Acacia berlandieri, an early-successional C₃ woody shrub legume, and Trichloris pluriflora, a mid-tall late-successional/climax C₄ grass, have both been documented in Texas for over 150 years, residing in similar habitats. This study examines resource mediated competition within and between these species, which may illuminate some of the dynamics involved in the encroachment and expansion 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.

Results/Conclusions

Acacia berlandieri, grown alone with a density of 1, 2 and 4 plants/pot, demonstrated a significant decrease in height with increased density (P=0.0228). Acacia also showed a significant decrease in basal diameter, height, and leaves with the addition of nutrients (P=0.0123, P=0.0032, and P=0.0202). When grown in combination with Trichloris, using densities of 1/3, 2/2, 3/1 and 4/0-Acacia/Trichloris plants/pot, Acacia height increased with its density (P<0.0001). Furthermore, there was a decrease in the number of Acacia leaves with the addition of nutrients (P=0.0479). Trichloris pluriflora, grown alone with a density of 1, 2, 4, and 8 plants/pot, demonstrated a significant decrease in culms, tillers, and height with an increase in density (P<0.0001). Furthermore, there was no change in these growth parameters with the addition of nutrients (P>0.05). When grown in combination with Acacia, using densities of 1/3, 2/2, 3/1 and 4/0-Trichloris/Acacia plants/pot, Trichloris culms and height decreased significantly with an increase in its density (P<0.0001). In addition, there was no change in Trichloris measurement parameters with an increase in nutrients (P>0.05). Both species clearly exhibited the effects of intra-specific competition. However, Trichloris seemed to out-compete Acacia in inter-specific competition. This may be a function of greater resource acquisition ability of Trichloris and possible nitrogen-fixation of Acacia, despite the absence of root nodules and lack of evidence for fixation in the literature. Furthermore, the addition of nutrients appearing to have a negative effect on the C₃ woody shrub and no effect on the C₄ grass, may also serve as a basis of this argument. Therefore, the expansion of Acacia berlandieri in Trichloris pluriflora grassland may also be the result of lack of nutrients due to overgrazing and erosion of these grasslands.