Junjun Zha, Muyi Li, Zaichun Zhu, Sen Cao, Yanan Zhang, Weiqing Zhao, Yue Chen

The long-term global Leaf Area Index (LAI) products are critical supports for characterizing the changes in land surface and its interactions with other components of the Earth system under the dramatic global change. However, intercomparisons between current available long-term global LAI products present significant spatiotemporal inconsistencies which have been a persistent source of uncertainties in global change ecology. Yet, a direct and systematic evaluation of current long-term LAI products is still lacking due to the absence of appropriate LAI references, especially before 2000. Here, we proposed a novel evaluation framework to directly evaluate the mainstream long-term global LAI products (GIMMS LAI3g, GLASS LAI, and GLOBMAP LAI) using massive high-quality LAI validation samples. The LAI validation samples, derived from the Landsat archive using machine learning and MODIS LAI, have a global distribution, a long temporal coverage (1982−2020), and a large amount of 4.9 million. They substantially address the issue of insufficient LAI reference data and can enable quantitative LAI assessments. The long-term global LAI products showed reasonable quality in terms of absolute value, with GIMMS LAI3g having better performance (R:0.96; MAE: 0.29 m^2 m^(-2); RMSE: 0.49 m^2 m^(-2)), followed by GLASS LAI (R:0.96; MAE: 0.31 m^2 m^(-2); RMSE: 0.51 m^2 m^(-2)) and GLOBMAP LAI (R:0.90; MAE: 0.52 m^2 m^(-2); RMSE: 0.91 m^2 m^(-2)). For all LAI products, the data quality after 2000 was better than before 2000. Their annual maximum LAI trends presented mediocre consistencies with the LAI validation samples (R: 0.20−0.29) which showed a significantly larger area of greening. The evaluation of ten state-of-the-art ecosystem models demonstrated varied capabilities in simulating global LAI trends, with the standard deviations ranging from ~0.01 to 0.04 m^2 m^(-2) a^(-1). Although the Multi-Model Ensemble Mean LAI agreed with satellite-based LAI products, they differed with vegetation biomes especially for the tropics. The Landsat LAI validation dataset produced in this study can facilitate the development of long-term global LAI products and provide a quantitative reference for vegetation dynamic studies.