Long-term monitoring of growing season characteristics and heat stress in Serbia.

Long-term monitoring of temperature thresholds and heat stress indicators reveals significant shifts in the growing season across Serbia. Using daily temperature data from 24 meteorological stations, this study analyzes changes in the onset, cessation, and duration of growing seasons defined by 5 °C, 10 °C, and 15 °C thresholds over two climatological periods (1961-1990 and 1991-2020). Trends in Growing Degree Days (GDD) and heat stress indicators, including the Heat Stress Index (HSI) and Days of Heat Stress (Dhs), further illustrate the impacts of rising temperatures. Results indicate a significant advancement in the start of the growing season, averaging - 7.8, - 4.4, and - 8.7 days for the respective thresholds. The duration of warm-season conditions (≥ 15 °C) has increased across all stations, with the most pronounced extensions in mountainous regions, where high-altitude ecosystems now exhibit thermal characteristics similar to past lowland climates. Rising GDD values suggest enhanced thermal accumulation, extending vegetation activity but also increasing risks such as hydrological stress, altered species composition, and shifting pest dynamics. The study highlights the intensification of extreme heat events, particularly in 2007 and 2012, marking peak heat stress conditions across Serbia. These findings align with broader Southeastern and Central European climate trends, underscoring the need for expanded environmental monitoring through meteorological networks, remote sensing, and cross-regional climate data integration. Strengthening long-term climate and phenological observation systems is essential for understanding climate-driven ecological changes and developing sustainable adaptation strategies.