Eleven years ago, in a review article, Clothier and Green (1997) assessed the hydrologic budget at the land surface and pronounced that plant roots are “the big movers of water and chemical” in the vadose zone. That assessment seems fair considering that all transpiration begins with the extraction of water from soil by roots and that transpiration sends approroximately 40% of global terrestrial precipitation back to the atmosphere (vs. about 20% by evaporation) (Gerten et al., 2005). Clothier and Green (1997) went on to note that considerable knowledge and understanding of roots had been gained from research begun...

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