Latitudinal Variation in the Phenology of the Emergent Species Phragmites australis and the Submerged Species Potamogeton crispus in Shallow Lakes of the North China Plain

Phenology, as a pivotal functional trait, serves as an important indicator for understanding vegetation responses to environmental variability. Against the backdrop of climate change, the latitudinal zonation of vegetation phenology has attracted widespread attention. However, how the phenology of different aquatic plants responds to latitudinal variation remains insufficiently understood. To characterize the contrasting responses of aquatic vegetation (submerged and emergent) along a latitudinal gradient, this study focuses on the submerged species Potamogeton crispus (P. crispus) and the emergent species Phragmites australis (P. australis), both of which are widely distributed across shallow lakes on the North China Plain. Based on dense time-series Normalized Difference Vegetation Index (NDVI) data from Sentinel-2 and a standardized phenological extraction method, we analyzed the spatial differentiation patterns of these two vegetation types, as well as their latitudinal phenological responses and environmental associations, including the Start of the growing Season (SOS), End of the growing Season (EOS), and length of the growing season (LOS). The results showed marked differences in the spatial distributions of P. crispus and P. australis, reflecting their distinct ecological niches within lake ecosystems. Their phenological metrics also exhibited contrasting latitudinal patterns. For P. australis, SOS showed no significant latitudinal trend, whereas EOS occurred progressively earlier and LOS became shorter with increasing latitude. For P. crispus, both SOS and EOS were delayed with increasing latitude, while LOS showed no significant latitudinal gradient. Among the linear predictors, annual mean temperature showed the highest overall importance, but its associations with the phenology of P. australis and P. crispus differed markedly. Other environmental and anthropogenic factors also showed species- and stage-specific associations with phenological variation. XGBoost-SHAP analysis (Extreme Gradient Boosting (XGBoost) combined with SHapley Additive exPlanations (SHAP)) further revealed nonlinear marginal associations between selected environmental predictors and phenological metrics, including an apparent change in the modeled EOS response of P. australis near 16 °C, which should be interpreted as an exploratory model-derived feature rather than a confirmed threshold. This study highlights contrasting latitudinal phenological responses of submerged and emergent macrophytes and provides scientific support for ecological monitoring and management of shallow lake ecosystems in the North China Plain.