Rangeland Pitting for Revegetation and Annual Weed Control

On the Ground Soil pitting is an ancient technique for concentrating soil moisture to enable plant establishment and promote plant growth. It is especially effective in arid areas where plant establishment is limited by water availability. Pits created by digging and mounding action have been shown to be effective. Small pits made by soil compression are not very durable. Larger pits last longer and store more moisture. In mesic areas or years with above-average soil moisture, pitting may not be needed for plant establishment, and better results may be obtained by drill seeding. Pitting can help control some non-native annual plants by hindering their seed dispersal and concentrating their seeds within pits. This increases intraspecific competition and limits seed production. The pitting technique has been demonstrated in several studies as an effective control tactic for cheatgrass. Pitting is a strategy that may become more important as climates become hotter and more variable, as it allows soils to stay wetter for longer periods of time. Many custom-built machines have been developed to create soil pits efficiently. Commercially available machines are less common. Currently, pitters that dig and mound soil and are suitable for rangelands are not commercially available in North America.


Pitting history and scope
The idea of planting seeds in pits or depressions is ancient.Tribes in arid zones, such as the A:shiwi ("Zuni") in North America, have used the technique of creating dams and soil depressions for over 1,500 years to strategically capture storm water and make agriculture successful in spite of extremely low annual rainfall (averaging 300-400 mm [12-16 inches]) and highly variable precipitation patterns. 1They have built what we today call "waffle gardens,"with pit walls surrounding small basins where seed is planted ( Fig. 1 ).In the experience of these farmers, pit walls shaded seedlings and funneled rainfall to their roots, increasing germination, survival, and productivity of their crops.By mid-century, expanding this gardening technique to landscape scales intrigued rangeland researchers.Today, revisiting the benefits of seeding within pits is worthwhile in light of current challenges in rangeland management and restoration.
Rangeland researchers have used pits to concentrate soil moisture since the 1930s and subsequently designed methods to efficiently create pits across large areas (e.g., multiple hectares). 2Working in the semiarid shortgrass steppe of eastern Wyoming, Anderson and Swanson 3 created a machine called the eccentric disk pitter.This device used disks that were mounted off-center, so that each disk would dig for only part of each rotation.These machines produced pits about 10 to 15 cm (4-6 inches) deep, about 45 cm (18 inches) long, and about 10 cm (8 inches) wide.By placing a seed box just after the gang of disks, managers could pit the range and broadcast seed in a single pass ( Fig. 2 A).Researchers concluded that the machine helped reduce risk of seeding failure given uncertain weather.In years with abundant precipitation, the pits would become inundated, and seedling establishment would occur mainly at the edges.In drier years, establishment would occur at the bottoms of pits. 3 This technique resulted in a 100% increase in the number of western wheatgrass ( Pascopyrum smithii ) plants compared with nonpitted range, and enabled increasing stocking rates for sheep grazing by 32%. 4 Pitting is only one of many types of surface manipulations for concentrating soil moisture, which can vary in both size and shape.Rainwater harvesting is a popular technique in extremely arid ecosystems.It involves manipulating water flow over a few acres so that a catchment zone receives the runoff of a contributing area, which is on average about 20 times 2024  larger than the catchment. 5In somewhat less arid ecosystems, a smaller-scale version of this idea is effective, and has been implemented by creating 3 m 2 (32 square feet) depressions that are deeper at one end, so that the deep end concentrates water captured by the entire depression. 6If the area is sloped, soil moisture can be concentrated very effectively by contour furrowing, which are linear depressions oriented perpendicular to the slope. 7At the smallest end of the scale, smaller than the aforementioned eccentric disk pitting, are imprinters and spike-tooth pitters, which create dents in the soil surface less than 20 cm (8 inches) in diameter.These have been found to be much less durable and less effective for water storage, 7 although they may still be helpful for improving seed-soil contact.
We focus on pits between 30 cm and 1.5 m (1-5 feet) in their surface dimensions and 10 to 50 cm (4-20 inches) deep, made by digging action and not merely by soil compression.Pits of these dimensions are large enough to concentrate soil moisture, yet small enough that they do not require recontouring and are therefore reasonable for rangeland reclamation.We will discuss the effects of pits on soil moisture, plant establishment, and annual weed control.We omit a discussion of the relationship between soil pitting and change in site-level erosion potential, which would most properly be addressed in a paper including larger-scale surface contouring.

Pitting effects on plant establishment and key considerations
Soil pits alter water retention and soil temperature in ways that can ultimately benefit plant establishment or survival in some climates.In a mine reclamation study in Utah, soil water potential was higher on average underneath gouged pits than in cultipacked locations (where soil was pressed and smoothed using heavy rollers), regardless of soil type and measurement year. 8A study in southern New Mexico found the duration of adequate soil moisture for grass establishment was longer in pits as compared with a flat area. 9Similar results were found in a study in Canada, in which water potentials were higher in the bottom of pits versus mounds at 2 of 3 sites (although the pattern was reversed at a third site). 10Cooler conditions contribute to this retention of soil moisture, and Naeth et al. 10 also documented lower soil temperatures in pits versus flats or mounds.Pit orientation may influence their internal soil temperature, where orienting the long axis of the pit, if there is one, in an east-west manner can create a shadier and cooler north slope.
The benefit of soil pitting on plant response has been noted in field studies in multiple arid locations.At seven sites in central Arizona, square pits 13 cm (5 inches) deep and 38 cm (15 inches) across promoted better native seedling establishment than either mulch or artificial nurse plants. 11In an arid Mediterranean climate in Western Australia, pit seeding increased alfalfa establishment and vigor 6-fold as compared with drill seeding. 12In western Colorado, seeded native species had higher cover in pits than on flat surfaces across 2 years receiving both low and average precipitation. 13Some have observed that the benefits of soil pits are limited to their footprint; for instance, although pits increased plant establishment within them during mine reclamation in southern Utah, very little plant establishment occurred on the mounded soil next to the pits. 8Others have found that plants respond differently to microsite position, where growth was greatest either in pit depressions or on the mounds between pits, depending on the species being observed. 10itting may have benefits to plant survival or productivity even while having neutral or negative effects on initial plant establishment.In southern Arizona, shallow basins 15 cm (6 inches) deep, 1.6 m (5 feet) wide, and 1.6 m (5 feet) long made with a modified bulldozer blade had slightly lower initial perennial grass establishment than smaller pits, but the shallow basins had double the grass productivity over a 4-year period. 6In western Colorado, initial grass density was similar between pit seeding and flat seeding, but surviving grass density was seven times higher in pitted plots than in drill seeded plots after a drought year with only 10 cm (4 inches) of annual precipitation ( Fig. 3 ). 14In a Utah mine reclamation study, frequency of grasses and shrubs at the end of the first growing season was lower in "gouged" (i.e., pitted) plots than in plots that were harrowed or cultipacked, but productivity 2 to 5 years post seeding was higher in most years with pitting. 8limate and short-term weather conditions are contributing factors influencing vegetation response to soil pitting ( Fig. 4 ).In cooler or more mesic climates, pits may not be needed to facilitate plant establishment, and seeding on a flat surface may result in a higher plant density.This was the case in a recent study conducted in Alberta, Canada, in which pits did not improve plant establishment or survival. 10Similarly, pits did not improve seedling emergence in a 1-year study with above-average rainfall in Nevada 15 and were not found to provide a suitable seedbed in northern Montana. 16Flat surfaces allow the use of drill seeders, which provide precise seed placement and facilitate seed-soil contact, whereas broadcast seeding over a pitted surface results in variable seeding depth, with some seed becoming deeply covered as soil sluffs from pit edges, and other seed remaining on the soil surface on the interspaces between pits.The relative benefits of these two seeding methods (drill seeding and pitting) vary by site.In a compilation of 11 sites in western Colorado and eastern Utah, sites with more than about 350 mm (14 inches) of annual precipitation sometimes had better grass establishment with drill seeding than with pit seeding. 14 , 17 , 18However, pit seeding with approximately 25 cm (10 inches) deep pits produced better grass establishment at more arid, lower elevation sites.
Soil texture has also been shown to influence the effectiveness of pits, in that it can mediate infiltration capacity vs. runoff potential ( Fig. 4 ). 19Across six western US states, the benefit of pits on increasing perennial grass productivity was higher on finer textured soils than on coarser soils. 7This is because coarser soils typically have high infiltration rates even in the absence of pits.Similarly, pits were found to increase grass production on clay or loam soils to a greater extent than on sandy soils in eastern Colorado. 20Another consideration is the presence of sterile arid subsoils, which are prevalent in some parts of North Africa and the Middle East.These often consist of calcareous or gypsum concretions.If these exist in your area, the pits should be made shallow enough, perhaps 8 to 10 cm (4 inches), to avoid disturbing the sterile layer (G.Gintzburger , Badia Consulting and S amarkand S tate University, personal communication, Sept. 24, 2022).
Pit size, shape, and spacing influence their potential water retention, durability, and effect on vegetation ( Fig. 4 ).Not surprisingly, larger or deeper pits can store more soil moisture than smaller or shallower pits.Pits created by digging and mounding of soil, at least 10 to 15 cm (4-6 inches) deep, increase infiltration and reduce runoff compared with unpitted land, 19 although some have considered ideal size to be roughly double that size. 21Pit spacing is another important consideration.Spacing pits more than 3.2 m (10 feet) apart can result in too little of the landscape affected to achieve restoration goals. 7One study recommended that if pits are placed in rows, the rows should be 60 to 90 cm apart (2-3 feet), as rows of pits > 1.5 m (5 feet) apart can reduce their effectiveness. 2oil pits are susceptible to becoming filled with soil, changing their functionality over time.Smaller gouged pits have been found to maintain their structure for at least 5 years on the landscape 8 , 22 but they can degrade more quickly on steeply sloped areas, 23 in areas where sediment movement is high, 24 or when subjected to frequent or intense rain events. 16-filling can result in lessened capacity to capture and retain soil moisture, 2 , 16 , 25 and pits that are actively filling can bury seeds and transplants. 10 , 23 , 24Some have found making pits with broader dimensions increases their likelihood of remaining intact for longer periods of time, 6 , 26 and larger structures are recommended to maximize vegetation benefits over decadal time scales. 25Regardless, even smaller soil pits that benefit plant establishment in the first few years but diminish thereafter can have lasting positive effects on vegetation composition and cover for many years following. 2

Soil pitting in the context of the broader environment
Pit seeding is a tool that can help rangeland managers as our climate continues to change.Even under current conditions, precipitation in rangelands is variable from year to year, and seedbed microclimates can differ even more than annual average precipitation. 27Although climate models vary in their specific predictions, increased temperatures and increased variability in precipitation are broadly anticipated conditions of future climates. 28Furthermore, persistent drought has already become the norm for many North American rangelands, 29 elevating the difficulties associated with restoration.In the Great Basin, where high soil temperatures constrain seedling success, an increase of 2 °C (3.6 °F) is predicted to lead to a 30% decrease in seedling establishment. 30These climate challenges have motivated ecologists to seek various solutions: refining seasonal weather predictions, 27 seeding over multiple years, and/or using seed technology to increase chances of germination occurring. 31Seeding over a pitted soil surface is another solution worth exploring to promote seedling establishment under warmer climate and more variable precipitation.
Climate change interacts with another huge challenge facing rangeland managers-invasive plants.Here again, pitting can have positive outcomes.In western North America, many of the undesirable weeds are annual grasses, such as cheatgrass ( Bromus tectorum ), medusahead ( Taeniatherum caput-medusae ), and ventenata ( Ventenata dubia ).The species share some key characteristics that make pitting a relevant management technique for their control.As annual plants, they produce seeds prodigiously, and some of those seeds must disperse to microsites with favorable conditions for the species to persist.Also, the seeds of these grasses have awns, which means that the movement of their seeds is greatly influenced by soil surface obstructions. 32Soil surface obstructions matter little to seeds with plumes, which catch wind easily, and they matter little to hard round seeds, which are unable to catch the wind.Seeds with awns, such as cheatgrass, can bump along the ground surface for tens of meters (30 or more feet) if the surface is bare and flat, 33 but they can be trapped in < 1 m (3 feet) if vegetation is dense. 34Depressions or pits provide soil surface obstruction and capture seeds effectively. 32imiting dispersal has big consequences for annual plants.For instance, when cheatgrass seeds are forced to compete in dense stands, each individual plant produces about 50 times less seed. 35Thus, trapping cheatgrass seeds near their parent plant not only limits the spatial extent of invasion, it also limits the number of seeds causing problems the following year ( Fig. 5 ).Although this technique is effective, soil depressions can increase exotic species germination and growth, 36 just as they do for more desirable plants.However, the net effect of depressions on cheatgrass is negative.In a collection of studies at five sites invaded by cheatgrass, pitting helped control cheatgrass at all five sites. 14 , 17 , 18At lightly invaded sites, pitting provided enough control to act as a stand-alone treatment, whereas at more heavily invaded sites, pitting provided an effective complementary technique to a light herbicide application. 14 , 17 , 18dditional environmental effects of pits are plausible and deserve further research.We know pits trap seeds 32 and gradually fill in over time due to deposition of soil and windblown debris.Do they trap enough organic material to meaningfully increase soil organic matter?Soil organic matter is an important component of soil health, increasing soil structure, fertil-it y, and stabilit y.Any increase in soil organic matter would have significant ecosystem benefits.Furthermore, we know seed predation is an underappreciated factor in plant establishment, 37 and we know litter depth influences habitat selection of seed-eating small mammals. 38How do pits influence the rate of seed predation in restoration plantings?Any change in seed predation could have a large influence on the outcome of seeding efforts.Given the notable effects that have already been documented for pits, further research into these topics is warranted.

Application of soil pitting on the landscape
Through the years, many different machines have been invented to create pits efficiently.In addition to the eccentric disk pitter, 4 researchers have designed machines with various names such as cut-out disk pitter, 39 range pitting machine, 20 rotary or Calkins pitter, 2 , 7 modified disk plows, 2 gougers, 26 Lee Pocket-Seeder, 15 bush and bog harrow, 16 dammer-diker, 19 pitting listers, 39 camel pitter, 40 or pothole seeder. 14In spite of the multitude of names and variation in design, most of these implements work by the rotation of an oddly shaped drum, wheel, or disk (notched or with eccentric rotation) that alternately digs and leaves the remaining ground undisturbed ( Fig. 2 ).Exceptions are gougers, 26 which involve blades or a moldboard plow that are alternatively raised and lowered.Many of the implements drop seed after the pits have been created, allowing efficient rangeland treatment with a single pass.Unfortunately, although the camel pitter is currently commercially available in Australia, in many areas there are no commercially available pitters suitable for rangelands.
With so many separate efforts to create custom pitting machines, and so many reliably demonstrated advantages relevant to today's challenges in rangeland restoration, why has pitting failed to become more widespread?In some cases, the degree of ground disturbance required for pitting is ecologically undesirable.Pitting can injure existing desirable vegetation, which may cause managers to wonder if the treatment might do more harm than good.Or, if no desirable vegetation is present, managers may deem the rangeland to be too severely degraded to be worth investment.Even so, the amount of exotic-annual invaded rangelands is increasing, and effective techniques for restoring them are needed.The camel pitter is a tool especially targeted for areas with low 2024 Figure 6.Reclamation studies on oil and gas well pads in the Uinta Basin in northern Utah.A, Concentration of James' galleta ( Pleuraphis jamesii ), a seeded native grass, in broad soil depressions created using the tread-twisting method.Plants were broadcast seeded November 2020; the photo was taken in June 2022.B, Snow concentrated in pits created using a mini-excavator.The photo was taken days after the depressions were created in November 2022.
tolerance for high costs.It can be built for less than $1,000 and be pulled by a truck, making it an attractive option for highly degraded, arid areas (G.Gintzburger , Badia Consulting and Samarkand State University, personal communication September 24, 2022).
Political and related economic factors may also be delaying the adoption of pitting.In the United States, the majority of the arid lands best suited for pitting are on public (federalor state-owned) land, which can face a suite of barriers for adopting new innovations. 41Five barriers can relate to the innovation itself: relative advantage, trialabilit y, observabilit y, complexity, and compatibility. 41Relative advantage of pitting has been demonstrated through decades of research, although application time in the field may be a factor, as seeding landscapes with a pitter is typically slower than drill seeding.Trialability and observability are not likely barriers, as creating small-scale trials of the technique and observing the results are straightforward activities.By contrast, complexity can be a barrier in areas where pitters are not commercially available.While the process of seeding with a pitter is no more complex than drill seeding, creating a custom tool is certainly a complex task, particularly if the user is seeking to optimize pit dimensions and spacing.Compatibility is another likely barrier.In the United States, federally owned land must be surveyed for archeological artifacts before any ground disturbing ac-tivities may take place, posing additional administrative costs (although the same rules would apply for disturbance caused from drill seeding as to that from soil pitting).There are also barriers to adoption of innovation that do not relate to the innovation itself. 41These can include a lack of trust and communication between agencies, as well as threats of litigation for operating outside an agency's policies.
Although certain barriers may currently constrain the widespread use of pitting or make the adoption of new innovations an economic hurdle, there are other scenarios in which soil pitting has been an achievable and even a recommended practice.For instance, land affected by resource extraction, including mining, oil and natural gas, and renewable energy, will already have been approved for soil-disturbing activity in adherence with requirements set by the National Environmental Policy Act and the National Historic Preservation Act.In these extremely disturbed areas, leaving soil roughened or pitted before reseeding has been tested 8 , 13 and recommended for industry practitioners. 42In these settings, larger pits can be created with machinery such as a trackhoe to gouge the soil 42 or a tractor via "tread-twisting," a technique of driving in tight twists to kick soil out to the sides of the wheels ( Fig. 6 A).Methods such as these have been feasible and cost-effective for industry use in reclamation, such as on oil and gas well pads in northern Utah ( Fig. 6 ).

Conclusions
Pitting is a technique with the potential to help establish desirable plants from seed in the face of two of the biggest challenges facing our rangelands: drought (exacerbated by climate change) and invasive species.As these problems continue to grow, our need for cost-effective techniques for rangeland restoration will also grow.Pit seeding is well suited to harsh sites with low and variable precipitation, conditions that are predicted to increase under climate change.While the reasons for reluctance to engage in new technology and ground disturbing activity are valid, re-examining the costs and benefits of pit seeding for restoration may ultimately add another valuable method to land managers' toolboxes.Pit seeding allowed ancient cultures to survive in harsh climates and has the potential to address the challenges of our changing climate.

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.The authors certify that they have no financial interest in the subject matter discussed in the manuscript.D.B.J. is an employee of Colorado Parks and Wildlife, an agency associated with management decisions or research regarding the topic of this manuscript.Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Figure 1 .
Figure 1.Gardens created by the A:shiwi (Zuni) Pueblo, ca.1927.Known today as waffle gardens, A:shiwi used a technique of creating small depressions to create shade, capture soil moisture, and farm effectively in arid southwestern North America.Credit: Edward Curtis, Library of Congress.

Figure 2 .
Figure 2. Examples of machines designed to create pits in rangeland: A, eccentric disc pitter (Anderson and Swanson 1949); B, pitting-seeding implement (Barnes 1950); C, Camel pitter, commercially available in Australia (Kimseed Australia Pty Ltd) creating pits in Kuwait (courtesy Stephen Hill); D) Pothole seeder, which uses Truax (Truax Co., Inc) drill seed boxes over a custom built gang of notched discs (Johnston 2022).Images used by permission.

Figure 3 .
Figure 3. Pits capture soil moisture at Escalante State Wildlife Area, Delta County, Colorado.Three years after seeding, survival of grass seedlings was about 6-fold higher in pit-seeded versus drill-seeded plots.

Figure 4 .
Figure 4. Examples of factors that can influence the response of desirable plants to soil pitting as a restoration strategy.These factors exist on gradients and generalities have been drawn from the scientific literature, although multiple interacting conditions can influence the overall benefits of using soil pitting for restoration.

Figure 5 .
Figure 5. Cheatgrass seed dispersal is influenced by soil surface topography.A, With a flat, bare soil surface, seeds disperse readily.Isolated cheatgrass plants are very productive, so the second generation of plants also produces many seeds, allowing cheatgrass to dominate sites within 2 years, even after severe reductions.B, Dispersal obstructions such as pits interrupt this process.Seeds are trapped near the parent plant, which forces the second generation to compete with each other.This reduces seed production and the spatial extent of invasion.