Changes in functional traits of the terricolous lichen <i>Peltigera aphthosa</i> across a retrogressive boreal forest chronosequence

Changes in the functional traits of the terricolous lichen Peltigera aphthosa with declining soil fertility during ecosystem retrogression were investigated. A well-documented retrogressive chronosequence of 28 forested islands in northern Sweden that differ greatly in fire history and which spans 5000 years was used. The abundance of cephalodia increased, indicative of higher N 2 -fixation rates resulting from lower N availability. Thallus δ 13 C values increased with ageing soils, in line with declining δ 13 C values of the humus substratum along this gradient. However, δ 13 C values were also driven by variation in factors that were at least partly independent of soil ageing. As such, δ 13 C values were mostly related to specific thallus mass (STM), possibly because a higher STM gives a thicker cortical layer and thus greater resistance to CO 2 diffusion, leading to higher δ 13 C values. STM and other measured traits (i.e. thallus N, P, secondary compounds and water-holding capacity) were unresponsive to the gradient, despite these traits being very responsive to the same gradient in epiphytic lichen species.