Global carry-over effects between the timing of spring leaf-out and autumn senescence within and across years

Climate warming alters the start (SOS) and end (EOS) of growing seasons, impacting biotic interactions and biogeochemical cycles. However, the global constraints between these two stages – how SOS influences EOS (SOS-EOS effect) and vice-versa (EOS-SOS effect) – remain poorly understood, hindering future growing-season projections. Using MODIS satellite-derived phenology data for deciduous vegetation and European ground observations for deciduous tree species, we show that earlier SOS typically advances EOS (on average by 0.19 ± 0.001 days per day [MODIS] and 0.12 ± 0.002 days per day [ground]), while EOS exerts a weaker influence on subsequent SOS (-0.05 days per day). The SOS-EOS effect often outweighed abiotic factors, with SOS being the top predictor of EOS in 34% of pixels (β = 0.27), while preseason temperature was the primary predictor of SOS in 58% (β = -0.33). More importantly, we identified a dampening interaction, where an increase in one carry-over effect reduced the other, with distinct and opposing geographic patterns: the SOS-EOS effect was twice as strong as the EOS-SOS effect in temperate deciduous forests, while the EOS-SOS effect was up to three times stronger in boreal taiga and tundra. Mechanistically, these patterns are likely to reflect developmental (cell and tissue growth) and stress-related constraints (SOS-EOS effect) and chilling requirements during dormancy (EOS-SOS effect at high latitudes). These findings highlight how plant-internal physiological feedbacks constrain phenological responses to climate change, emphasizing the need to integrate carry-over effects into future ecosystem models.