Wavelet coherence analysis to assess cross-correlation of Mediterranean vegetation and drought condition at local scale

Intensive droughts have become increasingly prevalent worldwide, with significant implications for ecosystems. The Mediterranean region, particularly vulnerable to climate change, has witnessed escalating aridity over recent decades. This study investigates the responses of two Mediterranean SIC sites in Italy, characterized by distinct vegetation types, to aridity. The analysis relies on historical time series data of the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI), obtained through remote sensing technology. The objectives of the study encompass examining the temporal cause-effect relationship between vegetation response and aridity, as measured by the Standardized Precipitation Evapotranspiration Index (SPEI). To achieve this, the application of cross-spectrum wavelet coherence (SP-WTC) analysis is employed. This technique enables the identification of frequency bands and time intervals exhibiting covariation between drought conditions (SPEI) and vegetation indices (NDVI or EVI). The research highlights the significant role of wavelet coherence in capturing both time and frequency localization, aiding the detection of vegetation changes resulting from extreme events like droughts. Additionally, the phase difference of wavelet coherence is computed to elucidate oscillation patterns between the time-series, providing insights into correlation, lagging, and leading relationships. The findings of this study will contribute to a better understanding of how different Mediterranean vegetation species and communities respond to aridity. The analysis of long-term vegetation dynamics and their interaction with aridity is crucial for informing policy and management practices in the Mediterranean region. Furthermore, this research showcases the potential of remote sensing techniques, such as wavelet analysis, for large-scale vegetation monitoring and assessment of ecosystem responses to changing climatic conditions.