The influence of forest duff (Oe +Oa) layer on Quercus rubra seedling establishment in Pennsylvania’s Ridge and Valley region: Implications for regeneration
Regenerating oak (Quercus spp.) is a common objective in eastern U.S. forests. In some areas of Pennsylvania’s Ridge and Valley region, however, attempts to establish competitive oak regeneration have failed despite controlling known inhibiting factors including light, competition, and deer browse. It has been suggested that oak seedling development may be related to edaphic conditions influenced by lithology. The current study focuses on two geologic formations from which the majority of soils in the region developed: Tuscarora (quartzitic sandstone) and Juniata (shale and siltstone). One site factor thought to be an impediment to seedling development, but with little supporting evidence, is a forest floor with thick fermentation and humus layers (Oe+Oa), here referred to as duff. We found duff at Juniata sites (n=4) to be 4.9 ± 0.35 cm thick and weigh 60.9 ± 4.58 Mg ha-1. Tuscarora duff (n=4) was thicker (P=0.0279) and heavier (P=0.0031) at 6.4 ± 0.35 cm and 91.9 ± 4.58 Mg ha-1, respectively. Three duff samples were collected from each site and placed over potting soil in a greenhouse. Pre-germinated northern red oak (Q. rubra L.) acorns were placed on the duff surface, 2.5 cm deep in the duff, or at the duff-soil boundary.
After a 14 week growth period, no statistical evidence was found to indicate that duff caused differences in seedling development between geologic formations. Differences were detected for relative height growth rate (P<0.0001) and the number of primary leaves (P<0.0001) among the eight sites. This result did not appear to be related to duff thickness, but we are currently exploring other possible factors including duff ectomycorrhizal inoculum potential. Seedlings from acorns planted at the duff-soil boundary took longer to emerge (P=0.0119), had lower total height (P=0.0005) and ground line diameter (P=0.0123) when compared to planting at the duff surface or 2.5 cm deep. In addition, planting at the duff-soil boundary resulted in fewer (P=0.0020) live seedlings at the end of the study period. These results indicate that in the greenhouse where neither moisture nor light were limiting to growth, seedlings’ roots are able to penetrate the even thickest duff layers. However, when acorns are planted deeply, the duff may impede shoot emergence often to the point of seedling mortality.