Seeds underhoof: can the soil seed bank facilitate restoration of sheep-grazed, cheatgrass-invaded rangelands?

Commodities

• Animals: sheep

Practices

• Pest Management: weed ecology
• Production Systems: agroecosystems

Ranchers and land managers in the United States spend billions of dollars annually to control invasive plants, such as cheatgrass – an invasive annual grass that pushes out native plant species and increases wildfire risk on western rangelands. Efforts to restore native perennial species through seeding projects are also costly, often with low and unpredictable germination rates. A growing body of evidence suggests that livestock grazing, particularly fall and spring targeted grazing, may reduce invasive annual grass abundance through biomass and litter removal of annual grass while perennial species are dormant. However, treatment of invasive species in isolation will be unsuccessful if there is no longer a sufficient seed source for native species to return without further assistance. With this project, we therefore examine relationships between soil seed bank composition and aboveground vegetation composition along a gradient of cheatgrass invasion.  In doing so, we set the stage to examine the potential for targeted grazing treatments to remove unwanted species and facilitate recovery of perennial species via regeneration from the seed bank.

The soil seed bank is a critical and understudied component of rangeland plant communities. Seed banks contribute to plant species persistence and recovery after disturbance and provide a signature of past, present, and future characteristics of the aboveground plant community. Furthermore, the seed bank can be used as a resource for restoration efforts after undesirable species (such as cheatgrass) have been removed. In relatively understudied, higher elevation areas at the leading edge of cheatgrass invasion, sheep grazing could play an integral role in both the cheatgrass removal and restoration processes, thereby sustaining the natural resource base on which livestock production depends.

The primary near-term objective of our study is to examine relationships between aboveground vegetation composition and belowground seed bank composition across a range of cheatgrass invasion levels. Our results will help rangeland managers better assess and incorporate the often neglected yet important belowground seed bank in their cheatgrass management decisions.

In addition to producing standalone research and education products, this project will build local capacity for researchers and students to conduct rangeland seed bank studies. It will also expand the scope of an existing funded WSARE targeted grazing project (SW22-938) by collecting baseline data on the soil seed bank and a heatmap of sheep use within the project’s targeted grazing treatments. These data will be used in a planned follow-up seed bank assessment to determine the efficacy of sheep grazing to decrease cheatgrass invasion directly (by reducing the abundance of cheatgrass and thereby cheatgrass seeds in the seed bank) and indirectly (by removing biomass and litter to make space for native seed germination), as well as through other potential mechanisms ripe for future investigation and collaboration.

Expected outcomes:

1. Improved understanding of the impacts of cheatgrass invasion on the soil seed bank.
2. Broader knowledge about the value of the soil seed bank and the opportunity to utilize it and sheep grazing as a tool for rangeland restoration.
3. Increased capacity to conduct rangeland seed bank studies to inform management decisions.

Research objectives

1. Quantify the differences in plant community composition between the soil seed bank and aboveground vegetation.
2. Assess the effects of cheatgrass invasion on the soil seed bank by quantifying differences in soil seed bank composition across a range of aboveground vegetation invasion levels.
3. Quantify the amount of sheep activity within targeted grazing treatment plots for more precise evaluation of sheep-vegetation interactions.

Education objectives

1. Increase knowledge of the soil seed bank and how it can be affected by annual grass invasion and sheep grazing.
2. Increase understanding and capacity to conduct seed bank research at the host institution.
3. Increase land managers’ knowledge of the use of the seed bank as a complement to grazing for restoration in rangelands.