Vegetation response to extreme drought events in northern Italy

The increase in drought conditions is one of the main consequences of climatic change, that affects both natural and socioeconomic systems. Northern Italy is historically rich in water resources, and one of the most fertile areas in Italy. However, in the last decades drought events increased also here, affecting the hydrological behaviour of the Po River and vegetation growth.

This study aims to quantify the spatial distributions of drought events and identify their effects on vegetation greenness in northern Italy during the 2000-2020 period using MODIS images at 1 km spatial resolution. For this purpose, correlation maps between fields of bi-weekly vegetation indices (NDVI and EVI) and drought indices (SPI and SPEI) were estimated.

The NDVI and EVI indices were extracted from the atmospherically corrected MODIS images and vegetation trends were investigated by mean on the Mann-Kendall test. To analyze drought events, 150 daily precipitation ground station series were collected, aggregated at bi-weekly scale, reconstructed, homogenised and spatialised at 1km resolution by mean of the Universal Kriging with auxiliary variables. Land Surface Temperature (LST), assumed as air temperature, was collected from MODIS images. Pixels with clouds were removed, and the accuracy was determined against the high resolution gridded temperature dataset available for northern Italy. The NDVI-LST space was investigated at yearly scale exploring the link between NDVI and LST for 6000 random points in the study area. Evapotranspiration was estimated by means of the Hargreaves equation and severe and extreme drought episodes were detected by means of drought indices (SPI and SPEI) calculated at 12-, 24- and 36-months aggregation time. Trends were analysed and the main drought events were characterised, identifying the percentage of area under drought, and the magnitude, duration and frequency of droughts. Each pixel was analysed to investigate the impacts of severe and extreme drought events on vegetation properties, and the Pearson’s correlation between NDVI/EVI and SPEI/SPI at different time scales was estimated. Finally, on the basis of the correlation maps and on the CORINE Land Cover 2020, drought impacts on different vegetation communities at medium (12 months) and long (24 and 36 months) time scales were detected as the percentage of vegetation under drought stress.

The study highlights the importance of applying multiple indices to study droughts, since even though positive temperature trends were recorded in northern Italy, in the last two decades the main trigger of droughts is the lack of precipitation. Moreover the western portion of northern Italy was mostly interested by drought intensification. The investigation on drought duration revealed that the longest extreme drought events were detected in the Po Valley, where the strongest negative impacts on vegetation were detected. The results also indicated that first droughts interested herbaceous vegetation, while subsequently affecting also sparse and open forests.