Ground and satellite phenology in alpine forests are becoming more heterogeneous across higher elevations with warming

Abstract

The role of temperature as a key driver for plant phenology is well established. However, an increasing lack of winter chilling may strongly slow down spring phenological advances in the course of warming. Along elevational gradients in the mountains, differential changes in winter chilling as well as more homogeneous leaf unfolding due to warming have been observed. In this study, we analyzed the elevation-linked lapse rates of phenological ground observations and remote sensing data in the pre-alpine and alpine regions of the Bavarian Alps, Germany. Seasonal start (SOS) and end of season (EOS) dates were extracted from time series data of 4-day maximum value composite Moderate Resolution Imaging Spectrometer (MODIS) sensor's Normalized Difference Vegetation Index (NDVI) for the years 2001-2016. Longer snow duration in the alpine region could be a possible reason of higher SOS elevational lapse rates as compared to the pre-alpine region. Significant and maximum differences in SOS rates between alpine and pre-alpine areas were observed in years with preceding warm winters with insufficient chilling. Minimum differences in SOS elevational lapse rates along the elevational gradients were found for cold spring and cold winter years. The MODIS-based SOS showed the highest correspondence when validated against the gridded German Meteorological Service (DWD) leaf unfolding data. However, EOS dates were in comparatively lower agreement with DWD data, and their lapse rates in the pre-alpine and alpine regions were difficult to validate. Contrary to the SOS, lower positive lapse rates of EOS were revealed in the alpine but not in the pre-alpine areas.