Investigating the phenology of two invasive macrophytes across environmental gradients using satellite time series

Remote sensing data enables the accurate tracking of short- and long-term vegetation changes and provides valuable information for assessing plant phenology at different scales. However, investigating plant phenology using satellites and relating it to the environmental context still presents challenges in natural, complex ecosystems such as wetlands. Within these systems, both anthropogenic disturbance and climate change pose a threat to habitat equilibrium, for example by facilitating the establishment of invasive alien species. To characterise the phenology of two invasive aquatic plant species (Ludwigia hexapetala and Nelumbo nucifera), which have a notable impact on wetlands across Europe and North America, we utilised a medium-term Sentinel-2 time series collected across seven sites, representing diverse wetland types and biogeographical regions. We then investigated the environmental drivers (meteo-climatic factors and water quality parameters) influencing different phenometrics linked to the invasion success of these plants across sites and growing seasons, using non-linear parametric models. The models demonstrated different strengths of linkage between environmental variables and selected phenological metrics. The phenometrics that were modelled most efficiently were the growth and senescence rates, and peak biomass for L. hexapetala, and the green-up and senescence start timing for N. nucifera. Assessing the most important inputs for the phenometrics models showed that the two species respond to environmental forcing at different spatial scales, reflecting their contrasting ecological strategies. Key phenology drivers are linked to climatic constraints, mainly temperature and light resources, for N. nucifera, or to local habitat features, such as residence time and wind disturbance, for L. hexapetala.