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The impact of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) on balsam fir and spruce in New Brunswick, Canada
Forest Ecology and Management 120 (1999) 77±87
The impact of hemlock looper (Lambdina ®scellaria ®scellaria (Guen.)) on balsam ®r and spruce in New Brunswick, Canada David A. MacLeana,b,*, Paul Ebert1,b b
a Canadian Forest Service, Atlantic Forestry Centre, P.O. Box 4000, Fredericton, NB, Canada, E3B 5P7 Faculty of Forestry and Environmental Management, University of New Brunswick, P.O. Box 44555, Fredericton, NB, Canada, E3B 5A3
Accepted 9 November 1998
Abstract In 1989, the ®rst recorded outbreak of hemlock looper (Lambdina ®scellaria ®scellaria (Guen.)) occurred in New Brunswick, Canada. Data were collected from ten plots established in an area infested from 1992±1994, to assess impacts of hemlock looper. Ocular and branch sample assessments of current defoliation and ocular assessments of total defoliation (all age classes of foliage) were conducted for balsam ®r (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.). Stand response was assessed and related to cumulative defoliation. Ocular assessments were found to accurately estimate defoliation, which was signi®cantly related to tree mortality. Ninety-two percent of balsam ®r trees that had cumulative defoliation >90% died. Mortality of balsam ®r was signi®cantly (p < 0.05) related to tree size, in both lightly and severely defoliated plots; trees with DBH <11 cm sustained 22±48% higher mortality than larger trees. Mortality of balsam ®r, in terms of both percent stems/ha and m3/ha merchantable volume, increased exponentially in relation to three estimates of cumulative (summed) plot mean defoliation. The strongest relationships (r2 0.75±0.79) were between mortality and the ocular defoliation assessment for 1990±1993 foliage. Tree mortality caused by the looper outbreak ranged from 4±14% stems/ha in lightly defoliated and from 32±100% in severely defoliated plots; merchantable volume killed was 3± 14 m3/ha and 51±119 m3/ha, respectively. Relationships between mortality and defoliation were similar when defoliation was assessed for 1987±1993 and 1990±1993 foliage age classes. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Defoliation; Hemlock looper; Mortality; Insect impact
1. Introduction The eastern hemlock looper (Lambdina ®scellaria ®scellaria (Guen.)) is an insect defoliator native to North America (Martineau, 1984). It is distributed *Corresponding author. Tel.: +1-506-452-3580; fax: +1-506452-3525; e-mail: [email protected] 1 Present address: TDB Forestry Services Ltd., RR#1, Site 31, Comp. 2, Prince George, BC, Canada V2N 2H8.
from Newfoundland to Alberta in Canada and south to Georgia in the eastern US (Drooz, 1985). The larvae feed on a variety of conifer and deciduous trees; however, tree mortality is restricted primarily to mature stands of the principal host trees, balsam ®r (Abies balsamea [L.] Mill.) and eastern hemlock (Tsuga canadensis [L.] Carr.) (Davidson and Prentice, 1967). In eastern Canada, the main host tree is balsam ®r (Martineau, 1984). Hemlock loopers are aggressive defoliators that feed on all age classes of foliage
0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 8 ) 0 0 5 2 7 - 1
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(Dobesberger, 1989). As a result, this insect can kill trees after only 1±2 years of severe defoliation (Hudak et al., 1978). Hemlock looper feeding is done openly from the third instar onward and all age classes of foliage are consumed (Carroll, 1956; Carolin and Lejeune, 1967; Otvos et al., 1971; Dobesberger, 1989). Older larvae are very wasteful feeders, typically eating a small part of a needle and then moving on to another (Otvos et al., 1971). Damaged stands have a characteristic reddish-brown color that results from the desiccation of damaged needles. Eight extensive hemlock looper outbreaks have been recorded in Newfoundland, Canada during the 20th century (Otvos et al., 1979). During the 1969± 1973 outbreak, which encompassed 1.5 million ha, 10 million cubic meters of wood was killed (Hudak et al., 1987). The 1983±1986 outbreak in Newfoundland started in two widely separated areas and resulted in 52 000±215 000 ha of moderate and severe defoliation (Clarke and Carew, 1987). It has been estimated that, from 1947 to 1991, the volume of trees killed by hemlock looper exceeded 25 million m3, enough supply for the three paper mills in Newfoundland for 7 years (Bowers, 1993). The ®rst recorded outbreak of hemlock looper in New Brunswick, Canada began in the Christmas Mountains area in the north-central part of the province in 1989 (Magasi, 1990; Hartling et al., 1991; Carter and Hartling, 1992). This outbreak resulted in 3500±3800 ha of defoliation each year from 1989 to 1991, 1500 ha in 1992, and no defoliation in 1993 (Magasi, 1990, 1993; Hartling and Carter, 1991; Carter and Hartling, 1992). The outbreak occurred in mature to overmature stands of balsam ®r and collapsed in 4 years (Carter and Hartling, 1992; Magasi, 1993), with smaller individual infestations lasting 1±2 years. The severity of the outbreak was indicated by the extent of the insecticide spray programs of 12 100± 17 000 ha conducted in New Brunswick in 1990, 1991, and 1993 (Hartling and Carter, 1991), as well as by harvest operations to salvage dead wood conducted by several forest industries. Little information exists to quantify the patterns of tree mortality resulting from hemlock looper outbreaks, although this is necessary for forest management planning (MacLean, 1990). The objective of this paper was to determine the impacts of the ®rst
recorded outbreak of hemlock looper in northcentral New Brunswick. Speci®c topics addressed, for balsam ®r, white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.) were: (1) a comparison of the accuracy of ocular versus detailed branch sample defoliation assessments for individual foliage age classes; (2) the relation of individual tree mortality to cumulative defoliation and to diameter class; (3) the temporal pattern of mortality; (4) the relation of mortality per stand, in terms of both percent stems/ha and m3/ha merchantable volume, to cumulative defoliation; and (5) an evaluation of the total merchantable volume (m3/ha) impact in the stands. 2. Materials and methods 2.1. Plot description Ten permanent sample plots (PSPs) were established near the Christmas Mountains area of northcentral New Brunswick, Canada in the spring of 1993, in a hemlock looper infestation that began in 1992, but which originated from the 1989 outbreak (Fig. 1). The PSPs were remeasured in the fall of 1993, 1994, and 1995. Locations were originally selected to represent different levels of 1992 and probable 1993 defoliation, based on 1993 population forecasts. Each PSP consisted of three variable radius prism points (BAF 2.0 m2/ha) located 40 m apart, with permanently numbered trees. 2.2. Defoliation measurements Defoliation was assessed using three different methods: ocular assessment for each tree of annual defoliation of each of the 1990±1993 age classes, ocular assessment for each tree of total defoliation (all age classes of foliage combined) each year, and branch sample assessment of defoliation of each of the 1987±1993 age classes of foliage. As hemlock looper feeds on all age classes, defoliation estimates in a given year re¯ect all cumulative feeding to date. 2.2.1. Ocular assessment of annual defoliation Using binoculars, the crown of each tree was assessed for defoliation of each of the three most
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2.2.2. Ocular assessment of total defoliation The total cumulative defoliation of each tree was also estimated directly using binoculars. Total defoliation on all age classes of foliage combined was estimated in seven classes: 0%, 1±10%, 11±25%, 26± 50%, 51±75%, 76±90%, and 91±100%, as used in previous spruce budworm (Choristoneura fumiferana (Clem.)) impact studies (Piene et al., 1981). Total defoliation was assessed four times: in the spring of 1993 for 1992 defoliation and in the fall of 1993, 1994, and 1995, for 1993±1995 defoliation, respectively. Average total defoliation each year was calculated, based on the midpoints of classes, per species per plot.
Fig. 1. Areas defoliated by the eastern hemlock looper from 1989 to 1993 in the first recorded outbreak in New Brunswick, Canada.
recent foliage age classes, rated in six classes (0±10%, 11±20%, 21±40%, 41±60%, 61±80%, and 81±100%). Mean defoliation per age class per tree was then calculated based on midpoints of the classes (MacLean and Lidstone, 1982). In the spring of 1993, defoliation was assessed for each of the 1990±1992 foliage age classes; in the fall of 1993, 1994, and 1995, each of the 1991±1993, 1992±1994, and 1993±1995 age classes were assessed, respectively. It was not possible to estimate defoliation of more than three age classes of foliage using binoculars, because newer foliage obscured older age classes. Most defoliation occurred in 1992, with small amounts in 1993 and virtually none in 1994 or 1995; therefore, for each tree, the highest observed defoliation on each of the 1990±1993 foliage age classes, where there was more than one measurement, were summed to calculate cumulative defoliation per tree. Average cumulative defoliation was then calculated per species per plot.
2.2.3. Branch sample assessment of annual defoliation The branch assessment of defoliation was by the shoot-count method developed by Fettes (1950). In the fall of 1993, one whole mid-crown branch sample was obtained, using pole pruners, from each of two balsam ®r trees per prism point, or six per plot. Branch samples from spruce also were collected when present: six white spruce from Plot 2, two from Plots 3 and 8, three from Plot 9, and four from Plot 10; ®ve black spruce branches from Plot 6, and four from Plot 10. Defoliation of each of the 25 shoots from each of the 1987±1993 foliage age classes was assessed on each branch, using the same classes as for ocular estimates of annual defoliation, and average defoliation per age class per branch was calculated. Defoliation then was summed for the 1987±1993 and for the 1990±1993 foliage age classes, to calculate two other cumulative defoliation estimates. Average branch sample cumulative defoliation from 1987 to 1993 and from 1990 to 1993 was calculated per species per plot. Since hemlock looper feeds on all age classes of foliage, defoliation assessments must represent feeding on all age classes of foliage. The maximum current defoliation that one age-class can incur is 100%. Therefore, the cumulative (summed) branch sample defoliation estimates calculated for four (1990±1993) and seven (1987±1993) age classes would have maximum values of 400% and 700%, respectively. 2.3. Plot volume and mortality determinations In the spring of 1993 and the fall of 1994, diameter at breast height (DBH, at 1.3 m) of each tree was
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measured with a diameter tape to the nearest 0.1 cm. The height of living trees was measured with a Suunto clinometer to the nearest 0.1 m in the fall of 1994. From the data collected, merchantable volume was calculated for each tree using volume tables from Honer et al. (1983). Calculated volume/tree was multiplied by the tree factor for each tree to determine m3/ha represented by each plot tree. Volume per hectare then was summed and divided by the number of sample points to calculate m3/ha for each plot. The number of trees for all points was summed and divided by the number of points to determine stand density per plot. The number of stems/plot was then multiplied by the tree factor to calculate stems/ha. All trees in each plot were checked for mortality at each measurement. Tree mortality was determined by the death of the inner bark at breast height (Hudak et al., 1978). Mortality was calculated in terms of both volume (m3/ha) and density (stems/ha). 2.4. Statistical analyses Ocular and branch sample assessments of defoliation were compared by regression analysis and by Freese's test of accuracy (Freese, 1960). The analysis
using y a bx compared ocular assessment (y, representing the method to be tested) versus branch assessment (x, representing the `true' value). Chi-square analyses were done to test the null hypothesis that mortality was not signi®cantly different among ®ve DBH classes (<11.1, 11.1±15, 15.1±19, 19.1±23 and >23.1 cm). This test was conducted separately for dead balsam ®r trees in plots with light defoliation (summed annual <200%) and with moderate to severe defoliation (summed annual 200%). Signi®cance level for all statistical tests was p 0.05. 3. Results and discussion 3.1. Stand characteristics The ten sample plots represented a range of stand conditions (Table 1). Softwood species composition ranged from 55% to 98% balsam ®r, 10±24% black spruce, and 2±37% white spruce. Hardwood species composition ranged from 2±16% white birch (Betula papyrifera Marsh.), 5±7% yellow birch (Betula alleghaniensis Britton), 0±6% red maple (Acer rubrum L.), and 0±2% trembling aspen (Populus tremuloides Michx). The plots were all softwood dominated, with
Table 1 Mensurational characteristics and mortality in ten variable-radius plots sampled in north-central New Brunswick, Canada Plot
A. Lightly 1 2 3 4 5 10
DBH (cm)
defoliated plots 15 13 25 18 19 13 15 12 17 12 20 14
B. Severely 6 7 8 9 a
Height (m)
defoliated plots 18 14 14 12 19 14 20 14
Density (stems/ha)
Mortality (% balsam fir stems/ha)a
Merchantable volume (m3/ha) Spruceb
Balsam fir Alive
Dead
2350 990 1470 1990 1170 1860
4 0 2 4 3 8
181 116 176 193 123 191
3 0 4 8 4 14
1210 2020 1200 1200
100 69 61 32
0 46 60 107
119 94 88 51
Hardwoodc
Alive
Dead
82 19
5
51 30 9 19
6
4 10 27 16 15 3 0 3 15 3
In addition to balsam fir mortality, there was 6% white spruce mortality in Plot 2 and 22% black spruce mortality in Plot 6. Spruce species were black spruce in Plot 6, 10% black and 8% white spruce in Plot 10, and white spruce in the remaining plots. c Hardwood species were 1±16% white birch (in all plots except No. 6), 6% red maple in Plot 2, and 5±7% yellow birch and 2% trembling aspen in Plots 3 and 4. b
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82±100% spruce-®r. Only three of the plots had >10% spruce. Hardwood composition was always <20%, with half of the plots having 10±20% hardwoods. Average DBH was <15 cm in three plots, 15±20 cm in six plots, and 25 cm in one plot (Table 1). Plot 2 was the most open (990 stems/ha) and had the largest trees; stand densities were 1100±1500 stems/ha in ®ve plots, 1800±2000 in two, and 2000±2500 stems/ha in two plots. Softwood merchantable volume ranged from 127±256 m3/ha and hardwood volume from 3±27 m3/ ha. Six plots sustained <10% mortality (% stems/ha), one had 32%, and three plots had 60±100% mortality. 3.2. Accuracy of ocular estimation of hemlock looper defoliation Defoliation estimation consisted of both ocular and branch sample defoliation assessments of individual foliage age classes. The branch assessment method is time consuming and while suitable for research purposes, is not feasible for large operational surveys (MacLean and Lidstone, 1982). Ocular assessments are faster, but they are of little use if they give inaccurate assessments of defoliation. Regression analyses showed that ocular estimates for defoliation of the 1990±1993 balsam ®r foliage age classes explained 97% of the variability in average branch sample defoliation assessments for balsam ®r (Fig. 2). Ocular estimates were overestimated by an average of 14% at all levels of defoliation. For white spruce, however, only 65% of the variability in branch samples was explained by the ocular estimates (Fig. 2), which were overestimated by 65±100% when defoliation exceeded 50%. MacLean and Lidstone (1982) determined that plot means of individual tree ocular assessments of defoliation by spruce budworm averaged 8% less than plot means of shoot-count (branch) assessments. This was concluded to be a suf®cient level of accuracy for surveys and some research purposes. Freese's test of accuracy (Freese, 1960) gave values of 26% accuracy for balsam ®r and 40% accuracy for white spruce. The ocular assessment would seem acceptable for assessing hemlock looper defoliation on balsam ®r but rather inaccurate on white spruce. Perhaps more accurate ocular assessments of defoliation on white spruce could be achieved if additional training of observers was conducted using trees with
Fig. 2. Comparison of plot mean ocular and branch sample cumulative (summed current) defoliation assessments for 1990± 1993 foliage age classes for balsam fir in ten plots in north-central New Brunswick. Data are also presented for the white spruce component, for the five plots that contained white spruce (Table 1). Accuracy is based on Freese's (1960) test.
known (branch sampled) defoliation. Also, there were only ®ve plots or data points for the white spruce relationship, and perhaps accuracy and r2 would be higher with a larger sample. 3.3. Patterns of cumulative defoliation Balsam ®r was present in all plots and the mean cumulative (summed) defoliation for 1990±1993 foliage age classes per plot ranged from 34% to 400%, where 400% represents all the foliage removed from four age classes (Fig. 3). No measurable defoliation occurred in 1994 or 1995. White spruce was present in ®ve plots and sustained mean cumulative defoliation of 27±200%. Black spruce was present in only two plots, which had mean cumulative defoliation of 32% and 180%. Based on the patterns of defoliation of balsam ®r, the plots were divided into two classes, lightly defoliated (cumulative defoliation <200%) and severely defoliated (200%). Six plots (Nos. 1±5 and 10) were lightly defoliated and four were severely defoliated. For ®r, ®ve of the lightly defoliated plots lost <1 age class of foliage while the sixth lost 1±2 age classes of foliage; in contrast, three of the severely defoliated plots lost 2±3 of the 1990±1993 foliage age classes and
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Fig. 4. The relation of individual tree balsam fir mortality to total defoliation class (ocular estimate for all age classes of foliage), for all ten plots combined.
Fig. 3. Plot mean cumulative current defoliation of 1990±1993 foliage age classes for ten plots. Severely defoliated plots are depicted with solid lines and lightly defoliated plots with dotted lines. Data are based on six branch samples per species per plot, and are grouped by species. Spruce is black for Plot 6, a mixture of black and white for Plot 10, and white for the other plots.
the fourth lost all four age classes. White spruce in all ®ve plots lost <1 age class of foliage. Black spruce in one plot lost <1 age class of foliage and slightly more than two age classes in Plot 6 (Fig. 3). Observed ranges of cumulative defoliation of ®r in severely defoliated plots were 310±400%, versus 46±108% in ®ve lightly defoliated plots and 207% in Plot 10 (Fig. 3). Comparable values for spruce were 80±200% in severely defoliated and 22±26% in lightly defoliated plots (Fig. 3). 3.4. The relation of individual tree mortality to total defoliation The relation of balsam ®r mortality to total defoliation class (ocular assessment for all age classes of foliage combined) is presented in Fig. 4. There was virtually no mortality (<1%) of balsam ®r trees with
total defoliation <50%. Five percent of the trees that sustained 50±75% and 11% sustaining 75±90% defoliation died. However, almost all (92%) ®r trees with 90±100% total defoliation died (Fig. 4). Sample sizes per defoliation class ranged from 46 to 99 trees, for all classes with >5% defoliation. When defoliation caused by hemlock looper is light, and no defoliation occurs the following year, the trees usually survive; however, annual increment will be reduced (Martineau, 1984). Hudak et al. (1978) noted in stands composed of more than 83% balsam ®r, that tree survival was 50±60% in the 40±50% defoliation class versus about 80±90% in the 20±30% class. With >80% total defoliation caused by the hemlock looper, most ®r trees died within 4 years of the initial defoliation (Hudak et al., 1978). On average, 64, 21, 14, and 1% of balsam ®r defoliated by the spruce budworm died with cumulative defoliation, respectively, of >90, 76±90, 51±75 and 26±50% (MacLean and Ostaff, 1989). Budwormcaused mortality for the >90% defoliation class recorded by MacLean and Ostaff (1989) was 28% lower than hemlock looper-caused mortality recorded in this study; however, 10% and 9% more budwormcaused mortality occurred with 50±75% and 76±90% defoliation, respectively. These differences probably resulted from the temporal pattern of defoliation, with spruce budworm feeding primarily on current foliage only for 6±8 consecutive years (MacLean and Ostaff,
D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
Fig. 5. Temporal patterns of (A) balsam fir and (B) spruce mortality (% stems/ha from 1992-1995) for ten plots. Severely defoliated plots are depicted with solid lines and lightly defoliated plots with dotted lines.
1989), versus hemlock looper feeding on all foliage age classes over a 2-year period. 3.5. Temporal patterns of mortality In the lightly defoliated plots (cumulative defoliation <200%), little balsam ®r mortality occurred (Fig. 5). From 0±11% of trees were dead in the ®rst measurement, and this remained virtually constant (4± 14%) until 1995 (Fig. 5(A)). As will be seen in the next section, these were small, suppressed trees that probably represent natural mortality. In the severely defoliated plots, 32±100% of the balsam ®r died. All the balsam ®r trees died in Plot 6 (the most severely defoliated, which had 29% dead in the ®rst measurement), 69±85% died in Plots 7 and 8, and 32% died in Plot 9 (Fig. 5). It is somewhat surprising that mortality was substantially less in Plot 9 than in Plots 7 or 8, since defoliation was generally
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similar (Fig. 3). Overall, only 4% of the 252 balsam ®r trees in the lightly defoliated plots died, versus 63% of the 142 ®r in the severely defoliated plots. Most of the mortality occurred between the fall 1993 and fall 1994 measurements, or 2 years after the most severe defoliation (Fig. 5). In severe outbreaks of hemlock looper, when feeding occurs on all foliage age classes, tree mortality typically occurs 1±2 years later (Kinghorn, 1954; Johnson et al., 1970; Hudak et al., 1978; Martineau, 1984). Kinghorn (1954) found that about 60% of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-®r (Pseudotsuga menziesii (Mirb.) Franco) trees with 70% or more total defoliation began to die 1 year after the collapse of the outbreak and mortality continued for another 3 years. Western hemlock looper outbreaks generally last 2±3 years and are less damaging than eastern hemlock looper outbreaks (Harris et al., 1982). Spruce mortality occurred in only two plots (Fig. 5(B)). The lightly defoliated Plot 2 had 12% white spruce mortality, due to competition or other causes, in the initial measurement, and this increased to 18% by 1995. Plot 6 sustained 47% black spruce mortality between 1993 and 1994 (Fig. 5(B)), and was the most severely defoliated, sustaining 100% ®r mortality. 3.6. The relation of mortality to DBH class Some defoliators cause mortality of intermediate and suppressed (smaller) trees at a higher rate than for dominant or co-dominant (larger) trees (Trail and Devine, 1994). Mortality in the lightly defoliated plots was consistently low, ranging from 0% for trees >19 cm DBH, 5% for trees 11.1±19 cm, and 27% for trees <11 cm (Table 2A). This probably represents natural, competition-caused mortality in the stands, and includes dead trees present in the initial measurement as well as trees that died from 1993 to 1995. There was a signi®cant difference in mortality between DBH classes, with the smallest trees (DBH class <11 cm) having higher (p < 0.05) mortality (27%) than trees in the larger classes (5%). Mortality in each DBH class in severely defoliated plots was high, ranging from 52% to 100% (Table 2B). Mortality differed between DBH classes, with trees in the smallest DBH class (<11 cm) having signi®cantly (p < 0.05) higher mortality (100%) than
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Table 2 Distribution of balsam fir mortality by DBH class in six lightly defoliated and four severely defoliated plots in north-central New Brunswick DBH class (cm)
Number of trees
Dead trees (%)
A. Lightly defoliated plots <11 11.1±15 15.1±19 19.1±23 >23.1
11 66 85 67 23
27a 5 5 0 0
B. Severely defoliated plots <11 11.1±15 15.1±19 19.1±23 >23.1
14 30 58 24 16
100a 73 52 63 56
a Indicates a percentage significantly different from all others within a defoliation class (c2, p < 0.05). A test on 2 5 contingency tables was followed by a test of the <11 class versus combined 11.1 to >23.1 cm.
ments for each of the 1990±1993 foliage age classes; summed branch sample assessments for each of the 1990±1993 age classes; and summed branch sample assessments for each of the 1987±1993 age classes. Relationships with all three defoliation variables were very similar, with an exponentially increasing curve form and r2 ranging from 0.74 to 0.79; because of this similarity, only the mortality versus ocular defoliation relationships are presented in Fig. 6. For both measures of mortality, slightly more of the variability (75± 79%) was explained by the ocular defoliation estimates than by the branch sample estimates (70±74%). Mortality was low (<10%) at cumulative defoliation <200% and then rapidly increased with defoliation (200% (Fig. 6). Branch assessment of defoliation on the 1987±1993 foliage had three additional age classes of foliage included in the cumulative defoliation assessment than for 1990±1993 foliage. However, it showed a similar trend in mortality as the curves for both branch and
trees in the larger classes (52±73%). Smaller trees were probably suppressed by competition and unable to withstand the defoliation, or possibly could have sustained higher defoliation than larger trees, if larval dispersal resulted in more larvae on understory trees. MacLean and Ostaff (1989) determined that spruce budworm-caused mortality was evenly distributed among DBH classes. Trail and Devine (1994) noted that mortality caused by hemlock looper was not related to tree size in severely defoliated areas. They concluded that hemlock looper populations and resulting damage were intense enough in the majority of their study area to cause mortality of all tree sizes when all related stress factors were combined. Otvos et al. (1979) demonstrated that hemlock looper infestations can be quite variable, resulting in uneven tree mortality over individual infestations. This would help to explain the wide range of mortality (32±100%) observed among severely defoliated plots (Table 2B). 3.7. The relation of mortality to cumulative current defoliation Average balsam ®r mortality per stand (percent stems/ha and m3/ha) was related to three measures of cumulative defoliation: summed ocular assess-
Fig. 6. The relation of balsam fir mortality (% stems/ha and m3/ha) to cumulative current defoliation, calculated as the sum of ocular estimates for each of the 1990±1993 foliage age classes.
D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
ocular cumulative 1990±1993 defoliation. The three additional older age classes included in the 1987±1993 cumulative defoliation assessment provided little increase in the ability to explain the mortality observed in plots. Hemlock looper defoliates all age classes of foliage (Carroll, 1956; Martineau, 1984; Dobesberger, 1989). On an average, defoliation of the three oldest age classes (1987±1989) was 110% for the lightly defoliated plots and 270% for the severely defoliated plots. It would appear that when defoliation occurs on the four most recent foliage age classes, older age classes will be defoliated in about the same proportion. Natural needle fall and declining photosynthetic capacity in the older age classes (Clark, 1961) also would tend to decrease the importance of older foliage, such that more recent age classes of foliage contribute most to tree growth. Therefore, a cumulative defoliation assessment for the four most recent foliage age classes gave an accurate assessment of defoliation and prediction of subsequent balsam ®r mortality. Balsam ®r merchantable volume mortality differed greatly between lightly and severely defoliated stands, at averages of 5.5 and 88 m3/ha, respectively (Table 1). Percentage tree mortality averaged 4 and 63% stems/ha in lightly and severely defoliated stands, respectively (Table 2). In both lightly and severely defoliated stands, mortality of balsam ®r was higher than for either white or black spruce, with the exception of Plot 2 (Table 1), which sustained no balsam ®r mortality and 6% white spruce mortality. 3.8. Total impact of the hemlock looper outbreak The ten plots examined in this paper represent a sample of mature to overmature ®r spruce stands in north-central New Brunswick. These plots all were defoliated to some degree in this ®rst recorded outbreak of hemlock looper in New Brunswick. The outbreak started in 1989 and collapsed in 1993 (Magasi, 1990, 1993; Carter and Hartling, 1992). Defoliation in the study area occurred mainly in 1992, with some in 1993. The overall outbreak consisted of several smaller individual infestations (Fig. 1). A total of 12 400 ha was defoliated by the hemlock looper during this outbreak, including approximately 1500, 4900, and 6000 ha of light, moderate, and severe defoliation, respectively
85
(Magasi, 1990, 1993; Carter and Hartling, 1992). In the study area, hemlock looper defoliated balsam ®r to a greater degree than white spruce or black spruce, consistent with results of Martineau (1984). Fir mortality (percent stems/ha) in severely and lightly defoliated plots averaged 63% and 4%, respectively. Combined ®r and spruce mortality, in terms of merchantable volume, averaged 57% in the severely defoliated and 2% in the lightly defoliated plots. Using the total areas in light, moderate, and severe defoliation classes in New Brunswick cited above, and rates of mortality from the plots in this study (severe with approximately 350% defoliation and 100 m3/ha killed, moderate with 200% defoliation and 10 m3/ha killed, light with negligible effects), an estimated 650 000 m3 of merchantable volume of balsam ®r was killed during the outbreak. The interval between eastern hemlock looper outbreaks in Newfoundland has varied from 7 to 18 years and usually lasted between 6 and 9 years. However, individual infestations may collapse in 2±3 years (Otvos et al., 1979). The interval between western hemlock looper outbreaks is 4±17 years and they usually last 2±3 years (Harris et al., 1982). Dry warm weather is favorable for the development of outbreaks, which usually start as isolated, small infestations in mature to overmature stands with a high content of balsam ®r (Otvos et al., 1979). These individual infestations under favorable conditions coalesce to form groups of infestations or to form a large irregular infestation forming part of an outbreak (Otvos et al., 1979). The implications of tree mortality resulting after just 1 year of severe defoliation have far reaching impacts on forest management decisions. The ability to salvage such areas is limited by harvesting, mill, and market capacity along with the ability to gain access to these dead stands. The impacts of large-scale outbreaks have serious environmental and economic rami®cations over both the short and long term (Carroll et al., 1995). Such impacts include decreased water quality, wildlife habitat, and limiting future supplies of wood for lumber along with pulp and paper production. A decision support system for the management of eastern hemlock looper populations in insular Newfoundland has been developed (Carroll et al., 1995). Individual models were constructed to predict probability of defoliation, timber mortality
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D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
and decay, risk of impending outbreaks, and larval phenology (Carroll et al., 1995).
from the ten plots in north-central New Brunswick, an estimated 650 000 m3 merchantable volume of balsam fir was killed during this outbreak.
4. Conclusions
Acknowledgements
1. Ocular assessments of cumulative (current) defoliation for the 1990±1993 foliage age classes explained 97% and 65% of the variability, in comparison with detailed branch sample estimates, for balsam ®r and white spruce, respectively. Ocular estimates were overestimated by an average of up to 14% for ®r, and over or underestimated by up to 40% for white spruce. We conclude that ocular assessments are acceptable for assessing hemlock looper defoliation on ®r and for predicting subsequent tree mortality. 2. Mortality of individual balsam fir trees was <5% at total defoliation levels below 75% and 92% when total defoliation exceeded 90% (ocular estimates for all age classes of foliage). Mortality resulting from hemlock looper feeding on all age classes of foliage over a 2-year period was 28% higher than for similar amounts of defoliation caused by spruce budworm over 6±8 years. 3. Fir mortality was significantly related to DBH class in both lightly and severely defoliated plots, with trees in the smallest DBH class (<11 cm) having a significantly higher mortality rate than trees in larger DBH classes. 4. Fir mortality (both percent stems/ha and m3/ha) was strongly related to increasing levels of cumulative (current) defoliation (r2 0.70±0.79) for ocular assessments on 1990±1993 foliage age classes, branch sample estimates for 1987±1993 foliage, and branch sample estimates for 1990± 1993 foliage. However, ocular assessment for 1990±1993 foliage showed the strongest relationship for both percent stems/ha and m3/ha mortality, with r2 of 0.79 and 0.75, respectively. 5. Balsam fir mortality in severely and lightly defoliated plots averaged 63% and 4% of the stems/ha, respectively. Merchantable volume mortality of balsam fir in severely and lightly defoliated plots averaged 88 and 5.5 m3/ha, respectively. 6. Overall, the eastern hemlock looper outbreak in New Brunswick from 1989±1993 totaled 12 400 ha of defoliation. Based on mortality rates calculated
We thank Drs. Allan Carroll, Jon Sweeney, Mr. Lester Hartling and two anonymous reviewers for helpful review comments on an earlier draft of this paper. References Bowers, W.W., 1993. Impact of eastern hemlock looper, Lambdina fiscellaria fiscellaria (Guen.), on balsam fir forests in Newfoundland. Proc. Northeastern Forest Pest Council and 25th Annual Northeastern Forest Insect Work Conf., March 8±10, 1993, Latham, NY, pp. 17±18. Carolin, V.M., Lejeune, R.R., 1967. The western hemlock looper, Lambdina fiscellaria lugubrosa (Hulst). In: Davidson, A.G., Prentice, R.M. (Eds.), Important Forest Insect and Diseases of Mutual Concern to Canada, The United States and Mexico. Can. Dept. of For. and Rural Devel., Ottawa, ON, pp. 123±124. Carroll, A.L., Hudak, J., Meades, J.P., Power, J.M., Gillis, T., McNamee, P.J., Wedeles, C.H.R., Sutherland, G.D., 1995. EHLDSS ± a decision support system for management of the eastern hemlock looper. In: Power, J.M., Strome, M., Daniel, T.C. (Eds.), Proc. Decision Support 2001. Am. Soc. Photogrammetry and Remote Sensing, Bethesda, MD, pp. 807±824. Carroll, W.J., 1956. History of the hemlock looper (Lambdina fiscellaria fiscellaria [Guen.]) (Lepidoptera: Geometridae) in Newfoundland and notes on its biology. Can. Ent. 88, 587±599. Carter, N., Hartling, L., 1992. Hemlock looper in New Brunswick. In: Proc. Annual Forest Pest Control Forum, November 17±19, 1992, For. Can., Ottawa, ON, pp. 161±164. Clark, J., 1961. Photosynthesis and respiration in white spruce and balsam fir. NY State Coll. For., Univ. Syracuse. Tech. Bull. No. 85. Clarke, L.J., Carew, G.C., 1987. Forest insect and disease conditions in Newfoundland and Labrador in 1987. Can. For. Serv., Nfld. For. Res. Cent., St. John's, Nfld. Inf. Rep. N-X-259. Davidson, A.G., Prentice, R.M., 1967. Important forest insects and diseases of mutual concern to Canada, the United States and Mexico. Dept. For. Rural Development, Ottawa, ON. Publ. No. 1180. Dobesberger, E.J., 1989. A sequential decision plan for the management of the eastern hemlock looper, Lambdina fiscellaria fiscellaria (Lepidoptera: Geometridae), in Newfoundland. Can. J. For. Res. 19, 911±916. Drooz, A.T. (Ed.), 1985. Insects of Eastern Forests. USDA For. Serv., Washington, DC. Misc. Publ. No. 1426. Fettes, J.J., 1950. Investigations of sampling techniques for population studies of the spruce budworm on balsam fir in Ontario. For. Insect. Lab, Sault Ste. Marie. Annu. Tech. Rep. 4. pp. 163±401.
D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87 Freese, F., 1960. Testing accuracy. For. Sci. 6, 139±145. Harris, J.W.E., Dawson, A.F., Brown, R.G., 1982. The western hemlock looper in British Columbia 1911±1980. Can. For. Serv., Pacific For. Res. Cent., Victoria, BC. Inf. Rep. BC-X-234. Hartling, L., Carter, N., 1991. Forest protection program against hemlock looper in New Brunswick in 1990. N.B. Dept. Nat. Resour. and Energy, Timber Manage. Branch, Fredericton, NB, 25 pp. Hartling, L., MacNutt, P.M., Carter, N., 1991. Hemlock looper in New Brunswick: notes on biology and survey methods. N.B. Dept. Nat. Resour. and Energy, Timber Manage. Branch, Fredericton, NB, 25 pp. Honer, T.G., Ker, M.F., Alemdag, I.S., 1983. Metric timber tables for the commercial tree species of central and eastern Canada. Can. For. Serv., Marit. For. Res. Cent., Fredericton, NB. Inf. Rep. M-X-140. Hudak, J., LaFlamme, G., Meades, J.P., 1978. Deterioration of balsam fir damaged by the eastern hemlock looper in Newfoundland. Can. For. Serv., Nfld. For. Res. Cent., St. John's, Nfld. Inf. Rep. N-X-157. Hudak, J., Raske, A.G., West, R.J., 1987. Some questions about the hemlock looper outbreak and its control. Woody Points 16(2), 5±11. Johnson, N.E., Shea, K.R., Johnsey, R.L., 1970. Mortality and deterioration of looper-killed hemlock in western Washington. J. For. 68, 162±163. Kinghorn, J.M., 1954. The influence of stand composition on the mortality of various conifers, caused by defoliation by the western hemlock looper (Ellopia fiscellaria lugubrosa [Hulst]) on Vancouver Island, British Columbia. For. Chron. 30, 380±400. MacLean, D.A., 1990. Impact of forest pests and fire on stand growth and timber yield: implications for forest management planning. Can. J. For. Res. 19, 391±404.
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MacLean, D.A., Lidstone, R.G., 1982. Defoliation by spruce budworm: estimation by ocular and shoot-count methods and variability among branches, trees, and stands. Can. J. For. Res. 12, 582±594. MacLean, D.A., Ostaff, D.P., 1989. Patterns of balsam fir mortality caused by an uncontrolled spruce budworm outbreak. Can. J. For. Res. 19, 1087±1095. Magasi, L.P., 1990. Forest pest conditions in the Maritimes in 1989. Can. For. Serv., Marit. For. Res. Cent., Fredericton, NB. Inf. Rep. M-X-177. Magasi, L.P., 1993. Forest pest conditions in the Maritimes in 1992. Can. For. Serv., Marit. For. Res. Cent., Fredericton, NB. Inf. Rep. M-X-183E. Martineau, R., 1984. Insects harmful to forest trees. Multiscience Publ. Ltd. and Can. For. Serv., Ottawa, ON. For. Tech. Rep. 32, 84±87. Otvos, I.S., Clark, R.C., and Clarke, L.J., 1971. The hemlock looper in Newfoundland: The outbreak 1966 to 1971 and aerial spraying, 1968 and 1969. Can. For. Serv., Nfld. For. Res. Cent., St. John's, Nfld. Inf. Rep. N-X-68. Otvos, I.S., Clarke, L.J., Durling, D.S., 1979. A history of recorded eastern hemlock looper outbreaks in Newfoundland. Can. For. Serv., Nfld. For. Res. Cent., St. John's, Nfld. Inf. Rep. N-X179. Piene, H., MacLean, D.A., Wall, R.E., 1981. Effects of spruce budworm-caused defoliation on the growth of balsam fir: experimental design and methodology. Can. For. Serv., Marit. For. Res. Cent. Fredericton, NB. Inf. Rep. M-X-128. Trail, J.H., Devine, M.E., 1994. The impact of the current hemlock looper (Lambdina fiscellaria [Guen.]) outbreak in selected severely damaged stands of eastern hemlock. Maine State For. Serv., Augusta, ME, 16 pp.
The impact of hemlock looper (Lambdina ®scellaria ®scellaria (Guen.)) on balsam ®r and spruce in New Brunswick, Canada David A. MacLeana,b,*, Paul Ebert1,b b
a Canadian Forest Service, Atlantic Forestry Centre, P.O. Box 4000, Fredericton, NB, Canada, E3B 5P7 Faculty of Forestry and Environmental Management, University of New Brunswick, P.O. Box 44555, Fredericton, NB, Canada, E3B 5A3
Accepted 9 November 1998
Abstract In 1989, the ®rst recorded outbreak of hemlock looper (Lambdina ®scellaria ®scellaria (Guen.)) occurred in New Brunswick, Canada. Data were collected from ten plots established in an area infested from 1992±1994, to assess impacts of hemlock looper. Ocular and branch sample assessments of current defoliation and ocular assessments of total defoliation (all age classes of foliage) were conducted for balsam ®r (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.). Stand response was assessed and related to cumulative defoliation. Ocular assessments were found to accurately estimate defoliation, which was signi®cantly related to tree mortality. Ninety-two percent of balsam ®r trees that had cumulative defoliation >90% died. Mortality of balsam ®r was signi®cantly (p < 0.05) related to tree size, in both lightly and severely defoliated plots; trees with DBH <11 cm sustained 22±48% higher mortality than larger trees. Mortality of balsam ®r, in terms of both percent stems/ha and m3/ha merchantable volume, increased exponentially in relation to three estimates of cumulative (summed) plot mean defoliation. The strongest relationships (r2 0.75±0.79) were between mortality and the ocular defoliation assessment for 1990±1993 foliage. Tree mortality caused by the looper outbreak ranged from 4±14% stems/ha in lightly defoliated and from 32±100% in severely defoliated plots; merchantable volume killed was 3± 14 m3/ha and 51±119 m3/ha, respectively. Relationships between mortality and defoliation were similar when defoliation was assessed for 1987±1993 and 1990±1993 foliage age classes. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Defoliation; Hemlock looper; Mortality; Insect impact
1. Introduction The eastern hemlock looper (Lambdina ®scellaria ®scellaria (Guen.)) is an insect defoliator native to North America (Martineau, 1984). It is distributed *Corresponding author. Tel.: +1-506-452-3580; fax: +1-506452-3525; e-mail: [email protected] 1 Present address: TDB Forestry Services Ltd., RR#1, Site 31, Comp. 2, Prince George, BC, Canada V2N 2H8.
from Newfoundland to Alberta in Canada and south to Georgia in the eastern US (Drooz, 1985). The larvae feed on a variety of conifer and deciduous trees; however, tree mortality is restricted primarily to mature stands of the principal host trees, balsam ®r (Abies balsamea [L.] Mill.) and eastern hemlock (Tsuga canadensis [L.] Carr.) (Davidson and Prentice, 1967). In eastern Canada, the main host tree is balsam ®r (Martineau, 1984). Hemlock loopers are aggressive defoliators that feed on all age classes of foliage
0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 8 ) 0 0 5 2 7 - 1
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(Dobesberger, 1989). As a result, this insect can kill trees after only 1±2 years of severe defoliation (Hudak et al., 1978). Hemlock looper feeding is done openly from the third instar onward and all age classes of foliage are consumed (Carroll, 1956; Carolin and Lejeune, 1967; Otvos et al., 1971; Dobesberger, 1989). Older larvae are very wasteful feeders, typically eating a small part of a needle and then moving on to another (Otvos et al., 1971). Damaged stands have a characteristic reddish-brown color that results from the desiccation of damaged needles. Eight extensive hemlock looper outbreaks have been recorded in Newfoundland, Canada during the 20th century (Otvos et al., 1979). During the 1969± 1973 outbreak, which encompassed 1.5 million ha, 10 million cubic meters of wood was killed (Hudak et al., 1987). The 1983±1986 outbreak in Newfoundland started in two widely separated areas and resulted in 52 000±215 000 ha of moderate and severe defoliation (Clarke and Carew, 1987). It has been estimated that, from 1947 to 1991, the volume of trees killed by hemlock looper exceeded 25 million m3, enough supply for the three paper mills in Newfoundland for 7 years (Bowers, 1993). The ®rst recorded outbreak of hemlock looper in New Brunswick, Canada began in the Christmas Mountains area in the north-central part of the province in 1989 (Magasi, 1990; Hartling et al., 1991; Carter and Hartling, 1992). This outbreak resulted in 3500±3800 ha of defoliation each year from 1989 to 1991, 1500 ha in 1992, and no defoliation in 1993 (Magasi, 1990, 1993; Hartling and Carter, 1991; Carter and Hartling, 1992). The outbreak occurred in mature to overmature stands of balsam ®r and collapsed in 4 years (Carter and Hartling, 1992; Magasi, 1993), with smaller individual infestations lasting 1±2 years. The severity of the outbreak was indicated by the extent of the insecticide spray programs of 12 100± 17 000 ha conducted in New Brunswick in 1990, 1991, and 1993 (Hartling and Carter, 1991), as well as by harvest operations to salvage dead wood conducted by several forest industries. Little information exists to quantify the patterns of tree mortality resulting from hemlock looper outbreaks, although this is necessary for forest management planning (MacLean, 1990). The objective of this paper was to determine the impacts of the ®rst
recorded outbreak of hemlock looper in northcentral New Brunswick. Speci®c topics addressed, for balsam ®r, white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.) were: (1) a comparison of the accuracy of ocular versus detailed branch sample defoliation assessments for individual foliage age classes; (2) the relation of individual tree mortality to cumulative defoliation and to diameter class; (3) the temporal pattern of mortality; (4) the relation of mortality per stand, in terms of both percent stems/ha and m3/ha merchantable volume, to cumulative defoliation; and (5) an evaluation of the total merchantable volume (m3/ha) impact in the stands. 2. Materials and methods 2.1. Plot description Ten permanent sample plots (PSPs) were established near the Christmas Mountains area of northcentral New Brunswick, Canada in the spring of 1993, in a hemlock looper infestation that began in 1992, but which originated from the 1989 outbreak (Fig. 1). The PSPs were remeasured in the fall of 1993, 1994, and 1995. Locations were originally selected to represent different levels of 1992 and probable 1993 defoliation, based on 1993 population forecasts. Each PSP consisted of three variable radius prism points (BAF 2.0 m2/ha) located 40 m apart, with permanently numbered trees. 2.2. Defoliation measurements Defoliation was assessed using three different methods: ocular assessment for each tree of annual defoliation of each of the 1990±1993 age classes, ocular assessment for each tree of total defoliation (all age classes of foliage combined) each year, and branch sample assessment of defoliation of each of the 1987±1993 age classes of foliage. As hemlock looper feeds on all age classes, defoliation estimates in a given year re¯ect all cumulative feeding to date. 2.2.1. Ocular assessment of annual defoliation Using binoculars, the crown of each tree was assessed for defoliation of each of the three most
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79
2.2.2. Ocular assessment of total defoliation The total cumulative defoliation of each tree was also estimated directly using binoculars. Total defoliation on all age classes of foliage combined was estimated in seven classes: 0%, 1±10%, 11±25%, 26± 50%, 51±75%, 76±90%, and 91±100%, as used in previous spruce budworm (Choristoneura fumiferana (Clem.)) impact studies (Piene et al., 1981). Total defoliation was assessed four times: in the spring of 1993 for 1992 defoliation and in the fall of 1993, 1994, and 1995, for 1993±1995 defoliation, respectively. Average total defoliation each year was calculated, based on the midpoints of classes, per species per plot.
Fig. 1. Areas defoliated by the eastern hemlock looper from 1989 to 1993 in the first recorded outbreak in New Brunswick, Canada.
recent foliage age classes, rated in six classes (0±10%, 11±20%, 21±40%, 41±60%, 61±80%, and 81±100%). Mean defoliation per age class per tree was then calculated based on midpoints of the classes (MacLean and Lidstone, 1982). In the spring of 1993, defoliation was assessed for each of the 1990±1992 foliage age classes; in the fall of 1993, 1994, and 1995, each of the 1991±1993, 1992±1994, and 1993±1995 age classes were assessed, respectively. It was not possible to estimate defoliation of more than three age classes of foliage using binoculars, because newer foliage obscured older age classes. Most defoliation occurred in 1992, with small amounts in 1993 and virtually none in 1994 or 1995; therefore, for each tree, the highest observed defoliation on each of the 1990±1993 foliage age classes, where there was more than one measurement, were summed to calculate cumulative defoliation per tree. Average cumulative defoliation was then calculated per species per plot.
2.2.3. Branch sample assessment of annual defoliation The branch assessment of defoliation was by the shoot-count method developed by Fettes (1950). In the fall of 1993, one whole mid-crown branch sample was obtained, using pole pruners, from each of two balsam ®r trees per prism point, or six per plot. Branch samples from spruce also were collected when present: six white spruce from Plot 2, two from Plots 3 and 8, three from Plot 9, and four from Plot 10; ®ve black spruce branches from Plot 6, and four from Plot 10. Defoliation of each of the 25 shoots from each of the 1987±1993 foliage age classes was assessed on each branch, using the same classes as for ocular estimates of annual defoliation, and average defoliation per age class per branch was calculated. Defoliation then was summed for the 1987±1993 and for the 1990±1993 foliage age classes, to calculate two other cumulative defoliation estimates. Average branch sample cumulative defoliation from 1987 to 1993 and from 1990 to 1993 was calculated per species per plot. Since hemlock looper feeds on all age classes of foliage, defoliation assessments must represent feeding on all age classes of foliage. The maximum current defoliation that one age-class can incur is 100%. Therefore, the cumulative (summed) branch sample defoliation estimates calculated for four (1990±1993) and seven (1987±1993) age classes would have maximum values of 400% and 700%, respectively. 2.3. Plot volume and mortality determinations In the spring of 1993 and the fall of 1994, diameter at breast height (DBH, at 1.3 m) of each tree was
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measured with a diameter tape to the nearest 0.1 cm. The height of living trees was measured with a Suunto clinometer to the nearest 0.1 m in the fall of 1994. From the data collected, merchantable volume was calculated for each tree using volume tables from Honer et al. (1983). Calculated volume/tree was multiplied by the tree factor for each tree to determine m3/ha represented by each plot tree. Volume per hectare then was summed and divided by the number of sample points to calculate m3/ha for each plot. The number of trees for all points was summed and divided by the number of points to determine stand density per plot. The number of stems/plot was then multiplied by the tree factor to calculate stems/ha. All trees in each plot were checked for mortality at each measurement. Tree mortality was determined by the death of the inner bark at breast height (Hudak et al., 1978). Mortality was calculated in terms of both volume (m3/ha) and density (stems/ha). 2.4. Statistical analyses Ocular and branch sample assessments of defoliation were compared by regression analysis and by Freese's test of accuracy (Freese, 1960). The analysis
using y a bx compared ocular assessment (y, representing the method to be tested) versus branch assessment (x, representing the `true' value). Chi-square analyses were done to test the null hypothesis that mortality was not signi®cantly different among ®ve DBH classes (<11.1, 11.1±15, 15.1±19, 19.1±23 and >23.1 cm). This test was conducted separately for dead balsam ®r trees in plots with light defoliation (summed annual <200%) and with moderate to severe defoliation (summed annual 200%). Signi®cance level for all statistical tests was p 0.05. 3. Results and discussion 3.1. Stand characteristics The ten sample plots represented a range of stand conditions (Table 1). Softwood species composition ranged from 55% to 98% balsam ®r, 10±24% black spruce, and 2±37% white spruce. Hardwood species composition ranged from 2±16% white birch (Betula papyrifera Marsh.), 5±7% yellow birch (Betula alleghaniensis Britton), 0±6% red maple (Acer rubrum L.), and 0±2% trembling aspen (Populus tremuloides Michx). The plots were all softwood dominated, with
Table 1 Mensurational characteristics and mortality in ten variable-radius plots sampled in north-central New Brunswick, Canada Plot
A. Lightly 1 2 3 4 5 10
DBH (cm)
defoliated plots 15 13 25 18 19 13 15 12 17 12 20 14
B. Severely 6 7 8 9 a
Height (m)
defoliated plots 18 14 14 12 19 14 20 14
Density (stems/ha)
Mortality (% balsam fir stems/ha)a
Merchantable volume (m3/ha) Spruceb
Balsam fir Alive
Dead
2350 990 1470 1990 1170 1860
4 0 2 4 3 8
181 116 176 193 123 191
3 0 4 8 4 14
1210 2020 1200 1200
100 69 61 32
0 46 60 107
119 94 88 51
Hardwoodc
Alive
Dead
82 19
5
51 30 9 19
6
4 10 27 16 15 3 0 3 15 3
In addition to balsam fir mortality, there was 6% white spruce mortality in Plot 2 and 22% black spruce mortality in Plot 6. Spruce species were black spruce in Plot 6, 10% black and 8% white spruce in Plot 10, and white spruce in the remaining plots. c Hardwood species were 1±16% white birch (in all plots except No. 6), 6% red maple in Plot 2, and 5±7% yellow birch and 2% trembling aspen in Plots 3 and 4. b
D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
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82±100% spruce-®r. Only three of the plots had >10% spruce. Hardwood composition was always <20%, with half of the plots having 10±20% hardwoods. Average DBH was <15 cm in three plots, 15±20 cm in six plots, and 25 cm in one plot (Table 1). Plot 2 was the most open (990 stems/ha) and had the largest trees; stand densities were 1100±1500 stems/ha in ®ve plots, 1800±2000 in two, and 2000±2500 stems/ha in two plots. Softwood merchantable volume ranged from 127±256 m3/ha and hardwood volume from 3±27 m3/ ha. Six plots sustained <10% mortality (% stems/ha), one had 32%, and three plots had 60±100% mortality. 3.2. Accuracy of ocular estimation of hemlock looper defoliation Defoliation estimation consisted of both ocular and branch sample defoliation assessments of individual foliage age classes. The branch assessment method is time consuming and while suitable for research purposes, is not feasible for large operational surveys (MacLean and Lidstone, 1982). Ocular assessments are faster, but they are of little use if they give inaccurate assessments of defoliation. Regression analyses showed that ocular estimates for defoliation of the 1990±1993 balsam ®r foliage age classes explained 97% of the variability in average branch sample defoliation assessments for balsam ®r (Fig. 2). Ocular estimates were overestimated by an average of 14% at all levels of defoliation. For white spruce, however, only 65% of the variability in branch samples was explained by the ocular estimates (Fig. 2), which were overestimated by 65±100% when defoliation exceeded 50%. MacLean and Lidstone (1982) determined that plot means of individual tree ocular assessments of defoliation by spruce budworm averaged 8% less than plot means of shoot-count (branch) assessments. This was concluded to be a suf®cient level of accuracy for surveys and some research purposes. Freese's test of accuracy (Freese, 1960) gave values of 26% accuracy for balsam ®r and 40% accuracy for white spruce. The ocular assessment would seem acceptable for assessing hemlock looper defoliation on balsam ®r but rather inaccurate on white spruce. Perhaps more accurate ocular assessments of defoliation on white spruce could be achieved if additional training of observers was conducted using trees with
Fig. 2. Comparison of plot mean ocular and branch sample cumulative (summed current) defoliation assessments for 1990± 1993 foliage age classes for balsam fir in ten plots in north-central New Brunswick. Data are also presented for the white spruce component, for the five plots that contained white spruce (Table 1). Accuracy is based on Freese's (1960) test.
known (branch sampled) defoliation. Also, there were only ®ve plots or data points for the white spruce relationship, and perhaps accuracy and r2 would be higher with a larger sample. 3.3. Patterns of cumulative defoliation Balsam ®r was present in all plots and the mean cumulative (summed) defoliation for 1990±1993 foliage age classes per plot ranged from 34% to 400%, where 400% represents all the foliage removed from four age classes (Fig. 3). No measurable defoliation occurred in 1994 or 1995. White spruce was present in ®ve plots and sustained mean cumulative defoliation of 27±200%. Black spruce was present in only two plots, which had mean cumulative defoliation of 32% and 180%. Based on the patterns of defoliation of balsam ®r, the plots were divided into two classes, lightly defoliated (cumulative defoliation <200%) and severely defoliated (200%). Six plots (Nos. 1±5 and 10) were lightly defoliated and four were severely defoliated. For ®r, ®ve of the lightly defoliated plots lost <1 age class of foliage while the sixth lost 1±2 age classes of foliage; in contrast, three of the severely defoliated plots lost 2±3 of the 1990±1993 foliage age classes and
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Fig. 4. The relation of individual tree balsam fir mortality to total defoliation class (ocular estimate for all age classes of foliage), for all ten plots combined.
Fig. 3. Plot mean cumulative current defoliation of 1990±1993 foliage age classes for ten plots. Severely defoliated plots are depicted with solid lines and lightly defoliated plots with dotted lines. Data are based on six branch samples per species per plot, and are grouped by species. Spruce is black for Plot 6, a mixture of black and white for Plot 10, and white for the other plots.
the fourth lost all four age classes. White spruce in all ®ve plots lost <1 age class of foliage. Black spruce in one plot lost <1 age class of foliage and slightly more than two age classes in Plot 6 (Fig. 3). Observed ranges of cumulative defoliation of ®r in severely defoliated plots were 310±400%, versus 46±108% in ®ve lightly defoliated plots and 207% in Plot 10 (Fig. 3). Comparable values for spruce were 80±200% in severely defoliated and 22±26% in lightly defoliated plots (Fig. 3). 3.4. The relation of individual tree mortality to total defoliation The relation of balsam ®r mortality to total defoliation class (ocular assessment for all age classes of foliage combined) is presented in Fig. 4. There was virtually no mortality (<1%) of balsam ®r trees with
total defoliation <50%. Five percent of the trees that sustained 50±75% and 11% sustaining 75±90% defoliation died. However, almost all (92%) ®r trees with 90±100% total defoliation died (Fig. 4). Sample sizes per defoliation class ranged from 46 to 99 trees, for all classes with >5% defoliation. When defoliation caused by hemlock looper is light, and no defoliation occurs the following year, the trees usually survive; however, annual increment will be reduced (Martineau, 1984). Hudak et al. (1978) noted in stands composed of more than 83% balsam ®r, that tree survival was 50±60% in the 40±50% defoliation class versus about 80±90% in the 20±30% class. With >80% total defoliation caused by the hemlock looper, most ®r trees died within 4 years of the initial defoliation (Hudak et al., 1978). On average, 64, 21, 14, and 1% of balsam ®r defoliated by the spruce budworm died with cumulative defoliation, respectively, of >90, 76±90, 51±75 and 26±50% (MacLean and Ostaff, 1989). Budwormcaused mortality for the >90% defoliation class recorded by MacLean and Ostaff (1989) was 28% lower than hemlock looper-caused mortality recorded in this study; however, 10% and 9% more budwormcaused mortality occurred with 50±75% and 76±90% defoliation, respectively. These differences probably resulted from the temporal pattern of defoliation, with spruce budworm feeding primarily on current foliage only for 6±8 consecutive years (MacLean and Ostaff,
D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
Fig. 5. Temporal patterns of (A) balsam fir and (B) spruce mortality (% stems/ha from 1992-1995) for ten plots. Severely defoliated plots are depicted with solid lines and lightly defoliated plots with dotted lines.
1989), versus hemlock looper feeding on all foliage age classes over a 2-year period. 3.5. Temporal patterns of mortality In the lightly defoliated plots (cumulative defoliation <200%), little balsam ®r mortality occurred (Fig. 5). From 0±11% of trees were dead in the ®rst measurement, and this remained virtually constant (4± 14%) until 1995 (Fig. 5(A)). As will be seen in the next section, these were small, suppressed trees that probably represent natural mortality. In the severely defoliated plots, 32±100% of the balsam ®r died. All the balsam ®r trees died in Plot 6 (the most severely defoliated, which had 29% dead in the ®rst measurement), 69±85% died in Plots 7 and 8, and 32% died in Plot 9 (Fig. 5). It is somewhat surprising that mortality was substantially less in Plot 9 than in Plots 7 or 8, since defoliation was generally
83
similar (Fig. 3). Overall, only 4% of the 252 balsam ®r trees in the lightly defoliated plots died, versus 63% of the 142 ®r in the severely defoliated plots. Most of the mortality occurred between the fall 1993 and fall 1994 measurements, or 2 years after the most severe defoliation (Fig. 5). In severe outbreaks of hemlock looper, when feeding occurs on all foliage age classes, tree mortality typically occurs 1±2 years later (Kinghorn, 1954; Johnson et al., 1970; Hudak et al., 1978; Martineau, 1984). Kinghorn (1954) found that about 60% of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-®r (Pseudotsuga menziesii (Mirb.) Franco) trees with 70% or more total defoliation began to die 1 year after the collapse of the outbreak and mortality continued for another 3 years. Western hemlock looper outbreaks generally last 2±3 years and are less damaging than eastern hemlock looper outbreaks (Harris et al., 1982). Spruce mortality occurred in only two plots (Fig. 5(B)). The lightly defoliated Plot 2 had 12% white spruce mortality, due to competition or other causes, in the initial measurement, and this increased to 18% by 1995. Plot 6 sustained 47% black spruce mortality between 1993 and 1994 (Fig. 5(B)), and was the most severely defoliated, sustaining 100% ®r mortality. 3.6. The relation of mortality to DBH class Some defoliators cause mortality of intermediate and suppressed (smaller) trees at a higher rate than for dominant or co-dominant (larger) trees (Trail and Devine, 1994). Mortality in the lightly defoliated plots was consistently low, ranging from 0% for trees >19 cm DBH, 5% for trees 11.1±19 cm, and 27% for trees <11 cm (Table 2A). This probably represents natural, competition-caused mortality in the stands, and includes dead trees present in the initial measurement as well as trees that died from 1993 to 1995. There was a signi®cant difference in mortality between DBH classes, with the smallest trees (DBH class <11 cm) having higher (p < 0.05) mortality (27%) than trees in the larger classes (5%). Mortality in each DBH class in severely defoliated plots was high, ranging from 52% to 100% (Table 2B). Mortality differed between DBH classes, with trees in the smallest DBH class (<11 cm) having signi®cantly (p < 0.05) higher mortality (100%) than
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Table 2 Distribution of balsam fir mortality by DBH class in six lightly defoliated and four severely defoliated plots in north-central New Brunswick DBH class (cm)
Number of trees
Dead trees (%)
A. Lightly defoliated plots <11 11.1±15 15.1±19 19.1±23 >23.1
11 66 85 67 23
27a 5 5 0 0
B. Severely defoliated plots <11 11.1±15 15.1±19 19.1±23 >23.1
14 30 58 24 16
100a 73 52 63 56
a Indicates a percentage significantly different from all others within a defoliation class (c2, p < 0.05). A test on 2 5 contingency tables was followed by a test of the <11 class versus combined 11.1 to >23.1 cm.
ments for each of the 1990±1993 foliage age classes; summed branch sample assessments for each of the 1990±1993 age classes; and summed branch sample assessments for each of the 1987±1993 age classes. Relationships with all three defoliation variables were very similar, with an exponentially increasing curve form and r2 ranging from 0.74 to 0.79; because of this similarity, only the mortality versus ocular defoliation relationships are presented in Fig. 6. For both measures of mortality, slightly more of the variability (75± 79%) was explained by the ocular defoliation estimates than by the branch sample estimates (70±74%). Mortality was low (<10%) at cumulative defoliation <200% and then rapidly increased with defoliation (200% (Fig. 6). Branch assessment of defoliation on the 1987±1993 foliage had three additional age classes of foliage included in the cumulative defoliation assessment than for 1990±1993 foliage. However, it showed a similar trend in mortality as the curves for both branch and
trees in the larger classes (52±73%). Smaller trees were probably suppressed by competition and unable to withstand the defoliation, or possibly could have sustained higher defoliation than larger trees, if larval dispersal resulted in more larvae on understory trees. MacLean and Ostaff (1989) determined that spruce budworm-caused mortality was evenly distributed among DBH classes. Trail and Devine (1994) noted that mortality caused by hemlock looper was not related to tree size in severely defoliated areas. They concluded that hemlock looper populations and resulting damage were intense enough in the majority of their study area to cause mortality of all tree sizes when all related stress factors were combined. Otvos et al. (1979) demonstrated that hemlock looper infestations can be quite variable, resulting in uneven tree mortality over individual infestations. This would help to explain the wide range of mortality (32±100%) observed among severely defoliated plots (Table 2B). 3.7. The relation of mortality to cumulative current defoliation Average balsam ®r mortality per stand (percent stems/ha and m3/ha) was related to three measures of cumulative defoliation: summed ocular assess-
Fig. 6. The relation of balsam fir mortality (% stems/ha and m3/ha) to cumulative current defoliation, calculated as the sum of ocular estimates for each of the 1990±1993 foliage age classes.
D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
ocular cumulative 1990±1993 defoliation. The three additional older age classes included in the 1987±1993 cumulative defoliation assessment provided little increase in the ability to explain the mortality observed in plots. Hemlock looper defoliates all age classes of foliage (Carroll, 1956; Martineau, 1984; Dobesberger, 1989). On an average, defoliation of the three oldest age classes (1987±1989) was 110% for the lightly defoliated plots and 270% for the severely defoliated plots. It would appear that when defoliation occurs on the four most recent foliage age classes, older age classes will be defoliated in about the same proportion. Natural needle fall and declining photosynthetic capacity in the older age classes (Clark, 1961) also would tend to decrease the importance of older foliage, such that more recent age classes of foliage contribute most to tree growth. Therefore, a cumulative defoliation assessment for the four most recent foliage age classes gave an accurate assessment of defoliation and prediction of subsequent balsam ®r mortality. Balsam ®r merchantable volume mortality differed greatly between lightly and severely defoliated stands, at averages of 5.5 and 88 m3/ha, respectively (Table 1). Percentage tree mortality averaged 4 and 63% stems/ha in lightly and severely defoliated stands, respectively (Table 2). In both lightly and severely defoliated stands, mortality of balsam ®r was higher than for either white or black spruce, with the exception of Plot 2 (Table 1), which sustained no balsam ®r mortality and 6% white spruce mortality. 3.8. Total impact of the hemlock looper outbreak The ten plots examined in this paper represent a sample of mature to overmature ®r spruce stands in north-central New Brunswick. These plots all were defoliated to some degree in this ®rst recorded outbreak of hemlock looper in New Brunswick. The outbreak started in 1989 and collapsed in 1993 (Magasi, 1990, 1993; Carter and Hartling, 1992). Defoliation in the study area occurred mainly in 1992, with some in 1993. The overall outbreak consisted of several smaller individual infestations (Fig. 1). A total of 12 400 ha was defoliated by the hemlock looper during this outbreak, including approximately 1500, 4900, and 6000 ha of light, moderate, and severe defoliation, respectively
85
(Magasi, 1990, 1993; Carter and Hartling, 1992). In the study area, hemlock looper defoliated balsam ®r to a greater degree than white spruce or black spruce, consistent with results of Martineau (1984). Fir mortality (percent stems/ha) in severely and lightly defoliated plots averaged 63% and 4%, respectively. Combined ®r and spruce mortality, in terms of merchantable volume, averaged 57% in the severely defoliated and 2% in the lightly defoliated plots. Using the total areas in light, moderate, and severe defoliation classes in New Brunswick cited above, and rates of mortality from the plots in this study (severe with approximately 350% defoliation and 100 m3/ha killed, moderate with 200% defoliation and 10 m3/ha killed, light with negligible effects), an estimated 650 000 m3 of merchantable volume of balsam ®r was killed during the outbreak. The interval between eastern hemlock looper outbreaks in Newfoundland has varied from 7 to 18 years and usually lasted between 6 and 9 years. However, individual infestations may collapse in 2±3 years (Otvos et al., 1979). The interval between western hemlock looper outbreaks is 4±17 years and they usually last 2±3 years (Harris et al., 1982). Dry warm weather is favorable for the development of outbreaks, which usually start as isolated, small infestations in mature to overmature stands with a high content of balsam ®r (Otvos et al., 1979). These individual infestations under favorable conditions coalesce to form groups of infestations or to form a large irregular infestation forming part of an outbreak (Otvos et al., 1979). The implications of tree mortality resulting after just 1 year of severe defoliation have far reaching impacts on forest management decisions. The ability to salvage such areas is limited by harvesting, mill, and market capacity along with the ability to gain access to these dead stands. The impacts of large-scale outbreaks have serious environmental and economic rami®cations over both the short and long term (Carroll et al., 1995). Such impacts include decreased water quality, wildlife habitat, and limiting future supplies of wood for lumber along with pulp and paper production. A decision support system for the management of eastern hemlock looper populations in insular Newfoundland has been developed (Carroll et al., 1995). Individual models were constructed to predict probability of defoliation, timber mortality
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D.A. MacLean, P. Ebert / Forest Ecology and Management 120 (1999) 77±87
and decay, risk of impending outbreaks, and larval phenology (Carroll et al., 1995).
from the ten plots in north-central New Brunswick, an estimated 650 000 m3 merchantable volume of balsam fir was killed during this outbreak.
4. Conclusions
Acknowledgements
1. Ocular assessments of cumulative (current) defoliation for the 1990±1993 foliage age classes explained 97% and 65% of the variability, in comparison with detailed branch sample estimates, for balsam ®r and white spruce, respectively. Ocular estimates were overestimated by an average of up to 14% for ®r, and over or underestimated by up to 40% for white spruce. We conclude that ocular assessments are acceptable for assessing hemlock looper defoliation on ®r and for predicting subsequent tree mortality. 2. Mortality of individual balsam fir trees was <5% at total defoliation levels below 75% and 92% when total defoliation exceeded 90% (ocular estimates for all age classes of foliage). Mortality resulting from hemlock looper feeding on all age classes of foliage over a 2-year period was 28% higher than for similar amounts of defoliation caused by spruce budworm over 6±8 years. 3. Fir mortality was significantly related to DBH class in both lightly and severely defoliated plots, with trees in the smallest DBH class (<11 cm) having a significantly higher mortality rate than trees in larger DBH classes. 4. Fir mortality (both percent stems/ha and m3/ha) was strongly related to increasing levels of cumulative (current) defoliation (r2 0.70±0.79) for ocular assessments on 1990±1993 foliage age classes, branch sample estimates for 1987±1993 foliage, and branch sample estimates for 1990± 1993 foliage. However, ocular assessment for 1990±1993 foliage showed the strongest relationship for both percent stems/ha and m3/ha mortality, with r2 of 0.79 and 0.75, respectively. 5. Balsam fir mortality in severely and lightly defoliated plots averaged 63% and 4% of the stems/ha, respectively. Merchantable volume mortality of balsam fir in severely and lightly defoliated plots averaged 88 and 5.5 m3/ha, respectively. 6. Overall, the eastern hemlock looper outbreak in New Brunswick from 1989±1993 totaled 12 400 ha of defoliation. Based on mortality rates calculated
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MacLean, D.A., Lidstone, R.G., 1982. Defoliation by spruce budworm: estimation by ocular and shoot-count methods and variability among branches, trees, and stands. Can. J. For. Res. 12, 582±594. MacLean, D.A., Ostaff, D.P., 1989. Patterns of balsam fir mortality caused by an uncontrolled spruce budworm outbreak. Can. J. For. Res. 19, 1087±1095. Magasi, L.P., 1990. Forest pest conditions in the Maritimes in 1989. Can. For. Serv., Marit. For. Res. Cent., Fredericton, NB. Inf. Rep. M-X-177. Magasi, L.P., 1993. Forest pest conditions in the Maritimes in 1992. Can. For. Serv., Marit. For. Res. Cent., Fredericton, NB. Inf. Rep. M-X-183E. Martineau, R., 1984. Insects harmful to forest trees. Multiscience Publ. Ltd. and Can. For. Serv., Ottawa, ON. For. Tech. Rep. 32, 84±87. Otvos, I.S., Clark, R.C., and Clarke, L.J., 1971. The hemlock looper in Newfoundland: The outbreak 1966 to 1971 and aerial spraying, 1968 and 1969. Can. For. Serv., Nfld. For. Res. Cent., St. John's, Nfld. Inf. Rep. N-X-68. Otvos, I.S., Clarke, L.J., Durling, D.S., 1979. A history of recorded eastern hemlock looper outbreaks in Newfoundland. Can. For. Serv., Nfld. For. Res. Cent., St. John's, Nfld. Inf. Rep. N-X179. Piene, H., MacLean, D.A., Wall, R.E., 1981. Effects of spruce budworm-caused defoliation on the growth of balsam fir: experimental design and methodology. Can. For. Serv., Marit. For. Res. Cent. Fredericton, NB. Inf. Rep. M-X-128. Trail, J.H., Devine, M.E., 1994. The impact of the current hemlock looper (Lambdina fiscellaria [Guen.]) outbreak in selected severely damaged stands of eastern hemlock. Maine State For. Serv., Augusta, ME, 16 pp.