The demonstration’s objective was to validate and demonstrate the efficacy of large-scale production of an alternative land cover, monoculture switchgrass, on military airfields and civil airports over a large portion of the eastern half of the U.S. as a means of reducing 1) wildlife strike risk (e.g., the likelihood of a wildlife strike with a particular species causing damage and the frequency of such strikes), and 2) economic and environmental costs associated with maintaining large expanses of managed grassland. Furthermore, one potential outcome of conversion of managed (i.e., mowed) airfield grasslands to switchgrass, despite the conditions of a monoculture habitat, would be an increase in population sizes of small, at-risk (threatened and endangered) bird species that are not hazardous to aircraft. Implementation of switchgrass monocultures was a primary technology demonstrated to participating installations concomitant with demonstration of wildlife sampling methodology including, but not limited to, bird point and line transects, mammal camera surveys, and switchgrass coverage estimates. Throughout the demonstration, installation personnel were kept informed of the project’s progress via direct interactions with the project manager and intermittent reports.
Switchgrass is a perennial cellulosic biofuel crop with high yields. Another advantage of switchgrass is that it is a high-quality animal forage (e.g., for beef cattle). Further, switchgrass is mowed (harvested) only once or twice per year, in contrast to most extant airfield grassland areas which are mowed multiple times each year. Finally, switchgrass is native and grows well over most of the eastern half of the U.S. (natural growth from 55° N to central Mexico) and can thrive on poor soils, which are common at military airfields and civil airports. Thus, switchgrass has the potential to be a regional solution for improving aircraft safety and generating revenue.
Implementation of switchgrass monocultures at airfields follows similar methodology to that of row crop agriculture. During the beginning of the demonstration, airfields and airports identified areas suitable for planting switchgrass (e.g., areas meeting Site Selection Criteria, on airport property, outside the Air Operations Area (AOA)). Plant competition suppression ensued using a broad-spectrum herbicide (e.g., glyphosate) and then areas were planted via seed-drill or broadcast seed at nine lbs/acre pure live seed (PLS), considered a very dense planting rate. Variations to this approach among installations included additional herbicides and/or mowing to reduce plant competition or improve seed drill access. These and post-planting switchgrass establishment techniques (e.g., additional plant competition) have been developed and well documented for programs ranging from wildlife conservation to biofuel production across the Midwestern and central United States. Primary challenges among past work and this demonstration included variable site conditions, dependence on weather patterns, and plant competition. However, the intended application of switchgrass monocultures in this demonstration is unique.
Despite an expected progression of switchgrass establishment mimicking past observations of 40%, 60%, and at least 80% coverage annually beginning with the planting year, switchgrass establishment failed to achieve intended coverages at most installations. Variability in switchgrass establishment was expected during the first few years based on the typical progression of native warm season grass establishment during which most fields transition through a fallow field stage until achieving a preponderance of the planted species. All attempts were made throughout this effort to improve switchgrass establishment success including additional planting, plant competition control, and planting earlier in the growing season. Despite variable establishment, however, all switchgrass sites experienced plant community changes (i.e., extant turfgrass progressing to a mixture of grasses and broadleaf weeds with or without preponderance of switchgrass) and were managed as tall-grass plots with only one to two mowings per year when mowing was used as competition control or for haying.
Bird responses varied substantially between breeding and non-breeding seasons, and whether assessed by installation or among installations. Overall, effect sizes (i.e., size of differences between switchgrass monocultures and controls) were small suggesting minimal differences in bird use between treatments. However, effect sizes did not meet minimum requirements for meeting defined success. Only two installations experienced significant cumulative hazard score responses to switchgrass establishment but were single year responses that conflicted between installations. Switchgrass establishment did not seem to cause any substantial increases or decreases in bird cumulative hazard scores between breeding and non-breeding seasons during the demonstration. Also, transitioning extant airfield grasslands to switchgrass monocultures did not cause substantial changes in bird use or hazards. Installation-specific mammal presence and responses also varied substantially. However, among year analysis suggested weak directional responses to switchgrass establishment with a slight decrease in coyote and deer use as switchgrass coverage increased. Overall, mammal responses suggest positive but weak support for establishing switchgrass at airfields and airports but did not meet performance objective success criteria. Some installation-specific investigations indicated beneficial outcomes of switchgrass establishment for reducing hazardous mammal use, but among-installation analysis suggested no overall effect.
Switchgrass monocultures offered an improvement to existing technology of extant airport grasslands and offered an alternative to leasing property for row crops. Typical airport grassland management outside the AOA involving periodic mowing may be improved by implementing less desirable conditions for wildlife hazardous to aircraft. Despite minimal change in the presence of hazardous wildlife, some switchgrass sites did not begin to transition to monocultures towards the end of the demonstration, a phase during which a greater wildlife response could have been observed.
Wildlife and vegetation surveys and meetings with airfield and airport personnel helped support monoculture switchgrass as a viable alternative land cover for airfields. Although switchgrass establishment failed at multiple locations and switchgrass monocultures were not realized until after the demonstration period (e.g., growing season 2018), discussions during final report presentations revolved around the alternative grassland management approach (i.e., tall grass with infrequent mowings versus frequently mowed short grass). Installations were asked if they would consider continuing to manage for switchgrass/tall-grass on their sites or otherwise leave the switchgrass plots “unmanaged.”
The majority (4 of 6) of participating installations supported maintaining switchgrass plots to differing degrees. Airport personnel for Detroit Metropolitan Airport (DTWA) and Gerald R. Ford International Airport (GRFI) plan to continue maintaining all switchgrass plots. Both installations will likely adopt a high-mow regime as a primary method of maintaining switchgrass coverage with limited additional herbicide applications for broadleaf weed control. The Ohio installations (Dayton International Airport (DAYT) and Wright-Patterson Air Force Base (WPAF)) will each maintain one switchgrass plot. The southern plot at DAYT will be converted to extant airport grassland likely through a frequent mowing regime due to its proximity to the airport entrance (i.e., aesthetics). At WPAF, the northern switchgrass plot was converted to a new gate construction project towards the end of the demonstration, but WPAF will continue maintaining the southern switchgrass plot as long as support continues from the Installation Commander. Columbus Air Force Base and Naval Air Station (NAS) Whiting Field (WHIT) experienced switchgrass failure and have both expressed the likelihood of applying periodic mowing to their switchgrass plots and not supporting the future growth and establishment of switchgrass.