Tree species identity and composition shape the epiphytic lichen community of structurally simple boreal forests over vast areas

Abstract

Greatly simplified ecosystems are often neglected for biodiversity studies. However, these simplified systems dominate in many regions of the world, and a lack of understanding of what shapes species occurrence in these systems can have consequences for biodiversity and ecosystem services at a massive scale. In Fennoscandia, ~90% of the boreal forest (~21Mha) is structurally simplified with little knowledge of how forest structural elements shape the occurrence and diversity of for example epiphytic lichens in these managed forests. One form of structural simplification is the reduction of the number and frequency of different tree species. As many lichen species have host tree preferences, it is particularly likely that this simplification has a huge effect on the lichen community in managed forests. In a 40–70 years old boreal forest in Sweden, we therefore related the occurrence and richness of all observed epiphytic lichens to the host tree species and beta and gamma lichen diversity at the forest stand level to the stand’s tree species composition and stem diameter. Picea abies hosted the highest lichen richness followed by Pinus sylvestris, Quercus robur, Alnus glutinosa, Betula spp., and Populus tremula. However, P. tremula hosted twice as many uncommon species as any of the other tree species. Stand level beta and gamma diversity was twice as high on stands with four compared to one tree species, and was highest when either coniferous or deciduous trees made up 40–50% of the trees. The stem diameter was positively related to lichen richness at the tree and stand level, but negatively to beta diversity. For biodiversity, these findings imply that leaving a few trees of a different species during forest thinning is unlikely as effective as combining life-boat trees for endangered species with an even tree species mixture.

The Role of Retention Trees in Providing a Habitat for Bryophytes and Lichens in Young Forest Stands: A Mid-Term Perspective

“…Larger trees hosted more diverse and abundant communities of epiphytes, due to a larger available surface for colonisation and higher diversity of available microhabitats [41]. Larger tree diameter is usually related to greater age of tree, thus providing a longer time for colonization [25]. In contrast, the richness and abundance of lichens was independent of DBH (Figure 2), which could be explained by higher environmental tolerance of this taxonomic group, particularly regarding tree size [25,44].…”

“…Larger tree diameter is usually related to greater age of tree, thus providing a longer time for colonization [25]. In contrast, the richness and abundance of lichens was independent of DBH (Figure 2), which could be explained by higher environmental tolerance of this taxonomic group, particularly regarding tree size [25,44]. Furthermore, the fact that the cover of bryophytes was negatively correlated with lichen species richness and cover, but positively with tree size (Figure 3) indicates that in the case of competition lichens could be sensitive to tree DBH.…”

“…Furthermore, many species are limited by regional distribution, therefore it is important to conduct studies in specific ecological conditions on different spatial scales. Due to differences in species' distribution each local case study contributes to better understanding of the efficiency of retention forestry [25].…”

Conifer tree species and age as drivers of epiphytic lichen communities in northern European production forests

“…However, there are a variety of options for landowners and authorities aiming to increase the biodiversity values of these stands. In addition to increasing the number of tree species (Klein et al 2021), promoting a variety of different aged trees, conserving dead wood and establishing set-asides; choosing Scots pine over Norway spruce and prolonging the rotation period could readily enhance the epiphyte diversity of these production forests. Due to the limited and fragmented occurrence of old and natural conifer forests in this region (Angelstam & Andersson 2001; Naturvårdsverket 2021), some older production stands may even provide an important source pool for set-aside establishment in the future.…”