Spatial and Temporal Variation in Competitive Effects on Soil Moisture and Pine Response

The effects of different plant life-forms, including a bunch grass species, Andropogon virginicus L. (broomsedge), and a sprouting deciduous tree species, Liquidambar styraciflua L. (sweetgum), on soil moisture and competitive responses of a transplanted coniferous tree seedling, Pinus taeda L. (loblolly pine), were investigated. Addition of the bunch grass and/or hardwood sprouts either had no effect or increased soil moisture in the surface soil (0-14 cm) depending on the time, while addition of sweetgum and/or broomsedge (greatest density alone) decreased soil moisture in deeper portions of the solum during the summer months. Soil moisture available to pine seedlings at various points in time was assessed by measuring predawn xylem pressure potential. Temporal variation in predawn xylem pressure potential was accounted for through a water stress integral approach. More than half of the variation in pine size after one growing season could be accounted for by the water stress integral.