One of the topic groupings for our posts is titled ‘Cool research’. The subject of today’s post has actually been around for a few years but I still think it’s pretty cool.
When we think of interactions between plants we usual think of negative interactions such as competition for water and nutrients or maybe allelopathy. But there are cases where plants can benefit each other. One of these is a phenomenon known as hydraulic lift. Hydraulic lift is the passive movement of water from roots into soil layers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. In essence, plants will large, deep root systems (usually trees) bring soil water from depth to the surface where it can be used by other plants. Hydraulic lift has largely been observed in arid and semi-arid ecosystems, though it can occur in wetter systems as well. For me, the research that went into discovering hydraulic lift is as fascinating as the process itself.
One of the key lines of evidence for hydraulic lift comes from studies of stable isotopes. As you may recall from college or high school chemistry, atoms of each chemical element have a certain number of protons and neutrons, which give it its mass. A small portion of each element has extra neutrons resulting in a ‘heavy isotope’. In the case of hydrogen, approximately 1 in 6400 atoms is heavy hydrogen or deuterium (2H). Interestingly, the amount of 2H in water can vary depending on the source of the water; this ratio is termed an isotopic signature. By comparing the isotopic signature of ground water and rain water in a given location, researchers can actually tell where certain plants are getting their water. One of the classic studies in this area was conducted by Todd Dawson at Cornell in the early 1990’s. Dawson analyzed isotopic signatures of groundwater, rainwater, and water in various plants around sugar maple trees and determined that many herbaceous plants contained a high proportion (up to 60%) of groundwater. But how do shallow-rooted plants obtain groundwater? The neighboring maple trees bring it to the surface from ground water as they are hydrating overnight. An efflux of water from the maple roots results in a localized increase in soil, which can be utilized by other plants: hydraulic lift.
How important in hydraulic lift in most landscapes? Probably not very. Demonstrating significant hydraulic lift requires the proper hydrology (shallow ground water accessible to trees or shrubs but not smaller plants) and limited rainfall. But the importance goes deeper (no pun intended!). Prior to the advent of stable isotope techniques, many would have been dismissive of the concept of hydraulic lift. Since 1993 over 100 papers have now been published on the subject. To me, the ultimate value of Dawson’s work and related studies is showing the importance of keeping an open mind and being receptive to new ideas.
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Cool post, Bert! Thanks for sharing.
Isotope tracing is very hard work but can produce pretty stunning results. I saw one where they used it to demonstrate water movement through hypothesized common mycorrhizal networks. I saw another cool study that showed that tree and shrub roots were reaching deep into (and using) water collected in caves. I
‘m on the side of doubting hydraulic lift is often important though – water from completely saturated soil won’t diffuse into nearby patches of dry soil.
Oops i didn’t mean water for the CMN study (prob was C), though maybe water moves too? Certainly it would through root grafts…
There’s so much we don’t know
So to way over simplify trees are straws, big beautiful straws.
@Foy, that’s the analogy I used to use when I taught plant physiology. The evaporative loss from leaves pulls the transpirational stream from the roots through the rest of the plant. Or to put it more succinctly, xylem sucks.
While we’re on the topic of water movement in plants, what ever happened to the Compensating Pressure Theory? I read about it in grad school, but am now out of the loop. I found the theory (and the heated published discussion over its validity) very interesting.
Hmmmmmm…. an AHA moment?…. have you never seen green grass under a tree, when the surrounding turf was brown. I had always attributed this very occasional phenomenon to shading of the soil surface, thus cooler, but could never reconcile it with the idea that the tree roots should have negated the impact of shading. Perhaps…..
I’ve always wondered if there’s been any competent research into using these principles to design a no-fuss irrigation system – I picture lots of very deep vertical tubes (or tiny straws) that transport groundwater to the root zone simply by capillary action.
I saw this in action a couple of years ago after I had pulled out some large tomato plants. The ground was loose and friable and the surface was very dry to about 4 inches deep. Several roots of the tomatoes broke off when I pulled them out and the still “rooted” parts were at the surface. A few hours later the ground surface for several inches around these roots was moist. It was just amazing. I have pictures, too!