Direct seeding is an efficient method to restore native grasslands, but to be successful the plants must grow and establish in the face of both inter- and intra- specific competition. Plants with resource use complementarity may be able to more completely utilize available resources than plants with more direct competition. A close spacing of mature plants may be more resistant to invasion by non-native species. If generalizations can be made about ecological behavior based on plant traits, or growth form, species mixtures can be chosen for communities with limited information on specific competitive interactions.  A greenhouse experiment was conducted to model competitive interactions during the first season of growth to address the following questions.

Question 1: Can a plant’s functional group or physiological traits be used as a surrogate for species identity when predicting ecosystem behavior?

Question 2: Is there evidence of resource use complementarity between different species?

The experiment involved two perennial grasses, two perennial forbs, and two annual forbs, all native to the prairies of Oregon’s Willamette Valley. Each species was grown alone or with an individual of its own or another species. After 90 days the aboveground biomass was harvested, dried, and weighed.

Analysis as an additive series experiment allowed calculation of relative competition indices (RCI) for answering question one.  Analysis as a diellele replacement series experiment provided information with respect to question two.

Results/Conclusions

The RCI values of each competitor species, each competitor functional group and each of the six functional group pairs were compared using ANOVA. The predictive values of the traits were examined using simple linear regression. Competition intensity differed between both species and functional groups. The competitor species’ relative growth rate was the best quantitative predictor.

Higher biomass would be expected in mixtures than single species plantings of the same density if the species utilize a different resource profile. The experiment provided no evidence that the growth in mixtures differed from what would be expected based growth in single species pots.

The results of this experiment suggest that the initial establishment phase of community development may be governed by a space capture model of resource use. This model predicts that the first plant to fill a particular volume of below or aboveground space will acquire the available resources at the expense of other plants in the area. In established communities heterogeneity of resource availability may allow plants to use resources complementarily.