Shoot facilitation contributes more than root facilitation to plant size inequality under environmental stress: theoretical insights from a two-layer zone-of-influence model

Positive plant-plant interactions (i.e. facilitation) often occur under stressful environments and regulate population and community dynamics. However, the relative importance of shoot vs. root facilitation in determining plant size inequality (coefficient of variation for biomass among individuals) remains poorly understood.

We used a two-layer “zone-of-influence” model to explore the effects of competitive size-asymmetry and facilitation on size inequality along stress gradients.

Stress level alone (without plant-plant interactions) had little influence on size inequality. When facilitation was not present, shoot, root, and shoot + root competition intensity generally decreased with increasing stress. Accordingly, size inequality under most interaction scenarios decreased with increasing stress. Size inequality was higher under asymmetric shoot competition (the largest individual obtains all the contested resources) than under completely-symmetric root competition. When either shoot or root facilitation was present, corresponding net effects of shoot or root interactions were positive at high stress levels. Facilitation led to larger size inequality under these interaction scenarios because larger plants tend to overlap more with and thus benefit more from neighbors. Furthermore, size inequality with shoot facilitation was greater than that with root facilitation, possibly because the former is generally size-asymmetric (i.e. larger shoots obtain disproportionately more benefits from overlapping areas), while the latter is more likely to be symmetric.

Our results highlight the role of shoot facilitation in amplifying plant size inequality. Agricultural and forest management in harsh environments may attempt to manipulate aboveground plant-plant interactions to achieve desired production goals.