1. Plant species differ in their competitive effects by decreasing resource availability via uptake, but in some cases may increase resource availability via non-uptake pathways. Here we explore differences between grasses and woody plants in their competitive effects, and relate these to differences in resource effects. 2. We grew five species each of grasses and woody plants in monocultures for eight years. In the final two growing seasons, competitive effects were measured by growing transplants in all monocultures and in plots without neighbours. 3. Total competitive effects were significantly greater for woody plants than grasses. In contrast, the competitive effect per gram of grasses was about 17 times greater than that of woody plants. 4. For grasses, soil water and soil available N decreased significantly with increasing biomass. In contrast, for woody plants, soil water and soil available N increased significantly with increasing biomass. The results suggest that the intense per-gram competitive effects in grasses is related to the uptake of soil resources, and that the significantly lower per-gram competitive effects of woody plants may be related to their positive effects on soil resources. 5. Synthesis. The results link differences in competitive effects between grasses and woody plants to differences in the direction of their effects on soil resources. These differences may contribute to the entrainment of negative feedback in grasslands, excluding trees by means of strong competition, and the entrainment of positive feedback beneath woody plants establishing in grasslands, resulting in a state change from grassland to woody vegetation.

Please see Methods.

Please see WilsonPeltzer2020ReadMe.