Anyone familiar with prairies has likely seen drawings and photographs showing the incredibly deep root systems of prairie grasses and other grassland plants. The prairie ecologist J.E. Weaver, in particular, is well known for his illustrations of long roots extending below prairie plants. That root depth is frequently held up as a major factor that influences the resilience of prairies in the face of summer drought. After all, deep roots allow those plants to draw water from far down in the soil profile when rainfall becomes scarce. It’s one of the defining components of prairie ecosystems.
There’s just one problem. Prairies don’t actually work that way.
Yes, prairie grasses and wildflowers have very deep roots, but research over the last decade or so has built a strong case against the idea that those plants use their deep roots to find moisture during times of scarcity. In fact, they might not be using them to draw moisture at all.
This revelation was first shared with me by Dr. Dave Wedin at the University of Nebraska-Lincoln and it has been reinforced during discussions I’ve had with Dr. Jesse Nippert and his graduate students at Kansas State University. Increased attention to this topic and continued improvements in technology have allowed researchers to measure how, where, and when plants are drawing water from the soil. What they’re learning doesn’t fit the story we’ve all bought into.
Here’s what we know. Despite having very deep roots, most prairie grasses pull water primarily from the top 10 inches or so of the soil. Grasses dominate that shallow root zone with a very dense web of roots. Those grass plants also have deeper roots, but researchers have shown that those deep roots are rarely, if ever, used to draw water or nutrients, even during periods of drought. Jesse says that’s been well documented within the Great Plains, but also in South Africa and Australia, so it seems to be a widespread phenomenon.
The way forbs use their roots is a little less well-known at this point, but progress is being made. Jesse says forbs seem to pull water from shallow depths when they can, but often get their moisture from below that zone of dense grass roots. However, even during droughts, they don’t seem to access water below the top 30 inches of soil. Work from Dave Wedin and his colleagues in Nebraska supports what the Nippert lab in Kansas has found. In the Nebraska Sandhills, researchers found that vegetation doesn’t have much impact on soil moisture below about three feet, even during drought years.
What about woody vegetation? Jesse’s students have found that shrubs pull water from much deeper in the soil than grasses and forbs, starting at about 18 inches and reaching down to 8 or 10 feet. As with forbs, shrubs can draw water from shallower depths during times of plenty, but they seem focus mainly water from depths below what grasses and forbs can reach. This, by the way, applies to shrub species such as sumac (Rhus sp) and dogwood (Cornus sp), but not to more forb-like shrubs such as leadplant (Amorpha canescens) or New Jersey tea (Ceanothus sp).
So, during droughts, grasses and forbs don’t seem to be drawing water from deep in the soil, but shrubs do. This probably gives them a major advantage during those times of stress. Prairie grasses can survive drought, but it’s not because they access water from great depths. Instead, at least some of them have just developed the capacity to continue functioning with very little available soil moisture. Shrubs, however, don’t seem to suffer much when the stop layers of soil are dry – they can just reach down deeper into the moist soil below.
Some of those shrubs have an additional advantage because they are clonal and can share water between the multitude of their aboveground stems, which are connected by underground rhizomes. The Nippert lab has shown that the more mature stems in the center of clone can pull water from deep in the soil and then transport it to the more shallow-rooted stems along the expanding outer edge of the clone. In that way, the young stems on the outside are better able to outcompete surrounding vegetation and allow the overall shrub clone to grow larger. As if that wasn’t enough, Jesse says it also appears that the big thick roots of shrubs alter water infiltration, speeding the passage of rainwater down through the soil to where only shrub roots can access it. This is especially true after those roots die and leave open channels behind.
Fortunately, while shrubs seem to have some serious advantages belowground, they still have a major disadvantage above ground, which is that their growing points are up in the air. Grasses produce new tillers (aboveground stems) from buds at or below the ground surface. That means that when they are grazed or burned off, they only lose the aboveground plant material they’ve invested in during the current growing season. If that defoliation occurs during the dormant season, it really doesn’t bother them at all because all their living biomass is safely belowground. Shrubs, however, put on new growth from the tips of their aboveground stems. When fire comes through and destroys all their aboveground tissue, they lose a considerable investment, even during the dormant season, and have to start rebuilding from the ground – where they have to compete for light with surrounding grasses. Frequent fire, then, creates big problems for shrubs, but grasses and forbs can more easily take it in stride.
So why have we been so wrong about how prairie plant roots work? Dave Wedin points out that some of it is because we’ve paid attention to Weaver’s drawings and ignored his data. Even in the 1940’s, Weaver was publishing data showing that the vast majority of grass root biomass was found in the upper 6-12 inches of the soil. However, people have focused more on the depth of those roots than where the bulk of their mass exists. In addition, the idea that prairie plants are pulling water from great depths is just an attractive – and logical – story. The accompanying illustrations are also really compelling. It’s easy to see how the myth has been perpetuated over time.
Our new understanding of prairie roots and how they work has important implications for prairie ecology and management. Over the last several years, I’ve found myself re-thinking the way plants are competing with each other belowground and how fire and grazing management can influence that competition. Of course, I also have lots of unanswered questions for researchers. For example, most work so far has focused on perennial plants – how do annuals fit into the equation? How much variation is there between grass and forb species in their rooting strategies? Most importantly, of course, what the heck are those deep roots for if they aren’t obtaining water?
Additional reading on this topic, if you’re interested:
Relationship between root system structure and resource use. Craine et al., 2002
Effects of depth and topography on water use of plants in Nebraska Sandhills. Wang et al., 2007 (see 2nd paragraph, pg 91)
Water uptake by encroaching trees and two grasses. Eggemeyer et al., 2009
Woody encroachment in grasslands and impact of clonal shrub root systems. Ratajczak et al., 2011
Challenging the idea that root depth equals drought resilience. Nippert and Holdo, 2015.
Water use by dogwood and goldenrod in tallgrass prairie. Muench et al., 2016
The Prairie Ecologist 
Great post! So what is the current thought on the deep grass roots? Are they there to anchor the plant into the ground? Maybe make it more difficult for grazers to uproot the entire plant when grazing?
Those are good ideas, also perhaps they gave these plants an edge in situations where/when water was only available at those lower levels in the soil.
Well, this changes how I’ll teach about prairie plants this year. Thanks for the information!
I’ll be using this info when I do volunteer teaching at Spring Creek Prairie!
Yes, what, then might be the advantage of having such a large percentage of total biomass in deep roots; particularly as in the C-4 tallgrasses. It’s even more of a problem to understand here in Ohio’s prairies. Our native tallgrass populations, (big bluestem, Indiangrass, switchgrass) are generally taller and denser than populations in the more arid prairies farther to the west. Ohio has adequate precipitation every month of the year. We seldom have an authentic, months-long drought; our soils have adequate moisture at all levels of depth year-round.
1988 was an exception, with a servere summer-long drought. But our tallgrasses were virtually unaffected. Forbs and cool-season grasses were stunted by mid to late summer.
The observed drought stresses in that year, in our native tallgrass prairies, seemed very clearly to indicate that, indeed, deep roots do provide water during periods of severe drought.
Again, if that is not their function, what is the selective advantage of the deep, dense roots of C-4 tallgrasses?
Of course, why do the populations of these tallgrasses maintain the genetics of deep roots out here in very moist Ohio; where there is seldom a soil moisture deficit or stress?
Interesting observation, John. I would note that here in eastern Missouri, the drought of 1988 did in fact stunt the tall C4 grasses. I first visited the prairie plantings at Shaw Nature Reserve near St. Louis at the beginning of September, 1988 and that first visit left me with a distinct image of big bluestem being only about chest height, compared to well over my head in wetter years. To a lesser extent this stunting was evident the summer of 2012, also.
A possibly related observation is that on a drive to Colorado during late August of a “normal” summer, I noted that big bluestem was gradually shorter while heading west, ending up at about 4ft tall near the Kansas-Colorado border.
Have they investigated (or can they investigate) if these deep roots are being used as nutrient storage mechanisms? I also wonder if they are mycorhizzal connects deep in the soil, which might also influence nutrient flow not directly from the soil. Very interesting post – and yes, I think most of us with an interest in prairie have ‘preached’ the deep root myth for years.
That is the great thing about science, always taking what is known and challenging that knowledge to pursue new knowledge.
That’s where my mind went. Perhaps the deep roots are below the main zone of underground herbivory. I recall hearing in the late 1980’s that the half-life of a prairie root was something like 90 days (or perhaps even shorter).
So what does this mean for the idea of carbon sequestration? Do prairies sequester as much carbon as a forest?
Oooo–good question!
Here’s a link to an article and some bullet points I drew from it, for a class I gave at my church for our climate action team. The upshot is that grasses leave more carbon sequestered in the ground after a fire than a forest does, which is pretty common-sense. Disclosure, I am an urban climate activist from Austin, Texas.
Grasslands More Reliable Carbon Sinks Than Forests (study from UC Davis, 7/9/18):
https://phys.org/news/2018-07-grasslands-reliable-carbon-trees.html
– Univ. of California, Davis study evaluates grasslands vs trees for cap-and-trade market
– Forests consume about ¼ of the CO2 produced by humans
– In all but most aggressive emissions reductions scenarios, grassland carbon sinks are more resilient.
o 4 scenarios were, 1) Carbon emissions largely stop; temp rises 1.7C by 2100.
2) Business as usual, 4.8C by 2100 3 )periodic drought intervals 4) Megadrought; lasts 100 yrs or more.
o In most models, grasslands store more carbon than forests because less impacted by wildfires.
– Semi-arid environments cover about 40% of the planet.
o Grasslands store most of their carbon underground in roots and soil.
o Trees store most on woody biomass and leaves.
o 130 million trees have died in California.
o 5 largest fire seasons have occurred since 2006.
So, I still don’t know WHY some species put down such deep roots, but still it looks like a huge and underrated potential mitigation strategy for the climate. I’m also a fan of managed grazing/rotational grazing/mob grazing/carbon farming–would love to see that huge hunk of BLM land that is leased for grazing done that way; especially considering the comments farther down about the difficulty of controlling shrubs on prairie preserves. (I don’t mean that the preserves should be grazed.)
Susan – for what it’s worth, here’s something I wrote about soil carbon in grasslands after talking to a number of experts. I have no confidence that mob grazing or other similar grazing techniques are going to help us much with carbon, but at this point, we just don’t have enough data to compare grazing approaches at all. In the meantime, it’s really frustrating to see anecdotes thrown out as evidence that one or another approach works. We sure don’t need more anecdotal evidence swamping out actual science these days.
Oh… here’s the link. https://prairieecologist.com/2019/03/11/what-we-know-about-managing-soil-carbon-in-prairies-a-complete-but-disappointing-guide/
Chris,
Any research on mycorrhizal fungi interactions with water/nutrients?
Rather than competition among species, is there symbiosis going on?
In the cover crop research, high diversity mixes use less water than monoculture plantings. The high diversity mixes look good under droughty conditions versus monoculture (think diverse prairie). Sharing of water among plant types or more diversity increases the microbial habitat that helps all plants?
Just pondering…
I watched my prairie during a two month drought along with my lawn and flowerbeds. The deep black prairie soil holds moisture and doesn’t stress til at least 6 or 7 weeks of no rain.
The prairie grasses still have deep roots and still sequester carbon, but the roots themselves appear to have distinct roles. If the shallow roots take up most of the water, it could be that the deep roots are mainly responsible for obtaining needed nutrients rather than water, but such roots may also be “redirected” to obtain moisture in times of drought. Distributed roles may be a useful adaptation, but I suspect they can be interchanged as needed. Life finds a way…
This article give me great pause on the implications of treating cut stumps on aggressive plants such as Sumac and Dogwood for those of us on private conservation easement properties who do not have access to fire and grazing. Should we be treating these cut stumps at all? Are we leaving those root channels available to drain the water away from the very plants we are trying to aid? Should we just cut or mow instead, more frequently for those plants that are overtaking our grasses and forbs?
My guess is that the freeze-thaw and wet-dry cycles would collapse any root channels as the roots slowly rot out. I still think treating the stumps makes sense for woody shrubs that would otherwise resprout and shade out the grasses/forbs.
Hi Patrick,
Does this collapse potentially occur even with Loess soil?
Kind Regards, and thanks! R
I am doing research on root reserve levels and regrowth response at given levels through the growing season on aspen and smooth sumac here in Minnesota right now. Preliminary observations are; lower root reserves=lower regrowth height/vigor Higher root reserves= higher regrowth/vigor at end of season. Next I am measuring weekly levels from spring pre-bud to leaf drop in fall. Trying to find cutting time sweet spot for both aspen and sumac. Keep posted!
When removing sprouts from girdled buckthorn, I had been waiting until right before the new green growth starts to become woody. The result is I need to remove the sprouts about once a month which correlates to four times a year. In savanna, it takes at least three and often four years of repeatedly removing sprouts to exhaust the energy reserves of buckthorn to the point that they stop sprouting. Of course, if herbicide is applied to a girdle all the follow-up work of removing sprouts can be avoided.
I could see how mowing might be the preferable to applying herbicide to 10,000’s of stems or reducing the recovery potential of an area with broadcast spraying. It will be interesting to see if the cutting time sweet spot you find matches the time of stems becoming woody that I have been using as a guide.
I’ve always thought the deeper roots were for temperature regulation. People have measured the cooling effect from prairie dock leaves. The grasses are most likely using their deep roots for the same purpose.
It makes sense that most roots would be near the surface because that is where nutrients are most abundant. Nutrients typically are what limits plant growth the most.
Of course, if a soil with high fertility was unusually deep then it is likely more roots would go deeper.
I doubt that. To provide meaningful temperature exchange the amount of liquid moved between top and root would have to be comparable to the mass of the above ground part of the plant on a time frame of hours. I don’t think sap moves anything like that speed.
You are welcome to experiment yourself and see if you get different results.
https://www.fieldmuseum.org/blog/prairie-dock-close-and-personal
I think we’re talking about two different things.
Yes, water evaporation from the leaves will cool them. The latent heat of evaporation of water is huge.
My understanding of your comment was that you proposed thermal exchange with lower soil — sending hot sap down to be cooled and brought back up as cold sap. This would require roughly 1080 times the liquid transport speed.
Going back to your comment, I can see that my interpretation made an assumption.
Evaporation is the dominant mechanism for cooling. However, if you consider a few feet down in the ground is about 50 degrees F in my location (Chicago Region) and leaf surfaces are often 120+ degrees F on the hottest days, the math I have done for a kg of both types of water transport follows.
(latent heat of water at 120 degrees F)
2,136,960 j/kg of water evaporated
(specific heat of water) * delta T
4179.6 j/(K*kg) * (120 degree F – 50 degree F) = 292,572 j/kg from sap flow
Therefore, the cooling effect from water evaporated would be roughly 7 times the cooling effect from an equivalent amount of water circulated on a hot day. Of course, this simple calculation does not consider that both types of transport are happening at the same time and the temperature of the sap would be continually changing with the counter current flow through the plant. This does match with observations that during drought and hot ambient temperatures the tips of the leaves die first. The tips of the leaves would receive the least amount of cooling from sap flow since they are furthest from the roots.
Not quite. You used F degrees. The 4.1 kJ is for C sized degrees. So you are comparing about 1.62 kJ instead of 292.
Out of curiosity what is the actual leaf temperature compared to the air temperature?
What is the transpiration rate of your plant on a hot day?
I don’t see water by either method being significant compared to other mechanisms. In full sun a square meter of leaf gets about 800 W. So over the course of an hour it gets 2.9 MJ. So 50% (guess) gets reflected. and 5% is converted into sugar. That leaves 1.3 MJ. 1 kg of water would last that m2 of leaf a bit over 2 hours, if all the cooling was done with water.
Once the leaf is cooler than the surrounding air, you have conductive heating too.
Many plants shut down starting at about 90 F (Tomatoes are one.) The water loss can’t be supported, so they close their stomata to reduce evaporation.
Another figure I remember: Tomatoes in the San Joachim Valley need 5 feet of water — 5 acre feet of water per acre per season.
Suppose that 3 acrefeet is actually used, and the other 2 feet is flow through to prevent salt buildup in th soil. Units: 1 m2 =~10ft 2 1cuft =~30 liters. So we use 3 * 10 *30 or about 1 cubic meter of water per square meter of field.
Anyway Let’s figure that the bulk of that water is transpired. 200 day growing season (a guess) = 5 mm of water per day, or 5 liters of water per square meter of field per day.
Hmm. That would be enough to cool it a few degrees all day. And if you are working on a scale that cools the entire air mass by the surface you can lower the conduction heating too.
That’s as far as I can take it with my slop shop back of the envelope work. Would have to start reading up on thermodynamics of plant systems to do better.
thank you for opening my mind.
Good catch on the units. I am rusty on doing calculations after being out of school for 20+ years.
With the correction, the cooling effect from water evaporated would be roughly 13 times the cooling effect from an equivalent amount of water transported from the roots on a hot day. However, it must be considered that when evaporation occurs the water is lost whereas if the water is recirculated it is retained to be used for cooling repeatedly.
I don’t know the transpiration rate of prairie dock leaves on a hot day. Your calculation looks like a good estimate.
I have used a thermometer placed on various surfaces during the middle of the day to measure temperature during the hottest day a few years ago. I live in the Chicago area. The temperature on the hottest day of that year was 105 ⁰F (40.6 ⁰C) plus or minus a degree. Below are my results.
Deck 158 ⁰F
Lawn Grass 116 ⁰F
Live Sphagnum Moss 115 ⁰ F
Live Sphagnum Moss 84.4 ⁰F
(in shade)
Sunflower Leaf 106.7 ⁰F
The temperature for a prairie dock leaf during a sunny day must be less than the leaf of a sunflower. The reflectivity of both are probably equivalent, but the prairie dock is drawing water with much deeper roots. However, the temperature is probably not a lot less since evaporative cooling is dominant when water is available.
If a prairie dock leaf was the temperature of a sunflower leaf, then the cooling effect from water evaporated would be roughly 16 times the cooling effect from an equivalent amount of water transported from the roots on a hot day.
The example you gave of tomatoes shutting down at 90 ⁰C must be for people who live in the south where the sunlight has a higher angle of incident. I don’t ever recall my tomatoes wilting like some other plants.
As you can see from the temperatures above, any plant is much cooler than my deck surface. If they weren’t, they would be dead. The cooling effect is significant. In shade the temperature of live sphagnum moss was 20.6 ⁰F cooler than ambient. Sphagnum moss does not even actively transport water.
The point of all this is that plants, especially deep-rooted prairie plants, help keep our environment livable. By turning ecosystems into streets, houses, shopping centers, etc. we become increasingly dependent on air conditioning for survival. A dependency that only adds to the problem.
Hi Chris,
I think you have a typo here: “Frequent fire, then, creates big problems for shrubs, but grasses and shrubs can more easily take it in stride.”
Did you mean grasses and forbs?
Yep! Thanks for catching it!
Amazing and useful Chris.
I wonder if by having a range of depths the grasses may be detecting soil moisture trends, enabling them to pull back their seed maturation and senescence dates. This would help them save water for their seeds – not that they can ‘think’. I can imagine mycorrhiza playing roles in this process, but I’ve forgotten most of my plant science.
This could function much like regulated deficit irrigation RDI (aka Partial Root Zone Drying – sort of) as used in fruit orchards where e.g. plants are planted above a membrane and watered both sides until you want to shut the plant down when you turn off one side which boosts levels of ABA (abscisic acid) and the plant shuts down saving water. I wouldn’t be surprised if nature got there first.
What about fungi and other organisms with a mutualistic relationship with plants? It’s my impression that plants sometimes provide a reservoir of moisture for their benefit. Having said that, I don’t know if their relationship is significant deep in the soil.
In thinking about Jesse’s presentation at GRN 2017 at Konza, am I remembering a mention that the exceptionally deep roots might be seeking out hard to find nutrients?
Excerpts below from: https://jacksonlab.stanford.edu/sites/default/files/bgc01.pdf
“Nutrients strongly cycled by plants, such as P and K, tend to be more concentrated in the topsoil (upper 20 cm) than are nutrients usually less limiting for plants such as Na and Cl.
Globally, the ranking of vertical distributions among nutrients was shallowest to deepest in the following order: P > K > Ca > Mg > Na = Cl = SO4”
Much great stuff in the comments! WIsh I had all the time in the world to read them all in-depth!!!
Great post Chris.
Lots of food for thought in this article, Chris. “Causes me to want to do a little more research on my own. Thanks.
Very interesting post! I’ve grown 48 Eastern gamagrass plants in a plot collected from various locations throughout Texas. Each year I randomly move about 5 percent of the mature clumps (more than two growing seasons in place) to another location in the plot. When I move the clump I get the soil and intact roots to a depth of 10 inches. I’ve moved the plants at two times, late summer and late winter, specifically Aug. 20-Sep. 20 (about 8 weeks before 1st frost) and Feb. 1-Mar. 10 (leaf green-up averages Mar. 10). All clumps receive supplemental irrigation. The clumps moved late winter are set back a growing season to reach full leaf size and display robust inflorescence, whereas the late summer clumps display no setback and appear mature the following season. Could it be the deeper roots re-establish in the late-summer moved clumps but not the late-winter clumps, and if so are the deep roots acting as a carbohydrate reserve?
At least with spruce trees, a reforestation technique used here in western Canada is to trick the spruce into early dormancy by using black shade cloth to shorten the days. They are planted out in August. In their new location they don’t grow at all above ground, but root activity continues until soil temps drop to about 6 C.
I’ve witnessed this also with hand dug spruce transplants. Transplanted in fall they can be planted in their new location immediatly. In spring, your survival goes way up if you dig them up, wrap in plastic and rest in the shade, keep moist until the buds start to swell.
My assumption is that the shade makes trees transpire less, so there is less water stress while the roots rebuild.
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Interesting. I live in east/central Illinois and am wanting to incorporate natives into my landscaping around the foundation of the house. However, I’ve been hesitant to do so, because many articles warn of plantings that draw too much moisture from the soil. In times of drought, some plantings can suck all the moisture and cause soil shrinkage leading to subsidence/foundation problems. Of course those articles were referencing plants typically used in landscaping, so I didn’t know how this might apply to natives. Your article reassures me if I stick to the mesic grasses it shouldn’t be too much of a problem.
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I appreciate this discussion and have benefited greatly from Jesse Nippert’s insights. I get it when it comes to grasses: narrow xylem vessels presumably create immense resistance to the long-distance flow of water.
However, I could show you 2 m tall Silphium integrifolium plants growing in a research plot in Argentina where precipitation is less than 200 mm. After the first year, these plants were not irrigated and they have been growing happily for several years. All other crops in this region require frequent flood irrigation. We know that there is a perched water table with damp soil at approximately 1.5 m below the surface. However, due to the very heavy clay, not even alfalfa is able to reach and use this soil moisture and requires surface irrigation. Very few trees can grow in this region except adjacent to irrigation canals.
I have also seen Silphium species in Kansas growing and setting seed relatively normally during dry years (e.g. 2012-2013) in which grasses and many forbs (including perennial Helianthus species) were stunted or otherwise severely drought stressed. I can not think of any way to explain this (especially in the second year of the drought) without invoking deep soil water extraction. Note that Weaver specifically mentioned forbs such as Silphium remaining green and flowering (though shorter than normal) during the Great Drought. A photo from one of your recent blog posts illustrates this!
Certainly, we have also seen that in climates where surface moisture is frequent and abundant, silphium gets much taller and produces greater biomass. Clearly there is a penalty to having to rely solely on deep moisture for at least part of the year (Kansas) or permanently (Argentina).
I will also note that older Silphium roots and rhizomes develop secondary growth and may thus potentially act more like woody shrubs (although this would be exactly the same as for the “forb-like shrubs” Ceanothus and Amorpha mentioned in the article). There is also a second non-woody fine, horizontal root system that can develop at the surface during wet seasons but I have come to suspect that in dry climates or seasons, these delicate, superficial roots do not persist. This altered root architecture might confound studies on soil water use? Yes, in certain seasons or situations perhaps most of the water acquisition is from top of the soil profile, due to the presence of these opportunistic, fine horizontal roots. However, in other seasons or situations, these fine roots may not exist and up to 100% of water is acquired from much greater depth.
Maybe not all forbs are created equal? I think we can all agree that deep root research remains extremely challenging and perhaps one of the “final frontiers” of botany, ecology and plant genetics.
This is confusing because the pictures don’t have any gauge on them. The grass roots seem to go down maybe to 3 feet max and yet the article suggests they go much deeper but don’t draw water any lower than 3′. I’m not convinced that grass would send roots down for no good reason.
Here’s an interesting article about deep-rooted trees in the Amazon rain forest. The deep roots of grass could be to store water. Not that the roots store much water themselves, but rather they modify the surrounding soil to help drain away water during periods of ample rain which is then passively drawn to the surface as surface moisture levels get depleted.
https://www.sciencedaily.com/releases/2006/01/060112035906.htm
I would assume that those deep roots provide a lot of storage for moisture and nutrients that are gathered in the upper soil. If the upper inches are packed with a dense mat of roots then space would seem to be a limiting factor. If you need to store moisture and nutrients for the lean times, then those resources have to go somewhere else. Where else but down?
Here in Ohio’s tallgrass prairies, on hot summer days, in mid-afternoon, I always have people on my field trips clasp their open hands together with a big prairie dock leaf between the hands. Everyone is amazed at how cool the prairie dock leaves are. This reduction in mid-day leaf temperatures can occur only because of evapotranspiration, with its evaporative cooling. Necessarily, the process pulls water out of the leaves and sends it into the atmosphere. The lost water is made up from absorption of water in the soil, in the deep root zone.
This summer, with an accurate digital imaging thermometer, I want to record prairie dock leaf temperatures, comparing them to ambient air temps. Then, if I can find the right people, I’ll let the physical chemists plot out the amount of water evapotranspired out of each prairie dock plant, plotted over many days or several weeks in hot summer.
I’m not familiar with any other prairie grass or forb that has equivalent leaf cooling. Nonetheless, water is evaporated out of them in hot summer. Simply, soil moisture is the source. For shallow-rooted plants, moisture below the root zone cannot be readily resourced. The deep-rooted prairie grasses and forbs have season-long water access.
Here in the eastern edges of the Prairie Peninsula (in Ohio), deep-rooted prairie plants create favorable rhizospheric (root-zone) environments. The long, deep roots microscopically open the often dense (clayey) soils, which allows deep percolation, infiltration of liquid water into the entire soil profile, often 6 to 8 feet deep. Then, in dry periods, those saturated deep soils provide water to the plants. I’m proposing that this mechanism is a major selective factor for deep roots in non-arid landscapes, as in Ohio. Grow deep roots, which open the soil. Precipitation than soaks down through the entire root zone, is held as a reservoir that the deep roots can tap in long dry periods.
You’ve completely overlooked one of the most important ways that plants obtain water from the soil: fungi! The billions of microorganisms that make up the complex living ecosystem within the soil (bacteria, fungi, protozoa and nematodes) are responsible for making nutrients and water available to plants. It’s not too surprising that the plants get most of their water and nutrients from the top 10″ of soil, since that is where most of the microbes live. It’s hard to believe that the invisible but vitally important soil microbiome is still being ignored by scientists in the early 21st century. If you want to understand the functioning of plants in the real world (i.e., the ecosystem), then you need to get out your microscopes and take a look at the interactions and relationships taking place in the other half of the ecosystem, underground.
K random theory from a household gardener… could the super deep roots be used to direct water AWAY from the plant when there’s too much? We like to think that roots only draw moisture but being able to use the long roots to help get rid of too much moisture like during a flood, would help fill underground aquafers and help keep the topsoil from washing away. If plants are planted in topsoil with a clay base, the water will sit and the roots will rot. Having those long roots deeper would allow water to be drawn down past the smaller sensitive nutrient gathering roots.
What do you think?
I’m not sure I follow the argument against the idea that deeper-rooted C4 species pull water from deeper layers in drought–especially from Dr. Nippert. Nippert & Knapp (2007) reported big bluestem pulling 47% of its water supply from “deep soil” during a drought in 2004, per Fig. 5 here: https://link.springer.com/content/pdf/10.1007/s00442-007-0745-8.pdf
It only pulled 15% the following month after rainfall. That suggests to me at least that the deeper roots do indeed prove useful in times of water stress, though not in times of adequate precipitation.
It would seem likely that some “minimum energy” principal is at work here. The plant will show preference for easier to extract materia. Transport requires energy => need for ATP => oxygen. Surface roots are better aerated.
I bet there is an optimum air/water mix in soil that minimizes the effort the plant makes. Too much air, and water is less mobile — higher water tension = more work. To little air, and the root has to import it’s oxygen from further up the plant.
Reblogged this on The Waterthrush Blog and commented:
Fascinating description here with implications for – among other things – how shrubs can be resilient to drought but susceptible to fire.
Great information, makes sense. Did I miss something about the shrubs. Root depth depends on oxygen, and the depth of oxygen depends on soil composition. For the most part anyhow. Adaptation plays a factor.
maybe the deep roots can store water. so when there is drought, the plants still have water saved inside the roots deep down below the dry upper soil layers.
What about the hydraulic lift model? Has that been tested in these systems?
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Chris,
Two questions:
1.) Do you get notices when people comment on five year old blog posts? :-p
2.) With this year’s drought I’ve been thinking about this article and other anecdotes from folks regarding the depth of frost in drought-year winters. A local forester here related deeper frosts during drought years with higher rates of tree mortality which caused my deranged tree-killing mind to jump for joy.
Knowing that grasslands developed during historically drier, cooler climate phases, I’ve been wondering if there has been any research specifically looking at the impacts of frost depth on prairie root systems versus those of woody plants. Do you know of any? What are your thoughts?
Maybe that’s three questions.
Hey Aric – great to hear from you. I made my first (ever!) visit to Hitchcock on Jan 1 this year and thought about you. What a cool place, and there was a lot of evidence of great stewardship going on.
1. Yep!
2. It’s an excellent question. I don’t really know the answer. I’m pretty sure I’ve heard that Kentucky bluegrass can experience winter mortality (mostly north of us, I think) if it has a weakened root system going into a severe winter, but that’s just something I remember – I don’t have a citation. I’ve not heard much talk about frost depth and root systems around here. I’ll definitely keep my ears open because you’re right that it’s something important for us to know about.
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hello , I live over by Seattle , right in the urban center. There’s quite a bit of clay all through with very limited topsoil depths. I’m wondering how to do a restart – complete make over , find the scheme where between the deep rooting clay busting shrubs or trees work right back with say a red fescue & wild flowering planting. Rockery & a circulating water feature is in with all this..We are looking for both shade in at least part & self sustaining results , a in ground drip system might be used with a soil moisture monitor attached. I am not anything more than an enthusiast to the cause of climate renewal , some community college chemistry & some cash crop orchardist work from way back. This said , I’ve read every post-comment here , causes great enthusiasm for me to get this moving..with the hot months upon us , using the time to prepare and review options. Eventually I would like to help others here locally with what I might call a climate flip ? I do not understand yet how to match plantings together that don’t fight each other so much for moisture , or where they try to kill off the other. I’ll err to native plantings strictly as possible, there are a few nw prairies left standing . Posted this here as a complete open question , now that Seattle & the state is allowing adu’s to be built in backyards & pretty much overriding local zoning everywhere I see this as a critical moment to maximize whats to be left in the natural world after this pending disaster.
All these smartie pants grass experts and not a single one of you said its got to do with bison and other big animals that used to be roaming around north america in the multi-multimillions. Have any of you ever seen what happens to livestock paddocks in the spring if the animals run in them when its muddy and the ground hasnt set? They get trampled, organically plowed and tilled. And that paddock in a draught? With the dryness the dirt can get as soft as sand and blow a shallow grass away after drying it all up, and then any flood that might follow and wash away all the topsoil leaving the soil deprived of nutrients and unable to sustain new growth. Prarie grasses had to survive constantly being torn apart both from the stalk and from the root. Deep roots are a safety anchor that many plants use, the deeper the roots the tougher the plant, (thats why we hate weeds). Prarie grass roots long and lives long and grows big enough and deep enough to survive anything that could ever possibly happen to it. Native grasses once sustained 30-60 million bison, I can promise that without suppliment we wouldn’t be able to maintain a fraction of that population of cattle today domestic grass is just a pet, bred to look pretty on parks and yards and golf courses, not meant to really deal with too much hardship so if it gets trampled it’s done, shredded, and the animals have to be kept off the entire time it regrows.
Is it possible that roots at deeper depths, make some nutrients (nutrient type depends on the plant species) more available for the plant and other species? I suspect that’s true for some species.
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