Soil temperature and weather factors as key drivers of flowering phenology and nectar production in black locust (Robinia pseudoacacia L.) in Hungary

The black locust (Robinia pseudoacacia L.) is the second most planted tree species worldwide, and the most common in Hungary. Phenotypic traits, particularly flowering patterns, are well-established indicators of the species' response to climate change. This study examined four forest subcompartments across three Hungarian regions: Northern-Central, Eastern and Southern-Central. The aim was to identify climatic factors correlating with the onset and duration of the flowering period. Additionally, the relationships between these factors and nectar weight and sugar concentration were defined. Results indicate a strong negative correlation between precipitation levels and flowering time: lower accumulated and average precipitation during the spring months of the preceding year was associated with a delayed flowering period in the following year (r = -0.922, r = -0.918, P = 0.05). However, when examining the 14-day period (r = 0.829) before blooming or examining from 1 January (r = 0.929, r = 0.890), the results indicate that other environmental factors may play a more dominant role. Furthermore, the number of chill and heat days was found to affect the starting date (R2 = 0.819, R2 = 0.765).