Woody plants adapt to global change droughts in part through their hydraulic traits, including vulnerability to cavitation and ability to adjust their sources of water uptake. Dimorphic root systems offer an individual plant the ability to switch from shallow to deep sources of water. Deep roots and low vulnerability to cavitation can be important traits for surviving drought, but they have high carbon and nutrient costs. We asked, “are deep roots a good investment or is it better for plants to have lower vulnerability to cavitation?” We addressed this question with biophysical modeling and physiological data from three woodlands: a) deep-rooted Mediterranean blue oak, b) a mixed species system subject to severe drought, and c) a pinon and juniper refugia exposed to experimental drought and heat. Two of the mixed species had vulnerable xylem with deep roots while the other two species had low vulnerability and relatively shallow roots. The pinon and juniper both had deep roots with access to groundwater. Our biophysical model is the Terrestrial Regional Ecosystem Exchange Simulator (TREES), which combines abiotic and biotic drivers of ecosystem response by coupling the physics of the segmented plant hydraulics to canopy physiology, carbon allocation, and root-rhizosphere dynamics.
Results/Conclusions
During seasonal drought, blue oak survival required only a small investment in deep roots with less than seven percent of the annual water uptake from deep groundwater. During prolonged drought, high vulnerability with deep roots was a better survival strategy compared to low vulnerability with shallow roots. Under experimental drought and heat juniper maintained its hydraulic health by switching between shallow and deep sources of water, while pinon accessed primarily shallow water at the expense of greater decline in hydraulic health. These alternative responses were explained primarily by instantaneous, passive responses to hydraulic gradients. There was also evidence of slow expansion of fine root area to maximize access to available water sources, but at the cost of unsustainable decline in carbohydrate reserves, suggesting that plant survival under rapid climate change would be unlikely. The results suggest that woody plants should invest in deep roots during hydraulically favorable periods so that they can acclimate during prolonged or severe drought.