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.
Highlights
Biodiversity studies are often performed in natural or near-natural ecosystems [1] with the aim to understand more about threatened species or ecosystems
The host tree diameter at breast height (DBH) had a positive effect on the occurrence of an average lichen species and thereby on species richness (Median = 0.08, 95% CI = 0.03– 0.14)
We know that having more tree species in managed boreal forests increases carbon storage, wood biomass and human food production [8, 52], as well as the diversity of birds and ectomycorrhizal fungi [53]
Summary
Biodiversity studies are often performed in natural or near-natural ecosystems [1] with the aim to understand more about threatened species or ecosystems. There is evidence that epiphytic lichen diversity in boreal forests is related to the tree species composition (e.g. number of tree species at the stand level [25] and density of hardwoods [26]), these studies either focus on natural forest systems or managed forests that include several forestry cycle stages in the study area. Complete tree level species inventories of epiphytic lichens in simplified systems such as managed boreal forests are extremely rare, and have until now not been performed in young managed forest stands (which in Fennoscandia constitute 60% of the managed forests and ~55% of the total forest area [27]). Tree species composition shapes lichen community in structurally simple boreal forest implications for biodiversity enhancing forestry methods such as life-boat tree retention [33] or mixed-wood forestry [34]?
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