Water is scarce at the Santa Rita Experimental Range in southern Arizona. The range receives less than 50 centimeters (20 inches) of precipitation each year, and much of that falls during the summer monsoon season from July to September. However, the aridity does not deter plant life from taking root and thriving in the baking Sun.
To survive the harsh conditions, the vegetation in the region employs savvy adaptations to make the best use of the water when it is available. Hydraulic redistribution is one such technique and refers to a plant’s ability to relocate water from wet to dry soil layers using its roots as conduits. Scientists have observed this water redistribution phenomenon in plants spanning climates and ecosystems around the world. In the savannas of Santa Rita, mesquite trees are masters of the practice.
Lee et al. explored how water in the savanna ecosystem is partitioned throughout the year and how mesquites use hydraulic redistribution to sequester some of that water for their own use. The researchers developed a “shared resource model” to evaluate how the trees and bunchgrass in the understory compete for the limited soil moisture. They validated the model with sap flow data collected in the field. The heat flow between sensors embedded in the roots serves as a proxy for flux of water in the tree, which is a measure of hydraulic redistribution.
Measurement of sap flow in the trunk and roots of mesquite trees (Prosopis velutina) in the savannas of southern Arizona helps scientists understand the direction and magnitude of sap flow, which leads to inferences regarding hydraulic redistribution. Credit: Praveen Kumar
The research showed that 83% of the water entering the ecosystem evaporated from the soil or escaped from plants as they absorbed carbon dioxide from the atmosphere, a process known as transpiration. The rest of the water was stored in the soil and used during the following dry season.
Of the water in the soil, 13% was transported to deep soil layers through hydraulic redistribution in the mesquite roots. More than half of that water later returned to the shallow soil layers during the dry season. The researchers found that the return of the water to the surface soil did not significantly benefit the understory grass, however, which indicated that mesquites use hydraulic redistribution to gain a competitive advantage over the neighboring bunchgrass.
The study offers novel insights into the mechanisms that drive the movement of water between plants and the environment. It also further elucidates how plants survive in the harsh conditions of arid and semiarid environments. (Water Resources Research, https://doi.org/10.1029/2017WR021006, 2018)
—Aaron Sidder, Freelance Writer
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