Modeling phenological phases in a Mediterranean environment often implies tangible challenges to reconstructing regional trends over heterogenous areas using limited and scattered observations. The present investigation aimed to project phenological phases (i.e., sprouting, blooming, and pit hardening) for early and mid–late olive cultivars in the Mediterranean, comparing two phenological modeling approaches. Phenoflex is a rather integrated but data-demanding model, while a combined model of chill and anti-chill days and growing degree days (CAC_GDD) offers a more parsimonious and general approach in terms of data requirements for parameterization. We gathered phenological observations from nine experimental sites in Italy and temperature timeseries from the European Centre for Medium-Range Weather Forecasts, Reanalysis v5. The best performances of the CAC_GDD (RMSE: 4 days) and PhenoFlex models (RMSE: 5–9.5 days) were identified for the blooming and sprouting phases of mid–late cultivars, respectively. The CAC_GDD model was better suited to our experimental conditions for projecting pit hardening and blooming dates (correlation: 0.80 and 0.70, normalized RMSE: 0.6 and 0.8, normalized standard deviation: 0.9 and 1.0). The optimization of the principal parameters confirmed that the mid–late cultivars were more adaptable to thermal variability. The spatial distribution illustrated the near synchrony of blooming dates between the early and mid–late cultivars compared to other phases.

Under a future changing climate, the Mediterranean is considered a region extremely prone to global warming and intensified climate events, which will possibly change the timing of phenological phases and alter conditions and risks for olive tree growth. This situation may restrict olive cultivation which is economically strategic in the Mediterranean countries. Since, the timing and management of agronomic practices (planting, irrigation, fertilization, crop protection, harvesting, etc.) are based on phenological phases and plant growth, accurate phenological projections are essential to assess climate risks and guide optimal management apt to mitigate climate change effects on olive development. Initially, the present investigation aims to introduce innovative phenological modeling, i.e., Chill, Anti-Chill, and Growing Degree Days combined model (CAC_GDD) applicable in heterogenous areas with limited and scattered observations. Then, we project future changes in olive phenological phases (i.e., sprouting, blooming, and pit hardening) and relevant agro-climate stressors during these phases over the Euro-Mediterranean for both early and mid-late bud break cultivars. For model parametrization and validation, the phenological observations were gathered from nine experimental sites in Italy and temperature time-series from the European Centre for Medium-Range Weather Forecasts, Reanalysis v5. To project the timing of phenological phases and then calculate the agro-climate stressors we used an ensemble of high-resolution climate projections at 0.11° from EURO-CORDEX (Coordinated Regional Climate Downscaling Experiment) repository, for two historical (1976-2005) and future (2036-2065) 30-year periods under three emission scenarios (i.e., RCP2.6, RCP4.5, and RCP8.5). The CAC_GDD modeling showed the best performance (RMSE: 4 days) for the blooming phase of mid-late cultivars, suiting similarly and in some cases even better than the more complex model to our experimental conditions. The spatial phenological projection illustrated that at least 75% of the Euro-Mediterranean area will experience significant phenological advances for olive crops. Meanwhile, current olive cultivations in the Mediterranean basin may face accelerated climate extremes mainly at blooming and pit hardening stages in the future. Hence, we expect possible future shifts in olive-growing areas from the Mediterranean to colder regions with more thermal suitability for the mid-late cultivars.