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; Harayama et al., 2023b). 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; Tomimatsu et al., 2011; 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 (Biring et al., 2003; Roy et al., 2010). 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, 2011f 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).