Artikkelit kirjoittajalta Eero Pelkonen

Since 1969 the effect, as well as the need and practical possibilities, of runoff regulation in peatland areas drained for forestry have been studied experimentally by the author (Pelkonen 1975, 1976, 1979). The drainage requirements of tree stands growing on peatlands have been shown to vary considerably during the growing season. When the water table was kept close to the soil surface during spring and early summer by damming (Fig. 1), no harmful effects were noticed. Instead, quite opposite tendencies were detected during dry and warm summers. As the project is now being expanded, some further aspects of damming are dealt with. Water consumption by trees in June usually exceeds the water input as rainfall. Hence the water table should be high. Owing to the greater oxygen requirements of the roots as well as the higher probability of high-rainfall periods in late summer and autumn, the water table should be deeper than in spring and early summer (Fig. 2). In 1976, a 15-ha peatland area was drained using 110-cm deep open ditches with a spacing of 20 m between the ditches. This is a rather effective combination compared with Finnish drainage norms. Peat dams fitted with plastic discharge tubes were constructed in part of the ditches (Fig. 3). The water level in the ditch was raised by plugging the inlet end of the plastic tubes in late autumn. By keeping the inlets closed untill the end of June, the water table was maintained at depth 15—20 cm above that in the undammed strips during early summer (Fig. 4). In spite of practically constant rain fall during July and August, damming did not prevent the watertable from dropping sufficiently during late summer. In order to obtain the maximum effect from damming, the ditch network described above was planned so as to have as small a slope as possible. During damming, the ditches were almost completely filled with water. As a consequence, no vegetation appeared in the ditches during the first four years (Fig. 7 and 8). In the undammed ditches the development of vegetation was fast (Fig. 5 and 6). Owing to the slight slope of the ditch network, no direct conclusions regarding the positive effect of damming on the deterioration of open ditches can be drawn. It is well known, however, that the problem of ditch deterioration is most pronounced in areas characterized by insufficient slope. As the construction of regulated ditch networks is easily carried out in such areas, the positive effect on ditch condition might prove one of the most important arguments when considering their practical use.

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

The root systems of rooted Scots pine and Norway spruce seedlings were kept in slow-flowing or stagnant ditch water for periods of varying length during different seasons. In the snowless period, the "flood" lasted from one week to three months. The seedlings were planted immediately after they hade been removed from water. If the treatment was started early in the spring before the beginning of root activity, both pine and spruce seedlings survived even if the treatment lasted until beginning of August. However, slight negative effects in growth and general development were observed. During late summer and autumn, pine and spruce seedlings appeared to suffer much more of temporary "flooding"" treatments than in spring and early summer. A treatment period of 2—3 weeks often was detrimental to a seedling. Norway spruce seedlings were less resistent in this respect than Scots pine seedlings. In November when soil temperature is approaching 0°C in Southern Finland, artificial flooding still caused damage to spruce seedlings. Part of the seedlings included in the experiments were kept in water throughout the winter. The effect of the treatment on the seedlings depended on the degree of ice formation in the water reservoir. If only a thin ice cover was formed (slow-flowing water), leaving major part of the root system in liquid water, both pine and spruce seedlings survived quite well; only a slight decrease in growth was observed during the following growth period. Instead, if the whole root system was surrounded by solid ice (stagnant water), more negative influences were observed, especially in the case of Scots pine. Also in the case of Norway spruce, a considerable decrease in growth during the next growing season was detected. Analogous experi-mets with larger individuals of the same two species (Pelkonen 1975, 1976 have shown corresponding results. "

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

According to results presented previously by the author (Pelkonen 1975), a high water table during the late summer and fall has a harmful effect on tree growth on peatlands drained for forestry purposes. No harmful effect seemed to be involved, when the water table was maintained close to the soil surface in spring and early summer. In this paper, more data is presented on the influence on tree growth of a high water table in spring and early summer. In addition, some points concerning practical runoff regulation are discussed. In two stands the water table was artificially maintained close to the soil surface for varying periods in the course of four summers (1972— 1975). In the pine stand, an increase in circumference growth was recorded irrespective of treatment duration (Fig. 4). However, the increase was the greater, the longer the treatment duration. In 1975, the difference between treatments of different duration became apparent during a prolonged dry period (Fig. 6). In the spruce stand, a significant decrease in growth was recorded in 1973 (Fig. 5). This was obviously due to abundant cone production in the stand during that particular year (c.f. Simpanen 1972). In the case of Norway spruce, circumference growth was not as clearly influenced by treatment duration as was Scots pine. However, even in this case the increase in circumference growth appears to be greatest on the plot with the longest treatment duration. Precipitation, depth of the water table, and cumulative circumference growth in 1975 are presented in Figure 7. It appears that no clear differences have developed between treatments of different duration in the course of the dry spell in July—August. The different response of Scots pine and Norway spruce might be due to site differences. On the basis of this experimental data, it seems probable that tree growth could be stimulated by maintaining the water table close to the soil surface in spring and early summer. In practice, the regulation of runoff from drained areas can easily be carried out by constructing peat dams in the ditches with plastic discharge tubes.

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

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