Microhabitat determines how grazing affects bryophytes in wood-pastures

Abstract

Wood-pastures are biodiverse biotopes where livestock graze among trees. The diversity of several species groups is related to the structural diversity created by herbivores and by trees, decaying wood and rocks. One of the greatest threats to wood-pastures is abandonment that results in decreased heterogeneity of soil microhabitats as well as encroachment by young trees. We studied the diversity of bryophytes on different microhabitat types (soil, rocks, trees and decaying wood) in grazed and abandoned boreal wood-pastures (48 sites). Most species grew on soil, rocks or both. For both soil and rocks currently grazed sites had higher bryophyte species richness and several individual species also preferred grazed sites over abandoned sites. The grazers increase biodiversity by creating patches of bare soil and exposed rock surfaces. Currently grazed and abandoned sites did not differ significantly in bryophyte species richness on decaying wood or trees, and the overall bryophyte richness on these substrates was quite low due to the scarcity of dead wood and large living deciduous trees with suitable bark conditions. Rare species were observed on all microhabitats, most abundantly on soil and decaying wood. Especially our result on higher richness on rock surfaces shows the complexity of grazing effects on biodiversity. Grazers may affect the biodiversity of a species group that is not consumed by them, on a surface which is not attractive to them. These unobvious interactions should be recalled when planning wood-pasture management and conservation.