Dendroecological analysis reveals long-term, positive effects of an introduced understory plant on canopy tree growth

Although the introduction of non-native species can have severe ecological and economic consequences, there is increasing awareness that introduced plants can also provide valuable ecosystem services. Here, we use tree-ring data from 30 sites in the southeastern US to demonstrate that non-native Lespedeza bicolor has long-term, positive effects on the growth of mature Pinus palustris trees. Trees growing near cultivated plots planted with nitrogen-fixing L. bicolor have experienced on average 56 % greater annual growth than trees growing near plots planted with non-nitrogen-fixing Secale cereale. Moreover, this fertilization effect has persisted for 25 years, since the local introduction of L. bicolor. This positive effect of L. bicolor provides evidence that non-native species may alter the growth of overstory trees that play important ecological and economic roles in southeastern forests, and suggests that subsidies from L. bicolor may influence carbon sequestration and change the nature of community dynamics at habitat edges. More generally, our results illustrate that non-native nitrogen-fixing understory plants may have strong, unappreciated effects on canopy tree growth, that the effects of non-native plants may have a distinct spatial extent, and that the net effect of non-native plants depends upon the interplay of ecological costs and benefits that may not be intuitive.