Comparing competition and solar radiation exposure as restricting factors in the growth of an understory plant

Background/Question/Methods

Verbesina virginica (Frost Weed) is a perennial herbaceous plant that occurs in areas across the eastern and central United States. In central Texas, V. virginica is found almost exclusively in the understory of woodland habitats. It does not seem to occur in open grassland areas, except near the drip line of adjacent woodland. The factors causing the distribution of V. virginica are not well understood. Competition with grasses and susceptibility to water loss with increased solar radiation are possible factors affecting its distribution. Density gradient experiments and a DeWitt replacement experiment were performed to evaluate the effect of competition on V. virginica and a perennial grass, Bothriochloa ischaemum (K.R. Bluestem). Light gradient experiments were performed in which V. virginica and B. ischaemum were grown in a monoculture at a constant population density and in a mixture at a constant ratio to evaluate their growth and ability to compete at specific light levels.

Results/Conclusions

Verbesina virginica aboveground, belowground, and total biomass per plant decreased significantly with increased density and with increased frequency in the replacement series. Bothriochloa ischaemum aboveground, belowground, and total biomass per plant displayed significant differences between densities but displayed no significant reductions between species frequencies. In both monoculture and mixture, Verbesina virginica biomass, stem length, and leaves per plant were highest at intermediate light levels ranging from 33% to 66% ambient light levels and Bothriochloa ischaemum biomass, leaf length, and leaves per plant were highest at 66% and 100% ambient light levels. The results show that growth of V. virginica and growth of B. ischaemum are inhibited at higher densities in intraspecific competition. The results also show that growth of V. virginica is inhibited by interspecific competition, but B. ischaemum is not significantly affected by interspecific competition. The results for V. virginica are consistent with an understory shade plant, while the results for B. ischaemum are consistent with a grassland species.