Lespedeza cuneata is encroaching into tallgrass prairie in Oklahoma.  To assess the mechanisms of its success, seasonal morphological and physiological traits were measured in this invasive forb, the native forb Ambrosia psilostachya and the native perennial grass Andropogon gerardii.  We hypothesized that L. cuneata has more favorable seasonal traits than other species, aiding in its success.  Rainfall was unusually high during the months of May through July 2007, nearly three times the average.  Compared to the other species, L. cuneata had greater total leaf area and specific leaf area throughout the growing season.  A. gerardii allocated more aboveground biomass to leaves than forb species early in the season, but was surpassed later on.  L. cuneata always had greater total aboveground biomass.  Photosynthesis, stomatal conductance, and chlorophyll fluorescence were measured at 0900, 1200 and 1500 hours twice per month May through September.  A. psilostachya had greater leaf-level photosynthesis and stomatal conductance for all months and time periods.  Maximum and achieved quantum yield of photosystem II in both forbs were greater than A. gerardii throughout the summer.  Early in the growing season, A. gerardii had less negative xylem pressure potentials at midday and predawn.  However, L. cuneata had less negative xylem pressure potential afterwards.  Though it seems that not all traits aid in the success of L. cuneata, favorable plant water status, light harvesting efficiency, and leaf area may contribute.