Mothers, nurses, or just hiding in tall grass? Routes to natural tree colonisation in a grazed upland grassland

Globally there is a drive to increase tree cover in former forested ecosystems. Assisted natural regeneration can be achieved through managing herbivore pressure, yet the relative role of different mechanisms controlling tree colonisation in grazed grasslands remain unclear. At a long-term and landscape-scale livestock grazing experiment in Scotland, we investigated natural tree colonisation using grassland monitoring data collected over 20 years. Tree colonisation was present across all livestock intensities from high density to no livestock. Of the three main colonising tree species, we found differing mechanisms of colonisation: Salix spp. (willow) required rank grass sward to colonise, Betula spp. (birch) required close mother trees and dual-mycorrhizal nurse trees (Salix spp.), and Sorbus aucuparia (rowan) benefited under low livestock pressure. Together the story is one of tree-species specific pathways to colonisation, yet our findings showcase tree colonisation can be achieved whilst sustaining low intensity livestock and deer grazing of upland grasslands.