Phenology of the beech forests in the Western Carpathians from MODIS for 2000-2015

The present paper introduces a satellite-based approach to the detection of phenology events in beech forests across Slovakia (the Western Carpathians) using the MOD/MYD09 products. Normalized vegetation index (NDVI) was used for determining the onset of the phenophases in spring and autumn. Double logistic sigmoid function was applied in order to fit the NDVI profile during the year. The satellite-derived phenological metrics was based on calculating the extreme values of the sigmoid function and its derivatives. Between 2000 and 2015, a time-series analysis using the linear regressions models revealed that the onset of leaf unfolding shifted at a rate of 0.8 day per decade, the onset of leaf fall was delayed at a rate of 1.9 day per decade, and the growing season (GS) extended at a rate of 1.1 day per decade. However, at a regional level, the trends were not found to be statistically significant in either case. Leaf unfolding/fall was significantly non-linearly delayed/advanced with the increase of altitude (p<0.01). GS duration varied extensively within the region. Theil-Sen estimation of GS trend revealed the median shift of 1.8 days, the range of shift being from -7.0 to + 12.1 days at the 5-95 % quantile for 2000-2015. A significant inverse correlation between GS shift and GS length (p< 0.01) was observed. The GS shift was positive in the sites with shorter GS and negative in the sites with longer GS.