Isolated trees may significantly enhance biodiversity at the landscape level. However, our understanding of their impacts is still poor, particularly in environments with high soil moisture where research on this topic has been comparatively limited. We examined understorey vegetation and soil oribatid mite assemblages under live and dead Scots pine trees and in open treeless areas, all within the same Scottish upland wet heath system, to determine whether isolated live trees affected the understorey and mite components of the ecosystem, and whether these effects occurred in parallel. We also explored whether these responses might result from tree-driven reductions in soil moisture content. Live trees reduced soil moisture (relative to wet heath and beneath dead trees) and appeared to change vegetation from wet heath to dry heath type communities. These effects were strongly related to tree trunk diameter (tree size). No major effects of dead trees on understorey vegetation or soil moisture were apparent. Higher mite species abundance and richness were found under live trees than in treeless open heath. Although mite abundances were lower under dead trees than live trees, richness remained similar, thus different factors seem to be regulating mite abundance and community composition. These findings indicate that landscape-level biodiversity responses to environmental change such as habitat fragmentation cannot be predicted from vegetation patterns alone, and that even in heavily fragmented landscapes comparatively small patches such as isolated individual trees can enhance biodiversity
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