In the forests of northeastern Sweden, Per-Anders Esseen stands on a ladder measuring something invisible to the casual observer: the delicate hair lichens draped across spruce branches like forgotten stockings. This lichen, often overlooked in discussions of forest health, is proving to be far more valuable than industrial forestry once assumed — and a new study offers hope that it can coexist with timber production after all.
For decades, Sweden's forests have suffered a quiet loss. Clearcutting, the dominant harvest method across industrial forestry, has decimated hair lichen populations that depend on the complex shelter of older trees. Yet hair lichens matter more than most people realize. These branching organisms feed reindeer in winter when snow covers ground-level plants, sustain birds and insects seeking nesting material and forage, and form part of the ecological web that keeps coniferous forests functioning. For Sámi herders whose reindeer depend on these lichens, their absence can mean the difference between survival and starvation through harsh Nordic winters.
A research team led by Esseen, professor emeritus at Umeå University, spent eleven years testing whether partial cutting — an alternative to clearcutting — could reverse this decline. Working in the Vindeln Experimental Forests from 2008 to 2019, they compared plots where one-third of trees were logged, plots where two-thirds were removed, and unlogged control forests. The question was deceptively simple: could careful thinning actually boost hair lichen recovery?
The answer surprised many. In moderately thinned forests — where one-third of trees were harvested — hair lichen biomass on individual trees increased up to four times over the study period, with the most dramatic growth in the lower branches. This growth was real and sustained, suggesting that partial cutting creates the conditions for lichens to flourish. The mechanism turned out to be straightforward: increased light penetration in thinned forests accelerates lichen photosynthesis and growth.
However, the study revealed an important threshold. When two-thirds of trees were removed in heavy thinning operations, hair lichens declined dramatically. The exposed forest lost too much protection from wind, and the sheer loss of host trees outweighed any benefit from increased light. For reindeer herds and the ecosystems that depend on hair lichens, this distinction between moderate and heavy cutting is crucial.
The research also uncovered an intriguing detail about lichen chemistry. Dark hair lichens — species containing melanin pigments that look almost black — recovered faster in thinned forests than their pale cousins. The dark lichens, which perform poorly in dense, shaded conditions, thrive when light increases, while pale lichens maintain steadier growth across different light regimes. This physiological variation offers foresters additional precision in understanding what their thinning operations will achieve.
"Our results show that continuous cover forestry can significantly increase the amount of hair lichens in managed forests, which is particularly important in important winter grazing areas for reindeer," Esseen concluded in the study, published in Forest Ecology and Management. The message is clear: Sweden's forests do not have to choose between timber production and ecological health. With knowledge of how thinning affects these ancient partnerships between fungi and algae, foresters can manage their lands in ways that regenerate the lichens that wildlife and human communities alike depend on surviving the winter.