Comparisons of interspecies field performance of Fagaceae (Castanea and Quercus) planted in the southeastern United States with attention to soil fungal impacts on plant performance

The loss of Fagaceae species is an increasing concern globally, including in North American where American chestnut (Castanea dentata) has been virtually eliminated by non-native pathogens, and oaks (Quercus) are experiencing widespread regeneration failures and declines. Tree improvement and breeding programs are producing trees for disease resistance or improved performance traits but require field testing to refine efforts. We established a study in 2015 on a xeric pitch pine (Pinus rigida) site in the Blue Ridge Mountains of North Carolina to regenerate American chestnut and interspecies hybrids (BC₃F₃) and the co-occurring species of white oak (Q. alba) through planting bare-root, quality-graded seedlings. Chinese chestnut (C. mollissima) was also tested as a control species. We used pedigreed seed sources from open-pollinated genetic families that were nursery grown (1-0 bareroot seedlings for chestnut, 2-0 bareroot seedlings for white oak) to maximize overall size and competitive ability. Though there was variability within and among plant families in performance, American chestnut and BC₃F₃ hybrids generally outperformed Chinese chestnut (at least 13% taller) and white oak (at least 29% taller) for the first three years, but intraspecies differences among genetic families were significant for nearly all traits tested. Initial seedling root morphology poorly explained field performance (R²<0.17), but this relationship was significant for both white oak families and the only northern BC₃F₃ seed source. American chestnuts and BC₃F₃ hybrids had higher stem height to ground diameter ratios compared to white oak (at least 11% greater), indicating that white oak likely concentrates more resources to root development while chestnut concentrates more resources to maintaining above-ground competitive advantages. Additionally, we investigated soil fungal communities, both pre- and post-tree establishment and tested if these fungal communities can be used to predict plant performance or health. Soil fungi did a poor job predicting plant performance. Our results indicate that co-occurring Fagaceae species can be established in restoration plantings using well developed quality seedlings on relatively xeric sites. Managers should use diverse seed sources to avoid planting poor performing families and expect that chestnuts bred for blight resistance will outcompete planted white oak, at least in the short-term.