Root hairs are always dying and being replaced. Only a small percentage of these actually grow into larger roots. Once a root is woody, most will have ability to store energy. If those reserves are depleted, roots or shoots can start to die. Roots grow where they will get right amount of pore space in the soil (that pore space is either air or water), and individual species of trees have certain preferences. That said, obviously roots that can't absorb enough water will die. There's more to it also, such as interactions with salts (fertilizers) while dry. I hope the link below will help.

Here is a link that might be useful: http://hort.ifas.ufl.edu/woody/roots.html

Not quite. Root hairs don't become roots. They are just extensions of root epidermal cells and they die as root growth puts them further back on the root and the root becomes woody. However new root tips form deep inside the root, and they branch, increasing in number. Most of these new roots have a short life, often only weeks. If mycorrizal fungi infect them, they live years. Root tips are always dying and being replaced, so there's a high turnover rate, and that's a major drain on the tree's carbohydrate balance.
Roots grow along moisture gradients, from drier into moister soil. In some woody plants the deeper roots bring up water from deeper in the soil, and the plant extrudes it through shallower roots into the upper soil, making it available later for shallower roots to take up; but that's new stuff and it's commonness is not yet known.

Moisture movement in plants is still being studied. You're right, I have read several things about trees releasing water in the upper soil. However, there seems to be a lot of different opinions on this, depending on if it's a forest situation or not. And keep in mind urban situations would place turf or other plants in that upper soil zone.

With the subject of drought though, mulch can be detrimental. It can take a while to thoroughly wet a dry mulch, especially if it has formed a crust. Just trying to get the word out that if irrigation is scheduled for a specific time, rake the mulch out and replace afterward. Same can be said before a rain.

I don't really agree with you on the mulch being detrimental. It's probably from watering the wrong way.

I think the problems with mulch are seen with very fine mulches, which could mat/pack down and become less permeable. I have seen this effect with the over-application of damp grass cuttings - they packed down, got anaerobic, and water ran off. It was fixed, messily, by removing it all, spreading out the grass to dry, and then reapplying it - a tedious job. As well, water will run off dry peat moss, until it has been well soaked. A coarse or mixed mulch would always have gaps/airspaces for water to flow through.

In addition, since the breakdown/fines from aging mulch are incorporated into the soil by earthworms and other beasties of the "micro-herd" (bugs, bacteria, etc.), if a goodly amount of chemical fertilizer is applied, this would effectively kill off or diminish the numbers and thus leave a lot of the fines at the soil surface. This could possibly crust over and block water, but I am not sure.

This is one link about drought and trees. I had a hard copy from a university about research they did on mulch effectiveness during drought. I will need to see if I still have it.

Here is a link that might be useful: http://hort.ifas.ufl.edu/woody/drought.html

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."

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Adapted from materials provided by Duke University.

Interesting but it may apply only to the unusual case of these very deep roots. Stay tuned.

As an aside, this continuous formation and subsequent dying of root hairs is a means by which soil organic matter increases over time just by dint of having plants growing in it. This is in addition to the more readily observable deposition of leaf and twig litter on the soil surface.

+oM

Wow, 60 to 75 feet! I'm praying my tree roots grow a little more than 2 feet down to hit the high water table. Very interesting article! Have a Quercus virginiana sapling, evergreen oak, about 3' directly over a PVC sewer line. It's growing a bit slowly. If the roots get that long it will just have to wrap around the sewer and go deeper. When they put these sewers in they dug trenches with gravel and water tends to collect in them underground, imagine the roots will grow along these trenches, slurp slurp. The Public Works guys assure me that PVC will not get invaded by roots.

Does anybody know what one can do to encourage and speed along the growth of the deep down roots? My water bill is insane ...

This is one of my favorite subjects on tree development. Interesting subject and article thanks. Another study on a similar subject is linked below.

"Unfertilized and water-stressed (F0W1) seedlings recorded the highest root length, indicating that root growth was promoted under limited water-stress treatments. The lowest root length was recorded for unfertilized plants under well-watered conditions (F0W0), indicating that low nutrient availability, coupled with well-watered conditions, depressed root growth .."

The article linked below shows the less water a seedling is given, the more root development occurs. To best compare, look at both well watered and water stressed graphs with no fertilizer in the equation. The trend definitely shows more root growth with less water. For seedlings, root development is vital. Yes, some trees will not make it due to extreme stress, but those that do have great root systems to aid in future growth.

A well respected local manager of a nursery with a degree in horticulture once told me when I bought several trees from him - "water the trees well when you plant it and don't ever water them again unless it doesn't rain for a month." The trees had a one yr warrantee, so there was no financial incentive for him to provide such advice.

I took that advice in mind when I planted a dawn redwood in the front yard 2 yrs ago. We had a severe drought that year. I held back on watering it. By the end of the brutal record drought that summer the tree looked bad. I thought it was a goner and started to dig it up to plant another in it's place that fall. I was astonished to see the roots (only 8 inches deep when planted) were extremely deep. I chased the roots (which were very alive and healthy) down nearly 3 feet deep and they weren't showing any sign of stopping. I wish now I would have chased them down to see how deep they went. The tree is now flourishing. I learned you cannot judge a young tree by the above ground growth.

Some will recommend watering whenever the top few inches dries out, but I think that does more long term harm than good. These studies and my experience shows tree roots "search" for water. Especially with seedlings, the better the root system, the better the future growth.

The lack of instant gratification (top growth) the first couple yrs after planting is difficult to overcome and leads many to water too much. Holding off on watering (while conserving water) may be the best approach for long-term growth - at least that is my theory.

John

Here is a link that might be useful: Watering Impacts on Root Growth

Stumbled on this thread:

May be a little off topic, because this thread is about seedlings. However, we are having a horrible drought. It has been the driest year since 1871. Mature native trees, planted by TX DOT are dying along the highway.

My bur oak and mature cedar elms started dropping leaves in October. Normally, they are dropping their leaves now.

Watering did not help my trees, however, tap water has ph of 8.5. However, maybe I didn't water them enough. Maybe the ph of the water was an issue. Maybe there was too much transpiration and maybe I should have been watering the leaves too.

Who knows?

I get my water from the Edward's Aquifer mentioned above.

The article from Florida is very helpful.

The article from Texas is interesting. We had always known that plants that are indigenous to the Edwards Escarpment have these deep type of tap roots. (This includes simple perennials such as salvias)

It is probably a geographical adaption. It probably applies to plants growing in native "soils", which are basically rock. Roots form in crevices.

This discussion has helped me to understand how my 6 year old Scarlet Oak sprouts, which I could never water are still surviving to this day. They are planted about 20 feet eastward of a residential street which runs along the Western boundry of this property. In the utility easment there is a drainage ditch, which runs almost halfway along the length of that west property boundry.

Water that flows through the drainage ditch comes mainly from a pond spillway. That Pond is located on the other side of the North boundry of the lot, and the lot gently slopes down in elevation the farther west that the boundry stretches from the pond. The lot's excavation angles ensure that most of the water feeding into the ditch mainly comes from the pond spillway. And most rain runoff that flows over the lot flows mostly Southward and does not flow into that drainage ditch.

The above description may be hard to visualize, but basically it helps describe why the trees I planted in two staggered rows along that 5 foot deep drainage ditch have experienced the most dry soil over the years compared to any other trees I planted near the eastern and the southern boundries on this lot.

For the first 3 years These Scarlet oaks experiences many dry periods, then 2 years of severe droughts followed.

When I planted these Scarlet oak sprouts that I had grown from acorns, they were in one gallon pots and only about a foot tall.

The first 3 years after they were planted they did not grow any taller. During the drought period a couple of them died back, but regrew new sprouts. Until reading this post, I have never before understood how these saplings could still be surviving. I even wondered it the trees would decline over the years from having experienced such drought contions, while they were so young.

Last spring and summer,and early fall of 2008, the drought period broke, and the lot experienced record amounts of rain events. That was when these Scarlet oaks more than doubled in height. They are still very small trees, but now- 6 years after first being planted- they are still alive. The neighbors have no idea that these trees, if never dug up, will one day look like real trees instead of just irregularly branched sticks.

All the while these Scarlet oaks have been growing, none of them have grown any taller than 18 inches, yet just think how deep their roots must have to be for them to still be alive. These trees remained mostly dry during the drought years, but during the 2008 rainy season water could always be seen, even if it was only a trickle flowing through and down the ditch into a 90 degree angled culvert which diverted the water under the street and on into another pond that starts on the other side of the street, just at that point.

I have often wondered if my scarlet oaks only survived, because I planted them when they were still so small. I planted them when they were only about a year to 18 months old from the time they had sprouted from acorns.

Since the roots of these trees have struggled so much to even put much growth on these tree's main leaders, I can only imagine that the roots could not have supported any taller of trunked oaks that I could have, instead, first planted in that location.

The same week I first planted these oaks, I also planted The same sized Summard oaks along the fenceline on the eastern border of the property which runs along a neighboring pasture, which some years is plowed and planted in crops. These trees received more rain runoff soil moisture for longer periods of time and they currently are 4 - 5 feet tall and have developed very good branching structures. These oaks's developmental rates are a stark contrast to the scarlet oaks I have been describing above. But if the reports are true that have been discussed in these posts, then it is reasonable to conclude that my larger Shumard oaks' roots are not likely to be nearly as deep as my slow growing scarlets' roots must be.