Grasslands across the globe currently experience numerous threats resulting in lost ecosystem services and global biodiversity. In the North American Great Plains, it is estimated that as little as 1% of the historical range of the tallgrass prairie ecosystem remains intact, and these tracts are threatened by a variety of global change phenomena. One of the greatest current threats to grasslands worldwide is the expansion of woody species. Many studies document aboveground consequences of woody species encroachment and have noted changes in multiple trophic levels. However, little research exists on belowground ecosystem-level effects following woody plant establishment, such as soil microbial community composition, including arbuscular mycorrhizal (AM) fungi abundance, and soil nutrient dynamics. Mycorrhizal interactions have previously been shown to play a critical role in native plant species dominance and contribute to healthy soil function of tallgrass prairie ecosystems. Woody species expanding their native ranges may competitively engage in mycorrhizal interactions but the potential competitive interactions between native grasses and encroaching woody species has not yet been determined. Our study assesses abiotic and biotic soil characteristics of native tallgrass prairie and adjacent areas with established stands of the following woody species: Juniperus virginiana, Cornus drummondii, Gleditsia triacanthos, and Rhus aromatica.
Our results indicate woody encroachment and establishment result in species-specific alterations in biotic and abiotic soil characteristics. For example, encroachment by J. virginiana resulted in increased plant-available P and more alkaline soils. However, while C. drummondii or G. triacanthos did not demonstrate significant shifts in nutrient availability, these species altered other soil abiotic characteristics including gravimetric moisture content and soil aggregate stability. Juniperus virginiana and G. triacanthos were associated with greater abundance of AM fungi, both within the roots and in surrounding soil, compared to native soils or soils associated with C. drummondii. (Data for R. aromatica are preliminary and forthcoming.) Alterations in AM fungal abundance may suggest preferential host-selection of the symbiont with potential cascading effects for competitive interactions between encroaching woody plants and native prairie grasses. Alteration in soil characteristics may be one mechanism facilitating the rapid conversion of grasslands to woodland species, and may provide challenges to ecosystem restoration following removal of the woody species.