DOT Project Number:  90-00-LRTF-402

Fiscal Year:  2004

Award:  $20,828.00

Principal Investigator:  Dr. Brian Wilsey, Department of Ecology, Evolution and Organismal Biology, Iowa State University, bwilsey@iastate.edu

Other Project Participants: Andrea Blong, Department of Ecology, Evolution and Organismal Biology, Iowa State University

Research Report:

NATIVE COVER CROPS: GERMINATION AND FIRST SEASON COVER AND ROOT BIOMASS

Introduction

There are presently many prairie reconstruction projects ongoing and getting started throughout Iowa and the United States (Mlot 1990, Smith 1998). In Iowa, many small, and a few large prairie restoration projects are being established, and the Integrated Roadside Vegetation Management program is at the forefront of prairie planting efforts. However, projects are somewhat hampered by a lack of knowledge on how to restore the high diversity found in prairies, while at the same time preventing the establishment of a large weedy component. Most information published on prairie reconstruction emphasizes the number of species, the proportion of grass and forb seed in the seed mixes, and how seed mixes should be planted (e.g. Packard and Mutel 1992, Shirley 1994). This information has been very useful in getting these reconstructions going, but the diversity of plantings is often much lower than the diversity of native prairie remnants. Many reconstructions are strongly dominated by a few grass species, contain very low diversity of forbs, take a long time to establish, and contain an abundance of weeds (Mlot 1990, Packard and Mutel 1994). This is probably because there has been less research emphasis on the ecology of reconstructed prairies than on the composition of the seed mixes. It is still largely unknown how to create the combinations of species and environmental conditions necessary to create and maintain high native plant diversity, while at the same time, preventing weeds from invading and establishing.

One method that has been proposed to simultaneously establish native seedlings and prevent weed invasion is to plant a cover crop prior to seeding the native prairie species (Shirley 1994). This idea is based on the assumption that the cover plant will act as a nurse plant to the prairie seedlings, and will have a positive effect on seedling recruitment by increasing weed suppression and by lowering the harmful effects of high evaporation and light availabilities. Plowed or bare ground can be a stressful site for seedlings due to very high light availabilities. High light in these conditions causes high surface soil evaporation and drought stress on seedlings, which favors warm season grass seedlings over cool season forbs. Furthermore, plowed ground is ideal habitat for a host of weedy species like foxtail, lamb’s quarters, velvet leaf and pigweed, which have higher germination and early growth rates (especially above ground) than do prairie species. Cover crops could also potentially reduce the amount of soil erosion that occurs during planting. However, the evidence supporting the benefits of cover crops is mostly anecdotal and has been challenged by Morgan (in Packard and Mutel 1994). Clearly, further scientific evidence is needed on the efficacy of cover plants, whether cover plants have a facilitative or competitive effect on prairie seedlings, as well as how these processes work (i.e. is the nurse plant effect caused by light suppression or water uptake).

Currently, the most common cover crops are oats, annual ryegrass, annual flax, and winter wheat, all of which are non-native (Shirley 1994). These plants are either seeded with the prairie seeds, or are established prior to seeding the prairie species. Since they are annuals, they are supposed to act as nurse plants during the first year, and then die out during the second or third years. However, these species sometimes inhibit prairie seedling establishment and only provide one season of cover (Funding memo). Furthermore, since these species are not native and are common crop plants, the prairie planting looks more like an agricultural field than a prairie during the first year. There are several native species that have great potential as cover crops.

Study sites:

Experimental plots were set up on slopes near roadsides at the Iowa State University farms near Ames (Horticulture Farm, a mesic site) and at the Western Research and Demonstration Farms (a dry site) near Castanea in Monona County.

Experimental design

Seed mixes (see more detailed section below) were added to plots that contain one of 6 cover crop treatments in November 2004. Cover crop treatments include:

1. No cover crop (control)
2. Canada wildrye (Elymus canadensis)
3. Partridge pea (Chamaecrista fasciculata)
4. Illinois bundleflower (Desmanthus illinoensis)
5. Black-eyed susan (Rudbeckia hirta)
6. Side-oats grama (Bouteloua curtipendula)

These species are all short-lived and range from annual (Partridge pea) and biennial (Black-eyed susan) to perennial (Illinois bundleflower, Canada wildrye, and Side-oats grama). They all are found in disturbed areas and along roadsides, have high germination rates and are fast growing (Christianson and Müller 1999). All species are cool season (C3) plants (with the exception of side-oats grama) that have been observed emerging early during prairie plantings. The inclusion of annuals and perennials will enable us to compare the longer term effectiveness of the perennial species as cover crops.

Canada wildrye is a relatively short-lived perennial grass that reaches 3-4 feet tall and has been used as a cover crop plant at Neal Smith National Wildlife Refuge (P. Drobney personal communication). It establishes well during the first year or two of planting at Neal Smith NWR (personal observation). It is included here because of its previous use as a cover crop in previous reconstructions (Packard and Mutel 1994). Thus, its effectiveness can be used as a benchmark in comparisons with other species.

Partridge pea and Illinois bundleflower are N-fixing legumes. Partridge pea achieves a height of 1.5-3 feet tall (Christianson and Müller 1999) in Iowa. Illinois bundleflower (also called Prairie mimosa) grows quickly to 3-4 feet, and is one of the first species emerging in our experimental prairie plantings in Monona County. These species are included here because of their potential to aid in the establishment of prairie seedlings due to their N fixing capabilities (Hooper 1998). Partridge pea is an annual, but readily re-seeds itself in subsequent years (personal observation).

Black-eyed susan (1-2 feet) is a biennial forb with high germination and quick establishment rates. Black-eyed susan was found to quickly establish in a prairie mesocosm experiment at the ISU Botany greenhouse using dry prairie mix on loess soil (Wilsey and Stirling in review). This species is included because they have attractive flowers, which would make them popular candidates as native cover crops.

Side-oats grama is a short perennial warm season grass with high germination and quick establishment rates. It has been observed to establish very quickly in restoration plots, and seems to be relatively non-aggressive for a warm season grass.

These 6 treatments were applied to experimental plots at each of the two sites (Story and Monona Counties). These two sites were selected because 1) they represent a mesic and a dry site and broader generalizations can be made as a result, and 2) they represent sites that are conveniently located for the PI, with the Ames site close to ISU and the Monona County site close to other projects. Six replicate plots were established for each treatment, for a total of 72 plots (2 sites x 6 treatments x 6 replicates). Plots are 5 m x 5 m in size and were planted in former brome grass fields. A 2 m mowed corridor is located between each experimental plot to maintain a constant background. All plots were placed on slopes so that they are more relevant to roadside plantings.

Each cover crop was seeded on April 9 (Monona County) and April 14 (Story County) at a rate of 10 lbs/acre and was allowed to establish for the entire 2004 growing season. Some mowing above the cover crop seedlings of weeds during the first month was done to help in their establishment. After the first month, weeds were allowed to come in to collect data on weed establishment among cover crop treatments. The ability to resist weed invasion is the first important part in how useful a species will be as a cover crop (the second part being how well the cover crop lets in prairie species) .

Seedling emergence

All species had fairly rapid seedling emergence. Days to first emergence were measured at the Horticulture Farm field site during 2004. Canada wildrye seedlings emerged after 12-15 days, Partridge pea after 12 days, Illinois bundle flower after 12-17 days, and Black-eyed susan after 14-22 days. Not surprisingly, seedlings of the warm season grass side-oats grama emerged later than other species, but still fairly quickly. They emerged 27 days after plots were first seeded.

Seedling density and relative percent cover in 2004

During July 2004, we measured seedling density and relative cover (percent cover of the cover crop/cover of all plant species) of each cover crop. At the mesic farm (Horticulture Farm): the number of seedlings per 50 x 50 cm quadrat (2 measurements per plot) were 7.0 (Canada wildrye), 4.5 (Illinois bundleflower), 1.9 (Partridge pea), 18.2 (Black-eyed susan) and 10.8 (Side-oats gramma). At the dry site (WRF), number of seedlings per 50 x 50 cm quadrat were: 1.8 (Canada wildrye), 2.0 (Ilinois bundleflower), 2.3 (Partridge pea), 26.8 (Black-eyed susan) and 22.8 (Side-oats gramma). Seedling counts were low in the legumes (Illinois bundleflower and Partridge pea) because of their relatively large size. (Individual plants within the plots are fairly large though.) Seedling counts were probably lower at the mesic site due to self thinning.

Relative percent cover was much higher for all species at the mesic Story County site (Horticulture Farm) than at the drier Monona County (Western Research Farm) site (Figure 1, top panels) in 2004. Relative covers varied from highs of 72.3 (Black-eyed susan) and 66.1 (Partridge pea) to a low of 22.9 (Illinois bundleflower) (Figure 1). Canada wildrye and Side-oats grama were intermediate (41.0 and 45.8%). At the drier site, relative covers ranged from 14.0% (Black-eyed susan) to 0.7% Canada wildrye. First-year establishment at the western (drier) site was probably lower due to dry soil conditions and a greater abundance of weeds.

Relative percent cover during spring, 2005

Establishment remained high at the Story County site during early 2005 with the exception of partridge pea. Partridge pea is an annual, and it failed to reseed itself during early 2005. This will help us to interpret cover crop effects in future years as the prairie species establish from seed. If partridge pea plots turn out to have different species compositions than other cover crop treatments, then we will know it is from the first year suppression of weeds rather than from facilitation by plants in later years.

Establishment was much higher in spring 2005 at the Monona County site than it was during 2004 (Figure 1 bottom right panel). Black-eyed susan had become the dominant species in its respective plots. Canada wildrye and side-oats grama are also coming in fairly well as of June 2005. Partridge pea and Illinois bundleflower had very poor establishment and would probably not make good cover crops in western Iowa. Thus, it appears that establishment at the Monona county site was delayed during the first year but is rapidly catching up to the Story County site for all species except the legumes.

Weed invasion into plots during 2004

There are two important characteristics that a cover crop should have for it to be useful in prairie restoration: 1) it should keep out weeds, and 2) it should increase the establishment of target prairie species. Both of these issues are important, and both will be addressed before recommending a cover crop because a cover crop species that is especially good at keeping out weeds may also keep out prairie species. The best cover crop species may be one that allows in an intermediate amount of weeds and also allows an intermediate amount of prairie establishment. We are measuring both characteristics in this study; the first part will be presented here, and the second part is ongoing and will be presented in future final reports.

Weed biomass in plots at the end of the first growing season (2004) are presented in Figure 2. These data were collected at the Story County site only. Again, weeds were clipped and mowed during the first month to allow the cover crop species to establish, but were then “let-go” to measure weed establishment. Highly significant differences in weed biomass were present (P < 0.01) among cover crop species. Black-eyed susan and partridge pea had much lower weed biomass than did the other three species. Canada wildrye, Side-oats grama, and Illinois bundleflower had much more weed biomass than black-eyed susan and partridge pea, but they did not differ among each other. The composition of the weed mix was also different among cover crops. Black-eyed susan tended to keep perennial weeds (e.g. crown vetch and smooth brome) out more than annuals (e.g. witchgrass), and Canada wildrye tended to let in more cool season grass weed species than other species.

In order to test for a mechanism for how Black-eyed susan and partridge pea are keeping out weeds, we tested for relationships between cover-crop aboveground biomass and weed invasion (weed biomass at the end of the growing season). Aboveground biomass is an excellent integrative measure of resource (nutrients, water, light) capture during the growing season. We split the weed data into two: perennial and annuals-biennials to see if they responded differently to cover crop biomass. They did. Weed biomass of perennial species declined significantly with cover crop biomass (Figure 3, top panel). Cover crop biomass accounted for 58% of the variation in perennial weed biomass (such as crown vetch and smooth brome). However, annual-biennial weed biomass (e.g. witchgrass) was unrelated to cover crop biomass (Figure 3, bottom panel). This suggests that cover crops will be ineffective in keeping out annual and biennial weeds, but that they will be effective at keeping out perennial weeds. The amount of perennial weed suppression will depend on how productive aboveground the cover crop species was, and black-eyed susan and partridge pea were clearly superior in that respect compared to other species. We will continue to measure weed establishment in plots as they develop further.

Seed mixes

Seed mixes of 29 species were added to all plots during November 2004. We did this so that the freeze-thaw cycle would work the seeds into the ground by the beginning of the 2005 growing season. Seed was supplied at a rate of 10 lbs. per acre. Thus, together with the cover crop seed, there is a total of 20 lbs. seed per acre. Seed mixes consist of common warm and cool season grass and forb species of mesic prairies (Ames) and dry prairies (Castanea) (Table 1). Comparisons of establishment among species is difficult if the same number of seeds are not added to each plot. For this reason, seed mixes were created with the same number of seeds for each species (based on a previously determined seeds/g value). Seeds were obtained from Allendon Seed company (Madison County) for the Story County site and from Heyne Custom Seed Services (Pottawattamie County) for the Monona County site.

Sampling in progress

Surface soil moisture has been measured in 2005 and we will be continue to measure it monthly in each plot during 2005 and beyond using time domain reflectometry rods which have been calibrated with gravimetric soil moisture measurements. Light availability at the soil surface has been measured with a 1 m long Decagon light already in 2005, and will be measured again at least once during the growing season. Light and water capture by the cover crop are predicted to be major factors in either promoting or inhibiting other prairie species. We are predicting that an intermediate amount of light and water at the soil surface will be associated with the greatest number and cover of other prairie species (Wilsey and Polley 2003), with weed invasion occurring at the high light end, and suppression of the prairie species by the cover crop occurring at the low light end. Measures of soil water, light at the soil surface, and plant biomass and seedling density will enable us to test this hypothesis.

New experiment for 2005

We have established a new set of plots during 2005 at each of the two sites. These new plots will provide a second year of establishment (replicating initial growing conditions) data, as well as test whether spring planting of cover crops differs from fall planting. A split-plot experimental design is being used. New plots, again 5 x 5 m, were marked out at each of the two study areas. Each plot was split up to four 2 x 2 m subplots, with 1 m corridors between subplots. Each subplot will receive one of four treatments: 1) spring 2005 planting of cover crop with prairie seed mix, 2) spring 2005 planting of cover crop with prairie seed mix added one year later (spring 2006), 3) fall 2005 planting of cover crop with prairie seed mix, and 4) fall 2005 planting of cover crop with prairie seed mix added the following spring (spring 2006). The cover crops will remain the same with the exception of Illinois bundleflower, which was dropped due to poor establishment in the first experiment. Canada wildrye, Side-oats gramma, Black-eyed susan, and partridge pea are being used again in the new experiment. These cover crops will be compared to control plots that will receive the prairie mix only. However, a sixth treatment was added to this new design: a mixture of all cover crops (but with the same overall seed mass). This will test the idea that having all of the early emerging species included as a cover crop will be better than having only one species. The same response variables will be measured on these new plots (except for soil moisture due to budgetary constraints), as explained above. These plots were established during Spring 2005 and were seeded with their respective treatments in April. They have been mowed once to reduce weeds and encourage cover crop establishment. Many cover crop and other prairie seedlings are emerging as of June, 2005. Results will be discussed in the 2006 final report.

Other activities:

We are also conducting two experiments in 1 x 1 m sub-plots within each 5 x 5 plot. The first is an 15N study. A trace amount of this stable isotope was added to plots and the amount of N retained in the plots are to be monitored over time and in future years. By knowing how much 15N was added (0.5 g N/m2), we can determine how much is retained in the prairie vegetation, in the weed component, and how much is lost from leaching and runoff (the difference between the amount added and the amount recovered). The second study involves planting and monitoring small transplants of 8 prairie species (purple prairie clover and round-headed bushclover, yellow coneflower and wild bergamot, big and little bluestem, and Scribners panicum and porcupine grass) in each plot. Transplants were planted in 1 x 1m subplots, with 2 subplots per plot. Seedlings were grown in greenhouses and planted into subplots during the 2004 growing season. These subplots are being weeded of everything but the cover crop and planted prairie seedlings.

Seedlings in one subplot per plot were harvested at the end of the 2004 growing season. Seedlings in the other subplot will be harvested at the end of the 2005 growing season. We are currently analyzing the data from the first year, and have data on the relative biomass of each species both above and belowground. Preliminary results suggest that the establishment of prairie species will differ between cover crop plots and controls (no cover crop), and that having a cover crop will affect the species composition of the forb community. For example, wild bergamot was ranked as the most common in cover crop plots based on relative biomass, possibly because of its shade tolerance. In control plots, yellow coneflower was ranked as the most common. These studies will provide additional information on prairie establishment success with and without native cover crops.

Root Biomass

Roots of all prairie species were harvested during Fall 2004 and will be again harvested in Fall 2005. These data will provide a good estimate of prairie root growth with and without cover crops, and among cover crop species. Root data are currently being analyzed and will be discussed in the 2006 annual report.

Conclusions

As mentioned earlier, cover crops should: 1) keep out weeds and 2) facilitate, or speed-up prairie plant establishment. During 2004, we collected data to address the first part. Black-eyed susan and partridge pea were much better at keeping out weeds than were other species. Whether they also keep out prairie species is unknown at this time, but will be better understood with continued sampling of our plots. Recommendations on which species will make the best cover crop, and whether cover crops are effective at keeping out weeds and facilitating prairie establishment will be made after another year of sampling.

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