Journal of Vegetation Science 5: 229-236, 1994© IAVS; Opulus Press Uppsala. Printed in Sweden229Impacts of drought on tree mortality and growthin a mixed hardwood forestElliott, K.J. & Swank, W.T.USDA Forest Service, Southeastern Forest Experiment Station, Coweeta Hydrologic Laboratory,Otto, NC, 28763, USA; Tel. +1 704 524 2128; Fax +1 704 369 6767Abstract. The tree and shrub species on a 16-ha watershed inthe Coweeta Basin were sampled in 1984 and again in 1991 todetermine the effects of drought on tree species compositionand basal area growth. Mortality and radial growth weredetermined for tree species within three community types thatrepresent a moisture gradient from moist to dry: cove-hardwoods > mixed-oak > oak-pine. Tree mortality from 1984to 1991 was 20 % and 23 % in the cove-hardwoods andmixed-oak communities, respectively, compared to only 12 %in the oak-pine type. With the exception of Oxydendrwnarboreum and Robinia pseudoacacia, the oaks had higherpercentage mortality than any other genus; Quercus velutinahad 29 %, 37 %, and 20 % mortality in the cove-hardwoods,mixed-oak, and oak-pine types, respectively; Quercus prinushad 23 % mortality in the mixed-oak type; Quercus coccineahad 36 % mortality in the mixed-oak type; and Quercusmarilandica had 27 % mortality in the oak-pine type. Mortal-ity occurred mostly in the small-size class individuals (< 10 cmin diameter) for all species, suggesting that thinning was stillan important process contributing to mortality 29 yr afterclearcutting. Although growth of Liriodendron tulipifera wasmuch higher than growth of either Quercus prinus or Quercuscoccinea, growth in Liriodendron was significantly reducedby the 1985-88 drought and no growth reduction was observedfor these two dominant Quercus species during the same timeperiod.Keywords: Coweeta Basin; Disturbance; Forest dynamics;Liriodendron tulipifera; Quercus; Southern Appalachians.Nomenclature: Brown & Kirkman (1990).IntroductionDisturbances such as fire, drought, and harvestingcan influence the composition and structure of forestcommunities. Changes in tree composition and struc-ture due to drought in the early stage of stand develop-ment may be difficult to understand because of thecomplex relationships associated with differential sen-sitivity to drought among species, competitive interac-tions, and within-site variability. However, long-termdata sets including climate and vegetation inventoriesmay provide some insight as to how drought influencestree mortality and growth in typically high rainfall re-gions of Southern Appalachian forests.Watershed 13 (WS13), a mixed hardwood forest inthe Coweeta Basin, was clearcut in 1939-1940 andagain in 1962 to determine the effects of vegetationmanipulation on the quantity and timing of streamflow(Swank & Helvy 1970; Swank et al. 1988). Streamflowincreased 360mm the first year after cutting and wasstill slightly above pretreatment levels for 23 yr aftercutting. After the second clearcut. annual water yieldincreased by 380mm the first year, but returned topretreatment levels after just 16 yr of forest regrowth(Swank & Helvey 1970). These disturbances and changesin watershed hydrology have allowed mesic-site speciessuch as Liriodendron tulipifera and Betula lenta toinvade drier sites dominated by oaks (Quercus spp.)(Parker & Swank 1982), probably in response to el-evated light, soil moisture and nutrients. However, asregrowth continues, these mesic species could be at acompetitive disadvantage against species that are betteradapted to lower resource levels. For example, as soilmoisture returns to predisturbance levels, the ridge andslope sites may become too dry to support mesic spe-cies.Parker & Swank (1982) suggested that reestablish-ment of the soil moisture gradient or a more abrupt eventsuch as drought, could restore preclearcut species distri-butions. Thus, it was hypothesized that mesic speciessuch as Liriodendron tulipifera and Betula lenta whichexpanded their range onto slopes after the 1962 clearcut,would decline on these sites in a prolonged drought(Leopold 1984). The influence of a gradual hydrologicshift on forest spatial patterns could cause both competi-tive and gradient stress to a variety of species, butLeopold & Parker (1985) and Leopold et al. (1985)concluded that growth data for major tree species duringthe first 21 yr after the second clearcut did not show thatspecies and community distributions returned to precutpatterns. Leopold et al. (1985) also suggested that L.230Elliott, K.J. & Swank, W.T.tulipifera would continue to dominate the slope areas inWS13 unless a severe disturbance such as a droughtoccurred.Historically, severe droughts have caused high treemortality in the Southern USA (Hursh & Haasis 1931;Tainter et al. 1984; Stringer et al. 1989; Starkey et al.1989). After the 1984 vegetation inventory conductedby Leopold (1984), a major drought was recorded from1985-1988 in the Coweeta Basin (Swift & Blood 1987).It was the most severe drought on record in terms ofduration and accumulated precipitation deficit. For thisperiod, precipitation averaged 136.4 cm, which was 24 %below normal. At the peak of the drought in 1986, totalprecipitation for the year was 123.9cm, 31% belownormal (unpubl. data, USDA Forest Service). The re-turn interval for this event, based on streamflow, wasestimated to be 233 yr (Swift et al. 1989). Clinton et al.(1993) showed an increase in canopy gap formation inthe Coweeta Basin during the 1985-88 drought.The purpose of this research was: (1) to assess theimpacts of drought on tree mortality and growth, and (2)to determine whether mesic tree species such asLiriodendron tulipifera and Betula lenta have returnedto their predisturbance locations (prior to the 1939-40clearcut) where they were restricted to the mesic cove-hardwoods forest.MethodsSite descriptionThe Coweeta Basin, a 16.1 ha-northeast-facing catch-ment, is located in southwestern North Carolina, 35°04'30" N, 83° 26' W. Elevations range from 724 to 853 m,mean land slope is 51 %, and mean annual precipitationis ca. 1800 mm. Mean annual temperature is 12.6 °C andaverage temperature ranges from 6.7 °C in the dormantseason to 18.5 °C in the growing season. Four soil typesare present on the watershed: (1) Cullasaja-Tuckasegee(cove bottoms), loamy-skeletal/coarse-loamy, mixed,mesic Typic Haplumbrepts; (2) Edneyville-Chestnut(lower slopes), coarse loamy, mixed mesic Typic Dystro-chrepts; (3) Plott (middle slopes), coarse loamy,