Nonlinear effects of elevated temperature on grapevine phenology

Phenological shifts are one of the most conspicuous biological effects of global warming; observational data need benchmarking against experimental data. Grapevine phenology was monitored weekly between budswell and maturity in three factorial field experiments during two seasons. Two thermal regimes (heated vs. control) were combined with two irrigation regimes (experiment 1), two source:sink ratios (experiment 2) and four varieties (experiment 3). Open-top chambers were used to elevate temperature. We present evidence of nonlinearity of thermal effects on phenology that is explained by two complementary factors. Firstly, temperature-driven shifts of sensitive events (e.g. onset of sugar accumulation in berries) moved the timing of subsequent events into cooler conditions, hence dampening warming effects. Secondly, thermal effects on phenology seemed to be modulated by the interplay between resource-driven growth and temperature-driven development; an enhanced thermal effect on berry ripening associated with high source:sink ratio supports this proposition. Measured shifts in maturity were smaller than expected from reported time series analysis. During berry ripening towards maturity, the phenological trajectories of heated and control vines largely overlapped on thermal time scales; differences in phenology between treatments were therefore attributed to actual thermal regimes. We thus conclude that thermal sensitivities from time series (6–9 d °C−1) over-estimate thermal effects on grapevine maturity.