Drought sensitivity of vegetation photosynthesis along the aridity gradient in northern China

Abstract

Satellite remotely sensed solar-induced chlorophyll fluorescence (SIF) enjoys the intrinsic superiority to the detection of ecosystem-level photosynthetic capacity under drought stress, yet little is known about how drought accumulation and legacy effects shape vegetation photosynthetic activities as approximated by SIF. Hence, we quantified drought sensitivity of vegetation photosynthesis (DSVP) by evaluating the degree and time to which SIF responded to prolonged and previous droughts with standardized precipitation evapotranspiration index (SPEI). Divergent SIF responses to SPEI across bioclimatic zones were examined in northern China for the period of 2000 to 2017. Our results suggested that the accumulation, legacy and their combined effects spread for 9.2, 25.0 and 44.3% of the vegetated land, respectively. The dominant lag and accumulation time of SIF responses to SPEI was 9.1 and 10.5 months for all biomes, respectively. In general, patterns of the cumulative and lagged effects along an aridity gradient were similar, displaying quicker and stronger SIF responses to SPEI in regions with a lower aridity index value, but the changing rate of magnitude and time duration varied among and within biomes. Besides, we detected a nonlinear response to increasing dryness in DSVP as reflected by multiple trend breaks, which were caused by the differences among vegetation types and their drought adaptation and vulnerability. Semi-arid ecosystems were found to be highly sensitive to drought, especially in the farming-pastoral zone with an aridity index value within 0.3–0.4. At the biome level, grasslands responded to prolonged and previous droughts quicker and stronger than croplands and deserts did, whereas forests were the least responsive. This study emphasizes the demand for a prompt assessment of drought impacts on vegetation photosynthesis in a drying climate, and to focus on regions where increasing dryness leads to substantial declines in photosynthetic capacity under global warming.