Suo - Mires and peat vol. 26 no. 2 | 1975

In the study of the effectiveness of forest drainage the conclusion has usually been made that trees grow better, the more effective the drainage. Thereby it has usually been understood that drainage should be effective all the year round. However, it is not self-evident that maximum effectiveness of drainage throughout the year is the best situation with regard to tree growth. Particularly the rapid discharge of water from melted snow leads often to overdrainage in the early summer in the case of effectively drained peatlands, and this may be unfavorable with regard to tree physiology and to the nutrient mobilization processes taking place in the soil. Likewise, the rapid discharge of melting water from drainage areas is an unfavorable phenomenon with regard to the water situation in watercourses. The aim of the present study was to assess the possible influence on the growth of Scots pine of a regulation of the runoff. In the expe-riments carried out the ground water table was artificially adjusted to a level very near the ground surface for periods of varying length in several subsequent years. Circumference growth of the trees included in the study was determined using the banding method. Fluctuations in the depth of the ground water table were also measured. According to the results so far obtained from these experiments, which are still very young, the requirements of trees on drainage are diffe-rent during different seasons of the year. A high ground water table, and on the other hand, effective drainage, influence tree growth in different ways during different seasons. In accordance with the results obtained so far effective drainage should be maintained at least during the late summer and fall. Keeping the ground water very near the ground surface for a period of some length in July-August affects the growth of pine to a considerable extent. A slight check of growth was observed even in those cases where the ground water table had been raised for a long time in September, October and November. On the other hand, at least up to the present time the experiments did not reveal any negative effect from ground water that had been kept at the ground surface in winter and spring. As a matter of fact, there are signs pointing toward the possibility that saving water in the site by damming during this season might even increase the growth of pine. On the basis of the results obtained from the present study the exact time when the water from melted snow should be released can not be told. It seems, however, that this moment is in the middle of June on the average. If the ground water table is kept at the ground surface from the spring until the month of July, a negative growth effect is recorded. The means required for the regulation of the runoff can be obtained at a relatively small additional cost. The possibility of regulating water discharging from forest drainage areas may be beneficial also in connection with reforestation of the drainage area, and with heavy fertilization, in fire control and for the promotion of berry production. Possible regulation of the runoff from drainage areas of course affects also the water situation in watercourses. This effect depends on the proportional share in the total area and the location of the dammed peatlands in the watershed area, and also, on the effectiveness of the measure.

• Pelkonen, Sähköposti: ei.tietoa@nn.oo

The article presents results of tensiometer measurements in peat soils. Measurements have been taken both in the field and in the greenhouse. The plots in the field experiment represented different stages of drainage in ordinary sedge bog. Plot 5 was in a virgin state, plot 6 had been drained in 1967, and plot 3, in the 1930's. Each plot had one tensiometer at each of the three measuring depths. (5, 20 and 40 cm). In the field the depth of the ground water table was also measured using self-registering gauges. In the greenhouse experiment the water table in the soil samples was artifically regulated by raising it straight from the deepest level to the topmost level, and lowering it after every 2-3 days (see Fig. 3). The results are presented in figures 1-4. The soil water tension and hydrostatic pressure values obtained with tensiometers show a close correlation with those concerning the ground water table in case when the measuring unit has been near the water table or below it. Evapo-transpiration and rainfall caused a variation to the relationship between the water table and soil water tension at the depth of 5 cm, particularly when the water table was deep (30-40 cm) (Fig. 2). The effect of evapotranspiration can be seen also in the results of the greenhouse experiment (Fig. 4). In this experiment the effect of the rise of the water table was not as great as expected. The reason for this may be too short a time for levelling (2 days). The results show that tensiometers can be used also for measurements on the depth of the ground water table if it has a sufficiently assu-rate manometer and if the measuring unit is kept near or below the ground water table.

• Laine, Sähköposti: ei.tietoa@nn.oo
• Mannerkoski, Sähköposti: ei.tietoa@nn.oo