The rise and fall of the silvicultural hypothesis in spruce budworm (Choristoneura fumiferana) management in eastern Canada

Forest Ecology and Management, 61 ( 1993 ) 171-189

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Elsevier Science Publishers B.V., Amsterdam

The rise and fall of the silvicultural hypothesis in spruce budworm (Choristoneura fumiferana) management in eastern Canada Alan Miller~,*, Paul Rusnockb aPsychologyDepartment, Universityof New Brunwick, Fredericton,N.B., E3B6E4, Canada bphilosophy Department, Universityof Waterloo, Waterloo, Ont., N2L3GI, Canada (Accepted 18 February 1993)

Abstract

The spruce budworm (Choristoneurafumiferana) is a major insect defoliator of softwood species in northeastern North America. Its commercial importance stems from periodic epidemics during which it damages extensively the spruce and fir on which the pulp and paper industry depends. From around 1920, numerous entomologists and foresters have suggested that the damage caused by budworm was due largely to the disruption of natural ecosystems by forestry practices, and could therefore be significantly reduced through appropriate forest management and silvicultural measures. However, this 'silvicultural hypothesis' was never implemented in any significant way by either government or industry. In later years, the hypothesis has been widely rejected on the grounds that it is based on fallacious assumptions about budworm-forest dynamics. In this paper, the arguments offered on both sides of the debate are assessed and the role of underlying assumptions clarified. It is concluded that, because these assumptions are relatively impervious to factual information, the debate may not be resolvable by reference to additional scientific information.

Introduction

The spruce budworm (Choristoneura fumiferana) is a major insect defoliator of softwood species in northeastern North America, balsam fir and white spruce being especially vulnerable. Periodic increases of spruce budworm populations to epidemic levels have, in the past, resulted in significant tree mortality over millions of hectares of spruce-fir forests. Since the species principally affected by the spruce budworm are also mainstays of the pulp and paper industry, a major component of the economies of the northeast, the insect has considerable economic importance. This importance has increased in recent years, due mainly to a shrinking wood supply (Baskerville, 1983; Webb and Irving, 1983 ) and an increase in the severity and extent of epidemics over the past century (Blais, 1983; Hardy et al., 1986). From around 1920, *Corresponding author.

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many entomologists and foresters have suggested that the damage caused by budworm was due largely to the disruption of natural ecosystems by forestry practices, and could therefore be significantly reduced through appropriate forest management and silvicultural practices. However, this 'silvicultural hypothesis' received little practical attention over the ensuing decades. The advent of effective aerial spraying of DDT in the 1950s pushed the silvicultural option even further into the background. Financial and political pressures surrounding spray programs in the northeast intensified greatly in the 1970s, leading to renewed interest in silviculture and forest management as alternative means of budworm control. However, the implementation of these methods has been limited due to shrinking revenues as well as scepticism concerning their efficacy (Baskerville, 1976a; Pearse, 1985; Irland, 1980). The debates over the merits of silvicultural approaches to spruce budworm management have been hindered by mixed messages emanating from the scientific community, which is split over the credibility of the underlying theory. What is interesting about this dispute is the fact that all involved have access to the same information but interpret it differently--largely, it seems, on the grounds of personal beliefs about the nature of budworm-forest ecology. The purpose of this paper is to provide a history of this difference of opinion within the scientific community, with a view to determining whether it might be resolved.

Spruce budworm-forestecology

Outbreak history Reconstructions of the history of past budworm outbreaks are based on four kinds of evidence: surveys of defoliation, budworm population sampiing, dendrochronologicalstudies (measurement of growth suppression due to budworm attack as inferred from annual growth rings), and documentary and anecdotal accounts of outbreaks. Of all these types of data, the first is probably the most complete, with fairly thorough defoliation records dating back to the late 1930s for many of the affected areas (Brown, 1970; Kettela, 1983; Hardy et al., 1986). Since population counts require considerably more resources, data are correspondingly less extensive, both in terms of time intervals and areas covered (Fleming, 1985 ). Dendrochronologicalstudies have the advantage of extrapolating data back to times when no records of defoliation were kept (Blais, 1983). Documentary/anecdotal evidence is rare but can be useful, especially if supported by other evidence (Swaine et al., 1924 ). Early reports by Tothill (1922) and Swaine et al. (1924) expressed the conviction that the spruce budworm problem had increased in importance in the century preceding the 1912-1920 outbreak. This judgment seems to have been based on a combination of evidence: observations of existing forest types,

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documentary and anecdotal accounts, some tree-ring samples, and inferences from the observed effects of logging on forest types. Of these types of evidence, it seems that the last was the most important. TothiU (1919) noted that budworm damage appeared to be heaviest in the fir-dominated stands found near abandoned logging camps. In addition, documentary and oral accounts suggested that earlier outbreaks were limited in extent and severity. Together, these observations provided some support for the claim that outbreaks were growing in size and intensity due to a shift in species composition caused by logging. With the advent of more thorough surveys of budworm defoliation in the 1930s, measurement of the severity and extent of budworm damage became considerably more accurate (de Gryse, 1947 ). Based on such surveys, it appears that budworm outbreaks have been increasing in size during this century. According to Brown (1970) and Kettela (1983), three outbreaks have occurred so far in the 20th century, each one larger than the last. The first ( 1910-1920) covered approximately 10 × 106 ha at its peak, the second (1939-1949) approximately 25× 106 ha, and the most recent (1970-1980) about 55 × 106 ha. In addition, it appears that outbreaks have been increasingly severe, judging by the higher rates of mortality for both fir and spruce and greater damage to immature stands (Blais, 1983 ). Dendrochronological studies for the most part support the thesis that outbreaks have been increasing in extent and severity, extending this observation back into the 19th century and, for some areas, even into the 18th (Blais, 1983 ). Criticisms of this view have come from various quarters on a number of grounds. Morris ( 1963 ) suggested that there was no evidence to support the claim, a contention repeated by BaskerviUe (1975a), who argued that budworm epidemics were not considered noteworthy in the past, since balsam fir was not then a commercially valuable species. With the development of a pulp and paper industry in the 20th century, fir became economically important, and the depredations of the budworm thus attained greater significance and hence increased visibility. Another line of criticism was put forward by Regni~re ( 1985 ) and Royama (1984), who observed that the main sources of data for the reconstruction of outbreak history (defoliation surveys and tree-ring studies) showed only the effects of very high population levels. More subtle population changes are therefore outside the scope of these methods. In particular, Royama argues, the apparent increase in the extent and severity of outbreaks may be the result of random variations in a relatively unchanging underlying process. Given the lack of resolution of the damage data and the small number of data points (i.e. past outbreaks), Royama concludes that the claim that outbreaks have been increasingly severe or frequent is unfounded. There are thus two strongly opposed views on outbreak history. The first suggests that the infestations of spruce budworm and the consequent damage

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to the forest have changed significantly in a relatively short time. It finds support in the defoliation records from 20th century outbreaks, as well as in the reconstruction of past outbreaks through radial growth studies and documentary evidence. The second view holds that historical reconstructions lack the precision required to make definitive statements. In this latter view, more evidence is needed before such changes can be established as having occurred. Without such evidence, there is no reason to suppose that the present budworm-forest relationship is markedly different from that which existed in the past.

Historical evidence of changes in budworm-susceptibleforests The earliest formulations of the silvicultural hypothesis identified changes in species composition, in particular an increased representation of fir, as the primary cause of the increased importance of budworm attacks (Tothill, 1919; Swaine et al., 1924). The evidence available at that time to support the claim of a widespread change in species composition was very limited however. In general, there are considerable difficulties involved in any attempt to reconstruct changes in forests in recent history. Records are, for the most part, sketchy, and other methods of reconstructing past species composition are not well developed. Most accounts are therefore quite general and note only larger qualitative features of changes to forests. Goldsmith (1980), for example, uses documentary evidence to support the claim that the forests of Nova Scotia were significantly changed by human activity as early as the 18th century, a trend which has continued unabated, and perhaps even intensified, to the present. Regier and Baskerville (1986), arguing from evidence of changing utilization standards in forest industries, claim that the forests of New Brunswick have been continually degraded, in terms of both quantity and quality of timber resources, from at least 1800. Recognition of this degradation has become commonplace in recent government literature (Baskerville, 1983; Canadian Forest Service, 1984). Changes in species composition may also be inferred from records of past timber cutting. Gordon ( 1985 ), for example, notes: 'The best stands of almost pure fir which occur in Quebec and northern New Brunswick might give the impression that this is predominantly what that country has produced. However, historical accounts of much of this area indicate that enormous volumes of spruce have been cut out there in the past...'. Historical accounts thus render credible the idea that the budworm susceptible forests of the Northeast have undergone significant changes in species composition. Extrapolations and inferences from relatively short-term observations also lend support to the thesis. It is well documented in the literature, for instance, that certain cutting methods, notably clear cutting, favor fir regeneration (Westveld, 1931, cited in Blais, 1968; Prebble and Morris, 1951;

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Hatcher, 1960; Blais, 1983). Other elements of forestry practice thought to increase the representation of fir include fire protection (as fir does not regenerate well after fire) and the spraying of insecticides and herbicides (Blais, 1974, 1983). Natural agencies such as birch dieback and the European spruce sawfly may also, by attacking competing species, increase the representation of fir (de Gryse, 1939; Blais, 1983; Gordon, 1985).

The ecological role of the spruce budworm The function of the spruce budworm in the forest ecosystems of the Northeast is a complicated question. Given the vastness of the affected areas, and the wide variety of forest, topographical and climatic conditions found across this range, as well as the different histories of human intervention in these forests, it should come as no surprise that a comprehensive view of spruce budworm-forest ecology has been difficult to achieve.

General relationships Swaine et al. ( 1924 ) provided the first comprehensive theory of the ecological role of budworm in spruce-fir forests. In their view, budworm outbreaks occur when the representation of balsam fir in the forest is high. This condition was generally due, they thought, to ecological disturbance (i.e. wind, fire, beetle attack and, most significantly, selective logging). Outbreaks resulted in more dead fir than spruce, allowing the spruce to increase its representation in the stand, and accelerating the conversion of mixed spruce-fir to pure spruce stands. This theory, developed mainly through experience with outbreaks in the southern and central range, has a certain plausibility for some forest types. However, their account does not deal with all of the conditions encountered in the budworm's range, and can be regarded at best only as a partial explanation. More extensive experience of outbreaks has led some entomologists to conclude that the role of spruce budworm in the forests varies considerably with each different forest and the physical conditions across its range (Mort, 1980; Blais, 1983, 1984, 1985a, 1985b; Hardy et al., 1983; Hardy, 1985). The contention of Swaine et al. that budworm outbreaks accelerate the conversion of spruce-fir forests to pure spruce was brought into question by de Gryse ( 1944, 1947 ) who noted that, in some stands, outbreaks did not result in any appreciable increase in the spruce content. Indeed, he noted that the fir content actually increased under some conditions (cf. Prebble and Morris, 1951 ). BaskerviUe (1975a,b) and MacLean (1984) reported similar results. Evidently caution is in order when discussing the impact of budworm outbreaks on subsequent species composition. As MacLean (1984) and Blais (1985a) justly remark, relationships observed under one set of conditions

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will not necessarily apply to others, and great care should be taken not to overgeneralize from a limited set of results. It is almost certain that the views of Swaine et al. (1924) and BaskerviUe (1975a,b) err in this respect. More probable is the hypothesis that the function of spruce budworm in the forest varies significantly with forest type and climatic conditions. Mort (1980), for instance, noted the differences between the fir-dominated forests of northern New Brunswick and the red spruce-fir forests of central and southern New Brunswick and Maine. The outbreak experiences in these two forest types have been quite different, and the role of spruce budworm most probably varies between the two. Finally, Blais (1985a), Hardy ( 1985 ) and Hardy et al. ( 1983, 1986) noted significant differences in budworm-forest ecology associated with variation in climatic conditions.

Stability and imbalance Two quite different ideas of ecological stability have been prominent in discussions on the spruce budworm problem. The older of the two is rooted in the notion of site-specific climax forest types which persist unless disrupted by external forces. Forests possessing this type of stability do not change radically in appearance over time. Instead, the same species composition is maintained and the forests are constantly renewed in small increments. Such associations are thought to be relatively immune to catastrophic disruption by disease, insect attack and various other natural dangers. When these associations are disrupted, however, they may become more susceptible to disease and insect attack (Zon, 1908; Swaine et al., 1924; Westveld et al. 1950; Graham, 1951, 1956, 1959; Prebble and Morris, 1951; Westveld, 1953a; Balch, 1958 ). The presettlement forests of the northeast (i.e. as they existed before European settlement) were thought to possess this kind of stability and with it the inherent resistance to insects and disease, including budworm. Losses may have occurred due to budworm attack, but they would be isolated, small and relatively non-disruptive, except perhaps in transitional forest types which followed major ecological disturbances (Swaine et al., 1924). In this view, the existence of a budworm problem is presumptive evidence of previous ecological disruption. This view of stability came under increasing scrutiny in the 1940s and 1950s, when outbreaks were observed in areas unaffected by human activity, suggesting that ecological disruption was not a prerequisite for budworm outbreaks (Prebble and Morris, 1951 ). It was further noted that outbreaks frequently had little or no effect on subsequent species composition (de Gryse, 1944, 1947 ). Combined, these observations suggested a new notion of stability, one in which periodic devastation of stands by budworm attack was an inherent part of the ecology of fir-spruce forests. Although the forests change radically in age-class distribution, their species composition remains relatively stable. In short, budworm attack ensures that mature and overmature

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fir-spruce stands are replaced by younger stands of the same composition (Morris, 1963; Blais, 1964; Baskerville, 1975a,b; MacLean, 1984). The existence of a budworm problem, in this view, provides no evidence of ecological instability; the notion of a budworm-proof forest is chimerical (Baskerville, 1975b; Irland, 1980). A compromise between these two versions of stability has appeared in some recent accounts. Hardy (1985), Hardy et al. (1986), and Blais (1985a), for example, suggest that the second type of stability is characteristic of many forests in the northern range while, further south, forestry activity has, by inducing changes in species composition, rendered formerly resistant forests more susceptible to budworm.

Damage and population theory Early workers appear to have made little distinction between population increases on the part of spruce budworm and damage suffered by host species. For instance, accounts of past outbreaks generally refer to observed defoliation of trees rather than directly to population counts. Since heavy damage caused by budworm feeding only occurs in conjunction with high population levels, this seems to be a reasonable practice. On the other hand, population counts obtained through field studies have shown that budworm numbers may be high, especially on spruce, before damage is obvious. Accordingly, later researchers have discerned two separate problems in budworm/forest ecology. The first is the question of the forest conditions which affect fluctuations in budworm populations, while the second is that of damage sustained by host species when attacked. Thus susceptibility refers to the characteristics which affect the likelihood that a stand will be attacked by spruce budworm, while vulnerability refers to the conditions which affect the degree of damage sustained by trees once attacked (Blum and MacLcan, 1985; Witter, 1985 ).

Factors affecting susceptibility Early speculation stressed the importance of food supply (presence of mature fir over wide areas) as the main cause of the development of outbreaks. A decline in food supply through host species mortality was correspondingly thought to explain the collapse of outbreaks (TothiU, 1922 ). More extensive experience of outbreaks caused researchers to modify this view. For example, high concentrations of mature fir are found in some regions (e.g. the Gaspdsie or Western Ontario) where outbreaks are relatively rare. Some outbreaks have collapsed with plenty of potential food still available (Blais, 1985a). Finally, outbreaks were found to persist in pure spruce stands containing little or no fir (Blais, 1983 ). A theory which seeks to explain budworm outbreaks exclusively in terms of a high proportion of fir is therefore not consistent with the observed phenomena. Nevertheless, food supply is still thought by some sci-

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entists (e.g. Blais, 1985a) to be a major factor in the origin and collapse of most outbreaks, an opinion which seems plausible given that the outbreaks result in the death of, on average, more than 80% of the fir and 30% of spruce in mature stands (Blum and MacLean, 1984). Later work elaborated the importance of climatic factors. Swaine (1928) and Graham (1939), for instance, suggested that favorable weather conditions might be critical to the development of outbreaks. This idea was further explored by Wellington et al. (1950), Wellington (1952), Pilon and Blais ( 1961 ), Greenbank (1963) and Hardy (1985). Generally speaking, the development of outbreaks was found to be associated with relatively dry, warm summers recurring over several years. Weather was also thought to be responsible for the collapse of some outbreaks before excessive damage occurred, i.e. before the food supply had been destroyed. The theory of budworm population dynamics implicit in this view, the socalled 'double equilibrium theory' (Morris, 1963; Mattson et al., 1988 ), seeks to account for only two levels of budworm population: very high (epidemic) and very low (endemic). Further work on budworm population dynamics based primarily on data gathered as part of the Green River Project (Morris, 1963 ) has led Royama (1984) to thoroughly reject this view. While some relatively crude data from defoliation surveys and tree-rings suggest that there are only two levels of budworm population, Royama claims that direct population counts reveal a continuous, cyclical fluctuation. He argues further that the available data do not support the claims of a causal connection between weather patterns or food supply and budworm population fluctuations. Instead, the pressure of predators, parasites, and a complex of unknown causes termed the 5th agent, which may involve a strain of Bacillus thuringiensis (Royama, 1990), is sufficient to explain the observed population fluctuations. Two different theories have thus been proposed to account for the fluctuations ofbudworm populations. For the first, patterns of gross damage due to budworm are of primary interest; they constitute the 'budworm problem'. For the second, the notion of budworm 'outbreaks' is too vague, since it refers only to obvious damage and ignores all other changes. These two approaches should probably be seen as complementary rather than competing. Detailed analysis of the microdynamics of budworm populations may provide important information, but will be relatively useless without a broader theoretical framework. For example, Royama's theory suggests that food supply and weather are not significant factors in the development of outbreaks. However, his data were gathered in areas where these factors were favorable to budworm populations. Without data from areas where, for example, host species are relatively scarce, or where the climate is particularly harsh, general pronouncements on the role of food supply or climate in the dynamics of budworm populations seem insufficiently supported.

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Factors affecting vulnerability The factors identified by early workers as contributing to the vulnerability of forests to spruce budworm damage have, by and large, been confirmed by subsequent research (van Raalte, 1972; Witter et al., 1984; Blum and MacLean, 1985 ). Early accounts noted the following factors as important in determining the degree of damage sustained: species composition (the higher the proportion of host species, such as balsam fir and spruce, the more damage sustained); stand vigor (the more vigorous the growth of host species, the less damage sustained); stand age (mature and overmature stands receive the most damage); size of susceptible areas (large, contiguous areas of susceptible forest suffer more damage than small, isolated ones); stand structure (stands where softwoods are overtopped by hardwoods suffer less damage than stands where softwoods overtop the hardwood canopy). Additional factors leading to greater damage have also been identified through subsequent research. These include: stands with more sun foliage (i.e. needles growing in full sunlight in the upper one third of dominant and co-dominant crowns) (Baskerville, 1975b); more flowering balsam fir (van Raalte, 1972); a higher density of host species (van Raalte, 1972; Witter et al., 1984); on abnormally wet or dry sites (Westveld et al., 1950; Witter et al., 1984); downwind of existing infestations; growing at elevations under 700 m and south of 50 ° latitude (Witter et al., 1984). In general, there is little controversy over the factors which affect the amount of damage sustained by different stands of trees when attacked by spruce budworm. Data are relatively abundant, and there are few speculative elements in the proposed relationships. Silvicultural remedies

What we have referred to as the 'silvicultural hypothesis' was developed most fully within the context of the climax theory of stability. In essence, it states that, since forestry practices have led to increasing susceptibility and vulnerability of the affected forests, these practices should be altered so as to minimize the conditions which favor budworm damage. This simple statement lies at the heart of all silvicultural or forest management solutions to the budworm problem. Most practical recommendations for minimizing damage due to spruce budworm are directly related to factors which have been identified as affecting the vulnerability of forests. The most important of these has been the presence of stands with a high proportion of mature balsam fir, over large, contiguous areas. Accordingly, many of the suggested silvicultural solutions have as their goal the elimination of such conditions. Reduction of the proportion of fir in susceptible stands through either selective cutting or management of regeneration (through planting, weeding/girdling and thinning) has been a constant theme through 70 years of recommendations (Swalne and Tothill, 1918; Craighead, 1922; TothiU, 1922; Swalne et al., 1924;

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Balch, 1946; de Gryse, 1947; Prebble and Morris, 1951; Graham, 1959; Frank and Blum, 1978; Frank, 1979, 1985; Robinson, 1983; Hardy, 1984; Blum and MacLean, 1985). Forest management strategies have also been aimed at breaking up large areas of susceptible forest (Graham and Orr, 1940; Balch, 1946; Blum, 1985; Blum and MacLean, 1984, 1985 ); at scheduling timber harvests so that fir is cut on a short rotation and does not become overmature (McLintock, 1947; Westveld, 1946, 1953a, 1980/1944; Blum and MacLean, 1984, 1985); and at managing spruce-fir forests for maximum vigor (Craighead, 1925; Westveld, 1953a; Balch, 1965; Blum and MacLean, 1985 ). Another series of recommendations addresses the problem of preventing the occurrence or spread of outbreaks, rather than attempting to limit the damage caused by outbreaks once they occur. Heimburger ( 1945 ) suggested that natural ecological and topological barriers should be cultivated and/or maintained in order to limit the spruce budworm to relatively isolated ranges. Hardy (1984), relying on the idea that outbreaks develop and spread from isolated epicenters, proposed that silvicultural measures be aimed specifically at the areas where epicenters usually occur. By reducing the representation of host species in these regions, outbreaks across the whole range of budwormsusceptible forest might be prevented. While measures aimed at reducing the vulnerability of forests to budworm may be pursued on a factor by factor basis, a more holistic approach is followed by some. One version of the silvicultural hypothesis is based upon the idea that, previous to human intervention, most of the forests of the Northeast were relatively immune to budworm epidemics. Accordingly, the structure and composition of these forests can be used as a guide for silvicultural measures in controlling the budworm, the basic idea being first to restore, and then maintain, the stability and balance of the presettlement forests. This approach was suggested by the work of Zon (1908), and worked out in more detail by Swaine et al. (1924), Westveld et al. (1950), Westveld (1953b, 1980/1944), Graham (1951, 1956, 1959), Frank (1979, 1985), and Hardy (1984). Generally, this strategy would require intensive forest management and silviculture. Timber harvests are selective and at relatively short intervals, with the aim of maintaining a continuous, all-aged forest of appropriate (climax) species composition. Although proposed silvicultural solutions have been accepted as credible by those who subscribe to some version of the climax view of stability, there remains some disquiet about the available evidence. Prebble (1955 ), for instance, pointed to the lack of empirical support for the proposed remedies: "...it should be noted that recommendations for reducing forest insect damage by means of silvicultural management are, in Canada, based on deductions drawn from outbreaks in various types of natural stands. At the present time, it is not possible to make a critical comparison of the population dynamics of destructive insect species in natural stands, on the one hand, and in stands

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managed for maximum resistance to insect damage, on the other hand, simply because stands of this latter type do not exist in Canada' (p.321). The lack of data should not be a deterrent to implementing silvicultural suggestions, however, as he argued elsewhere (Prebble, 1951 ): 'Interrelations between insects and forests under silvicultural management may only be outlined in broad terms until such forests are actually in existence. However, to defer application until full conclusions of silvicultural and entomological research are available is not only indefensible on practical grounds, but would almost certainly hold back, if not prevent, the possibility of progress in applied silviculture' (p. 12). Prebble's opinion that enough was known to justify implementation of silvicultural research on a large scale was also the consensus view among budworm scientists in the mid 1950s. Nonetheless, despite some practical trials (such as those pursued at the Green River Project in Northwest New Brunswick), the large-scale researches proposed were never implemented. The advent of new insecticide technology (DDT and the larger airplanes that became available in large numbers after World War II), provided a practical alternative to silvicultural methods and, doubtless, had a significant influence on the lack of action. Nonetheless, opinion remained strong that, though DDT might provide an effective short-term or emergency measure, silvicultural and forest management strategies would provide the only long-term solution (Leslie, 1945; Balch, 1952).

Criticisms of the silvicultural hypothesis Although silviculture was widely seen as the most effective long-term means of managing the spruce budworm problem in the 1950s, signs of doubt were evident even at this time. For instance, Prebble and Morris (1951) concluded that Swaine placed too much importance on human activity by assuming all budworm outbreaks resulted from logging. This was very likely the beginning of a considerable change of opinion as the cyclical view of budworm-forest interactions gathered momentum. Morris ( 1963 ), for example, wrote that: 'Periodic outbreaks of the spruce budworm are natural occurrences associated with the maturing of extensive areas of balsam fir and with climatic variation .... The influence of man on the forest is not a prerequisite to outbreaks; nor is there any evidence that man's influence has made them more frequent' (p. 311 ). These sceptical views about the silvicultural hypothesis were developed further by Baskerville (1975a,b; 1976a,b) who concluded that not only were its underlying assumptions unsubstantiated, but that remedies based on them would produce effects opposite to those desired. In his view, the budworm-forest system had cyclical stability, one in which mature fir-spruce forests were killed by budworm and replaced by young forests of similar composition. To attempt to change the forest composition to

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one less vulnerable to budworm would, he thought, run counter to normal ecological trends. The critique went further, questioning both the techniques and the goals of proposed forest management and silvicultural solutions. Relying on the experiences of selective cutting at the Green River Project (Croome, 1970), Baskerville remarked that, although selective cutting had considerably increased the proportion of spruce relative to fir in the overstory, the advance regeneration remained predominantly fir. He was therefore sceptical about the efficacy of selective cutting in changing species composition (Baskerville, 1975b). Finally, he contended that, even if silvicultural methods attained their goal (i.e. an all-aged, selection management forest), this would render the forests more rather than less vulnerable, as it would mean the preservation of large areas of mature forest, ideal conditions for the survival of budworm. In short, he argues that the budworm problem is an inherent, not a man-made feature of the forest, and that the silvicultural suggestions have, therefore, little hope of success. Baskerville is not alone in his scepticism, his views being shared to a greater or lesser extent by Blum and MacLean (1985), Fowle (1983), MacLean (1984) and Webb and Irving (1983). Baskerville's challenge to the silvicultural hypothesis, however, may be incorrect in many ways. It fails to explain the increasing severity and extent of outbreaks (Blais 1983, 1985a); it runs counter to evidence which suggests that the forests have been appreciably changed through human activity (Lorimer, 1977; Gordon, 1985 ); successful applications of selection management have been reported (Frank, 1985 ); and, finally, his conclusion that allaged selection forests would be highly vulnerable to budworm is as untested as the opposite contention. Despite the fact that both sides of the debate concerning the silvicultural hypothesis have access to the same information, they arrive at quite different conclusions. It is important to note that the data required to decide the question do not presently exist, and that there is little prospect of obtaining them. On the one hand, records are not sufficient to yield a detailed account of forest and outbreak history and, on the other, large-scale testing of silvicultural methods is unlikely due to the expense, time, and effort involved. Given these apparently intractable limitations on scientific information, it seems likely that other factors play an important role in determining differences of scientific opinion. While political factors (e.g. involvement with or opposition to spray programs) may account for some differences, it is very doubtful that they can account for more than a few. Instead, our research suggests that general beliefs concerning the nature of budworm-forest ecology may have a decisive influence, one side accepting the silvicultural hypothesis and the other rejecting it on the basis of the same incomplete data. While there is much variation in the general beliefs of budworm scientists, it is possible to describe those relevant to the debate over the silvicultural hypothesis in general terms. For the early proponents of the hypothesis, basic support was derived from the theory of climax species associations that were

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thought to be inherently resistant to insect outbreaks, disease, and most other violent disruptions. From this view, forest ecosystems had a number of properties in common with organisms, including the abilities to reproduce, to resist attack, and to repair damage when it occurs. It is thus very plausible to speak of 'forest health', or of 'the balance of nature'; violent changes in ecosystems were thought to occur only when extrinsic factors were able to overcome natural controls. In particular, the existence of a 'budworm problem' pointed to some previous disruption of the ecosystems affected. Other evidence suggested forestry practices as the culprit. That human activity is capable of disrupting natural ecosystems is abundantly proven through agriculture, but also supported by examples where forestry practice has given rise to unforeseen consequences, including outbreaks of insect pests (Pickett, 1949; Graham, 1956; Knight, 1976; Carrow, 1985). Finally, the measures recommended by many proponents suggest that they believe that forestry practices can be dramatically changed in order to bring them into line with ecological requirements (e.g. Westveld, 1953b; Hardy, 1984). Opponents of the silvicultural hypothesis are sceptical on all of these points. The ideas of a balance of nature or of forest health do not seem to be supported by evidence, but rather to arise from preconceptions. The existence of violent disruptions like budworm outbreaks may be perfectly 'natural' and require no particular explanation. Rather, periodic devastation may be part of the budworm-susceptible forest ecosystems, just as periodic destruction by fire is characteristic of other ecosystems. The fact that some human practices can create insect or other problems provides no particular support for the claim that forestry practices cause the budworm problem. Finally, critics of the silvicultural hypothesis may be struck with the impracticability of the suggested measures, especially given economic and other constraints on forestry practice (e.g. Baskerville, 1976b). Conclusions

The forest management and silvicultural measures suggested for dealing with the spruce budworm problem are all derived from observations of the damage suffered by existing stands. Despite repeated suggestions (e.g. Kemp, 1980; Prebble, 1959), no large-scale, long-term experiments have been carried out, the result being that there is little empirical basis from which to develop silvicultural methods. Speaking of proposed silvicultural methods of budworm control, Prebble (1975 ) wrote: 'Regardless of how sound or practicable these general recommendations may have been, little deliberate attempt to implement them has been made in the intervening years which have witnessed two massive waves of outbreaks since the 1920s' (p. 82). This judgment remains an accurate reflection of current practices, although it should be noted that this neglect of silviculture is not unique to the budworm

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situation. Generally, the forestry industry in eastern Canada has been, and continues to be, characterized by a lack of significant expenditure on silviculture and forest management, particularly with regard to Crown lands (Weetman, 1982, 1987, 1989, Pearse, 1985). De Gryse (1947)thought that this pattern would of necessity change: 'Operators generally have not arrived at a realization of either the necessity or the advantages of rational silvicultural practices. In other words, they do not consider them profitable under the present conditions. This is no doubt due, primarily, to the fact that virgin stands are still available. As time goes on and as the depletion of the forest progresses, the present attitude towards silviculture in commercial forestry will gradually become altered...' (p. 396). Despite this optimism, economic imperatives apparently still do not favor the type of forestry considered by de Gryse. Indeed, industrial demands on the forest appear to be exacerbating the spruce budworm problem. As Blais ( 1983 ) notes: 'The forest industry in eastern Canada is largely based on the exploitation of softwoods, but ironically the present methods of cutting and protecting fir-spruce stands result in their increasing susceptibility and vulnerability to budworm. Reducing the occurrence of extensive stands of fir at a time when man's interventions in the forest result in increasing the presence of this species poses a real problem' (pp. 545-546 ). Forestry practice continues, therefore, despite 70 years of exhortations, in the direction opposite to that suggested by the silvicultural hypothesis. This longstanding situation clearly presents a dilemma for the budworm scientist. The disaster predicted by the silvicultural hypothesis may indeed be occurring (the problem is apparently continuing to worsen) but technical ingenuity and the mobility of capital allows the economic effects of resource degradation to be minimized. Thus the suggestions of the silvicultural hypothesis, whatever their merits, may never be reinforced by economic incentive, as was often supposed in the past. This unwillingness on the part of government and industry to make the major commitments implied by the silvicultural hypothesis may be prudent given the differences of opinion among budworm scientists about the efficacy of proposed remedies. The actions prescribed require great expenditures of effort and money but do not promise any appreciable short-term returns. The risk of uncertain results due to such factors as fire, or migration of large numbers of moths from surrounding (untreated) areas, may simply be too great to justify the required expenditures (Batzer, 1976 ). Finally, a series of institutional and administrative arrangements discourage expenditure on silviculture in general, including '...property tax systems that generate disincentives to forest enhancement.... forest tenure arrangements that are insecure or dampen regional timber markets, and...regulations that impede or distort silvicultural effort' (Pearse, 1985 ). Given the complexity of budworm dynamics, the dearth of relevant empirical studies and the role of personal beliefs about forest ecology, it is possible that these contentious issues may never be

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18 5

resolved. In other words, attempts to evaluate the efficacy of large-scale silvicultural manipulation may face us with the limitations of scientific methods in resolving complex environmental problems. Perhaps the 'spruce budworm problem' is what Weinberg ( 1972 ) refers to as a 'trans-science' problem, a situation in which 'scientific' evaluation of silvicultural, or other remedies, may not be feasible. If this is so, then the management of the spruce budworm problem will remain dominated by economic and political considerations for the foreseeable future. Acknowledgement This paper was prepared with the help of a grant (No. 410-90-0012 ) from the Social Sciences and Humanities Research Council of Canada.

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