Barron-Gafford et al., 2017

Paper/Book

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

Barron-Gafford G.A., Sanchez-Cañete E.P., Minor R.L., Hendryx S.M., Lee E., Sutter L.F., Tran N., Parra E., Colella T., Murphy P.C., Hamerlynck E.P., Kumar P. and Scott R.L. (2017)
New Phytologist 215(4): 1451–1461  Cross-CZO

Plain English Summary

  • 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.

Illustration of potential rates and direction of hydraulic redistribution in overstory trees (hydraulic descent (HD) vs hydraulic lift (HL)) and the resulting effect on understory plants. HD by a tree, driven by a gradient of shallow wet to deep dry soil water potentials, creates a period of competition for water amongst the tree and understory plants. Alternatively, periods of HL, in response to deep wet and dry shallow soil water potentials, may facilitate understory plant function during periods of otherwise moisture stress. The x-axis represents time through a wet-to-dry transition. The axis is dimensionless because this may represent a period of time around a rain event or across an entire growing season of relatively available moisture and then drought.

Illustration of potential rates and direction of hydraulic redistribution in overstory trees (hydraulic descent (HD) vs hydraulic lift (HL)) and the resulting effect on understory plants. HD by a tree, driven by a gradient of shallow wet to deep dry soil water potentials, creates a period of competition for water amongst the tree and understory plants. Alternatively, periods of HL, in response to deep wet and dry shallow soil water potentials, may facilitate understory plant function during periods of otherwise moisture stress. The x-axis represents time through a wet-to-dry transition. The axis is dimensionless because this may represent a period of time around a rain event or across an entire growing season of relatively available moisture and then drought.

Citation

Barron-Gafford G.A., Sanchez-Cañete E.P., Minor R.L., Hendryx S.M., Lee E., Sutter L.F., Tran N., Parra E., Colella T., Murphy P.C., Hamerlynck E.P., Kumar P. and Scott R.L. (2017): Impacts of hydraulic redistribution on grass–tree competition vs facilitation in a semi-arid savanna. New Phytologist 215(4): 1451–1461. DOI: 10.1111/nph.14693

This Paper/Book acknowledges NSF CZO grant support.