Conservation of epiphytic lichens: significance of remnant aspen (Populus tremula) trees in clear-cuts

  • Published source details Hedenås H. & Hedström P. (2007) Conservation of epiphytic lichens: significance of remnant aspen (Populus tremula) trees in clear-cuts. Biological Conservation, 135, 388-395.


In many Swedish boreal forests where timber is extracted, aspen Populus tremula has been selected against in favour of conifers. The decrease in aspen is of concern as it has more host-specific species associated with it than other boreal tree species. Recently forest management systems have adopted 'green-tree retention' in order to maintain structural diversity. This present study was undertaken to ascertain if retained aspen trees provide habitats for epiphytic lichens and, if so, whether this varies according to lichen species characteristics. The abundance and vitality of five species, three foliose cyanobacterial lichens (Collema curtisporum, C.furfuraceum and Leptogium saturninum) and two crustose green-algal lichens (Biatora epixanthoides and (Mycobilimbia carneoalbida) were investigated. These lichens were examined on aspen within forest stands and on aspens left in clear-cuts at eight localities in northern Sweden.

Study areas: Two regions where the study species were relatively common were chosen: one coastal about 40 km west of Luleå and one inland about 10 km southeast of Vuollerim, in Norrbotten, northern Sweden. Four conifer-dominated forest localities were selected in each region, mostly consisting of one forest stand with a bordering clear-cut of similar age, ideally with more than 40 aspen trees with a diameter at breast-height (DBH) 20 cm in each stand type. Time since clear-felling ranged from 9 to 24 years.

Lichen monitoring: In August 2003 and September 2004, the occurrence of the five lichen species on aspen trees left in clear-cuts and that on aspen trees in forest were compared. At each site 40 randomly chosen aspen trees DBH 20 cm+ (the study lichen species seldom occur on smaller, younger trees) were examined in the forest stands and 36-40 in the clear-cuts. DBH of each study tree was recorded.

Lichen cover of each species on the bottom 2 m of each trunk, on the north and south sides was measured. Vitality of the thalli was classified as: 1 = healthy (no signs of damage, thalli of normal colour); 2 = suffering (thalli showing changes in colour with a little damage to the upper cortex due to desiccation and solar radiation); and 3 = dead.

In each stand at five different points, estimates of percentage canopy cover were made at eye height and basal area of each tree species present and total basal area were determined. The averages of the five estimates were used in the analyses.

Overall there was significantly higher canopy cover, total basal area, and basal area of Norway spruce P.abies, Scots pine P.sylvestris and birch Betula spp. in the forest compared to the clear-cuts. Total canopy cover was lower in the clear-cuts (16%) than in the forest stands (54%), this pattern was similar in all localities. The basal area of aspen did not generally differ between forest and clear-cuts. Average DBH of the sampled aspens was higher in the clear-cuts (41.4 cm) than the forests (27.7 cm)

Species responses: The two crustose lichens, B.epixanthoides and M.carneoalbida, were less abundant and less healthy on aspen in clear-cuts than on aspen in the forest; total cover and vitality were significantly lower on the south-side of aspen in the clear-cut compared to the north side, and any of the orientations on aspen trees in forest.

In contrast, highest cover and cover of healthy thalli of the three cyanolichens (C.curtisporum, C.furfuraceum and L.saturninum) were found on the northern sides of aspens in the clear-cuts. Of these three species, C.curtisporum occurred at relatively low abundances at all localities, C.furfuraceum was more common and L.saturninum, was the commonest species but for all three, total cover and cover of healthy thalli were higher on the clear-cuts than in the forests.

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