Competition and facilitation in mixtures of aspen seedlings, alfalfa, and marsh reedgrass

Increasing demand for aspen (Populus tremuloides Michx.) and related poplar species is generating interest in their intensive cultivation. Successful establishment of aspen plantations requires minimizing the negative effects of associated plant species. Competitive and facilitative effects were isolated in field plots containing fixed-density mixtures of aspen seedlings, alfalfa (Medicago sativa L.), and marsh reedgrass (Calamagrostis canadensis (Michx.) Beauv.) in central Alberta. Although aspen survival was unaffected in mixtures, damage to aspen leaf area was lower when grown with either herbaceous species than when grown in monoculture, possibly reflecting facilitation through plant defense guilds. Over the first two growing seasons, net competition was expressed as most aspects of aspen growth were reduced. Herbaceous species reduced photosynthetically active radiation, soil moisture, and soil N available to aspen. Moreover, relative yield totals from the species mixtures examined consistently indicated either neutral effects (combined yields equaled monoculture yields) or underyielding. Despite this, evidence of facilitation was also found when aspen was grown with alfalfa, including increases of overall available soil N and transient increases in soil moisture with pulsed precipitation during drought. These results indicate that short-term facilitative aspects of aspen–legume mixtures may be exploited through an agroforestry scheme by appropriately timed harvest of the herbaceous component. Conversely, aspen establishment has limited potential for integrated production with marsh reedgrass.