Canopy structure and microclimate drive long-term dynamics of hair lichens after partial cutting

Abstract

Hair lichens (Alectoria, Bryoria, Usnea) dominate old coniferous forest canopies across the boreal biome. These lichens play key roles in element cycling, hydrology, canopy microclimate and provide critical forage for reindeer/caribou and habitat for invertebrates. Clear-cutting has caused large-scale declines of hair lichens, highlighting the need for continuous cover forestry. We examined the long-term (2008–2019) effects of partial cutting on hair lichens in an old Picea abies-dominated forest using an experiment with three levels of basal area (BA) removal (0%, 33%, 67%). We compared pale Alectoria and dark Bryoria to understand how light-screening cortical pigments and other functional traits drive their responses to partial cutting. Forest structure, canopy profiles (using LiDAR data) and microclimate were monitored, and hair lichen mass was quantified at two heights (3−4 m and 6−7 m) on P. abies. Light intensity increased up to 2.6 times and radial growth of P. abies up to two times in partial cuts. Lichen mass per tree depended on lichen genus, height, level of BA removal and time. Alectoria dominated both heights, but Bryoria increased with height. Both genera declined in the first year after logging but thereafter increased 1.5−4.3 times the baseline over 10 years. Bryoria increased faster than Alectoria, except at 6−7 m after five years. The long-term changes in mass of Alectoria and Bryoria were positively correlated with canopy openness, showing that light is the major driver of hair lichen mass accumulation. The stand-level lichen mass after 10 years exceeded the baseline with 32−40% in the 33% BA removal but declined in the 67% removal. Our findings demonstrate that partial cutting, by altering canopy structure and microclimate, can significantly increase hair lichen mass and thus maintain their ecosystem functions in managed boreal forests.