Thirty-Year Analyses of Seasonal NDVI and Climatic Drivers Across Different Land Cover Types and Biogeographical Regions in Europe

Monitoring and analysing vegetation trends and their climatic drivers is useful to understand possible impacts of climate change on vegetation. In this study, we analysed Normalized Difference Vegetation Index (NDVI) trends at 1 km² resolution for spring (March–May), summer (June–August), and autumn (September–November) for individual biogeographical regions and land cover types in Europe for the 30-year period 1989–2018. Additionally, we determined the influence of precipitation, temperature, solar radiation, and vapor pressure deficit (VPD) on the NDVI and identified their influence in areas showing significant (p<0.05) negative and positive NDVI trends. Our results show varying seasonal NDVI trends for different regions, ranging between +0.004 and −0.001 NDVI units per year. Highest NDVI increase can be observed for land cover classes in the Pannonian region in autumn (>0.005 NDVI year<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>), strongest NDVI decrease (<−0.002 NDVI year<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>) for grassland (summer) and rainfed cropland (autumn) in the Steppic region. In spring, significant positive causal links between the NDVI and temperature are dominant in large areas of Europe, while VPD influences the NDVI in north-eastern (positive link) and southern Europe (negative link). In summer, precipitation shows positive causal links to NDVI in parts of the Anatolian and Mediterranean regions, while VPD influences NDVI regionally (negative link). In autumn, precipitation (positive link) and VPD (negative link) are the dominant controlling factors on the NDVI for central-eastern Europe and parts of the Iberian Peninsula. Across Europe, significant positive NDVI trends in spring can be mainly explained by increasing temperatures. Negative spring NDVI trends can be linked to decreasing VPD in some regions. In the Mediterranean, Steppic, and Anatolian regions, significant NDVI decrease in summer and autumn are mainly linked to decreasing precipitation. The results of this study assist in understanding and quantifying ongoing vegetation change in Europe and reveal the influence climate variables have on the vegetation activity for individual land cover types within different regions.