Differential effects of understory and overstory gaps on tree regeneration1

Journal of the Torrey Botanical Society, Jan/Feb 2008 by Beckage, Brian, Kloeppel, Brian D, Yeakley, J Alan, Taylor, Sharon F, Coleman, David C

Materials and Methods. STUDY AREA. Our study was conducted at Coweeta Hydrologie Laboratory, near Otto, North Carolina in the southern Appalachian Mountains (35°03' N, 83°25' W). Elevations in the Coweeta Basin range from 675 m to 1592 m and encompass a drainage area of 1626 ha. Mean annual precipitation is 1770 mm at the Coweeta base climate station. Vegetation in the lower elevations at Coweeta consists of secondgrowth mixed oak-hickory (Quercus-Carya) forest often with an understory of the evergreen ericaceous shrub Rhododendron maximum (Swank and Crossley 1988). Rhododendron can form a dense subcanopy (Baker and Van Lear 1998) with leaf area indices in the range of 4.8 to 6.6 and can have strong effects on the understory light environment (Beckage et al. 2000). Low light levels beneath Rhododendron subcanopies can have a dramatic impact on seedling regeneration, precluding nearly all seedling establishment (Beckage et al. 2000, Lei et al. 2002).

Our study resulted from the serendipitous occurrence of windthrown canopy trees in a study of the effect of Rhododendron removal on biogeochemical cycling. The original experiment examined watershed differences in nutrient fluxes due to shrub removal. We employed a watershed level experimental design (i.e., one treatment vs. one reference watershed), which is commonly used because of the high cost of watershed level studies, that relies on pre- and post-treatment data in a randomized intervention analysis that compares changes in temporal signals (Yeakley et al. 2003). Originally, two study sites, located within 100 m of each other were instrumented for the collection of data on nutrient fluxes and one year of pre-treatment data was collected prior to the removal of the Rhododendron understory layer in one of the sites (Yeakley et al. 2003). At the end of August 1995, the aboveground portions of all Rhododendron stems within a 10 m by 30 m area of one site were manually removed and the herbicide Roundup (Monsanto, Luling, LA) was applied once to the top of cut stumps. Sixty-five stems of Rhododendron were removed, representing approximately 30% of total above-ground woody biomass. This plot is referred to as the shrub removal treatment. On 4 October 1995, Hurricane Opal blew down 9 canopy trees in the adjacent site, which was originally intended as a control plot, while not damaging the shrub removal plot. We subsequently designated a 10 m by 30 m area within this blowdown site as the canopy removal plot. There was minimal damage to the shrub layer in the canopy removal plot, with forty-five intact stems of Rhododendron present. Undisturbed reference plots were located adjacent to both the shrub and canopy removal treatments, to maximize similarity between paired treatment and controls, and are referred to as the shrub and canopy controls, respectively. All four plots had similar pre-treatment overstory composition and Rhododendron density as well as aspect, topography, and soils (Yeakley et al. 2003).

We subsequently began monitoring the species richness and diversity of tree regeneration as well as the growth and survivorship of individual seedlings in all four of these plots. Our nomenclature follows Wofford (1989). Seedling censuses were conducted in ten l m by 1 m quadrats that were randomly located in each of the four plots and that were permanently marked with PVC corner posts. The height and species of all tree seedlings present in the quadrats were recorded during censuses conducted in the growing season of four consecutive years (1997-2000), and each seedling that was recorded in a prior census was scored as alive or dead in subsequent censuses. Species richness and the ShannonWiener index of species diversity (H') were calculated for each quadrat in every census year using the seedling counts. We calculated the relative growth rates (RGR) for species with seedlings that survived three years; i.e., from the first census in 1997 to the final census in 2000, in two or more treatment plots. RGR was calculated as log(Ht/H0) where H^sub T^ was the final height of the seedling in the 2000 census, H^sub 0^ was the initial height of the seedling in the 1997 census, and T was the elapsed time in years (i.e., 3 years; Beckage and Clark 2003).