Acacia berlandieri, an early-successional C₃ woody shrub legume, and Trichloris pluriflora, a late-successional C₄ mid-tall grass, reside in similar habitats and have both been documented in South Texas for over 150 years. This study examines the resource mediated intra- and interspecific competition of these species which may illuminate some of the dynamics involved in the encroachment of woody shrub species into grasslands. These species, started from seed, were grown outdoors (5 replicates) using sandy clay loam soil in plastic lined pots for 155 days and watered daily. Half of these pots received 12.5% Hoagland’s solution. Growth measurements were taken 3 times, after a 72 day establishment period and prior to harvest. The plants were then harvested intact and dried at 60°C. The above and below ground dry mass for each individual plant was separated and the roots were ashed at 650°C.

Results/Conclusions

The growth parameters of A. 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 T. pluriflora. 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. The growth parameters of T. pluriflora, culms and height, decreased with increasing density (P=0.0077 and P=0.0006), when grown in competition with A. berlandieri.  The culms, tillers, and height of T. pluriflora, when grown alone, decreased with increasing density (P<0.0001, P=0.0312, and P<0.0001). The measurement parameters of harvested A. berlandieri, above ground 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, root, and ash-free root biomass of harvested T. pluriflora, grown in competition with A. berlandieri, decreased with increasing 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 parameters). The addition of nutrients had little or no effect on the growth or harvest of A. berlandieri and T. pluriflora for either intra- or interspecific competition (P>0.0500). Density appeared to be the driving force for the intra- and interspecific competition of these two species and potentially an important variable in the disappearance of grasslands.