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Report, 2012

Regional övervakning av avrinningen från brukad skogsmark i Västra Götalands, Hallands och Jönköpings län - Utvärdering av perioden 1996-2009 och förslag till framtida utformning

Löfgren, Stefan


In this report, the runoff chemistry in five forest streams draining managed forests has been evaluated. The monitoring has been funded by the county administrations in Västra Götaland (Fallabäcken, Sågebäcken and Kvarnebäcken), Halland (Näver- bäcken) and Jönköping (Ramsjöbäcken). The assessment includes data from sur- veys beginning in 1996 - 2000 and ending in 2009. The aim of this work is to document the water chemical variations in forest streams in southwestern Sweden and to analyze how various forestry measures, mainly clear-felling, influenced this variation. The commission also includes proposals on how to conduct runoff monitoring in managed forest in the future. The catchment areas range from 331-698 hectares and the shares of forest and peat lands vary in the ranges of 82-95% and 5-13%, respectively. All the catchments are thus typical forest areas including elements of peat lands and small ponds. During the period 2004/05 to 2009/10, 4-14% of the catchment areas were clear-felled partly due to damages caused by the storms Gudrun (2005) and Per (2007). Branches and tops (slash, GROT) were harvested on a large proportion of the harvested areas in three catchments (76-92%) and scarification was also common in three catchments (80-83%). The average annual discharge during the investigation periods varied significantly between areas (279-765 mm) and the inter-annual variability was large. Depending on catchment it differed 2-4 times between dry and wet years. The export of nitro- gen and phosphorus from the catchment areas broadly followed the same ranking as runoff. The measured average annual transports were mainly within the ranges (1.4 to 4.2 kg N/ha year and 0.01 to 0.18 kg P/ha, year, respectively) previously measured in managed and natural forests in Sweden and Finland. The proportion of inorganic nitrogen ranged from 17-39% and the majority consisted of nitrate (76-84%). The proportion of phosphate varied between 22-33%. The flow-weighted annual average concentrations for total nitrogen and nitrate were in the ranges 419-667 μg/l and 64-115 μg/l, respectively, and for total phosphorus and phosphate 6-14 μg/l and 2-5 μg/l, respectively. All catchments showed an annual average export of buffer capacity (ANC), i.e. it exported more base cations (calcium, magnesium, sodium and potassium) than anions associated with mineral acids (sulfuric, nitric and hydrochloric acid). The export of ANC was dominated by organic anions (humus) while bicarbonate (alka- linity) were either missing completely or only represented a very small fraction (≤ 13%). The buffering of weak organic acids led to low pH values (4.8 to 5.7) and the export of protons. The catchments vicinity to the sea was reflected in the trans- ports of sodium and chloride. The recovery from acidification linked to the reduced sulfur deposition has likely slowed down due to an increased sea salt deposition. The importance of weak organic acids (humus) for the ANC and pH has increased over time. The transport of total organic carbon varied significantly between catchments (factor 2-3) and the metals iron, manganese and aluminum roughly followed the same patterns. The leaching of these substances was at similar levels as in the non-managed forests of Lake Gårdsjön and Aneboda. In the three catchments where it was estimated, the transport of inorganic aluminum accounted for 12-37% of the aluminum exports. Inorganic aluminum is the most toxic form for fish and other organisms that breathe with gills The flow-weighted average concentrations of total organic carbon, iron and aluminum were generally slightly higher at the end of the time series, which coincides with the general trends of increasing humus levels in Swedish lakes and streams. Based on data from 1-5 years, the flow-weighted average concentrations of inorganic aluminum were over (maximum level ca 100 μg/l) or within the range of 30-50 μg/l, which are considered harmful to e.g. fish. The 5-year time series from Ramsjöbäcken indicates decreasing concentrations of inorganic aluminum, which is consistent with observations from several other forest streams in southern Sweden. Some 10 other metals were analyzed in three of the areas. The transports and concentrations were at about the same levels as in the non-managed forest streams of Gårdsjön and Aneboda. The concentrations were generally much lower than the lowest levels where adverse biological effects can be expected. Forestry measures affected too small proportions of the catchment areas to safely distinguish their effects from the natural stream water chemistry variation. These results are consistent with previous studies, which compared data from streams in managed and non-managed forests in southern Sweden. The results are gratifying in that, on a spatial scale of a few hundred hectares, there were no indications on that forestry measures significantly increased the leaching of nutrients or otherwise affected the water-chemical state such as acidity. Forestry primarily seems to cause very local effects on the aquatic environment. The regional monitoring of the runoff chemistry from managed forests in the coun- ties of Västra Götaland, Halland and Jönköping have since start generated a large amount of valuable data. The results will be even more valuable as the time series are extended. The large spatial distribution of forests and peat lands in southwest- ern Sweden motivate why monitoring of the water quality in such areas should have high priority in order to study trends and provide a basis for various models. The monitoring of Fallabäcken, Kvarnebäcken, Sågebäcken, Näverbäcken and Ramsjöbäcken is therefore recommended to continue. The monitoring programs can to some extent be improved through the introduction of uniform methodology for analyzing inorganic aluminum and by supplementing the measurements with sampling in direct connection to silvicultural measures primarily harvesting

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Publisher: Länsstyrelsen Västra Götaland

    SLU Authors

    Associated SLU-program

    Lakes and watercourses
    Non-toxic environment

    UKÄ Subject classification

    Environmental Sciences related to Agriculture and Land-use
    Fish and Aquacultural Science
    Forest Science

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