The length of the growing season for deciduous trees in temperate and boreal forests is determined by the timing of bud burst and autumn senescence. It is generally assumed that a warmer climate leads to a longer growing season due to earlier bud burst and delayed autumn senescence and thereby increased gross primary production (GPP) of forests. In this study, we analysed past (1873–1951) and current (2008–2020) phenological observations on bud burst and senescence from aspen trees (Populus tremula) grown in Sweden. The observations indicated a reduction in temperature sensitivity of bud burst between the time periods, likely associated with warmer winters and reduced exposure to chilling. The phenological observations were used in the evaluation of an ecosystem model. Biases in modelling spring and autumn leaf cover development influenced the seasonal and annual GPP estimates. The overestimation of the modelled GPP was more pronounced in spring than in autumn, reflecting the GPP limitations by leaf cover development in spring, and by daylength and temperature conditions in autumn. Calibration of the spring phenology parameters, using the accumulated temperature sums and chilling days at the observed timing of bud burst, significantly improved model performance compared to the original parameterisation. The calibrated ecosystem model projections representing RCP8.5 suggested 15 days earlier timing of bud burst and enhanced mean annual GPP by the end of the century compared with current climate conditions in Sweden.

温带和寒带森林落叶乔木生长季节的长短取决于花蕾萌发和秋季衰老的时间。一般认为，由于较早的芽萌芽和延迟的秋季衰老，较温暖的气候导致较长的生长季节，从而增加了森林的初级生产总值 (GPP)。在这项研究中，我们分析了过去（1873-1951）和当前（2008-2020）对白杨树（Populus tremula) 生长在瑞典。观察结果表明，不同时期之间的芽萌芽的温度敏感性降低，这可能与温暖的冬季和减少的寒冷暴露有关。物候观察用于评估生态系统模型。春季和秋季叶盖发育建模的偏差影响了季节性和年度 GPP 估计值。春季模拟 GPP 的高估比秋季更明显，反映了春季叶盖发育以及秋季日长和温度条件的 GPP 限制。与原始参数化相比，使用累积温度总和和观察到的萌芽时间的寒冷天数校准春季物候参数，显着提高了模型性能。