The impact of tree and stand characteristics onspruce beetle (Coleoptera: Scolytidae) inducedmortality of white spruce in the Copper RiverBasin, Alaska

Patricia Doak

Abstract: I examined the relationships between individual and stand-level characteristics of white spruce, Picea glauca(Moench) Voss, and the incidence of spruce beetle, Dendroctonus rufipennis Kirby, induced mortality. The study re-gion, in the Kennicott Valley of the Copper River Basin, Alaska, has contained an active spruce beetle epidemic since1989. I investigated the relationship among the individual traits of host age, size (diameter at breast height, DBH), andgrowth rate (basal area increment, BAI) and mortality from the spruce beetle. I also examined the effects of stand den-sity, mean DBH, and mean BAI on percent mortality within plots. Survival was higher for younger, smaller, and faster-growing trees. However, the effect of age is not significant when included in a logistic regression model examining theeffect of individual host traits on host survival. Mortality increased with increasing DBH and decreasing BAI, and therewas a significant interaction between DBH and BAI. While the proportion of individuals killed by the spruce beetlesignificantly differed between stands, I found no significant relationships between stand-level characteristics and mortal-ity rate. This research suggests that the individual traits of host size and growth rate, as well as their interaction are thebest predictors of susceptibility to spruce beetle-induced mortality in this system.

Résumé : J’ai étudié les relations entre les caractéristiques des individus et des peuplements d’épinette blanche, Piceaglauca (Moench) Voss, et l’incidence de la mortalité causée par le dendroctone de l’épinette, Dendroctonus rufipennisKirby. La zone d’étude, qui est située dans la vallée Kennicott dans le bassin de la rivière Copper, en Alaska, est auxprises depuis 1989 avec une épidémie active du dendroctone de l’épinette. J’ai examiné les relations entre les caracté-ristiques individuelles, telles que l’âge de l’hôte, la dimension (diamètre à hauteur de poitrine, DHP) et le taux decroissance (accroissement en surface terrière), et la mortalité causée par le dendroctone de l’épinette. J’ai aussi étudiéles effets de la densité du peuplement, du DHP moyen et de l’accroissement moyen en surface terrière sur le taux demortalité dans les parcelles. Le taux de survie était plus élevé chez les arbres plus jeunes, plus petits et à croissanceplus rapide. Cependant l’âge n’a pas d’effet significatif lorsqu’il est inclus dans un modèle de régression logistique quiteste l’effet des caractéristiques individuelles sur la survie de l’hôte. La mortalité augmentait avec l’augmentation duDHP et la diminution de l’accroissement en surface terrière, et il y avait une interaction significative entre le DHP etl’accroissement en surface terrière. Alors que la proportion d’individus tués par le dendroctone différait significative-ment d’un peuplement à l’autre, je n’ai trouvé aucune relation significative entre les caractéristiques des peuplements etle taux de mortalité. Cette recherche indique que les caractéristiques individuelles, telles que la dimension de l’hôte etson taux de croissance ainsi que l’interaction entre ces facteurs, sont les meilleures variables pour prédire l’incidencede la mortalité causée par le dendroctone dans ce système.

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Introduction

Scolytid beetles are a major stand-replacing disturbance inNorth American coniferous forests (Stark 1982; Flamm et al.1988; Raffa 1988). Examination of the individual and stand-level characteristics predisposing trees to bark beetle attackis of interest for both management of forest resources andunderstanding natural forest dynamics.

In Alaskan boreal forests, the most important naturalcauses of tree mortality are fire, flooding, and outbreaks ofbark beetles (Viereck 1973; Werner and Holsten 1983;Kasischke and French 1995). Throughout Alaska, popula-tions of bark beetles reproduce in spruce trees recently killedby other agents (Holsten et al. 1991). Within stands, in mostyears, bark beetles kill few trees, but extremely high rates ofmortality occur during periods of epidemic infestations,when bark beetles mass-attack host individuals. In Alaska,the most damaging species is the spruce beetle, Dendro-ctonus rufipennis Kirby, which attacks white, Sitka, andLutz spruce (Picea glauca (Moench) Voss, Picea sitchensis(Bong.) Carrière, and Picea lutzii Little, respectively) (Hol-sten and Werner 1990).

Historical records provide evidence that spruce beetle out-breaks have occurred throughout the 20th century in areas

Can. J. For. Res. 34: 810–816 (2004) doi: 10.1139/X03-256 © 2004 NRC Canada

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Received 11 March 2003. Accepted 23 October 2003.Published on the NRC Research Press Web site athttp://cjfr.nrc.ca on 13 April 2004.

P. Doak. Institute of Arctic Biology, P.O. Box 757000,University of Alaska, Fairbanks, AK 99775, USA(e-mail: ffpd@uaf.edu).

south of the Alaska Range (Holsten 1990). Use of tree ringchronologies also provides evidence of large scale epidemicsin the Kenai lowlands back to 1810 (Berg 1998; C. Fastieand E. Berg, unpublished data). Current outbreaks have in-fested and killed spruce on approximately 500 000 ha of theKenai Peninsula since 1969 (Matthews et al. 1997; Wittweret al. 1998) and over 200 000 ha in the Copper River Basinsince 1989 (U.S. Forest Service 1998).

Insect herbivores respond to the individual nutritional anddefensive traits of their hosts (Cates and Alexander 1982;Raffa and Berryman 1983, 1987; Bernays and Chapman1994), as well as population-level attributes of host patches(e.g., density, age structure) (Cromartie 1975; Hard et al.1983; Hard 1985; Bach 1988a, 1988b; Raffa 1988; Lorio1993; Raffa et al. 1993; Wesser and Allen 1999). Under-standing why some hosts are attacked and others are spared,as well as predicting the magnitude and dynamics of out-breaks will necessarily have to consider processes at both ofthese scales.

Research aimed at understanding individual host charac-teristics related to bark beetle susceptibility has emphasizedthe roles of host species, size (often assumed to correlatewith age), growth rate, and phloem thickness (Cates and Al-exander 1982). Many bark beetle species have preferredhosts within their host ranges. The spruce beetle is capableof attacking all species of spruce (Cottrell 1978) and showsno preference for the three host species in Alaska, but wasfound to have higher brood production in white than in Sitkaor Lutz spruce (Holsten and Werner 1990). Black spruce(Picea mariana (Mill.) BSP) is seldom attacked.

The spruce beetle is generally believed to attack and killlarger diameter trees within forest stands (e.g., Holsten1984; Wesser and Allen 1999); however, some studies nowindicate a more complex relationship. When radial growthrate prior to infestation is considered along with tree diame-ter, some researchers find that it is growth rate rather thansize that contributes to risk or that there is a significant inter-action of the two factors (Hard et al. 1983; Hard 1985; VanHees and Holsten 1994). Werner and Holsten (1985) foundno direct relationship between phloem thickness or moisturecontent and the rate of spruce beetle brood development orsurvival; however, they note the correlation among lowerprecipitation, drier phloem, and the lack of spruce beetleoutbreaks in interior Alaska. The most robust generalizationseems to be that slowly growing trees are at the highest riskof spruce beetle-induced mortality, suggesting that radialgrowth rate or basal area increment may provide a goodproxy for a tree’s ability to defend itself against attack.

Population-level characteristics may also influence sus-ceptibility of individuals to attack. High stand density is of-ten correlated with slow growth and increased attack andmortality rates by the spruce beetle (Hard et al. 1983; Hard1985; Wesser and Allen 1999). Dense stands are apt to leadto decreases in photosynthesis, which will result in both de-creased growth and a decline in production of defensivecompounds (Lorio 1993). Dense stands may also better con-tain pheromone plumes and result in greater bark beetle ag-gregation (Lorio 1993; Raffa et al. 1993). Age and sizestructure of stands can influence attack rates, with even-agedold stands of large individuals sustaining the highest mortal-ity (Hard et al. 1983). Raffa (1988) points out that for Den-

droctonus ponderosae, uneven aged stands lead to more fre-quent but much smaller incidences of tree mortality from thebeetle in pockets of older trees.

Most of the research on spruce beetle in Alaska has fo-cused on the Kenai Peninsula epidemic. Examination of hostsusceptibility in distinct regions with a different climate willhelp address whether results from the Kenai can be general-ized to other Alaskan boreal forests.

Here I report on research done in the Wrangell Mountainsof the Copper River Basin, Alaska. While the Kenai Penin-sula has a predominantly maritime climate, the WrangellMountains are drier and experience the greater temperatureextremes typical of a continental climate (Winterberger andWesser 1999). A large U.S. National Park Service study ofspruce beetle impacts in Wrangell – St. Elias National Parkand Preserve indicated that mortality rates were higher inlarge diameter trees (Wesser and Allen 1999; also see Mat-suoka et al. 2001) and denser stands (Allen and Fleming1999). The current research will add to our understanding ofthe factors predisposing host individuals and stands to barkbeetle damage in the Copper River Basin. Specifically I ad-dress (i) the relationships among individual tree age, size,and growth rate; (ii) the effects of these individual traits onsusceptibility to spruce beetle-induced mortality; and(iii) the effects of stand density, mean diameter, and meangrowth rate on percent mortality caused by the spruce beetle.

Materials and methods

Study site and sampling methodsThis study was conducted within Wrangell – St. Elias Na-

tional Park and Preserve near the base of the Kennicott Gla-cier (61°N, 143°W). All sampled stands were situated alongthe west-facing slopes of the valley from 550 to 800 m ele-vation. Beetles in the Kennicott Valley typically have a2-year life cycle, although as in other regions, a 1-year cycleis possible in warm sites (Werner and Holsten 1985).

The forests on these slopes are dominated by white spruce(P. glauca), with only rare individuals of paper birch (Betulapapyrifera Marsh.) interspersed. The understory consists ofalder (Alnus crispa (Ait.) Pursh) stands, red currants (Ribestriste Pall.), club moss (Lycopodium annotinum L.), ferns(Gymnocarpium dryopteris (L.) Newm.), and isolatedpatches of bluejoint (Calamagrostis canadensis var. can-adensis (Michx.) Beauv.). Loso (1998) found no evidence ofstand-replacing fires within my study area. Very sparse, se-lective cutting for firewood and cabin logs has occurredthroughout the study area, but given that study plots con-tained no more than one cut stump, this has had negligibleimpact on stand characteristics.

Spruce beetle populations reached epidemic levels in theregion in 1989 or 1990 (U.S. Forest Service 1998; P. Doak,unpublished data). The outbreak was still active in the valleyin 2002, although the most heavily impacted stands do notappear to be experiencing any new mortality (personal ob-servation).

All research took place at five distinct sites distributedalong a 10-km stretch of the east side of the Kennicott Val-ley (Fig. 1). In 1998, I censused white spruce in five 20 mradius (1257 m2) plots, one at each of these sites. In 1999,an additional plot of 30 m radius (2827 m2) was added at

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each of the three northern sites. In each plot, all trees greaterthan 5 cm diameter at breast height (DBH) were censused.An increment core was taken from each tree and the heightof the core above the root crown was recorded; DBH wasalso measured. Each tree was examined for condition andstatus of bark beetle attack. If a tree was dead or had exitholes, I inspected it for spruce beetle galleries.

Cores were stored in plastic straws with ventilation holesto prevent molding. They were then mounted and sanded be-fore annual rings were counted. When the pith was missed,the distance to the pith was estimated from the curvature ofthe rings. When cores missed the pith by ≤1 cm, the numberof rings to the pith was estimated based on the curvature ofthe rings and the average ring width of the inner-most rings.

Tree ages were further corrected for the height of the core.I determined tree ages at multiple heights for 10 trees ateach of three sites (AZ, BN, NC). I used either cores orcross sections to count rings at from 3 to 11 different heights(ranging from 2 to 200 cm) for each tree. These data werethen used for a repeated measures analysis, examining therelationship between age and sample height controlling for

individual tree. The model yielded the following relation-ship: age = ring count + 0.184(height of the core) (r2 =0.97). This equation was used to correct for the fact that inthe current study, cores were obtained from varying heights.Some trees could not be aged because rot prevented the ac-quisition of complete cores; therefore, the data set of treeswith age estimates is smaller than the data set excluding age.

I measured the last 5 years of radial growth using a slid-ing bench micrometer with a precision of 0.01 mm. Thismeasurement was then used to calculate the 5-year basalarea increment (BAI), BAI = BAt – BA(t–5), where t refers totime in years.

During the summers of 2000 and 2001, I censused treedensity, measured DBH, and assessed bark beetle-inducedmortality in seven additional plots of 30 m radius (one ploteach at the AZ, BN, and NC sites and two plots at both thePY and SM sites; Fig. 1). This allowed for the examinationof the relationship between mortality and stand density. Alltrees that clearly died of causes other than spruce beetle at-tack were excluded from the analyses.

Data analysisData from the eight intensively studied plots were used to

examine the relationships between individual characteristicsand mortality. A general linear model (PROC GLM, SAS In-stitute Inc. 1992) was employed to examine whether DBHdiffered between sites. In this and all other models, residualplots were visually examined to judge whether model as-sumptions were met.

To examine whether DBH is a good predictor of tree age,I employed a linear mixed model (PROC MIXED, SAS In-stitute Inc. 1992) to evaluate the relationship among DBH,survival class (surviving vs. dead), and tree age. Next, theeffects of age, DBH, and survival class on BAI (BAI was lntransformed to normalize the residuals) were examined,again with the use of a linear mixed model. In both mixedmodel analyses, site (AZ, BN, NC, PY, and SM) was in-cluded as a random covariate.

I used logistic regression (PROC LOGISTIC, SAS Insti-tute Inc. 1992) to examine the influence of site, DBH, age,BAI, and all two-way interactions on the survival probabilityof individual trees. I used general linear models to examinethe effects of site and stand density on mean BAI, meanDBH, and percent spruce beetle-induced mortality. The firstanalysis includes data from the eight intensively studiedstands, and the other two include data from all 15 stands.

ResultsWithin the eight intensively studied plots, DBH ranged

from 5 to 68 cm (28.20 ± 0.43, mean ± SE; n = 427) and agefrom 50 to 360 years (180 ± 3.49, n = 306). DBH varied sig-nificantly between sites (F[4,422] = 2.94, P = 0.020), althoughmost sites displayed fairly similar size distributions (Fig. 2).

Considering the subset of trees that could be aged, treeskilled by the spruce beetle were slightly older than the sur-viving trees (F[1,299] = 10.49, P = 0.001; Fig. 3). Age signifi-cantly increased with increasing DBH (F[1,299] = 5.12, P =0.024); however, there was a great deal of scatter in this rela-tionship (Fig. 3), making DBH a poor predictor of age.

BAI was significantly higher for surviving trees (F[1,296] =48.73, P < 0.0001). BAI also significantly increased with

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Fig. 1. Locations of the five study sites on the east side of theKennicott Valley in the Wrangell Mountains, Alaska (modifiedfrom Loso 1998). Inset indicates general location of study region.

DBH (F[1,296] = 273.69, P < 0.0001) and significantly de-clined with age (F[1,296] = 37.73, P < 0.0001; Fig. 4). Therewere no significant interactions among the effects of sur-vival, DBH, and (or) age on BAI.

Survival of individual trees was significantly related toDBH, BAI, the interactions of DBH × site, and DBH × BAI(Tables 1 and 2). The logistic model was not significantlyimproved by the inclusion of age; therefore, I ran the analy-sis on the full data set (including trees that could not beaged). Large diameter trees and slow growing trees had ahigher risk of mortality (Table 1, Fig. 5). While there was asignificant interaction of DBH × site, the qualitative patternof lower survival in larger diameter trees held for all sites;therefore, I have provided figures for only two of the sites.

Considering all 15 plots, densities varied from 173 to 704white spruce per hectare (359 ± 35), and the level of mortal-ity ranged from 0.08 to 0.79 (0.43 ± 0.05). Mean BAI did

not differ between sites (F[4,2] = 4.38, P = 0.195) andshowed a decreasing but nonsignificant trend with standdensity (F[1,2] = 15.71, P = 0.058) (Fig. 6a). Mean DBH sig-nificantly differed between sites (F[4,2] = 227.24, P = 0.004)and significantly decreased with increasing density (F[1,2] =515.73, P = 0.002) (Fig. 6b). Spruce beetle-induced mortal-ity varied significantly between sites (F[4,9] = 10.21, P =0.002), but stand density did not have a significant impact(F[1,9] = 0.44, P = 0.524; Fig. 6c) nor was there a significantinteraction of site × density.

Discussion

Within the study area, spruce beetle caused the highestmortality in large diameter trees with relatively slow growthfor their size (Fig. 5). This result agrees with those forspruce stands on the Kenai Peninsula, Alaska (Hard et al.

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Fig. 2. White spruce diameter at breast height (DBH) by sampling site and year.

1983; Holsten 1984; Van Hees and Holsten 1994; Hard1985). As is often reported, this study finds that large diam-eter trees are more susceptible than small diameter trees;however, growth rate and its interaction with DBH are im-portant considerations when evaluating risk of individual at-tack.

Tree age was not significantly related to mortality, althoughafter correcting for the effects of DBH, surviving trees wereolder than those that were killed. This result may be con-founded by the fact that not all trees could be aged; old treeswith rot that precluded determination of age may also havebeen weakened and more susceptible to spruce beetle attack,leading to deflation of the age estimate of dead trees.

While age and DBH are positively related to each other,they have opposite impacts on BAI. Growth increases withdiameter and decreases with age; however, the effect of treediameter is of much greater magnitude (Fig. 4). Thus, astrees age and increase in DBH, they generally also increasein BAI.

While both BAI and DBH showed declines with standdensity, this did not translate into a significant relationshipbetween stand density and percent spruce beetle-inducedmortality in these stands. This may be because BAI andDBH have opposite impacts on susceptibility and are effec-tively countering each other; dense stands have low meanDBH but also low mean BAI that are related to low and highmortality, respectively.

The differences in mortality level between sites (Fig. 2)and the interaction between site and DBH, which impacts in-dividual mortality (Table 1), both suggest that sites vary in amanner that is significant to the spruce beetle – host interac-tion. Two major possibilities exist. First, sites may vary inbiotic or abiotic conditions that alter the suitability of thesite to the spruce beetle and (or) the susceptibility of thetrees to beetle attack. For instance, Wesser and Allen (1999)found north-facing aspects to be associated with the lowestmortality rates in the Copper River Basin. In contrast, thesecond possibility is that the site effect could be due to pat-

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Fig. 3. The relationship between white spruce diameter at breastheight (DBH) and age for surviving (circles and dashed line:y = 165.08 + 0.763x) and dead (×’s and solid line: y = 184.84 +0.763x) trees. Lines are based on estimates from the linearmixed model including site as a random covariate.

Fig. 4. Predicted relationship (back transformed from linearmixed model) among white spruce basal area increment (BAI),diameter at breast height (DBH), and age for surviving (grid:y = 2.24 – 0.005age + 0.083DBH) and dead (gray surface:y = 1.62 – 0.005age + 0.083DBH) trees.

Effect df χ2 P

Site 4 6.66 0.155DBHa 1 8.83 0.003BAIb 1 19.27 <0.0001DBH × Site 4 13.14 0.011DBH × BAI 1 10.32 0.001

Note: The logistic procedure excluded the BAI × site in-teraction because it did not improve model fit.

aDiameter at breast height.bBasal area increment.

Table 1. Type III analysis of effects for the logisticregression examining white spruce survival follow-ing a spruce beetle outbreak.

Parameter Estimate SE

Intercept 1.611 0.784AZ site 2.007 1.146BN site –1.181 0.891NC site 1.403 1.114PY site –0.891 1.144SM site 0 —DBHa –0.88 0.029BAIb 0.109 0.025AZ site × DBH –0.085 0.041BN site × DBH –0.030 0.034NC site × DBH –0.050 0.037PY site × DBH 0.047 0.038SM site × DBH 0 —DBH × BAI –0.002 0.001

aDiameter at breast height.bBasal area increment.

Table 2. Maximum likelihood parameter estimatesfrom the logistic regression examining white sprucesurvival following a spruce beetle outbreak.

terns of colonization and spatial spread from an epidemiccenter. While site-level traits such as slope, aspect, and soilmoisture were not recorded in this study, there were no ob-vious differences between sites in these characteristics, andall sites supported almost pure stands of white spruce withsimilar size distributions. I think it likely that in addition toany effects of site-level characteristics, the spatial organiza-tion of sites will prove important in explaining the pattern ofmortality between stands. It is also likely that the patternseen during this snapshot in time will change somewhatonce the epidemic actually ends. The spruce beetle is still

active in the area, and mortality rates will certainly increasebefore the epidemic subsides. Holsten et al. (1995) noteschanges in patterns of attack as an epidemic progresses. Adefinitive conclusion will have to wait until the infestationruns its course within the Kennicott watershed.

In conclusion, studies suggesting that tree diameter is agood indicator of spruce beetle-induced mortality in theCopper River Basin (Wesser and Allen 1999; Matsuoka etal. 2001) may be oversimplifying a more complex interac-tion. Of the factors measured in this study, the individual ef-fects of diameter and BAI and their interaction have thegreatest impact on survival. Furthermore, for this systemstand density does not appear to be related to rates of sprucebeetle-induced mortality.

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Fig. 5. Expected probability of survival for white spruce trees asa function of diameter at breast height (DBH) and basal area in-crement (BAI) for (a) the BN site and (b) the SM site. Thesesites provide examples of the general relationship seen in allstudy sites (see parameter estimates in Table 2). Expected valuesare from a logistic regression including the following effects:site, DBH, BAI, DBH × site, and DBH × BAI.

Fig. 6. Relationship between white spruce stand density and(a) mean basal area increment (BAI) in each plot (n = 8),(b) mean diameter at breast height (DBH) in each plot (n = 15),and (c) plot-level, spruce beetle-induced mortality (n = 15). Bestpredicted lines (with intercept averaged across sites) of the re-sponse on density are shown in (a) (y = 51.31 – 0.064x, r2 =0.90) and (b) (y = 33.33 – 0.017x; r2 = 0.78).

Acknowledgements

This project was funded by the Institute of Arctic Biologyand the University of Alaska Natural Resources Fund. Ithank two anonymous reviewers for feedback that improvedthis manuscript. I especially thank Tully for all the longhours and hard work she devoted to the field research.

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