Woody Plant Communities after 40 Years of Pollution Control and Restoration in Smelter-Denuded Landscapes

Abstract

Restoration of degraded ecosystems requires assessing the status of the recovery and identifying key factors limiting the recovery. Nearly 4 decades of the reduction in smelter emissions, soil amelioration, and planting of ∼10 million trees and shrubs have led to extensive revegetation of emission-denuded landscapes in the Sudbury area, ON, Canada. To evaluate the state of recovery and identify factors affecting it, we surveyed woody plants and environmental correlates (soil pH, soil nutrient content, and soil metal content) along gradients of historical smelting impacts. Species richness declined from reference sites to high-impact sites, with impact sites having 23–33% fewer species compared to reference sites. Species composition varied along gradients of historical smelting impacts, with the difference attributable to low abundances or absences of one third of the 43 species found in the reference sites. Species richness and composition were associated with soil toxicity (as measured by metal content and pH) but not soil nutrients. However, the negative relationships between soil toxicity and plant community were attributable to the mediation effect of canopy cover, rather than the direct effect of soil toxicity. Sixty three percent of species that were uncommon or absent in impact sites (Abies balsamea [Balsam Fir], Acer pensylvanicum [Striped Maple], Acer spicatum [Mountain Maple], Betula alleghaniensis [Swamp Birch], Fraxinus nigra [Black Ash], Gaultheria procumbens [American Wintergreen], Linnaea borealis [Twinflower], Lonicera canadensis [Fly Honeysuckle], Lonicera hirsute [Hairy Honeysuckle], and Ostrya virginiana [American Hophornbeam]) were shade-tolerant and declined with declining canopy cover. These species were reported to be absent or uncommon in impact sites nearly 4 decades ago. Together, these results suggest that woody plant community has undergone limited recovery and that the recovery is hampered by poorly developed canopy cover rather than direct soil toxicity or nutrient deficiency. Accordingly, strategic interventions of canopy cover-mediated processes (e.g., shading and soil moisture) is needed to facilitate the recovery of the woody plant community in the smelter-denuded landscapes.