Reconciling simulations of seasonal carbon flux and soil water with observations using tap roots and hydraulic redistribution: A multi-biome FLUXNET study.

Understanding the response of plants to soil moisture stress is important given a future climate subject to greater extremes, including drought. Nevertheless, major discrepancies still exist between observed and simulated seasonal carbon, water and energy fluxes at the vegetated land-surface. For tropical forest, these discrepancies have been reduced, especially during the dry season, by taking account of tap roots and hydraulic redistribution. The expanding FLUXNET open-access archive allows the current study to extend the investigation of seasonal drought-stress to ten different vegetation types. A state-of-the-art land-surface model is enhanced to take account of tap roots and hydraulic redistribution in order to compare with traditional simulations. Carbon fluxes and fractional soil water content are simulated and compared against observations. We find that a traditional approach, by neglecting tap roots, simulates a seasonal drought for trees and shrubs which is generally too severe compared to observed net carbon flux. The introduction of a tap root benefits tropical broadleaf forest and other ecosystems with high annual potential evapotranspiration in reducing observation-model discrepancies. Our simulations suggest a minor role for hydraulic redistribution, modifying weekly soil moisture rather than substantially changing seasonal water flux totals. [ABSTRACT FROM AUTHOR]