Water linking roots, stems, and leaves

Selection on plants has always worked to coordinate the functions of all the plant parts together. Demand must be coordinated with supply. The demand for N and water by leaves cannot outstrip the ability of roots to acquire them, or stems to move them. Root growth needs to be in balance with shoot carbon supply. 

Bucci et al. just published a nice study on the coordination between roots, stems, and leaves for moving water among Patagonian woody species. Deeply rooted species have access to lots of water at all times. Shallowly rooted species undergo periodic water stress as the shallow soils dry out. 

They found that deeply rooted species had low hydraulic conductivity (water moves slow through stems and leaves), low SLA, and high wood density. The shallower-rooted species, even though they were frequently under water stress, had high conductivity in stems and leaves, high SLA, and low wood density.

The patterns are great, but I think the authors interpret the patterns wrong. Plants with access to lots of water and no water stress should have high conductivity, not low. Why if the shallow species frequently experience severe water stress wouldn't they be more resistant to cavitation, which would lower conductivity? The authors state that "It appears that the marginal cost of having an extensive root system (e.g., high Rho_w and root hydraulic resistance) contributes to low growth rates of the deeply rooted species." 

More likely, all the nutrients are in the shallow soils and the deeply rooted species are adapted to low nutrient availability. The shallow species have periods of high nutrient availability and need to grow quick. It's the high resource strategy, which can end catastrophically if soils dry out too quickly, but also ends by the superior canopy of faster-growing competitors if they are built to withstand drying later.