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Carbon dioxide balance in early-successional forests after forest fires in interior Alaska

森林火災後の初期植生遷移段階にある内陸アラスカの北方林におけるCO$$_{2}$$収支

植山 雅仁*; 岩田 拓記*; 永野 博彦; 田原 成美*; 岩間 千絵*; 原薗 芳信*

Ueyama, Masahito*; Iwata, Hiroki*; Nagano, Hirohiko; Tahara, Narumi*; Iwama, Chie*; Harazono, Yoshinobu*

北米の北方林生態系において、森林火災は主要なかく乱要因であり、当該地域における炭素収支を決定する重要なプロセスである。本研究では内陸アラスカの2つの森林火災跡地において、合計13年間にわたり主要な温室効果ガスである二酸化炭素(CO$$_{2}$$)の交換量を渦相関法によって観測した。観測データから、火災後の生態系が植生の回復に伴ってCO$$_{2}$$の放出源から吸収源になるためには13年かかることが分かった。さらに、観測データを基に火災後の初期植生遷移段階におけるCO$$_{2}$$収支の広域評価を内陸アラスカ全土に対して行ったところ、1998年から2017年までの間に火災後の生態系から放出されたCO$$_{2}$$量(35-48 Tg C)は、火災に伴う燃焼によって大気へ直接放出されたCO$$_{2}$$量(156 Tg C)の1/3から1/4であると見積もられた。以上より、火災後に生態系から放出されるCO$$_{2}$$量は、州スケールでのCO$$_{2}$$収支に重要であることが示唆された。

Fire is the major disturbance in North American boreal forests, and is thought to be the most important process that determines the carbon balance in North American boreal forests. This study conducted four years of tower flux measurements in a burned ecosystem from one to four years after a fire, and nine years of measurements in a young regeneration from five to 13 years after a fire in interior Alaska. The fire scar acted as a source of 248 g C m$$^{-2}$$ yr$$^{-1}$$ one year after the fire, and the annual CO$$_{2}$$ emissions continuously decreased until seven years after the fire. At the final year of the study period, 13 years after the fire, the older forest became a CO$$_{2}$$ sink. During the 13 years after the fires, the total post-fire emissions were 767 g C m$$^{-2}$$ across both sites. Gross primary productivity (GPP) and ecosystem respiration (RE) recovered to those of mature black spruce forests 10 years after the fire. The successional recovery of GPP was mostly explained by the recovery of the leaf area index (LAI). Anomalous weather, such as a cold spring, hot summer, and high summer rainfall, increased the CO$$_{2}$$ emissions rather than the uptake. In interior Alaska, the post-fire CO$$_{2}$$ emissions (35-48 Tg C) were estimated to be approximately one third to fourth of the direct CO$$_{2}$$ emissions (156 Tg C) by combustions from 1998 to 2017, which indicates that post-fire emissions are important to the regional CO$$_{2}$$ balance. The forest successional trajectory at young age still contains large uncertainties due to lack of data, and thus adding new data improves our understanding of the post-fire CO$$_{2}$$ balance.

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