Under global warming, advances in spring phenology due to rising temperatures have been widely reported. However, the physiological mechanisms underlying the advancement in spring phenology still remain poorly understood. Here, we investigated the effect of temperature during the previous growing season on spring phenology of current year based on the start of season extracted from multiple long-term and large-scale phenological datasets between 1951 and 2018. Our findings indicate that warmer temperatures during previous growing season are linked to earlier spring phenology of current year in temperate and boreal forests. Correspondingly, we observed an earlier spring phenology with the increase in photosynthesis of the previous growing season. These findings suggest that the observed warming-induced earlier spring phenology is driven by increased photosynthetic carbon assimilation in the previous growing season. Therefore, the vital role of warming-induced changes in carbon assimilation should be considered to accurately project spring phenology and carbon cycling in forest ecosystems under future climate warming.

Global warming has been linked to declines in tree growth. However, it is unclear how the asymmetry in daytime and nighttime warming influences this response. Here, we use 2947 residual tree‐ring width chronologies covering 32 species at 2493 sites, between 1901 and 2018, across the Northern Hemisphere, to analyze the effects of daytime and nighttime temperatures, precipitation, and drought stress on the radial growth of trees. We show that drought stress was primarily triggered by daytime rather than nighttime warming. The radial growth of trees was more sensitive to drought stress in warm regions than in cold regions, especially for angiosperms. Our study provides robust evidence that daytime warming is the primary driver of the observed declines in forest productivity related to drought stress and that daytime and nighttime warming should be considered separately when modelling forest–climate interactions and feedbacks in a future, warmer world.

Under global warming, advances in spring phenology due to rising temperatures have been widely reported. However, the physiological mechanisms underlying the warming-induced earlier spring phenology remain poorly understood. Here, using multiple long-term and large-scale phenological datasets between 1951 and 2018, we show that warmer temperatures during the previous growing season led to earlier spring phenology in the Northern Hemisphere, with an average advancement of 2.5 days °C− 1. Correspondingly, we observed an earlier spring phenology with the increase in photosynthesis of previous growing season. Furthermore, we found a significant decline in the advancing effect of warming in previous growing season on spring phenology from cold to warm periods. These findings reveal that the observed warming-induced earlier spring phenology is driven by increased photosynthetic carbon assimilation in the previous growing season, while the slowdown in the advanced spring phenology may arise from decreased carbon assimilation when warming exceeds optimal temperatures for photosynthesis. Therefore, the vital role of warming-induced changes in carbon assimilation should be considered to accurately project spring phenology and carbon cycling under future climate warming conditions.

Under global warming, advances in spring phenology due to the rising temperature have been widely reported. However, the mechanisms underlying the warming-induced earlier spring phenology remain poorly understood. Here, using multiple long-term and large-scale phenological datasets between 1951 and 2018, we show that warmer temperatures during the previous growing season between May and September led to earlier spring phenology in the Northern Hemisphere. We also found that warming-induced increases in maximum photosynthetic rate in the previous year advanced spring phenology. Furthermore, we found a significant decline in the advancing effect of warming during previous growing season on spring phenology from cold to warm periods over the past decades. Our results suggest that observed warming-induced earlier spring phenology may be driven by increased photosynthetic carbon assimilation in the previous season, while the slowdown in the advanced spring phenology is likely due to decreased carbon assimilation when warming exceeding the optimal temperatures for photosynthesis. Our study suggested the vital role of photosynthetic carbon assimilation during growing season in spring phenology under global warming.

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