Effects of planted grasses and a pioneer tree on succession on a heavy metal contaminated mountainside
Restoring polluted landscapes for productive use is one of today’s major ecological challenges. While much effort has gone into revegetating disturbed sites, comparatively little is known about how succession proceeds after revegetation despite its importance for long-term management. The Palmerton Zinc Superfund Site is a >2000 acre mountainside devegetated and contaminated with zinc, lead, and cadmium from 80 years of zinc smelting emissions. Restoration efforts since 2003 have focused on planting C4 prairie grasses with low metal uptake rates in the hopes of eventually sequestering the metals under healthy surface soil. However, the metal hyperaccumulating tree Betula populifolia (gray birch) is rapidly colonizing the site. This species threatens to pump contaminants back aboveground, counteracting the restoration strategy and presenting management and public health concerns.
Here we examine how gray birch and the planted grasses may differentially affect long-term succession in this site. We planted Quercus velutina (black oak) and Acer saccharum (sugar maple) seeds and seedlings in field plots conditioned by gray birch or C4 grasses. We controlled for differences in aboveground competition by clearing aboveground vegetation from half of the plots and leaving the rest intact. We recorded germination after nine weeks and assessed seedling growth after 20 weeks.
For both target tree species, germination rates were <10% but seedling survival was >90%. In birch-conditioned soils, oak germination was greater with intact vegetation, but in grass-conditioned soils, oak germination was greater in cleared plots (generalized linear model, P<0.05). Oak seedling axial branch growth behaved similarly (ANOVA, P<0.05). However, oak seedling apical branch growth responded only to aboveground competition, growing higher with intact vegetation (ANOVA, P<0.05). Sugar maple germination and seedling growth did not respond significantly to either experimental treatment. We also found organic soil horizons to be 70% thicker under grasses compared to birch.
Taken together, these data suggest that soil may be more hospitable to black oak under grasses than under birch, but aboveground competition may be stronger under grasses. Implications for succession and management decisions depend on how much gray birch and the grasses are responsible for the difference in soil thickness or whether it predates their arrival, which we are currently testing. If gray birch causes its organic horizons to be thinner, land managers may want to remove it, but if it colonizes microhabitats with soils too thin to support the grasses, our results suggest it may benefit restoration by facilitating plant growth in these soils.