内蒙古草地关键光合过程参数的时空变化特征

Spatio-Temporal Variation in the Key Photosynthetic Parameters of Grassland in Inner Mongolia

地球表面的瞬时光合有效辐射(Photosynthetically Active Radiation,PAR)和植被吸收的光合有效辐射(Absorbed Photosynthetically Active Radiation,APAR)是许多集中于生物圈和大气之间交换过程研究的关键参量。光合有效辐射吸收比例(Fraction of Photosynthetically Active Radiation,FPAR)是APAR和PAR的比值,是许多气候模型及生产力模型的基本特征参量。因此,研究PAR/APAR/FPAR的时空变化规律,对准确估算草地NPP具有十分重要的作用。本文利用MODIS遥感影像和TOMS紫外波段反射率数据,在GIS和RS软件支持下,计算分析了内蒙古草地生长季的PAR/FPAR/APAR及其时空变化,并分析了气候因子对FPAR/APAR的影响。结果表明:内蒙古草地生长季平均PAR为1973.8MJ/m2,FPAR为0.275,APAR为516.64MJ/m2。内蒙古草地生长季的PAR/FPAR/PAR时空变化明显:FPAR及APAR空间上呈东北向西南地区递减的趋势,PAR与之相反。自生长季初期(4月份),随着时间的推移PAR/FPAR/APAR都开始增加,只是出现峰值的时间不同。内蒙古草地生长季不同草地类型的FPAR及APAR差异明显,在草地NPP估算及建模时应引起重视。

The amount of Photosynthetically Active Radiation （PAR, 400nm - 700 nm） incident on the earth＇s surface and the amount of PAR absorbed （APAR） by the vegetative surface of the earth are crucial variables for many studies focusing on exchange processes between the biosphere and the atmosphere. The fraction of photosynthetically active radiation （ FPAR ）, which is the ratio of APAR to PAR, is a key variable in many climatic models and models of net primary production （ NPP ） . FPAR is a measure of how large a fraction of sunlight leaves absorb in the 400nm - 700nm spectrum and FPAR thus expresses a canopy＇ s energy absorption capacity. FPAR is required for estimating the amount of PAR absorbed （ APAR ） by vegetation, which in turn allows for estimation of carbon uptake. China, which has a total area of natural grassland of about 400 million hm^2 , is one of the world＇ s richest countries in terms of grassland resources. Therefore, it is very important to study the spatiotemporal variations of PAR, APAR and FPAR for estimating the NPP of grassland. We used ARCGIS and ERDAS IMAGINE to process MODIS data and TOMS reflectivity of UV band, and estimated PAR, FPAR and APAR of grassland in Inner Mongolia during the growing season. We analyzed spatiotemporal variation and the impact of climate factors on FPAR/PAR. The parameter of PAR was calculated using the ultraviolet（UV） reflectance data at 370 nm which comes from the Total Ozone Mapping Spectrometer（TOMS）. FPAR can be retrieved from MODIS Normalized Difference Vegetation Index （NDVI） using the linear regression between the FPAR and NDVI. The results showed that the calculated PAR/FPAR and APAR values agreed well with previous studies. The method of calculating PAR/FPAR and APAR may be used to model and estimate grassland NPP. The mean value of PAR, FPAR and APAR is about 1 973.8 M J/ m^2, 0.275, and 516.64 MJ/m^2 respectively. The spatiotemporal variation during the growing season was distinctive, with the spatial distribution of APAR and FPAR gradually descending from southwest to northeast, with an opposite trend for PAR. From the beginning of the growing season, the PAR/FPAR and APAR gradually increase, but their values peak at different times. Rainfall is the main restricting factor of grassland FPAR/APAR in Inner Mongolia. The variation of FPAR and APAR for different grass types is very obvious, and should be taken into account for modeling or estimation of grassland NPP.