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Article

Analysis of Selection-Cutting Silviculture with Thujopsis dolabrata—A Case Study from Japan Compared to German Plenter Forests

by
Leonie Műnzer
1,2,
Kazuhiko Masaka
3,
Yuko Takisawa
3,4,
Sebastian Hein
1,*,
Christoph End
1,
Hisashi Sugita
5 and
Daisuke Hoshino
6
1
University of Applied Forest Sciences, Rottenburg, Schadenweilerhof 1, D-72108 Rottenburg am Neckar, Germany
2
State Forest Administration of Rhineland-Palatinate, D-72108 Rottenburg am Neckar, Germany
3
Iwate University, Ueda, Morioka 020 8550, Japan
4
Faculty of Agriculture, Kagoshima University, Kagoshima 890 8590, Japan
5
Snowy Forest Laboratory, Horikawakoizumi, Toyama 939 8081, Japan
6
Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba 305 8687, Japan
*
Author to whom correspondence should be addressed.
Forests 2023, 14(8), 1556; https://doi.org/10.3390/f14081556
Submission received: 20 June 2023 / Revised: 19 July 2023 / Accepted: 27 July 2023 / Published: 29 July 2023
(This article belongs to the Section Forest Ecology and Management)
Browse Figures
Review Reports Versions Notes

Abstract

:
(1) Background: In Japan, single-tree selection-cutting silviculture with hiba (Thujopsis dolabrata var. hondai) is a traditional silvicultural system and is well documented. We evaluated an experimental forest regarding past structural dynamics and future development while comparing it to the German multilayer coniferous “Plenterwald/plenter forest”. (2) Methods: Classical growth and yield data were recorded since 1995 from two hiba-mixed species plots, one managed since Matsukawa’s (the founder) times in 1931 and the other unmanaged since 40–50 years before 1931. (3) Results: Clear structural differences appeared, with the managed plot having a reverse J-shaped DBH and tree height distribution curve, and a higher percentage of hiba, also typical for German plenter forests’ intensely multi-storied structure. The unmanaged plot was composed of trees with large stem diameters, even though they were more evenly distributed, together with less admixed species. In both plots, the growing stock increased in the past. While the managed plot’s growing stock in 2019 was 561 m3 ha−1, the unmanaged plot reached 982 m3 ha−1 with large portions accumulated in DBH classes >60 cm. (4) Conclusions: When compared to today’s clearcutting system in Japan, selection silviculture shows advantages and may thus serve as a valuable inspiration for silviculture in Japan as it was with plenter forestry for Germany.

1. Introduction

Selection-cutting silviculture is repeatedly reported from some places in Europe [1,2,3] or North America [4]. Such silviculture (i.e., plenter forest/Plenter system or generally multi-layered forest management as variations in continuous cover silviculture, e.g., Mason et al. [5]) is expected to be more natural or close to nature [6,7] and to have a high level of sustainability due to its long-term equilibrium of stand structure [3]. Furthermore, plenter forestry represents a silvicultural system that is very flexible with respect to the amount of natural regeneration, level of stocking, species composition, and stand structure while still ensuring sustainability [3,8,9,10,11].
However, selection-cutting systems also developed traditionally in some regions of Japan [12,13,14], for example, the natural Thujopsis dolabrata SIEB. Et ZUCC var. hondai MAKINO forests in Aomori, northern Japan. Such regional silvicultural concepts are considered an outstanding alternative to the prevailing clearcut forestry present today in Japan [14,15,16,17]. For instance, in the early 1930s, Matsukawa documented and refined such regional forest management by setting up an experimental forest in the corresponding region [18,19]. In a time of silvicultural change and the rising importance of sustainability, such alternative concepts in forest management could turn out to be role models and serve as an inspiration for mainstream plantation forestry in Japan but also for other environmental conditions.
Thujopsis dolabrata (hereafter referred to as “hiba”, the Japanese traditional name) is essential to the selection-cutting silviculture defined by Matsukawa. This high-grade coniferous timber species has been and still is an important revenue source of the local government [20,21]. Wood has traditionally been used for shrines and temples [22]. The oldest wooden structure made of hiba wood currently standing is the World Heritage Temple Chuson-ji, built in 1108. Its long-term durability is due to antifungal activity and insecticidal effects due to hinokitiol-related compounds [22,23,24]. From a silvicultural viewpoint, hiba seedlings have a very high shade tolerance [25]. The saplings are often observed in dark closed forests where other plant species cannot survive [26,27]. Some hiba saplings have survived about 50 years under a closed canopy, a striking parallel to Abies alba Mill. (silver fir) from European forests [3]. Similar to silver fir, selection-cutting silviculture takes advantage of hiba’s tolerance to shade, thus securing its own natural regeneration and providing an important element for uneven-aged silviculture, consistent with the principles of close-to-nature or continuous-cover silviculture [28].
However, to date, only limited research has been carried out on selection-cutting silviculture of hiba forests in Japan [29,30]. Lacking information on stand dynamics and the effects of silvicultural prescriptions and missing long-term perspectives may prevent landowners and forest policy from adopting or modifying such alternative forms of silviculture in other regions. Furthermore, hiba selection-cutting forests show several similarities to European selection forests. However, no evaluation of Matsukawa’s silvicultural system with a comparison to the European plenter systems has been conducted thus far. Finally, even though hiba selection-cutting stands and European plenter stands are praised for their closeness-to-nature [31], hiba selection-cutting forests have never been compared to unmanaged hiba forests.
Therefore, our objectives focus on four questions, following viewpoints of practical silviculturists: (1) How did hiba selection-cutting plots develop in the last 25 years? (2) How do hiba stands managed according to Matsukawa’s selection-cutting system differ from hiba stands left unmanaged in the long term? (3) How do hiba stands managed according to Matsukawa’s selection-cutting system differ from European plenter forest stands, as described in the literature? Finally, (4) which perspectives can be derived for further developing hiba selection-cutting silviculture?

2. Materials and Methods

2.1. Brief History of the Ōhata Experimental Forest

Most hiba forests of Shimokita Peninsula (Aomori Pref., northern Japan) are national forests and managed in a selection-cutting system. The stands of Ōhata have a long history. Natural hiba forests were managed by the Nanbu Domain after the mid Edo period (from 1711 on) until the collapse of Edo Shogunate, i.e., the Meiji Restoration in 1868 [19]. In 1711, the Nanbu Domain instituted a law for the silviculture. Until then, local people were free to cut trees [19]. During those times, the Nanbu Domain conducted extensive selection cuttings. After restoration, the estates were incorporated into the national forests. The new government adopted regional silvicultural principles, with some minor interruptions only. In the early 1930s, Kyosuke Matsukawa (1892–1980), a forester of the Aomori Regional Forest Office, refined the selection-cutting system specific to hiba and set up an experimental forest divided into 20 compartments with a cutting cycle of 10 years. He shaped the local management by developing a silvicultural guideline and established an experimental forest at Ōhata on the Shimokita peninsula (Figure 1) [19,32].
With the establishment of Matsukawa’s system, large trees of poor quality were removed from the forests in an individual tree cutting system, which was conceived to improve the growth of the remaining trees and make way for high-quality hiba [16]. However, the experimental forest also included unmanaged stands consisting of hiba and broad leaves, especially Fagus crenata Blume (Japanese beech, buna), Magnolia obovata Thunb. (bigleaf magnolia, hōnoki), and Quercus mongolica Fisch. Ex Lebed. (Mongolian oak, mizunara). The experimental forest is still under the authority of the Regional Forest Service following the same traditional system.

2.2. Matsukawa’s Concept of Selective Cutting

The silvicultural concept published by Matsukawa in 1935 defined silvicultural objectives, stated silvicultural principles, and offered management options for various forest types, while not being limited to selection-cutting systems. The key elements of Matsukawa’s concept are offering an alternative to a clearcut system by splitting a stand into single trees or tree groups of (supposed) similar age and continuously ensuring new waves of natural regeneration [33]. By combining both elements, the main silvicultural principle is defined as the transformation of one age-group type towards larger-diameter tree classes through thinnings and the continuous creation of new patches of natural regeneration through canopy openings. Such single-tree-wise and group-wise silviculture can be expanded to the idea of one stand being equivalent to one group, for example, in even-aged one-species stands, that will be cut and regenerated completely. However, the system is ideally applied to structured forests with several groups, each with a size of 15–30 m in diameter, present in one stand, thus applying selection-cutting silviculture [18]. Matsukawa also recommended the continuous harvesting of older tree groups to continuously achieve hiba regeneration and to continuously promote groups of regrowth. Canopy openings are made with individual trees or single groups and not through the cutting of all groups homogeneously within one stand. Applying such principles, it is supposed to continuously yield timber while maintaining the desired unevenly aged structure with trees of a large DBH range standing in close vicinity to each other [18].
Within his silvicultural system, Matsukawa [18] described pruning as an important element for growing high-quality hiba and a practice with multiple positive effects (pp. 47–48). It improves timber quality and offers growing space inside the stand that can later be claimed by medium-size trees. It changes light conditions gently with the surrounding remaining canopy cover, promoting the natural regeneration of hiba in shady conditions of the understorey. Unfortunately, however, the pruning has not been carried out in hiba selection cutting forests.
Matsukawa [18] required trees without good stem growth or stem quality to be removed first, as they prevent the good-quality remaining trees from growing and because they may cause damage if harvested after reaching a larger diameter. He also recommended a stronger harvest within tree groups of large stem diameters. In the case of steep slopes, removals should be reduced, and large-diameter trees should be harvested with care. Admixed broadleaves are expected to stabilise conifer stands and prevent erosion. In the past, up to 30% of the growing stock were occasionally harvested. Today, harvests are conducted in a much more moderate way, aiming to hold a constant level of the growing stock [34].
According to the Shimokita District Forest Office [34], Matsukawa’s silvicultural objective is defined at the stand level by holding a stable growing stock of 400 m3 ha−1 and, at the tree level, by growing “large size timber”. Nevertheless, no precise final harvesting diameter is given. No budgeting of harvested trees by tree size or timber assortment is available to date. Matsukawa also defined an ideal distribution for DBH frequencies and ideal shares of tree volume by DBH classes that we later use for comparison.

2.3. Study Plots

The experimental forest of Ōhata (41°23′ N, 141°03′ E) stretches from 65 to 404 m a.s.l. in the north of Shimokita peninsula, about 7 km west from Ōhata (Figure 1). The bedrocks are green tuff, pyroxene andesite, and volcanic mudflow; the soil types are 90% brown soil and 6% podzol [32]. According to the Japan Meteorological Agency [35], long-term values (1981–2010) of medium annual precipitation at Ōhata have been recorded as 1342 mm y−1, and the mean monthly air temperature during the warmest (August) and coldest (January) months is 21.7 and −1.4 °C. Snow cover lasted from November until April, with a maximum depth of 65 cm.
Shortly after Matsukawa set up his concept, the Regional Forest Service installed the experimental forest and divided it into 20 compartments, some having plots for detailed investigations. Among the plots, we set up two study plots (un-/managed) in 12-3 and 12-7, respectively. Both have the most comprehensive data, even though no replicates are available and the plots are very limited in size as is typical for experimental designs from those early times.
In plot 12-3, recordings have been available since Matsukawa’s times (Table 1). Table 1 shows selective cutting data for full compartment 12-3 including the managed plot [34,36]. The total growing stock varied between 353 m3 ha−1 in 2001 (min. value) and 550 m3 ha−1 at the beginning (max. value). Over the decades, the portion of hiba in the growing stock increased from 57.4 to 92.1%, while the total amount of broadleaves decreased almost continuously. According to the Shimokita District Forest Office [34] and Toyama [36], the tree volume removed by harvesting fluctuated strongly between 1931 and 2001.
On average, 17.1% of the growing stock was harvested between 1933 and 2011. In the initial year 1933, the standing volume was over the target volume of 400 m3 ha−1, as set by Matsukawa. In the following two decades, strong harvests with a focus on broadleaved trees were conducted, which led to a considerable decrease in volume remaining especially until the 1961 inventory. The low-growing stock in 1961 might have been caused by the strong harvest during the previous decade, where up to 24.5% of the volume was harvested. It was 40 years later, at the 2001 inventory, that growing stock reached a level of >400 m3 ha−1 again. The rise in volume was caused by cutting down any harvests to a very low level (23.5 m3 ha−1 from 1970 to 2001), enabling a considerable rise in the standing volume of hiba trees.
Our study plots were installed in 1995 and are both located close to the southern border of Matsukawa’s study site. The managed plot faces the river terrace of the Ōhata River at its southern border situated at a distance of about 200 m westwards at 80 m a.s.l. (Figure 1), with a size of 30 × 30 m. The unmanaged plot is situated close to the managed plots at 70 m a.s.l. in flat terrain. It has a size of 50 × 50 m. The absence of management has been proven since 40–50 years before 1931, with a longer history being possible [19]. Measurements were conducted in 1995, 2001, 2006, 2011, and 2019. In both plots, tree girth was measured in trees higher than 1.3 m, and then trees were numbered permanently. Tree height was measured using a height-measuring pole in 1995 and Vertex IV (Haglöf Sweden) in 2019. Both plots were embedded in buffer zones to reduce the negative edge effects of small plot size on per-hectare values.

2.4. Data Analysis

DBH: For plots and measurements, all girths (G) were converted to DBH using the following equation: DBH = G/π.
Tree height (H)–stem diameter relationship: For both plots, the formula by Prodan [37] was fitted to hiba and broad-leaved trees in 2019 as follows:
H = 1.3 + D B H 2 a 0 + a 1 D B H + a 2 D B H 2
where ai (i = 0, 1, 2) are regression coefficients.
H–DBH ratio: For hiba trees over 3 m in tree height, slenderness was calculated in both plots as the coefficient = H/DBH × 100.
DBH and tree height distribution: For calculating DBH distributions, only trees with a DBH ≥6 cm were recorded, using size classes of 2 cm: 6.0–7.9, 8.0–9.9, 10.0–11.9, etc. For reasons of compatibility, the classes were adjusted to data from Köstler [38] to the historical diameter classes used for stands in Uchimappe at Tsugaru Peninsula [18] (p. 74). The distribution by Matsukawa is available in percentages of the number of trees/ha and growing stock/ha. Little attention has been paid to the vertical structure of stands, although diameter frequency distribution has commonly been used to describe the structure of selection forests [39]. As the height distribution contributes to better revealing the hierarchy of the stand compared to stem diameter distribution, the distribution of tree height classes was added with classes of 2 m, including all trees with a diameter ≥6 cm.
Tree volume and growing stock: All trees with a stem diameter ≥6 cm were included in the calculation of the stands’ growing stock (m3 o. b./over bark). The volume (V) of individual hiba trees was calculated by multiplying the cross-sectional area with factors from hiba tree volume tables [40,41]. Measured data were provided by the Shimokita District Forest Office.
As natural mortality was recorded incompletely in both plots, no calculation of volume increment was performed. Only the volume of the growing stock in the corresponding surveys was recorded.
For visual and statistical data analysis, DeltaGraph ver. 7.5.2 J (Red Rock Software, Inc., Salt Lake City, UT, USA) was used.

3. Results

3.1. Tree Species Share

Both plots were dominated by hiba, with shares of 86.3% for the managed plot and 79.4% for the unmanaged plot (trees with DBH ≥6 cm, by stem number ha−1, inventory in 2019). Other tree species, mainly broadleaves (Fagus crenata, Magnolia obovata, Quercus mongolica, in order of importance), held shares of 13.7% in total for the managed plot (unmanaged plot: 20.6%). In 1995, when the first survey on tree species was conducted, the corresponding share differed (90.8% hiba in the managed plot and 71.5% unmanaged). In this year, the species shares of trees with a diameter <6 cm were similar (share of hiba 91.1% in the managed plot, 75.0% in the unmanaged plot). Between the surveys of 2001 and 2019, there was a strong change in the small trees’ species occurrence in the unmanaged plot. With a strong increase in stem number, the share of broadleaves rose to 86.5% because of collapsing canopy trees of mizunara-oak creating large gaps in approximately 2004.

3.2. DBH and Tree Height Distribution

In the managed plot, the DBH distribution showed a clear reverse J-shaped curve, consistent with the first measurements in 1995 until 2019. There were between 433 and 444 trees ha−1 in the lowest DBH class (<8.0 cm), dropping quickly to frequencies of <50 trees ha−1 at DBH classes >18 cm, at each survey, and hiba continuously dominated in all size classes (Figure 2).
The highest measured DBH in the managed plot through all surveys was a hiba tree with 70.3 cm in 2019. The number of trees <6 cm diameter decreased continuously from 4100 in 1995 to 1267 trees ha−1 in 2019 (Table 2). In the unmanaged plot, the shape of the DBH distribution remained unchanged throughout the study period (Figure 2). However, the distribution pattern showed a flatter and unimodal form compared to the managed stand, with maximum values <40 stems ha−1 at DBH classes around 18 cm. The frequency of trees at DBH classes <12 cm was around 10% in relation to the frequencies of managed stands. In unmanaged stands, there were many trees of larger diameters, with 104 trees ha−1 showing DBH values between 60 and 112 cm. The number of trees with a diameter <6 cm strongly increased from 8 in 1995 to 504 in 2019 (Table 2).
The tree height distribution of the managed plot in 2019 showed a pattern similar to the DBH distribution (Figure 3), with a sharp decreasing trend as is known from selection-cutting forests with a multi-storied stand structure. There were 911 trees ha−1 in the lowest class (<12 m), dropping down to frequencies of <400 trees ha−1 at tree height classes >12 m. The tallest tree had a height of 30.3 m. In tree height classes between 12 and 22 m, broadleaves and shrubs played an important role. The tree height distribution of the unmanaged plot had a very low number of trees (32 trees ha−1) in the lowest class (<12 m), followed by tree height classes >12 m with frequencies of 332 trees ha−1.

3.3. Tree Height–DBH Relationship

The curves of the H–DBH relationship for hiba showed similar shapes in the managed and unmanaged plots, as exemplified for 2019 (Figure 4) similarly to the previous measurements. Tree height development had a steep and early rise at low DBH classes (<40 cm) and then a considerable flattening of the curve. Tree height development of broadleaves was slightly faster until a DBH of <30–40 cm, but later never reached a maximum tree height, similar to hiba. On the managed plot, the maximum height of broadleaves levelled off at 20 m compared to unmanaged plots (25–30 m).

3.4. Tree Height/DBH Ratio

The slenderness coefficient of hiba and broadleaves showed high variability along the DBH gradient (Figure 5). In the managed plot, the H/DBH ratio exceeded threshold values of 85 at very young developmental stages, declining later to values of around 50 at DBH classes of >30 cm. In the managed plot, hiba trees reached values indicating low slenderness values, i.e., a good stability, much earlier than hiba trees in the unmanaged plots.

3.5. Growing Stock

Since the first recordings of the managed plot in 1995, the growing stock increased from 356.1 to 560.8 m3 ha−1, thus being considerably above the target set by Matsukawa’s concept. The share of hiba constantly remained at >85% with only small fluctuations (Table 3).
The distribution of the growing stock by DBH classes had clear overstockings with trees ≥42 cm in DBH; however, they varied since 1995 (Figure 6). We included the trees with 5 cm ≤ DBH for comparison with Matsukawa’s concept. The high share of the largest size class in 2019 indicates that over the years, trees grew considerably large diameters, contributing much to the high-growing stock. During the same period of observation, the growing stock in the unmanaged plot also increased from 883.6 to 981.7 m3 ha−1 (Table 3).

3.6. Comparison to Köstler’s Plenter Forest

When contrasting the DBH distribution of the managed plot with the findings by Köstler [38] (Figure 7), large similarities were obvious. The number of trees in the DBH class from 6 to 14 cm was considerably over Köstler’s suggestions, independent of stocking level, whereas the other DBH classes varied around the required values. Köstler, by summarising results from a multitude of Bavarian plenter forests (Germany), which are dominated by Abies alba Mill. and Picea abies (L.) Karst., suggested levels of growing stock; he classified 650–785 m3 ha−1 as “rich in stock”, an optimum level of 550–600 m3 ha−1 was called “good in stock”, and finally a level “poor in stock” was 250–320 m3 ha−1. In our stand dominated by hiba (Table 3), the total growing stock (356.1 m3 ha−1, Table 3) of the managed plot in 1995 was slightly above the values for “poor in stock”. With the increasing volume until 2019, the situation has finally almost reached the category “good in stock”. In contrast, the growing stock of the unmanaged plot always exceeded the category “rich in stock”, and in 2019, it even approached Köstler’s final level “overstocked” ranging from 950 to 1000 m3 ha−1.

4. Discussion

Thujopsis dolabrata is a tree species endemic to Japan with a wide distribution latitude (32° to 42° [42]), even though its share is limited; it made up 2.2% of all conifers’ standing volume in Japan in 1953 [43] and today holds a good share as a regionally important conifer species in the northern Japanese archipelago. Hiba is very shade-tolerant and can thus survive for a very long time in shady conditions. However, it can still reach tree heights of 30 m, as observed by the German scientist Mayr (1906 [44]) during his trip to Aomori. Both Mayr [44] and Akinaga [42] confirmed its slow growth during its youth by describing hiba trees reaching a DBH of 70 cm at 205 years. However, hiba saplings require some small canopy gap openings for successful regeneration [45,46,47]. If the light conditions later improve, hiba will quickly exhibit faster height growth [42,45,48], contributing to hiba being considered a climax species in some regions of Japan. Especially on the northern Honshu Island/Japan and the peninsulas of Tsugaru and Shimokita/prefecture of Aomori (Figure 3), hiba is known for both the best growing conditions and the highest hiba stands and, specifically, is very closely linked to a historic and still-applied-today silvicultural system focusing on individual tree management instead of clearcut and age-class forestry.
From a national level, in Germany, even-aged forest management with conifers is still dominant [49]. Although there have been new shifts in forest policy in recent years [50], they have limited impact, so, for example, the area of plenter forest in Germany is “neglectable”, as stated by the Third National Forest Inventory [51]. In Japan, the situation is similar, especially for pure stands of Cryptomeria japonica (L. F.) D. Don and Chamaecyparis obtusa Sieb. Et Zucc. (Jpn.: “hinoki”), installed on large areas as single species plantations after World War II [20,52]. Moreover, during WWII and since the mid-1950s, clearcut systems have been applied in hiba forests [16,20]. Both conditions may contribute as major reasons for the lack of research about selection-cutting silviculture, especially with hiba.

4.1. Managed and Unmanaged Stands

Even though our unmanaged plot was left untouched at least since 40–50 years before 1931 and combined with hiba as a long-living tree species [19], our plot was not set aside for sufficiently long to show “old growth attributes” [53]. However, some main trends in stand structure were deduced. The growing stock of the unmanaged plot is still on the rise. Meanwhile, the stand does not show any signs of decay and disintegration or phase of saturated stock, neither at the plot level nor in the surrounding landscape. The total growing stock of the managed plot reaches only 57.1% of the unmanaged plot (derived from Table 3). Primary forests often accumulate large growing stocks [53]; however, no other untouched hiba stands are available in Japan for comparison. In our plots, no firm data were available on natural mortality; thus, no analysis on per-hectare productivity nor on volume increment were made. However, Kajimoto et al. [54] gave a range of volume increments for hiba hiba-plantation forests from 2.0 to 8.6 m3 ha−1 yr−1.
DBH (Figure 2) and, to some lesser extent since they are much steeper, tree height distribution (Figure 3) in the managed plot showed a clear reverse J-shaped distribution. However, the DBH curve in the unmanaged plot was unimodal with the highest values around 20 cm in DBH, as well as a flatter number of trees per DBH class and many large-diameter trees, compared to the managed plot. In the unmanaged plot, the distribution of tree height implies that there are very many hiba trees with a uniformly small height surviving in low stand layers and waiting for some sudden canopy opening to quickly stretch up into the gap (Figure 3). Such differences contrast with the uniformity of the tree height–stem diameter relationship in both managed and unmanaged stands for hiba trees. In both plots, hiba trees reached a height of approximately 30 m at a DBH of 60 cm, indicating that hiba trees surpass growth trajectories of tree height and stem diameter almost uniformly. However, the height growth of broadleaved trees in unmanaged plots is levelling off to 27 m at a DBH of 40 cm, whereas this occurs earlier (DBH = 25 cm) in the managed plots at a lower level (tree height = 20 m), which is a clear effect of selection-cutting silviculture favouring hiba and the removal of fast-growing broadleaves that would reach into the upper canopy layer.
The H/DBH ratio [55] is a measure of tree stability often used for risk assessment in silvicultural systems [56]. It is influenced by the horizontal and vertical competitive environment of trees but is also altered by technical equipment, such as tree shelters [57]. Threshold values of >85 indicate unstable trees, thus requiring tending or thinning operations as stabilising measures [55]. The analysis of the slenderness coefficient (Figure 5) of hiba allows a more magnifying insight into the effect of selection cutting on the combined growth dynamics of tree height and DBH. Even though there is a large variability within each zone of DBH, in the unmanaged stand, the H/DBH ratio is above the threshold mentioned above and peaks at a stem diameter of 7 to 8 cm. We interpret this pattern as an indication that, in unmanaged stands, there is some slight favouring of height growth over stem diameter growth with hiba at young stages of individual tree development in shaded understorey conditions. In the managed plot, no such peaking pattern is visible, as there is just a decreasing H/DBH ratio when trees pass to higher-diameter classes. Especially for hiba, which is said to be vulnerable to windthrow [20], unstable situations at early tree development, as observed in our unmanaged plot, may affect long-term stability. This supports the need for appropriate silvicultural interventions to improve tree and stand stability.

4.2. Evaluating Matsukawa’s Concept

A key element of Matsukawa’s silvicultural concept is to perceive the forest stand as individual trees or tree groups of (supposed) similar ages to continuously ensure new waves of natural regeneration. Such an alternative to clearcut or age-class silviculture is best coined by Matsukawa when using the term “canopy opening”. In this way, he demonstrated a shift from previous stand-wise towards tree-wise silviculture. Nevertheless, canopy openings are not applied in a homogeneous way throughout the stand, such as in a shelterwood system, but in an irregular way to create small patches of hiba regeneration and to prevent competing tree species from sprouting in shady conditions.
In the description of his concept, Matsukawa does not use the term plenter forest or Dauerwald/Dauer forest (continuous cover forest), even though his concept was very close to the ideas of Alfred Möller, a leading German Forest Scientist with a high visibility at the international level. This is even more interesting, as in 1927, the first translation of the famous book “Der Dauerwald” by Möller [58] was published in Japanese (Kōzokurin shisō, transl. by K. Hirata. Tōkyō Eirinkyoku, 1927. アルフレート・メーラー, 平田慶吉訳『恒續林思想』. 東京営林局); however, no reference was found in Matsukwa’s documents to date. Thus, there is a strong indication that his concept was original, as well as also based upon traditional silvicultural solutions.
Selection-cutting silviculture, as applied on the managed plots, has a significant impact on the amount of natural regeneration (Table 2). In the low-diameter classes (DBH <6 cm), there were 15–450 times more saplings (e.g., hiba) compared to the unmanaged plot. Therefore, only strong punctual growth after tree mortality made the number of saplings temporarily exceed that of the managed plot. The tree species composition in those regenerating spots of the unmanaged plot strongly shifted towards broadleaf species.
According to Dengler et al. [59], wave-like deviations from the equilibrium stem diameter distribution and slight changes in growing stock were known from European selection forests. Similarly, in the Ōhata managed stand, there have been fluctuations in the amount of regeneration in the smallest DBH class (high values in 1995, low values in 2019), followed by varying values in higher classes along the subsequent surveys. A potential reason for such variability is, for example, the omitted harvest in 2001, the generally increasing growing stock (2019: in total 560.8 m3 ha−1), and significant overstocking in larger DBH classes (≥42 cm, Figure 6) or excessive regrowth of seedlings after thinnings caused by layering regeneration from branches, a phenomenon typical to hiba [60].
A specific remark is necessary for the growing stock in 1995 (Figure 6), as the diameter class ≥62 cm contains only very few observations. With the establishment of Matsukawa’s system, large-diameter trees of poor quality were removed first. Later, after WWII, according to the Shimokita District Forest Office [34], during a period of rising need for timber and increasing popularity of clearcutting systems, large-diameter hiba might be harvested [16,20]. The forest was exploited, reducing the available resource of hiba (Table 1), serving here as an explanation for lacking trees in this highest-diameter class, while in the unmanaged plot, trees did reach diameters of over 100 cm (Figure 2).
From our data, a difference in species dynamics was visible when analysing the number of trees at small-diameter classes (DBH <6 cm). Broadleaves benefit from sudden changes in the light situation in the unmanaged plot due to canopy trees’ mortality. Thinning operations in the managed plot reduce broadleaves in the canopy layer and give only moderate changes in the light situation, which is represented by reduced shares in the growing stock (2019: 13.7% in managed vs. 20.6% in unmanaged stand).
In the managed plot, the growing stock in 2019 was above the target values defined by Matsukawa (Table 3). O’Hara and Gersonde [39] emphasised the option of continuous control of the growing stock in selection forests by balancing harvest volume. Growing stock affects the per-hectare volume increment, the stem diameter growth of individual trees, and the structure of natural regeneration [3]. The rising volumes of both hiba (484.0 m3 ha−1) and broadleaves (76.7 m3 ha−1) contribute to overstocking in the managed plot. Even though the values lie significantly below the corresponding values of the unmanaged plots (in total: 981.7 m3 ha−1), corrective harvestings are necessary, including close monitoring of their effects on the regeneration dynamics, competing vegetation, and volume increment of individual trees.
In Matsukawa’s concept, no precise final harvesting diameter nor target quality is given, with the objective simply stated as growing “large size timber” and “ensuring natural regeneration”. From today’s perspective [61,62], the choice of a final harvesting diameter has a strong influence on stand volume, volume distribution, and shares of volume increment by DBH classes, as well as on the amount of natural regeneration, including tree species. Some range of a DBH target, e.g., 60 to 80 cm, could be a flexible solution to better steer harvesting and the influencing factors mentioned above, which also include desired timber assortments and revenues [62].

4.3. Is the Managed Plot a Plenter Forest?

Even though there is no clear and consistent definition of plenter forest in the literature [3,4,31,63,64], some key elements can be given; they are vertically and horizontally structured forest stands with potentially several admixed tree species. They are made up of individual or group-positioned trees of very different dimensions assembled in confined areas. They are unevenly aged, at which the exact age, however, remains unknown. Plenter silviculture is characterised by an individual tree or group-wise selection-cutting system, with each silvicultural intervention having a simultaneous function as the harvesting, tending, thinning, and particularly continuous promotion of natural regeneration to balance out the above-mentioned complex structure in the long run.
In the sense of Köstler’s classification, a standard investigation still valid as a benchmark for Europe to date, all characteristics of the Ōhata-managed plot qualify as a true plenter forest: DBH and tree height distribution, tree height–DBH relationship, and the growing stock with its distribution along classes of stem size. This is particularly interesting as the Köstler plenter forests from Bavaria, Southern Germany, are made up of different tree species, such as silver fir, Norway spruce, and, more rarely, European beech (Fagus sylvatica L.). Obviously, such forest types are not necessarily linked to taxonomic entities but more to structural patterns and dynamics; silver fir and hiba are both shade-tolerant climax species, as well as most appreciated on the timber market and thus by the forest owner. This is supported by Ammon [31], an early investigator of plenter forests, who confirmed the term plenter forest to suit the type of silviculture regardless of country, site conditions, or tree species. This is also in line with Köstler’s perception of a plenter forest as a—by itself—heterogeneous group of multistorey forest stands ranging from almost devastated to under- or overstocked and high-volume stands, depending on the balanced share of stem diameter classes: either “natural regeneration/small diameter trees—DBH <24.9 cm”, “medium size trees” or “large size trees—DBH ≥50 cm”. Any of them may be appropriately in balance, understocked, or lacking almost completely.
Additionally, the Ōhata-managed stands are a result of steady human silvicultural activities. Giving up management will lead to a loss in the structure in confined areas typical for plenter forests. In the medium and long term, such overstocked stands will finally end up in phases of partial stand collapse and gap pattern decay as part of the natural disturbance cycling [65,66]. The selection-cutting forests of Ōhata/Japan, as well as those from Köstler [38], may thus be seen as a point in time within forest cycling dynamics that meets human needs through preserving a specific and fragile structure.

4.4. Perspectives of Selection-Cutting Silviculture in Japan

Even though selection-cutting silviculture is still of minor importance in Japan (the exact share among Japanese forests is unknown [67]), it has been the subject of repeated scientific studies and many positive statements [68,69,70]. However, there are many examples of partial or complete failure, mainly referring to failing natural regeneration: many non-commercial accompanying species impede the natural regeneration of high-grade valuable conifers, as similarly known, e.g., from the Appalachian Mountains [71] or Canadian forest ecosystems [72].
Indeed, in our study plots, we can observe several broad-leaved tree species in the canopy layer, e.g., beech, oaks, horse-chestnut (Aesculus turbinata Blume), or wingnut (Pterocarya rhoifolia Sieb. et Zucc.). Masaka and Utsumi [73] analysed the stand dynamics of natural hiba forest in Hokkaido, northern Japan, and argued that such broadleaves coexist with hiba, due to their traits as gap species, and do not completely impede the natural regeneration of hiba, especially when the canopy layer is kept closed. From a long-term perspective, enough hiba seedlings will survive under the dense canopy layer due to their shade tolerance.
In addition, Japanese forestry also struggles with abundant herbaceous or gramineous vegetation competing with commercial conifers, e.g., shading by dense dwarf bamboo (Sasa spp.) often prevents tree regeneration [74,75]. Later in stand development, tall-forb species can also dominate after clearcutting and suppress the tree seedlings [76,77]. Here, Matsukawa’s silviculture with canopy gaps from single-tree or group selection cuts offers significant advantages, especially in forests with dominant or admixed hiba; as such, competitors cannot survive under the dark understorey [78]. In addition, such alternatives to clearcutting systems can offer pathways to reduce the workload of costly weeding and precommercial tending.
Almost one century ago, Yoshida [79] confirmed the impracticability of selective cutting for private owners due to the high financial, technical, and silvicultural requirements needed for such complex systems. Similar to the Shimokita Peninsula, Tsugaru Peninsula, situated just across Mutsu Bay (Figure 1), where our investigation took place, has experienced another example of failing natural regeneration; hiba forests were exploited during the feudal period [20]. After heavy overuse and transport of the timber to Osaka until the late-18th century, Tsugaru’s forests with hiba were strictly protected. However, this finally led to overmature stands with limited hiba regeneration for advanced growth [20] because of not enough light even for shade-tolerant hiba. Dr. Karl Hefele, a Bavarian academic forest engineer who held the chair for forest science and erosion control at the University of Tokyo from 1900 to 1903, visited various forests around Japan and Korea. He also mentioned the hiba stands on the Tsugaru Peninsula as examples of misunderstood silvicultural systems of natural regeneration [80]. Much later, Watanabe [15] confirmed such early examples and extended them to the general statement that failing natural regeneration is one of the major threats to successfully implementing close-to-nature forest management in Japan. Finally, most recently, Boneberger et al. [12] stated that the privately owned hinoki cypress stands in Gifu Prefecture, central Japan, were losing their plenter-like structure because of heavy overstocking and consequent insufficient natural regeneration due to the same socioeconomic and silvicultural reasons as Yoshida [79] pointed out.
Therefore, managed plots in Ōhata have been tended in the long term according to the principles of Matsukawa and have resulted in a plenter-like structure with abundant regeneration and successful advanced growth of hiba. They hold a special position as a scientific experimental area and a living laboratory for practical close-to-nature or even continuous cover forest management.

5. Conclusions

In Europe, numerous regions are known to have well-established plenter or plenter-like forest stands. However, in Japan, where they are usually referred to as 択伐林 (takubatsu-rin), “selection cutting”, and sometimes as 複層林 (fukusou-rin), “multi-layer forest”, the situation is different.
Even though some structures of such forest types show similarities across continents and biomes, such comparisons have several weak points and can thus be criticised: (a) Comparing forest structures across tree species without considering underlying ecological functions and processes may not contribute enough to understand why and how such develop. Specifically, eco-physiological traits of Thujopsis dolabrata are not sufficiently known to explain its contribution to establish and maintain such forests. (b) Such comparative analysis on DBH-distributions and standing volume over time do not give enough insight into what needs to be done in either of the compared forests in order to improve their forest management in terms of steering stem growth mortality and per-hectare volume productivity. Finally, (c) societal factors setting the objectives for producing timber often remain hidden, even though their impact may be important.
However, the first two critics may show a pathway to future investigations on the results of Matsukawa’s concept with a more detailed resolution. From this perspective, selection-cutting silviculture as implemented in Ōhata, Shimokita Peninsula, may then further contribute to further differential analysis.

Author Contributions

Conceptualisation, L.M., K.M. and S.H.; methodology, L.M. and K.M.; software, K.M.; validation, L.M., K.M., S.H., Y.T., C.E., H.S. and D.H.; formal analysis, L.M. and K.M.; investigation, L.M., K.M. and S.H.; resources, K.M., H.S. and D.H.; data curation, L.M. and K.M.; writing—original draft preparation, L.M.; writing—review and editing, L.M., K.M. and S.H.; visualisation, K.M.; supervision, K.M. and S.H.; project administration, C.E.; funding acquisition, K.M., S.H. and C.E. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the German Federal Ministry of Food and Agriculture & Federal Office for Agriculture and Food within the German–Japanese project “3Arrows” under the Grant (28I-038-01). The article processing charge was funded by the Baden-Württemberg Ministry of Science, Research and Culture and the University of Applied Forest Sciences Rottenburg in the funding programme Open Access Publishing.

Acknowledgments

The authors express their greatest gratitude to the Shimokita District Forest Office for their help to conduct the study; to Akira Hiyane who dug up forgotten old data of the study plot in a reference room of the Forest Office and contributed to the redevelopment of the plots in 1995; to Yoshihiko Itoya for the organization of data in Table 1; and to Yuko Toyama for the contribution to the establishment of plots and field works in 1995.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 14 01556 g001
Figure 1. Location of the Ohata experimental forest on Shimokita Peninsula (Pen.), including the study plots. Numbers display Matsukawa’s original compartments in his experimental forest. Light grey and dark grey areas indicate the plots set up by the Regional Forest Service, within which our study plots (installed in 1995, 50 × 50 m resp. 30 × 30 m) are located: 12-3 (12を)—managed, selective cut, 12-7 (12い)—unmanaged.
Figure 1. Location of the Ohata experimental forest on Shimokita Peninsula (Pen.), including the study plots. Numbers display Matsukawa’s original compartments in his experimental forest. Light grey and dark grey areas indicate the plots set up by the Regional Forest Service, within which our study plots (installed in 1995, 50 × 50 m resp. 30 × 30 m) are located: 12-3 (12を)—managed, selective cut, 12-7 (12い)—unmanaged.
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Forests 14 01556 g002
Figure 2. Tree diameter (DBH) frequency distribution by year of inventory for managed and unmanaged plots. Black bars: hiba; white bars: other (mostly broadleaved, some Cryptomeria japonica) species.
Figure 2. Tree diameter (DBH) frequency distribution by year of inventory for managed and unmanaged plots. Black bars: hiba; white bars: other (mostly broadleaved, some Cryptomeria japonica) species.
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Figure 3. Tree height frequency distribution in 2019 for managed and unmanaged plots. Black bars: hiba; white bars: other (mostly broadleaved, some Cryptomeria japonica) species.
Figure 3. Tree height frequency distribution in 2019 for managed and unmanaged plots. Black bars: hiba; white bars: other (mostly broadleaved, some Cryptomeria japonica) species.
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Figure 4. Stand height curve for hiba (▬●▬) and broad-leaved trees (▬○▬) in 2019. Coefficients of the regression lines: a0 = 4.1741, a1 = 1.0061, a2 = 0.0199, R2 = 0.976, p < 0.001 for hiba in the managed plot; a0 = 1.8072, a1 = 0.2690, a2 = 0.0389, R2 = 0.880, p < 0.001 for broad-leaves in the managed plot; a0 = 16.5446, a1 = 0.0551, a2 = 0.0284, R2 = 0.963, p < 0.001 for hiba in the unmanaged plot; a0 = 47.7243, a1 = −1.6252, a2 = 0.0475, R2 = 0.899, p < 0.001 for broad-leaves in the unmanaged plot.
Figure 4. Stand height curve for hiba (▬●▬) and broad-leaved trees (▬○▬) in 2019. Coefficients of the regression lines: a0 = 4.1741, a1 = 1.0061, a2 = 0.0199, R2 = 0.976, p < 0.001 for hiba in the managed plot; a0 = 1.8072, a1 = 0.2690, a2 = 0.0389, R2 = 0.880, p < 0.001 for broad-leaves in the managed plot; a0 = 16.5446, a1 = 0.0551, a2 = 0.0284, R2 = 0.963, p < 0.001 for hiba in the unmanaged plot; a0 = 47.7243, a1 = −1.6252, a2 = 0.0475, R2 = 0.899, p < 0.001 for broad-leaves in the unmanaged plot.
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Figure 5. Slenderness (H/DBH ratio) for hiba over DBH measurements in 2019. The horizontal dotted line is the guideline of H/DBH ratio = 85. Black dots: managed plot; white dots: unmanaged plot.
Figure 5. Slenderness (H/DBH ratio) for hiba over DBH measurements in 2019. The horizontal dotted line is the guideline of H/DBH ratio = 85. Black dots: managed plot; white dots: unmanaged plot.
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Figure 6. Shares of growing stock (volume o.b./ha) by years of inventory for managed and unmanaged plots. Matsukawa [18] (p. 74) did not show the ideal distribution for the growing stock but showed a well-managed stand as a good example obtained in Uchimappe at Tsugaru Peninsula. Note: DBH was measured at 8 cm intervals except for the 6–10 cm class, e.g., DBH class of 6–10 cm covers the range of 5.0–10.9 cm. Matsukawa considered that the growing stock of the stand after 10 years will show the distribution drawn as a thick black line (“After 10 years”), if the distribution of initial growing stock is drawn as a dotted grey line (“Initial state”).
Figure 6. Shares of growing stock (volume o.b./ha) by years of inventory for managed and unmanaged plots. Matsukawa [18] (p. 74) did not show the ideal distribution for the growing stock but showed a well-managed stand as a good example obtained in Uchimappe at Tsugaru Peninsula. Note: DBH was measured at 8 cm intervals except for the 6–10 cm class, e.g., DBH class of 6–10 cm covers the range of 5.0–10.9 cm. Matsukawa considered that the growing stock of the stand after 10 years will show the distribution drawn as a thick black line (“After 10 years”), if the distribution of initial growing stock is drawn as a dotted grey line (“Initial state”).
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Figure 7. Frequency distribution of DBH for the managed and unmanaged plots (1995, 2001, 2011, 2019) in comparison to Köstler’s [38] selection-cutting forest types. Köstler [38] classified the tree sizes as follows: DBH class of 6–14 cm covers the range of 6.0–13.9 cm.
Figure 7. Frequency distribution of DBH for the managed and unmanaged plots (1995, 2001, 2011, 2019) in comparison to Köstler’s [38] selection-cutting forest types. Köstler [38] classified the tree sizes as follows: DBH class of 6–14 cm covers the range of 6.0–13.9 cm.
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Table 1. Growing stock and harvest volume in percent in compartment 12-3.
Table 1. Growing stock and harvest volume in percent in compartment 12-3.
YearHibaBroad-Leaved TressTotal
Growing Stock
(m3 ha−1)		Harvest
(%)		Growing Stock
(m3 ha−1)		Harvest
(%)		Growing Stock
(m3 ha−1)		Harvest
(%)
1933315.37 (57.4)13.4234.51 (42.6)39.4549.88 (100.0)24.5
1943312.32 (68.3)6.3144.66 (31.7)36.9456.98 (100.0)16.0
1961312.82 (81.3)10.272.04 (18.7)49.3384.86 (100.0)17.5
1971336.45 (88.2)18.844.87 (11.8)25.2381.32 (100.0)19.6
1981326.97 (89.3)10.239.02 (10.7)45.3365.99 (100.0)13.9
1991292.43 (82.9)16.360.33 (17.1)6.6352.76 (100.0)14.7
2001407 (92.1)6.635 (7.9)0.0442 (100.0)6.6
2011------
Mean ± SD319.7 ± 58.69.0 ± 5.172.0 ± 76.78.1 ± 7.8391.7 ± 48.517.1 ± 9.2
Values in the brackets of the growing stock indicate % in hiba and the broad-leaved trees. SD indicates the standard deviation for 1933–2001. Hyphen (-) indicates no data. Table shows selective cutting data for full compartment 12-3 including the managed plot.
Table 2. Number of saplings (DBH <6 cm and tree height >1.3 m) in the managed and unmanaged plots.
Table 2. Number of saplings (DBH <6 cm and tree height >1.3 m) in the managed and unmanaged plots.
Year
19952001200620112019
Managed plot (no. ha−1)
 Hiba36332989254422441167
 Broad-leaved trees467289211189100
Total41003278275624331267
Unmanaged plot (no. ha−1)
 Hiba824--68
 Broad-leaved trees08--436
Total832--504
Table 3. Growing stock in the managed and unmanaged plots.
Table 3. Growing stock in the managed and unmanaged plots.
YearManaged Plot (m3 ha−1)Unmanaged Plot (m3 ha−1)
HibaBroad-Leaved TreesTotalHibaBroad-Leaved TreesTotal
1995323.2 (90.8)32.9 (9.2)356.1 (100.0)632.2 (71.5)251.4 (28.5)883.6 (100.0)
2001366.0 (89.4)43.3 (10.6)409.3 (100.0)653.2 (68.2)305.1 (31.8)958.2 (100.0)
2006374.2 (88.1)50.5 (11.9)424.7 (100.0)721.9 (74.2)250.5 (25.8)981.7 (100.0)
2011412.0 (87.7)57.6 (12.3)469.6 (100.0)724.7 (77.4)211.4 (22.6)936.1 (100.0)
2019484.0 (86.3)76.7 (13.7)560.8 (100.0)778.9 (79.4)202.7(20.6)981.7 (100.0)
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Műnzer, L.; Masaka, K.; Takisawa, Y.; Hein, S.; End, C.; Sugita, H.; Hoshino, D. Analysis of Selection-Cutting Silviculture with Thujopsis dolabrata—A Case Study from Japan Compared to German Plenter Forests. Forests 2023, 14, 1556. https://doi.org/10.3390/f14081556

AMA Style

Műnzer L, Masaka K, Takisawa Y, Hein S, End C, Sugita H, Hoshino D. Analysis of Selection-Cutting Silviculture with Thujopsis dolabrata—A Case Study from Japan Compared to German Plenter Forests. Forests. 2023; 14(8):1556. https://doi.org/10.3390/f14081556

Chicago/Turabian Style

Műnzer, Leonie, Kazuhiko Masaka, Yuko Takisawa, Sebastian Hein, Christoph End, Hisashi Sugita, and Daisuke Hoshino. 2023. "Analysis of Selection-Cutting Silviculture with Thujopsis dolabrata—A Case Study from Japan Compared to German Plenter Forests" Forests 14, no. 8: 1556. https://doi.org/10.3390/f14081556

APA Style

Műnzer, L., Masaka, K., Takisawa, Y., Hein, S., End, C., Sugita, H., & Hoshino, D. (2023). Analysis of Selection-Cutting Silviculture with Thujopsis dolabrata—A Case Study from Japan Compared to German Plenter Forests. Forests, 14(8), 1556. https://doi.org/10.3390/f14081556

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