Population temporal stability in grasslands increases with root outsourcing to mycorrhizae

Understanding which plant belowground foraging strategies promote long-term population temporal stability is critical for understanding species coexistence in grassland ecosystems. Root trait variation is increasingly framed within a multidimensional root economics spectrum that contrasts do-it-yourself (DIY) soil exploration via fine, acquisitive roots with outsourcing of resource acquisition to mycorrhizal symbionts. While these strategies are known to shape belowground resource use, their consequences for temporal population stability remain poorly understood.
Here, we tested whether root strategies predict long-term population stability using 16 years of annual vegetation data from 150 plots in three regions of temperate grasslands in Germany. Temporal stability was quantified as the coefficient of variation in species cover and related to a comprehensive set of belowground and aboveground functional traits measured for 82 species. 
Populations investing in outsourcing showed higher temporal stability, while populations with stronger DIY strategies exhibited lower temporal stability. At the trait level, collaboration-related anatomical traits, particularly cortex fraction, consistently promoted stability, while acquisitive fine-root traits were associated with higher variability. In contrast, aboveground leaf economic traits showed little explanatory power.
Our results demonstrate that root outsourcing to micorrhizal symbionts enhances long-term population stability in temperate grasslands, identifying belowground collaboration as a key axis of temporal niche partitioning and offering new insight into mechanisms underpinning species coexistence and grassland ecosystem functioning.