Stronger fertile island patterns enhance plant facilitation in drylands, regardless of overall ecosystem fertility

Abstract

Plant facilitation is a central process structuring arid ecosystems, where soil nutri-ents play a key role. However, it remains unclear whether the heterogeneity andavailability of soil nutrients are linked to facilitation intensity at a global scale. Here, we tested two hypotheses: (i) that facilitation intensity increases withstronger fertile island patterns (higher nutrient heterogeneity at the micrositelevel) and (ii) that it decreases with increasing overall ecosystem soil fertility(higher nutrient availability at the ecosystem level) across drylands. To test these hypotheses, we conducted a meta-analysis based on 94 observa-tions from 27 studies that jointly measured plant facilitation and the fertile islandeffect for nitrogen and/or phosphorus in drylands. According to our first hypothesis, we found that facilitation intensity increasedwith stronger fertile island patterns of nitrogen and phosphorus. However, con-trary to our second hypothesis, facilitation intensity did not respond to ecosys-tem fertility. This indicates that facilitation intensity increases as the difference insoil nutrient concentration between nurse and open microsites becomes larger,independently of overall ecosystem fertility. Synthesis. Our study provides novel insights into the role of soil nutrients in plantfacilitation in dryland ecosystems. Our findings highlight that the strength of thefertile island effect is a key ecosystem feature mediating facilitation intensity,regardless of overall ecosystem fertility. These findings emphasize the need toinvestigate how soil degradation processes could alter fertile island patterns andfacilitation processes in drylands.