Comparative Study of Thermal Accumulation and Adaptive Responses of Forsythia × intermedia Zabel and Kerria japonica (L.) DC. 'Pleniflora' Under Conditions of Climatic Fluctuations.

The study establishes a locally specific basis that may serve as an indicator for the selection of climate-adaptive taxa in urban landscape design through a comparative evaluation of the effects of thermal anomalies on the flowering phenodynamics of Forsythia × intermedia and Kerria japonica 'Pleniflora' in Belgrade (2007-2026). By integrating empirical phenological parameters into a long-term climatological retrospective, the study bridges a methodological gap and distinguishes anthropogenic influences from regional climatic fluctuations. Non-parametric analyses identified a stable long-term trend, while the absence of statistically significant macro-scale effects confirms the predominant role of natural climatic variability. Nevertheless, microclimatic modifications induce distinct, taxon-specific phenological responses. Non-linear regression models identified two divergent adaptive mechanisms during warmer years: a phenological compression model in F. × intermedia (shortening of the flowering cycle accompanied by an annual increase in growing degree days (GDDs) of 0.71%) and a non-linear expansion model in K. japonica 'Pleniflora' (extension of the flowering period by more than one month, with the end of flowering delayed by 8.4 days per decade). Circular statistical analyses confirmed a high degree of temporal synchronisation and genetically determined phenological rhythmicity in both taxa, while revealing asynchrony in the terminal phenophases that was independent of growing degree day (GDD) accumulation. The synchronous shift towards warmer thermal classes positions both taxa as bioindicators of microclimatic change, although K. japonica 'Pleniflora' exhibited greater adaptive capacity. These findings highlight the importance of applying non-linear models in phenological forecasting and a contextualised taxonomic and ecological basis for the planning, introduction, and management of green spaces, supporting the sustainability of shrub taxa within the analysed urban core.