e. old forest, since we assumed that species composition on the clear-cut reflects the species composition

in the old forest) relating to the positive link between environmental heterogeneity and species richness (Ellis, 2012). The stem of a retained tree is exposed to large microclimatic changes and becomes a more diverse habitat than before logging; the south side is exposed to sunlight, while the north side is shadier. The environment on a clearcut is overall windier, drier and the temperature variation is higher than that inside a forest (Chen et al., 1999). Stem shape may also be altered due to increased wind and destabilization of the root system at clear-cutting and scarification, Selleck GSK1349572 and we observed that several trees in the young forest were leaning. Most probably

structural heterogeneity is larger on the stem of a leaning tree, and the variation in bark pH between upper and lower side is large due to differences in basic run-off water (i.e. a drip zone effect). Some lichens may establish on the more rain-exposed and therefore more acidic upper side; one example is Hypogymnia physodes which we found only on 28 aspens on clearcuts but on 105 aspens in the young forest, and there mainly on leaning trees (F. Jonsson, personal observation). It is known that epiphytes shift their vertical positions upwards with Non-specific serine/threonine protein kinase time (McCune, 1993 and Sillett and Neitlich, 1996) and consequently there might be a source PD-1/PD-L1 inhibitor 2 of propagules from light-demanding species that can shift their vertical position downwards after clear-cutting when light and moist conditions change. The result of increasing species richness with time is also in line with what can be expected from knowledge about natural disturbance dynamics; aspen is a pioneer species promoted by fire disturbance,

resembling conditions after logging. Thus, many aspen-associated species are evolutionary adapted to exposed trees. Aspen-dependent species, i.e. those that have aspen as their main substrate, increased with time since clear-cutting but also with increasing diameter of the host tree. Since there is a strong positive correlation between diameter and age of aspens (Hedenås and Ericson, 2000) this indicates that old aspens are important for many aspen-dependent lichens, probably because an older tree has had time to develop a suitable habitat and species have had a longer time to colonize. There is no shortage of available area on the stems (i.e. the habitat is unsaturated); therefore it is less likely to be an effect of competition for space. Another explanation could be development of smooth bark due to increased tree growth-rate of retained trees after logging, thus an effect of increased habitat heterogeneity.