Interesting article on tree roots : DURHAM, N.C. -- Taking advantage of a unique labyrinth of Texas caves festooned with tree roots, Duke University biologists have given trees the most exacting root-to-twig physical of their circulatory system yet. The scientists' findings reveal the impressive adaptive engineering of deep-rooted trees in adjusting the size and structure of their piping, or xylem, to maximize water uptake, maintain their integrity and avoid flow-blocking embolisms. In particular, the findings reveal how the deepest roots develop the largest conduits in order to draw deep water most efficiently. The scientists' technique of studying tree roots in place offers a far more realistic look at how trees manage their water circulation than researchers have had before. The approach is like studying the plumbing of a house by carefully tracing its pipes, valves and faucets. In contrast, the usual approach -- drilling core samples and separating tree root samples for study -- is like taking a wrecking ball to the house and sifting through the debris to understand the plumbing system. The researchers, led by biologist Robert Jackson, published their findings in the September 2004 issue of the journal "New Phytologist." Lead author on the paper was Andrew McElrone, formerly of Duke and now at St. Joseph's University in Philadelphia. Other co-authors on the paper were William Pockman of the University of New Mexico and Jordi Martez Vilalta of the University of Edinburgh in the United Kingdom. Jackson is a professor of environmental sciences and biology in the Nicholas School of the Environment and Earth Sciences and a professor in the Department of Biology at Duke. The scientists' work was supported by the National Science Foundation, the Mellon Foundation and the U. S. Department of Agriculture. In their study, the researchers took advantage of the fact that a well-mapped system of limestone caves runs beneath the Edwards Plateau in central Texas. Thrusting into those caves from above are the roots of trees, some of which reach depths of 60 feet. The Texas state government is interested in understanding the effects of such trees on water uptake because the Edwards aquifer supplies a large area of central Texas with drinking water, including the city of San Antonio. The water supply from this aquifer has been affected by the change in the landscape above from a savanna to forest, due to grazing and human prevention of natural wild fires over the last 150 years. In their experiments, Jackson and his colleagues descended into the caves and sampled roots of four tree species -- a juniper, an evergreen oak, a deciduous oak and the deciduous gum bumelia, which is also known as the chittamwood. "The evergreen oak we studied is the dominant oak on the Edwards Plateau," said Jackson. "Because it needs a lot of water to maintain its year-round foliage, it has the deepest roots, going some 60 to 75 feet into the bedrock to find water." In their studies, the researchers did genetic fingerprinting on both the roots and above ground trees to match the roots to specific trees. The fingerprinting is the same technique used in criminal forensics. They then measured the diameters and wall thicknesses of the water-conducting root xylems. And in laboratory experiments, they determined how vulnerable specific sections of root were to "cavitation," in which mechanical stress or drought induces bubbles that block water flow. The researchers found that trees adjust their root anatomy very effectively to work in different environments. Shallow roots and especially stems have thicker walls and smaller interior diameters because they must support the tree more, and because they experience more drying and temperature extremes that cause cavitation. Deeper roots, more protected from such demands, can have larger diameters and thinner walls to maximize the conduit size and enhance water flow. "The data are interesting because they show that the same individual plant can adjust its anatomy and physiology to maximize water transport deep underground where the plant is less likely to experience drought or freezing conditions which cause cavitation," said Jackson. "The surprise was the extent to which individual trees can make these developmental changes," he said. "There has long been data showing that roots tend to have larger conduits than shoots. But there has not been an analysis like this looking across the gradient of differences within the same plant. And nobody has analyzed roots that go as deep as these," said Jackson. According to Jackson, such insights will improve theoretical models of water transport within trees. "If you're building a model of water transport and you don't take into account these differences among roots, you could severely underestimate the importance of those deep roots," he said. "Their biomass isn't that great, but they are extremely efficient in transporting water." In further studies, Jackson and his colleagues are installing instruments on trees to measure their water flow in real time to determine the dynamic effects of the anatomy they have discovered. "These studies will allow us to ask what proportion of the water in trees comes from different depths and how that proportion changes during drought conditions," said Jackson. "Ultimately, the question behind this work is how much of the water balance of the Edwards Plateau is driven by the vegetation. And for that question, we need more than static physiological measurements. We're already teaming up with hydrologists and geologists in the region to find the answer." -------------------------------------------------------------------------------- Adapted from materials provided by Duke University.

Because the side of a pot, especially a pot made of porous material, is more or less continuous, it has a lot of potential for water molecules to adhere to the pot wall. Since there is more adhesion at the interface between the pot wall and particles vs the amount of adhesion between soil particles, you would expect a greater tendency for there to be a very thin film of water on the pot walls that is slightly higher than the PWT say 1/8" inside of the pot wall. Even soils that aren't capable of supporting a PWT can support this light film of water on pot walls. I suppose that, technically, you could call the water that remains as a microscopic film on soil particles and water that is retained in the immediate interface where soil particles contact each other perched water, but that's not water we're concerned with. We're concerned with the water that fills potential air spaces between particles because the size of the particles is small enough to increase adhesion to beyond the point where water will defy gravity and remain in the smaller soil pores instead of the pores draining. Example, if you have a container of a given volume filled with 100 1" diameter balls, the balls would have 314 sq in of surface area for water to stick to. If you had 1,000 1/10" balls, the surface area increases to 502 sq in, but 1,000 1/10" balls wouldn't come nearly as close to filling the container because of the reduction in pore size. I have to admit that I can't recall how to determine how many of the smaller balls it would take to fill the pot, but it would be considerably more than 1,000, which would significantly increase the surface area and in doing so increase o/a adhesive forces. You can thwart the sum of adhesion and cohesion causing perched water, and even force waters tendency to be retained at the interface where soils touch by employing Newton's First Law of Motion. Hold the pot with perched water and a drain hole over the sink and start moving it up and down. You'll find that each time you reverse directions from moving downward to moving upward, a good measure of perched water exits the drain hole. You'll soon learn that a gentle downward rhythm followed by a very sharp reversal upward will fling your plants against the ceiling, so unbridled enthusiasts should try this maneuver when they are about to fall asleep. Actually, the sharp reversal upward is the most effective way to force water from pots. It works great for small plants, and there's no mess involved unless you try it on the couch while watching your favorite TV show. Summed: The water you asked after isn't the 'bad' water. Water that remains in potential air spaces between soil particles is the water that limits root health and function. Al

I think we are in disagreement over what perched water is. When a medium is saturated with water bulk of the water will flow out of the drain holes. But there will be a portion of soil in the bottom that will remain 100% saturated unless an external force is applied. The height of 100% saturated zone is called the perched water height. That is a property of the medium. All air space in saturated zone is occupied by water. Above that the soil is less than 100% saturated. Basically some of the spaces between soil particles are not fully occupied by water. When you tilt the pot or shake it disturbs the equilibrium of forces and some/most of the extra water in the bottom saturated zone will flow out. Instead of repeating it here again look at this post: https://www.gardenweb.com/discussions/5868967/how-can-one-determine-the-level-of-the-perched-water-in-a-container The post illustrates what perched water is using a sponge. A wick also acts the same way. A good wick will have better perched water properties or even more. The wicking material does not need to have exact same properties as the soil . As long it has better wicking properties than the medium it will draw the water out. The target for wicking is draw out the water that occupies the air spaces between particles. For a good medium we need good water holding capacity and air porosity. Perched water kills the air porosity. The whole purpose of wicking is to take out the water from the saturated zone. It has nothing to do with water held by and within the particles by adhesive forces and charges. Your glass analogy, in this case, doesn't have any capillary action to the soil, so wouldn't apply. No idea what that means. Also, try that with a good cotton wick and see how long it takes to drain a half ounce from the glass. To drain the perched water from a pot will take much less than half an hr. Otherwise add more wick. It is immaterial. A wick may be slower than shaking the pot but gets the job done especially if you have many pots. Now try it with a soil that's exactly saturated--but not draining actively. There will be a static bind of water to soil just by the charges exposed. Static bind of water to soil is of no consequence. We want water to be held by soil medium so that roots can use it. It is the water in the supposed air space between soil that displaces air is of consequence in a medium. In the glass analogy, as long as the level of water is within the perched water height of wick it will draw the water out. When the wick is inserted from the bottom of a pot, the perched water level is always above the wick and so will always work to draw the water out.

Appreciate the additional comments folks! Gardengal: Thank you for the clarification on PWT, I think I probably overestimated how much 5-1-1 would reduce the PWT. In time I will get deeper pots, but for now I think I will just regularly rock the pot after watering. I think these pots are borderline too big for these trees right now, which is maybe why water is staying in them longer than I expect. The trees are only starting their second season with me, they are not big trees. Ken: I probably shouldn't have tagged this in the houseplants forum because they are outdoors. I did so because I saw much of the 5-1-1 and PWT chatter seemed to happen in that forum, and I thought the concepts would apply whether indoor or outdoor. You are correct that there are many variables here and I'm less concerned about solving this particular instance of YLD because I think on the whole my trees are healthy. I was mostly wondering if my drainage hole situation could be causing a higher PWT or if I just need to adapt a little to the properties of my mix. Based on posts above, I am going with the latter. Silica: Yes, both trees are covered in flower buds and/or new leaves. Only a few buds have opened so far. This is my first season of active flowering with both trees, so I'm not sure what to expect in terms of YLD related to flowering. It's not hard for me to imagine that reduced fertilizing over the winter left them under prepared for this new growth. We also got several days of rain (rare for Phoenix) that may have contributed to them getting a little water logged. I assumed 5-1-1 mix, they could take it. I also haven't added any vinegar to my hard water, so there could be pH issues as well. 😕