Spring phenology projections for apples in southwestern Germany indicate persistent frost risk levels

Abstract

Spring frost poses a significant risk to apple production, prompting growers to invest in mitigation measures. Climate change is expected to affect both the occurrence of frost and the timing of phenological stages sensitive to frost. While warmer winters may reduce frost occurrence, earlier phenology could increase frost exposure. The combined effect on potential frost damage remains uncertain. We updated the PhenoFlex model to project multiple phenological stages—budbreak, first and full bloom—and assessed frost risk during the period from budbreak to full bloom for three apple cultivars in Ravensburg, southern Germany. We chose a frost threshold of -1 °C, as temperatures below trigger automated frost irrigation. Using the RMAWGEN weather generator, we simulated temperature scenarios under historical (2008–2022) and future (2035–2065, 2070–2100) conditions across four Shared Socioeconomic Pathways (SSP1–2.6, SSP2–4.5, SSP3–7.0, SSP5–8.5). Phenology advanced by 2050, more strongly for full bloom (3.9 ± 2.8 days under SSP1–2.6, 6.3 ± 2.6 days under SSP5–8.5) than budbreak (0.4 ± 3.8 days under SSP1–2.6, 2.7 ± 3.3 days under SSP5–8.5). Frost risk dropped notably in early February (75 % in 2015 to 49–40 % by 2050), but less so in mid-March (32 % in 2015 to 23–31 % by 2050). Despite more frost-free seasons, the chance of 1–10 frost hours during the budbreak-to-bloom phase remains stable (2015: 31 %; 2050: 33 %; 2085: 21–27 %). The share of frost-free seasons remains similar (67 % in 2015; 54–57 % in 2050), though up to three frost nights per season remains possible. These findings inform future frost protection planning under climate change.