EFFECTS OF CONTROLLED BURNS ON MERCURY SEQUESTRATION AND VOLATILIZATION IN THE VEGETATION OF THE OSSIPEE PINE BARRENS PRESERVE, NEW HAMPSHIRE, USA

Over the past 165 years the global mercury pool has undergone a five-fold increase, and the vast majority of the global mercury budget originated anthropogenically. Much of this Hg accumulates in forest soils far from industrial sources. The forest canopy plays a critical role in capturing and cycling Hg into soils via throughfall, dry deposition and litterfall. The Ossipee Pine Barrens Preserve (OPBP) is a 2,800 acre, fire-dependent, pitch pine - scrub oak woodland where natural wildfires used to occur every 50-100 years. In 2007, after decades of fire suppression the Nature Conservancy began conducting prescribed burns to restore the pine barren ecosystem. An emergent issue with regard to this forest management strategy involves what happens to Hg in soil and vegetation during a prescribed burn. This research focuses on the role of vegetation in capturing and cycling Hg during these events.

Vegetation and soil samples were collected in October of 2014 from ten OPBP sites, spanning a range of burn histories. Vegetation samples included: bark, cones and needles of Pinus rigida (pitch pine); Vaccinium angustifolium (lowbush blueberry) leaves; Quercus ilicifolia (scrub oak) leaves; Cladonia (reindeer lichen), Usnea (old man’s beard), and Flavoparmelia (greenshield). All samples were air dried at room temperature and analyzed for total mercury content (THg) by Thermal Decomposition/Cold Vapor Atomic Adsorption, using a Teledyne Leeman Labs Hydra IIC Mercury Analyzer.

Concentrations of THg in leaves and pitch pine from unburned sites were two to three times higher than those from a site burned in 2010. This anomaly could be due to vegetative uptake of Hg volatized from the soil during the September 2014 burn, when a persistent temperature inversion spread smoke over much of the Pine Barrens. Total mercury concentrations in lichens were appreciably higher than all other vegetation types—even exceeding concentrations in the organic horizons of the soil in some areas. These results correlate to the fact that lichens receive nutrients directly from the atmosphere and have life spans that can average one hundred years. This makes them very efficient bioaccumulators and demonstrates the importance of understory organisms in cycling Hg in Pine Barren ecosystems.