Contrasting effects of elevated ozone and climate warming on photosynthetic phenology in the Yellow River Basin

Photosynthetic phenology has become a key indicator for the detection of land-atmosphere carbon cycle processes. Meanwhile, surface ozone (O 3 ) is a phytotoxic air pollutant that adversely influences the growth and photosynthetic activity of various vegetation types. Although shifts in vegetation phenology have been widely attributed to climate warming, the contrasting effects of Ozone and water-heat factors under different climatic backgrounds remain unclear. In this study, by introducing a long-term series of solar-induced chlorophyll fluorescence (SIF) data, we investigated the spatiotemporal pattern, interannual variability, and response of the start of photosynthetic growing season (SOS) to elevated O 3 levels and climate change in the Yellow River Basin (YRB). The results indicated that area-averaged SOS occurred between 128 and 153 days of the year (DOY), with an advanced trend at an average rate of 0.45 DOY ( p < 0.05), from 2000 to 2022. Spatially, the SOS occurs earlier in the southern and southeastern regions, whereas a later case is mainly located in the northern and western areas of the YRB, indicating that the SOS of low-latitude areas advances faster than that of high-latitude areas. Based on partial derivatives and ridge regression analysis, we found that SOS is negatively correlated with minimum temperature (TMN), but positively correlated with maximum temperature (TMX), which indicates that TMN and TMX play asymmetric roles in the trend of SOS across the YRB. Spatially, the effect of TMN on SOS was concentrated in the central and western parts of the YRB, whereas the responses to TMX and soil moisture varied across different climate zones. Remarkably, we confirmed a strong negative effect of elevated O 3 on SOS in the YRB, which is almost universal for all vegetation types. Furthermore, the differences in the sensitivity and relative contribution of climatic factors and elevated O 3 to SOS were determined, suggesting a combined effect of TMN (22.9%) and O 3 (26.9%) on the advancing trends of SOS in the YRB. These findings highlight the key role of O 3 in that the meteorological variables alone may not fully determine advanced SOS, which should be incorporated into the photosynthetic phenology model under climate warming and O 3 pollution environments.