Role of co-occurring competition and facilitation in plant spacing hydrodynamics in water-limited environments

Significance Plant communities exist in a continual state of transition as the spacing between individuals in each successive generation changes in response to environmental conditions. We developed a conceptual model based on quantitative experimental data and established ecological theories to provide insight into plant performance and community spatial pattern development from a hydrodynamic perspective. We demonstrate that the roles of co-occurring competition for soil water and facilitation of abiotic stresses (i.e., microclimatic) vary with spacing distance between individual plants, impacting soil moisture patterns and local water availability. Our conceptual model can provide insight into the design of future field and modeling efforts focused on pertinent climatological and ecohydrological problems related to land–atmosphere fluxes, agricultural best practices, and ecosystem productivity and recovery. Plant performance (i.e., fecundity, growth, survival) depends on an individual’s access to space and resources. At the community level, plant performance is reflected in observable vegetation patterning (i.e., spacing distance, density) often controlled by limiting resources. Resource availability is, in turn, strongly dependent on plant patterning mediated by competitive and facilitative plant–plant interactions. Co-occurring competition and facilitation has never been specifically investigated from a hydrodynamic perspective. To address this knowledge gap, and to overcome limitations of field studies, three intermediate-scale laboratory experiments were conducted using a climate-controlled wind tunnel–porous media test facility to simulate the soil–plant–atmosphere continuum. The spacing between two synthetic plants, a design consideration introduced by the authors in a recent publication, was varied between experiments; edaphic and mean atmospheric conditions were held constant. The strength of the above- and belowground plant–plant interactions changed with spacing distance, allowing the creation of a hydrodynamic conceptual model based on established ecological theories. Greatest soil water loss was observed for the experiment with the smallest spacing where competition dominated. Facilitation dominated at the intermediate spacing; little to no interactions were observed for the largest plant spacing. Results suggest that there exists an optimal spacing distance range that lowers plant environmental stress, thus improving plant performance through reduced atmospheric demand and conservation of available soil water. These findings may provide a foundation for improving our understanding of many climatological, ecohydrological, and hydrological problems pertaining to the hydrodynamics of water-limited environments where plant–plant interactions and community self-organization are important.