Aims Light-use efficiency(LUE)is an important tool for scaling up local CO_(2)flux(F_(CO_(2)))tower observations to regional and global carbon dynamics.Using a data set including F_(CO_(2))and environmental factors ob...Aims Light-use efficiency(LUE)is an important tool for scaling up local CO_(2)flux(F_(CO_(2)))tower observations to regional and global carbon dynamics.Using a data set including F_(CO_(2))and environmental factors obtained from an alpine meadow on the Tibetan Plateau,we examined both diurnal and seasonal changes in LUE and the environmental factors controlling these changes.Our objectives were to(i)characterize the diurnal and daily variability of LUE in an alpine meadow,(ii)clarify the causes of this variability,and(iii)explore the possibility of applying the LUE approach to this alpine meadow by examining the relationship between daily LUE and hourly LUE at satellite visiting times.Methods First,we obtained the LUE—the ratio of the gross primary production(GPP)to the absorbed photosynthetically active radiation(APAR)—from the flux tower and meteorological observations.We then characterized the patterns of diurnal and seasonal changes in LUE,explored the environmental controls on LUE using univariate regression analyses and evaluated the effects of diffuse radiation on LUE by assigning weights through a linear programming method to beam photosynthetically active radiation(PAR)and diffuse PAR,which were separated from meteorological observations using an existing method.Finally,we examined the relationships between noontime hourly LUE and daily LUE and those between adjusted noontime hourly and daily LUE because satellites visit the site only once or twice a day,near noon.Important Findings The results showed that(i)the LUE of the alpine meadow generally followed the diurnal and seasonal patterns of solar radiation but fluctuated with changes in cloud cover.(ii)The fraction of diffuse light played a dominant role in LUE variation.Daily minimum temperature and vapor pressure deficit also affected LUE variation.(iii)The adjusted APAR,defined as the weighted linear sum of diffuse APAR and beam APAR,was linearly correlated with GPP on different temporal scales.(iv)Midday adjusted LUE was closely related to daily adjusted LUE,regardless of the cloud cover.The results indicated the importance of considering radiation direction when developing LUE-based GPP-estimating models.展开更多
The accurate measurement of the dynamics of photosynthesis in China’s subtropical evergreen forest ecosystems is an important contribution to carbon(C) sink estimates in global terrestrial ecosystems and their respon...The accurate measurement of the dynamics of photosynthesis in China’s subtropical evergreen forest ecosystems is an important contribution to carbon(C) sink estimates in global terrestrial ecosystems and their responses to climate change. Eddy covariance has historically been the only direct method to assess C flux of whole ecosystems with high temporal resolution, but it suffers from limited spatial resolution. During the last decade, continuous global monitoring of plant primary productivity from spectroradiometer sensors on flux towers and satellites has extended the temporal and spatial coverage of C flux observations. In this study, we evaluated the performance of two physiological remote sensing indices, fluorescence reflectance index(FRI) and photochemical reflectance index(PRI), to measure the seasonal variations of photosynthesis in a subtropical evergreen forest ecosystem using continuous canopy spectral and flux measurements in the Dinghushan Nature Reserve in southern China.The more commonly used NDVI has been shown to be saturated and mainly affected by illumination(R^2=0.88, p <0.001), but FRI and PRI could better track the seasonal dynamics of plant photosynthetic functioning by comparison and are less affected by illumination(R^2=0.13 and R^2=0.51, respectively) at the seasonal scale. FRI correlated better with daily gross primary production(GPP) in the morning hours than in the afternoon hours, in contrast to PRI which correlated better with light-use efficiency(LUE) in the afternoon hours. Both FRI and PRI could show greater correlations with GPP and LUE respectively in the senescence season than in the recovery-growth season. When incident PAR was taken into account, the relationship between GPP and FRI was improved and the correlation coefficient increased from 0.22 to 0.69(p < 0.001). The strength of the correlation increased significantly in the senescence season(R^2=0.79, p < 0.001). Our results demonstrate the application of FRI and PRI as physiological indices for the accurate measurement of the seasonal dynamics of plant community photosynthesis in a subtropical evergreen forest, and suggest these indices may be applied to carbon cycle models to improve the estimation of regional carbon budgets.展开更多
基金Supported by the projects‘Integrated Study for Terrestrial Carbon Management of Asia in the 21st Century Based on Scientific Advancements’and‘Early Detection and Prediction of Climate Warming Based on the Long-Term Monitoring of Alpine Ecosystems on the Tibetan Plateau’funded by the Ministry of the Environment,Japanresearch fund from the Program for New Century Excellent Talents in University,from Ministry of Education,China,to J.C.
文摘Aims Light-use efficiency(LUE)is an important tool for scaling up local CO_(2)flux(F_(CO_(2)))tower observations to regional and global carbon dynamics.Using a data set including F_(CO_(2))and environmental factors obtained from an alpine meadow on the Tibetan Plateau,we examined both diurnal and seasonal changes in LUE and the environmental factors controlling these changes.Our objectives were to(i)characterize the diurnal and daily variability of LUE in an alpine meadow,(ii)clarify the causes of this variability,and(iii)explore the possibility of applying the LUE approach to this alpine meadow by examining the relationship between daily LUE and hourly LUE at satellite visiting times.Methods First,we obtained the LUE—the ratio of the gross primary production(GPP)to the absorbed photosynthetically active radiation(APAR)—from the flux tower and meteorological observations.We then characterized the patterns of diurnal and seasonal changes in LUE,explored the environmental controls on LUE using univariate regression analyses and evaluated the effects of diffuse radiation on LUE by assigning weights through a linear programming method to beam photosynthetically active radiation(PAR)and diffuse PAR,which were separated from meteorological observations using an existing method.Finally,we examined the relationships between noontime hourly LUE and daily LUE and those between adjusted noontime hourly and daily LUE because satellites visit the site only once or twice a day,near noon.Important Findings The results showed that(i)the LUE of the alpine meadow generally followed the diurnal and seasonal patterns of solar radiation but fluctuated with changes in cloud cover.(ii)The fraction of diffuse light played a dominant role in LUE variation.Daily minimum temperature and vapor pressure deficit also affected LUE variation.(iii)The adjusted APAR,defined as the weighted linear sum of diffuse APAR and beam APAR,was linearly correlated with GPP on different temporal scales.(iv)Midday adjusted LUE was closely related to daily adjusted LUE,regardless of the cloud cover.The results indicated the importance of considering radiation direction when developing LUE-based GPP-estimating models.
基金National Key Research and Development Program of China(2017YFC0503803)National Natural Science Foundation of China(41571192)+1 种基金Natural Science Foundation of Hebei,China(D2016302002)Science and Technology Planning Project of Hebei,China(17390313D)
文摘The accurate measurement of the dynamics of photosynthesis in China’s subtropical evergreen forest ecosystems is an important contribution to carbon(C) sink estimates in global terrestrial ecosystems and their responses to climate change. Eddy covariance has historically been the only direct method to assess C flux of whole ecosystems with high temporal resolution, but it suffers from limited spatial resolution. During the last decade, continuous global monitoring of plant primary productivity from spectroradiometer sensors on flux towers and satellites has extended the temporal and spatial coverage of C flux observations. In this study, we evaluated the performance of two physiological remote sensing indices, fluorescence reflectance index(FRI) and photochemical reflectance index(PRI), to measure the seasonal variations of photosynthesis in a subtropical evergreen forest ecosystem using continuous canopy spectral and flux measurements in the Dinghushan Nature Reserve in southern China.The more commonly used NDVI has been shown to be saturated and mainly affected by illumination(R^2=0.88, p <0.001), but FRI and PRI could better track the seasonal dynamics of plant photosynthetic functioning by comparison and are less affected by illumination(R^2=0.13 and R^2=0.51, respectively) at the seasonal scale. FRI correlated better with daily gross primary production(GPP) in the morning hours than in the afternoon hours, in contrast to PRI which correlated better with light-use efficiency(LUE) in the afternoon hours. Both FRI and PRI could show greater correlations with GPP and LUE respectively in the senescence season than in the recovery-growth season. When incident PAR was taken into account, the relationship between GPP and FRI was improved and the correlation coefficient increased from 0.22 to 0.69(p < 0.001). The strength of the correlation increased significantly in the senescence season(R^2=0.79, p < 0.001). Our results demonstrate the application of FRI and PRI as physiological indices for the accurate measurement of the seasonal dynamics of plant community photosynthesis in a subtropical evergreen forest, and suggest these indices may be applied to carbon cycle models to improve the estimation of regional carbon budgets.
