COMPARATIVE GROWTH AND INTERFERENCE BETWEEN CHEATGRASS AND YELLOW STARTHISTLE SEEDLINGS

Annual grasslands in the Pacific Northwest are being invaded by Eurasi an weeds, such as yellow starthistle (Centaurea solstitialis L.). Plan t-plant interactions influence community dynamics and plant establishm ent. The objectives of this study were to quantify the effects of inte rference between seedlings of cheatgrass (Bromus tectorum L.) and yell ow starthistle and to compare growth of isolated individuals of these species. Isolated individuals and addition series mixtures with total stand densities ranging from 20-20,000 plants m-2 were grown in an env ironmental chamber (10-degrees-C, 12-hour daylength). Individuals were harvested on 4-day intervals between 10 and 46 days, and mixtures wer e harvested 37 days after planting. Shoot weight, root weight, leaf ar ea, and total root length of isolated individuals were similar. Yellow starthistle roots penetrated deeper into the soil than did cheatgrass roots 22 days after planting. Intraspecific interference was greater than interspecific interference for both species, and resource partiti oning via rooting depth was evident. The yellow starthistle root:shoot ratio and the cheatgrass lower (below 200 mm): upper (above 200 mm) r oot ratio increased with increasing densities. Yellow starthistle and cheatgrass minimize interspecific interference as seedlings through di fferential growth and rooting depth. Invasion of cheatgrass rangelands by yellow starthistle increase resource partitioning and reduce our a bility to revegetate rangelands by conventional means.