Effects of habitat conditions and disturbance on lichen diversity

This thesis includes five papers from four studies on lichen diversity in its broad sense. The overall objectives were to examine species richness, composition, distribution, and abundance of lichens at tree and stand-level, after forest fire, and after prescribed burning. Based on these studies I develop two themes in the thesis: 1) The epiphytic lichen metacommunity – how tree-level lichen diversity depends on local and regional processes, whose importance may vary over time. 2) How disturbance induced habitat loss and mortality may affect lichen diversity and the extinction risk of rare species. Nemoral, epiphytic lichen communities were examined in Uppland, Sweden. Species distribution, richness, and composition were related to factors at both tree and stand-level, e.g. tree age, stand size (number of trees), surrounding landscape, and isolation. Species abundance was mainly related to tree size and stand size (number of trees). For Pleurosticta acetabulum the results indicated an effect of connectivity on stand-level abundance, i.e. a ‘mass-effect’. For species richness, the relationship with tree age was asymptotic and levelled off at ca. 65 years of age. Boreal, epiphytic lichen communities were examined eight years after a forest fire in Minnesota, USA. Lichen abundance was proportionally lower than species richness at burned sites compared with unburned sites. Post-fire colonization was related to species post-fire abundance, and was higher for common species and dead wood preferring species. Crustose, shade preferring, and rare lichens seemed especially vulnerable to the fire. Grassland lichens were surveyed before and after prescribed burning of two old, abandoned fields in central Minnesota. After a low intensity fire, cover of Cladonia spp. was still relatively high and positively related to pre-fire cover. After a high intensity fire no such relationship was seen, and all lichens had experienced high mortality rates. The studies demonstrate that lichen community structure depends on a multitude of local and regional factors, whose importance may vary between diversity measures. They also demonstrate that lichen community response to disturbance depends on disturbance intensity and may vary between diversity measures. Post-disturbance colonization rates are higher for species with high post-disturbance population size, which in turn, may be a function of pre-disturbance population size. However, if disturbance intensity exceeds critical thresholds for species mortality, population size does not matter – all individuals are killed.