Impacts of hydraulic redistribution on grass–tree competition vs facilitation in a semi-arid savanna

Greg A. Barron-Gafford, Enrique P. Sanchez-Cañete, Rebecca L. Minor, Sean M. Hendryx, Esther Lee, Leland F. Sutter, Newton Tran, Elizabeth Parra, Tony Colella, Patrick C. Murphy, Erik P. Hamerlynck, Praveen Kumar, Russell L. Scott

Research output: Contribution to journalArticlepeer-review


A long-standing ambition in ecosystem science has been to understand the relationship between ecosystem community composition, structure and function. Differential water use and hydraulic redistribution have been proposed as one mechanism that might allow for the coexistence of overstory woody plants and understory grasses. Here, we investigated how patterns of hydraulic redistribution influence overstory and understory ecophysiological function and how patterns vary across timescales of an individual precipitation event to an entire growing season. To this end, we linked measures of sap flux within lateral and tap roots, leaf-level photosynthesis, ecosystem-level carbon exchange and soil carbon dioxide efflux with local meteorology data. The hydraulic redistribution regime was characterized predominantly by hydraulic descent relative to hydraulic lift. We found only a competitive interaction between the overstory and understory, regardless of temporal time scale. Overstory trees used nearly all water lifted by the taproot to meet their own transpirational needs. Our work suggests that alleviating water stress is not the reason we find grasses growing in the understory of woody plants; rather, other stresses, such as excessive light and temperature, are being ameliorated. As such, both the two-layer model and stress gradient hypothesis need to be refined to account for this coexistence in drylands.

Original languageEnglish (US)
Pages (from-to)1451-1461
Number of pages11
JournalNew Phytologist
Issue number4
StatePublished - Sep 2017


  • drylands
  • hydraulic lift
  • mesquite (Prosopis velutina)
  • photosynthesis
  • sap flow
  • woody plant encroachment

ASJC Scopus subject areas

  • Physiology
  • Plant Science


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