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8-1 Chapter 8 Creating Climate- Smart Pollinator Habitat Along Roadsides 8.1 IntroductionÂ Â More than 98 percent of climate scientists agree that climate change is occurring and that human activities, primarily the burning of fossil fuels and deforestation, are the cause. The effects are already being felt across the country in rising temperatures and increased frequency of extreme weather events. Climate change presents challenges for department of transportation (DOT) managersâextreme events, heatwaves, and sea level rise can all significantly affect transportation infrastructure. Climate change also affects pollinators and the plants that rely on them. This chapter presents an overview of the effects that climate change can have on pollinators, the components of climate-smart pollinator roadside habitat, and additional benefits of such habitat for mitigating the climate crisis. 8.2 ClimateÂ ChangeÂ ProjectionsÂ Â Climate change will have a variety of effects on climate, many of which are already occurring (see Box 8-1). Increases in average temperatures will continue nationwide, with increased frequency of extreme temperature events, including heatwaves. Weather variability will increase in most regions, with extreme weather events becoming more frequent and intense. Drought will become more common in many regions, which can play a role in increased wildfire frequency. Flooding will also become more common as a result of more frequent extreme weather events and sea level rise. Sea level rise will increase flood risk in coastal areas, while extreme weather events may increase flooding in both coastal areas (such as with increased number and intensity of hurricanes) and inland, especially near rivers and floodplains. Climate change will also have many impacts on the nationâs infrastructure and create additional challenges for DOT staff who manage and maintain safe roadways. In many areas, increases in temperature will reduce the amount of time that employees can safely spend working outdoors, especially during heatwaves. However, the need to update and/or repair infrastructure in response to climate change will also create opportunities for revegetation projects that support pollinators and other wildlife. According to the Fourth National Climate Assessment (USGCRP 2018): ï· Increased temperatures, combined with more frequent extreme weather events (including flooding, drought, and fire) and sea level rise, will compromise infrastructure. Unprecedented rains and flooding in summer 2022 in Yellowstone National Park led to the destruction of roads and other infrastructure. Extreme weather events like this will become more common as a result of climate change. Photo Credit: AP via NPS
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-2 ï· Increased temperatures can cause rutting, cracking, and buckling of roads. ï· Nationally, 60,000 miles of bridges and roads run through coastal floodplains, which are at increased risk from flooding and storms. ï· Increases in inland flooding will threaten 2,500 to 4,600 bridges nationwide by 2050 and 5,000 to 6,000 bridges by 2090. Box 8-1. Climate projections for the Great Lakes Region Climate projections for this region include: ï· Increases in average temperatures are expected. o The number of days above 35Â°C (95Â°F) will increase, with a greater increase at southern latitudes. ï· Precipitation will increase. o Winter and spring precipitation could increase by up to 30 percent by 2100. o Extreme rainfall events will become more common and more severe. o Increases in precipitation and extreme rainfall events will increase flood risk. o The risk of erosion will increase. o The region will experience an increase in the number of days with heavy precipitation. ï· Extreme weather events will become more common, including an increase in severe winter weather events. References: USGCRP 2018. 8.2.1 Where Do These Numbers Come From? Scientists use models to project the effects of climate change in the future (usually up to 100 years). Predicting how climate change will affect future climate requires information about how greenhouse gas emissions and human population size will change over time. The Intergovernmental Panel on Climate Change (IPCC) has developed a set of scenarios that describe a range of possible future greenhouse gas emissions levels and human population sizes for use in climate models. In addition to using multiple IPCC scenarios, scientists typically use multiple climate models that differ in their sensitivity to different parameters. By using multiple models and multiple IPCC scenarios, scientists can bracket the potential effects of climate change to provide a range of likely expected conditions. A recent analysis shows that such models have been successful in predicting changes in climate over time (Hausfather et al. 2020). 8.3 HowÂ WillÂ ClimateÂ ChangeÂ AffectÂ Pollinators?Â Creating climate-resilient pollinator habitat first requires an understanding of the variety of ways that climate change can affect pollinators. Below is a list of effects of climate change
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-3 on pollinators. These are not mutually exclusive, as pollinators are likely to experience multiple effects of climate change simultaneously. While some pollinator species may fare better under climate change, many species will be negatively affected. ï· Range shifts. Species may change their distributions to track more optimal climate. In general, species are expected to shift poleward or to higher elevations, but not all species will respond the same way. Differences in future distributions of pollinators and host/nectar plants may lead to spatial mismatches. Range shifts require habitat corridors for species to move through. ï· Altered phenology. Phenology is the timing of biological events such as flowering time in plants and adult flight time in butterflies. Shifts in phenology in response to climate change may be especially problematic if pollinators and their host plants respond differently, leading to phenological mismatches. For example, a bee that specializes on the pollen of a particular group of plants may emerge after its host plants have begun to bloom, missing out on resources they need to provide for their young. ï· Physiological responses. Temperature is extremely important in influencing a variety of physiological processes in insects, including metabolism, growth rate, and digestion. But temperature can also affect behaviors, such as foraging rates and the amount of time each day that pollinators are active. Therefore, changes in temperature can affect many aspects of pollinator performance (e.g., survival, reproduction, size at maturity). ï· Altered species interactions. The outcome of species interactions, such as competition, predation, or disease can be affected by changing climate, especially temperature change. ï· Changes to the diversity, quantity, and quality of floral resources. Plants will also respond to climate change. Changes in plant diversity or community composition will affect competitive relationships among pollinators. Specialist pollinators may be especially sensitive to such changes in plant communities. Drought, heatwaves, and increases in temperature, as well as increasing atmospheric carbon dioxide (CO2) concentrations can all affect the quantity and quality of pollen and nectar, as well as floral attractiveness to pollinators. ï· Combined stressors. Climate change may exacerbate the effects of other stressors, such as habitat loss, pesticide use, and pathogen exposure, magnifying effects on pollinators. 8.4 WhatÂ SpeciesÂ AreÂ MostÂ VulnerableÂ toÂ ClimateÂ Change?Â Â Species most likely to be negatively affected by climate change include resource specialists (e.g., species that use only a narrow subset of plants as larval host plants or for pollen), species that are already declining, species that occur at high latitudes or high elevations, and species with limited distributions or narrow habitat requirements. Many of the species featured in Chapter 3, Imperiled Pollinator Profiles, fit into several of these categories and will be at increased risk as the effects of climate change continue to become apparent.
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-4 8.5 IncreasingÂ ClimateÂ ResiliencyÂ forÂ PollinatorsÂ Â Increasing climate resilience for pollinators requires a multifaceted approach. Creating and restoring habitat, increasing habitat connectivity, and reducing other stressors are all key components for increasing pollinator climate resilience. Roadside habitat has an especially important role to play in increasing habitat connectivity. 8.5.1 Habitat Enhancing and restoring pollinator habitat is crucial for improving climate resilience of pollinator communities. Habitat with abundant pollinator-attractive floral resources that bloom from spring through fall is required to support large, stable, and diverse pollinator communities. Larger populations, in general, will better withstand bad years and extreme weather events that become more frequent with climate change. Maintaining a diverse pollinator community will also be key to ensuring that the valuable ecosystem service performed by these animals remains resilient in the future. Habitat with a diversity of plant species helps to buffer pollinators from extreme weather events by creating important refugiaâspaces where they can be protected from heat waves or other extreme weather eventsâfor pollinators. Habitat with a diversity of plant species also provides important pollinator nesting sites and larval host plants that may be less abundant in developed areas. 8.5.2 Habitat Connectivity Improving habitat connectivity is another key aspect of creating a climate-resilient environment. Habitat corridors act as pathways, connecting larger natural areas that act as reservoirs of pollinator diversity. Because roads and other rights-of-way create vast networks crisscrossing the landscape, linear habitats created or maintained alongside them are ideal for improving habitat connectivity for pollinators. This is an area where DOTs and other managers of roadsides can make especially meaningful contributions to climate resilience of wild pollinators. Habitat corridors and stepping-stones (patches of habitat that are close enough to allow movement among them) are important because they allow bees, butterflies, and other insect pollinators to move around the landscape. This increases gene flow and helps prevent pollinator populations from becoming too small by enabling individuals to move among patches. Habitat corridors and stepping-stones also enable species to migrate, facilitating range shifts in response to climate change. While not all species will change distributions in response to climate change, increasing habitat connectivity provides the opportunity for those that will. Specialist pollinators that use a narrow range of plants for foodâsuch as the Hermes copper, which only feeds on spiny redberryâare generally expected to be more vulnerable to climate change than generalist pollinators. Photo Credit: Michael Klein SR/USFWS
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-5 8.5.3 Reducing Additional Stressors Reducing stressors (i.e., drivers of pollinator declines; see Chapter 3) to pollinators such as pesticide exposure, habitat loss, and invasive species is important because these different stressors can interact with each other and with climate change to magnify negative effects on pollinators. For example, exposure to a particular pesticide may be non-lethal in ordinary circumstances, but exposure to that same pesticide may become lethal during a heatwave or drought. Therefore, it is important to reduce other stressors as much as possible to increase climate resilience of pollinators. 8.5.4 Building Climate-Smart Roadside Pollinator Habitat The most important steps to increasing climate resilience of pollinators are protecting and enhancing pollinator habitat, increasing habitat connectivity, and reducing other stressors. Creating pollinator habitat along roadsides and other DOT land holdings can contribute significantly to this effort. When creating roadside habitat for pollinators, following a few basic principles can help make that habitat more resilient to climate change. Resilient habitat will help to foster resilient pollinator communities. Pay Attention to Flowering Phenology Ensure that a minimum of three species of nectar plants bloom at all times during the season of activity for bees and butterflies (generally from spring through fall; see Chapter 7, Revegetation and Pollinators: Design and Implementation, for a plant list, example bloom time charts, and other information on revegetation). This practice will ensure that important food resources are always available for pollinators. This practice may also help reduce effects of phenological mismatches between hosts and pollinators by ensuring that some floral resources will be available (OlliffâYang et al. 2020). For example, early warm springs may lead to truncated bloom periods for some plant species, meaning spring-emerging pollinators will have fewer resources and less time to find food. More pollinators can be supported in habitat with abundant, diverse blooms compared to habitat with few flowering plants. However, this may be more effective for generalist pollinators than for specialists that need specific plant species. If possible, work to better understand what specialist pollinators might be found on the property in question, including imperiled pollinators (Chapter 3), and include specific flowering resources for those species if possible. Whenever possible, include more than three species per bloom period; plantings that are more diverse can support more species of pollinators and have greater resilience. Provide Nesting Habitat for Native Bees When many people think about bees, they think about honey bee hives or bumble bee nests; however, the vast majority of bees are solitaryânesting in narrow tunnels in the ground or in wood. About 70 percent of native bees nest in the ground, and installing vegetation like bunch grasses that can provide access to small patches of earth will provide areas for these species to build nests (Chapter 2, Pollinator Biology and Roadsides). About Incorporating biodiversity into revegetation projects is key to increasing carbon sequestration services as well as for increasing climate resilience of the planting. Photo Credit: Dianne Kahal-Berman
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-6 30 percent of native bees nest in wood or pithy-stemmed plants. Retaining downed logs and snags where possible will provide nesting habitat for some of these species, while planting native, pithy-stemmed plants like goldenrod or wild rose will provide nesting habitat for others (see Chapter 7 for a plant list that includes species used for nesting by cavity-nesting bees). In general, a diverse plant community is more likely to provide necessary nest sites and nesting materials for a diverse community of pollinators. Biodiversity Is KeyâInclude a Variety of Native Plants Biodiversity is the variety and variability of life on earth. This concept includes not only the number of species in a given location, but the amount of genetic variation within each species or population at that location. Adapting to climate change and conserving biodiversity go hand-in-hand. Indeed, peopleâs ability to adapt to climate change and retain important ecosystem services such as pollination will depend on their ability to protect biodiversity. When creating climate-smart pollinator habitat, incorporating biodiversity in the planting is key to creating sustainable habitat that will support a variety of pollinators. Incorporate as many native plant species as possible into habitat projects. Habitat with a diverse native plant community can support more species of pollinators than habitat with only a few plant species. Many species of insects, including pollinators, exhibit some degree of resource specialization during their lives. For example, the monarch butterfly feeds only on species of milkweed during its larval stage (Chapter 3). Incorporating a diversity of native plants into habitat projects means the habitat will be more likely to support some of the many specialist pollinators in this region. Finally, a diverse native plant community will have greater habitat heterogeneity that can provide an array of microclimates, which serve as important refugia for pollinators during heatwaves and other extreme weather events. A biodiverse habitat (both in terms of species richness and genetic variation) has many additional benefits for climate resilience. First, generally speaking, more-diverse ecological communities are better able to provide ecosystem services such as erosion control, carbon sequestration, and pollination over time than species-poor ecological communities (Tilman et al. 2006). Second, genetically diverse populations are generally more likely to contain the traits that will enable them to adapt to climate change than populations with low genetic diversity. Third, diverse communities will be better able to withstand bad years and extreme weather events than species-poor communities. For example, a roadside habitat with a variety of plant species is more likely to include plants that can persist during both drought and flooding than a habitat with only a few plant species. Given that climate change will lead to more extreme weather events and more variability in weather, having a diverse plant community will better ensure that some plants will be able to persist and thrive, supporting the pollinators that depend on them. This may be especially important in regions where opposing conditions will both become more common. For example, in many regions both flooding and drought will become more common. It can be difficult to find plant species that can thrive in the range of expected conditions, so including a variety of plant species will help to ensure that at least some will do well in any given year or season. For similar reasons, diverse plant communities may be more resistant to pressure from invasive species or diseases. Climate change is likely to also affect the quantity and quality of floral resources for pollinators. Abiotic factors associated with climate change, such as increased temperature or increased atmospheric CO2 concentrations, may also affect the quantity and quality of
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-7 nectar and pollen as well as floral attractiveness to pollinators (Rusetrholz and Erhardt 1998; Burkle and Runyon 2016; Ziska et al. 2016; Glenny et al. 2018; Russo et al. 2019). Because the effects of climate change on these plant traits are likely to be species specific, having a diverse array of flowering plants will help ensure pollinators have the resources they need. Choosing Climate-Smart Plants An additional challenge for creating pollinator habitat is trying to anticipate what plant materials will be resilient to future climate scenarios. As stated above, incorporating biodiversity, both in terms of genetic diversity within species and in terms of species richness within plantings is a good strategy for creating plantings that will persist over time as climate changes (Wilsey 2020). Using local ecotypes (Chapter 7) can help ensure the plants used will be adapted to local conditions. However, as local conditions change, incorporating as much genetic diversity as possible may be beneficial. Therefore, try to get local ecotypes from several different source populations when possible (St. Clair et al. 2020) and consider working with the local native plant industry to increase genetic diversity of important pollinator plants used in revegetation projects. Another possibility is to incorporate a small percentage of seed from lower elevation or lower latitude populations that more closely mimic future climate conditions for the area (SgrÃ² et al. 2011; Ramalho et al. 2017; Camarretta et al. 2020). This may be more important for long-lived shrubs and trees than for annuals. It can be difficult to predict what plant species will be most successful in the future. Plants will also respond to climate change, and plant characteristics such as phenology and nutritional value of pollen and nectar may vary with climate change. Additional research is needed to understand what traits or characteristics make plants more resilient to extreme weather events and to increases in weather variability. Research is also needed to understand how competitive relationships between key invasive plants and native plants will be altered by climate change. However, choosing native plants that are adapted to future climate projections for this region is vital (Box 8-1). For example, many regions will experience increased drought, so choosing drought-tolerant plants is a wise choice. Table 8-1 lists some plant traits that are generally associated with adaptations to different climatic conditions. Use this list to select plant species for revegetation projects, along with the list of workhorse pollinator plants in Chapter 7, as well as any plants that support imperiled pollinators occurring in the revegetation project area (Chapter 3). As discussed above, the best approach for creating resilient, sustainable roadside habitat is to harness the benefits of biodiversity, planting a diverse assemblage of plant species. Table 8-1. Plant traits that will generally be beneficial for adapting to different conditions associated with climate change. PlantÂ traitÂ ClimateÂ variableÂ TallerÂ grassesÂ IncreasedÂ temperaturesÂ ShorterÂ treesÂ IncreasedÂ temperaturesÂ
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-8 PlantÂ traitÂ ClimateÂ variableÂ ThickerÂ leavesÂ Drought,Â increasedÂ temperaturesÂ GreaterÂ belowgroundÂ biomassÂ DroughtÂ HighÂ waterâuseÂ efficiencyÂ DroughtÂ DeeperÂ rootsÂ DroughtÂ HigherÂ woodÂ densityÂ DroughtÂ ThickerÂ barkÂ FireÂ AbilityÂ toÂ resproutÂ FireÂ Source: Willis 2017. This region will become wetter and warmer (see Box 8-1). Flood-tolerant plants and heat- tolerant plants may be better able to persist. Recommended plants include: Virginia mountain mint (Pycnanthemum virginianum), Cup plant (Silphium perfoliatum), New England aster (Symphyotrichum novae-angliae), flat-top goldenrod (Euthamia graminifolia), wild bergamot (Monarda fistulosa), swamp verbena (Verbena hastata), and common milkweed (Asclepias syriaca). 8.5.5 Additional Benefits of Climate-Smart Pollinator Habitat Creating climate-smart pollinator habitat along roadsides has many additional benefits beyond pollinator conservation. Pollinator habitat also provides benefits such as erosion control and reduced runoff. This helps reduce the impacts of flooding, which will be critical in many areas where increased flooding is projected, and in coastal areas where sea level rise will increase the frequency of coastal flooding. Pollinator habitat can also help to mitigate the climate crisis. Creating diverse plantings along roadsides will increase soil health, which in turn increases carbon sequestration. Studies of grassland soils show that increasing plant diversity increases the amount of carbon stored by the soils (Lange et al. 2015; Chen et al. 2018; Yang et al. 2019). A recent study estimated the value of carbon sequestration services of roadside habitat in Florida at $39 million/year, and this number could increase with the sale of carbon credits (Harrison 2014). Thus, diverse plantings created for pollinator conservation also help to mitigate climate change by increasing carbon sequestration services of those habitats (Ament et al. 2014). This is the idea behind ânature-based climate solutions,â which encourages the restoration and protection of natural habitats as a means of combating the climate crisis. Recent studies suggest that nature-based climate solutions alone could provide one-third of the climate mitigation required to keep warming below 2Â°C (warming of 2Â°C causes extreme heat and droughts, as well as increased water stress and sea levels,
ChapterÂ 8.Â CreatingÂ ClimateâSmartÂ PollinatorÂ HabitatÂ AlongÂ RoadsidesÂ 8-9 altering the livability of millions of acres of land and escalating risks to millions of lives) (Griscom et al. 2017; Fargione et al. 2018; IPCC 2018). DOTs are in a position to make real contributions to nature-based climate solutions through the creation of roadside pollinator habitat. 8.6 AdditionalÂ ResourcesÂ ï· Engineering with Nature website: https://ewn.el.erdc.dren.mil/. ï· U.S. Forest Service Transportation Resiliency Guidebook: Addressing Climate Change Impacts on U.S. Forest Service Transportation Assets: https://www.volpe.dot.gov/FS- Transportation-Resiliency-Guidebook. ï· Federal Highway Administrationâs Climate Change Adaptation Guide for Transportation Systems Management, Operations, and Maintenance: https://ops.fhwa.dot.gov/publications/fhwahop15026/fhwahop15026.pdf. ï· National Oceanic and Atmospheric Administrationâs State Climate Summaries: https://statesummaries.ncics.org/. ï· U.S. Global Change Research Programâs Fourth National Climate Assessment, Volume II: Impacts, Risks, and Adaptation in the United States (includes regional information as well as a chapter on transportation and infrastructure): https://nca2018.globalchange.gov/. ï· The Seedlot Selection Tool (matches seedlots with climatic parameters using a map- based program): https://seedlotselectiontool.org/sst/. ï· The Transportation Research Boardâs Critical Issues in Transportation 2019: http://www.trb.org/Main/Blurbs/178402.aspx.