1. Introduction
Continuous cover forestry (CCF) on drained boreal peatlands is receiving increasing interest, as it is expected to have fewer adverse environmental and climate effects than traditional rotation forestry [
1,
2]. In some cases, CCF can also be economically more profitable, because it necessitates less investment in stand regeneration and ditch cleaning [
2,
3,
4]. Selection cutting, where the dominant timber-sized stems are felled and the mid-sized trees and undergrowth are retained, is the prevailing CCF harvesting method. Regeneration is based on natural seedlings established and developed in the gaps formed by selection harvest [
5]. Selection cutting is generally regarded as suitable for growing shade-tolerant tree species such as Norway spruce (
Picea abies Karst.), but it is less or even nonapplicable for Scots pine (
Pinus sylvestris L.) or other shade-intolerant species. Strip cutting, where narrow regular or irregular rectangular strips are clear-cut and part of the stand is left unharvested or only slightly managed, has been proposed as a better CCF approach for shade-intolerant trees. In the case where a considerable share of the stand is left unharvested, strip cutting is considered to belong to CCF rather than rotation forestry [
3].
Adequate drainage (i.e., maintaining the water table (WT) at a depth of 0.3–0.4 m below the soil surface during the late growing season (July–August)) is a prerequisite for profitable forestry, as it enables good tree growth [
6,
7] and successful forest regeneration [
8]. Maintaining a sufficiently deep WT after harvesting also reduces methane emissions [
9,
10] as well as mitigates the leaching of redox-sensitive water pollutants such as phosphorus, iron, and dissolved organic carbon [
11]. The WT can be controlled by ditch depth and spacing as well as by biological drainage through stand evapotranspiration (ET), the magnitude of which depends on a stand’s properties such as tree species and leaf area [
12,
13]. The WT also strongly depends on factors that cannot be managed such as weather conditions and peat hydraulic properties [
7,
12,
14]. In boreal conditions, the average late growing season WT typically varies from 0.1 to 0.8 m below the soil surface [
7,
13].
As strip cuttings create an uneven forest structure within the peatland, it can be assumed that they also generate significant spatial variations in the ET and the WT [
15,
16,
17]. The removal of trees leads to reduced interception losses and transpiration [
18,
19,
20,
21], which is expected to locally elevate the WT in the harvested strip. However, the water use of trees in the adjacent unharvested forest extends into the harvested treeless strip and may lower the WT. Therefore, ditch network maintenance, which is a necessary and costly measure after clear-cuts, may not be needed after strip cutting. However, whether and under which conditions this desired effect can be achieved is currently completely unknown, and a methodology to analyze the dependency of WT variations among strip cutting regimes is lacking.
This study aimed to address this fundamental knowledge gap and assess whether, and under which conditions, strip cutting is a hydrologically feasible CCF approach to manage boreal drained peatland forests. Based on earlier literature, we considered strip cutting to be hydrologically feasible when the mean of a long-term late-summer (July–August) WT remains deeper than approximately −0.35 m from the soil surface. This deep WT should ensure undisturbed tree growth after harvesting [
6,
7] while not significantly increasing methane emissions [
9] and exports of redox-sensitive nutrients [
11]. We addressed our aim by conducting a set of hydrological experiments established to quantify the effect of strip cutting on the WT at harvested strips and the adjacent unharvested tree stand. The research encompassed two main questions: (1) What kind of drying and wetting effects emerge in unharvested tree stands and harvested strips? (2) How does the strip cut layout affect the drainage of the harvested strips and the adjacent unharvested stand? Due to the large potential variations in ditching and initial stand attributes, strip cut layouts, peat properties, and climatic conditions, these questions cannot be solved purely empirically. Lately, process-based hydrological models have been applied for other practical management issues in boreal drained peatland forests with promising results [
7,
12,
14,
17,
20]. However, these models are not directly suitable for simulating strip cut forest hydrology. By combining the features of existing models [
14,
22,
23] and extending them for a spatially distributed grid, we were able to apply a process-based hydrological model to test how multiple combinations of strip cut layouts and peat types affect the WT. We used modeling to extend our predictions beyond what is generally possible with empirical data.
4. Discussion
The measured WTs in the strip cut drained peatland forests showed that in the harvested strips, the WTs remained much closer to the surface than in the adjacent unharvested tree stands, especially during dry growing seasons (
Figure 3). The WT differences between the strips and unharvested stands produced by the model were of a similar magnitude (
Table 3). The larger difference in the observed WTs than in the simulated WTs at the four sites can be explained by the fact that the observed WTs covered only the tube locations, while the simulated WT differences were calculated from the mean WT within the whole unharvested or harvested domains. In one of the sites (i.e., Häädetjärvi 3), the difference in the observed WTs between the unharvested stand and the harvested strip was approximately 50% smaller than the other sites (
Table 3). This was most likely related to the better drainage of the harvested strip compared to the other sites (
Figure 3a–c). Because the dimensions of the drainage structures were similar, probable explanations include different peat hydraulic properties or field layer vegetation with high ET capacity.
The results suggest strip cutting had limited capacity to keep the WT below the desired −0.35 m level in the harvested strips (
Figure 7c–d), especially in wet conditions and in the
Sphagnum peat sites.
Sphagnum peat had low hydraulic conductivity, and the harvested strips could not be adequately drained, even if the LAI (i.e., the stand volume of the adjacent forest) was large (
Figure 7d). However, compared to clear-cutting, strip cutting may be a better alternative. This is especially true when the adjacent stand has an LAI ≥ 2, and the site is characterized by
Carex peat with high hydraulic conductivity. In these conditions, strip cutting yielded a notable improvement in the share of the well-drained area compared to clear-cutting (
Figure 7c). Narrower strip widths improved drainage on the harvested strips, as the drying effect of the adjacent stand was the highest near the strip edges and faded further away (
Figure 6).
The simulated edge effects in different weather conditions (
Figure 6) and the sensitivity analysis (
Table 4) jointly indicate that the drainage on the harvested strips was better in southern than northern Finland, and this is mostly related to climatologically driven differences in precipitation and ET. It seems that in northern Finland, forest regeneration with strip cuttings is hydrologically feasible only in
Carex peat sites. Similar conclusions have been made also for selection cuttings [
12]. However, as the importance of ET in controlling the WT is likely to increase, especially in northern Finland, in the future climate [
12], the hydrological feasibility of strip cuttings might increase in future elevated temperatures.
Observed WT data from our experimental sites showed a clear WT gradient between and within unharvested stands and harvested strips in the dry summer conditions (
Figure 3). The WTs in the adjacent unharvested stands were clearly lower the farther the distance from the edge of the harvested strips. However, the vicinity of the ditches also affected the WT. This makes it difficult to interpret the edge effects from the observed WT data, because it is not possible to separate them from the drainage effect of the ditches. In the harvested strips, the measured gradient was not as clear as in the adjacent unharvested stands.
Edge effects were studied by comparing the simulated WT of a strip cut area to the WT in a case where either no cuttings or clear-cutting was applied to the whole area. Edge effects smooth the WT differences between unharvested stands and harvested strips. Simulation results showed that this was most affected by the stand density (
Figure 6). Secondary impacts were caused by weather or climate conditions. There were also several other factors affecting the magnitude of the edge effect: distance to ditches, ditch depth, peat hydraulic properties, and strip cut width. The magnitude of the edge effect was strongest in dry conditions. When the soil hydraulic conductivity was low and the water retention capacity was high (such as in
Sphagnum peat,
Figure 6), the edge effect did not reach as far into the unharvested stands or harvested strips as in the soil with higher hydraulic conductivity (such as
Carex peat,
Appendix C). Ditch drainage strongly affected the WT near the ditches and, as a result, suppressed the edge effects (see
Appendix D for an example). In drained peatlands, the extent of the edge effects depends on the ditch depth and peat hydraulic properties that control ditch drainage.
There is evidence that transpiration can be higher near the forest edge than in the inner forest [
34,
35]. This could be the result of better water availability [
34] or differences in the light environment [
35]. However, with Scots pine, the needle mass is thought to explain the variability in transpiration more than the distance and diameter of neighboring trees [
36]. In our model, transpiration may be limited by water availability, although in drained peatlands, such dry conditions are rare [
37]. Differences in needle mass in edge and inner forests were not accounted for in this study. Higher transpiration near the forest edge caused by different light environments was also not accounted for in the current model structure. Thus, near the forest edge, our model predictions may have yielded WTs that were closer to the surface than if the above effects were accounted for fully.
In addition to hydrology, edge effects also affect stand growth and allometry [
38,
39] and understory vegetation composition [
40]. Trees growing close to the forest edges receive more light and have better access to nutrient resources in the harvested strip. However, WT rise (
Figure 7a,b) within the edges of an unharvested stand might also reduce tree growth compared to completely forested peatland with a deeper WT. Forest regeneration in the harvested strip is also affected by the competition inflicted by the surrounding unharvested tree stand. Competition can reduce height growth as far as approximately half of the dominant height of the surrounding forest [
3,
39]. From the point of view of maintaining satisfactory drainage conditions after harvesting, narrow strips would be optimal, but too narrow strips can be impractical because of higher competition and unfavorable light conditions for seedlings established in the harvested strip. A balance between the WT, vegetation competition, and other factors, such as the direction of the strip in relation to the optimal amount of solar radiation, is thus needed in selecting proper strip cut width.
Strip cutting reduced increases in the WTs of harvested strips and resulted in smaller annual runoff (not shown), especially smaller runoff peaks, compared to clear-cuts. Compared to selection harvestings, WTs in the harvested strips would be higher but runoff would not differ much between the methods if the average stand volumes were similar. Thus, from an environmental point of view, strip cutting can be beneficial compared to clear-cuts, as it may mitigate the export of nutrients and carbon-sensitive anoxic redox reactions [
2] as well as have a positive environmental impact by reducing runoff and ditch erosion [
41].
In strip cuttings, high water levels will likely occur in parts of the harvested strips and also in the unharvested stands, particularly during wet years. Similarly, WTs will likely drop down to deep peat layers, particularly in forest stands during dry summers. To diminish GHG emissions from drained peatland forests, WTs should be kept at a suitable level, not too high so as to enhance CH
4 emissions and not too low so as to increase CO
2 and N
2O emissions [
2,
42]. Nevertheless, compared to clear-cut-based forestry, where the entire area will be either completely forested or clear-cut, significantly smaller areas will be subjected to extremely high or low water levels in the strip cut areas. This may decrease both water quality impacts and GHG emissions [
2]. Even though strip cutting may not be as effective a means of controlling water levels as selection harvesting [
12], more research is still needed to compare the overall environmental and economic effects among the different management types. Strip cutting may be a more feasible management option than selection harvesting, for example, because of the better growth of shade-intolerant tree species and lower forestry harvesting costs.