Dynamic properties of subalpine coniferous forests in Nepal Himalaya and Hokkaido

Abstract

Subalpine forests form the highest forest ecosystems on mountains. Subalpine forests dominated by coniferous species are commonly observed in the northern Hemisphere, which provide a range of ecosystem services to not only local human communities but also those in downstream regions. In this study, I focused on two contrastive subalpine forests in East Asia, a high altitude subalpine forest in Nepal Himalaya under subtropical climate, and a low altitude forest in northern Hokkaido under cool-temperate climate. I examined how forest structure is determined by environmental factors and whether or not forests experience change in biomass dynamics in a subalpine forest dominated by Abies spectabilis in Langtang National Park, Nepal Himalaya and a subalpine forest co-dominated by Abies sachalinensis and Picea glehnii on Mount Oakan, northern Hokkaido so as to quantify the structure and dynamics of two subalpine coniferous forests. In Chapter 1, I quantified the change in forest stand variables due to geographic and anthropogenic factors in Abies spectabilis forest of Langtang National Park. Woody species composition was changed with altitude and trampling intensity. The number of species decreased with altitude and increased with slope inclination and the number of cut stumps. Frequency distribution of tree size in stem diameter at breast height (D) showed monotonous decrease with D. Stands in higher altitude showed lower tree density, basal area and maximum D, while higher density of juvenile trees (D < 1.0 cm). Juvenile density of A. spectabilis was reduced in plots with high basal area. Stands on steeper slopes had higher tree density with smaller maximum D on poorer soil. With increasing cut stumps, basal area and soil carbon content decreased while woody species richness and tree density increased. These results showed that Abies population is vulnerable to livestock trampling, and that altitude-dependent management is needed. In Chapter 2, I estimated the aboveground biomass (AGB) developing an allometric equation between stem diameter D and tree height H of which change with altitude, and D2H versus aboveground total tree mass W using the data of Abies spectabilis in eastern Nepal (Yoda 1968). For canopy trees with records of present D in 2015 and past D estimates in 2010, I estimated past H employing D-H allometry, and past W as well. Based on the change in W for canopy trees per plot, I estimated aboveground coarse wood production rate (CWP) for each plot, as the annualized increment of AGB for survived trees. Tree height of Abies spectabilis at any given diameter was increased with altitude. Growth rate of sapling height was increased with altitude for taller saplings (> 50 cm). AGB and CWP tended to be decreased with altitude. CWP relative to AGB was also decreased with altitude, suggesting slower turnover in high altitude. These results showed that there is huge biomass storage in subalpine Abies spectabilis forest in eastern Himalaya, and that biomass is maintained by slow turnover rate (ca. 1% per annum in CWP/AGB) relative to usual forest ecosystems (1.5 to 2%). Altitude-dependent decline in canopy height, AGB and CWP/AGB suggests adaptation to ambient conditions for the maintenance of forest structure. In Chapter 3, I quantified the species-dependent architectural and demographic properties of Abies sachalinensis and Picea glehnii populations using repeated census data of trees in a 1-ha plot in years 2004 and 2014. Based on D-H allometry for the two species, I estimated AGB with allometry between D2H and W using the data for subalpine Abies veitchii (Tadaki et al. 1970). Based on the change in W over 10 years, I estimated CWP and annualized wood loss rate. At the same D, H of Abies sachalinensis was larger for understory trees with D < ca. 50 cm, while H of Picea glehnii was larger for canopy trees both in year 2004 and 2015. Growth rate of understory trees (D < ca. 30 cm) was larger for A. sachalinensis, whereas that of canopy trees (D > ca. 30 cm) was larger for P. glehnii. Mortality rate of understory trees (D < ca. 30 cm) was lower in A. sachalinensis, whereas that of canopy trees was lower for P. glehnii. There was gradual change abundance from P. glehnii to A. sachalinensis on young volcanic substrate. AGB of ca. 13.9 kg m–2 was maintained with a high demographic turnover rate of 3% per year with CWP of ca. 0.40 kg m–2 year–1, suggesting that stands of the 1-ha plot experienced high disturbance, and recovery enhancing the increasing abundance of A. sachalinensis. The overall results suggest that relatively mild winter in Himalayan subalpine forests may bring about lower disturbance compared to chilly-winter in subalpine forest in Hokkaido. Relatively higher biomass turnover rate in Hokkaido forest can be attributed to the recovery from such frequent disturbances. Co-dominance of coniferous species is likely to be associated with high disturbance in severer climate, which enhance inter-specific differentiation in successional niche.