Investigating phenological characteristics of two invasive macrophytes across gradients using satellite data time series

Invasive macrophytes threaten shallow aquatic ecosystems by outcompeting native species and causing significant ecological and economic damage. This study examines two species that are widespread in the northern hemisphere and have colonised a number of sites in Europe and North America in the last decade: Nelumbo nucifera Gaertn. (sacred lotus, native to East Asia) and Ludwigia hexapetala (Hook. & Arn.) Zardini, H.Y. Gu & P.H. Raven (water primrose, native to Central and South America), comparing their phenological traits and productivity across different environmental gradients: native vs. non-native ranges and different climatic regions. Sentinel-2 satellite data (10 m spatial resolution, 12 spectral bands) were used to generate time series of the Water Adjusted Vegetation Index (WAVI), a proxy for canopy density and biomass, over six years (2017-2022) at seven study sites: Mantua lakes system and Lake Varese (humid subtropical climate, non-native range for both species) in Italy, Lake Fangzheng, Lake Bayangdian and Lake Xuanwu (humid continental, cold semi-arid and humid subtropical climate, respectively, native range for N. nucifera) in China, Lake Grand-Lieu in France and Santa Rosa Lagoon in the USA (temperate oceanic and warm-summer Mediterranean climate, respectively, non-native range for L. hexapetala). Seasonal dynamics parameters (phenological metrics and productivity) were extracted from WAVI time series, and their meteorological and environmental drivers were analysed using parametric generalised additive mixed models (GAMMs). The results indicate that N. nucifera has higher canopy density and productivity in non-native than in native sites, and that the onset and termination of the growing season follow a latitudinal gradient within the native range. For L. hexapetala, differences among invaded sites appear to be driven more by the timing of species establishment and its adaptation to local conditions. The GAMMs models showed that climate and site differences are key to understanding the phenology of N. nucifera - driven by temperature and summer radiation, which favours longer seasons and increased productivity - whereas water quality and local conditions determine that of L. hexapetala, whose growth rate is influenced by site turbidity, while senescence is accelerated by wind conditions and summer radiation. This approach can be easily extended to other macrophytes and can benefit studies on the variability of the eco-physiological characteristics of invasive macrophytes under climate change scenarios.