DOI: 10.5061/dryad.hmgqnk9hh
title: The tempo of greening in the European Alps: Spatial variations on a common theme

description: The long-term increase of satellite-based proxies of vegetation cover is a
 well-documented response of seasonally snow-covered ecosystems to climate
 warming. However, observed greening trends are far from being uniform and
 substantial uncertainty remains concerning the underlying causes of this
 spatial variability. Here, we processed surface reflectance of the
 moderate resolution imaging spectroradiometer (MODIS) to investigate
 trends and drivers of changes in the annual peak values of the Normalized
 Difference Vegetation Index (NDVI). Our study focuses on the above
 treeline ecosystems in the European Alps. The NDVI changes of these
 ecosystems are highly sensitive to land cover and biomass changes and are
 marginally affected by anthropogenic disturbances. We found a widespread
 greening for the period 2000-2020, a pattern that is consistent with the
 overall increase of summer temperature. At the local scale, the spatial
 variability of greening was mainly due to the preferential response of
 north-facing slopes between 1900 m and 2400 m. Using high resolution
 imagery, we noticed that the presence of screes and outcrops locally
 magnified this response. At the regional scale, we identified hotspots of
 greening where vegetation cover is sparser than expected given the
 elevation and exposure. Most of these hotspots experienced delayed
 snowmelt and green-up dates in recent years. We conclude that the ongoing
 greening in the Alps primarily reflects the high responsiveness of
 sparsely vegetated ecosystems that are able to benefit the most from
 temperature and water-related habitat amelioration above treeline.
description: We exploit available time series of the moderate resolution
 imaging spectroradiometer (MODIS) to provide a comprehensive picture of
 recent greening and its spatial variability in the European Alps and to
 improve our understanding of its drivers. Our study utilizes annual peak
 values of the Normalized Difference Vegetation Index (NDVI) as a proxy of
 land surface greenness. We quantified the significance and magnitude of
 NDVI trends for high elevation ecosystems that are located between the
 treeline and the permanent snowline. We first selected
 a set of 250-m resolution MODIS pixels having non-forested land cover
 classes with a tree cover density below 5%, an elevation above 1400 m and
 an average NDVImax above 0.15. We discarded pixels exhibiting more than
 10% of settlements (ski resorts) or permanent water using data layers of
 European settlements
 (https://land.copernicus.eu/pan-european/GHSL/european-settlement-map/)
 and of permanent water
 (https://land.copernicus.eu/pan-european/high-resolution-layers/water-wetness/status-maps/water-wetness-2018). We also removed pixels for which we found significant abrupt changes of NDVImax within the period 2000-2020, as this might be indicative of a physical or anthropogenic disturbance unrelated to climate trend. We ended up with 511,375 pixels - see the file data_GCB.csv - of which 284,346 (55.6%) exhibited an average NDVImax below 0.65. Our main findings are based on this data subset to avoid saturation effect of NDVImax for closed canopies.