A Mesic Maximum for Vegetation Water Use Across Biomes

Abstract:

How efficiently different biomes are at using locally available resources is a critical determinate of overall ecosystem structure and function; yet in the case of water, a fundamental biologic necessity, the transpiration to precipitation ratio remains poorly characterized across climates. Since development in the 1970’s, the Budyko framework has provided a simple, first-order, approach to partitioning total rainfall into runoff and evapotranspiration across climates. However, this classic paradigm provides little insight into the strength of biological mediation (i.e. transpiration flux) of the hydrologic cycle. Based on both a synthesis of field-based studies and ecohydrolgoic models we extend this framework to further partition evapotranspiration across climates and demonstrate that the fraction of rainwater used by vegetation reaches a maximum when potential evapotranspiration is approximately equal to rainfall. This ‘mesic maximum’ in the transpiration to precipitation ratio arises because plant water availability is limited in xeric ecosystems due to elevated soil evaporation driven by high surface energy loading, while transpiration is limited in hydric ecosystems due to elevated interception caused by large canopy holding capacity. The amount of rainwater that vegetation is able to transpire directly determines the total productivity of ecosystems, and this extension of the Budyko framework outlines the connection between current and future climate regimes, hydrologic flux partitioning, and macro-system ecology.