Above- and below-ground effects from alfalfa and marsh reedgrass on aspen seedlings

Partitions were used to separate above- and below-ground effects of alfalfa (Medicago sativa L.) and marsh reedgrass (Calamagrostis canadensis (Michx.) Beauv.) on aspen (Populus tremuloides Michx.) seedlings. Surrounding herbaceous vegetation reduced soil water, available soil nitrates and photosynthetically active radiation. Although aspen survival was not affected, net competition was expressed with competitor biomass correlated negatively with most aspects of aspen growth. Competitive differences between herbaceous species were primarily related to individual plant sizes. Herbaceous species increased soil moisture when shoots were intermixed and root barriers were in place, but did not enhance soil moisture when below-ground effects were also present. This suggests herbaceous canopies increase soil moisture by blocking evaporation, but concomitant root uptake negates this increase. Aspen made little use of the N pool outside the immediate zone around the seedling, suggesting retention of herbaceous vegetation between trees could minimize N-leakage from the system. Thus, there may be some benefit to agroforestry systems in balancing the short-term competitive effects from herbaceous species presence against longer-term negative effects on soil fertility.