Microclimate drives growth of hair lichens in boreal forest canopies after partial cutting.

Hair lichens in the genera Alectoria and Bryoria dominate old-growth circumboreal coniferous forests and have important ecosystem functions, particularly for reindeer and caribou. These lichens are sensitive to changes in climate and are unable to maintain a high standing crop in industrial forestry based on clear-cutting, highlighting the need of management models based on continuous cover forestry. We examined how dry mass (DM) growth and CO 2 exchange in hair lichens depended on the balance between growth (carbon gain from photosynthesis) and losses (both carbon loss from respiration and mass loss from fragmentation). Partial cutting trials were conducted in a Picea abies -dominated forest by three levels of basal area (BA) removal (0 %; 33 %; 67 %), with five 80 m × 80 m plots per level. We compared two species with similar functional traits but with different cortical pigments, the pale Alectoria sarmentosa and the dark Bryoria fremontii. Lichens were transplanted within the lower canopy using net cages over a 1-year period to evaluate net growth, loss by thallus fragmentation and gross growth. Canopy openness and transmitted radiation during the growing season were estimated from hemispherical photographs. Canopy temperature, relative humidity, and photosynthetic photon flux density were monitored, with microclimate data subsequently used to model net CO 2 exchange using previously published response matrices describing net photosynthetic and respiratory activity. Net DM growth of A. sarmentosa was higher than in B. fremontii , and increased with level of BA removal, being twice as high in the 67 % BA removal as in the control. In contrast, B. fremontii responded weakly to partial cutting due to high rates of thallus fragmentation. However, gross growth of both species increased with canopy openness and transmitted radiation. The modelled net assimilation showed large seasonal variation, with the largest difference among levels of BA removal in autumn. The estimated DM growth agreed well with observed gross growth in A. sarmentosa but was underestimated in B. fremontii. Modelling of CO 2 exchange can provide a mechanistic understanding of how hair lichens respond to partial cutting and climate change. The response of hair lichens to microclimate in partial cuts depends on the trade-off between growth and losses. Results suggest that the faster lichen growth on residual trees in the one-third removal partial cuts compensated to a significant degree for the loss of lichen mass by the removal of host trees. [Display omitted] • Net growth of Alectoria was doubled in plots with 67 % basal area removal. • Bryoria responded weakly to partial cutting due to high thallus fragmentation. • Gross growth of both species increased with canopy openness and transmitted light. • Gross growth of hair lichens can be predicted from microclimate and CO 2 gas exchange. • Partial cutting has large potential to improve growth of hair lichens on residual trees. [ABSTRACT FROM AUTHOR]