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

The dynamics of understory plant communities in forest ecosystems with different tree productivities that are reclaimed after surface mining is poorly studied. In the Athabasca oil sands region of Canada, the cover, composition, diversity and foliar nutrient concentrations of understory vascular plant communities were examined on reclaimed sites (at least 15 years old since site reconstruction) with low, medium and high productivity that were planted to lodgepole pine (Pinus contorta) and white spruce (Picea glauca). Understory plant communities showed different responses to tree productivity classes in both pine and spruce stands. On pine sites, higher shrub and grass covers contributed to higher total cover on medium than on low and high productivity sites; however, low and high productivity sites had higher species richness and Shannon-Wiener index than medium productivity sites. In addition, shrub, grass and total covers were all positively correlated with cover soil thickness, soil dissolved organic carbon and inorganic nitrogen concentrations, but there was no relationship between any understory plant community variable and tree growth parameters. On spruce sites, lower shrub and forb covers and higher grass cover on high than on low and medium productivity sites resulted in no difference in total cover among productivity classes; however, species richness and Shannon-Wiener index were higher on medium than on low and high productivity sites. Additionally, forb cover, species richness and Shannon-Wiener index were all negatively correlated with leaf area index, and grass cover and species evenness were positively correlated with tree growth parameters such as height, diameter at breast height and aboveground biomass increments. We conclude that the relationships between tree productivity and understory plant communities in reclaimed forest ecosystems were site specific. Reclamation strategies such as adopting a proper planted tree spacing or fertilization should be used on sites with different tree productivities to balance overstory tree growth with understory plant community development in reclaiming surface-disturbed mining areas.