pr7 7 AL Plant Ecology 140: 245-253, 1999. 245 rim © 1999 Kluwer Academic Publishers. Printed in the Netherlands. Growth pattern of Picea rubens prior to canopy recruitment Xinyuan Wu l , J. Frank McCormick2 Richard T. Busing3 1 Department of Rangeland Ecology and Management Texas AM University, College Station, TX 77843, USA; 2Department of Ecology and Evolutionary Biology, The University of Tennessee Knoxville, TN 37996, USA; 3Forestry Sciences Laboratory 3200 SW Jefferson Way Corvallis, OR 97331, USA Received 22 July 1997; accepted in revised form 6 October 1998 Key words: Canopy disturbance regime, Great Smoky Mountains, Radial growth, Red spruce, Suppression and release Abstract A majority (72%) of Picea rubens Sarg. (red spruce) trees in an old-growth spruce-fir forest in the Great Smoky Mountains underwent episodes of radial growth suppression and release before they reached the forest canopy. Prior to canopy recruitment, trees experienced an average of 1.43 and a maximum of 7 suppression periods with an average ring width of 0.257 mm. Duration of suppression periods ranged from 4 to 79 years with an average of 19.05 years, which was significantly shorter than the average duration of release periods (29.00 years). Mean ring width in a suppression period was negatively correlated with duration of the suppression period. The opposite was true for release periods. The severity of suppression had no significant effect on mean ring width in subsequent release periods. Greater suppression was observed in the recent growth pattern of current non-canopy trees than in the historical growth pattern reconstructed from current canopy trees. Growth releases prior to canopy recruitment, triggered by small gaps or neighboring gaps, displayed a continuous pattern over the last two centuries and had consistent frequency in recent decades. By contrast, canopy recruitment resulting from large-gap disturbances was discontinuous over the last two centuries, and less frequent after 1850. Introduction (Adelges piceae) an exotic insect (Eagar 1984). This and other recent disturbances have stimulated research on spruce-fir forest dynamics in the southern Ap- Study of understory growth patterns in shade-tolerant palachians (Busing Clebsch 1987; Busing et al. trees has lead to a better understanding of their ecol- 1988; Busing Wu 1990; Busing Pauley 1994; ogy and of canopy disturbance regimes in deciduous Nicholas et al. 1992; Peart et al. 1992; Stephenson forests (Canham 1985, 1989; Frelich Lorimer 1991; Adams 1984; White et al. 1985). Several of these Lorimer Frelich 1989). Multiple episodes of sup- studies have considered the population dynamics of pression and release prior to canopy recruitment have Picea rubens (Sarg.). Patterns of radial growth of been inferred from radial increments of shade-tolerant Picea rubens in the canopy are also well documented species. The frequency of such episodes is often (e.g., LeBlanc et al. 1992; Reams et al. 1993; Van high suggesting frequent disturbances to the canopy Deusen 1988). However, studies describing the growth (Canham 1990). pattern of this species prior to canopy recruitment are Southern Appalachian spruce-fir forests are the few (e.g., Mount Grove 1952, reported in McLin- southernmost high-elevation coniferous forests in tock Bickford 1957). Information on growth and eastern North America (Oosting Billings 1951; canopy recruitment is basic to an understanding of the White 1984; White Cogbill 1992; Whittaker 1956). population dynamics of Picea rubens, the community In recent decades, the southern Appalachian en- structure and dynamics of spruce-fir forest stands, and demic, (Pursh.) Poir. (Fraser fir), has Abies fraseri the effects of disturbances upon them. been severely affected by the balsam woolly adelges 246 The pattern of growth by which shade-tolerant stands with a Picea overstory. Stand I was on a south- trees reach the canopy can be described in terms of facing slope. The point on the trail nearest (about periods of suppression and release they undergo (Can- 75 m) to the center of stand I was approximately 260 m ham 1985). There are two basic modes of canopy east of the observation tower along the trail. Stand II recruitment. Young trees may start in large canopy was on a north-facing slope. The point on the trail openings or successively expanding smaller openings nearest (about 120 m) to the center of stand H was (Foster Reiners 1986; Runkle 1985) and grow di- approximately 425 m east of the tower along the trail. rectly into the canopy without any growth suppression. A 10 x 50 m transect perpendicular to the ridge was Alternatively, individuals may persist under a closed conducted in each stand to characterize stand com- canopy with suppressed growth, and respond with position and structure. Live stems of all tree species released growth to canopy openings created by the with a dbh (diameter at breast height, 1.37 m) of death of canopy trees or the falling of large branches. 3 cm or greater were sampled. Picea rubens, Abies They may reach the canopy in a single period of re- fraseri, Betula alleghaniensis Britton (yellow birch). lease, or they may undergo several periods of release Acer spicatum Lam. (mountain maple) were present and suppression before reaching the canopy. The rel- in both stands. Prunus sp. (cherry) was also present ative importance of these two modes for Picea in in stand I. Picea dominated both stand I (680 stems two old-growth stands of the southern Appalachians ha-1 , 85.89 m 2 ha— I basal area) and stand II (660 is addressed in this study. stems ha-1 , 57.23 m2 ha— I basal area). It represented Growth patterns may be used to infer distur- 89.3% and 93.4% of the basal area of all tree species bance regimes over several decades or centuries (e.g., in stand I and stand II, respectively. Abies had the Lorimer Frelich 1989; Frelich Lorimer 1991). lowest density (40 stems/ha) and basal area (0.05- Radial increment series can be used not only to in- 0.07 m2 ha-1 ) among tree species in both stands. The fer the frequency and severity of local canopy dis- overwhelming dominance of Picea and low impor- turbances, but also to detect changes in disturbance tance of Abies were typical of stands at this elevation regimes. The constancy of regimes can be explored (Busing et al. 1993). using series from different periods. For comparison to recent growth patterns of trees that have not reached Sampling and measurements the canopy, the historical growth pattern of Picea can be reconstructed from tree ring series of canopy trees. Field sampling was conducted in 1986. Increment Differences between recent and historical growth pat- cores were extracted at breast height from 47 canopy terns may reflect environmental changes in recent Picea trees (25 in stand I and 22 in stand II) and 50 centuries. non-canopy Picea trees (29 in stand I and 21 in stand The primary objectives of this study are: ( 1 ) to re- II). In this study, canopy trees included superdomi- construct the growth pattern of Picea from tree ring nants, dominants and co-dominants visually classified series of canopy trees; (2) to compare the recent based on their height and position relative to trees growth pattern of non-canopy trees with the histori- in the main canopy. All other trees (subcanopy and cal growth pattern of canopy trees; and (3) to make understory) were considered non-canopy trees. One inferences on the canopy disturbance regime. Our core was taken in the direction parallel to terrain con- findings contribute to the understanding of Picea ecol- tour. Trees <5 cm or >70 cm in dbh were not sampled ogy, spruce-fir forest dynamics, and forest disturbance due to the concern of the effect of coring on small history in the southern Appalachians. trees, and the limitation of the equipment available at the time (the bit length of the borers) for coring very large trees. Trees of abnormal growth form (bent, Methods twisted or hollow) were excluded from sampling. A tree ring measuring system (Robinson Evens Study site 1980) was used to determine ring width (annual in- crements of radial growth) by ring number at breast Two stands were selected along Noland Divide within height of all trees sampled. Picea cores had dis- the spruce-fir forest zone in the Great Smoky Moun- tinct boundaries between rings, even during strongest tains National Park (35°35 N, 83°28 W), between periods of suppression. Tree ring series were not cross- 1620 m and 1 700 m elevation. Both were old-growth dated and standardized. This study focused on the 247 section of a tree ring series when the tree was in the pared to the growth pattern based on the threshold of understory, and on the growth pattern in this sec- 0.40 mm yr-1 . None of the growth pattern parameters tion related to micro-environmental stand dynamics based on each of the four additional thresholds was rather than macro-environmental factors. The lack of significantly different from that based on the threshold cross-dating, an important step for establishing precise of 0.40 mm yr-1 , with two exceptions. As one would chronologies, might have introduced some error but expect, averages of ring width in suppression peri- should not have significantly affected characterization ods based on the thresholds 0.35 and 0.45 mm yr-1 of suppression and release periods (Canham 1985). were significantly different from the corresponding av- erage based on the threshold of 0.40 mm yr -1 . With Growth pattern an increased threshold value, all the growth pattern parameters increased slightly except for the duration The term growth pattern refers to the alternating of release periods which decreased.