干旱对亚热带人工针叶林碳交换的影响

DROUGHT EFFECTS ON CARBON EXCHANGE IN A SUBTROPICAL CONIF-EROUS PLANTATION IN CHINA

干旱对陆地生态系统的影响已成为全球变化研究的焦点问题之一。该研究基于生态系统过程模型——CEVSA2,结合涡度相关通量观测,分析了不同程度干旱对亚热带人工针叶林碳交换的影响及其关键控制因素。结果表明:1)干旱使生态系统碳交换显著下降,2003和2004年的干旱使得年净生态系统生产力(Net ecosystem produc-tion,NEP)相比无干旱影响情景的模拟结果分别减少了63%和47%;2)光合和呼吸对干旱具有不同的响应,干旱时光合的下降比呼吸更为显著,这导致了NEP的显著下降;3)当饱和水气压差(Vapor pressure deficit,VPD)达到1.5kPa以上时,生态系统的光合、呼吸和净碳吸收均开始下降,当VPD大于2.5kPa、土壤相对含水量(土壤含水量/土壤饱和含水量)(Relative soil water content,RSW)低于40%时,生态系统的碳收支由碳汇转为碳源;4)土壤干旱是造成碳交换下降的主要驱动因素,对年NEP下降的平均贡献率为46%,而大气干旱的贡献率仅为4%。

Aims Drought effects on terrestrial ecosystems are a key issue in global change research. This study was designed to 1） analyze effects of drought on carbon exchange in a subtropical coniferous plantation; 2） elucidate the sensitivity of carbon exchange to different degree of water deficit and the critical values when the ecosystem converts from carbon sink to source and 3） investigate the main factors that control ecosystem carbon exchange when drought occurs. Methods The CEVSA2 model, which incorporated several significant modifications based on the CEVSA process-based ecosystem model and has been tested by using eddy covariance observation in different forest ecosystems, was parameterized by using site-specific ecophysiological measurements. Drought scenarios were designed to analyze effects on annual carbon budget and to elucidate the main control factors. Important findings Drought decreases ecosystem production and carbon exchange significantly. Compared with simulation of no drought effect scenario, the droughts in 2003 and 2004 decrease annual net ecosystem production （NEP） by 63% and 47%, respectively. Ecosystem photosynthesis and respiration respond to drought differently, and the more rapid decrease of gross ecosystem production （GEP） than ecosystem respiration （Re） lead to the decrease of NEP when drought occurs. As daily average vapor pressure deficit （VPD） rises above 1.5 kPa, GEP, Re and NEP begin to decrease; When VPD rises above 2.5 kPa and relative soil water content （RSW; soil water content/saturated soil water content） decreases below 40%, the ecosystem converts from a carbon sink to source. Soil water deficit, which is the main factor controlling the ecosystem carbon exchange, accounts for 46% to the decrease of total annual NEP in 2003 and 2004, and atmospheric drought accounts for only 4%.