<b>Deciphering the carbon uptake period: unraveling climatic influences on gross primary productivity in the Northern Hemisphere</b>

The carbon uptake period (CUP), defined as the duration of the growing season between its onset (CUPstart) and termination (CUPend), is a critical determinant of ecosystem productivity and carbon cycling. However, the geographic variability, interannual dynamics, and climatic drivers that influence CUP remain poorly understood. We analyzed gross primary productivity (GPP) observations from 104 FLUXNET and ChinaFLUX sites, encompassing 862 site-years across the Northern Hemisphere. We estimated the CUP as a proxy for the growing season length, based on CUPstart and CUPend at each site year. There were significant spatial heterogeneity: tropical ecosystems exhibited the longest and most stable CUP, while the arid, polar and alpine regions showed shorter and more variable CUPs. In particular, both CUPstart and CUPend demonstrated latitudinal gradients, with higher latitudes experiencing later CUPstart and earlier CUPend, resulting in a shorter CUP. From 1995 to 2014, we observed an extension in both CUP and CUPend, along with an earlier CUPstart, primarily driven by shifts in temperature and precipitation patterns. The mean annual temperature (MAT) was the dominant climatic factor influencing CUPstart and the overall length of CUP, while the mean annual precipitation (MAP) had a significant effect on CUPend. CUPstart had the strongest impact on the duration of CUP. Additionally, CUP directly influenced GPP, with MAT and MAP exerting both direct and indirect effects on GPP through their modulation of CUPstart and CUPend. This study improves our understanding of how climate change influences carbon fluxes by affecting the timing and duration of the growing season.碳吸收期（carbon uptake period, CUP）被定义为生长季自起始点（CUPstart）至终止点（CUPend）的持续时长，是调控生态系统生产力与碳循环的关键因子。然而，当前学界对碳吸收期的空间变异特征、年际动态规律及其气候驱动因子的认知仍较为有限。 本研究分析了来自FLUXNET与ChinaFLUX的104个观测站点的总初级生产力（gross primary productivity, GPP）观测数据，覆盖北半球范围内共计862个站点年序列。我们以各站点年的CUPstart与CUPend为依据，估算得到碳吸收期以表征生长季长度。 研究区域存在显著的空间异质性：热带生态系统的碳吸收期最长且稳定性最强，而干旱、极地与高山区域的碳吸收期则相对更短且波动幅度更大。具体而言，CUPstart与CUPend均呈现出清晰的纬度梯度特征：高纬度地区的CUPstart出现更晚、CUPend结束更早，进而导致碳吸收期总时长更短。 1995年至2014年间，我们观测到碳吸收期与CUPend均出现延长，同时CUPstart有所提前，这一变化主要由气温与降水格局的改变所驱动。年平均气温（mean annual temperature, MAT）是影响CUPstart以及碳吸收期总时长的主导气候因子，而年平均降水量（mean annual precipitation, MAP）则对CUPend具有显著调控作用。其中，CUPstart对碳吸收期时长的影响最为显著。 此外，碳吸收期直接作用于总初级生产力，而年平均气温与年平均降水量则通过调控CUPstart与CUPend，对总初级生产力产生直接与间接的双重影响。本研究增进了学界对气候变化如何通过调控生长季的时间进程与持续时长来影响碳通量的认知。