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Does clearcut forestry influence aquatic greenhouse gas emissions?
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Forest clearcutting generally increase exports of carbon and nitrogen to downstream aquatic systems. Although these losses affect the greenhouse gas budget of managed forests, it is unknown if they modify greenhouse gas emissions of recipient inland waters. To assess this question, we quantified atmospheric fluxes of carbon dioxide (CO2) methane (CH4) and nitrous oxide (N2O) of humic lakes and their inlet streams in four boreal catchments of which two were treated with forest clearcuts (18% and 44% of the catchment area) using a Before-After/Control-Impact-experiment. We measured gas concentrations and hydrological and physicochemical water characteristics in hillslope groundwater, along stream transects and at multiple locations in lakes at 2-hourly to biweekly intervals throughout the snow-free season over a four year period. These measurements were combined with atmospheric gas transfer measurements and models to calculate aquatic greenhouse gas emissions. Forest clearcutting did not change greenhouse gas emissions from streams or lakes, despite significant increases of CO2 and CH4 concentrations in hillslope groundwater. Clearcut effects on groundwater were likely buffered in the riparian zone. Hence, the greenhouse gas budget of forests initially after clearcutting is unlikely to be confounded by aquatic greenhouse gas emissions. However, our findings should be extrapolated with caution to other environments. Here, site-specific conditions makes our study system representative for systems where clearcutting causes only a limited initial impact on catchment hydrology and biogeochemistry.

National Category
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-135003OAI: oai:DiVA.org:umu-135003DiVA: diva2:1095737
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-18
In thesis
1. Land use effects on greenhouse gas emissions from boreal inland waters
Open this publication in new window or tab >>Land use effects on greenhouse gas emissions from boreal inland waters
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Anthropogenic activities perturb the global carbon and nitrogen cycle with large implications for the earth’s climate. Land use activities deliver excess carbon and nitrogen to aquatic ecosystems. In the boreal biome, this is mainly due to forestry and atmospheric deposition. Yet, impacts of these anthropogenically mediated inputs of carbon and nitrogen on the processing and emissions of greenhouse gases from recipient streams and lakes are largely unknown. Understanding the ecosystem-scale response of aquatic greenhouse gas cycling to land use activities is critical to better predict anthropogenic effects on the global climate system and design more efficient climate change mitigation measures.

This thesis assesses the effects of forest clearcutting and nitrate enrichment on greenhouse gas emissions from boreal inland waters. It also advances methods to quantify sources and sinks of these emissions. Short-term clearcut and nitrate enrichment effects were assessed using two whole-ecosystem experiments, carried out over four years in nine headwater catchments in boreal Sweden. In these experiments, I measured or modeled air-water fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), combining concentration, ebullition and gas-transfer velocity measurements in groundwater, streams and lakes. By using Swedish national monitoring data, I also assessed broad-scale effects of forest clearcutting by relating CO2 concentrations in 439 forest lakes to the areal proportion of catchment forest clearcuts. To improve quantifications of CO2 sources and sinks in lakes, I analyzed time series of oxygen concentrations and water temperature in five lakes on conditions under which whole-lake metabolism estimates can be inferred from oxygen dynamics given the perturbing influence of atmospheric exchange, mixing and internal waves.

The experiments revealed that aquatic greenhouse gas emissions did not respond to nitrate addition or forest clearcutting. Importantly, riparian zones likely buffered clearcut-induced increases in groundwater CO2 and CH4 concentrations. Experimental results were confirmed by monitoring data showing no relationship between CO2 patterns across Swedish lakes and clearcut gradients. Yet, conclusions on internal vs. external CO2 controls largely depended on whether spatially or temporally resolved data was used. Partitioning CO2 sources and sinks in lakes using time series of oxygen was greatly challenged by physical transport and mixing processes.

Conclusively, ongoing land use activities in the boreal zone are unlikely to have major effect on headwater greenhouse gas emissions. Yet, system- and scale specific effects cannot be excluded. To reveal these effects, there is a large need of improved methods and design of monitoring programs that account for the large spatial and temporal variability in greenhouse gas dynamics and its controls by abiotic and biotic factors.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2017. 46 p.
Keyword
greenhouse gas, boreal forest, carbon cycling, whole-ecosystem experiment, limnology, metabolism, forest clearcutting, nitrogen enrichment
National Category
Physical Geography
Identifiers
urn:nbn:se:umu:diva-134767 (URN)978-91-7601-716-6 (ISBN)
Public defence
2017-06-09, Lilla Hörsalen, KBC huset, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2017-05-19 Created: 2017-05-11 Last updated: 2017-05-19Bibliographically approved

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