Exploring the root economics space in shrub-encroached subalpine grasslands

Shrub encroachment, a global phenomenon caused by land abandonment and shifts in traditional land use practices, is particularly prevalent in subalpine grasslands. This ecological shift is characterized by increased woodiness, which leads to changes in biogeochemical cycles and microbial composition. These changes in turn impact the soil's abiotic environment, particularly on carbon and nitrogen availability. While the influence of these changes on aboveground plant traits is well recognized, a substantial knowledge gap remains regarding their effects belowground. Understanding how shrub encroachment affects root morphological traits and mycorrhization is crucial, as they play a key role in nutrient uptake and transfer. This study focuses on the effects of shrub encroachment on root morphological traits and arbuscular mycorrhiza fungi (AMF) colonization at the levels of both herbaceous plants and of communities, i.e. including herbaceous and dwarf shrub plants, along a gradient of shrub encroachment in subalpine grasslands. We also aimed to describe the root economics space in encroached grasslands and to identify key soil changes correlated with changes in root traits. In herbaceous plants, shrub encroachment decreases AMF colonization and specific root length (SRL), and increases root tissue density (RTD). At the community level, AMF colonization, SRL, and RTD all decrease with shrub encroachment. Surprisingly, the observed root economics space at the community level does not follow the already established negative correlations of “do-it-yourself” strategies with high SRL and “outsourcing” strategies with increased root diameter and AMF colonization. Moreover, we observed a negative correlation between RTD and AMF. Our results highlight the importance of soil characteristics, specifically the carbon/nitrogen ratio (C:N) and soil pH, for changes in root traits. We conclude that shrub encroachment promotes the development of shorter and less dense roots and causes a decrease in AMF colonization through changes in the soil abiotic environment, such as an increase in C:N and a decrease in pH. This research provides valuable insights by expanding our understanding of belowground responses to shrub encroachment and highlights the importance of considering root traits in the broader context of ecosystem functioning.