The Effect of Competition and Availability of Water and Light on the Distribution of Verbesina virginica
Verbesina virginica (frost weed) is a perennial herbaceous plant found almost exclusively under the canopies of woodland and forest ecosystems. It does not seem to occur in the grassland areas, except near the drip line of an adjacent woodland habitat. The factors affecting its distribution are not well understood. Prior research suggests that Verbesina virginica can tolerate full sunlight. In addition to light saturation, water stress caused by evapotranspiration and competition with local grasses are possible factors which may contribute to the restriction of Verbesina virginica from grassland ecosystems. To better understand the effect of these factors and their interaction, a series of growth experiments and field studies were performed on Verbesina virginica and a common perennial grass with which it competes, Bothriochloa ischaemum (K.R. bluestem). Additive series experiments and a DeWit replacement series were performed on both species at the West Campus greenhouse at the UTSA 1604 campus. Verbesina virginica was negatively affected by competition with other individuals of its own species and benefitted from reduced frequency in the replacement series, despite competition with Bothriochloa ischaemum. Bothriochloa ischaemum was not significantly affected by competition with individuals of its own species or with Verbesina virginica. A light gradient experiment was performed in which both species were grown in monoculture and in combination to evaluate competition at different light levels. Verbesina virginica displayed optimum growth at intermediate light levels but tolerated full sunlight. Bothriochloa ischaemum displayed optimum growth at full sunlight and declining growth with decreasing light levels. Verbesina virginica was negatively affected by competition with Bothriochloa ischaemum and Bothriochloa ischaemum benefitted from competition with Verbesina virginica at full sunlight but was negatively affected at lower light levels. Gas exchange measurements were performed at a field site in northern Bexar County over 19 intervals from 0 to 2000 mol ∙ m-2 ∙ sec-2. Photosynthesis rates, stomatal conductance, intracellular CO2, and transpiration of Verbesina virginica were consistent with those of an understory plant, while those of Bothriochloa ischaemum were consistent with a sun plant. A reciprocal transplant experiment was performed in which Verbesina virginica and Bothriochloa ischaemum were planted under a woodland canopy, at the drip line of the canopy, and in an adjacent grassland habitat. Both species displayed optimum growth in their habitats, moderate stress at the drip line, and severe stress in the opposite habitats. Although competition with Bothriochloa ischaemum generated a slightly significant effect on Verbesina virginica in the light gradient experiment, the effect did not appear sufficient to limit the distribution of Verbesina virginica without additional factors. Given its tolerance to ambient light, it is not likely that excessive light levels are a limiting factor for Verbesina virginica. The results suggest that soil moisture availability is a major potential limiting factor in the growth and distribution of Verbesina virginica which could be the primary factor restricting Verbesina virginica from grassland habitats.