Ecohydrological Impacts Of Woody-Plant Encroachment: Seasonal Patterns Of Water And Carbon Dioxide Exchange Within A Semiarid Riparian Environment

  • Sites: US-LS1
  • SCOTT, R. L., HUXMAN, T. E., WILLIAMS, D. G., GOODRICH, D. C. (2006/02) Ecohydrological Impacts Of Woody-Plant Encroachment: Seasonal Patterns Of Water And Carbon Dioxide Exchange Within A Semiarid Riparian Environment, Global Change Biology, 12(2), 311-324.
  • Funding Agency: USDA-ARS

  • Across many dryland regions, historically grass-dominated ecosystems have been encroached upon by woody-plant species. In this paper, we compare ecosystem water and
    carbon dioxide (CO2) fluxes over a grassland, a grassland–shrubland mosaic, and a fully
    developed woodland to evaluate potential consequences of woody-plant encroachment
    on important ecosystem processes. All three sites were located in the riparian corridor of
    a river in the southwest US. As such, plants in these ecosystems may have access to
    moisture at the capillary fringe of the near-surface water table. Using fluxes measured by
    eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net
    ecosystem exchange of carbon dioxide (NEE) increased with increasing woody-plant
    dominance. Growing season ET totals were 407, 450, and 639 mm in the grassland,
    shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and
    473 mm. This excess was derived from groundwater, especially during the extremely dry
    premonsoon period when this was the only source of moisture available to plants. Access
    to groundwater by the deep-rooted woody plants apparently decouples ecosystem ET
    from gross ecosystem production (GEP) with respect to precipitation. Compared with
    grasses, the woody plants were better able to use the stable groundwater source and had
    an increased net CO2 gain during the dry periods. This enhanced plant activity resulted
    in substantial accumulation of leaf litter on the soil surface that, during rainy periods,
    may lead to high microbial respiration rates that offset these photosynthetic fluxes.
    March–December (primary growing season) totals of NEE were 63, 212, and
    233 g C m2 in the grassland, shrubland, and woodland, respectively. Thus, there was
    a greater disparity between ecosystem water use and the strength of the CO2 sink as
    woody plants increased across the encroachment gradient. Despite a higher density of
    woody plants and a greater plant productivity in the woodland than in the shrubland, the
    woodland produced a larger respiration response to rainfall that largely offset its higher
    photosynthetic potential. These data suggest that the capacity for woody plants to exploit
    water resources in riparian areas results in enhanced carbon sequestration at the expense
    of increased groundwater use under current climate conditions, but the potential does
    not scale specifically as a function of woody-plant abundance. These results highlight the
    important roles of water sources and ecosystem structure on the control of water and
    carbon balances in dryland areas.