Spruce budworm defoliation and growth loss in young balsam fir: relationships between volume growth and foliage weight in spaced and unspaced, defoliated and protected stands

Forest Ecology and Management 179 (2003) 37–53

Spruce budworm defoliation and growth loss in young balsam fir: relationships between volume growth and foliage weight in spaced and unspaced, defoliated and protected stands Harald Pienea,*, David A. MacLeanb, Mike Landrya a

Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, P.O. Box 4000, Fredericton, NB, Canada E3B 5P7 b Faculty of Forestry and Environmental Management, University of New Brunswick, P.O. Box 44555, Fredericton, NB, Canada E3B 6C2 Received 9 April 2002; accepted 2 September 2002

Abstract Foliage weight (FW) and tree volume, volume increment (VI), and specific volume increment (SVI) were examined throughout a spruce budworm (Choristoneura fumiferana (Clem.)) outbreak cycle from 1976 to 1984, for young balsam fir (Abies balsamea (L.) Mill.). Treatments were spaced and unspaced, defoliated and protected (annual insecticide spraying to prevent defoliation). Six years into the outbreak, FW in defoliated plots reached its minimum levels, representing a reduction of 83.3 and 84.3% for spaced and unspaced stands, respectively, compared with the protected stand. For surviving trees, FW recovery was rapid and substantial (threefold to fivefold over 3 years), due to the release of suppressed buds. Still, at the end of the outbreak, FW was reduced by 37.5 and 68.5% for spaced and unspaced stands, respectively. VI was reduced by 92.5 and 91.1% and SVI by 92.0 and 89.3% for spaced and unspaced stands, respectively, compared with the protected stand. This severe budworm outbreak reduced growth rates by about the same level, regardless of initial differences in growth rates and density. By the end of the outbreak, volume was reduced by 32.6 and 54.1 m3/ha for spaced and unspaced stands, respectively, or a decrease of 51.7 and 35.7% compared with the protected stand. Relationships between VI and FW were, in general, linear for spaced trees and a mixture of linear and non-linear for unspaced trees. On an individual-tree basis, VI: FW relationships were strong at the beginning and end of the spruce budworm outbreak, but weaker in the middle. Combining all years, on a per hectare basis, the relationships between VI and FW were strong (r2 ¼ 0:78 for spaced and 0.80 for unspaced stands), and still held despite defoliation. Published by Elsevier Science B.V. Keywords: Abies balsamea; Choristoneura fumiferana; Defoliation; Foliage weight; Volume

1. Introduction *

Corresponding author. Tel.: þ1-506-452-3537; fax: þ1-506-452-3525. E-mail addresses: [email protected] (H. Piene), [email protected] (D.A. MacLean). 0378-1127/02/$ – see front matter. Published by Elsevier Science B.V. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 4 8 4 - X

Spruce budworm (Choristoneura fumiferana (Clem.)) is a common defoliating insect across northern North America, from Newfoundland in the east to British Columbia in the west, and south to the

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midwestern United States. In the 20th century, three major spruce budworm outbreaks occurred in the spruce (Picea sp.)–balsam fir (Abies balsamea (L.) Mill.) forest of eastern North America (Blais, 1983). The most recent outbreak started in the late 1960s, peaked around 1980, and then declined. An estimated 37 million hectares of spruce–fir forest were defoliated by spruce budworm in 1981, and volume losses amounted to an estimated 44 million cubic meters per year from 1977 to 1981 (Sterner and Davidson, 1982). This spruce budworm outbreak was characterized by a particularly severe infestation over large areas in Cape Breton Island, NS, and in Newfoundland. Losses caused by spruce budworm have negatively influenced timber supply in many regions during a time of increased demand for wood (MacLean, 1985). Numerous field studies have attempted to quantify the relationship between insect defoliation and growth loss and, to a lesser extent, recovery after insect defoliation has ended (Kulman, 1971; MacLean, 1985). However, it has been difficult to obtain accurate relationships because corresponding data on tree growth without defoliation were generally not available during outbreak periods. Results have been variable due to variation in the intensity, distribution, and history of insect attacks, as well as variation in the age, species, and physiological condition of host trees. Furthermore, insect damage has typically been assessed based on percentage defoliation, a measure of the proportion of foliage removed. As tree growth is functionally related to foliage remaining on a tree after damage, and not to the proportion removed, we seek relationships between volume growth and foliage weight (FW) under a variety of defoliation scenarios. Such information will serve not only as a basis for constructing dynamic models linking growth to FW and defoliation, but also as a validation tool for other models. Given the need for more detailed relationships between volume growth and defoliation, a study was started in 1976 in 25- to 30-year-old balsam fir stands on the Cape Breton Highlands, NS, Canada (Piene et al., 1981; Piene, 1989a,b). The main objective was to relate volume increment (VI) to FW under different defoliation scenarios created by foliage protection (Piene, 1989a,b). The specific objectives of the present paper are to (1) quantify FW and VI

and (2) relate VI to FW in spaced and unspaced, defoliated and protected stands throughout a spruce budworm outbreak.

2. Methods 2.1. Study area The study area was located in the southernmost part of the Cape Breton Highlands, NS, Canada. The area was covered with an almost pure 25- to 30-year-old balsam fir forest, parts of which were spaced in 1971 to approximately 2:4 m  2:4 m. For a detailed description of the study area, see Piene et al. (1981). Spruce budworm populations started to increase in the study area in the early 1970s and reached outbreak levels in 1976, when about 85% of the current foliage was destroyed, with no backfeeding on older age classes of foliage (Piene, 1989a). Due to a dramatic increase in spruce budworm populations in 1977– 1978, the current foliage was destroyed in the early stages of shoot elongation in both years, combined with severe backfeeding on older age classes. The spruce budworm population decreased thereafter and only current foliage was consumed until the population decreased to low levels in 1983. An exception was in 1980, when defoliation also occurred on 1-year-old foliage (Piene, 1989a). Tree mortality began in 1980 and by 1984, 40 and 18% of the trees were killed in the defoliated spaced and unspaced stands, respectively (MacLean and Piene, 1995). 2.2. Plot establishment Eight 0.025 ha plots were established in 1976 within 300 m of each other on similar forest sites (Piene et al., 1981). Four of the plots were in a spaced stand and four in an adjacent unspaced stand. An insecticide spray program was started in the early spring of 1977, 1 year after the start of the budworm outbreak. Four plots, two spaced (P1 and P2) and two unspaced (P3 and P4), were protected (sprayed with insecticide) to serve as controls for the remaining two defoliated, spaced plots (D1 and D2) and two defoliated, unspaced plots (D3 and D4). Each plot was sprayed with Dylox from the ground in the early spring of each year from 1977 to 1984, prior to spruce

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Annual estimation of FW (in the present study FW means total FW of all age classes unless otherwise noted) by age class was conducted on a subset of trees in each experimental plot representing the full range of DBH classes present. A total of 14, 10, 10, and 10 detailed sample trees were selected from P1, P2, P3, and P4, and 19, 13, 24, and 13 trees were selected in D1, D2, D3 and D4, respectively (Piene and MacLean, 1999). On these trees, FW was estimated each year from 1976 to 1984 for plots P1, P3, D1, and D3, but only from 1980 to 1984 for plots P2, P4, D2, and D4. Estimation of FW was based on a non-destructive sampling procedure (Piene, 1983), because the need to keep the sample trees intact precluded destructive sampling. To account for variability in foliage parameters with tree height and aspect, one branch was sampled from each whorl starting at the top and continuing in a spiral down the full crown length. The same branches were sampled each year, and foliage parameters were sampled from all second-order branches (laterals growing from the main axis) (Piene, 1989a; Piene and Eveleigh, 1996). Estimation of FW by age class was based on sampling four key foliage parameters annually: mean needle length, mean shoot length, number of shoots per branch, and percent needlefall (protected plots) or defoliation (defoliated plots) (Piene, 1983). For a description of the foliage sampling procedures, see Piene (1989a) and Piene and MacLean (1999). FW for an age class for a particular year on each sample branch was based on interrelationships between average needle length, density, weight, and shoot length, multiplied by the total number of shoots in that age class, and reduced by needlefall or defoliation (Piene, 1983). FW for each sample branch was then obtained by summing the weights for the individual age classes. This has proved to be a reliable method for estimating FW of balsam fir (Piene, 1983). Total length of each sample branch, from 1976 to 1984, was also determined.

plot each year based on the branches from the detailed sample trees within that plot. As there were no significant differences between the equations developed for P1 and P2 (spaced plots), equations developed for P1 were also used for P2 because of the larger sample size. Similarly, equations developed for P3 were also used for P4 (unspaced plots). Spacing resulted in substantial differences in crown length and foliage density and hence in different FW to branch length relationships. A strong curvilinear relationship existed between FW and branch length, with mean r2’s (log– log transformation) of 0.89 and 0.84 for the spaced and unspaced plot, respectively. The bias of taking the antilogarithms of the log–log transformation to obtain FW was corrected according to Beauchamp and Olson (1973). When estimating FW by age class over time, we wanted as much as possible to account for the actual changes that occur in the field: the production of new foliage each year, combined with the decrease in FW, due to needlefall, of older age classes as they age. Details of the procedure used to determine age-specific FW per branch were described in Piene and MacLean (1999), but in general it consisted of: (1) estimating FW per branch based on an equation relating FW to branch length and (2) partitioning FW into age classes based on percentages calculated from detailed sample branches. FW per age class per tree was then obtained by summing across whorls. For small trees in the unspaced plots (DBH < 6 cm), FW was obtained by multiplying FW per age class for a branch of average length by the number of branches in the whorl, and then summing across the whorls. We also compared the results of our calculations with those of Baskerville (1965), who destructively estimated FW for trees from balsam fir stands of similar age and density. Mean FW per tree for all trees in the protected plots P1, P2, P3, and P4 in 1984 was 1.86 kg per tree, which compares well with 1.75 kg per tree, or a difference of 6.3%, based on Baskerville’s (1965) equation. FW was not estimated for the trees that died during the outbreak in the unspaced plots. These were, almost without exception, suppressed trees with DBH < 6 cm.

2.3.1. Protected trees Allometric relationships between FW per branch and branch length were developed for each protected

2.3.2. Defoliated sample trees In 1976, about 85% of the current foliage was consumed, with no bud destruction (Piene, 1989a).

budworm feeding (Piene, 1989a). These spraying operations were very successful and usually less than 10% of the current foliage was defoliated annually. 2.3. Estimation of FW

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Because shoot destruction was universally severe in 1977–1978, the normal symmetrical growth pattern of the balsam fir branches was maintained in the plots from 1976 to 1978. A decline in budworm population density in 1979 triggered the development of prolific epicormic shoot production (Piene and MacLean, 1999). New shoots appeared anywhere along the branch, and this altered the normal symmetrical growth pattern thereafter (Piene, 1989a). Due to this change in branch structure, estimation of FW for the sample branches of defoliated spaced and unspaced trees differed by period. For 1976–1978 and those age classes (1978 and older) on branches in the years from 1979 to 1984, FW was estimated as described for the protected trees, but using the respective percentage defoliation for each age class by tree and year instead of needlefall. For the post-1978 age classes from 1979 to 1984, FW was estimated as described previously from interrelationships between shoot length, needle length, weight, density, number of shoots, and defoliation (Piene, 1983). To estimate FW by age class for the remaining (nonsample) branches in a whorl, various models were tried to relate FW to branch length. However, only poor relationships (r2 ranging from 0.11 to 0.48) were found, reflecting the high variability in foliage production among sample branches, and thus between whorls. Therefore, FW per centimeter branch length calculated from the sample branch in a whorl was multiplied by the length of each of the branches in that whorl. FW per age class of a tree in a particular year was then obtained by summing the weight from each whorl. Support for this approach was provided by MacLean and Lidstone (1982), who found no significant difference in spruce budworm defoliation between branches sampled from crown quadrants (the four cardinal directions). 2.3.3. Defoliated non-sample trees FW was not estimated for the remaining non-sample trees in the plots during the spruce budworm outbreak. However, for those trees that were alive at the end of the outbreak, FW was estimated destructively in 1984, based on one branch of average length per whorl collected during tree harvest. FW per age class from 1976 to 1984 was estimated as described above for the defoliated sample trees, but using average defoliation by whorl and age class for the sample

trees in that plot. FW per age class for a whorl was obtained by multiplying the FW by the number of branches in each whorl for the spaced trees, and for the unspaced trees, where all branch lengths were measured in a whorl, by multiplying the FW per centimeter branch length by branch lengths and summed. FW per age class per whorl was then summed for the whole tree. Estimates of percent defoliation by age class have demonstrated relatively small variations among trees. For example, the coefficient of variation of percent defoliation from 1976 to 1980 of spaced trees that died during the outbreak and for cumulative percent defoliation in 1980 averaged 13.6 and 2.3%, respectively (Piene, 1989a). Therefore, the estimate of FW by age class for trees that died during the outbreak was assumed to be similar to that of the sample trees that died. Estimation of FW from 1976 to 1978, and for those age classes (1978 and older) from 1979 to tree death, was as described for the sample trees. FW of post-1978 foliage age classes to the time of tree death was obtained by using average FW per centimeter branch length based on the dead sample trees, and multiplying by length of the one branch per whorl sampled at the time of tree harvest. The FW per whorl was then obtained by multiplying by the number of branches per whorl, and then summing to obtain total FW for a particular year. In summary, estimation of FW by age class for a branch was based on interrelationships between shoot length, needle length, weight, and density, multiplied by the number of shoots in that age class, and reduced by needlefall (protected trees) or defoliation (defoliated trees). FW for the branch was then obtained by summing the weights for each individual age class. For the protected trees, FW for the remaining branches in a whorl was obtained from relationships between branch length and FW. For defoliated trees, FW for the remaining branches in a whorl was obtained by either multiplying FW per centimeter branch length for the sample branch by the length of the remaining branches in that whorl (for sample trees and unspaced non-sample trees), or by multiplying FW per branch by the number of branches in that whorl (spaced non-sample trees and for spaced and unspaced trees that died during the outbreak). The FW per whorl was then summed to obtain FW per tree and plot.

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2.4. Measurements at tree harvest At the time of tree harvest, the following data were recorded for each tree: (1) total height and height to each whorl and disk for all trees in the experimental plots, (2) lengths of top-kill (boundary line between dead and live portion was determined by removing the bark), (3) lengths of new tops, (4) crown length and width, and (5) DBH. Branch lengths were measured by two methods: (1) lengths of each branch were measured each year from 1976 to 1984 for all sample trees, for non-sample trees in the spaced protected plots, and for larger trees (>6 cm DBH) in the unspaced plots and for the non-sample trees in the unspaced defoliated plots and (2) the length of one average branch per whorl was measured and the number of branches per whorl was recorded for the many small protected (<6 cm DBH) unspaced trees, and for spaced defoliated nonsample trees. All branches were recorded as dead or alive. 2.5. Volume growth Annual growth of trees in the eight plots was determined by stem analysis. Trees were harvested between the end of the growing season in 1984 and in early June 1986. One disk was cut from the middle of every third internode, at DBH, at 60 cm from the base, and at stump height from all the sample trees, all remaining non-sample trees in the spaced plots, and trees in the unspaced plots with DBH > 6 cm. For trees that were recovering following the spruce budworm outbreak and that had developed a new leader, one disk was also cut from every third internode of the new top and at the base of the new leader. The 5–18 disks sampled from each tree allowed VI to be estimated to within 5% of the volume based on disks sampled from every internode (Piene et al., 1981). Because of the high density of small trees (DBH < 6 cm) in the unspaced plots, and their minimal contribution to stand volume, the number of disks sampled from trees was reduced. A total of three (top, DBH, and stump height), and four (top, mid-crown, DBH and stump height) disks were sampled from the protected and defoliated trees, respectively. The disks from each tree were placed in a plastic bag and stored at 30 8C until measured. On each

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thawed disk, ring widths were measured along one (protected trees) and two (defoliated trees) average diameters, or two or four radii; ring widths were more variable in defoliated trees, so the sample was increased. Analyses have shown that measuring ring widths along one or two average diameters estimates VI to within 9.7 and 5.4%, respectively, of a value based on five diameters (Piene, unpubl.). Trees in the protected plots had undergone only 1 year of defoliation (in 1976) of the current foliage, and showed little variation in ring width from year to year. In contrast, most trees in the defoliated plots had partial or entire growth rings missing. Ring dating did not present much difficulty. For trees in the defoliated plots, the dating was aided by a typical combination of a wide ring in 1976 and a narrow one in 1977, representing the first year of spruce budworm feeding in 1976, and a dramatic increase in defoliation levels in 1977. In addition, the spaced trees showed a wide ring in 1972, as a result of the spacing treatment in 1971 (Piene, 1981). For trees that died during the outbreak, the ring widths after 1977 were increasingly narrow and, in many cases, ring widths were missing for the period 1981–1984. Recovering trees typically had a wide ring in 1984. Possible missing rings were identified by the decreasing ring width pattern after 1977 meeting the increasing ring width pattern leading up to 1984. Ring widths were measured to the nearest 0.01 mm using a Holman DIGI-MIC (Holman Electric Controls, NB, Canada) ring-measuring machine (Jordan and Ballance, 1983; Clarke and Murchison, 1987) connected to a computer. Total volume (V, cm3) was calculated by summing the volumes of frustums of a cone and using a cone to represent the top volume. In addition, VI (cm3 per year), cambial surface area (CSA, cm2), and specific volume increment (SVI, cm3/(cm2 year)), were calculated for each year. SVI for a specific year, t, was calculated as SVIt ¼

VIt ðCSAt1 þ CSAt Þ=2

(1)

SVI, which is a measure of annual VI in relation to the cambial surface area from which it is derived (Duff and Nolan, 1957), allows for direct comparison of growth rates between different-sized trees, and has been found to be a sensitive measure for assessing growth responses (Piene, 1981).

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2.6. Statistical analyses

3. Results and discussion

The SVI data were subjected to Tukey’s (HSD) multiple range test ðP < 0:05Þ for differences between treatments for dominant, suppressed, protected and defoliated, spaced and unspaced trees. Where transformations (arcsine) failed to normalize the data, the non-parametric Kruskal–Wallis test was used. Likewise, differences between spaced and unspaced protected trees for the percentage of FW contained in current foliage and for 5-year-old foliage and older, and for differences between defoliated and protected trees for FW contained in current combined with 1year-old foliage, was analyzed by the non-parametric Kruskal–Wallis test, since transformations (arcsine) failed to normalize the data. We wanted to explore the relationship between VI and FW, and in particular whether these were linear or non-linear, and how the strength of these relationships changed throughout the spruce budworm outbreak. We used a quadratic model of the form VI ¼ b0 þ b1 FW þ b2 FW2 . The backward elimination procedure (Draper and Smith, 1966) was applied to find the best regression equation. We removed parameters that were not significant ðP > 0:05Þ and refitted the reduced models. Distribution of residuals was examined to verify that the regression assumptions were satisfied. Linear relationships between VI and FW and between current foliage weight (CFW) and FW on a per ha basis, were determined by model II regression and the data were fitted by Bartlett’s three-group method (Sokal and Rohlf, 1981).

Tree and stand characteristics of the eight plots are summarized in Table 1. This clearly shows differences in average tree size (DBH, crown length, and crown width) between spaced and unspaced and between protected and defoliated trees. Mortality from 1976 to 1984 was 0% in spaced protected, 10–15% in unspaced protected (all small suppressed trees), but 26–54% in defoliated plots (Table 1). 3.1. Foliage weight Total FW of all trees in a plot on a per hectare basis from 1976 to 1984 for the protected plots showed a curvilinear relationship with time, especially for the spaced trees (Fig. 1). FW increased relatively slowly from 1976 to 1980 and then increased more rapidly thereafter. There may be several reasons for this: first, the current foliage was severely defoliated (about 85%) in 1976 in the study area, resulting in a ‘‘missing age class’’ for several years. By 1980, the effect of the loss of much of the 1976 age class had diminished (Fleming and Piene, 1992), allowing FW to increase rapidly thereafter. This effect was most pronounced for the protected spaced trees, which had the least current foliage present in 1976 (Fig. 1). Second, due to defoliation of the current foliage in 1976, the developing buds were probably deprived of resources (e.g., La˚ ngstro¨ m et al., 1990), possibly resulting in decreased production of needle primordia, and thus

Table 1 Density, total volume (live and dead trees), mortality, and mean DBH, crown length and width (live trees) in 1984 for the eight plots in immature balsam fir stands, Cape Breton Highlands, NS, Canada Treatment

Plot

Density Per plot

Volume (m3/ha)

Mortalitya (%)

DBH (cm)

Crown length (m)

Crown width (m)

Per hectare

Spaced, protected

P1 P2

60 46

2,403 1,842

62.0 64.2

0.0 0.0

10.5 11.8

4.3 5.1

2.6 2.9

Unspaced, protected

P3 P4

260 361

10,413 14,458

173.4 129.8

9.5 14.9

7.3 6.0

3.2 2.3

1.8 1.5

Spaced, defoliated

D1 D2

83 86

3,324 3,444

50.9 42.7

54.2 32.6

9.2 7.6

2.9 2.5

2.0 1.8

Unspaced, defoliated

D3 D4

329 273

13,176 10,934

125.2 111.3

26.4 30.0

6.8 6.8

2.1 2.2

1.4 1.5

a

Total mortality, percentage of number of trees dead, from 1976 to 1984.

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Fig. 1. FW distribution among foliage age classes for P1 and P2 (protected, spaced), D1 and D2 (defoliated, spaced), P3 and P4 (protected, unspaced) and D3 and D4 (defoliated, unspaced) plots from 1976 to 1984 (P1, P3, D1, D3) and from 1980 to 1984 (P2, P4, D2, D4).

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Table 2 The percentage of foliage (in 1984) contained in current, 1-, 2-, 3-, 4-, and 5-year-old and older age classes in protected plots from the present study and from Baskerville (1965) Age class

P1

P2

P3

P4

Average

Baskerville (1965)

Current 1-year-old 2-year-old 3-year-old 4-year-old 5þ years old

22.0 19.7 17.3 14.5 11.8 14.7

21.0 19.3 17.4 14.4 10.0 17.9

25.0 22.1 19.2 14.5 9.4 9.8

27.3 23.6 19.0 12.2 6.7 11.1

23.8 21.2 18.2 13.9 9.5 13.4

26.2 24.7 20.7 12.5 7.2 8.7

shorter shoots in the following years (e.g., Kulman, 1965). Also, the developing foliage in 1977 lacked its main source of carbohydrates (the 1976 foliage) (Kozlowski and Winget, 1964). All of these factors presumably contributed to reduced FW. Analyses of the amount of foliage in each age class for the protected trees was based on the relationships from 1984, to minimize the effect of the defoliated 1976 age class. On average, 23.8, 21.2, 18.2, 13.9, 9.5, and 13.4 % of the FW was in current, 1-, 2-, 3-, 4-, and 5year-old and older age classes, respectively. These values are similar to those obtained for immature balsam fir by Baskerville (1965) (Table 2). As the density increased going from spaced to unspaced stand conditions (Table 1), the percentage of the FW contained in the current foliage significantly ðP < 0:0001Þ increased, on average, from 21.5 to 26.2%, while the percentage of 5-year-old and older foliage significantly ðP < 0:0001Þ decreased from 16.3 to 10.5%. This was due to increased needlefall from older foliage in the unspaced plots (Piene and Fleming, 1996). In the defoliated plots, FW decreased rapidly, from 4083–6135 kg/ha in 1976 to 525–1034 kg/ha in 1981, when it reached its lowest values (Fig. 1). This represents mean decreases of 87.1 and 83.2% for spaced and unspaced plots, respectively, or 83.3 and 84.3% compared to protected plots. FW recovery for trees that survived the outbreak was rapid for both spaced and unspaced plots (Fig. 1). From 1981 to 1984, FW increased about fivefold and threefold for the spaced and unspaced plots, respectively (Fig. 1). This resulted from a rapid increase in number of shoots from the release of suppressed buds caused by complete bud destruction in 1977–1978 (Piene and MacLean, 1999). By 1984, there were essentially three foliage age classes present (Fig. 1), compared with about 11 age classes on protected trees (Fleming and Piene,

1992). The current and 1-year-old foliage, which have the highest photosynthetic rates (Clark, 1961), comprised on average 57.8 and 71.6% of the FW for spaced and unspaced defoliated plots, respectively, which was significantly ðP < 0:0001Þ greater compared to protected plots averaging 41.0 and 49.0%, respectively. This recovery in FW is quite remarkable, as the trees had undergone 6 years of severe defoliation and must have been weak with low levels of stored energy (e.g., Webb, 1980). Aside from the increase in shoot production, other compensatory mechanisms may have contributed to the rapid recovery in FW. Percent nitrogen was significantly higher for defoliated than protected trees in 1979 and 1981 for the spaced trees and in 1981–1982 for the unspaced trees (Piene unpubl.), possibly due to a given amount of nitrogen being distributed over a smaller quantity of foliage. Higher foliar nitrogen levels have previously been associated with increased photosynthetic rates (Brix, 1971; Lavigne et al., 2001). For the unspaced plots, the rate of recovery was generally slower, but nonetheless significant, especially considering the high density levels (Table 1). However, these plots had undergone significantly less defoliation than the spaced ones (Piene, 1989a), and had the ability to expand their crowns because tree mortality acted like a spacing treatment. This reduced tree competition and improved light conditions for the lower crown. Information on changes in FW resulting from artificial defoliation or insect outbreaks is very scarce and, to our knowledge, there are no previous data documenting changes in FW during entire insect outbreaks. When young balsam fir were defoliated of all age classes of foliage by spruce budworm, and then allowed to recover by spraying insecticide, there was no significant difference in FW from undefoliated

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trees after 3 years of recovery (Piene, 1989b). This rapid recovery in FW was due to the release of suppressed buds following bud destruction. This compares with reductions in FW of 37.5 and 68.5% for spaced and unspaced plots following recovery in the present study (Fig. 1). Presumably, this difference in recovery is explained by the number of years that defoliation occurred, 6 in the present study compared with 2 in Piene’s (1989b) study. A longer period of severe defoliation reduces stored reserve energy (e.g., Webb, 1980) and thus the ability to recover in FW. Debudding of the leader shoot in the main axis of young Scots pine (Pinus sylvestris L.) increased FW of shoots close to the leader shoot (Honkanen et al., 1994). Following partial removal of current and 1-year-old shoots from the upper crown of young Scots pine, FW was significantly less than for control trees after three growing seasons (La˚ ngstro¨ m et al., 1990). The pruned section showed no recovery, which is probably related to the pruning procedure that left the leading shoot of the stem untreated, thus keeping apical dominance intact. 3.2. Volume growth Before the start of the spruce budworm outbreak in 1976, VI for the four spaced plots followed similar patterns (Fig. 2). Tree density before spacing was very high (averaging 53,000 stems/ha for the protected plots—Piene, 1981), which resulted in slow growth due to competitive stress from neighboring trees. Following the spacing treatment in 1971, VI increased rapidly until the start of the budworm outbreak in 1976 (Fig. 2). Severe defoliation from 1976 to 1980 resulted in progressively lower FW for both spaced and unspaced plots (Fig. 1). VI of spaced and unspaced plots reached its lowest levels of 0.4 and 0.8 m3/ (ha year) in 1981 (Fig. 2), when budworm populations were at their lowest levels (Piene, 1989a). This represents mean decreases of 92.5 and 91.1% for spaced and unspaced plots, respectively, compared with protected plots. As FW rapidly increased for the surviving defoliated trees from 1981 to 1984 (Fig. 1), VI increased about fivefold and fourfold for spaced and unspaced trees, respectively. SVI is a sensitive measure of the balance between supply and demand of raw materials. This reflects changes in the activity of the meristematic tissues and thus serves as an index of tree growth (Shea and

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Armson, 1972). Changes in SVI for dominant and suppressed, spaced and unspaced trees from 1965 to 1984 were generally similar to those for VI (Figs. 2 and 3), however, there were some noteworthy differences. Before the start of the budworm outbreak in 1976, VI was higher for the spaced, defoliated plots than for the protected ones because of higher tree densities (Table 1). In general, however, there were no significant differences in SVI between plots before 1976, for both dominant and suppressed trees. An exception was for dominant trees in P1 from 1968 to 1971, when SVI was significantly lower than for the other spaced plots (Fig. 3). Similarly, for unspaced plots, VI was lower for P4 before 1976 (Fig. 2). In general, however, there were no significant differences in SVI, except for the suppressed trees in P3 from 1970 to 1975, when SVI was significantly higher than for the other plots (Fig. 3). Similar growth rates would be expected because tree densities were similar between plots (Table 1). In general, there were no significant differences in SVI from 1976 to 1984 between the two plots within a treatment (protected and defoliated, spaced and unspaced), but protected were significantly different from defoliated in almost all cases. SVI for both dominant and suppressed trees reached its minima in 1981, ranging from 0.00 to 0.03 cm3/ (cm2 year) for spaced defoliated and 0.00 to 0.02 cm3/ (cm2 year) for unspaced defoliated trees. This represents mean decreases of 92.0 and 89.3% compared to protected trees, respectively, or similar to the reductions in VI discussed previously. Thus, this severe spruce budworm outbreak reduced growth rates by the same level, regardless of initial differences in growth rates and density. The minimum growth rates reported in this study compare well with minimal SVI of 0.03 cm3/(cm2 year) after 7 years of defoliation (3 years severe) by spruce budworm (Karsh, 1996). Volume for the protected plots increased steadily from 1965 to 1984 for both spaced and unspaced plots (Fig. 4). Volume in defoliated plots declined from 1976 to 1984, in particular from 1981 to 1982 and thereafter because of tree mortality (Fig. 4). By 1984, volume loss due to the budworm outbreak amounted to 32.6 and 54.1 m3/ha compared to protected plots, for spaced and unspaced plots, of which 50.1 and 38.4% were due to tree mortality. Overall volume reductions caused by the budworm outbreak were 51.7% for spaced and 35.7% for unspaced plots.

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Fig. 2. Changes in volume increment from 1965 to 1984 for protected and defoliated, spaced and unspaced stands. See Fig. 1 for description of treatments.

The impact of spruce budworm on forest growth in eastern North America has been the subject of numerous studies in the past. However, usually only tree mortality has been investigated (MacLean, 1980) or, when growth changes were assessed either on a tree or stand basis, the studies only covered parts of an out-

break (MacLean, 1981, 1985). Following a severe spruce budworm outbreak in Newfoundland from 1972 to 1985, average total volume loss was 112 m3/ha, equivalent to 45% of potential volumes determined based on growth prior to defoliation (Karsh, 1996). These volume losses are substantially

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Fig. 3. Mean changes in specific volume increment for dominant (DBH  8 cm) and suppressed (DBH  6 cm) trees from 1965 to 1984, for P1 (52, 2: number of dominant and suppressed trees), P2 (41, 1), P3 (102, 98), P4 (73, 211), D1 (46, 13), D2 (37, 29), D3 (74, 144), and D4 (74, 122). See Fig. 1 for description of treatments.

greater than the average of 43.4 m3/ha in the present study, however, the percent losses were similar, 45 versus 36–52% in the present study. The main reason for these higher volume losses was that the stands in Newfoundland were mature and mortality was substantially higher, 72% compared with an average of 44% in the present study. In comparison, following a budworm outbreak in the 1950s in northern New Brunswick, average volume loss amounted to 57%, of which 51% was from tree mortality (Baskerville and MacLean, 1979).

3.3. Relationships between volume increment and FW Relationships between tree or stand growth and leaf area have been examined in numerous studies (e.g., Waring, 1983; Kollenberg and O’Hara, 1999). Prior to crown closure, volume growth is related linearly to leaf area and thus FW, because competition for growing space and light is favorable, allowing for full expansion of tree crowns (Smith and Long, 1989). Although the P1 (spaced, protected) trees in the present study

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Fig. 4. Changes in total live volume for spaced and unspaced stands from 1965 to 1984. See Fig. 1 for description of treatments.

approached crown closure in 1984, a linear relationship existed between VI and FW, with r2 ranging from 0.73 to 0.86 (Fig. 5). In mature forest stands after crown closure, VI has been reported to be related non-linearly to leaf area (Smith and Long, 1989; Long and Smith, 1990), possibly due to an increase in the cost of

respiration as tree size increases in a stand (Long and Smith, 1990; Roberts and Long, 1992). Such a relationship was not shown consistently in the present study for the P3 (unspaced protected) trees, perhaps because of the young age of the stand (about 30 years old), where marked differences in tree size had not yet

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Fig. 5. Relationships between volume increment and FW for individual sample trees in P1 (protected, spaced) and D1 (defoliated, spaced) from 1977 to 1984.

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Fig. 6. Relationships between volume increment and FW for individual sample trees in P3 (protected, unspaced) and D3 (defoliated, unspaced) from 1977 to 1984.

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Fig. 7. Relationships between volume increment and FW (all age classes) and between CFW and FW from 1977 to 1984 (for D1 and D3 CFW data from 1977 to 1980 was not presented due to an almost complete destruction of the current foliage by the spruce budworm) on a per hectare basis for spaced (P1 and D1) and unspaced (P3 and D3) plots. See Fig. 1 for description of treatments.

been established (Piene et al., 1981). Defoliated plots D1 and D3 both showed a mixture of linear and nonlinear relationships between VI and FW. Typically, the relationships were weak in the middle of the budworm outbreak, and were substantially different than for protected trees (Figs. 5 and 6). Towards the end of the budworm outbreak, however, from 1982 to 1984, production of current foliage particularly for trees in D1 was approaching that of protected trees (Fig. 1), and the relationships for defoliated trees re-approached those for protected trees (see 1984 in Figs. 5 and 6).

Relationships between VI and FW, combining information from all treatments and years, indicated a linear relationship on a per ha basis for spaced and unspaced stands (Fig. 7). Different symbols were used to differentiate 1977–1980 from 1981 to 1984 data, for two reasons. First, during the defoliation period from 1976 to 1980, virtually all current foliage was removed, and the remaining foliage comprised only the oldest age classes. This foliage has low photosynthetic capacity compared to younger age classes (Clark, 1961), and thus production of resources would

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be lower compared with the same weight of foliage from undefoliated trees. Second, during the recovery period from 1981 to 1984, resources were channeled into foliage rather than volume production (Figs. 5 and 6) (Ericsson et al., 1980), and CFW was approaching that of protected trees (Fig. 1). Strong relationships between VI and FW and between CFW and FW (particularly for unspaced trees) on a per hectare basis were shown in this study. This indicates that if FW, which can be obtained from existing allometric models (e.g., Baskerville, 1965), is known, VI and CFW can be predicted for young spaced and unspaced balsam fir stands (Fig. 7). During insect outbreaks, such estimates must include effects of defoliation on FW. Non-linear relationships have been observed between basal area increment and FW for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and grand fir (Abies grandis (Dougl.) Lindl.) (Nichols, 1988), and between reduction in tree growth and defoliation for Douglas-fir defoliated by the western spruce budworm (Choristoneura occidentalis Freeman) (Alfaro et al., 1982).

4. Conclusions Periodically the spruce-fir forests of North America are defoliated by spruce budworm. The results from this study show that FW was reduced by 83.3 and 84.3% in spaced and unspaced stands, respectively, and that VI was reduced by 92.5 and 91.1%, respectively. However, surviving trees refoliated quickly, due to the release of suppressed buds. By the end of the budworm outbreak, total volume was reduced by 32.6 and 54.1 m3/ha for spaced and unspaced stands, respectively, or a decrease of 51.7 and 35.7%, while FW was reduced by 37.5 and 68.5%, respectively. Strong relationships between VI and FW and between CFW and FW existed on a per hectare basis, indicating that VI and CFW can be predicted from FW for both spaced and unspaced young balsam fir stands despite defoliation by the spruce budworm.

Acknowledgements We thank W.F.A. Anderson, J.E. D’Amours, M.G. Morgan, and J.C.E. Farrell for excellent technical assistance during this study.

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