Ditch networks in drained peatland forests are maintained regularly to prevent water table rise and subsequent decrease in tree growth. The growing tree stand itself affects the level of water table through evapotranspiration, the magnitude of which is closely related to the living stand volume. In this study, regression analysis was applied to quantify the relationship between the late summer water table depth (DWT) and tree stand volume, mean monthly summertime precipitation (Ps), drainage network condition, and latitude. The analysis was based on several large data sets from southern to northern Finland, including concurrent measurements of stand volume and summer water table depth. The identified model demonstrated a nonlinear effect of stand volume on DWT, a linear effect of Ps on DWT, and an interactive effect of both stand volume and Ps. Latitude and ditch depth showed only marginal influence on DWT. A separate analysis indicated that an increase of 10 m3·ha–1 in stand volume corresponded with a drop of 1 cm in water table level during the growing season. In a subsample of the data, high bulk density peat showed deeper DWT than peat with low bulk density at the same stand volume.
In Finland nearly 6 million hectares of peatlands are drained for forestry purposes. Ditch network maintenance in the drained peatlands, i.e. cleaning old ditches or digging complementary ditches, deteriorates surface water quality by increasing the export of dissolved elements and suspended solids (SS). Effect of ditch network maintenance on the export of SS, dissolved organic carbon (DOC), and dissolved nitrogen (N), phosphorous (P), iron (Fe), aluminum (Al) and manganese (Mn) was studied in nine pairs of treated and control (no maintenance) catchments located in southern and central Finland. In this study we extended the paired catchment approach by combining data from several catchments and identifying the treatment effect on SS and element loads from the entire dataset. Following the method of Laurén et al. (2009) we identified how uncertainty in correlation between treatment and control catchments during pre-treatment period is reflected in the estimated treatment effect on SS and element loads. In the experiment, the export of SS increased significantly for the four year study period following the ditch network maintenance and Al export increased for one year. The export of N, P and Fe was not significantly changed and DOC and Mn export decreased after the ditch maintenance operation.
Abstract. The most common forest management method in Fennoscandia is rotation
forestry, including clear-cutting and forest regeneration. In clear-cutting,
stem wood is removed and the logging residues are either removed or left on
site. Clear-cutting changes the microclimate and vegetation structure at the
site, both of which affect the site's carbon balance. Peat soils with poor
aeration and high carbon densities are especially prone to such changes, and
significant changes in greenhouse gas exchange can be expected. We measured
carbon dioxide (CO2) and energy fluxes with the eddy covariance method
for 2 years (April 2016–March 2018) after clear-cutting a drained
peatland forest. We observed a significant rise (23 cm) in the water table
level and a large CO2 source (first year: 3086±148 g CO2 m−2 yr−1; second year: 2072±124 g CO2 m−2 yr−1). These large CO2 emissions resulted from the very low gross
primary production (GPP) following the removal of photosynthesizing trees
and the decline of ground vegetation, unable to compensate for the
decomposition of logging residues and peat. During the second summer
(June–August) after the clear-cutting, GPP had already increased by 96 %
and total ecosystem respiration decreased by 14 % from the previous
summer. The mean daytime ratio of sensible to latent heat flux decreased
after harvesting from 2.6 in May 2016 to 1.0 in August 2016, and in 2017 it
varied mostly within 0.6–1.0. In April–September, the mean daytime
sensible heat flux was 33 % lower and latent heat flux 40 % higher in
2017, probably due to the recovery of ground vegetation that increased
evapotranspiration and albedo of the site. In addition to CO2 and
energy fluxes, we measured methane (CH4) and nitrous oxide (N2O)
fluxes with manual chambers. After the clear-cutting, the site turned from a
small CH4 sink into a small source and from N2O neutral to a
significant N2O source. Compared to the large CO2 emissions, the
100-year global warming potential (GWP100) of the CH4 emissions was
negligible. Also, the GWP100 due to increased N2O emissions was
less than 10 % of that of the CO2 emission change.
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