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.

Multilevel logistic regression models were constructed to predict the 5-year mortality of Scots pine (Pinus sylvestris L.) and pubescent birch (Betula pubescens Ehrh.) growing in drained peatland stands in northern and central Finland. Data concerning tree mortality were obtained from two successive measurements of the National Forest Inventory-based permanent sample plot data base covering pure and mixed stands of Scots pine and pubescent birch. In the modeling data, Scots pine showed an average observed mortality of 2.73% compared to 2.98% for pubescent birch. In the model construction, stepwise logistic regression and multilevel models methods were applied, the latter making it possible to address the hierarchical data, thus obtaining unbiased estimates for model parameters. For both species, mortality was explained by tree size, competitive position, stand density, species admixture, and site quality. The expected need for ditch network maintenance or re-paludifi cation did not infl uence mortality. The multilevel models showed the lowest bias in the modeling data. The models were further validated against independent test data and by embedding them in a stand simulator. In 100-year simulations with different initial stand conditions, the models resulted in a 72% and 66% higher total mortality rate for the stem numbers of pine and birch, respectively, compared to previously used mortality models. The developed models are expected to improve the accuracy of stand forecasts in drained peatland sites.

Interactions between soil water conditions and forest stands in boreal forests with implications for ditch network maintenanceSikström U., Hökkä H. (2016). Interactions between soil water conditions and forest stands in boreal forests with implications for ditch network maintenance. Silva Fennica vol. 50 no. 1 article id 1416. 29 p. Highlights• Ditch network maintenance (DNM) may influence soil water conditions less than initial ditching due to reduced hydraulic conductivity of the peat. • Stand stocking and management substantially influence soil-water conditions. • DNM can lower the GWL and increase tree growth.• DNM growth responses of 0.5-1.8 m 3 ha -1 yr -1 during 15-20-years in Scots pine peatland stands reported. • Greatest need for DNM in the early phase of a stand rotation.• Need for better understanding of the link between soil water and tree growth. AbstractAt sites with either peat or mineral soils in large areas of boreal forests, high soil-water contents hamper tree growth and drainage can significantly increase growth. Hence, areas covering about 15 × 10 6 ha of northern peatlands and wet mineral soils have been drained for forestry purposes. Usually ditches gradually deteriorate, thus reducing their functionality as drains, and ditch-network maintenance (DNM) might be needed to maintain stand growth rates enabled by the original ditching. This article reviews current knowledge on establishing the need for DNM in boreal forest stands, subsequent growth responses, and the financial outcome of the activity. The issues covered in the review are: (i) ditching, changes in ditches over time and the need for DNM; (ii) interactions between soil water and both stand properties and stand management; (iii) ground-water level (GWL) and tree growth responses to DNM; and (iv) financial viability of DNM. Conclusions about the current understanding of issues related to DNM are drawn and implications for DNM in practice are summarized. Finally, gaps in knowledge are identified and research needs are suggested.

Drained peatland forests form an important timber resource in Finland. They also form a sink for atmospheric carbon (C) because of the increased growth and C sequestration rates following drainage. These rates have, however, been poorly quantifi ed. We simulated the tree stand dynamics for drained peatland stands with and without cuttings over two stand rotations. Simulations were done on four peatland site types and two regions in Finland with different climatic conditions, using recently published peatland tree growth models applied in a stand simulator. We then calculated the amount of C stored in the stands on the basis of previously published tree-level biomass and C content models. Finally, we developed regression models to estimate C stores in the tree stands using stand stem volume as the predictor variable. In the managed stands, the mean growth (annual volume increment) ranged from 2 to 9 m 3 ha -1 a -1 , depending on the rotation (fi rst/second), site type and region. Total yield during one rotation varied from 250 to 920 m 3 ha -1 . The maximum stand volumes varied from 220 to 520 m 3 ha -1 in the managed stands and from 360 to 770 m 3 ha -1 in the unmanaged. By the end of the fi rst post-drainage rotation the total C store in the managed stands had increased by 6-12 kg C m -2 (i.e. 45-140 g C m -2 a -1 ) compared to that in the undrained situation. Averaged over two rotations, the increase in the total C store was 3-6 kg C m -2 . In the corresponding unmanaged stands the C stores increased by 8-15 kg m -2 over the same periods. At stand level, the C stores were almost linearly related to the stem volume and the developed regression equations could explain the variation in the simulated C stores almost entirely.

No abstract

No abstract

The availability of phosphorus (P) and potassium (K) commonly limits tree growth on drained peatlands. The nutritional status and volume growth of Scots pine (Pinus sylvestris L.) after varying doses of wood ash fertilization were investigated in seven field experiments located on deep-peated sites in Finland between latitudes 63°N and 67°N. Needle samples were taken 14–20 years after fertilization and trees measured 15 years after fertilization. Annual growth rates and differences in foliar nutrients were analyzed with a two-way ANOVA model. A regression model was used in the analysis of periodic relative volume growth. High doses of wood ash (100–265 kg P·ha–1, 225–450 kg K·ha–1) resulted in greater changes in foliar nutrient concentrations and in a stronger growth response than low doses (5–50 kg P·ha–1, 14–210 kg K·ha–1). Significant increases in needle mass and concentrations of P, K, and B were observed in the ash-fertilized stands, with a good status for these elements 20 years after fertilization. Depending on the ash dose, the mean annual volume growth was 0.5–1.4 m3·ha–1 higher than in the control plots during the 15 year postfertilization period.

Using the Finnish MELA model, a set of scenarios were produced and used to map the possibilities and risks surrounding the utilisation of peatlands in wood production in Finland. One of the scenarios was an estimate of allowable-cut calculated by maximising the net present value of the future revenues using a four per cent interest rate subject to non-decreasing flow of wood, saw logs and net income over a 50-year period, and net present value after the 50 year period greater or equal than in the beginning. The estimate for maximum regionally sustained removal in 1996-2005 was 68 million m 3 per year -approaching 74 million m 3 during the next decades. In this scenario, 14 per cent of all cuttings during the period 1996-2005 would be made on peatlands, which comprise ca. 31 per cent of the total area of forestry land. By the year 2025, the proportion of peatland cuttings would increase to over 20 per cent. The increase in future cutting possibilities on peatlands compensated for a temporary decrease in cuttings and growing stock on mineral soils. The allowable-cut effect was especially pronounced in northern Finland, where peatlands play an important role in wood production. In addition, the sensitivity of cutting possibilities for assumptions related to growth and price were analysed. The estimate of maximum sustainable yield as defined here seems to be fairly robust on the whole, except in northern Finland where the cutting scenarios were sensitive to the changes in the price of birch pulpwood. The proportion of peatland stands that are profitable for timber production depends on the interest rate: the higher the rate of interest the less peatland stands are thinned. The effect of cutting profile on future logging conditions and resulting costs were analysed in two forestry centres. If clear cuttings on mineral soils are to be cut first, an increase in future logging costs is inevitable.