Response of forest photosynthesis to dry-wet change based on solar-induced chlorophyll fluorescence over eastern...

Solar-induced chlorophyll fluorescence (SIF) has been widely used in different area, such as estimating forest gross primary productivity (GPP), monitoring drought, estimating evapotranspiration and tracking vegetation phenology. Based on the Global OCO-2 SIF product (GOSIF) and the standardized precipitation evapotranspiration index (SPEI) at different temporal scales (1, 3, 6, and 12 months), we explored the responses of forest photosynthesis to dry-wet change over eastern monsoon China during 2001-2021. The results showed that there were differences in drought intensity and frequency among forests in different geographical regions. Forests in the North China and East China experienced higher drought intensity, while the southern part of Northwest China had lower drought intensity. Forests in the North China experienced more frequent droughts, while the Northeast China and Southwest China had lower drought frequencies. About 74.1% of the area where forest GOSIF was significantly and positively correlated with SPEI, and the response of photosynthesis to SPEI showed the most pronounced at the 1-month scale. In different geographical regions, photosynthesis in the Northeast China was the most sensitive to SPEI, whereas in the North China it was the least sensitive. The drought resistance of forests in the southern part of Northwest China exhibited the strongest, while in the Northeast China it was the weakest. Meanwhile, in different forest types, deciduous broad-leaved forests were the most sensitive to SPEI, followed by mixed forests, evergreen broad-leaved forests, evergreen needle-leaved forests and deciduous needle-leaved forests. Evergreen needle-leaved forests had the strongest resistance to drought stress, followed by deciduous needle-leaved forests, evergreen broad-leaved forests, deciduous broad-leaved forests and mixed forests. During the growing season (May-September), the response sensitivity of photosynthesis to SPEI was strongest in June and weakest in July. Dry-wet changes at the 1 and 3-month scales exerted the main impact on photosynthesis, while in the mid-season (June-August) and late season (September), the impact of dry-wet changes at the 6 and 12-month scales on photosynthesis increased.