Veteranising Scots pine trees by initiating tree hollowing: Inoculation with the fungal keystone species Porodaedalia pini

Hollow trees are crucial for forest biodiversity but are becoming increasingly rare in many ecosystems, including the Scots pine forests of northern Europe. Here, we inoculated heartwood of live Scots pine trees with the fungal keystone species Porodaedalia pini to initiate tree hollowing. The fungus was inoculated in 50-, 110- and 170-year old stands, using wood dowels containing mycelia. Three different strains were used to test for intraspecific variation. Molecular analysis of samples from inoculated trees seven years after treatment showed that 67% were successfully colonised, with no differences between stands. Fungal strain had no effect on colonisation success. Our findings suggest that inoculation with P. pini has the potential to be an efficient method to restore a key ecological process, tree hollowing, in degraded Scots pine forests. The possibility of initiating the process even in young trees may be a way to accelerate the formation of hollow pines in younger forests.