Can Relationships Between Ground-Layer Plant Cover and Biomass be Used to Follow Succession in Boreal Riparian Forests?

The Forest Watershed and Riparian Disturbance (FORWARD) project examines the movement of water and nutrients from Canadian boreal forests before and 5 years after harvest. Plant biomass is the ideal metric for abundance, as it approximates productivity and is the basis to which other resources (e.g., nutrients) are related. However, these data are difficult and destructive to collect and therefore are not suitable for investigations of vegetation change over time. Plant cover data are easier and non-destructive to collect, but may not be proportional to the resources used by individual plants. The objectives of this study were to (1) develop allometric equations using vegetation cover for rapid and non-destructive estimates of biomass and (2) use these equations to approximate ground-layer biomass change over time, following harvest. We collected cover data from ground-layer riparian plant communities in permanent plots at buffered, cut-to-shore and control sites before and five years after harvest treatment. In addition, similar plots were established for destructive sampling of the ground-layer vegetation so that estimates of cover preceding the harvest of aboveground plant parts could be modelled according to dry weight of functional groups (i.e. dwarf shrubs, bryophytes, graminoids, ferns and forbs). Linear relationships were identified (P \u3c0.001), with slope factors depending on functional group and consequently applied to pre- and post-harvest vegetation cover data. Relative to the pre-harvest condition, no differences were detected in the control and buffer treatment in all growth forms five years after harvest. However, on average, graminoids increased by 27 g/m2 and bryophytes decreased by 73 g/m2 in the cut-to-shore treatment. Results suggest that estimating biomass, rapidly and non-destructively, from allometric equations allows for an important characteristic in boreal riparian vegetation community structure to be followed during succession

Metal–Polycyclic Aromatic Hydrocarbon Mixture Toxicity in <i>Hyalella azteca</i>. 2. Metal Accumulation and Oxidative Stress as Interactive Co-toxic Mechanisms

Mixtures of metals and polycyclic aromatic hydrocarbons (PAHs) are commonly found in aquatic environments. Emerging reports have identified that more-than-additive mortality is common in metal–PAH mixtures. Individual aspects of PAH toxicity suggest they may alter the accumulation of metals and enhance metal-derived reactive oxygen species (ROS). Redox-active metals (e.g., Cu and Ni) are also capable of enhancing the redox cycling of PAHs. Accordingly, we explored the mutual effects redox-active metals and PAHs have on oxidative stress, and the potential for PAHs to alter the accumulation and/or homeostasis of metals in juvenile <i>Hyalella azteca</i>. Amphipods were exposed to binary mixtures of Cu, Cd, Ni, or V, with either phen­anthrene (PHE) or phen­anthrene­quinone (PHQ). Mixture of Cu with either PAH produced striking more-than-additive mortality, whereas all other mixtures amounted to strictly additive mortality following 18-h exposures. We found no evidence to suggest that interactive effects on ROS production were involved in the more-than-additive mortality of Cu-PHE and Cu-PHQ mixtures. However, PHQ increased the tissue concentration of Cu in juvenile <i>H. azteca</i>, providing a potential mechanism for the observed more-than-additive mortality

Metal–Polycyclic Aromatic Hydrocarbon Mixture Toxicity in <i>Hyalella azteca</i>. 1. Response Surfaces and Isoboles To Measure Non-additive Mixture Toxicity and Ecological Risk

Mixtures of metals and polycyclic aromatic hydrocarbons (PAHs) occur ubiquitously in aquatic environments, yet relatively little is known regarding their potential to produce non-additive toxicity (i.e., antagonism or potentiation). A review of the lethality of metal–PAH mixtures in aquatic biota revealed that more-than-additive lethality is as common as strictly additive effects. Approaches to ecological risk assessment do not consider non-additive toxicity of metal–PAH mixtures. Forty-eight-hour water-only binary mixture toxicity experiments were conducted to determine the additive toxic nature of mixtures of Cu, Cd, V, or Ni with phen­anthrene (PHE) or phen­anthrene­quinone (PHQ) using the aquatic amphipod <i>Hyalella azteca</i>. In cases where more-than-additive toxicity was observed, we calculated the possible mortality rates at Canada's environmental water quality guideline concentrations. We used a three-dimensional response surface isobole model-based approach to compare the observed co-toxicity in juvenile amphipods to predicted outcomes based on concentration addition or effects addition mixtures models. More-than-additive lethality was observed for all Cu-PHE, Cu-PHQ, and several Cd-PHE, Cd-PHQ, and Ni-PHE mixtures. Our analysis predicts Cu-PHE, Cu-PHQ, Cd-PHE, and Cd-PHQ mixtures at the Canadian Water Quality Guideline concentrations would produce 7.5%, 3.7%, 4.4% and 1.4% mortality, respectively