Impact of Silvicultural Treatment on Chestnut Seedling Growth and Survival

Putatively blight-resistant advanced backcross chestnut seedlings will soon be available for outplanting on a regional scale. Few studies have examined the importance of silvicultural treatment or seedling quality to chestnut reintroduction in the U.S. This paper examines results from a silvicultural study of high-quality chestnut seedlings on the Cumberland Plateau of southeastern Kentucky. Three hundred American (Castanea dentata), three hundred advanced backcross (BC2F3) and one hundred fifty Chinese chestnut (Castanea mollissima) seedlings were planted in three silvicultural treatments, ranging from low-light to high-light, on the Daniel Boone National Forest in Mar 2009. Seedlings were planted in a completely randomized design with a split-plot treatment arrangement, with silvicultural treatments as whole plots, and species in a randomized block design in the sub-plot. After three years, chestnut seedlings in the high-light treatment sites grew significantly more in height and root collar diameter, on average, compared to seedlings in the moderateand low-light treatments. Survival did not differ among silvicultural treatments and averaged 64% over all sites. Low survival was due in part to the non-native root-rot disease organism, Phytophthora cinnamomi, which was confirmed at the site. This study suggests that while chestnut grows best in highlight environments, the species can become established under varying light-levels, which will give forest managers flexibility when choosing management strategies for chestnut reintroduction. INTRODUCTION American chestnut (Castanea dentata), once a dominant timber tree throughout eastern North America, has been nearly eradicated by the chestnut blight fungus, Cryphonectria parasitica, and ink disease, Phytophthora cinnamomi, both non-native pathogens accidentally introduced from Asia (Anagnostakis, 2006; Crandall et al., 1945). The American Chestnut Foundation (TACF) and The Connecticut Agricultural Experiment Station each utilize the backcross breeding program (Burnham, 1988), incorporating resistance from Chinese or Japanese chestnuts in efforts to develop blightresistant American-hybrid chestnut trees. In anticipation of widespread planting of blight-resistant backcross chestnuts, an understanding of the silvics and competitive ability of the species for successful reintroduction to eastern North American forests is critical. Forestry publications written Proc. V Int. Chestnut Symposium Eds.: M.L. Double and W.L. MacDonald Acta Hort. 1019, ISHS 2014 192 while American chestnut was still canopy dominant describe the species’ rapid growth and prolific sprouting (Ashe, 1911; Mattoon, 1909; Zon, 1904); however, they lack indepth analysis of chestnut silvics. Although American chestnut has been planted for many years (Hough, 1882) there have been few experimental studies on chestnut silviculture and artificial regeneration of the species (Anagnostakis, 2007; Clark et al., 2011; Jacobs and Severeid, 2004; MCament and MCarthy, 2005; MNab, 2003; Rhoades et al., 2009). At the time this study was established, no other studies had examined silvicultural impact on backcross-chestnut establishment. Understanding which silvicultural techniques appropriately manipulate light levels for backcross hybrid chestnut establishment and growth is necessary to guide reintroduction efforts. The overall goal of this study was to evaluate three-year field performance of American, Chinese and BC2F3 chestnut under three silvicultural treatments. Specifically, we aimed to evaluate the effects of silvicultural and species treatments on chestnut survival and growth. MATERIALS AND METHODS Study Site This study was located on the London Ranger District of the Daniel Boone National Forest (DBNF) on the Cumberland Plateau in Southeastern Kentucky. The forest type, classified as upland hardwood, is dominated by mixed oak species (Schweitzer et al., 2008). Braun (1950) described this area of Kentucky as part of the mixed-mesophytic forest region, abundant with beech (Fagus grandifolia), white oak (Quercus alba) black oak (Quercus velutina) and hickory (Carya spp.). Other common hardwoods included chestnut oak (Quercus prinus), particularly on ridges, maple (Acer spp.) and black gum (Nyssa sylvetica). Before chestnut blight, American chestnut was a dominant timber tree on the Cumberland Plateau, particularly at higher elevations (Braun, 1950). Silvicultural Treatments This study was nested within a larger USDA Forest Service study, referred to as the Cold Hill Study, which was established with the goal of improving oak regeneration and forest health prior to the anticipated arrival of gypsy moth (Lymantria dispar) to the area (Schweitzer et al., 2008). Our study utilized three of the five silvicultural treatments implemented in the Cold Hill study: oak shelterwood (OS), thinning (TH), and shelterwood with reserves (SW). For the OS treatment sites, all stems greater than 3 cm diameter at breast height in the midstory were killed using triclopyr herbicide injection (Loftis, 1990), leaving a basal area of 22 m2/ha of intact overstory (Schweitzer et al., 2008). This treatment increased light on the forest floor to favor oak regeneration while retaining enough overstory to inhibit shade intolerant species, such as yellow poplar (Liriodendron tulipifera). The overstory will be removed four-to-five years following midstory removal. The TH treatment thinned stands to the B-level of Gingrich stocking (Gingrich, 1967), with a basal area of 18 m2/ha of overstory (Schweitzer et al., 2008). While thinning is not a standard regeneration treatment, this may provide adequate light for seedling establishment or recruitment of species that are moderate in shade tolerance (Quercus spp.), while discouraging shade intolerant species. The SW treatment was a commercial tree harvest that left a residual basal area of 5 m2/ha of overstory. The overstory trees that promoted “good forest health conditions” and wildlife habitat were left uncut (Schweitzer et al., 2008). All harvest treatments were completed between Aug. 2007 and Feb. 2009. Experimental Materials American (C. dentata), BC2F3 generation (Hebard, 2001) and Chinese chestnut (C. mollissima) seedlings were used in this study. The open-pollinated American and advanced backcross chestnuts were harvested at The American Chestnut Foundation’s Meadowview Research Farms, Meadowview, VA in the fall of 2007 and manually sown at the Georgia Forestry Commission’s Flint River Nursery in Byromville, GA in Jan.