Elevational variation in abundance of coarse woody debris in subalpine forests, central Japan
Introduction
In the context of global warming due to the increase of CO2 emissions, forest ecosystems play an important role as a carbon sink in worldwide carbon flux (Goulden et al., 2011). Coarse woody debris (CWD), a long-term pool and source of carbon in forests, is attracting substantial attention from researchers (Harmon et al., 1986, Harmon et al., 2013, Holeksa et al., 2007, Olajuyigbe et al., 2011). The amount of carbon stored in CWD, including dead stems, branches, and roots, varies widely, ranging between 1 and 240 Mg ha−1 (Harmon et al., 1986, Harmon et al., 1990, Delaney et al., 1998, Alberti et al., 2008). Many factors such as climate, stand age, tree taxa and logging affect the amount of CWD (Harmon et al., 1986, Harmon et al., 1990). Therefore, CWD is important for forest management to calculate CO2 absorption in forests. Such information is indispensable for forest managers and policy makers because of carbon trading. CWD mass could greatly increase after disturbances by forest fires, insect attacks, and hurricanes (Westerling et al., 2006, Chambers et al., 2007, Kurz et al., 2008). Climate change will also increase the mass of CWD by increasing the frequency of disturbances (Harmon et al., 2013). Rapid environmental changes have become a particular concern in recent years, so it is important to investigate the mechanism of regional variations in CWD mass within forest ecosystems to predict future global carbon budgets.
In general, the aboveground biomass (AGB) and aboveground net primary production (ANPP) of living trees and the decomposition rates of soil organic matter decrease at high elevations (Kitayama and Aiba, 2002, Raich et al., 2006, Kurganova et al., 2018, Ivanov et al., 2018). Furthermore, the maximum size of trees decreases and the relative frequency of small trees increases at high elevations (Miyajima and Takahashi, 2007). CWD mass is determined by the balance between the input rate (i.e., AGB and ANPP) and the output rate of carbon (i.e., the decomposition rate of CWD) (Grove, 2001). CWD mass may vary depending on the location because of changes of environmental conditions. Thus, prediction of global carbon balance requires a large data set of CWD mass in each region. However, few studies have investigated the CWD mass in Asian temperate forests, compared with the cases on the west coast of North America and for boreal ecosystems in Europe and Russia (Fukasawa et al., 2014).
In mountainous areas, environmental factors such as temperature and precipitation vary greatly along elevational gradients; CWD mass is also considered to vary. Approximately 25% of the land surface of the Earth is covered by mountains (Körner, 2007), so investigating the variation of CWD mass along elevational gradients is necessary for forest management in order to predict future changes in forest carbon flux through global climatic change.
Only a few studies have investigated variations of CWD mass along elevational gradients. Furthermore, the findings in previous studies regarding elevational variations in CWD mass were not consistent. For example, Holeksa et al. (2007) showed that CWD volume decreased slightly from 1100 m to 1600 m above sea level in Slovakia and Poland. Kueppers et al. (2004) also showed that CWD mass decreased from 3000 m to 3500 m a.s.l. in the Rocky Mountains. By contrast, a meta-analysis focusing on Europe reported that CWD mass was greater at higher elevations (Christensen et al., 2005). Against this background, the variations of CWD mass along elevational gradients and the mechanisms causing these variations are not yet well understood, partly because of limited elevational ranges in previous studies (i.e., several hundred meters). Therefore, variations of CWD should be investigated along wide ranges of elevations.
Total CWD mass of a forest stand is affected by the frequency distribution of decay classes of CWD (Merganičová and Merganič, 2010). The wood density (mass per volume) of CWD decreases from fresh CWD (low decay class) to old CWD (high decay class) (Kueppers et al., 2004, Rice et al., 2004). Frequency distributions of decay classes of CWD are expected to vary along elevational gradients because the decomposition rate of CWD is lower at higher elevations due to lower temperature. Therefore, it is necessary to measure frequency distributions of decay classes of CWD in order to correctly estimate CWD mass along an elevational gradient.
We investigated the variation of CWD mass along a wide elevational range (1600–2800 m a.s.l.) in subalpine coniferous forests, central Japan. In this study, we examined (1) AGB, ANPP and CWD mass along an elevational gradient and (2) frequency distributions of CWD in size and decay classes. In this study, we show the decrease of the ratio of CWD mass to AGB at high elevations, and show the reason for this from the perspective of the elevational change of CWD size.
Section snippets
Study site
This study was conducted at five elevations (1600 m, 2000 m, 2300 m, 2500 m, 2800 m a.s.l.) on the eastern slope of Mount Norikura (36°06′N, 137°33′E; summit elevation 3026 m a.s.l., Fig. 1), central Japan, in 2017. Subalpine conifers Abies mariesii Mast., A. veitchii Lindle, and Tsuga diversifolia (Maxim.) Mast. and a deciduous broad-leaved species Betula ermanii Cham. were the dominant tall tree species (Miyajima et al., 2007, Ohdo and Takahashi, 2020). Canopy height was approximately 20–25 m
Plot census, litterfall, CWD volume and mass
In this study, plots were established at 1600, 2000, 2300, 2500, and 2800 m a.s.l., and the plot sizes were 100 m × 100 m, 120 m × 50 m, 100 m × 100 m, 10 m × 50 m, and 2 m × 20 m, respectively. The year of plot establishment was 2004 for 1600 m, 2300 m, and 2500 m a.s.l., 2006 for 2000 m a.s.l., and 2011 for 2800 m a.s.l. Dwarf pine Pinus pumila dominated at the two plots of 2500 m and 2800 m a.s.l. The scrub height of dwarf pine was about 2 m and 1 m at 2500 m and 2800 m a.s.l., respectively.
AGB, ANPP and CWD abundance
AGB significantly decreased with elevation (R = −0.940, P < 0.05, n = 5; Fig. 2a). The maximum AGB among the five plots was 282.4 Mg ha−1 at 1600 m a.s.l. and the minimum AGB was 29.0 Mg ha−1 at 2800 m a.s.l. The AGB of 2800 m was 10.3% of that at 1600 m a.s.l. ANPP significantly decreased with elevation (R = −0.940, P < 0.05, n = 5, Fig. 2b). ANPP of 2800 m was 24.1% of that at 1600 m a.s.l.
CWD volume significantly decreased with elevation (R = −0.984, P < 0.01, n = 5; Fig. 2c). The maximum
Discussion
We showed the significant reduction of AGB at high elevations, which is consistent with several studies (Kitayama and Aiba, 2002, Leuschner et al., 2007, Alves et al., 2010: Girardin et al., 2010, Wang et al., 2014). These previous studies demonstrated that the decrease in forest production due to low temperature was the main cause of the decrease of AGB at high elevations. In this study area, the mean annual growth period (daily mean temperature > 5 °C) decreases from 0.52 years at 1600 m to
Conclusion
Global warming will affect carbon cycling in forest ecosystems along elevational gradients. In particular, CWD is important role on the long-term pool and source of carbon. Nevertheless, the distribution of CWD was unknown along elevational gradients. This study examined the distribution of CWD in subalpine coniferous forests along an elevational gradient (1600–2800 m a.s.l.), in central Japan. Although the observed values of CWD masses (1.04–46.8 Mg ha−1) in this study ranged within reported
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was partially carried out by the joint research program of the Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Japan. Funding for this study was partially supported by the grant from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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