Sakari Sarkkola

Trends in hydrometeorological conditions and stream water organic carbon in boreal forested catchments

Temporal trends in stream water total organic carbon (TOC) concentration and export were studied in 8 forested headwater catchments situated in eastern Finland. The Seasonal Kendall test was conducted to identify the trends and a mixed model regression analysis was used to describe how catchment characteristics and hydrometeorological variables (e.g. precipitation, air and stream water temperatures, and atmospheric deposition) related to the variation in the concentration and export of stream water TOC. The 8 catchments varied in size from 29 to 494 ha and in the proportion of peatland they contained, from 8 to 70%. Runoff and TOC concentration were monitored for 15-29 years (1979-2006). Trends and variation in TOC levels were analysed from annual and seasonal time series. Mean annual TOC concentration increased significantly in seven of the eight catchments. The trends were the strongest in spring and most apparent during the last decade of the study period. The slopes of the trends were generally smaller than the variation in TOC concentration between years and seasons and between catchments. The annual TOC export showed no clear trends and values were largely determined by the temporal variability in runoff. Annual runoff showed a decreasing trend in two of the eight catchments. Mean annual air and stream water temperatures showed increasing trends, most clearly seen in the summer and autumn series. According to our modeling results, stream water temperature, precipitation and peatland percentage were the most important variables explaining annual and most seasonal TOC concentrations. The atmospheric deposition of SO4, NH4, and NO3 decreased significantly over the study period, but no significant link with TOC concentration was found. Precipitation was the main hydrometeorological driver of the TOC export. We concluded that stream water TOC concentrations and exports are mainly driven by catchment characteristics and hydrometeorological factors rather than trends in atmospheric acid deposition.

Iron concentrations are increasing in surface waters from forested headwater catchments in Eastern Finland

Observations of increased water colour have been made in lakes and rivers all across the northern mid-latitudes of Europe and North America, particularly during the last 10-20 yr. This water browning or brownification has been attributed to the increased organic carbon concentrations due to climate change and decreased acid atmospheric deposition. Given that iron (Fe) may also increase water colour, the contribution of Fe to water brownification has received small attention. Our aim was to study the temporal trends of Fe in forested headwater catchments in eastern Finland, where an increasing air temperature and total organic carbon (TOC) trend had been observed in an earlier study. We found a statistically significant increasing trend also in stream water Fe concentrations and a strong correlation between the trends of TOC and Fe. The average increase in TOC and Fe concentrations between 1995 and 2006 was 0.5 mg l(-1) yr(-1) (2.5%), and 34.6 μ gl(-1) yr(-1) (3.5%), respectively. These results indicate that the increased water colour or brownification in Northern Europe may not only be due to increased concentrations of organic matter but also increased concentrations of Fe. The change in precipitation and temperature conditions, particularly during late autumn and early winter periods, appeared to be the main environmental factor behind increasing Fe trends. The strong correlation between the trends of Fe and TOC indicated that the increased Fe-organic matter complexation is the mechanism behind increasing Fe trends, but further research is needed to assess the chemical forms of increased Fe that coupled with increased TOC concentrations would enhance water brownification.

Changes in structural inequality in Norway spruce stands on peatland sites after water-level drawdown

Size structural dynamics of naturally established Norway spruce (Picea abies (L.) Karst.) stands growing on peatlands drained for forestry were investigated. The study was based on modelling of diameter at breast height (DBH) distributions of repeatedly measured stands in southern Finland. The Weibull function was used to parameterize the DBH distributions and mixed linear models were constructed to characterize the impacts of different ecological factors on stand dynamics. Initially, the positive skewness of the DBH distributions increased after drainage as a result of in- creases in stem numbers and a reduction in mean diameters. Simultaneously, the size inequality among trees increased. These changes were due to regeneration and (or) ingrowth and indicated only little competition from the larger trees. Subsequently, the DBH distributions changed from positively skewed to normal and finally to negatively skewed result- ing from tree growth and a reduction in the number of small DBH trees. This indicated increased asymmetric intertree competition. Size inequality did not change during this later stage in stand development, suggesting a concurrent com- ponent of symmetric competition. Thinnings had little impact on DBH distribution trends. The observed stand dynam- ics allow the allocation of growth resources to the desired crop component by appropriate silvicultural treatments.

Depth of water table prior to ditch network maintenance is a key factor for tree growth response

Abstract In boreal-drained peatland forests, tree growth is retarded by the gradual deterioration of drainage ditch networks. In order to avoid the development of suboptimal growth conditions, ditch network maintenance (DNM) operations (ditch cleaning and/or complementary ditching) are annually conducted on an area of about 70,000 ha in Finland. The previous studies indicate that the depth of the water table prior to DNM may influence the magnitude of the growth response to DNM. Tree growth does not necessarily increase after DNM at sites with large stand volume and subsequent low water levels due to tree stand evapotranspiration. We investigated how the pre-treatment water table depth (pre-WTD) in late summer relates to the growth of Scots pine (Pinus sylvestris L.) stands after the DNM operation in 12 field experiments. The increase in mean annual volume growth caused by DNM was negatively related to the pre-WTD, with the highest growth response in stands where the pre-WTD was less than 25–30 cm. DNM did not clearly increase stand growth in sites where the pre-WTD was more than 35–40 cm below the soil surface. There was a high variation in growth response to DNM between the water levels from 20 to 35 cm below the soil surface, indicating that factors other than water table depth are also needed as decision criteria for assessing the appropriate timing of ditch network maintenance.

Restoration of nutrient-rich forestry-drained peatlands poses a risk for high exports of dissolved organic carbon, nitrogen, and phosphorus

Restoration impact of forestry-drained peatlands on runoff water quality and dissolved organic carbon (DOC) and nutrient export was studied. Eight catchments were included: three mesotrophic (one undrained control, two treatments), two ombrotrophic (one drained control, one treatment) and three oligotrophic catchments (one undrained control, two treatments). Three calibration years and four post-restoration years were included in the data from seven catchments, for which runoff was recorded. For one mesotrophic treatment catchment only one year of pre-restoration and two years of post-restoration water quality data is reported. Restoration was done by filling in and damming the ditches. Water samples were collected monthly-biweekly during the snow-free period; runoff was recorded continuously during the same period. Water quality was estimated for winter using ratios derived from external data. Runoff for non-recorded periods were estimated using the FEMMA model. A high impact on DOC, nitrogen (N) and phosphorus (P) was observed in the mesotrophic catchments, and mostly no significant impact in the nutrient-poor catchments. The DOC load from one catchment exceeded 1000kg (restored-ha)-1 in the first year; increase of DOC concentration from 50 to 250mgl-1 was observed in the other mesotrophic treatment catchment. Impact on total nitrogen export of over 30kg (restored-ha)-1 was observed in one fertile catchment during the first year. An impact of over 5kg (restored-ha)-1 on ammonium export was observed in one year in the mesotrophic catchment. Impact on P export from the mesotrophic catchment was nearly 5kg P (restored-ha)-1 in the first year. The results imply that restoration of nutrient-rich forestry-drained peatlands poses significant risk for at least short term elevated loads degrading the water quality in receiving water bodies. Restoration of nutrient-poor peatlands poses a minor risk in comparison. Research is needed regarding the factors behind these risks and how to mitigate them.

Nitrogen and phosphorus concentrations in discharge from drained peatland forests are increasing

The current understanding, based on previous studies, is that increased discharge nutrient concentrations from boreal peatlands drained for forestry return to similar levels as those of pristine peatlands within about 20years after their drainage. As an implicit consequence of this finding, it has been assumed that there are no long-term increasing trends in nutrient exports from these peatlands after the establishment of forestry. We analysed discharge total nitrogen (TN) and phosphorus (TP) concentration data from 54 catchments with undrained pristine peatlands and 34 catchments with drained peatlands using data with considerably longer drainage history than in previous studies. Our results agree with previous studies in that discharge TN and TP concentrations in areas drained 20-30years ago did not differ much from those in pristine sites. However, we also observed that the TN and TP concentrations were increasing with years since drainage of these catchments. Discharge TN and TP concentrations were over two times higher in areas drained 60years ago when compared with more recently drained areas. Our results challenge the current perceptions by showing that forestry-drained peatlands may contribute to water eutrophication considerably more than previously estimated.

Impact of Re-wetting of Forestry-Drained Peatlands on Water Quality—a Laboratory Approach Assessing the Release of P, N, Fe, and Dissolved Organic Carbon

A laboratory column study with peats from four sites from south-central Finland and two sites from blanket peats in the west of Ireland was established to assess the factors contributing to P, N, Fe, and dissolved organic carbon (DOC) transfer to receiving water courses from restored forestry-drained peatlands. The study indicated that the DOC and Fe release from re-wetted peats are likely governed by the amount of Fe in peat and the degree of Fe reduction upon re-wetting. In contrast to our other hypothesis concerning DOC, high degradability of organic matter was not related to high DOC release. Nitrate release was found to largely cease along with oxygen depletion, but ammonium release was considerable from a site with high nitrification potential before wetting. The release of P from anoxic peat was complicated in the sense that it appeared to be controlled by many factors simultaneously. In the nutrient-poor sites, the P release increased following re-wetting, probably because of their high easily soluble peat P content and low Al and Fe content, resulting in high anoxic P mobilization, but limited re-sorption of the mobilized P. Among the three nutrient-rich sites, there was either no P release upon re-wetting or higher P release than from the nutrient-poor sites. Low risk for P release following re-wetting in nutrient-rich sites was associated with low content in peat of easily soluble P and a high molar Fe/P ratio.

A synthesis of the impacts of ditch network maintenance on the quantity and quality of runoff from drained boreal peatland forests

Drained peatlands are an important source of forest biomass in boreal regions and ditch network maintenance (DNM) operations may be needed to restore the drainage functions of ditches. By reviewing the available literature, as well as utilizing an existing hydrological model and analyzing the characteristics of eroded sediments, we assessed the impacts of DNM on runoff and exports of suspended solids (SS), dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P). In general, DNM had minor impact on runoff and dissolved N and P, and it decreased rather than increased DOC exports. To increase the understanding of the hydrochemical impacts of DNM, future research should focus on the characteristics of SS and particulate nutrient exports. A major gap in knowledge is also the very limited regional representativeness of the available studies. High erosion risk in the ditches reaching the mineral soil below peat should be acknowledged when planning mitigation measures.

Whole-tree, stem-only, and stump harvesting impacts on site nutrient capital of a Norway spruce-dominated peatland forest

About 15 million hectares of peat soils have been drained for forestry in temperate and boreal zones. Increasing interest in wood biomass as a source of bioenergy has led to more intensive harvests also in peatland forests. These harvestings remove branches, needles, and stump/root systems that would earlier have remained on-site. However, in drained peatland forests, some key growth-limiting nutrients, such as potassium (K) and boron (B), are largely stored in the living tree biomass, while the stores in peat in tree rooting layer are small. A concern has thus been raised that bioenergy harvesting may induce nutrient insufficiency and decrease second rotation forest productivity. We studied the effect of whole-tree and stump harvesting on site nitrogen (N), phosphorus (P), K, and B capitals in final harvesting of a well-productive, Norway spruce-dominated peatland forest. Harvesting of stumps and above-ground harvest residues resulted in minor depletion of site N and P stores, but significant depletion of K and B. Maximizing harvest residue biomass recovery, but minimizing nutrient depletion, our results indicated that stump harvesting is a more feasible option than harvesting of above-ground residues. If above-ground harvest residues are to be harvested for bioenergy, K and B depletion may be decreased by letting the branches dry out somewhat and the needles fall down before branch harvesting. Combining our results with data on waterborne K losses suggested that K depletion may become a serious problem in second rotation forests on drained peatlands, even if harvest residues and stumps were left completely non-harvested.

Ditch network maintenance in peat-dominated boreal forests: Review and analysis of water quality management options

The objective of this study was to evaluate the potential of different water management options to mitigate sediment and nutrient exports from ditch network maintenance (DNM) areas in boreal peatland forests. Available literature was reviewed, past data reanalyzed, effects of drainage intensity modeled, and major research gaps identified. The results indicate that excess downstream loads may be difficult to prevent. Water protection structures constructed to capture eroded matter are either inefficient (sedimentation ponds) or difficult to apply (wetland buffers). It may be more efficient to decrease erosion, either by limiting peak water velocity (dam structures) or by adjusting ditch depth and spacing to enable satisfactory drainage without exposing the mineral soil below peat. Future research should be directed towards the effects of ditch breaks and adjusted ditch depth and spacing in managing water quality in DNM areas.