Evaluation of MODIS-based gross primary production by flux tower measurements (Case study: Temperate forests of England)

MODIS-based Gross Primary Production (MODIS-GPP) as a proxy of the potential of vegetation cover in the atmospheric carbon sequestration was evaluated using Flux Tower measurements (Flux Tower-GPP) across three sites, broadleaved temperate deciduous woodlands, in the south of England. Ensuing to the normal evaluation of data distribution, the mean value of MODIS-GPP (cell size: 1km2) compared with the Flux Tower-GPP through the Two-Tailed T-Test (P < 0.05). A significant difference between two groups, MODIS-GPP vs. Flux Tower-GPP, was found that there is a measurable difference between the groups and that, statistically. The results show that Flux Tower-GPP values for both Alice Holt and Wytham Woods sites are higher than the MODIS-GPP, while in Pang Lambourne the opposite is true due to land cover heterogeneity. The trend of seasonal changes of both groups is the same and the increase of GPP in plant phenology is observable in all sites. GPP values gradually increase to early of May, then a prompt rising observed in May that extend to late of summer which arisen from development of solar radiation, day length and photosynthetic activity. In late of summer, GPP values gradually decrease so that by advent of autumn a rapid decline come about in the early of October that corresponds with senescence and abscission, due to decreasing temperature, which reflects the seasonal variation in terms of carbon uptake. The research finding reveals that the MODIS-GPP alone is unable to estimate accurately the potential of vegetation cover in the atmospheric carbon sequestration due to sub-pixel variability in plant functional type. On the other hand, despite at certain sites the meteorological bias influences estimates of GPP significantly and flux towers provide an excellent means to estimate accurately GPP across their footprint, they are sparse worldwide, even at the regional scale. Thus, the Flux Tower GPP may not truly be used to estimate a continuous GPP map at a larger area. Therefore, Flux Tower and MODIS individually have large uncertainty but their combination as a complementary could result in a robust estimation of GPP.