Response of understory vegetation to long-term occupancy by introduced Pinus species in temperate deciduous hardwood forests

In many temperate regions of the world, conifer species have been planted to stabilize soils and promote site recovery on former hardwood sites that were cleared for agriculture. In many areas of eastern North America, these conifer plantings consisted of introduced (non-native) Pinus species planted on abandoned agricultural land once dominated by mesophytic hardwood species. These plantings constitute a shift in overstory composition away from native hardwood species with nutrient-rich litter that decomposes more quickly towards Pinus species with recalcitrant litter that may alter soil chemistry and nutrient availability. To examine how edaphic conditions associated with long-term introduced Pinus species occupancy are related to forest regeneration and herbaceous-layer diversity and composition, we sampled a total of 97 plots in planted Pinus echinata and Pinus strobus stands and naturally regenerated hardwood stands growing on two ecological landtype phases (ELTPs) of southern Indiana, USA forests, Fagus-Acer saccharum/Arisaema Mesic Ridges, and Acer saccharinum/Boehmeria Bottomlands. We collected vegetation data and analyzed soil samples to examine herbaceous-layer species distribution across gradients using non-metric multidimensional scaling (NMS) ordination. Two-way ANOVA was used to examine differences in individual species and species functional groups across stand types (P. echinata, P. strobus, and native hardwoods) and ELTPs. Our results show that differences in soil chemistry resulting from Pinus spp. occupancy were associated with differences in the composition and distribution of herbaceous-layer species in ordination space. Species across stand types and ELTPs were distributed across dominant gradients related to litter depth, cation exchange capacity, cation content, and soil aluminum concentration. Hardwood sites had significantly greater herbaceous-layer cover (139.6 ± 8.0 %) than P. echinata (48.5 ± 6.2 %) or P. strobus sites (81.7 ± 7.7 %), as well as greater herbaceous-layer species richness and diversity (mean species richness was 46.9 ± 2.1 on hardwood stands vs. 33.3 ± 1.6 and 36.4 ± 2.0 on P. echinata and P. strobus stands, respectively). Pinus echinata stands contained a greater density of woody regeneration, including Quercus spp. (201 ± 48 saplings ha⁻¹) and Fagus grandifolia stems (415 ± 82 saplings ha⁻¹), both of which occurred in greater density than Acer saccharum (163 ± 93 saplings ha⁻¹) and A. rubrum (70 ± 64 saplings ha⁻¹). Our results suggest that pine occupancy has created divergent successional trajectories in comparison to hardwood stands. These differing trajectories may offer both challenges and opportunities for restoration efforts. For example, the greater abundance of Quercus reproduction under P. echinata on ridges, combined with less productive soils, may allow Quercus stems to be promoted into the canopy with less competition from mesophytic competitors.