Tropical forests and savannas are responsible for the largest proportion of global Gross Primary Productivity (GPP), a major component of the global carbon cycle. However, there are still deficiencies in the spatial and temporal information of tropical photosynthesis and its relations with environmental controls. The MOD17 product, based on the Light Use Efficiency (LUE) concept, has been updated to provide GPP estimates around the globe. In this research, the MOD17 GPP collections 5.0, 5.5 and 6.0 and their sources of uncertainties were assessed by using measurements of meteorology and eddy covariance GPP from eight flux towers in Brazilian tropical ecosystems, from 2000 to 2006. Results showed that the MOD17 collections tend to overestimate GPP at low productivity sites (bias between 111% and 584%) and underestimate it at high productivity sites (bias between −2% and −18%). Overall, the MOD17 product was not able to capture the GPP seasonality, especially in the equatorial sites. Recalculations of MOD17 GPP using site-specific meteorological data, corrected land use/land cover (LULC) classification, and tower-based LUE parameter showed improvements for some sites. However, the improvements were not sufficient to estimate the GPP seasonality in the equatorial forest sites. The use of a new soil moisture constraint on the LUE, based on the Evaporative Fraction, just showed improvements in water-limited sites. Modifications in the algorithm to account for separate LUE for cloudy and clear sky days presented noticeably improved GPP estimates in the tropical ecosystems investigated, both in magnitude and in seasonality. The results suggest that the high cloudiness makes the diffuse radiation an important factor to be considered in the LUE control, especially over dense forests. Thus, the MOD17 GPP algorithm needs more updates to accurately estimate productivity in tropical ecosystems.

热带森林和热带稀树草原在全球初级生产力（GPP）（全球碳循环的主要组成部分）中占最大比例。但是，热带光合作用的时空信息及其与环境控制的关系仍然存在缺陷。基于光使用效率（LUE）概念的MOD17产品已更新，可以在全球范围内提供GPP估算值。在这项研究中，通过使用2000年至2006年巴西热带生态系统中的八个通量塔的气象学和涡度协方差GPP的测量，对MOD17 GPP集合5.0、5.5和6.0及其不确定性来源进行了评估。结果表明，MOD17集合倾向于在低生产率站点（偏倚在111％和584％之间）偏高估计GPP，而在高生产率站点（偏见-2％至-18％之间）偏低GPP。总体而言，MOD17产品无法捕获GPP的季节性变化，尤其是在赤道站点。使用特定于站点的气象数据，更正的土地利用/土地覆盖（LULC）分类以及基于塔的LUE参数对MOD17 GPP进行的重新计算显示出某些站点的改进。但是，这些改进不足以估计赤道森林站点的GPP季节变化。在LUE上基于蒸发分数的新的土壤水分约束条件的使用，刚好显示出缺水地区的改善。对算法的修改以考虑多云和晴朗天的单独LUE，在幅度和季节性方面，在所研究的热带生态系统中均显着提高了GPP估计值。结果表明，高云度使漫射辐射成为LUE控制中要考虑的重要因素，尤其是在茂密的森林上。因此，MOD17 GPP算法需要更多更新才能准确估计热带生态系统的生产力。