Hydrological responses to vegetation-climate interactions at two subtropical forested watersheds of Taiwan

Forested upstream watersheds support clean freshwater and maintaining stable hydrological conditions of ecosystem services. The associations between vegetation growth and climatic variations play a vital role on hydrological regimes that are region-dependent, but the associations of climate-phenology-hydrology have rarely been investigated in tropical/subtropical regions particularly. In this analysis, the hydroclimate records (1991-2020) at two long-term studied forest watersheds, Fushan (FS) and Leinhuachi (LHC) experimental forest, Taiwan were used, and showed that the incidences of meteorological and hydrological droughts are becoming prominent after 2001. We further investigated the effects of monthly climate variables (temperature and precipitation) on vegetation growth using monthly PV (photosynthetic vegetation fraction) of a watershed derived from MODIS (Moderate Resolution Imaging Spectroradiometer), and examined the effects of spring and summer rainfall on the variations of vegetation phenological patterns and subsequent watershed streamflow during 2001–2020. The PV and temperature showed a linear relationship without time-lag effect (R² = 0.51-0.57, p < 0.001), whereas PV and precipitation exhibited no time-lag in FS but a log-linear relationship with 2-month lag (R² = 0.15-0.59, p < 0.001) existed in LHC, indicating the accumulation of rainfall during relatively dry season (winter-spring) was critical for vegetation growth. Structural equation modeling (SEM) revealed that earlier start of growing season (SOS) caused by relatively high spring rainfall (February-March) led to longer growing season (LOS) and higher P-Q deficit (precipitation minus runoff) during the growing season in LHC. Nevertheless, the large amount of precipitation during growing season has no effect on the end of growing season (EOS), LOS and P-Q deficit. Neither EOS has influence on LOS and P-Q deficit. However, these patterns were not found in FS. Understanding the vegetation responses to climatic variations is required for future hydrologic regime projections, especially under changing climate.