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 represent a useful indicator of environmental change. Despite its ecological and economic importance, long-term studies on black locust flowering behaviour remain limited in Italy. This study analyses a multi-decadal time series of flowering onset dates collected in Piedmont (1981–2025) and assesses the robustness of the observed relationships by incorporating data from the Italian Phenological Network (IPHEN, 2011–2025). Seasonal thermal indices, derived from the AgERA5 dataset, were used to assess the influence of autumn, winter, and spring temperatures on flowering timing. Over the study period, flowering onset advanced by about 0.5 days per year (p-value < 0.01), in parallel with a significant warming trend. Spring temperatures emerged as the main drivers: a 1 °C increase in the spring maximum temperature index was associated with an advance of more than 4 days in flowering onset. The winter temperatures also showed a significant, though weaker, negative association, suggesting a modulatory role on interannual phenological variability. In contrast, correlations with autumn temperatures reflected shared long-term trends rather than independent climatic control. Analyses on the pooled Montaldo + IPHEN dataset, including internal k-fold cross-validation, confirmed the robustness of the spring signal and highlighted the operational value of the winter index, which can be computed by late February, and predicts flowering onset with an average error of about 5 days. These results clarify the relative role of seasonal thermal signals in regulating the black locust flowering and provide useful information to support apicultural planning under ongoing climate change.