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Observations of photosynthetically active radiation (PAR) and global solar radiation ( G) at Wuhan, Central China during 2005-2012 were first reported to investigate PAR variability at different time scales and its PAR fraction ( F) under different sky conditions. Both G irradiances ( I) and PAR irradiances ( I) showed similar seasonal features that peaked in values at noon during summer and reached their lower values in winter. F reached higher values during either sunrise or sunset; lower values of F appeared at local noon because of the absorption effects of water vapor and clouds on long-wave radiation. There was an inverse relationship between clearness index ( K) and F; the maximum I decreased by 22.3 % (39.7 %) when sky conditions changed from overcast to cloudless in summer (winter); solar radiation was more affected by cloudiness than the seasonal variation in cloudy skies when compared with that in clear skies. The maximum daily PAR irradiation ( R) was 11.89 MJ m day with an annual average of 4.85 MJ m day. F was in the range of 29-61.5 % with annual daily average value being about 42 %. Meanwhile, hourly, daily, and monthly relationships between R and G irradiation ( R) under different sky conditions were investigated. It was discovered that cloudy skies were the dominated sky condition in this region. Finally, a clear-sky PAR model was developed by analyzing the dependence of PAR irradiances on optical air mass under various sky conditions for the whole study period in Central China, which will lay foundations for ecological process study in the near future. [ABSTRACT FROM AUTHOR]

Net primary productivity (NPP) is an important component of the carbon cycle and a key indicator of ecosystem performance. The aim of this study is to construct a more accurate regional vegetation NPP estimation model and explore the relationship between NPP and climatic factors (air temperature, rainfall, sunshine hours, relative humidity, air pressure, global radiation, and surface net radiation). As a key variable in NPP modeling, photosynthetically active radiation (PAR) was obtained by finding a linear relationship between PAR and horizontal direct radiation, scattered radiation, and net radiation with high accuracy. The fraction of absorbed photosynthetically active radiation (FPAR) was estimated by enhanced vegetation index (EVI) instead of the widely used normalized difference vegetation index (NDVI). Stress factors of temperature/humidity for different types of vegetation were also considered in the simulation of light use efficiencies (LUE). The authors used EVI datasets of Moderate Resolution Imaging Spectroradiometer (MODIS) from 2001 to 2011 and geographic information techniques to reveal NPP variations in Wuhan. Time lagged serial correlation analysis was employed to study the delayed and continuous effects of climatic factors on NPP. The results showed that the authors' improved model can simulate vegetation NPP in Wuhan effectively, and it may be adopted or used in other regions of the world that need to be further tested. The results indicated that air temperature and air pressure contributed significantly to the interannual changes of plant NPP while rainfall and global radiation were major climatic factors influencing seasonal NPP variations. A significant positive 32-day lagged correlation was observed between seasonal variation of NPP and rainfall (P , 0.01); the influence of changing climate on NPP lasted for 64 days. The impact of air pressure, global radiation, and net radiation on NPP persisted for 48 days, while the effects of sunshine hours and air temperature on NPP only lasted for 16 and 32 days, respectively. [ABSTRACT FROM AUTHOR]

As the difference between photosynthesis, or gross primary productivity (GPP), and autotrophic respiration (RA), net primary productivity (NPP) is a key component of the terrestrial carbon cycle. The temporal and spatial response of NPP to climate change is thus one of the most important aspects in the study of climate-vegetation relationship. In this study, we developed a new method to estimate NPP accurately by finding a linear relationship between solar radiation and photosynthetically active radiation (PAR) and improving maximum light use efficiency (LUE) of vegetation, which could be adopted and used in other regions of the world. We utilize normalized difference vegetation index (NDVI) datasets of Moderate Resolution Imaging Spectroradiometer (MODIS) from 2001 to 2010 and geographic information system (GIS) techniques to reveal the monthly and interannual change of NPP in Wuhan, China. We also applied the lagged cross-correlation analysis method to study the delayed and continuous effects on monthly and interannual variations of NPP to climatic factors (air temperature, precipitation, total radiation and sunshine percentage). The result showed that precipitation and total radiation were the major climatic factors influencing monthly variation of NPP, and sunshine percentage mostly determined the interannual variation of NPP for different vegetation. Monthly NPP showed significant positive correlation with total radiation of that month, and the effect could persist for one month; significant positive one month lagged correlation was also observed between monthly variation of NPP and precipitation, and the influences of changing climate on NPP would last for two months. [ABSTRACT FROM AUTHOR]