Trends and driving forces of spring phenology of oak and beech stands in the Western Carpathians from MODIS times series 2000-2021

This study focused on trends and driving forces of the leaf unfolding (LU) on-set of oak and beech forests in the Slovak Carpathians along elevational gradients in the period 2000-2021. Particular attention was paid to improving the modelling of the LU onset using the MOD/MYD09 Moderate Resolution Imaging Spectroradiometer (MODIS) products. The LU onset was derived from the annual Normalized Difference Vegetation Index (NDVI) trajectories fitted with a double logistic function. An improved estimate of the onset was obtained by calculating 6 parameters of the logistic function and by comparing with the LU onset from phenological field observations. Between 2000 and 2021, we found a trend towards an earlier LU onset at the national level by similar to 0.39 day year(-1) for oak stands (p = 0.13) and similar to 0.08 day year(-1) for beech stands (p = 0.48). The analysis of trends in three elevation zones showed a difference in the LU onset of oak and beech stands as a function of elevation. For oak in 100-350 and 350-500 m zones was found a shift towards an earlier onset by similar to 0.41 day year(-1) (p = 0.12). This corresponds to a shift of 8.6 days for the entire observation period 2000-2021. In the elevational zone above 500 m, the trend was milder, similar to 0.27 day year(-1) (p = 0.21), i.e., 5.6 days for the entire analysed period. The shift towards an earlier onset at lower elevations and a later onset at higher elevations for beech was not statistically significant, with p-values between 0.44 and 0.51. The atypical year 2021, with the latest onset of LU during the entire observation period, fundamentally affected the significance of all trends. Nevertheless, the pixel-level analysis revealed a significant trend towards an earlier LU onset (p < 0.05) in 20.3% of oak stands. The same result was found only in 0.8% of beech stands. Strong negative correlations with R-2 = 0.72 for oak and R-2 = 0.81 for beech (p < 0.001) were found between the LU onset and March and April temperature deviations from the long-term normal. Temperature changes are the main driving force affecting the LU onset in the studied region.