title: A depth-controlled tracer technique measures vertical, horizontal and temporal patterns of water use by trees and grasses in a subtropical savanna.
abstract: • As described in the two-layer hypothesis, woody plants are often assumed to use deep soils to avoid competition with grasses. Yet the direct measurements of root activity needed to test this hypothesis are rare. • Here, we injected deuterated water into four soil depths, at four times of year, to measure the vertical and horizontal location of water uptake by trees and grasses in a mesic savanna in Kruger National Park, South Africa. • Trees absorbed 24, 59, 14 and 4% of tracer from the 5, 20, 50, and 120  cm depths, respectively, while grasses absorbed 61, 29, 9 and 0.3% of tracer from the same depths. Only 44% of root mass was in the top 20 cm. Trees absorbed tracer under and beyond their crowns, while 98% of tracer absorbed by grasses came from directly under the stem. • Trees and grasses partitioned soil resources (20 vs 5  cm), but this partitioning did not reflect, as suggested by the two-layer hypothesis, the ability of trees to access deep soil water that was unavailable to grasses. Because root mass was a poor indicator of root activity, our results highlight the importance of precise root activity measurements.
authors.affiliation: Department of Biological Sciences, University of Alaska, Anchorage, AK 99508, USA. afak@uaa.alaska.edu
authors.forename: Andrew
authors.initials: A
authors.lastname: Kulmatiski
authors.forename: Karen H
authors.initials: KH
authors.lastname: Beard
authors.forename: Richard J T
authors.initials: RJ
authors.lastname: Verweij
authors.forename: Edmund C
authors.initials: EC
authors.lastname: February
mesh_terms: D018533:Biomass; D017666:Deuterium Oxide / Q000378:metabolism*; D017753:Ecosystem; D007553:Isotope Labeling / Q000379:methods*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000378:metabolism / Q000502:physiology; D018517:Plant Roots / Q000378:metabolism; D006109:Poaceae / Q000378:metabolism*; D011891:Rain; D012987:Soil; D013019:South Africa; D013997:Time Factors; D014197:Trees / Q000033:anatomy & histology / Q000378:metabolism*; D014329:Tropical Climate; D014867:Water / Q000378:metabolism*
chemical_list: D012987:Soil; D014867:Water; D017666:Deuterium Oxide