Woody plant encroachment alters the vegetative and reproductive phenology of a Vereda palm swamp in Cerrado

Abstract

Woody plant encroachment (WPE) is widespread in savanna and wetland ecosystems, yet its effects on phenological dynamics at fine spatial scales remain poorly quantified. Here, we assessed how woody plant encroachment alters vegetative and reproductive phenology in a Vereda palm swamp ecosystem of the Brazilian Cerrado by integrating high-resolution unmanned aerial vehicle (RPAS; Remotely Piloted Aircraft System) imagery with ground-based phenological monitoring. Ten regions of interest representing WPE and non-invaded native Vereda vegetation were monitored monthly for twelve consecutive months using drone imagery, with 1 × 1 m sampling grids (≈ 260 m² per region). Vegetation greenness was quantified from monthly orthomosaics using the Green Chromatic Coordinate (GCC) and analyzed with linear mixed-effects models including vegetation type, time, and their interaction. Flowering phenology was monitored biweekly from January to December 2023 in transects located in the middle region of the same Vereda area. Three WPE plant species and the native Vereda community occurring in the same area were analyzed using circular statistics and permutation tests. Woody plant encroachment areas exhibited significantly higher GCC values than native Vereda vegetation, indicating greater and more persistent greenness throughout the year. A significant vegetation type × time interaction revealed distinct phenological trajectories, with WPE areas maintaining elevated greenness during dry–wet seasonal transitions and possibly resulting in greater evapotranspiration and water use during the end of the dry season. Flowering phenology of WPE species was highly synchronized (mean angle = 3.45 rad; r = 0.65), whereas native Vereda species showed a more diffuse flowering pattern (mean angle = 1.99 rad; r = 0.27). Although a temporal flowering peak shift of approximately three months was observed between groups, this difference was not statistically significant. Overall, woody plant encroachment substantially modifies vegetative phenological dynamics and tends to homogenize flowering timing in Vereda ecosystems, highlighting the value of integrating RPAS-based monitoring and field observations to detect functional changes in Cerrado wetlands.