Resource Mediated Competition of two South Texas Natives: Acacia berlandieri and Trichloris pluriflora
Authors: Roach Sara , Jurena Paul 
Published: September 2009

Abstract
Background/Question/Methods
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.

Results/Conclusions
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.