Comparison of climate-phenology-hydrology associations at two long-term studied forest watersheds in subtropical mountainous Taiwan

Forested watersheds provide clean water and stabilize hydrological services. Associations between vegetation growth and climatic variation significantly influence hydrological regimes, which are region-dependent. However, the climate-phenology-hydrology nexus has rarely been investigated in subtropical forested watersheds, which remain underexplored due to their year-round vegetation activity complicating the phenological shift detection, and highly variable seasonal rainfall introducing uncertainty in hydrological modeling. Understanding these dynamics provides insights into subtropical forests' buffering capacities against climatic fluctuations and their water regulation. This study examined monthly temperature and precipitation impacts on vegetation growth using monthly photosynthetic active vegetation cover fraction (PV) from satellite imagery, assessing the effects of spring and summer rainfall (2001-2020) on vegetation phenology and streamflow in subtropical Fushan (annual precipitation >4000 mm) and tropical Leinhuachi (annual precipitation >2300 mm) Experimental Forests. PV and temperature exhibited linear correlations without time-lag effect (R-2 = 0.51-0.57, p < 0.001). However, PV and precipitation had no time-lag in Fushan, but demonstrated a log-linear relationship with two-month lag in Leinhuachi (R-2 = 0.15-0.59, p < 0.001), highlighting rainfall accumulation during the relatively dry season (winter-spring) as critical for vegetation growth. Structural equation modeling (SEM) revealed that an earlier start of growing season (SOS), driven by high spring rainfall (February-March), led to an extended growing season and higher P-Q deficit (precipitation minus runoff) during Leinhuachi's growing season (Goodness of fit index = 0.988, chi(2) = 0.825, df = 2, p = 0.662). Surprisingly, abundant growing season precipitation had no significant impact on season end, length, or P-Q deficit. These patterns were absent in Fushan (Goodness of fit index = 0.997, chi(2) = 0.137, df = 2, p = 0.934). Integrating seasonal precipitation variability into watershed management is critical for water security, particularly in drought-prone subtropical regions. Furthermore, incorporating these climate-phenology-hydrology patterns into climate change models will enhance predictions of ecosystem responses, especially where seasonal precipitation affects vegetation productivity, water budgets, and carbon cycling. Understanding subtropical forest regulation of water and carbon cycles is essential to improve climate projections and conservation policies.