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Introduction

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Bees and other native pollinators perform vital ecosystem functions within the grasslands of eastern Oregon by pollinating flowering plants, serving as food for vertebrates, and making up a large portion of biodiversity (). Populations of native bees and other pollinators in grasslands are declining in eastern Oregon, as are their native grassland habitats. In recent years, both grassland restoration projects and severe wildfires in eastern Oregon have increased in number, independently of each other. However, little is known about howthese different landscape changes affect bee populations and Supporting pollinators is not a common objective when planning grassland restoration projects ().

In order to investigate how grassland restoration affected pollinators, especially native bees, researchers from Oregon State University set up a study at The Nature Conservancy Boardman Grasslands Preserve and collected data on the abundance and diversity of native bees and key habitat characteristics. Researchers selected a degraded grassland in eastern Oregon where the current and most popular restoration technique involves controlling invasive grasses with glyphosate or other common herbicides and re-seeding the area with native grasses and forbs. Grassland restoration projects are primarily focused on increasing biomass of common grass species, which provides nesting habitat for some bee species but falls short in holistically supporting bee populations by providing abundant and diverse blooms throughout the season. While the study to determine the impacts of grassland restoration on pollinators was underway, a severe wildfire raged through half of the study site early in the fire season. This wildfire was an opportunity for the team to measure how the bee population responded to both grassland restoration and wildfire. Throughout their study, researchers paid close attention to environmental variables influencing the native bee community, such as nesting site availability, litter cover, and flower abundance. 

Key Issues Addressed

The increase in grassland restoration projects in eastern Oregon has provided an opportunity to study how current restoration practices affect bee populations. The glyphosate/re-seed method effectively controls invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species , but may not benefit pollinators. In this study, researchers investigated the effect of glyphosate/re-seed restoration on declining bee populations in Oregon, which would benefit from grassland restoration practices that also consider pollinator needs including abundance and diversity of nesting substrate and other habitat needs.

Wildfires are becoming increasingly common and severe across the western USA, including eastern Oregon, as climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change increases the severity of drought. The effect of wildfires on bees also requires further attention. It is unknown if fire could be utilized as a tool to help restore and enhance native bee population because it is difficult to predict future fire locations and collect pre-fire data at study sites. However, this study allowed for data collection before a fire burned through the study area, so researchers were able to study how wildfire impacted both bee populations and grassland vegetation.

Project Goals

• Bridge the knowledge gap on how bees are affected by glyphosate/re-seed restoration work in the grasslands of eastern Oregon
• Identify which habitat elements to improve in grassland restoration projects to better support pollinators
• Assess how bees are impacted by increasingly common and severe wildfires in grasslands of the eastern Oregon and similar grassland across the Pacific Northwest

Project Highlights

Protection From Fire: Unburnt patches of vegetation and lightly burned areas likely allowed pollinators to find refuge from the fire and prevent populations from declining one year after the fire. 

• Restoration Practices Do Not Affect Bees: Researchers found that common glyphosate/re-seed methods did not reduce abundance or diversity of grassland bees. They also did not increase abundance or diversity of native bee populations, indicating that grassland restoration techniques might be improved by considering multiple pollinator needs such as nesting sites, vegetation height, abundance and diversity of blooming plants in project design to benefit the pollinator community.
• More Flowers ≠ More Bees: An increase in flowers alone was not found to increase native bee populations. Researchers suggest that nesting site availability, diversity and seasonality of blooms, and diversity and quantity of litter are important features of pollinator habitat to restore in addition to the number of flowering plants.
• Valuable Pre-Fire Data: After the fire, native bee diversity and richness increased compared to pre-fire conditions. By analyzing key environmental variables that impact bees before and after the early season fire at the study site, researchers were able to understand how fire could potentially be used as a restoration tool in the future. Researchers suggest this might be due to additional nesting sites and/or increased floral resources after burns, but more direct investigations are necessary. 

Lessons Learned

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Researchers identified key habitat variables that benefit native bees that could be incorporated into current grassland restoration methods to better support grassland pollinators. Unrestored sites with taller vegetation and more blooming flowers exhibited greater bee species richness but lower diversity. Current restoration methods could be supplemented by re-seeding sites with a diverse array of flowering plants that include multiple species, different bloom times, and tall plants. 

Sites with more varied and abundant litter cover had higher bee populations. For example, some bee species use twigs or cavities in larger plants to nest, while others require bare soil as nesting sites. By including diverse types of vegetation and managing for some bare ground in grassland restoration projects, managers could help provide for the diverse needs of the native pollinator community.

Fire has the potential to be a useful tool for grassland restoration that is also beneficial to native pollinators. Environmental variables that benefit pollinators– including forb abundance and diversity of nesting habitat– improved after the early season wildfire, as did the diversity and richness of native bee populations. Forbs that were more abundant after fire included non-native Douglas’ knotweed (Polygonum douglasii) and Russian thistle (Salsola tragus); and native Canadian horseweed (Conyza canadensis), tall willowherb (Epilobium brachycarpum), and hoary tansyaster (Machaeranthera canescens). 

Contrary to many other studies, the cover of invasive annual grasses, such as cheatgrass (Bromus tectorum) decreased significantly at burnt sites. This may be due to the early season of the fire and low cover of invasive grasses before the fire. Restoration practitioners may be able to reduce invasive grass cover by conducting early season burns in areas with moderate to low invasive grass cover.

Next Steps

• Apply findings about increasing litter cover, blooming plant diversity, and nesting site availability to future grassland restoration projects, and study impacts on native pollinator communities.
• Expand on observations about the positive effects of early season fire on pollinators by comparing how different seasonality and severity of burns impacts grassland pollinator communities to help inform use of prescribed burns for grassland restoration.

Funding Partners

Resources

• Smith DiCarlo, L. A., DeBano, S. J., & Burrows, S. (2020). . Restoration Ecology, 28, A54-A64.
• Smith DiCarlo, L. A., DeBano, S. J., & Burrows, S. (2019). . Rangel. Ecol. Manag, 72, 551-560.

Contacts

• Lauren DiCarlo, Westfield State University, ldicarlo@westfield.ma.edu
• Sandy DeBano, Oregon State University, sandy.debano@oregonstate.edu

CART Lead Author

• Marty Salamone,  CART Student Intern, Georgetown University

Suggested Citation

Salamone, M. (2022). “Impacts of Grassland Restoration and Wildfire on Bee Populations in Eastern Oregon.” CART. Retrieved from /project/grassland-restoration-and-wildfire-bee-populations.