Spatio-temporal Dynamics Of Tibet Plateau Net Primary Production Using CASA Model

Climate change is increasingly affecting land surface processes of terrestrial ecosystem, which has becomes one of research focuses in terrestrial carbon cycle. Estimating vegetation production and monitoring spatio-temporal dynamics of Tibet Plateau, which a sensitive region to climate change, is crucial to study of global climate change and carbon cycle. Using remote sensing and GIS technologies, based on the light utility efficiency principle and CASA model, we estimated the CASA-based Absorbed Photosynthetic Active Radiation (APAR) and the light utility efficiency (ε), and set up a model to estimate the terrestrial Net Primary Production (NPP) of Tibet Plateau region utilizing the MODIS data and meteorological data to drive the model. Meanwhile, we estimated the terrestrial NPP of Tibet Plateau region during2003-2010. And the temporal and spatial distribution and change of NPP was analyzed. The conclusions are as following:(1) The precision of NPP estimation model, validated by field investigation data, can reach by72.2%in Tibet Plateau region. APAR on Tibet Plateau is between0and3250MJ/m2a, mainly distributes from200MJ/m2a to1600MJ/m2a, the average value is560MJ/m2a and the average annual APAR is1.46109MJ. The spatial distribution of APAR shows a rising tendency from northwest to southeast gradually.ε on Tibet Plateau is between0.252g C/MJ and0.510g C/MJ, the average is0.34g C/MJ. It presents a fluctuant decreasing tendency from2003to2010.(2) The accumulation of vegetation NPP is related to its growth period closely, and it appears a normal single-peak curve in a year. The curve reaches the peak value of15.83g C/m2in July and the lowest value of0.88g C/m2in December. The NPP on Tibet Plateau is between0g C/m2a and1118g C/m2a, mainly distributes from70g C/m2a to800g C/m2a, the average value is298g C/m2a and the average annual NPP is3.541014g C. The spatial distribution of NPP shows a rising tendency from northwest to southeast gradually, maily distributes in the areas where the altitude is between3000m and5000m, and the NPP in the areas accounts for79.48%of the overall NPP in Tibet Plateau region.(3) It is obviously different in average annual NPP and overall NPP between different grassland classes. The average annual NPP of alpine meadow and alpine steppe is217.33g C/m2and85.75g C/m2, and the overall NPP is1.391014g C and3.691013g C, respectively; the total of the two grassland classes accounts for49.78%.(4) The overall NPP on Tibet Plateau is increasing slowly from2003to2010. Compared to the seasonal changes of NPP in Tibet Plateau region, the NPP is increasing slowly in spring and summer, and decreasing in autumn and winter. The total NPP of spring, summer, autumn and winter accounts for18.78%,56.04%,17.56%and7.62%of average annual NPP, respectively.(5) The spatial dynamic change of NPP from2003to2010shows that NPP has no obvious variation in northwest, and there is a downtrend of NPP in southwest and a uptrend in southeast of Tibet Plateau.(6) Vegetation NPP on Tibet Plateau is influenced by precipitation, accumulated temperature and average temperature, in which the influence of precipitation is more significant.