_Acacia berlandieri_, an early-successional C~3~ woody shrub legume, and _Trichloris pluriflora_, a late-successional/climax C~4~ mid-tall grass, reside in similar habitats and have both been documented in Texas for over 150 years. This study examines the resource mediated above and below ground competition within and between these species, which may illuminate some of the dynamics involved in the encroachment of woody shrub species into the former grasslands. These species, started from seed, were grown outdoors (5 replicates/treatment) using sandy clay loam soil in plastic lined 15×15 cm pots for 155 days and watered daily. Half of these pots received 12.5% Hoagland’s solution as a nutrient source. Growth measurements were taken 3 times, after a 72 day establishment period and prior to harvest. The plants were then harvested intact, dried at 60°C, and above and below ground dry mass for each individual plant was separated. The roots were ashed at 650°C and measurements were taken for the above and below ground biomass.
The growth parameters of _Acacia berlandieri_, basal diameter, height, and number of leaves increased with density (P = 0.0024, P < 0.0001, and P < 0.0001), when grown in competition with _Trichloris pluriflora_, using the densities of 1/3, 2/2. 3/1, and 4/0 _Acacia/Trichloris_ plants/pot. None of these growth parameters for _A. berlandieri_ had an effect on the variables of density, nutrients or and their interaction (P > 0.0500), when grown alone with densities of 1, 2, and 4 plants/pot. The growth parameters of _T. pluriflora_, culms and height decreased with density (P = 0.0077 and P = 0.0006), when grown in competition with _A. berlandieri_, using densities of 1/3, 2/2. 3/1, and 4/0 _Trichloris/Acacia_ plants/pot. The culms, tillers, and height of _T. pluriflora_, when grown alone with densities of 1, 2, 4, and 8 plants/pot, decreased with density (P < 0.0001, P = 0.0312, and P < 0.0001). The measurement parameters of harvested _A. berlandieri_, above ground biomass and ash-free root biomass increased with density (P = 0.0463 and P = 0.0389), when grown in competition with _T. pluriflora_. The root biomass of harvested _A. berlandieri_, grown alone, decreased with the interaction of density and nutrients (P = 0.0068). The total plant dry mass, above ground biomass, root dry mass, and root dry mass of harvested _T. pluriflora_, grown in competition with _A. berlandieri_, decreased with density (P < 0.0001, P < 0.0001, P = 0.0235, and P = 0.0145). These same measurement parameters of harvested _T. pluriflora_, grown alone, increased with density (P < 0.0001 for all four). The addition of nutrients had no effect on the growth or harvest of _A. berlandieri_ and _T. pluriflora_ for either intra- or interspecific competition. Instead, density appeared to be the driving force for the competition between and among these two species for both growth and harvest. Although woody shrub and grassland interactions should be mediated by resource availability, that was not the case in this study. Clearly, density is an important variable involved in the disappearance of the grasslands.