Increase in Ponderosa Pine Density in the Nebraska Sandhills: Impacts on Grassland Plant Diversity and Productivity

Increase in woody species encroachment into semi-arid grasslands and savannas has been of great concern at a global level. Temperature and precipitation are key factors determining vegetation cover; however other factors, such as fire regime and grazing may be at play in semi-arid ecosystems. Historically, in Nebraska’s semi-arid grasslands, woody species have been controlled by periodic fires. Changes in social values and land use, fire suppression, overgrazing, increased N deposition, and climate change, have attributed to the observed shift from grasslands to shrublands or woodlands. The primary objectives of this study were to determine the impacts of Ponderosa pine ( Pinus ponderosa P. & C. Lawson) expansion on grasslands productivity and species composition, and associated soil water content in the Nebraska Sandhills. Ten long-term vegetation plots, ranging from open grassland (basal area = 0 m2ha-1) to dense stands of ponderosa pine (basal area = 49.5 m2ha-1), were established in Nebraska National Forest (NNF) at Halsey, NE. Measurements at each site included basal area, LAI (leaf area index), understory species composition, biomass, and monthly soil water content. With increasing basal area, and thus, tree canopy cover, declines were reported in species composition, forage production, and volumetric soil water content, and cool-season (C3) grasses began to replace warm-season (C4) grasses before they both declined in dense canopy cover. The consequences of this shift from grassland to woodland are likely to include significant ecological, hydrological, and economical impacts