The magnitude of hydraulic redistribution by plants: A laboratory and modeling investigation

Background/Question/Methods

Hydraulic redistribution, the movement of water from moist to dry soil layers through the root system of plants, was discovered more than 20 years ago in sagebrush¹, and is known to occur in over 60 tree and grass species that grow in xeric, semiarid, temperate and tropical biomes^2,3. Modeling studies suggest that water moved via hydraulic redistribution (HR) can increase whole-stand transpiration rates and net ecosystem carbon exchange^4,5. However, the actual amount of water redistributed by plants is likely quite variable; estimates from empirical studies span more than an order of magnitude, and estimates from most modeling studies match or exceed the larger empirical estimates⁶. These disparities challenge our ability to ascertain and predict the extent of HR in various ecosystems.

Results/Conclusions

Utilizing wild sunflower species with roots divided between two pots, we found that the amount of redistributed water measured with balances was a factor of three to an order of magnitude lower than that expected from determinations of soil–root conductivity and water potential differences between pots. The disparity is explained by a variety of physical and biological factors, including nighttime transpiration, root tissue capacitance, a slow rebound of soil–root conductivity after watering, and the depletion of water potential gradients as water moves out of dense root systems into soils where small changes in water content result in large changes in water potential. Variations in these factors among species and ecological settings likely contribute to observed difference in HR magnitude, while neglecting these factors in models likely contributes to mismatches between empirical measures and modeled predictions.

References

1.   Richards, J. & Caldwell, M. Oecologia 73, 486-489 (1987).

2.   Jackson, R. B., Sperry, J. S. & Dawson, T. E. Trends Plant Sci 5, 482-488 (2000).

3.   Caldwell, M. M., Dawson, T. E. & Richards, J. H. Oecologia 113, 151-161 (1998).

4.   Domec, J. et al. New Phytol. 1-13 (2010).

5.   Lee, J. E., Oliveira, R. S., Dawson, T. E. & Fung, I. P Natl Acad Sci Usa 102, 17576-17581 (2005).

6.   Neumann, R. B. & Cardon, Z. G. New Phytol. In preparation.