热浪发生期提前对草甸草原碳水循环关键过程影响研究

One of the surprising characteristics of global warming is the increasing number and frequency of heat waves, particularly those occurring in springs and early summers at high latitudes that have greatly intensified. Undoubtly, they would change the ecological and physical processes responsible for ecosystem functions such as carbon and water cycles. Unfortunately, we have almost no knowledge on the degree and mechanism of their impacts on key ecosystem processes. Through a manipulative experiment, we propose to test a central hypothesis that early occurring heat waves would alter key processes of carbon and water cycles that in turn would result in significant different seasonal and inter-annual dynamics. Our full factorial experiment will be conducted in a representative Stipa baicalensis meadow steppe, located at high latitude of Inner Mongolia where early occurring heat waves will likely produce a much stronger impacts than those during the summer. Three level of heat waves during early-summer and mid-summer with absence of heat wave control will be simulated with two levels of mowing -- the most widely-spread human disturbance in the region. We will install a continuous monitoring system to quantify the changes in processes of the carbon and water cycles. Specifically, short-, medium-, and long-term manipulations of early- and mid- summer heat wave, and absence of heat wave will be implemented at two levels of mowing treatments (mowed and unmowed) at four replicated plots to explore the underline mechanism on various measures of carbon and water. At the same time, two eddy covariance systems with >10 years continuous data, one at fenced and another one at mowed meadow steppe, will be maintained throughout the duration of this project to analyze the relationship between different period heat waves and carbon and water key processes. We anticipate some ground breaking results from this study on how the meadow steppe may respond to the interactive driving forces of heatwaves and mowing. Lessons learned from this study will greatly enhance the leadership role of China in global change ecology.

全球变暖导致热浪频发背景下，热浪发生期有明显提前趋势，这势必会影响生态系统碳水循环，由于该方面研究十分匮乏其影响程度和机理尚不明晰，因而成为当前全球变化研究方向紧迫和前沿问题之一。内蒙古属高纬度半干旱生态脆弱区，遭受提前热浪侵袭几率更大、危害更强。本项目拟以内蒙古贝加尔针茅草甸草原为研究对象，结合该区域最常见的人类干扰（刈割），通过野外原位人工模拟热浪控制实验，结合微生态环境自动监测系统，系统研究热浪发生期提前和刈割及其交互作用对碳水循环关键过程（光合作用、呼吸作用和蒸发散等）的短、中、长期影响及其调节机制。并结合同区域围封和刈割2个通量站点各10年的涡度相关数据，捕捉历史热浪事件，全面阐释热浪发生期提前对生态系统碳水循环关键过程造成的影响及其机理。本项目不仅为草地畜牧业有效经营管理提供理论依据，还将极大地提升我国在全球变化生态学研究领域中的引领作用。

热浪（Heat Wave，HW）作为近年来频发的一种极端气候事件，预计未来将会有更高频率、强度和更广的范围，并且热浪发生期有明显提前的趋势，这必将影响生态系统碳水循环。本研究以内蒙古羊草草甸草原为研究对象，结合该区域最常见的人类干扰（刈割，M），自2018（预实验）年至2021年进行了为期4年的野外原位人工模拟热浪控制实验，来研究热浪发生期提前与刈割及其交互作用对碳水循环关键过程的瞬时、过后与遗留效应的影响及其调节机制。主要结论如下：①热浪显著减低了碳交换。无论热浪发生期提前（HW7）或正常热浪期热浪（HW8），均导致生态系统碳交换下降，分别降低净生态系统CO2交换（NEE）和生态系统总生产力（GEP）为37%和 15%。②热浪显著降低了土壤含水量（SWC）。HW7和HW8分别比对照降低SWC1.9%和2.3%。③热浪发生期提前对生态系统带来更大的影响。热浪发生期提前更大程度地降低了生态系统碳交换。与HW8相比，HW7热浪影响更强，使生态系统NEE下降 41%~56%，HW8碳吸收降低20%~31%。此外，在热浪期间增加了生态系统呼吸（Re），而在热浪结束后，Re低于正常水平。同时HW7能够抑制土壤呼吸、土壤自养呼吸和土壤异养呼吸，HW8促进了土壤呼吸、土壤自养呼吸和土壤异养呼吸。④刈割处理降低了生态系统对热浪的敏感性，相比HW8，这种降低热浪敏感性效果对HW7更明显。刈割能够显著增加生态系统NEE、Re和GEP，分别增加了20%、15%和18%（2019-2020年），而刈割处理与热浪相互作用时，刈割降低了生态系统对热浪的敏感性，使HWM7负面影响降低18%，然而这种减缓作用在HW8时并不明显。刈割处理对土壤呼吸不尽相同，刈割处理降低了HW7热浪的土壤呼吸，却促进了HW8的土壤呼吸。⑤刈割能够增加而热浪降低了生物量积累。刈割降低建群种羊草重要值，增加了次年生态系统群落物种多样性以及丰富度。热浪处理同样能够增物种丰富度，与单纯的HWs相比，刈割将HWs对生态系统碳通量的负面影响降低了约15%，促进了豆科物种的增加，进行适度刈割处理是一种抵御热浪对碳通量负面影响的有效方式。

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