Mortality Trends in a Southern Appalachian Red Spruce Population

Forest Ecology and Management ELSEVIER Forest Ecology and Management 64 (1994) 41-45

Mortality trends in a southern Appalachian red spruce population

R.T. Busing *'a, E.F. Pauley b "USDA Forest Service, Pacific Northwest Research Station, 3200 Jefferson Way, Corvallis, OR 97331, USA bGraduate Program in Ecology, University of Tennessee, Knoxville, TN 37996, USA (Accepted 15 September 1993)

Abstract

Mortality of red spruce (Picea rubens Sarg.) was assessed in an old-growth spruce-fir forest before and after adelgid-induced mortality of canopy fir at about 1985. Mortality rates of red spruce canopy trees remained low (less than 2% year- ~) following the loss of fir from the canopy. However, the proportion of mortality attributable to wind increased from less than 60 to 90%. Transects placed in areas of concentrated wind damage revealed that uprooting or snapping accounted for 94% of the mortality of spruce canopy trees in the winter of 1987-1988. The transect data were biased toward wind-damaged areas and revealed the potential impact of wind-related mortality on spruce. There was no evidence of atypically high wind velocities in the general area, or of a decline in the radial growth of spruce trees prior to death. It was concluded that the loss of Fraser fir (Abiesfraseri (Pursh) Poir. ) from adelgid infestation has increased wind exposure to the remaining canopy trees. The increased exposure of southern Appalachian red spruce to wind must be considered in studies of its health.

Key words: Mortality; Growth; Picea; Wind

1. Introduction (Hornbeck and Smith, 1985; McLaughlin et al., 1987; Hamburg and Cogbill, 1988; Cook and Ze- Patterns of red spruce (Picea rubens Sarg.) daker, 1992; Johnson et al., 1992 ). In the south- mortality in the Appalachian Mountains are of ern Appalachians, comparably elevated spruce increasing concern. Some northern and central mortality levels have not been detected (Busing Appalachian red spruce populations have in- et al., 1988; Busing and Wu, 1990; Peart et al., curred high mortality or declining growth over 1992 ). However, sharp radial growth reductions the last 3 decades (Siccama et al., 1982; Scott et have been found in some high elevation stands al., 1984; Vogelmann et al., 1985; Adams and (over 1900 m elevation; McLaughlin et al., 1987; Stephenson, 1989; Peart et al., 1992). Spruce LeBlanc et al., 1992 ) and crown deterioration has growth decline appears to involve stand dynam- been observed throughout spruce-fir forests of ics (Hornbeck et al., 1986; Van Deusen, 1987) the Great Smoky Mountains (Peart et al., 1992 ). and high winds (Harrington, 1986; Worrall and Until recent years, the disturbance regime of Harrington, 1988 ). Possible contributing factors old-growth southern Appalachian spruce-fir was include air pollutants and climatic changes dominated by small canopy gaps (White et al., 1985 ). Gaps were typically created by the death *Corresponding author. of one to three canopy trees. Large-scale wind-

SSDI0378-1127(93)03309-7 42 R.T. Busing, E.F. Pauley / Forest Ecology and Management 64 (1994) 41-45 falls were uncommon except on exposed peaks 1960 and last sampled in 1986 (Busing et al., and ridges. On less exposed sites, such as Mt. 1988 ). Approximately half of the plots were lo- Collins in the Great Smoky Mountains, wind cated on ridge crests; others were on slopes within events resulting in uprooted, snapped, or top- 0.3 km of a ridge crest. The mode of death pled boles accounted for less than half of the (standing, bole uprooted by wind, snapped by spruce mortality (Busing and Wu, 1990). Large- wind, toppled by another tree, or uprooted by soil scale disturbances have recently impacted the disturbance from an adjacent tree fall) was re- old-growth spruce-fir forest at Mt. Collins. From corded for all canopy trees (over 30 cm diameter 1982 to 1986, the codominant Fraser fir (Abies at breast height (DBH) ) dying over the interval. fraseri (Pursh) Poir.), was almost completely Mortality rates were calculated as the average eliminated from the canopy as a result of balsam number of trees dying per year divided by the woolly adelgid (Adelges piceae Ratz.) infesta- original number of live canopy trees. Radial tion (Busing et al., 1988). Red spruce remained growth rates were calculated from 10 year DBH as the sole canopy dominant. It has been hypoth- remeasurements ( 1983-1993 ) for live canopy esized that the resulting exposure to high light trees in the 0.1 ha plots. intensity and strong winds has adversely affected To further characterize wind-related mortal- spruce trees (Peart et al., 1992), but quantita- ity, spruce trees (over 1.37 m tall) that had been tive evidence has been lacking. windthrown in the previous winter were tallied In this study we examine the mortality and in 40-m-wide belt transects of varying length growth of red spruce trees at the Mr. Collins long- ( 100-500 m) during the summer of 1988. Tran- term research site before and after the demise of sects totaling 1100 m in length (4.4 ha area) were Fraser fir. Our objectives are to determine placed along ridge crests (400 m total) and across whether rates of spruce mortality and growth slopes within 0.3 km of ridge crests (700 m to- have changed markedly, and whether there has tal). Areas of noticeably high wind damage were been a shift toward wind-related mortality. Be- selected for transect sampling. Mode of death (as cause adelgid-induced fir mortality is wide- above), diameter at breast height (DBH), tree spread in the southern Appalachian Mountains height, direction of fall, and location were re- (Eagar, 1984), information on the effects of fir corded for each tree. The final 10 year radial in- elimination on spruce growth and mortality is crement was determined for each tree over 10 cm essential to understanding trends in the condi- DBH. tion of southern Appalachian red spruce. Wind speed records were obtained from a me- teorological station on Noland Divide, 2 km south of Mt. Collins. Wind data were available 2. Methods after 1985 and included hourly mean and hourly maximum 1-min mean wind speeds. The study site is located on the north slope of Mr. Collins in the Great Smoky Mountains Na- tional Park ( 35 ° 35'N, 83 ° 28' E), Tennessee. El- 3. Results and discussion evations of the site range from 1675 to 1850 m above sea level. Oosting and Billings (1951), Changes in forest composition following heavy White et al. (1985), and Busing et al. (1988) fir mortality included a 78% decrease in basal provide descriptions of vegetation, climate, and area of live Fraser fir and a 28% decrease in total soils of the site. Recent spruce mortality was as- basal area of live trees (Table 1 ). Prior to infes- sessed by resampling permanent plots and by tation, Fraser fir was a substantial component of placing belt transects across the site. Two sets of the canopy with 36% of the total live basal area. permanent plots were reinventoried in 1993: ( 1 ) By 1986, red spruce remained as the sole canopy a set of six 0.1 ha plots established in 1983 (Bus- dominant, comprising 72% of the total live basal ing, 1985 ); (2) two 0.4 ha plots established about area. R. T. Busing, E.F. Pauley / Forest Ecology and Management 64 (1994) 41-45 43

Table 1 normally high for a long-lived tree species, and Forest composition at Mt. Collins before (about 1960) and they remained well below those reported for de- after (1986) adelgid-induced mortality of Fraser fir~ clining stands (Peart et al., 1992). The shift to- Density Basal area wards wind-related mortality was marked, how- (stems ha -1 ) (m2ha -~ ) ever, and it was apparently a result of the loss of canopy fir from Mr. Collins in the early 1980s. Before After Before After Ninety-nine incidents of wind-related mortal- Red spruce 402 933 26.2 26.6 ity of spruce were observed in the 4.4 ha area Fraser fir 1012 1140 18.4 4.1 surveyed by transects. This level of mortality was Yellow birch 123 121 6.8 6.0 not catastrophic as an average of only 22.5 wind- Mountain maple 7 51 < 0.1 0.1 related spruce deaths per hectare (about 15% by Mountain ash 40 26 < 0.1 0.1 density) occurred in these areas of compara- Pin cherry 0 23 0 < 0.1 Totals 1584 2294 51.4 36.9 tively high wind damage. There was a tendency of high mortality on ridge crests (31 deaths ha- ~) aSummaries are based on all live stems more than 1.37 m tall versus slopes ( 18 deaths ha- ~). Mortality tended from two 0.4 ha plots. toward a clumped dispersion pattern. As many as 15 incidents of individual tree mortality oc- Table 2 Mortality of red spruce canopy trees at Mt. Collins following curred in a 50 m segment along the 40-m-wide heavy mortality of Fraser fir transects, while none were encountered in three of the 50 m segments. Clumping resulted, in part, Plot set Original live Wind Other Annual from toppling and ground disturbing effects of canopy mortality mortality mortality large windthrown trees on adjacent individuals. spruce rate (%) Highly localized gusts of wind may also have contributed to the clumped mortality pattern. Six 0.1 ha Mortality was size-dependent and most fre- (1983-1993) 70 4 0 0.6 quent among trees with small (less than 10 cm) Two 0.4 ha and large (over 30 cm) diameters (Fig. 1 ). A (1986-1993) 87 5 1 1.0 majority (57%) of the small individuals were uprooted by ground disturbance while most A total of four spruce canopy trees out of 70 (94%) large individuals were uprooted or died in the 0.1 ha permanent plots from 1983 to snapped (Fig. 1 ). Only 7% of the windthrown 1993 (Table 2 ). Of these, three were snapped and spruce had rotten bole wood. one was uprooted. All three snapped trees were The final radial growth rates of uprooted and on ridge crests. In the 0.4 ha plots, six out of 87 snapped spruce were similar to those of standing spruce canopy trees died from 1986 to 1993 (Ta- live trees (Table 3 ). Clearly, mortality by wind ble 2). Of these, three were uprooted, two were was not associated with a decline in radial snapped, and one died standing. All but two, an growth. The average radial growth of live canopy uproot and a bole snap, occurred on a ridge crest spruce had not changed significantly and there site. The annual mortality rate for spruce canopy was no evidence of a sharp radial growth decline trees ranged from 0.6% for the 0.1 ha plot set to throughout the spruce population at this site. 1.0% for the 0.4 ha plot set. These rates were Also, the loss of fir did not seem to have had an somewhat above the rate of 0.5% for the preced- immediate effect on the growth of red spruce ing 2 decade period at Mt. Collins (roughly canopy trees. 1960-1986; Busing and Wu, 1990). In addition, In the southern Appalachians, winds are typi- the fraction of mortality events attributable to cally strongest in late winter and early spring wind damage has shifted from less than 60% (Saunders, 1979 ). The frequency of hourly max- (Busing and Wu, 1990) to 90%. imum 1-min mean wind speeds over 10 m s- 1 in The recent spruce mortality rates were not ab- the Great Smoky Mountains has increased 44 R.T. Busing, E.F. Pauley / Forest Ecology and Management 64 (1994) 41-45

14.1

I,- t,i. 0 20

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25 45 65 DBH MIDPOINT (cm) Fig. 1. Mode of wind-related spruce mortality by size class. Solid black indicates uprooting by wind, unshaded indicates snapping, shaded indicates toppling, and hatched indicates uprooting by soil disturbance from an adjacent tree fall.

Table 3 Student's t-test comparisons of final radial growth incre- Similar episodes of wind-related spruce mor- ments of standing live and windthrown red spruce at Mr. tality were infrequent in the past at the study site, Collins. Windthrown trees include only those trees snapped and most spruce canopy trees died standing or uprooted (Busing and Wu, 1990). The similarity of final radial growth rates between windthrown and live Mean 10 year 95% confidence increment limits spruce suggested that a recent growth decline (mm) (e.g. McLaughlin et at., 1987) was not a factor in the wind-related mortality studied here. The Live (1986) 70 7.04 6.96-7.13 wind data did not indicate a recent increase in Live (1993) 61 5.65 4.40-7.27 maximum wind speeds, although high winds were Windthrown (1988) 55 6.77 5.80-7.89 more frequent in recent winters. Live (1986) vs. Live (1993): t= - 1.697, P=0.092 These findings support the hypothesis that Live (1986) vs. Windthrown: t=0.359, P=0.720 synergistic disturbance (sensu White and Pick- Live (1993) vs. Windthrown: t= - 1.352, P=0.179 ett, 1985) is operating in this case. Stand-wide Fraser fir mortality has presumably exposed the remaining canopy trees to the full force of wind Table 4 Frequency and magnitude of high winds at Noland Divide, Great episodes, thereby contributing to increased dam- Smoky Mountains National Park (elevation 1730 m) age and windthrow of spruce. The small gap disturbance regime, once char- Period Percentage of hours Maximum 1 min with 1 min mean mean wind acteristic of the study site (White et at., 1985 ), wind speeds > 10 speed (m s- 1) is changing. Canopy gaps are now expanding as ms -1 in a northern Appalachian subalpine forest Nov. 1986-Feb. 1987 13.1 22 (Foster and Reiners, 1986 ). Windthrow of can- Nov. 1987-Feb. 1988 18.3 21 opy-size spruce is likely to continue. Increased Nov. 1988-Jan. 1989 22.5 23 crown and root damage to spruce may also be ex- pected (Harrington, 1986; Peart et at., 1992 ). We predict that wind will become a major cause of slightly in recent winters, but the highest 1-min spruce damage and mortality in spruce-fir for- mean wind speeds have not shown a trend (Ta- ests severely affected by the balsam woolly ble 4). adelgid. R.T. Busing, E.F. Pauley / Forest Ecology and Management 64 (1994) 41-45 45

4. Acknowledgments Johnson, A.H., McLaughlin, S.B., Adams, M.B., Cook, E.R., DeHayes, D.H., Eagar, C., Fernandez, I.J., Johnson, D.W., We thank H.S. Adams, E.E.C. Clebsch, J. Kohut, R.J., Mohnen, V.A., Nicholas, N.S., Peart, D.R., DeCoster and J.A. McLean for comments on Schier, G.A. and White, P.S., 1992. Synthesis and conclu- sions from epidemiological and mechanistic studies of red earlier drafts of the manuscript. David Silsbee spruce decline. In: C. Eagar and M.B. Adams (Editors), provided access to the wind data files. The 1993 Ecology and Decline of Red Spruce in the Eastern United field work was funded by a Michaux grant from States. Springer, New York, pp. 385-411. the American Philosophical Society. LeBlanc, D.C., Nicholas, N.S. and Zedaker, S.M., 1992. Prevalence of individual-tree growth decline in red spruce populations of the southern Appalachian Mountains. Can. J. For. Res., 22: 905-914. McLaughlin, S.B., Downing, D.J., Biasing, T.J., Cook, E.R. References and Adams, H.S., 1987. An analysis of climate and com- petition as contributors to decline of red spruce in high Adams, H.S. and Stephenson, S.L., 1989. Old-growth red elevation Appalachian forests of the eastern United States. spruce communities in the mid-Appalachians. Vegetatio, Oecologia, 72: 487-501. 85: 45-56. Oosting, H.J. and Billings, W.D., 1951. A comparison of vir- Busing, R.T., 1985. Gap and stand dynamics of a southern gin spruce-fir forest in the northern and southern Appa- Appalachian spruce-fir forest. Ph.D. Dissertation, Uni- lachian system. Ecology, 32: 84-103. versity of Tennessee, Knoxville, 198 pp. Peart, D.R., Nicholas, N.S., Zedaker, S.M., Miller-Weeks, Busing, R.T. and Wu., X., 1990. Size-specific mortality, M.M. and Siccama, T.G., 1992. Condition and recent growth, and structure of a Great Smoky Mountains red trends in high-elevation red spruce populations. In: C. Ea- spruce population. Can. J. For. Res., 20:206-210. gar and M.B. Adams (Editors), Ecology and Decline of Busing, R.T., Clebsch, E.E.C., Eagar, C.C. and Pauley, E.F., Red Spruce in the eastern United States. Springer, New 1988. Two decades of change in a Great Smoky Moun- York, pp. 125-191. tains spruce-fir forest. Bull. Torrey Bot. Club, 115: 25- Saunders, P.R., 1979. The vegetational impact of human dis- 31. turbance on the spruce-fir forests of the southern Appa- Cook, E.R. and Zedaker, S.M,, 1992. The dendroecology of lachian mountains. Ph.D. Dissertation, Duke University, red spruce decline. In: C. Eagar and M.B. Adams (Edi- Durham, NC, 177 pp. tors), Ecology and Decline of Red Spruce in the Eastern Scott, J.T., Siccama, T.G., Johnson, A.H. and Breisch, A.R., United States. Springer, New York, pp. 192-231. 1984. Decline of red spruce in the Adirondacks, New York. Eagar, C., 1984. Review of the biology and ecology of the bal- Bull. Torrey Bot. Club, 111: 438-444. sam woolly aphid in southern Appalachian spruce-fir for- Siccama, T.G., Bliss, M. and Vogelmann, H.W., 1982. De- ests. In: P.S. White and J. Wood (Editors), The Southern cline of red spruce in the Green Mountains of Vermont. Appalachian Spruce-Fir Ecosystem: its Biology and Bull. Torrey Bot. Club, 109: 162-168. Threats. Research/Resource Management Reports SER- Van Deusen, P.C., 1987. Testing for stand dynamics effects 71, USD1, National Park Service, Atlanta, GA, pp. 36- on red spruce growth trends. Can. J. For. Res., 17: 1487- 50. 1495. Foster, J.R. and Reiners, W.A., 1986. Size distribution and Vogelmann, H.W., Badger, G.J., Bliss, M. and Klein, R.M., expansion of canopy gaps in a northern Appalachian 1985. Forest decline on Camels Hump, Vermont. Bull. spruce-fir forest. Vegetatio, 68:109-114. Torrey Bot. Club, 112: 274-287. Hamburg, S.P. and Cogbill, C.V., 1988. Historical decline of White, P.S. and Pickett, S.T.A., 1985. Natural disturbance red spruce populations and climatic warming. Nature, 331: and patch dynamics: an introduction. In: S.T.A. Pickett 428- 431, and P.S. White (Editors), The Ecology of Natural Distur- Harrington, T.C., 1986. Growth decline of wind-exposed red bance and Patch Dynamics. Academic Press, New York, spruce and balsam fir in the White Mountains. Can. J. pp. 3-13. For. Res., 16: 232-238. White, P.S., MacKenzie, M.D. and Busing, R.T., 1985. Nat- Hornbeck, J.W. and Smith, R.B., 1985. Documentation of ural disturbance and gap phase dynamics in southern Ap- red spruce growth decline. Can. J. For. Res., 15:1199- palachian spruce-fir forests. Can. J. For. Res., 15: 233- 1201. 240. Hornbeck, J.W., Smith, R.B. and Federer, C.A., 1986. Growth Worrall, J.J. and Harrington, T.C., 1988. Etiology of canopy decline in red spruce and balsam fir relative to natural gaps in spruce-fir forests at Crawford Notch, New Hamp- processes. Water, Air,'Soil Pollut., 31: 425-430. shire. Can. J. For. Res., 18: 1463-1469.