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.
