Rasmus Sørensen

Short-term Effects of Clear-cutting on the Water Chemistry of Two Boreal Streams in Northern Sweden: A Paired Catchment Study

Abstract The effects of clear-cutting on stream-water chemistry in northern Sweden remain largely unexplored. Here we report data collected during a reference period and the first two years after logging in two typical partially harvested northern catchments; the objective was to compare water chemistry along the stream with and without a forest buffer. Two typical uncut reference catchments are included for comparison. Runoff was measured at the outlet of each catchment, and water samples were generally taken every second week and analyzed for 20 constituents. Logging resulted in increased runoff and increased concentrations of sodium, potassium, chloride, total nitrogen, total phosphorus, and suspended material from both catchments. Nitrate (NO3−) leaching increased only from the catchment without a forest buffer. It has not yet been possible to evaluate fully the effects of the forest buffer on the NO3− leaching because the uphill clear-cut area leached minimal amounts of NO3−.

The Effects of Forestry on Hg Bioaccumulation in Nemoral/Boreal Waters and Recommendations for Good Silvicultural Practice

Abstract Mercury (Hg) levels are alarmingly high in fish from lakes across Fennoscandia and northern North America. The few published studies on the ways in which silviculture practices influence this problem indicate that forest operations increase Hg in downstream aquatic ecosystems. From these studies, we estimate that between one-tenth and one-quarter of the Hg in the fish of high-latitude, managed forest landscapes can be attributed to harvesting. Forestry, however, did not create the elevated Hg levels in the soils, and waterborne Hg/MeHg concentrations downstream from harvested areas are similar to those from wetlands. Given the current understanding of the way in which silviculture impacts Hg cycling, most of the recommendations for good forest practice in Sweden appear to be appropriate for high-latitude regions, e.g., leaving riparian buffer zones, as well as reducing disturbance at stream crossings and in moist areas. The recommendation to restore wetlands and reduce drainage, however, will likely increase Hg/MeHg loadings to aquatic ecosystems.

Forest Harvest Increases Runoff Most during Low Flows in Two Boreal Streams

Abstract To understand how forest harvest influences the aquatic environment, it is essential to determine the changes in the flow regime. This paper presents changes in the hydrological regime during the first 2 y after harvest in two catchments of the Balsjö Catchment Study in Sweden. The changes were judged relative to a reference catchment, calibrated during an 18-mo pretreatment period starting in September 2004. From August 2006 through March 2008, there was an average of 35% more runoff from the harvested catchments relative to the reference. The flow increased most during the growing seasons and at base flows (<1 mm d−1; 58–99% increase), followed by dormant season and intermediate flows (30–43%). No significant changes were observed during the highest flows (over 5 mm d−1), except for the spring flood a few weeks after harvest, which was delayed and attenuated. Large relative changes in low flow may influence the ecosystem by altering the aquatic habitat.

Response of dissolved organic carbon following forest harvesting in a boreal forest.

Abstract To determine if forestry affects stream water dissolved organic carbon (DOC) concentrations, we conducted high frequency water sampling at a clear-cut catchment experiment in northern Sweden 1 year after harvesting. The overall finding was that harvesting significantly increased stream water DOC in these boreal forest catchments, at least during the growing season. The results indicate a DOC concentration increase of up to 50% during early summer on the two harvested catchments relative to the two control catchments. The analysis supports the hypothesis that a raised groundwater level following harvesting caused the increased DOC concentration during both hydrological episodes and low flow conditions. Harvesting resulted in a 70% increase in DOC export due to the combined effect of runoff and DOC concentration during the June–October study period. Given the extent of forestry activity in the boreal landscape, these results demonstrate that tree harvesting will affect the water quality of the region.

The Effects of Forest Harvest Operations on Mercury and Methylmercury in Two Boreal Streams: Relatively Small Changes in the First Two Years prior to Site Preparation

Abstract Forest harvest is hypothesized to increase the mercury (Hg) load in aquatic ecosystems. The Balsjö paired catchment study examined the outputs of methylmercury (MeHg) and total mercury (Hgtot) from two boreal catchments during the 2 y following forest harvest but prior to site preparation. This enabled us to separate the effect of the two operations that followed best management practices. Hgtot concentrations increased by approximately 15%, and fluxes by 20–30%. The MeHg concentrations and fluxes either declined or increased by up to 60%, depending on whether annual MeHg peaks during summer low flows were considered to have been influenced by forest harvest. The lack of a severalfold increase in Hg outputs after forest harvest, as reported from other sites, may be the result of minimal soil disturbance during the winter forest harvest operations. If so, there may be a greater Hg response after soil scarification to prepare for planting.

Impact of Forestry on Total and Methyl-Mercury in Surface Waters: Distinguishing Effects of Logging and Site Preparation

Forestry operations can increase the export of mercury (both total and methyl) to surface waters. However, little is known about the relative contribution of different forestry practices. We address this question using a paired-catchment study that distinguishes the effects of site preparation from the antecedent logging. Runoff water from three catchments, two harvested and one untreated control, was sampled biweekly during one year prior to logging, two years after logging, and three years after site preparation. The logging alone did not significantly increase the concentrations of either total or methyl-mercury in runoff, but export increased by 50-70% in one of the harvested catchments as a consequence of increased runoff volume. The combined effects of logging and site preparation increased total and methyl-mercury concentrations by 30-50% relative to preharvest conditions in both treated catchments. The more pronounced concentration effect after site preparation compared to logging could be related to site preparation being conducted during summer. This caused more soil disturbance than logging, which was done during winter with snow covering the ground. The results suggest that the cumulative impact of forest harvest on catchment mercury outputs depends on when and how forestry operations are implemented.

Using landscape characteristics to define an adjusted distance metric for improving kriging interpolations

Interpolation of point measurements using geostatistical techniques such as kriging can be used to estimate values at non-sampled locations in space. Traditional geostatistics are based on the spatial autocorrelation concept that nearby things are more related than distant things. In this study, additional information was used to modify the traditional Euclidean concept of distance into an adjusted distance metric that incorporates similarity in terms of quantifiable landscape characteristics such as topography or land use. This new approach was tested by interpolating soil moisture content, pH and carbon-to-nitrogen (C:N) ratio measured in both the mineral and the organic soil layers at a field site in central Sweden. Semivariograms were created using both the traditional distance metrics and the proposed adjusted distance metrics to carry out ordinary kriging (OK) interpolations between sampling points. In addition, kriging with external drift (KED) was used to interpolate soil properties to evaluate the ability of the adjusted distance metric to incorporate secondary data into interpolations. The new adjusted distance metric typically lowered the nugget associated with the semivariogram, thereby better representing small-scale variability in the measured data compared to semivariograms based on the traditional distance metric. The pattern of the resulting kriging interpolations using KED and OK based on the adjusted distance metric were similar because they represented secondary data and, thus, enhanced small-scale variability compared to traditional distance OK. This created interpolations that agreed better with what is expected for the real-world spatial variation of the measured properties. Based on cross-validation error, OK interpolations using the adjusted distance metric better fit observed data than either OK interpolations using traditional distance or KED.