Effects of rooting volume and nutrient availability as an alternative explanation for root self/non-self discrimination

An increasing number of studies have shown that plants produce more root mass when sharing rooting space with an intraspecific neighbour as compared with plants growing alone. This so-called self/non-self discrimination has been suggested as a mechanism by which plants may prevent wasteful competition with their own roots and enhance their competitive ability for nutrients with roots of neighbouring plants. The overproduction of root biomass is said to result in a 'tragedy of the commons', because it appears to occur at the expense of reproductive biomass. Studies on self/non-self root discrimination have commonly used a split-root design to distinguish self from non-self competition, while keeping the total amount of nutrients available per plant the same. This design has recently been criticized because the rooting volume differs between treatments. Here, we use three general hypotheses to explain the published results without invoking the mechanism of self/non-self discrimination. The hypotheses propose that differences in root mass are due to differences in rooting volume, and differences in nutrient availability determine whole plant growth. More root mass without more growth results in less reproductive biomass. A reanalysis of the results of root self/non-self discrimination confirms these hypotheses. Root overproduction in the presence of another plant, as found in nearly all studies, is consistent with effects of a larger soil volume available to these plants as compared with plants growing alone. Under the same total nutrient availability, total plant weight was the same or higher when more roots were produced. Inevitably, a larger root production with the same total biomass implies that less reproductive biomass is produced. Although our analysis can explain most of the results of the split-root experiments, we cannot rule out the possibility that self/non-self root discrimination did take place. We discuss a limited set of experiments for which volume effects cannot explain the results, suggesting, in fact, that direct self/non-self root interactions have operated. We suggest experimental designs that can demonstrate their ecological significance in the future. We conclude that there is ample evidence that plants can sense the volume of available rooting space, and a limited number of studies on individual roots show that plant roots may sense the identity of neighbouring roots and respond accordingly. The significance of these responses for whole plant growth and reproduction in relation to well-known resource competition effects is yet largely unknown and in urgent need of further study.