Explore less to control more: why and when should plants limit the horizontal exploration of soil by their roots?

Abstract

In ecosystems limited by soil nutrients, some plants show a restricted horizontal distribution of their roots. We explored the hypothesis that this particular pattern is a foraging strategy emerging from trade-offs between soil exploration (that increases the pool of nutrients available for plants) and the local control of nutrient cycling within the soil that we call soil occupation. We developed two general analytical models of the cycling of a limiting nutrient in a plant population that is not limited by water. They allowed to explore how plant productivity is affected when roots do not exploit the whole soil available and to determine the conditions for which plant nutrient stock is maximized when plants limit their exploration of soil. We predict that a restricted exploration strategy can be beneficial when (1) there is at least one trade-off between a nutrient cycling parameter and soil exploration, (2) nutrient availability in the unexplored soil is poor and (3) the area of soil explored by plants is stable over time. The exploration limitation strategy results in spatially heterogeneous and nutrient-conservative ecosystems. Our results should apply well to perennial tussock grasses within tropical nutrient-limited ecosystems and raises interesting cues for the construction of more sustainable agro-ecosystems. Overall, our study underlines the importance of considering the multiplicity of root-soil interactions and of their scales when considering root foraging strategies.