Centennial changes in forest cover and water yield in a flatwoods watershed

Forest cover has changed globally over the last century, with important impacts on catchment hydrological function. To better quantify changes in forest density, we integrated in-situ leaf area index (LAI) measurements, historical aerial images starting in the late 1930s, and satellite images starting in the mid 1980s to construct a 90-year time series of canopy cover (CC) and LAI for a flatwoods watershed in north Florida. We applied an existing stand-level model of Florida's forest evaporation to predict water yield (catchment annual discharge) and compared these estimates to observed discharge spanning the same period. The centennial time series of forest cover reconstruction indicates substantial increases in upland LAI, rising rapidly from less than 2.0 in the late 1930s until it stabilized in the 1980s and settled at ∼2.7 by the more recent decade (2015-2024). In contrast, wetland LAI, which is consistently higher than upland values, remained stable through the entire record except for marked variability between 1984 and 1992. Temporal fluctuations in predicted annual water yield, driven by variation in LAI and climate, aligned well with observed water yield records (R² = 0.60) and outperformed a climate-only model (R² = 0.55). While our results suggest significant forest cover changes over the last century, the timing of these changes does not align with the timing of dramatic declines in observed water yield evident starting in the late 1990s. However, incentivized reductions in forest density to match historical conditions could increase river flows by as much as 5 cm yr^-1, or roughly 20 %, meaningfully offsetting recent observed flow declines (∼7.2 cm yr^-1). Overall, our centennial-scale reconstruction of CC and LAI demonstrates the utility of multi-source data fusion for long-term forest assessment, quantifies the clear increases in upland forest density over time and highlights the resulting impacts of upland forest densification on catchment water balances, offering new evidence for programs incentivizing integrative natural resource management.