Aboveground biomass increments over 26\xa0years (1993\u20132019) in an old-growth cool-temperate forest in northern Japan

Mahoko Noguchi,Kazuhiko Hoshizaki,Tomoyuki Saitoh,Tsutomu Yagihashi,Katsuhiro Osumi,Tomohiro Itabashi,Takashi Masaki,Kazunori Takahashi,Ohta Kazuhide,Mitsue Shibata,Daisuke Hoshino,Wajirou Suzuki,Michinari Matsushita,Daiki Sugiura

doi:10.1007/s10265-021-01358-5

Abstract

Assessing long-term changes in the biomass of old-growth forests with consideration of climate effects is essential for understanding forest ecosystem functions under a changing climate. Long-term biomass changes are the result of accumulated short-term changes, which can be affected by endogenous processes such as gap filling in small-scale canopy openings. Here, we used 26 years (1993–2019) of repeated tree census data in an old-growth, cool-temperate, mixed deciduous forest that contains three topographic units (riparian, denuded slope, and terrace) in northern Japan to document decadal changes in aboveground biomass (AGB) and their processes in relation to endogenous processes and climatic factors. AGB increased steadily over the 26 years in all topographic units, but different tree species contributed to the increase among the topographic units. AGB gain within each topographic unit exceeded AGB loss via tree mortality in most of the measurement periods despite substantial temporal variation in AGB loss. At the local scale, variations in AGB gain were partially explained by compensating growth of trees around canopy gaps. Climate affected the local-scale AGB gain: the gain was larger in the measurement periods with higher mean air temperature during the current summer but smaller in those with higher mean air temperature during the previous autumn, synchronously in all topographic units. The influences of decadal summer and autumn warming on AGB growth appeared to be counteracting, suggesting that the observed steady AGB increase in KRRF is not fully explained by the warming. Future studies should consider global and regional environmental factors such as elevated CO2 concentrations and nitrogen deposition, and include cool-temperate forests with a broader temperature range to improve our understanding on biomass accumulation in this type of forests under climate change.

Highlights

Old-growth forests are widely recognized to play an important role in the carbon cycle (Harmon et al 1990)
We examined the effects of climatic condition in each measurement period, canopy gap formation and topography on local-scale aboveground biomass (AGB) gain using two linear mixed-effect models (LMMs)
In model 1, we aimed to investigate whether the amount of AGB gain differed among the measurement periods with the effects of topographic unit and gap formation in the current and previous measurement periods

Summary

Introduction

Old-growth forests are widely recognized to play an important role in the carbon cycle (Harmon et al 1990). It has been commonly accepted that old-growth forests are carbon neutral (Odum 1969) and their living biomass is at ‘steady state’ (Bormann and Likens 1979). Continuous increases in aboveground biomass (AGB) have been found in temperate (e.g., Foster et al 2014; Keeton et al 2011) and tropical (e.g., Baker et al 2004; Lewis et al 2009; Phillips et al 1998; Qie et al 2017) old-growth forests. Lewis et al (2004) argued that recent remarkable AGB increases in old-growth Amazonian tropical forests was induced by climate change.

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