Abstract
* Shrub encroachment into grasslands modifies ecosystem functions, species composition and soil properties. However, knowledge about its detailed below-ground effects and specifically on concurrent changes in root traits and mycorrhizal associations is missing.
* We studied the effects of shrub encroachment on the herbaceous and the whole subalpine grassland community (i.e. including herbaceous and shrub species) in the Austrian Alps. We analysed changes in root traits and mycorrhizal associations, and their interactions, along two gradients of shrub cover, one dominated by deciduous and the other by evergreen dwarf shrubs.
* As shrub cover increased, in the herbaceous community root tissue density (RTD) increased, while specific root length (SRL), arbuscular mycorrhizal fungi (AMF) colonization and arbuscule abundance, where nutrient exchange occurs, all decreased. In the whole community, RTD unexpectedly decreased with shrub cover, as shrubs showed lower RTD than neighbouring herbaceous swards. The decrease in RTD, SRL, root diameter and AMF colonization was only compensated by a slight increase in the relative abundance of ericoid mycorrhizal fungi (ErM).
* Shrub encroachment in subalpine grasslands significantly modifies root traits and mycorrhizal associations. Herbaceous plants shift towards more conservative root strategies with increasing shrub cover. Concurrently, the whole community shows compensation between ErM and AMF colonization, while root resistance and soil exploration ability decrease. Our results suggest that changes along a gradient of shrub cover concern individual traits or trait–trait interactions, rather than following the ‘root economics space’ framework focused primarily on AMF.