Resource mediated competition of two Texas natives: Acacia berlandieri, a C3 shrub, and Trichloris pluriflora, a C4 grass

Acacia berlandieri, an early successional C₃woody shrub, and Trichloris pluriflora, a late successional/climax C₄ tall grass, reside in similar habitats and have both been documented in Texas for over 150 years. This study examines resource mediated above and below ground competition within and between these species, which may illuminate some of the variables involved in the expansion of woody shrub species into the former grasslands. These species were started from seed and grown outdoors (five 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. Using a factorial design, growth measurements were taken 3 times, spaced evenly apart, after a 72 day establishment period. The plants were then harvested intact, rinsed with water to remove the soil, and dried at 60°C. The above and below ground dry masses were separated, and the roots were ashed at 650°C to remove all foreign organic debris. Measurements were then taken for the above and below ground biomass. Then by utilizing the ANOVA, the effects of the growth and harvest parameters on each species was determined for both intraspecific and interspecific competition. The addition of nutrients had little or no effect on the intra- or interspecific competition for either species growth or harvest. However, nutrients did appear to have a negative effect on Acacia berlandieri for harvested intraspecific competition. Although this legume is not known for nodulation, the possibility of N₂ fixation occurring in the root system must be considered because of this negative response. The variable of density appeared to be the driving force for the intra- and interspecific competition of these two species for both growth and harvest. Although grassland and woody shrub interactions should be mediated by resource availability, nutrients did not mediate the intra or interspecific competition of Acacia berlandieri and Trichloris pluriflora in this study. Clearly, density is an important variable involved in the disappearance of the grasslands.