Detecting the Drought-Response Difference in Semiarid Ecosystems on the Mongolian Plateau: 10-Year Observation Using GOSAT SIF and MODIS PRI

Atmospheric aridity and soil drought control vegetation water use and affect the terrestrial water and carbon cycles. Separating these two types of drought is difficult but crucial because they relate to different ecosystem properties and their impacts depend on vegetation types. We examine how well satellite-observed solar-induced chlorophyll fluorescence (SIF) captures the drought responses of taiga and steppe in semiarid areas on the Mongolian plateau, which have experienced a historic drought since the late 1990s. Ten-year records of the GOSAT SIF and the MODIS-band-1 photochemical reflectance index (PRI, also known as chlorophyll/carotenoid index) from Aqua consistently suggest that the taiga is sensitive to vapor-pressure deficit but insensitive to surface-soil drought, and that the opposite is the case for the steppe. The MODIS PRI changes reasonably with temperature and drought stress, and also with canopy shades, which is attributable to the xanthophyll cycle. However the most influential factor on the PRI is leaf area in both vegetations, and temperature is the second in the taiga, indicating the dominant effect of its pigment-pool size. The leaf area index of the taiga has almost similar values and seasonal patterns in each year, while the SIF shows remarkable interannual changes. The SIF yield in the taiga decreases 36–48% on average with the increase of vapor-pressure deficit from 1 kPa to 3 kPa under high-PRI conditions. Almost all detectable information from the SIF yield in the steppe is correlated with PRI on a monthly basis. The SIF in the steppe decreases nonlinearly when the surface-soil-water content fell below ∼0.154 m³ m⁻³, which agrees well with an eddy-covariance result in this region and implies the capability of satellite-based wilting-point estimation. We will further clarify which biological factors affected the observed results of SIF and PRI using the process-based model 'Soil-Canopy-Observation of Photosynthesis and Energy fluxes' (SCOPE) and show the impact of drought on the gross primary production in the past decade.