Reconstruction of late spring phenophases in Poland and their response to climate change, 1951–2014

Phenology is primarily seen as an indicator of the impacts of climate change. The strongest biological signal of climatic change is revealed by phenological data from the period after 1990. Unfortunately, the Polish nationwide network of phenological monitoring was terminated in 1992, and was only reactivated in 2005. Here, we attempt to reconstruct late spring phenophases of flowering of Syringa vulgaris L. and Aesculus hippocastanum L. across several sites in Poland from 1951 to 2014 using the GIS-based approach (if observations from neighboring stations were available) and multiple regression modeling with stepwise screening and bootstrap resampling. It was found that the air temperature and its indices explain over 60% of the variance, giving an accuracy of 3.0–3.4 days (mean absolute error) and correlation coefficients of 0.83 and 0.78 for lilac and horse chestnut, respectively. Altogether, both plant species showed a statistically significant advancement in the onset of flowering with an average rate of 1.7 days per decade. We also found that the final trend is the result of rapid acceleration of the increase in air temperature after the 1990s, while most of the trends for late spring were ambiguous before that period