Distribution and nitrogen fixation of terricolous lichens in a boreal forest fire chronosequence

Abstract: Given that nitrogen (N) is a limiting nutrient in boreal ecosystems, biological N fixation can have a large influence on primary production and other processes in this ecosystem. To date, most studies focused on N fixation in boreal ecosystems have focused on bryophyte-cyanobacteria associations, whereas relatively few studies have investigated the significance of N fixation by lichens. In this study I examined how biomass and aerial N fixation rates of two common terricolous lichens, Nephroma arcticum (L.) Torss. and Peltigera aphthosa (L.) Willd., varied across a boreal chronosequence, where time since fire varied from 43 to 362 years. The main objective of the study was to observe how stand age influenced biomass and N fixation rates of terricolous lichens, which has been extensively described in previous studies for bryophyte-cyanobacteria associations. The study revealed that biomass and consequently N fixation rate per unit area for both lichen species significantly increased with time since fire; whereas, lichen N fixation rates per unit mass was unaffected by stand age. Peltigera aphthosa, but not N. arcticum, demonstrated a significantly higher N fixation rate per unit biomass than the Pleurozium-cyanobacteria association. However, on a per unit area basis, both lichen species demonstrated several orders of magnitude lower N fixation rates relative to the Pleurozium-cyanobacteria association, which was largely driven by differences in biomass per unit area of lichens and bryophytes. This study demonstrates that lichen biomass and N fixation rates per unit area respond positively to the long term absence of wildfire, and further suggest that the contribution of lichens to total forest N fixation rates is minor relative to bryophytes.