The effect of planting, seeding and soil preparation on the regeneration success of Scots pine (Pinus sylvestris L.) on drained peatlands – 10-year results

Most of the drainage of peatlands for forestry, totaling 4.7 million ha in Finland (Hökkä et al., 2002; Päivänen & Hånell, 2012), was done in the 1960s and 1970s. Now the area of peatland forests that have reached maturity is almost 400,000 ha and it is expected to reach 700,000 ha within ten years (Saarinen, 2013).

Scots pine (Pinus sylvestris L.) is the most dominant tree species in poor and medium-productivity peatlands. In recently drained peatlands the natural regeneration of Scots pine stands was earlier recommended, and studies showed that it was generally successful (Lukkala, 1938; Heikurainen, 1954; Seppälä, 1966; Moilanen & Issakainen, 1984). However, the water level drawdown due to drainage gradually changes the ground vegetation, which increases the coverage of forest moss species. These species are considered more difficult for seedling germination than Sphagnum mosses (Kaunisto, 1984; Kaunisto & Päivänen, 1985; Saarinen, 2013). Sphagnum mosses prevent the formation of the litter layer on peatlands and form a good substrate for seedling germination. However, mosses disappear from efficiently drained peatlands already in the early phases of post-drainage succession (Place, 1955; Heinselman, 1957; Sarasto & Seppälä, 1964; Wood & Jeglum, 1984; Groot & Adams, 1994; Saarinen, 2002). At the same time, the accumulation of tree and vegetation litter on top of the peat starts to form a so-called ‘raw humus layer’ which breaks the capillary connection between the ground water table and the seed bed (Kaunisto & Päivänen, 1985; Saarinen & Hotanen, 2000; Saarinen, 2013). This leads to reduced seedbed receptivity and an increased need for soil preparation. Thus, the planting of Scots pine seedlings in older drainage areas has been a commonly used regeneration method.

In older transformed drainage areas site preparation is seen as a prerequisite for successful establishment and leaving the soil unprepared is not considered a viable option (Kaunisto, 1984; Mannerkoski, 1975; Kaunisto & Päivänen, 1985). Site preparation removes competing surface vegetation and the possible raw humus layer, and uncovers the original moist peat surface. The decision of which soil preparation method to use is one of the key issues in forest regeneration. Mounding has been the most common method, especially in drained peatland forests, and can be simultaneously associated with ditch network maintenance (Saarinen, 1997; Saarinen et al., 2013). Seedlings planted on mounds are not at risk of being waterlogged in rainy seasons. However, the vegetation layer, which is a major hindrance to the development of seedlings, can be removed also by scalping (a form of patch scarification done by an excavator). Scalping also maintains the capillary connection between the soil surface and the water table, thus preventing excessive drying of the peat, which may occur in mounds due to wind and sun exposure, especially during dry periods (Saarinen, 2013). However, in scalps, especially if they are too deep, the water level may rise too high during rainy seasons (Saarinen et al., 2013). Scalping could be more environmentally friendly compared to mounding, since it does not involve as severe soil disturbance as mounding. Furthermore, shallow scarification by rotavation can be used if the raw humus layer is shallow (Kaunisto & Päivänen, 1985; Saarinen, 1993; Helenius & Saarinen, 2013).

Stand regeneration after clearcutting is the most expensive investment in forest management. Seeding, if successful, is a less expensive method than planting. Additionally, scalping is considered less expensive than mounding in soil preparation. The profitability of forestry on peat-lands is hampered by lower harvesting removals and higher harvesting costs than on mineral soils as well as additional costs such as ditch network maintenance and in some cases also fertilization. Concerning the afforestation of treeless swamps, Keltikangas & Seppälä (1966) concluded that seeding may economically be the best alternative to planting. If successful, seeding or natural regeneration could thus be a very attractive alternative regeneration method instead of planting for minimizing regeneration costs.

The aim of this study was to investigate regeneration methods (planting or seeding) and soil preparation methods (no soil preparation, mounding, or scalping) on the regeneration results of Scots pine on drained peatlands. We hypothesized that planting would lead to higher seedling density and faster development than seeding and that site preparation (scalping, or mounding) would increase the number and growth of Scots pine seedlings and result in higher stand stocking compared to no soil preparation.

We studied the effect of soil preparation and regeneration methods (planting or sowing) on the regeneration of Scots pine on drained peatlands. We were interested in the overall regeneration result defined as the number of crop seedlings after ten years from planting or sowing. A sufficient regeneration result was obtained at all sites and with all of the studied treatments in terms of silvicultural recommendations (Äijälä et al., 2014). However, the number of planted or sown Scots pine crop seedlings was low on non-prepared plots and on scalped plots, but even with these treatments the target density of crop seedlings (2,000 per ha) was achieved when naturally regenerated Scots pine or Norway spruce seedlings were also accepted as crop seedlings. In three cases, 100–250 downy birch seedlings were needed to reach the target of 2,000 seedlings per ha.

After ten years the number of surviving Scots pine seedlings was independent of the establishment method (planting or sowing; only one seedling per sowing spot accepted) at two of the study sites. At the Sievi site, sowing gave poorer results than planting. However, it seems that in terms of the number of surviving Scots pine seedlings, planting and seeding could be alternative methods. This study confirms earlier results (e.g. Mannerkoski, 1975; Kaunisto, 1984; Moilanen & Issakainen, 1984; Kaunisto, 1986; Hånell, 1992: Moilanen et al., 1995) that soil preparation is needed in order to achieve a satisfactory result in artificial regeneration. Without soil preparation the number of original planted or seeded seedlings was very low (350–600 per ha). On scalped plots the number of seedlings was generally only a little higher (550–900 per ha) and at the Sievi site, scalping did not increase the number of seeded seedlings at all. On the mounded plots the number of original planted or seeded seedlings was high (1,325–2,350 per ha), with the exception of the seeded plots at the Sievi site where mounding did not increase the number of seeded seedlings. Similarly, Pearson et al. (2011) reported that the survival of Scots pine seedlings was 90% on mounded plots and 33% on scalped plots three years after planting. In this study, the number of naturally born Scots pine seedlings was high and not affected by the soil preparation treatments (at the Vaala and Sievi sites, there were 5,300–5,400 seedlings per ha, while at the Simo site, there were 850 seedlings per ha). Also a high number of downy birch seedlings (7,900–31,000 per ha) was found in the regeneration areas regardless of the soil preparation method.

The average number of crop seedlings reached the target of 2,000 seedlings per ha in all treatments. Thus, the soil preparation or establishment methods (planting or sowing) did not affect the total number of crop seedlings when naturally born seedlings were also included. However, the share of planted or sown seedlings of the total number of crop seedlings considerably varied among the soil preparation treatments. On unprepared soil, only 23% of the crop seedlings originated from planting or sowing and the rest were naturally regenerated seedlings. Scalping would have led to an almost as poor result if naturally born seedlings were not included. The share of planted or seeded Scots pines on scalped plots was on average 30% out of 2,000 crop seedlings per ha, the rest being naturally regenerated seedlings. Therefore, less than one third of the seedlings originated from planting or sowing.

The best result was achieved with mounding. If the sown plots at the Sievi site were excluded, the average share of planted or sown Scots pines would have been 84% of the total of 2,000 crop seedlings per ha. At the Sievi site, mounding failed to increase the number of sown seedlings. The number of sown seedlings was low already after the first growing season in the follow-up study in the Sievi experiment. The majority of the naturally regenerated seedlings accepted as crop seedlings were Scots pines or also Norway spruces at the Simo site. Thus, the share of downy birch needed to reach the target of 2,000 seedlings per ha was low and only in three cases it was 100–250 downy birch seedlings per ha. Earlier we have shown that natural regeneration can also yield a high share of crop seedlings even without soil preparation in eight years if downy birch seedlings were also included (Hytönen et al., 2019). However, in natural regeneration with seed trees, soil preparation considerably increased the numbers of Scots pine crop trees, hence leading to a reduced share of downy birches among crop seedlings. Consequently, in natural regeneration without soil preparation, a much higher birch admixture was needed to reach full density (Hytönen et al., 2019), which may lead to stands of lower value reducing the financial outcome of the next generation of trees. Kaunisto (1984) found that soil preparation (scalping or mounding) increased the number of pine-occupied sowing spots by two to four folds after one to two years from sowing in a Scots pine regeneration experiment.

Contrary to the Vaala and Simo experiments at the Sievi site, seeding failed to increase the number of seedlings on prepared (mounding, scalping) soil. At the Sievi site, the number of seeding spots with at least one seedling after the first growing season was very low (mounding; 450 seedlings per ha, scalping up to 675 seedlings per ha). The very dry first summer could have increased the mortality of seedlings on the mounded plots, but since almost similar weather conditions prevailed at other sites as well this was probably not the main reason. Since at the Simo site, sowing was done only a week after soil preparation the mounded soil was still moist resulting in good survival. On mounds the effect of drought is highly dependent on the type and bulk density of the peat. During extended dry periods, the outer portions of mounds dry rather quickly resulting in a crust several centimeters thick. Upon drying to a specific water content, the peat becomes water-resistant, i.e., hydro-phobic, after which it rewets very slowly. In practice, hydrophobicity means that the dry, crusty peat mounds in the regeneration area require long-lasting rainfall in order to become rewetted (Saarinen et al. 2013). According to some rough visual observations at the Sievi site, quite many of the mounds consisted of poorly decomposed Carex-peat which is susceptible to excessive desiccation. This might be one possible reason for the poor sowing result on mounds at the Sievi site compared to that in the other experiments. However, also high seed predation at the Sievi site could explain the poor outcome of seeding. Seed predation on prepared soil can jeopardize some seeding operations (Nilsson & Hjältén, 2003). Vegetation (as in the control treatment) decreases the risk of seed predation due to the fact that covered soil (vegetation, unprepared soil, cutting residues) gives shelter to the seeds (Helenius et al., 2015). Also, covering seeds immediately after seeding is seen as a cost-effective way to reduce seed predation and increase seedling emergence (Heikkilä, 1977; Nilsson & Hjältén, 2003).

The results of this study suggest that naturally established Scots pine seedlings gradually fill up the regeneration area, although no artificial seeding or planting is conducted. However, the distance to forest edge and its tree species composition affect natural regeneration and its spatial distribution in clearcut areas. Even in the control treatments without soil preparation a very high total number of seedlings was found when 10 years had passed since regeneration. This stresses the good natural susceptibility of peat soils for seedling germination, which enables continuous Scots pine and Norway spruce seedling establishment for at least ten years and downy birch establishment for six years after harvest (Figure 3). It also raises a question of whether investments in soil preparation and seeding or planting pay off if a good regeneration result without any investments can be achieved in the same time. In order to cover the higher investment costs of artificial regeneration and soil preparation, the share of Scots pines as crop seedlings should be higher, the height development of planted seedlings on prepared soil should be clearly faster or the planted seedlings should otherwise be superior (e.g. originating from genetically improved seed) compared to natural regeneration without soil preparation.

Pine weevils (Hylobius abietis (L.)) caused significant damage in the experiment at the Sievi site and probably also in other experiments after planting on unprepared soil. The damage (46% of seedlings) was likely the cause for the fast decline in the number of living planted seedlings on unprepared soil. Soil preparation considerably reduced pine weevil damage not just on mineral soils but also on peatlands one year after planting (Luoranen & Viiri, 2012). Thus, the planting of Scots pine seedlings without soil preparation, even though they have been treated with an insecticide, is questionable. Pearson et al. (2011) found pine weevil damaged Scots pine seedlings even in scalped and mounded areas, with the damage being much more substantial on scalped plots. At the age of ten years both the planted Scots pine seedlings and seedlings originating from sowing had suffered from moose damage, especially at the Simo site (65% of seedlings). Additionally, at the Sievi site, damage caused by the competing growth of downy birches was noted.

Generally, soil preparation is expected to increase the height growth of seedlings due to better aeration of the mounds, higher temperature, and less competition from ground vegetation. This assumption was confirmed by the results of Mannerkoski (1975) and Kaunisto (1984) showing that the height growth of Scots pine seedlings at 4–5 years from planting was higher on mounded plots than on non-mounded plots. In this study, the mean height advantage of planted seedlings on mounds (69 cm, 33 cm, and 12 cm at the Sievi, Vaala, and Simo sites, respectively) over seedlings growing on unprepared soil was not statistically significant. Also, according to Silfverberg (1995), mounding did not significantly increase the height of seedlings on nutrient-poor drained peatlands after 7 growing seasons. Furthermore, Pearson et al. (2011) found no differences in seedling height between mounds and unprepared planting spots three years from planting. The significant height advantage of planted seedlings over the sown seedlings in our study was partially due to the one-year age gap of the planted seedlings.

Mounding produced better results in this study than scalping when the regeneration results were assessed as the total number of planted or sown seedlings or the number of Scots pine crop seedlings. The poor outcome of planted and seeded seedlings in the scalps was probably related to their filling with water during the third especially wet growing season. Such is the case particularly in years when a fluctuating water table level in a scalped regeneration area rises during a late summer (Saarinen, 2013). Excess moisture in scalps has also been fatal in natural regeneration (Hytönen et al., 2019) as well as in planting (Pearson et al. 2011), but it does not cause problems at coarse mineral soil sites where water perforates into the soil. In well-humified Carex-dominated peat, as in the study sites, the water conductivity is low, which may transform scalps into water ponds after longer periods of rain. With scalping the water table may also be too close to the scalp surface for young seedlings to grow efficiently (Mueller-Dombois, 1964; Lieffers & MacDonald, 1990; Pearson et al., 2011; 2013). Scalping could also increase the risk of frost heaving for small seedlings after planting or seeding, especially when the peat is well humified (Kaunisto & Päivänen. 1985 citing Multamäki, 1939).

However, in natural regeneration, mounding and scalping have produced equal results (Hytönen et al., 2019). This may be due to the longer establishment phase in natural regeneration using the seed tree method. The most rapid increase in the seedling number takes place within three years of treatment (Hökkä et al., 2016), but pine seedlings can emerge even 6–7 years from seed tree cuttings and site preparation (Saarinen, 2002). The weather conditions vary annually, and scalps may be good surfaces for establishment in dry seasons. In mounding, seeds establish themselves on an elevated surface, which is generally not too moist for seedling germination. The mound surface can, however, become too dry and prove an unfavorable substrate for seeds to germinate in (Saarinen, 2013). In this study, the very dry first summer was probably the reason for the low survival of sown seedlings due to excessive desiccation of the surface peat in mounds. However, planted seedlings fared much better during the dry summer probably because their roots were in the moist peat deeper in the mounds. Mounding has also the advantage of restricting competition from ground vegetation more effectively and for a longer time than lighter soil preparation methods, such as scalping (Mannerkoski, 1975; Moilanen & Issakainen, 1981; 1984; Saarinen et al., 2009). This study strengthened the evidence of the positive effects of mounding for the regeneration of Scots pine trees by planting or sowing. Even on nutrient-poor drained peatlands which are subject to slow vegetational changes, mounding considerably increased the number of seedlings after sowing (Silfverberg, 1995).

In line with our earlier studies (Hytönen et al., 2019), besides increasing the abundance of Scots pines, site preparation also seems to increase the number of downy birch seedlings. This has been observed in other studies, as well (e.g. Moilanen & Issakainen, 1981; 1984; Silfverberg, 1995; Saarinen, 2002). The dense stocking of downy birch increases the need for young stand treatment, and if neglected, the further development of pine regeneration becomes hampered.

Similarly to some other studies (e.g. Saarinen, 2002; 2013), attempts to generalize the results of site preparation effects come up against the uncertainty inflicted by the variation in soil moisture. In the light of this study, the risk of drought on mounds during dry seasons is a smaller problem for regeneration than the possibility of excess moisture in scalps during rainy summers and sometimes also summers with average precipitation. Thus, it is better to restrict the use of scalping in natural regeneration where seedlings emerge over several years (Hökkä et al., 2016) only and to make mounds when using artificial regeneration. All methods of site preparation have increased the number of Scots pine seedlings on drained peatlands, excluding such drained peatlands where original Sphagnum moss vegetation still dominates the ground vegetation (Saarinen, 2002).

This study suggests that seeding and planting can achieve similar regeneration results when the number of crop seedlings is considered but that the height of planted seedlings is greater than that of seeded seedlings at a ten-year time point. Mounding proved to be a superior soil preparation treatment, leading to a higher number of surviving planted seedlings or seedlings in seeding spots. However, mounding did not increase significantly the height growth of the seedlings. On the other hand, scalping only had small advantages compared to non-prepared soil. The poor result of the scalping treatment was probably due to the scalps filling with water during rainy seasons. It is better to restrict the use of scalping to natural regeneration in which seedlings emerge over several years. Since natural regeneration was abundant and when naturally regenerated seedlings were also included, a target density of 2,000 seedlings per ha was achieved with all treatments. On two of the study sites the number of naturally born Scots pine seedlings (even on non-prepared plots) was so high (4,200–6,500 per ha) that reforestation would have been secured even without any planting or seeding. Some years after clearcutting the number of downy birch seedlings was high, suggesting an increasing need for young stand tending and precommercial thinning.