Seasonal climate signals driving flowering of Robinia pseudoacacia in Piedmont (Italy): implications for apiculture under climate change

Flowering dynamics of Robinia pseudoacacia L. (black locust), a key honey plant in southern Europe, are highly sensitive to climate variability and thus serve as a reliable indicator of environmental change. Despite the ecological and economic importance of this species, long-term studies on its flowering behaviour remain limited in Italy. This study analyses a multi-decadal time series of flowering onset dates collected in Piedmont (1981–2025) and validated at the regional scale using data from the IPHEN – Italian Phenological Network (2011–2025). Seasonal thermal indices, derived from the AgERA5 dataset, were evaluated through linear regressions and Monte Carlo cross-validation. During 1981–2025, flowering onset advanced by about 0.5 days per year (p-value < 0.01), consistent with the local warming trend observed over the same period. Among the thermal indices tested, spring temperatures were the main drivers: a 1°C increase in the spring maximum temperature index (mTmax_spring) advanced flowering onset by more than 4 days on average. The winter index (mTmax_winter) also showed a significant, though weaker, negative association, suggesting that milder winters may contribute to earlier flowering, likely by modulating dormancy processes. In contrast, autumn correlations appeared to result from spurious trends linked to global warming. Validation with the IPHEN dataset confirmed the robustness of the spring signal and the predictive role of the winter index, which allows estimates as early as late February, with an average error of about 5 days. The results help clarify the climatic mechanisms driving flowering onset in black locust and provide practical insights for apicultural planning in a changing climate.