Inter-annual variations in biometric-based net primary production and heterotrophic respiration among three different forest types over 7 years in a cool-temperate zone, Japan.

Forest ecosystems play an important role as a carbon sink in global carbon cycle. The carbon balance in forest ecosystems is affected by forest structure such as age, tree density and dominant species. This study aimed to clarify the differences in net primary production (NPP) and heterotrophic respiration (HR) among three different forest types under same climate condition over 7 years. Our study area is located on the southern slope of Mt. Asama, central Japan. We chose three middle-aged stands (about 50 years old) dominated by deciduous broad-leaved tree (Quercus serrata ;Q), de ciduous conifer (Larix kaempferi ;L), and evergreen conifer (Pinus densiflora ;P), respectively. A permanent 30×30 m quadrat was established in each forest. Net ecosystem production (NEP) was estimated biometrically by the difference between NPP(= tree biomass increment ( ΔB) ＋current year litter fall (LF) ) and estimated HR using trenching method from 2012 to 2018. In addition, we analyzed the relationships between carbon fluxes and climate conditions (solar radiation, temperature, precipitation and frequency of disturbance events).

Inter-annual variations of LF were mostly within small range in three forests. But LF in P forest was slightly high in 2013 and 2017. It was probably because our study area was hit by a big typhoon and young pine leaves would fall down in these years. On the other hand, inter-annual variations of ΔB showed different patterns among three forests. ΔB in L and P forests showed similar fluctuations and high values in 2015, probably due to high temperature around the leaf expansion period. Whereas, ΔB in Q forest was much higher in 2017 and 2018 and there was strong positive correlation between ΔB and annual daylight hours (R=0.78). In contrast to the NPP variation, HR showed similar inter-annual variations among three forests and it especially depended on summer soil temperature. As a result, NEP showed similar fluctuation pattern to that of ΔB and the annual NEP values in Q, L, P forests were estimated to be 2.2 ±0.5, 2.2 ±0.4, 2.0 ±0.3 tC ha ^-¹ yr ^-¹, respectively.

These results revealed that inter-annual variation in NEP was strongly affected by fluctuation of ΔB and the major environmental factors influencing the ΔB would differ among the forest types even under the same climate condition.