The legacy effect of competition

Background/Question/Methods

Kentucky bluegrass (Poa pratensis L.) is a perennial seed crop of primary importance in eastern Oregon (Union County). Approximately 90% of grass seed crops in Union County are grown under contract with seed companies as certified seed (Walenta, pers. comm., 2000). In sodic areas of Kentucky bluegrass fields, alkaligrass (Puccinellia spp.) is a dominant perennial weedy species. The common name ‘alkaligrass’ actually refers to either one of two species of alkaligrass: Nuttall’s alkaligrass (Puccinellia nuttalliana (Schult.) Hitchc.) a native grass to the region or weeping alkaligrass (Puccinellia distans (Jacq.) Parl.) an introduced species from Eurasia. Grass seed producers of eastern Oregon are concerned over the potential for either species of alkaligrass to expand from their sodic habitats and compete with Kentucky bluegrass on agriculturally productive sites. An addition series competition study was designed to study the competitive interactions and survival rates, over a two year period, among the three species when grown in pair-wise matrices to evaluate the potential for either species of alkaligrass to affect Kentucky bluegrass seed production in agriculturally productive soils of eastern Oregon.

Results/Conclusions

In year 1, weeping alkaligrass was more competitive than Nuttall’s alkaligrass and both species were far more competitive than Kentucky bluegrass. Both Nuttall’s and weeping alkaligrass exhibited high mortality following harvesting, 40% and 60%, respectively. There was a shift in competitive effects in year 2; such that weeping alkaligrass was equally competitive with Kentucky bluegrass, and both were far more competitive than Nuttall’s alkaligrass. While weeping and Nuttall’s alkaligrass survival into year 2 may be affected by standard harvesting procedures, the effects of their initial densities may have long term implications for Kentucky bluegrass productivity, especially weeping alkaligrass. The legacy effect of weeping alkaligrass will likely have a lasting effect on the seed production of a Kentucky bluegrass crop, even if removed in the first year. While Kentucky bluegrass farmers should consider weeping alkaligrass to be a greater threat to seed production, these results should not diminish the effects of Nuttall's alkaligrass. Thus, diligent weed control of both species early in the growing season will reduce direct competition, the legacy effect of competition, seed bank development, and weed spread through Kentucky bluegrass seed contamination