利用叶片荧光参数估算油蒿灌丛群落生态系统生产力

Estimating ecosystem productivity from leaf chlorophyll fluorescence parameter in Artemisia ordosica shrub community

生态系统生产力(GEP)在全球碳循环中具有重要意义,但其准确估算仍然是一个挑战。近年来,叶绿素荧光和冠层GEP的关联成为生态学的研究热点,关系尚不清楚且存在广泛争议。于2015年对宁夏盐池毛乌素沙地荒漠灌木油蒿(Artemisia ordosica)灌丛生态系统碳交换(NEE)特征进行连续观测,使用多通道荧光监测仪对通量贡献区内油蒿叶片的实时荧光(F_(s))和光下荧光(F_(m)′)进行原位连续监测,叶面积指数(LAI)、归一化植被指数(NDVI)以及环境因子同步观测。利用光合有效辐射(PAR)、LAI和实际光化学效率(Φ_(PSII))等参数构建基于叶绿素荧光的生态系统生产力(GEP_(ChlF))模型,探究叶片和冠层尺度不同参数对环境因子波动的响应,比较分析GEP_(ChlF)和基于涡度相关法监测生态系统生产力(GEP_(EC))相关性及GEP_(ChlF)的适用性。研究发现,Φ_(PSII)和NEE日变化规律一致,Φ_(PSII)、GEP_(ChlF)和GEP_(EC)由PAR控制,受空气温度(T_(a))和饱和水汽压差(VPD)调控,土壤含水量(SWC)和Φ_(PSII)呈显著正相关(P<0.01)。PAR处于400—800μmol m^(-2 )s^(-1)时,GEP_(ChlF)与GEP_(EC)线性关系最优,斜率为0.627(R^(2)=0.67,P<0.01);弱光下GEP_(ChlF)的低估可能是由于冠层实际光能拦截率高造成;强光下GEP_(ChlF)显著高于GEP_(EC),呈非线性关系,可能是GEP_(EC)基于夜间温度敏感性所拟合的生态系统呼吸(R_(e))无法预测光呼吸部分所导致。GEP_(ChlF)具有荧光参数的特性,对环境波动更加敏感,和环境因子相关性优于GEP_(EC)。结果表明,高辐射、极端温度、高蒸腾和干旱是限制叶片和冠层尺度下油蒿光合过程的主要胁迫因素。本研究构建的荒漠生态系统光合过程模型GEP_(ChlF)能够替代GEP_(EC)作为一个良好的冠层尺度生态参数,所提出的空间尺度上推方法可为促进区域可持续发展提供数据支撑和决策参考。

Accurate estimation of gross ecosystem productivity(GEP)remains a challenge despite its importance in the global carbon cycle.Recently,the relationship between chlorophyll fluorescence and canopy GEP becomes an important scientific concern in plant ecology.However,it remains unclear how they are linked at multiple spatial scales across the growing season.In this study,continuous measurements of net ecosystem CO_(2) exchange(NEE)were made in a desert shrub(Artemisia ordosica)ecosystem using eddy covariance(EC)technique from May to October 2015 in Mu Us desert.The real⁃time fluorescence(F_(s))and fluorescence under light(F_(m)′)were also measured in situ using the Monitoring⁃PAM multi⁃channel fluorometer.Leaf area index(LAI),the normalized difference vegetation index(NDVI),and environmental factors including photosynthetically active radiation(PAR),air temperature(T_(a)),vapor pressure deficit(VPD),soil water content(SWC)and precipitation(PPT)were measured simultaneously within the range of carbon flux contribution.We developed a chlorophyll fluorescence⁃based model with input variables of photochemical efficiency(Φ_(PSII)),PAR,and LAI to estimate ecosystem productivity(GEP_(ChlF)).We then compared the applicability of GEP_(ChlF) with EC⁃based GEP(GEP_(EC))and examined the responses of different parameters to environmental factors at leaf and canopy scales.As a result,the diurnal pattern ofΦ_(PSII) was similar to that of NEE,being mainly controlled by PAR and modified positively by T_(a) and VPD(P<0.01).TheΦ_(PSII) had a significantly positive correlation with SWC(P<0.01).The GEP_(ChlF) agreed well with GEP_(EC) when PAR was 400-800μmol m^(-2) s^(-1),with a slope of 0.627(R^(2)=0.67,P<0.01).The GEP_(ChlF) was likely to be underestimated under low PAR,leading to a high light interception rate of canopy,thus resulting in a low absorbed photosynthetically active radiation(APAR)in the model.The GEP_(ChlF) was significantly higher than GEP_(EC)under high PAR and their relationship was nonlinear,which may be due to Artemisia ordosica released excessive sunlight energy for photo⁃respiration,nitrogen metabolism,Miller reaction and other processes.It was noted that photo⁃respiration was difficult to predict accurately because the daytime ecosystem respiration(R_(e))was simulated on the basis of nocturnal temperature sensitivity.Compared with GEP_(EC),chlorophyll fluorescence⁃derived GEP_(ChlF) was more sensitive to environmental fluctuations and had a better relation with environmental factors.Our results confirmed that the GEP_(ChlF) from chlorophyll fluorescence⁃based model could replace GEP_(EC) as a good ecological parameter at canopy scale in consideration of the plant physiological status.We found that excessive radiation,extreme temperature,high transpiration and drought were the main stress factors limiting the photosynthetic process of Artemisia ordosica at different spatial scales.The study provides a method for estimating ecosystem productivity under the stressed environmental conditions on the basis of monitoring chlorophyll fluorescence in relation to environmental factors.The present results may scientifically support decision⁃making for promoting regional sustainable development.