Water uptake and redistribution during drought in a semiarid shrub species.

In arid systems, most plant mortality occurs during long drought periods when water is not available for plant uptake. In these systems, plants often benefit from scarce rain events occurring during drought but some of the mechanisms underlying this water use remain unknown. In this context, plant water use and redistribution after a large rain event could be a mechanism that allows deep-rooted shrubs to conservatively use water during drought. We tested this hypothesis by comparing soil and plant water dynamics in Artemisia tridentata ssp. vaseyana (Rydb.) Beetle shrubs that either received a rain event (20mm) or received no water. Soil water content (SWC) increased in shallow layers after the event and increased in deep soil layers through hydraulic redistribution (HR). Our results show that Artemisia shrubs effectively redistributed the water pulse downward recharging deep soil water pools that allowed greater plant water use throughout the subsequent drought period, which ameliorated plant water potentials. Shrubs used shallow water pools when available and then gradually shifted to deep-water pools when shallow water was being used up. Both HR recharge and the shift to shallow soil water use helped conserve deep soil water pools. Summer water uptake in Artemisia not only improved plant water relations but also increased deep soil water availability during drought.

Hydraulic lift promotes selective root foraging in nutrient-rich soil patches.

Hydraulic lift (HL) - the passive movement of water through plant roots from deep wet to shallow drier soil layers - can improve root survival in dry soils by providing a source of moisture to shallow roots. It may also enhance plant nutrient capture, though empirical evidence for this is scarce and whether HL promotes the selective placement of roots in nutrient-rich soil enhancing nutrient capture in dry soils remains unknown. We tested this with a split-pot design in which we separated the root system of Retama sphaerocarpa (L.) Boiss shrubs into two pot compartments: a lower, well-watered one; and an upper, drier one. Half the shrubs grew under natural light conditions hence allowed to perform HL, whereas the other half had impaired HL by maintaining continuous illumination at night. Resource-rich (organic matter enriched in 15N and P) and resource-poor soil patches were inserted in the upper compartment after a drought treatment was imposed. Artificial illumination did impair HL at night. Soil moisture in both the whole upper compartment and in soil patches was lower in plants illuminated at night and reduced the allocation of roots to nutrient-rich soil patches at the expense of root growth in nutrient-poor patches (i.e. root foraging precision). Plant nitrogen capture was also lower in shrubs with impaired HL. Overall, these results demonstrate that HL favoured the selective placement of roots in nutrient-rich patches as well as nutrient capture under drought, a process that may secure nutrient capture and maintain plant performance during drought periods.