Permalink : http://hdl.handle.net/2115/76213

JaLCDOI : 10.14943/EJFR.21.1.1

KEYWORDS : allometry;coarse wood production;height;stem diameter

Himalayan subalpine forests with a wide altitudinal range of distribution are ideal target for quantifying the change in biomass dynamics along altitude. We estimated aboveground biomass and coarse wood production rate of subalpine Abies spectabilis forest on a north-facing slope in Langtang National Park, Nepal, over the entire altitudinal range from 3170 to 3820 m a.s.l. We established 36 plots (3251 m2 in total) for closed-canopy stands, and additional sapling plots in open-canopy sites (772 m2) in October 2015. We recorded stem diameter at breast height D and top height H for all trees (H ≥ 2.0 m) and saplings (2.0 >H ≥ 0.2 m). We measured recent five-year radial growth in D for all canopy trees in the plots from stem-core samples and recorded recent three-year height growth of all saplings by annual bud scars on leader shoot. We quantified altitude-dependent change in D-H relationship, by extended allometric equation with asymptotic H. We estimated aboveground biomass (AGB) using an allometric equation between D2H and aboveground tree biomass W. For canopy trees with past D estimates in 2010, we estimated past H from D-H allometry, and past W as well. Based on the change in W, we estimated aboveground coarse wood production rate (CWP) as the annualized increment of AGB for surviving trees. Tree height H of Abies spectabilis at any given D decreased with altitude. Relative growth rate (RGR) of W decreased with W and altitude. RGR of sapling height increased with altitude for taller saplings (> 0.5 m), whereas it decreased with altitude for shorter ones. AGB of Abies trees in 36 plots was 489 Mg ha–1 and CWP was 4.88 Mg ha–1 year–1, indicating relatively slow biomass turnover rate by tree growth (CWP/AGB) at 1% per year. AGB and CWP decreased with altitude. CWP relative to AGB also decreased with altitude. Altitude-dependent decline in canopy height, AGB and CWP/AGB suggests adaptation to ambient conditions for the maintenance of forest structure.