Abstract: Climate changes in cold-temperate zones are increasingly altering the state of climatic constraints on photosynthesis and growth, leading to adaptive changes in plant phenology and subsequent seasonal carbon assimilation. However, the spatio-temporal patterns of climatic constraints and seasonal carbon assimilation are poorly understood. In this study, the timing of peak photosynthetic activity (DOY_pmax) was employed as a proxy for plant adaptive state to climatic constraints on growth to examine the spatio-temporal dynamics of DOY_pmax. By using multiple remote sensing metrics, DOY_pmax was characterized with changes in the solar-induced chlorophyll fluorescence (SIF) and leaf area index (LAI) from 2000 to 2018. Based on SIF, the DOY_pmax was generally around day 190, while based on LAI was about 10 d later. Peak photosynthetic activity of forests occurs earlier compared to other vegetation types. Overall, the advanced DOY_pmax were observed based on both SIF and LAI, with annual rates of 0.2 (P = 0.31) and 0.3 (P < 0.05) d, respectively. DOY_pmax dynamics were influenced by hot temperature extremes and vapor pressure deficits (VPD) during the early growing season, regardless of sub-zone and different vegetation type. The generalized linear mixed model (GLMM) showed the largest contribution by hot extremes to DOY_pmax dynamics accounted for 55.5% (DOY_{pmax_SIF}) and 49.1% (DOY_{pmax_LAI}), respectively, followed by VPD (DOY_{pmax_SIF}: 23.1%; DOY_{pmax_LAI}: 29.5%). These findings highlight the crucial role of climate extremes in shaping seasonal carbon dynamics and regional carbon balance.

Key words: Peak photosynthetic activity, Climate change, Hot extremes, Leaf area index, Solar-induced chlorophyll fluorescence