Effects of planting season and disturbance on the competition between a non-native grass and two native forbs

Background/Question/Methods

Replacing non-native species with natives is often a goal of restoration projects.  However, the establishment of native species may be limited by competition from non-native species.  Additionally, competitive interactions may shift under different disturbance frequencies and environmental conditions.  We examined effects of bahiagrass (Paspalum notatum) competition on two congeners, Coreopsis leavenworthii and C. lanceolata.  Bahiagrass is an introduced rhizomatous grass; both Coreopsis species are native to Florida.  We hypothesized that bahiagrass would be the more competitive species, and that an intermediate frequency of disturbance would result in the highest likelihood of species coexistence.  Coreopsis seedlings were grown in a monoculture (low and high density) or in a mixture surrounded by bahiagrass seedlings.  To simulate disturbance, shoot biomass taller than 10 cm was clipped throughout the study every 2 weeks (frequent), every 4 weeks (intermediate), or never (none).  The experiment was initiated in fall 2006 and repeated beginning spring 2007.  Plants were grown outdoors in full sun in Gainesville, FL.  Biomass from all plants was harvested 8 months after planting.  Shoots and roots were separated at the soil line, washed, and dried at 60-70 degrees C for 1 week. 

Results/Conclusions

Biomass of Coreopsis roots and shoots was highest when plants were grown alone and lowest when grown with individuals of the same species.  Interspecific competition from bahiagrass also negatively affected Coreopsis biomass but not as much as intraspecific competition.  Disturbance frequency and planting season affected competitive interactions between species.  In both Coreopsis species, shoot biomass was greatest in the treatment with no disturbance.  The same trend was observed in root biomass of C. lanceolata, but C. leavenworthii root biomass was not affected by disturbance.  The root and shoot biomass of bahiagrass responded negatively to disturbance in the spring-established plants only; we observed no response in fall-established bahiagrass.   Percent survival was much greater in fall-established plants than spring-established plants for both Coreopsis species but C. lanceolata percent survival was greater than that of C. leavenworthii.  Both root and shoot biomass of spring-established C. lanceolata plants were at least three times less than those of fall plants.  Based on these results, C. lanceolata may result in more successful establishment in a restoration context than C. leavenworthii.  While intraspecific competition limits Coreopsis establishment more than interspecific competition, planting in the fall and limiting disturbance may increase Coreopsis survival.