Effects of forest dieback on wood decay, saproxylic communities, and spruce seedling regeneration on coarse woody debris
Introduction
Picea is one of the most dominant conifer genera in boreal, montane and subalpine forests in the Northern Hemisphere and consists of 28–56 species, depending on the classification system: the highest species diversity is found in East Asia (Ran et al., 2006). Because of their dominance, Picea play key roles in ecosystems and are important for timber production and reducing the risks of avalanches, rockfall, and soil erosion in certain localities (Panayotov et al., 2011). Although shifts in the Picea distribution range under global warming are of major concern, studies to date have been restricted to the northern limits of the forests in Europe (Seppä et al., 2009), North America (Mimura and Aitken, 2007), and Russia (Kremenetski et al., 1998), with few having been conducted in the southern limits of the forests in East Asia (Aizawa et al., 2009). Investigating regeneration mechanisms in Picea forests along the southern border of their range is important for understanding the future dynamics and distribution of Picea forests under global climate change.
It is well known that some Picea species require coarse woody debris (CWD) as a seedbed for regeneration (Mori et al., 2004; Bače et al., 2012). Given that the physicochemical properties of CWD change constantly as it decomposes, the decay class (DC; decay stage) of CWD and differences in the decay types of fungi—the organisms primarily responsible for wood decay in forest ecosystems—are important for the regenerative success of Picea (Mori et al., 2004; Fukasawa, 2012; Fukasawa et al., 2017; Fukasawa and Ando, 2018a). Fungal wood decay type is traditionally grouped into one of three categories: white rot, brown rot, or soft rot, reflecting fungal preference for lignocellulose decomposition and wood moisture content (Eaton and Hale, 1993). Bače et al. (2012) reported that Picea abies seedlings preferentially regenerate on logs in which white rot rather than brown rot fungi dominate, possibly due to the nutrient-poor, acidic, and fragile nature of brown-rotted wood. Furthermore, Fukasawa (2015) found a clear increase in the frequency of brown-rotted wood in pine CWD from north to south. These results suggest that Picea may face more challenges in seedling regeneration on CWD in their southernmost localities.
The southernmost populations of Picea (Picea jezoensis var. hondoensis) exist on the main island of Japan, distributed as fragments among several subalpine areas as remnants of the vegetation of the last glacial period (Aizawa et al., 2009). In one of those populations, a serious decline in the forest was observed after severe blowdown disturbance caused by a large typhoon in 1959 (Akashi and Nakashizuka, 1999). Numerous studies have focused on the negative effects of grazing by increased deer and mouse populations on seedlings in the affected forest areas, and as a result, many fences have been put in place to prevent deer grazing (e.g. Akashi and Nakashizuka, 1999; Shibata et al., 2008; Kisanuki et al., 2009). However, successful colonisation of Picea seedlings on CWD has not yet been recorded in this area.
From a microbial point of view, forest disturbance should be a major factor affecting fungal communities in CWD and wood decay because canopy openness, which generally increases after forest disturbance, has been reported to be an important factor affecting fungal communities in CWD (Bässler et al., 2010, Bässler et al., 2016; Lehnert et al., 2013; Horak et al., 2016; Krah et al., 2018). Vogel et al. (2017) reported the marked dominance of a brown rot fungus Fomitopsis pinicola after severe dieback in a P. abies forest in central Europe. Because the CWD decay process of subalpine conifer tree species are known to take several decades (Yin, 1999), it is expected that the effects of forest disturbance on fungal communities and CWD decay as well as the knock-on effects on seedling establishment would be long-lasting. Furthermore, our previous study found that wood decay type affects not only tree seedlings but also bryophyte communities on CWD (Fukasawa et al., 2015), which have a pivotal role in Picea seedling establishment (Iijima and Shibuya, 2010; Ando et al., 2017; Fukasawa and Ando, 2018a). It has been reported that bryophyte communities were severely damaged after forest dieback in the focal area (Oishi and Doei, 2015), and thus this damage might also affect seedling establishment. In addition to the direct effects of the microclimatic changes after disturbance (Jonsson and Esseen, 1990), the effect of wood decay should also be considered when predicting bryophyte and seedling colonisation changes (Fukasawa et al., 2019). The effect of wood decay type on Picea seedling regeneration may be a complex phenomenon including interactions with bryophyte species.
In the present study, we aimed to evaluate the effect of forest dieback on Picea seedling establishment in one of its southernmost populations in central Japan, which was severely damaged by a large typhoon in 1959. We hypothesised that dieback intensity affects the fungal communities, wood decay type, and bryophyte and seedling communities of CWD. Potential links between variables associated with spruce seedling density were evaluated by structural equation modelling.
Section snippets
Study sites
This study was conducted in a subalpine coniferous forest (34˚ 11′ N, 136˚ 06′ E, 1550–1650 m a.s.l.) on Odaigahara Plateau, central Japan (Fig. S1). The mean annual temperature of the study area is 6.4 °C and the mean annual precipitation is over 4500 mm (Shibata et al., 2008). The bedrock is sandstone. The area was formerly dominated by P. jezoensis var. hondoensis, but the forest in this area experienced severe damage caused by a Category 5 typhoon, named Vera or the Isewan typhoon, in
Fungal communities in decay class II snags
The mean diameter at breast height of the investigated DC II snags was 18.9 cm, 23.7 cm, and 25.2 cm for the control, mid-level, and intensive dieback sites, respectively, and was significantly smaller at the control site than the two dieback sites (one-way ANOVA, F = 9.531, P < 0.001; Tukey's HSD, P < 0.05).
A total of 1,714,800 reads were obtained using Illumina MiSeq sequencing after filtering. The obtained 653 OTUs consisted of 413 Ascomycota, 231 Basidiomycota, 4 Chytridiomycota, 4
Discussion
The present study clearly shows that forest dieback intensity has a significant effect on fungal communities in dead wood. A noteworthy finding was the increase in the frequency of brown rot fungi in dieback sites as compared with the control site. The dominance of brown rot fungi in the disturbed forest was also reported in a previous study (Vogel et al., 2017). Even though there are various causes of dieback (e.g. blowdown, pests, clear-cutting, and forest fires), a common feature of forests
Conclusions
In the present study, forest dieback that began due to severe blowdown disturbance in 1959 had marked impacts on the current wood decay type, fungal communities in wood, bryophyte cover and Picea seedling establishment. No significant direct link between forest dieback intensity and Picea seedling density was detected in our SEM. This is likely because of the strong relationship between seedling density and bryophyte cover, which is strongly affected by dieback intensity. In addition to the
Acknowledgements
We wish to thank Kayo Honobe and Takashi Higuchi for providing vegetation data of the study site, and to Masakuni Kimura and Masaki Michimori for their help in bryophyte identification in the field. We are also grateful to Syuichi Shichimeki and Kosuke Kanno in Kinki branch office of Ministry of the Environment Japan for the permission for our fieldwork. This study was financially supported by Japan Society for the Promotion of Science KAKENHI Grant Number 26850093 to YF.
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