Assessing phenological patterns and impacts of Ailanthus altissima using COPERNICUS satellite mission on invaded ecosystems in a Mediterranean island

Invasive alien plants (IAPs) may impact ecosystems through occupying vacant phenological or functional niches and disrupting ecosystem functioning. The Copernicus mission delivers free remote sensing data, facilitating cost-effective and timely monitoring of invaded areas. This study deploys multispectral (Sentinel-1) and thermal (Sentinel-3) Copernicus satellites to characterize ecophysiological and biophysical parameters of patches invaded by the alien tree Ailanthus altissima in Sardinia. Furthermore, it tracks the ecophysiological and biophysical temporal changes in the native vegetation caused by invasion. A total of 176 invaded patches and their non/invaded buffer areas were identified on aerial orthophotos, digitized and rasterized at the resolution of 20 x 20 m. Invaded cells dominated by A. altissima and Not Invaded cells dominated by native vegetation were classed according to the second level of the legend of the regional vegetation map (Carta della Natura). This vegetation map is based on the legend Corine Biotopes (CORINE). In particular, A. altissima tends to invade in Sardinia six vegetation classes: Mediterranean maquis (32.11, 32.12, 32.211, 32.23, 32.3, 32.4), Mediterranean sub nitrophilous grassland (34.81), Deciduous woody vegetation (41.72, 41.732), Evergreen woody vegetation (45.1, 45.21, 45.317), Agricultural herbaceous areas (82.1, 82.3), Agricultural woody areas (83.11, 83.15, 83.16, 83.21, 83.31, 84.6). After, we calculated a set of uncorrelated spectral indices for invaded patches and buffer areas as proxy of leaf chlorophyll and carotenoid content (CVI, SIPI), productivity and canopy biomass (EVI, LAI), leaf water content (NMDI, MSI), soil features (CI), and daily evapotranspiration. We analyzed the monthly trends of these indices in invaded patches and buffer areas as proxies for analyze phenological trends of A. altissima and its seasonal impacts on native vegetation, using linear mixed models (LMMs), two-way ANOVA, and Tukey’s post-hoc test. Our results highlighted the potential of Copernicus mission in capturing the temporal trends of ecophysiological and biophysical spectral parameters’ changes in the patches invaded by A. altissima, as the high conditional R2 values of LMMs ranged from 0.522 of CVI to 0.776 of LAI. Furthermore, invaded patches featured as most affected by A. altissima invasions during summer, in particular for higher productivity and canopy biomass, for greater leaf water content, for lower leaf carotenoid content, for lower bare soil presence when comparing invaded cells to native vegetation in the buffer non-invaded cells. These results confirmed that A. altissima can strongly interfere with native vegetation, especially during the summer drought period of the Mediterranean basin.