Exploring Populus phenological response to climate change using observational data and ecosystem modelling

Abstract

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.