Substratum diversity explains landscape-scale co-variation in the species-richness of bryophytes and lichens

Though the habitats and substrata of bryophytes and lichens overlap, their diversity has not been comparatively explored at large scales. We studied lichens and bryophytes on 30 random 2-km transects in a forest landscape in Estonia. For each vegetation type on each transect, large broad-leaved trees, decorticate snags, logs and windthrows were randomly sampled and their species composition recorded. A total of 108 bryophyte and 127 lichen species were found in 781 sample plots. At the transect scale, larger numbers of lichen species were accompanied with more bryophyte species (r = 0.49; P = 0.006). Substratum diversity explained 38% of the variation in species-richness for lichens and 44% for bryophytes; when this effect was taken into account, the transect-scale co-variance of the species-richness of the two taxa disappeared. Indeed, other diversity-promoting factors differed for these species groups: the residual species-richness of lichens depended strongly on the relative area of mature forest on transects, while the residual species-richness of bryophytes was higher in commercial than protected forests. The latter was due to the lack of bryophyte-rich substrata in the reserve, since the small-scale richness of the same substrata there was similar to that in commercial forests. For cryptogam conservation, direct consideration of substratum diversity and co-occurrence is extremely important. If the substrata have scattered distributions or the substrata of different taxa are aggregated to different sites, conservation measures should not be based on one surrogate species group only.