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 better 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 lichens under an industrial forestry regime may not necessarily be determined by low dispersal efficiency only.

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This study aims to investigate patterns of species richness and abundance in relation to stand age in 71 Picea abies plantations, aged between 9 and 85 years, situated in the oceanic region of Central Norway. The study has shown that plantations within the oceanic spruce forests can support a relatively high number of epiphytic lichen species. Some of the oldest plantations hosted several old-forest associated species, e.g. Hypogymnia vittata, Lobaria pulmonaria, Pseudocyphellaria crocata and Ramalina thrausta. The number of species was influenced significantly by stand age and increased rapidly in stands <20 years old. Stands >30 years old showed no clear increase in species number, except for a high number of species in the two oldest stands. The colonization pattern could be characterized as an additional entrance of species, rather than by a replacement sequence. The probability of occurrence increased steeply at young stand ages (<20 years) for Bryoria spp., Cavernularia hultenii, Platismatia glauca, Parmelia sulcata and Usnea spp. A lower rate of colonization was characteristic for Alectoria sarmentosa, Parmelia saxatilis and Platismatia norvegica. The cover of foliose lichens on the branches showed an almost unimodal response to stand age. The cover of lichens was highest on branches in middle-aged plantations. The reason for the lower lichen cover in late successional stages, compared to middle-aged stands, could be due to reduced light in the lower canopy of mature plantations. Increased rotation cycle, creation of gaps and short distance to sources of propagules are factors suggested to promote species richness and abundance in forest plantations.

The distribution of epiphytic lichens on branches of a stand of Picea abies has been mapped, and patterns of succession and community structure are described. Many crustose species are of particular interest since little is known about their ecological requirements (e.g. Fuscidea pusilla Tønsb., Japewia subaurifera Muhr & Tønsb. and Gyalideopsis alnicola Noble & Vězda). Numerical treatment (correspondence analysis and canonical correspondence analysis) of the species data was used to study the lichen distribution in relation to measured environmental variables. Tree age, tree height and branch height above ground are shown to be the most important variables to explain the species distribution. No significant relationship was found between branch compass point and the distribution of lichens. The species composition and cover changed from young to old trees. The largest variation in the lichen vegetation was found on the branches of young trees. A more homogeneous and stable lichen community appears on branches of mature trees. A clear zonation of the epiphytic vegetation develops as the branches grow, resulting in the occurrence of typical ‘branch-tip’ and ‘branch-base’ species. The highest number of species was recorded on the outermost part of branches before lichen cover reached the maximum. Hyperepiphytic thalli were mainly located in specific areas with high lichen cover on the branches. Lichens with reduced vitality were most common at the base of branches.

Summary• Relationships between thallus size and growth variables were analysed for the foliose Lobaria pulmonaria and the pendulous Usnea longissima with the aim of elucidating their morphogenesis and the factors determining thallus area (A) versus biomass (dry weight (DW) gain.• Size and growth data originated from a factorial transplantation experiment that included three boreal climate zones (Atlantic, suboceanic and continental), each with three successional forest stands (clear-cut, young and old).• When A was replaced by the estimated photobiont layer area in an area-DW scatterplot including all thalli (n = 1080), the two separate species clusters merged into one, suggesting similar allocation patterns between photobionts and mycobionts across growth forms. During transplantation, stand-specific water availability boosted area gain in foliose transplants, consistent with a positive role of water in fungal expansion. In pendulous lichens, A gain greatly exceeded DW gain, particularly in small transplants. The A gain in U. longissima increased with increasing DW:A ratio, consistent with a reallocation of carbon, presumably mobilized from the dense central chord.• Pendulous lichens with cylindrical photobiont layers harvest light from all sides. Rapid and flexible three-dimensional A gain allows the colonization of spaces between canopy branches to utilize temporary windows of light in a growing canopy. Foliose lichens with a two-dimensional photobiont layer have more coupled A and DW gains.

Juvenile development from sown vegetative diaspores of the cyanobacterial lichen Lobaria scrobiculata and the green algal lichens Platismatia glauca and P. norvegica was studied through a four‐year field experiment in a boreal spruce forest. All three species developed juvenile thalli within the period of observation. The largest lobules of L. scrobiculata and Platismatia were 0.4 mm and 1.3 mm, respectively, four years after sowing the diaspores. The first very tiny lobule (0.1 mm) of L. scrobiculata was observed 29 months after sowing the soredia. This period of juvenile development is the longest period so far reported for lichens. A long phase of dormancy, after the diaspores were attached to the substrate, was characteristic for the investigated species. Apical growth of the isidia of Platismatia glauca and P. norvegica was not observed until 21 months after sowing. Low light levels, low temperatures and snow covering the diaspores during winter, causes a short growth season in boreal spruce forests. These factors might explain the long period for juvenile development. The results indicate a slow rate of colonization of a new habitat for the investigated species since they need several years to grow to visible thalli.

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