Sweetgum and broomsedge response to competition across a range of soil organic matter during the first year of plant establishment

Sweetgum (Liquidambar styraciflua L.) seedlings and broomsedge (Andropogon virginicus L.) rooted tillers were planted in a factorial combination of densities (additive series) in east central Alabama on a formerly cultivated field which varied in soil organic matter (soil OM). Mean plant aboveground biomass of sweetgum one growing season after planting declined in a hyperbolic fashion as a function of increasing density of either itself or broomsedge. Broomsedge biomass over the same time period also declined in a hyperbolic fashion as a function of increasing density of itself and sweetgum, except for sweetgum densities from 0 to 1 m(-2), where biomass did not change. Sweetgum response was positively correlated with soil OM across all species-density combinations, whereas broomsedge response was correlated with soil OM only at higher densities of broomsedge. Nonlinear models including both species density and soil OM explained a nearly equal amount of variation in response of each species: 74 to 75% for sweetgum and 70% for broomsedge. Relative effects of intraspecific versus interspecific competition on the response of each species was dependent on the density of one or both species. Effect of sweetgum on broomsedge response was two to three times that of broomsedge on itself, with the effect decreasing slightly as soil OM decreased. Over a common range of densities of each species, effects of broomsedge on sweetgum were slightly greater than or nearly equal to effects of sweetgum on itself. The greater interspecific effects on the response of each species suggests no niche differentiation between broomsedge and sweetgum through one growing season.