Hydraulic redistribution

Hydraulic redistribution is a passive mechanism where water is transported from moist to dry soils via subterranean networks.[1] It occurs in vascular plants that commonly have roots in both wet and dry soils, especially plants with both taproots that grow vertically down to the water table, and lateral roots that sit close to the surface. In the late 1980s, there was a movement to understand the full extent of these subterranean networks.[2] Since then it was found that vascular plants are assisted by fungal networks which grow on the root system to promote water redistribution.[1][3][4]

Process

Hot, dry periods, when the surface soil dries out to the extent that the lateral roots exude whatever water they contain, will result in the death of such lateral roots unless the water is replaced. Similarly, under extremely wet conditions when lateral roots are inundated by flood waters, oxygen deprivation will also lead to root peril. In plants that exhibit hydraulic redistribution, there are xylem pathways from the taproots to the laterals, such that the absence or abundance of water at the laterals creates a pressure potential analogous to that of transpirational pull. In drought conditions, ground water is drawn up through the taproot to the laterals and exuded into the surface soil, replenishing that which was lost. Under flooding conditions, plant roots perform a similar function in the opposite direction.

Though often referred to as hydraulic lift, movement of water by the plant roots has been shown to occur in any direction.[5][6][7] This phenomenon has been documented in over sixty plant species spanning a variety of plant types (from herbs and grasses to shrubs and trees)[8][9][10] and over a range of environmental conditions (from the Kalahari Desert to the Amazon Rainforest).[8][9][11][12]

Causes

The movement of this water can be explained by a water transport theory throughout a plant. This well-established water transport theory is called the cohesion-tension theory. In brief, it explains the movement of water throughout the plant depends on having a continuous column of water, from the leaves to roots. Water is then pulled up from the roots to the leaves moving throughout the plant's vascular system, all facilitated by the differences in water potential in the boundary layers of the soil and the atmosphere. Therefore, the driving force for moving water through a plant is the cohesive strength of water molecules and a pressure gradient from the roots to the leaves. This theory is still applied when the boundary layer to the atmosphere is closed, e.g. when plant stomata are closed or in senesced plants.[13] The pressure gradient is developed between soil layers with different water potentials causing water to move by the roots from wetter to drier soil layers in a similar manner as when a plant is transpiring.

Fungal associations
Main article: Mycorrhizal network

It has been understood that hydraulic lift aids the host plant and its neighboring plants in the transportation of water and other vital nutrients.[2] At that time, the hydraulic lift described as the movement of water and soil nutrients from a vascularized host into the soil during at night mostly.[2] Then after studies in the 2000s, a more comprehensive word was taken into consideration where it described a bi-directional and passive movement exhibited by the plant roots and further assisted by mycorrhizal networks.[2][3][14] A 2015 study then described a "direct transfer of hydraulically redistributed water" between the host and fungi into the surrounding root system.[3] As mentioned, hydraulic redistribution not only transports water but nutrients as well.[14] The fungi most likely to form water and nutrient networks are Ectomycorrhizae and Arbuscular mycorrhizae.[3]

Significance

The ecological importance of hydraulically redistributed water is becoming better understood as this phenomenon is more carefully examined. Water redistribution by plant roots has been found influencing crop irrigation, where watering schemes leave a harsh heterogeneity in soil moisture. This influencing process also assist in seedling success.[3][4] The plant roots have been shown to smooth or homogenize the soil moisture. This sort of smoothing out of soil moisture is important in maintaining plant root health. The redistribution of water from deep moist layers to shallow drier layers by large trees has shown to increase the moisture available in the daytime to meet the transpiration demand.

The implications of hydraulic redistribution seem to have an important influence on plant ecosystems. Whether or not plants redistribute water through the soil layers can affect plant population dynamics, such as the facilitation of neighboring species.[15] The increase in available daytime soil moisture can also offset low transpiration rates due to drought (see also drought rhizogenesis) or alleviate competition for water between competing plant species. Water redistributed to the near surface layers may also influence plant nutrient availability.[16]

Observations and modeling

Due to the ecological significance of hydraulically redistributed water, there is an ongoing effort to continue the categorization of plants exhibiting this behaviour and adapting this physiological process into land-surface models to improve model predictions.

Traditional methods of observating hydraulic redistribution include Deuterium isotope traces,[7][9][12][17] sap flow,[8][11][18][19] and soil moisture.[6][9] In attempts to characterize the magnitude of the water redistributed, numerous models (both empirically and theoretically based) have been developed.[20]

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See also

References
  1. Allen, Michael F.; Vargas, Rodrigo; Prieto, Iván; Egerton-Warburton, Louise M.; Querejeta, José Ignacio (2012-06-01). "Changes in soil hyphal abundance and viability can alter the patterns of hydraulic redistribution by plant roots". Plant and Soil. 355 (1–2): 63–73. doi:10.1007/s11104-011-1080-8. ISSN 1573-5036.
  2. Meinzer, Frederick C.; Clearwater, Michael J.; Goldstein, Guillermo (2001-06-01). "Water transport in trees: current perspectives, new insights and some controversies". Environmental and Experimental Botany. 45 (3): 239–262. doi:10.1016/S0098-8472(01)00074-0. ISSN 0098-8472. PMID 11323032.
  3. Querejeta, José I.; Navarro-Cano, José A.; Alguacil, María del Mar; Huygens, Dries; Roldán, Antonio; Prieto, Iván (2016-09-01). "Species-specific roles of ectomycorrhizal fungi in facilitating interplant transfer of hydraulically redistributed water between Pinus halepensis saplings and seedlings". Plant and Soil. 406 (1–2): 15–27. doi:10.1007/s11104-016-2860-y. ISSN 1573-5036.
  4. Simard, Suzanne; Bingham, Marcus A. (2012-03-01). "Ectomycorrhizal Networks of Pseudotsuga menziesii var. glauca Trees Facilitate Establishment of Conspecific Seedlings Under Drought". Ecosystems. 15 (2): 188–199. doi:10.1007/s10021-011-9502-2. ISSN 1435-0629.
  5. Burgess, S. S. O.; Adams, M. A.; Turner, N. C.; Beverly, C. R.; Ong, C. K.; Khan, A. A. H.; Bleby, T. M. (2001). "An improved heat pulse method to measure low and reverse rates of sap flow in woody plants". Tree Physiology. 21 (9): 589–598. doi:10.1093/treephys/21.9.589.
  6. Richards, J. H.; Caldwell, M. M. (1987). "Hydraulic lift: Substantial nocturnal water transport between soil layers by Artemisia tridentata roots". Oecologia. 73 (4): 486–489. Bibcode:1987Oecol..73..486R. doi:10.1007/BF00379405. PMID 28311963.
  7. Smart, D. R.; Carlisle, E.; Goebel, M.; Nunez, B. A. (2005). "Transverse hydraulic redistribution by a grapevine". Plant, Cell and Environment. 28 (2): 157–166. doi:10.1111/j.1365-3040.2004.01254.x.
  8. Oliveira, Rafael S.; Dawson, Todd E.; Burgess, Stephen S. O.; Nepstad, Daniel C. (2005). "Hydraulic redistribution in three Amazonian trees". Oecologia. 145 (3): 354–363. Bibcode:2005Oecol.145..354O. doi:10.1007/s00442-005-0108-2. PMID 16091971.
  9. Dawson, Todd E. (1993). "Hydraulic lift and water use by plants: Implications for water balance, performance and plant-plant interactions". Oecologia. 95 (4): 565–574. Bibcode:1993Oecol..95..565D. doi:10.1007/BF00317442. PMID 28313298.
  10. Schulze, E.-D.; Caldwell, M. M.; Canadell, J.; Mooney, H. A.; Jackson, R. B.; Parson, D.; Scholes, R.; Sala, O. E.; Trimborn, P. (1998). "Downward flux of water through roots (i.e. Inverse hydraulic lift) in dry Kalahari sands". Oecologia. 115 (4): 460–462. Bibcode:1998Oecol.115..460S. doi:10.1007/s004420050541. PMID 28308264.
  11. Burgess, S.; Bleby, T. M. (2006). "Redistribution of soil water by lateral roots mediated by stem tissues". Journal of Experimental Botany. 57 (12): 3283–3291. doi:10.1093/jxb/erl085. PMID 16926237.
  12. Schulze, E. -D; Caldwell, M. M.; Canadell, J.; Mooney, H. A.; Jackson, R. B.; Parson, D.; Scholes, R.; Sala, O. E.; Trimborn, P. (1998). "Downward flux of water through roots (i.e. Inverse hydraulic lift) in dry Kalahari sands". Oecologia. 115 (4): 460. Bibcode:1998Oecol.115..460S. doi:10.1007/s004420050541. PMID 28308264.
  13. Leffler, A. Joshua; Peek, Michael S.; Ryel, Ron J.; Ivans, Carolyn Y.; Caldwell, Martyn M. (2005). "Hydraulic Redistribution Through the Root Systems of Senesced Plants". Ecology. 86 (3): 633–642. doi:10.1890/04-0854.
  14. Allen, Michael F. (2009). "Bidirectional water flows through the soil–fungal–plant mycorrhizal continuum". New Phytologist. 182 (2): 290–293. doi:10.1111/j.1469-8137.2009.02815.x. ISSN 1469-8137. PMID 19338631.
  15. Caldwell, Martyn M.; Dawson, Todd E.; Richards, James H. (1998). "Hydraulic lift: Consequences of water efflux from the roots of plants". Oecologia. 113 (2): 151–161. Bibcode:1998Oecol.113..151C. doi:10.1007/s004420050363. PMID 28308192.
  16. Ryel, R.; Caldwell, M.; Yoder, C.; Or, D.; Leffler, A. (2002). "Hydraulic redistribution in a stand of Artemisia tridentata: Evaluation of benefits to transpiration assessed with a simulation model". Oecologia. 130 (2): 173–184. Bibcode:2002Oecol.130..173R. doi:10.1007/s004420100794. PMID 28547139.
  17. Brooks, J. R.; Meinzer, F. C.; Coulombe, R.; Gregg, J. (2002). "Hydraulic redistribution of soil water during summer drought in two contrasting Pacific Northwest coniferous forests". Tree Physiology. 22 (15–16): 1107–1117. doi:10.1093/treephys/22.15-16.1107. PMID 12414370.
  18. Burgess, Stephen S. O.; Adams, Mark A.; Turner, Neil C.; Ong, Chin K. (1998). "The redistribution of soil water by tree root systems". Oecologia. 115 (3): 306–311. Bibcode:1998Oecol.115..306B. doi:10.1007/s004420050521. PMID 28308420.
  19. Meinzer, F. C.; James, S. A.; Goldstein, G. (2004). "Dynamics of transpiration, sap flow and use of stored water in tropical forest canopy trees". Tree Physiology. 24 (8): 901–909. doi:10.1093/treephys/24.8.901. PMID 15172840.
  20. Neumann, Rebecca B.; Cardon, Zoe G. (2012). "The magnitude of hydraulic redistribution by plant roots: A review and synthesis of empirical and modeling studies". New Phytologist. 194 (2): 337–352. doi:10.1111/j.1469-8137.2012.04088.x. PMID 22417121.

Further reading
  • Horton, Jonathan L.; Hart, Stephen C. (1998). "Hydraulic lift: a potentially important ecosystem process". Trends in Ecology and Evolution. 13 (6): 232–235. doi:10.1016/S0169-5347(98)01328-7. PMID 21238277.
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