Plants maximize competition while minimizing competitors belowground: a theoretical analysis of incentives for root competition in space

Recent research shows that shared access to belowground resources drives plants to overproliferate fine roots competitively, limiting community-level aboveground biomass. Models of this phenomenon are commonly based on an assumption that belowground resources and fine roots are thoroughly well mixed. In reality, of course, fine roots are spatially structured by individual. Here we investigate how costs of sending roots through horizontal space influence incentives for fine-root overproliferation. We find that these costs restrain overproliferation to the net benefit of community aboveground biomass. And further, the costs eliminate incentives for individuals to grow fine roots beyond their closest neighbors. Plants that interact with the fewest competitors benefit the most in relative fitness from overproliferation of fine roots. Effectively, individual-based optimization of root allocation in space increases the effects of competition while decreasing the number of individual competitors for each individual. Because an individual’s optimal competitive network consists of only the closest neighbors, we predict the full effects of competition are achieved just shortly after disturbance, making competition belowground an almost inescapable pressure on plants. Together these results have important implications for predicting plant interaction networks, patterns of carbon allocation, and ecosystem carbon storage.