油蒿灌丛生态系统碳交换对降水年际变异响应的滞后性研究

Arid and semiarid ecosystems are widely-distributed both globally and in China, and are highly sensitive to climatic variability. Climate models project a future scenario characterized by larger climatic variability, frequent transitions between wet and dry years and more climate extremes in arid and semiarid areas worldwide. Previous studies have focused primarily on the effects of current-year climatic conditions on carbon exchange between the atmosphere and biosphere. However, past precipitation can have long-term or carryover effects on multiple biotic and abiotic factors, thus leading to lagged responses of ecosystem structure and functioning to inter-annual changes in precipitation. Currently, the importance of current- relative to previous-year precipitation in controlling ecosystem carbon balance remains largely unknown. Furthermore, a large uncertainty exists on the direction and magnitude of precipitation legacy effects, indicating our lack of understanding on how ecosystems respond to past moisture conditions. These knowledge gaps limit our ability to accurately predict ecosystem structure and functioning under a changing climate. This study aims to investigate the responses of ecosystem carbon exchange to the inter-annual variation in precipitation, with particular emphasis on the relative importance of current- vs. previous-year precipitation. For this purpose, we intend to use a combination of methods, including long-term continuous eddy-covariance and soil surface CO2 efflux measurements, manipulation experiments and process-based model simulations, to examine the responses of shrub ecosystems dominated by Artemisia ordosica to either natural or manipulated transitions between wet and dry years. Various vegetation- and soil-related factors will be measured simultaneously for disentangling the mechanisms underlying precipitation legacies. This study will provide insights into a mechanistic understanding of the carbon cycling in arid and semiarid ecosystems, and will aid in the accurate prediction of regional carbon balance under climate change.

干旱/半干旱生态系统是陆地生态系统的重要组分，其对环境波动极为敏感。关于荒漠生态系统碳过程的研究多关注于碳交换对当年气候因子的响应，却忽略了过往气象条件(尤其是降水)的遗留效应。荒漠生态系统诸多生物、非生物因子往往受过去降水持续影响，引起其碳交换的滞后响应。项目拟以毛乌素沙地油蒿(Artemisia ordosica)灌丛生态系统为研究对象，结合碳通量原位观测、降水控制实验和模型模拟等途径，量化生态系统碳交换对降水量年际变异滞后响应的方向和强度，厘清当年降水和过去降水对碳交换的相对影响，进而揭示碳交换对降水变异的滞后响应机理，建立综合当年降水和过去降水的生态系统碳收支估算方法。预期结果将有助于解释荒漠生态系统碳平衡与当年降水的解耦现象，可为气候变化背景下干旱/半干旱生态系统碳收支精准预测提供理论依据。

气候变化影响下，干旱/半干旱区对全球碳水平衡长期变化趋势和短期年际变异具有重要贡献。降水和干旱事件的时间、强度、频率发生改变，将诱发一系列生态系统响应。目前对典型沙生灌丛生态系统碳水交换响应水热条件变化的认识仍十分有限。项目采用野外观测实验、样品分析、数据整合等手段，揭示毛乌素沙地油蒿灌丛生态系统碳水交换及其组分(净生态系统生产力、总生态系统生产力、生态系统呼吸、土壤呼吸、植物光合、生态系统蒸散、植物蒸腾)对降水和干旱事件(土壤干旱、大气干旱)的响应阈值、滞后性和敏感性。结果发现，(1)生态系统净碳交换(NEE)日变化主要受光合有效辐射驱动，提前于气温(Ta)和饱和水汽压差(VPD)约3.5小时；NEE季节变化主要由Ta驱动，但响应滞后约19天；NEE中等时间尺度(周-月)变化主要受降雨和干旱事件导致的土壤含水量变化影响。(2) 春旱导致生态系统生产力、蒸散和水分利用效率受到强烈抑制，而夏旱对碳水交换的影响较小。(3)非生长季降水通过影响春季冠层发育影响碳水平衡年际变异。(4)降水事件不仅通过水分条件影响生态系统碳交换，其导致的辐射组分变化也对植被光合固碳能力产生重要影响。(5)生态系统呼吸受水热因素交互影响，随温度先升高后下降，降水变化导致的生产力变化驱动着生态系统呼吸的季节和年际动态。(6) 土壤呼吸与温度的时滞关系受水分调控，并受到水分条件介导的底物限制，植物光合作用产物向地下的转运可能是土壤有机碳积累、沙化土壤恢复和微生物活性维持的重要碳源。(7)油蒿光合固碳特征(气体交换和光合荧光参数)对干旱、强光、高温等胁迫表现出较强的调节和适应能力。(8)油蒿茎干液流(蒸腾)的季节和年际变异受温度、辐射、土壤水分、VPD和叶面积指数的交互影响。项目研究获得的结果、模型和积累的长期观测数据可为气候变化背景下干旱/半干旱生态系统碳水功能的预测与评估提供科学支撑。