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Sunday, October 2, 2011

Getting To The Root of the Problem With Desert Trees and Water Use

Deep tree roots are important for large trees. We know that these roots help to stabilize large trees, keeping them anchored during high winds, and we know that it is important to get deep rooting established in landscape trees when transplanted into urban residential environments. Early research at the University of Arizona traced deep roots of native mesquite to depths below 200 feet. More recent research suggests that these deep roots are important for other reasons.

Mesquite roots called sinkers in the Sonoran Desert
in Jerez, Mexico, near a river
that periodically overflows
Global research which surveyed maximum rooting depth of plants (trees, shrubs, perennial grasses) in natural settings (290 observations, 253 species) found a range in average maximum rooting depths from one foot (plants growing in northern tundra regions) to over 200 feet in deserts like the Kalahari; 194 species had roots at least six feet deep, 50 species had roots fifteen feet or more and 22 species had roots more than 30 feet deep.

When the researchers grouped these plants by similar natural habitats, they found the average maximum rooting depth to be 6 feet for cropland, 30 feet for deserts, 12 feet for conifer forests, and nine feet for deciduous forests. When plants were again regrouped into three groups based upon growth habit, then trees had an average maximum depth of 20 feet, shrubs 15 feet and herbaceous plants (nonwoody) 7.5 feet. This research showed that deep rooting is quite common in woody and herbaceous species in natural habitats, far deeper than the traditional view held up until now.

Deep rooting is suspected, and research supports it, to be more important than just structurally anchoring plants in the landscape. Research supports that they could be very important for moving and releasing nutrients and water, both up and down, and redistributing water and nutrients among different soil profiles. Water movement up roots into drier surface soils may affect water use estimates trees and other plants growing in their vicinity.

Many woody plants utilize deep roots for water uptake, particularly when surface soils are dry, but how they do this is not well understood. It was thought to be a combination of water “pulled” up through the tree by evapotranspiration and capillary action (like a soda straw) and little understood process called “root pressure” (like a submersible pump). Measuring water moved from deep in the soil by roots has always been difficult without disturbing the roots and accessing these roots.

However a plant process for moving water deep in the soil profile to upper soil profiles through plant roots has been identified more recently. Research found that during periods without rain, upward flow through deep roots was continuous during both day and night using a plant process researchers call hydraulic lift. Researchers identified that this process contributed up to 20% of daily water movement from that depth with no evidence of nighttime transpiration and no water storage inside the plant.

Research done in Texas on tree roots of two native trees found that roots growing at 20 to 60 feet below the soil surface contributed 20 – 50% of daily transpiration, depending on the water content of surface soils. As surface soils dried, more water was taken from deeper sources. All of this water from deeps soils was attributed to the plant’s hydraulic lift. Large quantities of this water are lifted at night. When hydraulic lift occurs at night then it is termed nocturnal hydraulic lift.

The question then becomes, if available water is moved from deep sources through deep roots during nighttime, when the plant is not transpiring for transpiration the next day, then where is this water stored? Other research indicates that water lifted from the deep soil profile is redistributed to dry, shallower soils where it is stored and used in the future. Deep root water transport varies with changes in the environment. When shallow soils become wetted again due to rain and/or the plant’s need for water decreases, hydraulic lift stops or is reduced dramatically.

Hydraulic lift is the passive movement of water from roots where water is more available to roots or root compartments where the soil is drier. It does not require plant energy. While the majority of documented cases for hydraulic lift are in native plants in desert or arid climates, recent studies (such as those in the Northeast with Sugar Maple) indicate that hydraulic lift is not restricted to desert or arid species or regions.

Release of water into the upper soil layers has been shown to benefit plants neighboring roots responsible for hydraulic lift. Because soils tend to dry from upper soil profiles downward and nutrients are usually more plentiful in the upper soil layers, lifted water may provide moisture to dry surface soils and enhance mineral uptake, beneficial microorganism growth such as mycorrhizae, and uptake of nutrients by feeder roots which typically occupy shallow soils. Some researchers feel that this is a form of plant parasitism and may have been the primary selective force in the evolution of this process. Hydraulic lift may also prolong or enhance root hair activity by keeping them hydrated.

The direction of water movement in deep roots may be upward, downward or horizontal depending on where soil moisture is more limiting. The transfer of water downwards by root systems, from lets say roots growing in wet shallow soils to dry deep soils, has been termed downward siphoning or inverse hydraulic lift; the reverse of hydraulic lift.

Such downward movement through the root system may allow growth of roots in otherwise dry soil at greater depths, permitting more rapid establishment of some plants. The amounts of water stored deep in the soil are not likely to be significant contributions if plant drought is severe. However, downward transfer of water may be important to plant establishment and the reduction of waterlogging in certain soil types. Inverse hydraulic lift may facilitate root growth into deep soil layers and transfer water away from neighboring, shallower-rooted competitors.

In addition to hydraulic lift, where water is redistributed from moist depths to dry topsoil, or inverse hydraulic lift which transfers water downward, the process of "hydraulic redistribution" includes the transfer of water horizontally, from areas that are moist to areas which are drier. In some locations and at some times of the year the subsurface transfer of water through roots may actually represent more water than the amount needed for transpiration.

How much water can this represent? Researchers aren’t really sure but field measurements of hydraulic lift in sugar maple in the Eastern U.S. have put these estimates as high as 25 gallons per night but other studies indicate much higher values. It is suspected that there is a great deal of competition for this water by neighboring plants.

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