refers to the degree to which biomass is eliminated during an outbreak. It ranges from the occurrence of small gaps in the forest, to widespread mortality of dominant trees and the replacement of entire stands. Stand-replacing outbreaks, i.e., infestations that cause extensive overstorey mortality, create the need for prompt salvage before weathering and fungal deterioration reduces the commercial value of the timber. Such intense outbreaks disrupt planning for sustained timber yield because the long-term wood supply is affected.
Many factors determine the amount of biomass removed by budworm during an outbreak, including length of outbreak and proportion of susceptible host trees in the stand. For example, although white spruce and balsam fir are equally susceptible to infestations by the spruce budworm, balsam fir is more likely to die than is spruce. In addition to tree mortality, repeated defoliation by budworms causes surviving trees to lose diameter, height, and volume, all of which lead to economic losses.
Impacts of budworm on non-timber values have received little attention but are important considerations in an ecosystem-based management system. Impacts are probably large but transitory. Forests dominated by large, mature host trees are highly susceptible to budworms. Therefore, forests that are protected because of their old-growth characteristics may be at greatest risk to severe disturbance by budworms.
Transitory effects of defoliation last until the forest recovers to pre-outbreak conditions, and include species shifts and changes in canopy structure, which affect the biodiversity of vertebrate and invertebrate species that inhabit the forest; the recovery process can take decades.
Susceptibility refers to the probability that a stand would be infested by budworm, while vulnerability refers to the probability of tree mortality once a stand has been infested by budworm. Several researchers have related stand vulnerability and susceptibility to the proportion of balsam fir in the stand (MacLean 1980; Blais 1983). In western Canada, susceptibility to defoliation is related to site quality, degree of crown closure, and stand age (Alfaro et al. 2001). At the landscape level, Bergeron et al. (1995) concluded that mosaics containing high proportions of mixed wood or deciduous are less susceptible to budworm outbreaks.
This refers to the size of the area affected by defoliation. This variable can range from a few hectares to millions of hectares and depends on ecosystem characteristics that render stands susceptible to defoliation. Notable are the large spruce budworm outbreaks that began in eastern Canada in the 1970s and reached 55 million ha by the mid-1980s.
The development of a plan to manage budworm requires estimates of the possible positive or negative effects on budworm populations of other disturbances operating in the area of interest. For example, in British Columbia we have uncovered a positive relationship between budworm defoliation and the likelihood of spruce beetle attack, and we have recorded episodes of defoliation by the Douglas-fir tussock moth associated with defoliation by the western spruce budworm. Tree mortality and top kill resulting from budworm defoliation creates fuel for intense wildfires (Fleming et al. in press). Root infection by Armillaria spp., has been associated with an increased risk of mortality to trees damaged by the jack pine budworm (Mallett and Volney 1990). These interactions drastically increase the disturbance beyond what would have been caused by budworm alone.
Succession is the natural change of species which occurs on a unit of land after a stand-replacement disturbance event such as fire or intense budworm defoliation. The succession pathway is determined by the severity of the disturbance, the size and diversity of the patches surviving the disturbance (biological legacies), and local ecological conditions. Disturbance by budworm is a key driver of these natural succession and renewal processes, which are integral to maintaining forest ecosystems (Fleming 2000). For example, because true firs seem more vulnerable to budworm defoliation, intensive defoliation by the spruce budworm or two-year cycle spruce budworm may cause a species shift from a balsam-spruce mixture to a predominantly spruce forest with a balsam understorey.
In this section we describe a systematic approach to budworm management under the expectation of periodic outbreaks and within the context of ecosystem management.
Budworm management requires short-term and long-term planning. Annual monitoring using pheromone traps provides an early warning of impending outbreaks. Once an outbreak is forecast, budworm management calls for a landscape analysis of resources at risk.
Site-specific and short-term information about the status of budworm populations and the risk of defoliation can be gained from pheromone traps or by using egg-mass or larval sampling. With the exception of jack pine budworm, the technology of this monitoring tool is very advanced. A network of pheromone traps over much of the susceptible area of eastern North America currently provides an early-warning system for broad-scale increases in population density of this species. However, the potential for using such a system to monitor other budworm species has not been fully investigated.
Forest-management plans typically apply to spatial units ranging from several thousand hectares to over a million hectares. Therefore, a successful planning system needs to be concerned with processes at these scales. Geographic information systems are able to provide graphical displays and statistical analyses of the resources, including area of defoliation and characteristics such as timber types, old-growth stocks, patch sizes, riparian zones, biodiversity values, connectivity for wildlife corridors, hydrological concerns, economic and social issues. The intersection of defoliation and these resource values can be examined and described.
This analysis will provide indications as to whether current landscape conditions meet desired management objectives, or are on a trajectory to do so. The objective here is to maintain the landscape within its historical range of variability for each characteristic of the landscape, including natural disturbances. This analysis will indicate possible departures from the historical range of variability of an ongoing outbreak in terms of frequency or intensity. It will also indicate if budworm may be disproportionately affecting certain components of the ecosystems, such as wildlife or fish habitat, or timber scheduled for harvest under a sustainable management plan. Excess budworm frequency or intensity, relative to historical levels, may warrant control actions if unacceptable economic or ecological damage is expected.
These forces subject the landscape unit to internal processes that confer stability as well as to inwards and outwards flows of resources. For example, fauna may move into or out of the landscape unit as a result of canopy defoliation. Other effects on flow may include reductions in seed production and tree dispersal because of feeding on staminate flowers and cones. Increased coarse woody debris and standing snags in defoliated stands will augment the habitat of some birds and small mammals.
This requires the establishment of a time frame for the projection, usually 100 years or more, and includes estimates of the outbreak recurrence rate, outbreak extent, and impacts on forest values specific to the landscape under study. The analysis includes consideration of the shifting context in which budworm disturbances will likely be affecting future forest landscapes, i.e., as a result of direct and indirect influences of global change. Scenarios are created using different levels of budworm disturbance under different future landscape conditions.
The final step of the budworm analysis system is to compare the various scenarios analyzed with the desired landscape condition. If the desired condition has not been achieved, management practices, including budworm control, may need to be modified and the analysis re-iterated. Only through this process can we reach defensible decisions regarding budworm management options.
Managing the landscape for budworm requires systems capable of reflecting the emerging vision of ecosystem management. The systems need to accommodate the analysis and projection of a range of values and ecological processes, including society's sometimes-antagonistic need for forest products, biodiversity, wildlife habitat, and other values. The systems require that land managers understand the ecological processes that operate over large spatial and temporal scales.
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Figure 1. Spruce budworm defoliation at Kledo Creek, British Columbia, 1999. Photo: Troy Lockhart, British Columbia Ministry of Forests.
1 Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, 506 West Burnside Road, Victoria, British Columbia, V8Z 1M5, Canada. [email protected]; Website: www.pfc.cfs.nrcan.gc.ca