Trifolium repens and Lolium perenne were exposed as both monocultures and two-species mixtures to an episodic rural ozone regime in large, well-watered containers within solardomes for 12 weeks. There were reductions in biomass for T. repens, but not L. perenne, and the proportion of T. repens decreased in ozone-exposed mixtures compared to the control. In addition, leaf biomass of T. repens was maintained at the expense of biomass partitioning to the stolons. The decreased growth corresponded with decreased photosynthetic capacity for T. repens, however, by the end of the exposure there was also decreased photosynthetic capacity of L. perenne, a species previously considered insensitive to ozone. The observed decreases in photosynthetic efficiency and capacity in elevated ozone indicate that the ability of such ubiquitous vegetation to act as a sink for atmospheric carbon may be reduced in future climates. Ozone causes changes in biomass partitioning, and photosynthetic efficiency and capacity that could decrease the ability of plants to act as a carbon sink.

Atmospheric mercury deposition by wet and dry processes contributes mercury to terrestrial and aquatic systems. Factors influencing the amount of mercury deposited to boreal forests were identified in this study. Throughfall and open canopy precipitation samples were collected in 2005 and 2006 using passive precipitation collectors from pristine sites located across the Superior National Forest in northern Minnesota, USA. Samples were collected approximately every 2 weeks and analyzed for total (THg) and methyl mercury (MeHg). Forest canopy type and density were the primary influences on THg and MeHg deposition. Highest THg and MeHg concentrations were measured beneath conifer canopies (THg mean = 19.02 ng L⁻¹; MeHg mean = 0.28 ng L⁻¹) followed by deciduous throughfall (THg mean = 12.53 ng L⁻¹; MeHg mean = 0.19 ng L⁻¹) then open precipitation (THg mean = 8.19 ng L⁻¹; MeHg mean = 0.12 ng L⁻¹). The greater efficiency of conifers at scavenging THg and MeHg from the atmosphere may increase the risk of mercury related water quality issues in conifer-dominated systems.