Drought and heat waves on Mediterranean forests

A study published on the international journal Land (MDPI), conducted by researchers of the LCD&D (Land Cover Dynamics and Degradation) group of CNR-IMAA together with some researchers of the University of Basilicata, explores the response to a prolonged dry period (summer 2017) of five different forest areas in Basilicata. In particular, the work analyzes the relationship between the trend of the NDVI (Normalized Difference Vegetation Index) values elaborated from data coming from the Sentinel-2 satellite sensor and the in situ observations conducted on trees through the Mannerucci and Raunkiær methods.

© JodyDellDavis on Pixabay

In recent years, Mediterranean areas, although considered rather resistant to drought events, have been characterised by increasingly frequent and important drought events. In particular, forests are among the ecosystems most significantly impacted, especially if drought occurs during a prolonged period of abnormal heat. Several studies have shown that forest species respond differently to extreme events depending on the intensity and duration of the event. Often this response is aggravated by specific local environmental conditions and is even more complex where vegetation cover is very heterogeneous. However, the high cost of in-situ measurements does not allow the collection of sufficient data to assess the large-scale effects of the natural event.

This exploratory study analyses the response of five forest areas located in Basilicata (South of Italy) following a long period in the summer of 2017 characterised by low rainfall and high temperatures. It highlights a substantial consistency between the decrease in the mean NDVI value in the study areas (estimated with the Sentinel-2 sensor) in the post-event period and the vitality of individual trees and other non-trees (meadows, shrubs, etc.) assessed by the Mannerucci and Raunkiær methods, respectively.

In particular, the comparative analysis between satellite and in situ data was carried out considering the percentage of damaged trees (assessed by the Mannerucci method), regardless of species.

The results show that the tree layer appears to be decisive in the decrease of NDVI values; in fact, the latter is proportional to the number of damaged trees (R2=0.98 for the period July-October 2017). However, the analysis of the remaining vegetation layers can not only be considered a valid support in interpreting the response of forest ecosystems in the analysed period, but it also shows that the structure/composition of the undergrowth represents a crucial element in the recovery phase.

© Francesco Ripullone – Oak groves (Gallipoli-Cognato Park, Basilicata, South Italy) heavily damaged by the prolonged dry season of 2017

In general, the work shows the suitability of the Sentinel-2 NDVI index in estimating the damage suffered by vegetation at an adequate spatial resolution (10 metres) and in accounting for the damage suffered by other types of vegetation (undergrowth) that make up forest areas. This is an important result because it shows how Sentinel-2 data, distributed free of charge and with a good acquisition frequency (about 5 days), are particularly useful in the continuous monitoring of forests, especially during extreme events, as they are able to provide information on large areas while taking into account the specific characteristics of local variability.

© Rosa Coluzzi – (a) viability classes according to the Mannerucci method; (b) percentage of damaged trees according to the Mannerucci method (classes 2-6) and Sentinel-2 NDVI values observed in the period July-October 2017.

For more information: Coluzzi, Rosa; Fascetti, Simonetta; Imbrenda, Vito; Italiano, Santain S.P.; Ripullone, Francesco; Lanfredi, Maria. 2020. “Exploring the Use of Sentinel-2 Data to Monitor Heterogeneous Effects of Contextual Drought and Heatwaves on Mediterranean Forests” Land 9, no. 9: 325.

The article was included in the list of “notable articles” compiled by Land MDPI magazine to mark International Forest Day on 21st March 2021.

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