Influence of tree plantations on the phylogenetic structure of understorey plant communities

The effect of non-native and native tree species plantations on plant understorey diversity and functioning has gained major interest among ecologist and conservationists. The majority of studies, however, has been focused on changes in species richness and composition in plantations in comparison with natural forests. Recently, the phylogenetic community structure (PCS), i.e. the degree of phylogenetic relatedness within a community, came into the focus of interest. As species richness and PCS are not equivalent, the question of how plantations change PCS and thus the evolutionary history conserved in the associated understorey community is important to fully assess the biodiversity of tree species introductions. In this study we used phytosociological data from the Cedynia Landscape Park in western Poland containing anthropogenic mono-cultures of Scots pine and introduced trees (black locust and Douglas fir) as well as natural sessile oak stands to study the influence of plantations on species richness, composition, and phylogenetic diversity of understorey vegetation. We also added relevés from mixed forests naturally occurring on wetter and richer soil to obtain the full regional species pool. Our results revealed that plantations do not consistently change PCS; they either increased (Douglas fir) or decreased (black locust) the phylogenetic relatedness of understory plant communities. Moreover, our data suggest that environmental conditions are more important drivers of understorey community composition than dominant tree identity. In this respect we identified light regime, pH, and nitrogen availability as major drivers of phylogenetic diversity.