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:
Effects of depth and topography on water use of plants in Nebraska Sandhills. Wang et al., 2007 (see 2nd paragraph, pg 91)