Growth of epiphytic old forest lichens across climatic and successional gradients Y. Gauslaa, K. Palmqvist, K.A. Solhaug, H. Holien, O. Hilmo, L. Nybakken, L.C. Myhre, and M. Ohlson Abstract: This paper aims to assess the influence of canopy cover on lichen growth in boreal forests along a regional forest gradient. Biomass and area gain, and some acclimation traits, were assessed in the old-forest lichens Lobaria pulmonaria (L.) Hoffm., Pseudocyphellaria crocata (L.) Vain., and Usnea longissima Ach. transplanted 110 days in three successional Norway spruce (Picea abies (L.) Karst.) forest stands (clearcut, young, and old forest) repeated along a rainfall gradient (continental, suboceanic, and Atlantic zones) in Scandinavia. Lichen growth peaked in Atlantic rainforests with mean dry matter (DM) gain up to 36%–38%. The alectorioid lichen U. longissima showed the widest range of growth responses and no signs of chlorophyll degradation. Its highest DM gain consistently occurred in clearcuts, whereas the DM gain was close to zero in the shadiest young forest. The two foliose lichens L. pulmonaria and P. crocata exhibited maximal growth rates in old forests, but apparently growth was limited by low light even in old forests. Their DM gain was reduced in the most sun-exposed clearcuts due to chlorophyll degradation and was relatively high under closed young canopies, suggesting a bet- ter adaptation to shade. The lichen responses show that a high frequency and dominance of young and dense fast-growing forest stands at a landscape level are not compatible with large populations of these old-forest lichens and that a lack of li- chens under an industrial forestry regime may not necessarily be determined by low dispersal efficiency only. Re ´sume ´: Cet article a pour but d’e ´valuer l’influence du couvert de la canope ´e sur la croissance des lichens dans les fore ˆts bore ´ales le long d’un gradient forestier re ´gional. Le gain en biomasse et en superficie ainsi que certains caracte `res d’accli- matation ont e ´te ´e ´value ´s chez les lichens des fore ˆts a ˆge ´es, Lobaria pulmonaria (L.) Hoffm., Pseudocyphellaria crocata (L.) Vain. et Usnea longissima Ach., qui ont e ´te ´ transplante ´s pendant 110 jours dans trois peuplements d’e ´pice ´a commun (Picea abies (L.) Karst.) rendus a ` diffe ´rents stades de succession (fore ˆt coupe ´e a ` blanc, jeune et vieille fore ˆts). L’expe ´- rience a e ´te ´ re ´pe ´te ´e le long d’un gradient pluvial (zones continentale, suboce ´anique et atlantique) en Scandinavie. La crois- sance des lichens a culmine ´ dans les fore ˆts pluviales atlantiques avec un gain moyen en matie `re anhydre allant jusqu’a ` 36 % – 38 %. Le lichen alectorioı ¨de U. longissima a montre ´ la plus vaste gamme de re ´ponses en croissance ve ´ge ´tative et aucun signe de de ´gradation de la chlorophylle. Son gain maximal en matie `re anhydre est survenu essentiellement dans les coupes a ` blanc alors qu’il e ´tait presque nul dans la jeune fore ˆt la plus ombrage ´e. Les deux lichens foliace ´s, L. pulmonaria et P. crocata, montraient des taux maximum de croissance dans les vieilles fore ˆts, mais la croissance e ´tait apparemment lim- ite ´e par la faible luminosite ´, me ˆme dans les fore ˆts a ˆge ´es. Leur gain en matie `re anhydre e ´tait re ´duit dans la plupart des coupes a ` blanc expose ´es au soleil a ` cause de la de ´gradation de la chlorophylle alors qu’il e ´tait relativement e ´leve ´ sous les jeunes can- ope ´es ferme ´es, ce qui indique une meilleure adaptation a ` l’ombre. La re ´ponse des lichens montre qu’une fre ´quence e ´leve ´e et une forte dominance de jeunes peuplements forestiers a ` croissance rapide a ` l’e ´chelle du paysage ne sont pas compatibles avec de grandes populations de ces lichens, typiques des fore ˆts matures, et que le manque de lichens sous un re ´gime forestier industriel pourrait ne pas ne ´cessairement e ˆtre du ˆ seulement a ` la faible efficacite ´ de dispersion des lichens. [Traduit par la Re ´daction] Introduction An unmanaged boreal forest landscape, which is often rich in epiphytic lichens (e.g., Esseen et al. 1996), is a mo- saic of environmental conditions and successional stages created by disturbances on various spatial and temporal scales. Recurring wild fires, insect outbreaks, windthrow, death, and fall of mature trees (Bonan and Shugart 1989) are determinants for the structure and function of such for- ests. The industrial forestry has, however, transformed vast areas of boreal forests by replacing the natural disturbance regimes with clear-cutting, planting of seedlings, and thin- Received 10 November 2006. Accepted 5 March 2007. Published on the NRC Research Press Web site at cjfr.nrc.ca on 18 October 2007. Y. Gauslaa, 1 K.A. Solhaug, L. Nybakken, and M. Ohlson. Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432 A ˚ s, Norway. K. Palmqvist. Department of Ecology and Environmental Science, Umea ˚ University, SE-90187 Umea ˚, Sweden. H. Holien and L.C. Myhre. Faculty of Social Sciences and Natural Resources, Nord-Trøndelag University College, P.O. Box 2501, NO-7729 Steinkjer, Norway. O. Hilmo. Department of Biology, Faculty of Natural Science and Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. 1 Corresponding author (e-mail: yngvar.gauslaa@umb.no). 1832 Can. J. For. Res. 37: 1832–1845 (2007) doi:10.1139/X07-048 # 2007 NRC Canada