森林生态系统碳汇:概念、时间效应与提升途径

Carbon sink of forest ecosystems:Concept,time effect and improvement approaches

工业革命以来,广泛使用化石燃料向大气中排放大量二氧化碳(CO_(2)),导致气候变暖、极端气候事件频发。为有效缓解气候变化,国际社会在减少碳排放和清除大气中CO_(2)等方面做了各种努力。2020年9月我国政府郑重承诺:CO_(2)排放力争于2030年前达到峰值、2060年前实现碳中和(“双碳”目标)。根据目前已有预测结果,至2060年碳中和时,在化石能源使用、生产、生活所有排放均达到最低条件下,我国不得不排放的CO_(2)约为目前排放总量的1/4,这些碳排放主要由生态系统吸收;即便达到碳中和,工业革命以来大气中增加的约140 ppm CO_(2)仍然需要由生态系统清除。森林是陆地生态系统的主体,其碳汇能力占陆地生态系统碳汇的80%以上;但是,由于森林生态系统的长周期性、复杂性与动态变化性,生态系统碳汇存在基本概念不清、时间效应不明等问题,导致森林碳汇能力提升技术受限、碳汇交易市场混乱等诸多问题。为此,本文首先从光合作用吸收CO_(2)固定在植物体内的过程,梳理了森林生态系统碳汇概念,结合地上-地下过程,介绍了森林生态系统碳汇形成机理。其次,分析了依赖时间尺度的森林生态系统碳汇变化过程,讨论了森林碳汇的时间效应,建议在尚未明确碳汇对缓解气候变化的时间效应前,使用“吨-年”(t-year)计量碳汇,并将3~6个月作为碳汇的最小计量时间(即,当年固碳的开始)。第三,明确了提升森林生态系统碳汇能力的主要途径:最大限度增加森林生态系统的碳汇能力(扩大森林面积、提高森林质量、增加森林土壤碳储量等);尽量延长森林生态系统碳汇维持时间,即降低“临时碳汇”(固定在森林生态系统中的碳一定时间内重新排放到大气中),增加“持久碳汇”(森林生态系统从大气中清除CO_(2),并在一定时间内不再排放到大气中;根据《巴黎协定》相关内容,碳汇计量的上限时间为2100年);推广维持持久碳汇的措施(用木材取代钢筋、水泥、塑料等,减少林火等干扰排放,将森林生物质以“生物炭”形态固定下来等)。最后,提出发展以人工智能(AI)为核心技术的“气候智慧林业(climate-smart forestry)”,以期为提升森林生态系统碳汇能力和可持续管理提供理论与技术支撑。

The widespread utilization of fossil fuels has emitted large amounts of CO_(2) into the atmosphere since the Industrial Revolution,leading to climate warming and frequent occurrence of extreme climate events.To effectively alleviate climate change,the international community has made various efforts to reduce carbon emissions and eliminate CO_(2) from the atmosphere.In 2020,the Chinese government announced that carbon emission peaking and car-bon neutrality will be achieved by 2030 and 2060,respectively.According to the current forecast,by the time car-bon neutrality is achieved in 2060,even under the minimum conditions of fossil energy use,production,and living emissions,China will still have to emit about 1/4 of the current total emissions.These carbon must primarily be absorbed by ecosystems.Furthermore,approximately 140 ppm increase in CO_(2) in the atmosphere since the Industrial Revolution still needs to be removed by ecosystems.Forests are the main component of terrestrial ecosystems,con-tributing more than 80%of the carbon sequestration capacity of all terrestrial ecosystems.However,due to the long periodicity,complexity and dynamic variability of forests,the basic concepts of ecosystem carbon sink and its time effect are still unclear,leading to problems,such as lacking technologies for improving carbon sink capacity and disorganized rules in the carbon sink trading market.In this review,we introduced carbon sink concept according to the processes of absorbing and fixing CO_(2) by plant photosynthesis in forest ecosystems.Then,we analyzed the processes of time-scale-dependent carbon sinks of forest ecosystems,discussed the time effects of forest carbon sinks,and suggested using“t-year”as the unit of carbon sink(taking 3-6 months as the minimum measurement time,i.e.,the beginning of carbon sequestration).Third,we proposed the approaches to improve the carbon sink capacity of forest ecosystems.One way is to improve the carbon sink capacity(expanding forest area,improving for-est quality,and increasing forest soil carbon storage)of forest ecosystems.Another approach is to maintain the car-bon sink of forest ecosystems as long as possible,i.e.,to reduce temporary carbon sink(definition:carbon in the forest ecosystems emit into the atmosphere for a certain period)and to increase persistent carbon sink(definition:carbon in the forest ecosystems no longer emit into the atmosphere for a certain period;according to the relevant provisions of the Paris Agreement,the upper time limit for carbon sink measurement can be considered to be the year 2100.In order to maintain the persistent carbon sink,strateges such as efficient use of wood products(replace steel,cement,plastic with wood),control of forest fires or other disturbances-induced emissions,and turning for-est biomass into biochar should be taken.Finally,we proposed to develop climate-smart forestry driven by artificial intelligence(AI),which would provide new theoretical and technical support for improving the carbon sink of for-est ecosystems and facilitating sustainable forest management.