Eight-year survival and growth of Sakhalin fir (Abies sachalinensis) seedlings with one weeding operation: Impact of mechanical site preparation, vegetation release, summer planting, stock type, and forwarder trail

doi:10.21203/rs.3.rs-3336224/v2

In Hokkaido, northern Japan, bareroot Sakhalin fir seedlings are conventionally planted in short periods during spring and fall following strip site preparation that alternates managed and unmanaged strips to reduce labor requirements. Postplanting, seven years of weeding are imperative owing to the relentless encroachment of evergreen dwarf bamboo from unmanaged strips. Given diminishing forest labor availability, this protocol poses increasing challenges. We hypothesized that comprehensive mechanical site preparation (MSP) to eradicate dwarf bamboo across the entire site could reduce weeding frequency by delaying the regeneration of competing vegetation. Furthermore, using container seedlings could extend the planting season to summer. Over an eight-year period, we examined the survival and growth of summer-planted bareroot and container seedlings in a fully MSP-treated site, with a single weeding operation conducted in the fifth year. Whole-site MSP induced a shift from dwarf bamboo to deciduous woody and herbaceous species, fostering consistently high Sakhalin fir seedling survival and growth rates despite the lone weeding operation. Container seedlings exhibited superior establishment after summer planting and maintained higher survival rates over eight years relative to bareroot seedlings. Planting in the forwarder trail detrimentally impacted overall seedling growth and survival. These findings advocate for container seedling summer planting alongside comprehensive MSP as an effective Sakhalin fir reforestation technique, mitigating weeding frequency and broadening the planting timeframe.

bareroot seedlings

container seedlings

soil compaction

weeding

Vegetation management is a crucial silvicultural practice for forest establishment, regeneration, and growth (Wagner et al. 2006; Willoughby et al. 2009; McCarthy et al. 2011). The survival and growth of target tree seedlings can be profoundly affected by vegetation competition (Richardson et al. 1996; Biring et al. 2003; Kitao et al. 2022). Evergreen dwarf bamboo, with its low environmental requirements, robust clonal reproduction through rhizomes, and adaptability to heavy snow, often sustains prolonged dominance in the understory of cool temperate forests in Japan (Figure S1a, b). This competitive strength prevents tree regeneration in natural forests and hampers the survival and growth of planted seedlings in plantation forests (Figure S1c–e) (Oshima 1961; Nakashizuka and Numata 1982; Noguchi and Yoshida 2007; Nakagawa 2013). Consequently, dwarf bamboo management is paramount for successful regeneration and reforestation in Japan. The use of herbicides for forest vegetation management is increasingly prohibited or restricted regionally and globally owing to environmental concerns (Ammer et al. 2011; Thiffault and Roy 2011; Ramantswana et al. 2020; Saha et al. 2020), including in Japan. Vegetation release using motor-manual brush cutters is more socially acceptable than herbicides, causing fewer disruptions to forest ecosystems (Fortier and Messier 2006; Wyatt et al. 2011; Iddris et al. 2023). However, this method tends to be less effective and costlier in competition control compared with herbicides (Mallik et al. 2002; Fortier and Messier 2006). Therefore, in regions with thriving weeds due to humid climates, the cost for vegetation release using motor-manual brush cutters can be prohibitively high, accounting for up to half of the reforestation cost in the initial decade. This financial burden discourages forest owners from reforestation after clearcutting in Japan (Masaki et al. 2017).

Sakhalin fir (Abies sachalinensis (F.Schmidt) Mast.) is an important timber tree species in Hokkaido, northern Japan (Ishizuka and Goto 2012; Tsuyama et al. 2020), alongside Japanese larch (Larix kaempferi (Lamb.) Carrière). Given its slower growth rate relative to Japanese larch, Sakhalin fir typically undergoes seven years of consecutive weeding from planting until surpassing the surrounding vegetation height (Nakagawa 2013), whereas Japanese larch requires four consecutive years of weeding (Nakagawa et al. 2017). To minimize labor and weeding costs, Sakhalin fir reforestation operations are typically undertaken in recurring strip areas comprising managed and unmanaged zones, as opposed to whole-site plantation (Figure S1f). Site preparation, planting, and subsequent weeding transpire within a 3 m wide strip, with the adjacent 4 m wide strip remaining unaltered. After clearcutting and site preparation, dwarf bamboo predominantly takes over unmanaged strips as competing vegetation, with heights of 3 m for Sasa kurilensis (Rupr.) Makino & Shibata, 2 m for Sasa senanensis (Franch. & Sav.) Rehder, and 1 m for Sasa nipponica (Makino) Makino & Shibata, contingent on snow cover depth (Nakagawa 2013; Kayama and Koike 2018). For Sakhalin fir reforestation, 2000–2500 seedlings per hectare are planted in two rows within each managed strip. This maintains a mere 0.75 m distance between the unmanaged strip’s border and the planted seedlings (Figure S1f). Dwarf bamboo exhibits leaf physiological traits adaptable to both well-lit and shaded environments (Lei and Koike 1998) and quickly adapts its leaf physiology to abrupt light intensity changes following overstory tree removal (Tobita et al. 2010). Consequently, the planted seedlings are susceptible to being overshadowed by vigorously growing dwarf bamboo in unmanaged strips, leading to its rapid invasion of the managed strip via rhizome spreading.

Mechanical site preparation (MSP) before reforestation entails clearing vegetation and logging debris from the forest floor to facilitate planting and modifying the soil to support seedling survival and growth while managing competing vegetation. This approach serves as a chemical herbicide alternative on a global scale (Thiffault and Roy 2011; Löf et al. 2012; Fargione et al. 2021). In Japan, MSP is occasionally implemented through topsoil scarification using an excavator-mounted bucket and mulching with a forest mulcher attached to an excavator for postlogging vegetation management (Yamada et al. 2018; Oya et al. 2021; Harayama et al. 2023a), although manual site preparation remains predominant. In birch forests where the upper trees had been lost due to logging or natural disturbances in Hokkaido, both bucket scarification MSP and mulching MSP have effectively suppressed dwarf bamboo regeneration, enabling the emergence of deciduous perennials and shrubs, previously stifled by dense dwarf bamboo cover (Yamazaki and Yoshida 2020; Harayama et al. 2023b). As Sakhalin fir is highly shade-tolerant (Kitao et al. 2018; Kitao et al. 2019), similar to other American and European Abies species (Claveau et al. 2002; Dobrowolska et al. 2017), its seedlings’ survival and growth may be less impacted by deciduous vegetation compared with evergreen dwarf bamboo. Thus, applying MSP across the plantation and restraining dwarf bamboo regeneration could substantially curtail the need for frequent weeding in Sakhalin fir plantations.

Various factors influence seedling survival and growth during reforestation, including seedling type and quality (Grossnickle and El-Kassaby 2016; Grossnickle and MacDonald 2018a; Harayama et al. 2023a), as well as soil compaction resulting from heavy forestry vehicle use (Cambi et al. 2015; Labelle et al. 2022). In Japan, the past decade has seen the introduction of container seedlings, aiming to extend the planting season to include summer and counteract the decline in forest labor availability (Masaki et al. 2017; Harayama et al. 2021). Container seedlings generally suffer less transplant shock and exhibit greater drought resistance than bareroot seedlings (Grossnickle and El-Kassaby 2016). Nevertheless, information on Sakhalin fir container seedling survival and growth, particularly after summer planting, remains insufficient. Additionally, Hokkaido’s forested regions, featuring gentle slopes and the largest collection of high-performance forestry machinery in Japan, frequently employ a ground-based cut-to-length (CTL) system combining a harvester and a forwarder for logging efficiency (Sasaki 2014). This operational focus, while improving efficiency, often neglects the adverse impact of machinery on Japanese forest soils, subsequently affecting seedling performance postreforestation (Sugai et al. 2020, 2022). Notably, Sakhalin fir seedlings displayed greater resilience to soil compaction in our previous nursery experiment (Sugai et al. 2023).

In this context, we hypothesized that all-encompassing reforestation site MSP targeting dwarf bamboo removal could reduce weeding demands for planted Sakhalin fir seedlings by delaying regeneration of rival vegetation. We also examined the prospect of extending the planting season into summer through container seedling deployment. To address these aims, we explored the eight-year survival and growth of summer-planted Sakhalin fir, using both bareroot and container seedlings. We restricted weeding to a single intervention in the fifth year during eight years in the reforested area, with whole-site MSP approaches after harvest by a harvester and forwarder CTL system. The main objective of this study was to determine whether Sakhalin fir seedlings could flourish with markedly reduced weeding frequency, accomplished by suppressing the resurgence of the robust competitor, dwarf bamboo, through comprehensive site-wide MSP. Furthermore, we conducted quantitative assessments of summer planting effects on seedling survival and growth, distinguished by seedling type (bareroot vs. container), as well as the effects of weeding on vegetation release and the repercussions of forwarder trails using generalized linear models (GLMs) and linear mixed models (LMMs).

Study site, site preparation, and planting design

The study site, with a haplic brown forest soil type, is situated in a ~50-year-old Sakhalin fir plantation with a slope of ~14°–20° in Shimokawa town, Hokkaido (44°16'40''N, 142°41'42''E; 370 m above sea level). It has a humid continental (hemiboreal) climate (Dfb), as per the Köppen classification. The 1991–2020 records from the nearest weather station (44°18' N, 142°37'E; 140 m above sea level) indicated an average annual air temperature of 5.4°C. Monthly mean daily maximum and minimum air temperatures were 25.0°C in August and −8.8°C in January, with an annual precipitation of 965 mm. The heaviest snow cover, 116 cm, occurred in March.

Between June 30 and July 7, 2015, approximately 0.3 ha (30 m wide × 100 m long) underwent CTL harvesting. This system used a harvester (350.1, Komatsu Ltd., Tokyo, Japan; base machine: PC138US, Komatsu Ltd.) and a forwarder with front-wheel and rear-crawler (F801, IHI Co., Tokyo, Japan). Subsequently, two MSP methods were applied: 0.5 m³ capacity bucket scarifying MSP (PC120, Komatsu Ltd.) or mulching MSP (MINI-BMS 125, Seppi, Mezzolombardo, Italy) using excavators (Figure S2). The harvested area was divided into three plots (each approximately 30 m wide × 30 m long): 1) lower-slope bucket scarification MSP (hereafter, called the lower-bucket plot; Figure S2), 2) mid-slope mulcher MSP without raking (middle-mulcher plot), and 3) upper-slope mulcher MSP with raking (upper-mulcher + rake plot). In the latter plot, planting spots were perforated with a rake attachment to the mulcher for manual planting facilitation, involving cutting dwarf bamboo rhizomes and soil loosening. Notably, machine operations occurred in rainy weather with occasional interruptions during the first half of the June 30 to July 4 operational period (Figure S3).

The study site was vertically divided along the slope. Bareroot Sakhalin fir seedlings were placed on one side, whereas container seedlings were positioned on the other (Figure S2). On July 8, 2015, post-MSP, planting occurred at a 2.58 m × 2.58 m spacing (equivalent to 1503 plants ha⁻¹). Container seedlings grown in JFA300 (cell capacity: 300 mL; density: 178 seedlings m⁻²; Zenbyouren, Tokyo, Japan) and JFA150 (cell capacity: 150 mL; density: 292 seedlings m⁻²; Zenbyouren) were planted each in alternating rows. This led to six rows of each seedling type (three each of JFA300 and JFA150) along the slope direction. Weeding occurred solely in summer 2019, the fifth-year postplanting, using motor-manual brush cutters in line with the previous year’s survey findings of dwarf bamboo height matching Sakhalin fir seedlings.

Field measurements

For each stock type (bareroot, JFA300, and JFA150), study individuals were selected from the two rows nearest the study site’s centerline (n = 71 for bareroot; n = 70 each for JFA300 and JFA150). Seedling height (measured to 1 cm accuracy with a ruler) and root collar diameter (RCD; measured to 0.1 mm accuracy with a digital caliper) were assessed on July 8, 2015, postplanting, and at each growing season’s conclusion (late fall October–November or the ensuing early spring May) until the eighth-year postplanting (final measurement on October 4, 2022). Additional data, including vitality status, animal damage, and motor-manual brush cutter miscutting during weeding, were recorded for each seedling. The height:RCD (H:D) ratio was calculated to assess stem shape. For annual relative growth rates of height (RGR_H) and RCD (RGR_D) in each year, RGR = ln(S₂) − ln(S₁) (Hunt, 1982) was used, where S₂ and S₁ represent the size (height or RCD) of the seedling at season’s end and start, respectively. Notably, data equivalence was assumed between late fall and the subsequent early spring, given minimal seedling height and diameter change over winter, i.e., the nongrowing season. Therefore, RGR was presented as an annual figure for convenience, despite calculations not being founded on precise one-year intervals. Notably, RGR for the first planting year spanned from July 8, immediately after planting, to the growing season end, a period shorter by two months compared with the second through eighth years due to summer planting.

Recovery of vegetation removed during MSP in 2015 was assessed three years later in June 11, 2018. No weeding was performed between MSP and vegetation assessment. For the central two rows of container seedlings where survival and growth were examined, we documented the species and the maximum height of the most dominant plant covering surviving Sakhalin fir seedlings (n = 63). Competing vegetation was categorized as evergreen dwarf bamboo and deciduous trees, shrubs, and herbs.

During the October 2017 survey, we observed suppressed vegetation and reduced planted seedling growth on forwarder trails where multiple return trips had been made during the harvest (Figure S4a). Subsequently, we noted whether the studied seedlings grew on the forwarder trail. In June 2018, aerial photographs of the study site were captured using an unmanned aerial vehicle (Phantom 4 Pro, DJI, Shenzhen, China), yielding an orthoimage to confirm suppressed vegetation growth locations on the forwarder trail (Figure S4b).

Statistical analyses

Table 1 provides the initial count of measured seedlings. Distribution of seedlings within and outside the forwarder trail was unbalanced, especially with a limited number of bareroot seedlings inside the trail (Figure S4b). The effect of the trail was unintended in the original experimental design. Owing to evident forwarder trail impact on seedling growth, the statistical analysis was divided into two patterns. First, only container seedling data were analyzed to assess the forwarder trail effect, excluding bareroot seedlings seldom placed on the trail. Second, analysis of stock type differences (bareroot vs. container seedlings) considered only data representing planting outside the trail. It was not possible to differentiate the MSP method and slope location effects owing to site size limitations and thus the plot effects included both MSP and slope location effects. Preliminary analysis indicated minimal plot effects on height, RCD, H:D ratio, RGR_H, and RGR_D over the eight-year study period (Figures S5, S6). Hence, the MSP method and slope location effects were not nearly discussed in this study, although their results were described. R version 4.2.2 (R Core Team 2022) was used to conduct all statistical analyses. Significance was set at α = 0.05.

Table 1. Summary of the initial seedling counts per plot, categorized by combined mechanical site preparation (MSP) methods, slope position, and forwarder trail.
Plot name	Lower-bucket		Middle-mulcher		Upper-mulcher + rake		Total
MSP	Bucket		Mulcher		Mulcher + Rake
Slope position	Lower		Middle		Upper
Forwarder trail	Outside	Inside	Outside	Inside	Outside	Inside
Bareroot	21	0	26	0	22	2	71
Container-JFA150	14	6	17	9	17	7	70
Container-JFA300	14	6	17	9	16	8	70
Subtotal	49	12	60	18	55	17
Total	61		78		72		211

Height, RCD, H:D ratio, RGR_H, and RGR_D differences among stock types per year were evaluated using Tukey’s multiple comparison test through the “multcomp” package’s glht function (Hothorn et al. 2008) for data outside the forwarder trail. Additionally, differences in RCD, H:D ratio, RGR_H, and RGR_D across years were examined via Tukey’s multiple comparison test for year effects in gaussian GLMs. These models featured year and stock type as fixed effects. The container seedling dataset was used to analyze differences between the outside and inside of the forwarder trail in terms of height, RCD, H:D ratio, RGR_H, and RGR_D. This analysis included using LMMs with random effects of plot and container type. To analyze the relationship of the sixth-year survival decline in the forwarder trail with seedling size, LMMs were applied to container seedlings surviving in the forwarder trail until the fifth year, with height and RCD differences assessed between sixth-year surviving and deceased container seedlings. In addition, LMMs were used to investigate RGR_H and RGR_D differences in the fifth year between seedlings competing with dwarf bamboo and those with deciduous vegetation. This analysis included random effects of plot and stock type. Three seedlings in competition with deciduous herbs, Carex sp., were excluded owing to substantial soil moisture and thus small seedling size. All LMMs were analyzed using the “lmerTest” package’s lmer function (Kassambara 2023). Notably, although weeding was done in the summer of the fifth year, the height growth of Sakhalin fir seedlings in that year should have been completed prior to weeding and thus not affected by vegetation release, as the height growth of Sakhalin fir seedling typically reaches a plateau by late June (Funakoshi 1985).

Survival rates in the second and eighth years postplanting were assessed through GLMs with binomial distributions using the “brglm2” package’s glm function (Kosmidis and Firth 2021), based on Firth’s bias correction for complete separation (Firth 1993). Survival in the second year was analyzed to determine the effect of summer planting, whereas eighth-year survival was analyzed to determine the effects of silvicultural practices. Height and RCD eight years postplanting were analyzed using gaussian GLMs through the lm function in R. Height and H:D ratio at planting, seedling type (bareroot or container seedlings), inside or outside the forwarder trail, and plot were explanatory variables, and explanatory variable multicollinearity was confirmed using adjusted generalized standard error inflation factors (values < 3) through the “car” package’s vif function (Fox and Weisberg 2019). H:D ratio was chosen over RCD as the initial seedling shape value owing to limited RCD overlap between bareroot and container seedlings. For the second and eighth-year survival GLMs and eighth-year height GLM, Akaike’s information criterion (AIC)-based model selection was performed using the “MuMIn” package’s dredge function (Bartoń 2023). No significant difference emerged between JFA150 and JFA300 container seedlings in preliminary analyses (JFA150 and JFA300 values, respectively, with p-values for type II analysis of deviance: survival in the second year: 96% and 99%, p = 1.00; survival in the eighth year: 79% and 74%, p = 0.61; height after eight years: 206 cm and 216 cm, p = 0.16; and RCD after eight years: 43.8 mm and 46.4 mm, p = 0.20). Consequently, JFA150 and JFA300 were treated as undifferentiated container seedlings.

Competing vegetation

In 2018, three years post-MSP, competing vegetation included, in order of frequency, the deciduous tree Betula platyphylla Sukaczev, the deciduous shrubs Rubus idaeus L. subsp. melanolasius Focke and Aralia elata (Miq.) Seem, followed by the evergreen dwarf bamboo S. senanensis. Deciduous shrubs and trees represented 43% and 30% of the total frequency, respectively, with dwarf bamboo accounting for only 16% (Fig. 1a). Vegetation height was ~80–150 cm for trees, shrubs, and dwarf bamboo, comparable to Sakhalin fir seedling height. In contrast, herb height was ~30–80 cm (Fig. 1b). Across plots, the upper-mulcher + rake plot showed higher dwarf bamboo occurrence (Fig. 1c). As of May 2021, six years post-MSP and planting, the lower-bucket plot exhibited no dwarf bamboo regeneration (Figure S7a), whereas the middle-mulcher and upper-mulcher + rake plots experienced regeneration (Figure S7b, c). Planted Sakhalin fir seedlings maintained a marked height advantage over dwarf bamboo even after eight years at the end of the study period.

Survival rate

Sakhalin fir seedlings planted in July outside the forwarder trail in standard reforestation soil exhibited >10% reduced survival in the first year for bareroot seedlings, whereas both container seedling types exhibited high survival rates (Fig. 2a). From the second to eighth year of planting, all stock types consistently achieved survival rates ≥95% (Fig. 2a), leading to >75% survival at the study’s end (Fig. 2b). Plot-wise survival varied, with the upper-mulcher + rake plot exhibiting relatively lower survival in the first year (Fig. 2c). After eight years, survival was around 75% in the lower-bucket and upper-mulcher + rake plots, whereas the middle-mulcher plot showed around 90% survival (Fig. 2d). Comparing container seedling survival inside and outside the forwarder trail, annual survival rates tended to be lower within the trail in the sixth and seventh years (Fig. 2e), resulting in a cumulative survival rate of 65% inside the trail, contrasting with 82% outside the trail after eight years (Fig. 2f). Regarding container seedlings inside the forwarder trail, analysis of surviving and dead seedlings in the sixth year showed that surviving seedlings had significantly greater height and RCD than their deceased counterparts (height: p = 0.0016; RCD: p = 0.0016; Table 2).

Table 2. Table 2. Comparison of height and root collar diameter (RCD) five years after planting between surviving and dead container seedlings on the forwarder trail in the sixth year. Mean ± standard error with corresponding p-values from the linear mixed model.
	Height (cm)	RCD (mm)	n
Surviving	87 ± 6	22.1 ± 1.4	32
Dead	39 ± 9	11.7 ± 1.5	7
p-value	0.0016 **	0.0016 **

The GLM for two-year survival with the lowest AIC included stock type and plot effects as explanatory variables (Table 3). Container seedlings showed a significant positive effect on two-year survival compared with bareroot seedlings (p < 0.001; Table 3). The upper-mulcher + rake plot exhibited a nonsignificant negative effect compared with the lower-bucket plot. For eight-year survival, GLM with the lowest AIC included stock type and forwarder trail effects as explanatory variables. Inside the forwarder trail exhibited a significantly negative impact on eight-year survival compared with outside (p = 0.006)), whereas container seedlings showed a positive nonsignificant effect on eight-year survival.

Table 3. Explanatory variable results in the best general linear models for two-year and eight-year survival, selected based on Akaike’s information criterion. Significant effects are indicated in bold.
		Two-year survival				Eight-year survival
Fixed effects	Category	Estimate	SE	z-value	p-value	Estimate	SE	z-value	p-value
Intercept		1.61	0.53	3.00	0.002	−0.05	0.47	−0.12	0.908
Stock type (vs. bareroot)	Container	2.07	0.56	3.69	<0.001	0.54	0.37	1.45	0.143
Plot	Middle-mulcher	0.25	0.71	0.35	0.728	Not selected
(vs. lower-bucket)	Upper- mulcher + rake	−0.87	0.62	−1.40	0.163
Forwarder trail (vs. outside trail)	Inside trail	Not selected				−1.08	0.39	2.74	0.006
Initial height		Not selected Not selected				Not selected
Initial H:D ratio		Not selected Not selected				Not selected

Seedling growth

For seedlings planted in standard soil outside the forwarder trail, bareroot seedlings exhibited significantly higher height and RCD at planting compared with container seedlings. This disparity persisted for eight years postplanting, although no significant difference compared with JFA300 seedlings was noted in the sixth and eighth years (Fig. 3a, b). Although JFA150 seedlings were significantly taller than JFA300 seedlings at planting, the difference became insignificant after one year. The H:D ratio was significantly higher in container seedlings relative to bareroot seedlings at planting, subsequently declining in container seedlings by the second year, and maintaining minimal differences among seedlings or years after the third year (Fig. 3c).

Owing to apical shoot death induced by summer planting, bareroot seedlings demonstrated a remarkably negative RGR_H in the first year, significantly lower than container seedlings (Fig. 4a). From the second year onward, all stock types displayed predominantly positive RGR_H until the eighth year, with negligible differences among them after the third year. RGR_D was significantly lower in bareroot seedlings compared with the two types of container seedlings in the first and second years, maintaining relatively consistent values among stock types after the third year (Fig. 4b). In terms of year-wise differences, RGR_H reached its lowest and highest levels in the first and 3rd year, respectively (Fig. 4a), whereas RGR_D was lowest and highest in the first and second year, respectively (Fig. 4b). There was no discernable trend for higher RGR_H and RGR_D in the fifth year with weeding or the subsequent year compared to the preceding year.

Comparing container seedlings positioned inside and outside the forwarder trail, seedling height was significantly greater outside the trail from the second to the eighth-year postplanting (Fig. 5a). RCD was significantly higher outside the trail from the second to the fifth-year postplanting, with no significant difference observed from the sixth to the eighth year (Fig. 5b). H:D ratio also showed significance, with values outside the trail being higher from the fourth to eighth year (Fig. 5c). Although RGR_H was significantly higher outside the trail from the second to fourth year postplanting, no consistent trend emerged after the fifth year (Fig 6a). RGR_D was significantly higher outside the trail in the second year after planting, with no significant differences observed in other years, except for the fifth year (Fig. 6b).

For seedling height eight years postplanting, the best GLM comprised initial height and forwarder trail effects, indicating positive and negative effects from initial height and planting inside the forwarder trail, respectively (Table 4). The best GLM for RCD eight years postplanting included initial height and H:D ratio effects but excluded the forwarder trail effect. Notably, stock type and plot effects were not selected in the best models for height and RCD.

Table 4. Explanatory variable results in the best general linear models for seedling height and root collar diameter (RCD) eight years postplanting, selected based on Akaike’s information criterion. Significant effects are shown in bold.
		Height after eight years				RCD after eight years
Fixed effects	Category	Estimate	SE	t-value	p-value	Estimate	SE	t-value	p-value
Intercept		112.2	31.4	3.58	<0.001	39.15	9.56	4.09	<0.001
Initial height		3.3	0.8	4.02	<0.001	0.42	0.18	2.31	0.022
Forwarder trail (vs. outside trail)	Inside trail	−42.6	14.4	-2.96	0.004	Not selected
Initial H:D ratio		Not selected				−0.13	0.08	−1.5	0.136
Stock type		Not selected				Not selected
Plot		Not selected				Not selected

In a comparison of growth in the fifth year between seedlings competing with deciduous woody and herbaceous vegetation versus evergreen dwarf bamboo, RGR_H was significantly lower with evergreen dwarf bamboo compared to deciduous vegetation (Fig. 7a). However, RGR_D exhibited no significant differences between the two vegetation types (Fig. 7b).

Success in Sakhalin fir reforestation with one weeding operation in eight years

In this comprehensive study, where the entire reforested area was subjected to MSP, the planted Sakhalin fir seedlings exhibited no significant decline in survival or growth rate over an eight-year period (Figs. 2–4) despite facing competitive vegetation with a single weeding operation in the fifth year. The successful reforestation, without conventional weeding sustained for seven consecutive years (Nakagawa 2013), can be attributed to the extensive implementation of MSP, which effectively eradicated the potentially robust competitor, evergreen dwarf bamboo (S. senanensis). This intervention involved bucket scarification and mulching via a forestry mulcher across the site, leading to the replacement of dwarf bamboo with deciduous trees and herbaceous growth post-MSP (Fig. 1). Previous studies also highlighted the efficacy of intense scarification treatments in curbing S. senanensis resurgence, thereby enhancing Sakhalin fir regeneration and survival in a mixed boreal temperate forest under selective logging (Nakagawa and Kurahashi 2005; Yoshida et al. 2005; Yoshida et al. 2022).

Although this study included a less dense proliferation of dwarf bamboo (Fig. 1a, b) compared with typical growth patterns (Figure S1), the competitive influence of dwarf bamboo still resulted in reduced height growth of Sakhalin fir seedlings, especially in comparison to competing with deciduous vegetation (Fig. 7a). Had whole-site MSP not been implemented and weeding executed only once over the eight-year planting period, a denser dwarf bamboo cover would have likely ensued, intensifying the coverage of planted seedlings and markedly hampering their growth and survival.

Conversely, the post-MSP regeneration of deciduous trees, shrubs, and herbs throughout the reforested area showed diminished competitiveness with the established Sakhalin fir seedlings (Fig. 7a). Evidently, RGR_H and RGR_D displayed no consistent increase following vegetation release after fifth-year weeding (Fig. 4). Notably, the competing vegetation, largely composed of deciduous trees and herbs (76%; Fig. 1a), achieved a height similar to that of Sakhalin fir seedlings (Fig. 1c) during this period. Renowned for being highly shade-tolerant (Iijima et al. 2015; Kitao et al. 2018; Kitao et al. 2019), Sakhalin fir’s resilience to vegetation competition is consistent with findings suggesting that shade-tolerant tree species experience minimal growth variation before and after vegetation release (Wright et al. 2000). Moreover, the evergreen nature of Sakhalin fir seedlings enables them to use bright light for photosynthesis before leaf expansion in early spring and after leaf shedding in late autumn when the deciduous vegetation canopy remains underdeveloped. This feature is particularly advantageous as the growth of the current-year shoots in evergreen conifers depends on photosynthates assimilated by preexisting shoots during spring to early summer (Hansen and Beck 1990, 1994; Wyka et al. 2016). The abundant light availability during spring, when deciduous species pose as competitors, augments Sakhalin fir seedling growth. These combined factors contribute to the success of Sakhalin fir reforestation, with minimal weeding performed over eight years, ensuring little impact on their survival and growth.

Negative effects of forwarder trail

The forwarder trail yielded significant negative effects on the survival and growth of implanted Sakhalin fir seedlings, with timing variations noted for survival, RGR_H, and RGR_D (Fig. 2e, f; Fig. 5a, b; Tables 3 and 4). Although heavy machinery typically exerts its most pronounced influence immediately after operation, without a cumulative increase over time (Rab 2004; Cambi et al. 2015; Sohrabi et al. 2019), a noteworthy finding was the significant negative survival effect appearing in the sixth year following the operation (Fig. 2e), rather than promptly postoperation. The sixth-year survival downturn on the forwarder trail may be attributed to soil desiccation due to low July precipitation in that year (2020), totaling a mere 44 mm, approximately one-third of the normal level (Figure S8b), accompanied by only four days of rainfall in July (Figure S8c). The repeated passage of heavy machinery can induce soil compaction, diminishing water permeability, hydraulic conductivity, soil macropore space, and root expansion and elongation, and ultimately reducing plant water availability (Kozlowski 1999; Startsev and McNabb 2000; Sutherland 2003; Labelle et al. 2022). In the present study, the forwarder traversed wet soils during light rain or immediately after rainfall on a relatively steep slope of approximately 20°, implying severe soil compaction (Cambi et al. 2015; Picchio et al. 2020). The recovery of severe soil compaction generally takes more than a decade and may not reach full recovery (Rab 2004; Cambi et al. 2015; Sohrabi et al. 2019; DeArmond et al. 2021), whereas mild compaction can be fully restored (Zenner et al. 2007; Goutal et al. 2012). Moreover, the adverse effects of compaction on tree seedlings in brown forest soil, as in the current study, are documented elsewhere (Picchio et al. 2019; Sohrabi et al. 2019; Sugai et al. 2020, 2022), although the impact of soil compaction on tree seedlings varies by soil type (Mariotti et al. 2020).

Significantly smaller individuals died on the forwarder trails in the sixth year (Table 2), implying that compacted soil inhibited both root elongation and height growth (Naghdi et al. 2016), thereby impeding water absorption during the drought in that summer and causing desiccation-induced death (Picchio et al. 2020). Despite similarly low July precipitation levels in the seventh and eighth years (2021 and 2022, respectively; Figure S8), survival rates were not significantly reduced in these years (Fig. 2e). This result may be attributed to robust sixth-year survivors possessing deep roots in the moist soil, enabling water absorption during subsequent droughts, and safeguarding against mortality in the sixth to eighth years, which were marked by summer dry spells.

Regarding growth dynamics, the forwarder trail negatively affected RGR_H up to the fourth year postplanting (Fig. 6a), whereas RGR_D was negatively affected only in the second year (Fig. 6b). This dissimilarity in duration may be attributed to tree seedlings favoring height growth over diameter expansion under competitive conditions (Richardson et al. 1996; Collet and Chenost 2006; Pinto et al. 2018; Sugai et al. 2023). Although soil compaction can directly impede tree seedling growth (Kozlowski 1999; Cambi et al. 2015; Mariotti et al. 2020), it can indirectly promote growth by inhibiting the growth of competitive vegetation (Figure S4a) (Brais 2001). Consequently, growth reduction due to soil compaction was absent outside the forwarder trail, whereas negative effects from competing vegetation were more pronounced for RCD than for height growth. Contrastingly, within the forwarder trail, discernible growth reduction owing to soil compaction was prominent in competing vegetation. Thus, RCD growth within the forwarder trail possibly reached levels akin to those outside the trail earlier than height growth. The relatively limited influence of soil compaction on diameter growth compared with height growth in meta-analyses examining soil compaction effects (Ampoorter et al. 2011; Mariotti et al. 2020) may be due in part to the suppression of competing vegetation overgrowth by soil compaction, as shown in the present study.

Effects of stock type and summer planting

The influence of summer planting on the growth and survival of Sakhalin fir seedlings exhibited stock type divergence, with container seedlings showing greater resilience to negative effects compared with bareroot seedlings (Figs. 2 and 3). This aligns with the general tendency for container seedlings to endure transplant shock and challenging site conditions more effectively (Grossnickle and El-Kassaby 2016). A study summarizing Sakhalin fir container seedling planting trials in Hokkaido also revealed robust survival rates exceeding 90% in 13 out of 16 cases for June–September planting, a period deemed unsuitable for planting bareroot seedlings (Harayama and Tsuyama 2020). These findings support the feasibility of summer planting for Sakhalin fir container seedlings.

Despite July being considered unfavorable for bareroot seedling planting, a notably high cumulative survival rate exceeding 80% was evident in the second planting year (Fig. 2b). This outcome can be attributed to significantly higher than average preplanting and postplanting rainfall in this study (Figure S3), with July precipitation in the planting year double the historical average for that month (Figure S8). This increased soil moisture effectively mitigated planting shock, explaining the observed high survival rate in bareroot seedlings. Nonetheless, a noticeable reduction in height due to apical shoot dieback was observed exclusively in bareroot seedlings (Fig. 4a), suggesting that, under typical rainfall conditions, bareroot seedling survival would likely be even lower than in this study.

After eight years of summer planting, seedling height and RCD tended to be greater for bareroot seedlings than for container seedlings (Fig. 3a, b). However, growth rates exhibited minimal divergence between stock types beyond the third year (Fig. 4a, b). GLM analyses revealed that the observed height advantage of bareroot seedlings after eight years was primarily attributed to their larger initial height, rather than a simple stock type effect (Table 4). These findings are consistent with the general trend that once acclimatized in the field, both bareroot and container seedlings show comparable growth rates, with larger seedlings at planting yielding better growth (Grossnickle and El-Kassaby 2016). In the present study, the influence of initial seedling height on height and RCD after eight years may have been diminished compared with conventional site preparation methods, given that MSP shifted competitive vegetation from highly competitive dwarf bamboo to less competitive deciduous vegetation (Fig. 1). In other words, the typical advantage of taller seedlings outgrowing competing vegetation (Rose and Scott Ketchum 2003; Thiffault et al. 2014; Grossnickle and MacDonald 2018b) may have been diluted in the presence of less competitive vegetation due to the strong shade tolerance of Sakhalin fir.

Planting bareroot seedlings during the optimal spring or fall seasons would likely lead to a more pronounced height difference between bareroot and container seedlings after eight years, because there will be no top-shoot withering due to summer planting. While container seedlings are often smaller than bareroot seedlings globally as demonstrated in this study (Fig. 3a, b), advancements of seedling production programs could yield larger container seedlings (Grossnickle and El-Kassaby 2016). Equivalently sized container seedlings can perform equally well (Thiffault et al. 2014) or even outperform bareroot seedlings (Morrissey et al. 2010). The smaller container seedlings with a higher H:D ratio at planting tend to prioritize diameter growth over height until reaching a more balanced ratio (Yagihashi et al. 2016; Ivetic et al. 2017; Harayama et al. 2023a). In this study, container seedlings with higher H:D ratio (Fig. 3c) exhibited high RGR_D and low RGR_H in the second year (Fig. 4a, b), suggesting potential for improved growth with optimized production practices leading to larger and more suitable H:D ratios in Sakhalin fir container seedlings.

MSP can alter vegetation from highly competitive dwarf bamboo to less competitive deciduous trees and herbs. Consequently, even when the weeding frequency is reduced from the conventional once a year for seven consecutive years to once in the fifth year, the survival and growth of shade-tolerant Sakhalin fir seedlings can be maintained, significantly reducing the weeding costs. Additionally, planting within compacted soil forwarder trails negatively impacts seedling survival and growth, with reduced survival due to low summer rainfall, even six years postplanting. Furthermore, summer planting of container Sakhalin fir seedlings minimally affects survival and growth, and taller container seedlings with the appropriate H:D ratio exhibit improved postplanting growth.

Acknowledgments
We would like to thank Tomohiro Saitou of the Shimokawa Town Government for providing and managing the study site, and Mitsutoshi Umemura, Qingmin Han, Takami Saito, Tatsuya Sasaki, Daisuke Kabeya, Toshimitsu Nagasawa, and Haruka Yamamoto of the Forestry and Forest Products Research Institute for helping with field investigations and data entry.

Funding: This study was supported by the Ministry of Agriculture, Forestry, and Fisheries of Japan (25093C, 18064868) and the Forestry Insurance Center (“Advanced evaluation of meteorological damage risk based on the elucidation of the damage process”).

Availability of data and material: The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Author contributions
Conceptualization: HH, HU; Methodology: HH; Formal analysis and investigation: HH, IT, TY, MK, NF, KY, TS, AU, SS, HU; Writing - original draft preparation: HH; Writing - review and editing: HH, IT, TY, MK, NF, KY, TS, SS, HU; Funding acquisition: HH, SS, HU. All authors contributed to the article and approved the submitted version.

Statements and Declarations

Competing Interests: The authors declare that they have no conflict of interest.

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2Supplement.pdf

Eight-year survival and growth of Sakhalin fir (Abies sachalinensis) seedlings with one weeding operation: Impact of mechanical site preparation, vegetation release, summer planting, stock type, and forwarder trail

Status:

Journal Publication

Version 2

Abstract

Figures

Introduction

Materials and Methods

Study site, site preparation, and planting design

Field measurements

Statistical analyses

Results

Competing vegetation

Survival rate

Discussion

Success in Sakhalin fir reforestation with one weeding operation in eight years

Negative effects of forwarder trail

Effects of stock type and summer planting

Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 2