On the balance between positive and negative plant interactions in harsh environments

Positive interactions between plants typically occur where the presence of a species ameliorates the abiotic environment for another. However, there i s also a potential for resource competition to act at the same time, which creates a situation where the net outcome is a balance between positive and negative interactions. We present data from a nine-year study in two extre me high alpine habitats that was designed to test whether the effects of es tablished Ranunculus glacialis individuals on germination and growth of Oxy ria, digyna are primarily positive or negative at the altitudinal limit of vascular plants. We show net effects ranging from neutral to negative, but no positive effects were detected. We also argue that close associations be tween plants in these harsh environments may both ameliorate and deteriorat e the abiotic environment, and that experimental manipulations are necessar y to tell the difference.