Lichens Contribute to Open Woodland Stability in the Boreal Forest Through Detrimental Effects on Pine Growth and Root Ectomycorrhizal Development

Abstract

In the boreal forest, open lichen woodlands have been described as an alternative stable state to closed-crown feather moss forest. In this study, we addressed the role of terricolous lichens in stabilizing open woodlands by hindering tree regeneration and/or growth. Based on field and greenhouse experiments, we compared germination and growth of jack pine (Pinus banksiana) on feather mosses (primarily Pleurozium schreberi) and lichens (primarily Cladonia stellaris), using bare mineral soil as a control. Drivers were investigated by (1) manipulating nutrient supply, (2) simulating shade of a closed canopy on the ground layer with the assumption this would mitigate lichen influence on pine growth, and (3) examining pine root ectomycorrhizal colonization and diversity as indicators of pine ability to take up nutrients. Total growth of 6-month-old greenhouse and 2–3-year-old field seedlings, as well as belowground growth of 2-year-old greenhouse seedlings, was significantly greater in moss than in lichen. Seed germination was not affected by ground cover type. Although field phosphorus and base cation availability was greater in mosses than in lichens, fertilization did not entirely compensate for the negative effects of lichens on pine growth in the greenhouse. Ground layer shading had no impact on pine growth. Lichens were associated with reduced abundance and modified composition of the root ectomycorrhizal community. By suggesting that terricolous lichens constitute a less favorable growth substrate than mosses for pine, our results support the hypothesis that lichens contribute to open woodland stability in the potentially closed-crown feather moss forest.