A Deep-Rooted Prairie Myth

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 prairie has survived droughts, grazing, and fire. The root system beneath these plants plays a major role in that resilience, but not in the way many of us have been taught.

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. 

Prairie grasses have a dense mass of roots in the upper reaches of the soil profile. They dominate the access to water within that zone, especially during times of drought. However, while they have deeper roots too, they don’t seem to use them to access water from deep in the soil.

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).

Grasses dominate the upper most zone of the soil profile with a dense mass of roots and rarely get water from greater depths. Forbs may pull water from near the surface when it is available, but usually get moisture from below the “grass zone” (but still within the top 30 inches). Shrubs can obtain water from any depth, but mainly get it from 18 inches and below – and down to deep as 8 to 10 feet or more. The purpose of deep roots for forbs and grasses is still under investigation.

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.

Two students of Jesse Nippert (Marissa Zaricor and Seton Bachle) examine prairie roots as part of a project at our Platte River Prairies.

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

This entry was posted in Uncategorized by Chris Helzer. Bookmark the permalink.

About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

28 thoughts on “A Deep-Rooted Prairie Myth

  1. 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?

  2. 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.

  3. 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.

  4. 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…

  5. 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.

  6. 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…

  7. 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.

    • 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.

  8. 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.

  9. 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?

  10. 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.

  11. 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.

  12. 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”

  13. 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?

PLEASE COMMENT ON THIS POST!

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.