Plant feedbacks on small-scale nutrient heterogeneity when resources recycle at different rates

Background/Question/Methods

Previous theoretical work suggests that nutrient recycling through plant litter may allow plants with a larger foraging footprint to facilitate smaller plants, promoting coexistence of the two strategies.  Separate smaller 'patches' within a large plant's foraging footprint may differ in the relative rates at which different resources are supplied.  Large plants may reduce spatial resource heterogeneity, by preferentially taking up each resource from patches in which that particular resource is in relatively high supply and depositing it (via litter) into patches in which it is in relatively low supply.  This benefits plants with smaller foraging footprints, because growth of smaller plants is often limited by the resource that is scarce within a given patch.   

These results were obtained from a resource-ratio model which assumed equal rates of recycling across all resources.  In that model, recycling always reduced spatial resource heterogeneity across two patches occupied by a single larger plant, while it had no effect on heterogeneity when each patch was occupied by a smaller plant.  Here, we explore the effects of relaxing this assumption, allowing resources to be recycled at different rates.   

Results/Conclusions

Allowing resources to recycle at different rates introduced several new potential outcomes.  Recycling may decrease spatial resource heterogeneity (as in the earlier model), but it may also increase it.  The relative effects of small vs. large plants on resource heterogeneity may also be reversed in the new model. Resource heterogeneity is often more sensitive to recycling with small plants than with large plants, and recycling can even lead to large plants increasing heterogeneity while small plants decrease heterogeneity.  Resource heterogeneity under large plants is more sensitive to the recycling rate of their limiting resource than that of their non-limiting resource.  Differences in recycling rates generally have the most pronounced effects when a plant (large or small) is nearly co-limited by the two resources, so that small differences between resources in recycling rate affects which resource is limiting. 

These results provide new insights into how differences in plant size, and thus the spatial scales at which they interact with the environment (in terms of both nutrients uptake and litter deposition), may affect the distribution of soil resources.  This in turn has implications for how plant size and nutrient recycling may interactively affect plant competition and coexistence.