Projecting In-stream Dissolved Organic Carbon and Total Mercury Concentrations in Small Watersheds Following Forest Growth and Clearcutting
2016
Zhang, Chengfu | Jamieson, Rob C. | Meng, Fan-Rui | Gordon, Robert | Bourque, Charles P.-A.
Forest is an important vegetation type on the globe, and clearcutting is the main forest management method. This paper presents a process-based model developed to project the impact of forest growth and clearcutting on dissolved organic carbon (DOC) and total mercury (THg) export from forest-dominated watersheds over two forest-growing cycles. The modelling of THg is based on the observation that THg export from terrestrial to aquatic ecosystems occurs with the binding and subsequent in-stream transport of THg by DOC. From the results generated with the integrated model, DOC and THg export follows two main trends; (i) a multiple-year trend, associated with forest harvesting and re-growth patterns over the lifetime of the forest, and (ii) an annual trend, associated with the seasonal dynamics in forest litter production and decomposition. During a forest rotation, DOC and THg concentration decreases following clearcutting, reaches a minimum at about 15 years after forest regeneration and then gradually increases with forest ageing. Large debris pools left on site following clearcutting can provide a significant pulse in DOC production and within-watershed THg export during the first 2–3 years after harvest. In a single year, the integrated model predicts that DOC- and THg-concentration peaks after leaf fall in autumn, decreases to a minimum in April, increases to another maximum in June and finally decreases to a second minimum just before leaf fall. This seasonal cycle is repeated every year. Conifer species and wetland-dominated watersheds are anticipated to release a greater amount of DOC and THg to aquatic ecosystems than deciduous and dryland-dominated watersheds. The long-term and seasonal DOC production is consistent with field measurements.
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