Warming‐Induced Earlier Greenup Leads to Reduced Stream Discharge in a Temperate Mixed Forest Catchment

• Published in: Journal of geophysical research. Biogeosciences 2018-06, Vol.123 (6), p.1960-1975
• Main Authors: Kim, Ji Hyun, Hwang, Taehee, Yang, Yun, Schaaf, Crystal L., Boose, Emery, Munger, J. William
• Format: Article
• Language: English
• Subjects: Catchment area; Catchments; Climate change; Computer simulation; Coniferous forests; Deciduous forests; Deciduous trees; Discharge; Dormancy; Dynamics; Ecohydrology; Evapotranspiration; Fluxes; Forest watersheds; Forests; Growing season; growing season length; Hydrologic models; hydrologic nonstationarity; Hydrologic regime; Hydrology; Mixed forests; Moisture content; Phenology; Primary production; RHESSys; Rivers; Senescence; Soil; Soil water; Stream discharge; Temperate forests; Vegetation; vegetation phenology
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2018JG004438

The phenological response of vegetation to ongoing climate change may have great implications for hydrological regimes in the eastern United States. However, there have been few studies that analyze its resultant effect on catchment discharge dynamics, separating from dominant climatic controls. In this study, we examined the net effect of phenological variations on the long‐term and interannual gross primary production (GPP) and evapotranspiration (ET) fluxes in a temperate deciduous forest, as well as on the catchment discharge behavior in a mixed deciduous‐conifer forest catchment. First, we calibrated the spring and autumn leaf phenology models for the Harvard Forest in the northeastern United States, where the onsets of greenup and senescence have been significantly advanced and delayed, 10.3 and 6.0 days respectively, over the past two decades (1992–2011). We then integrated the phenology models into a mechanistic watershed ecohydrological model (RHESSys), which improved the interannual and long‐term simulations of both the plot‐scale daily GPP and ET fluxes and the catchment discharge dynamics. We found that the phenological changes amplified the long‐term increases in GPP and ET driven by the climatic controls. In particular, the earlier greenup onsets resulted in increases in annual ET significantly, while the delayed senescence onsets had less influence. Consequently, the earlier greenup onsets reduced stream discharge not only during the growing season but also during the following dormant season due to soil water depletion. This study highlights the importance of understanding vegetation response to ongoing climate change in order to predict the future hydrological nonstationarity in this region. Key Points Both carbon and water fluxes were better simulated when phenological variations were incorporated into a watershed hydrology model The long‐term increase of GPP and ET fluxes due to climate drivers were amplified by the extended growing season Earlier greenup onset reduced catchment discharge not only during growing season but also during the following dormant season