Understory plant communities vary with tree productivity in two reclaimed boreal upland forest types in Canada

•Understory plant communities were studied on sites with different tree productivity.•Medium productivity pine sites had higher understory cover but lower diversity.•Total understory cover did not vary with productivity class on spruce sites.•Understory cover increased with cover soil thickness and soil DOC on pine sites.•Understory cover also increased with inorganic N availability on pine sites.•Species evenness was positively correlated with tree growth parameters on spruce sites.