March 30, 2019
Protecting Native Biodiversity Isn’t Easy
By Dr. Helen Rowe, McDowell Sonoran Conservancy Field Institute Director
Republished from McDowell Sonoran Conservancy Mountain Lines newsletter”
A sea of fountain grass (Pennisetum setaceum) fills the Preserve’s Quartz Wash crowding out native desert plants. Photograph by Steve Jones.
Over half of Scottsdale’s McDowell Sonoran Preserve’s 80-mile boundary is surrounded by urban development, making it easily accessible to incursion from nonnative plants. Recent surveys by the McDowell Sonoran Conservancy Field Institute show that over 60.5 acres of the Preserve are currently dominated by two invasive grasses—bufflegrass (Pennisetum ciliare) and fountain grass (Pennisetum setaceum). These invasive grasses present two major problems—they increase fire hazard and displace native plants.
Humans brought these grasses to Arizona, along with many other nonnative plants and animals over many decades. Buffelgrass, introduced into the United States in the 1930s as livestock forage, did not initially thrive in the desert. Then in the 1980s, it began to expand rapidly. Fountain grass, introduced into Arizona as an ornamental landscape plant, was well established in the Tucson area by the 1940s.
Buffelgrass is a perennial bunchgrass considered to be a major threat to biodiversity in the southwestern United States and Australia. Buffelgrass is apomictic, meaning it produces seeds without using sexual reproduction. The seeds are dispersed by the wind or dropped to the ground. It also reproduces vegetatively sending rhizomes (lateral shoots) underground and stolons (lateral stems) along the surface. New plants grow from the rhizomes that send grass shoots up to the soil surface, and from the stolons that send their roots below the surface. The preferred habitat of buffelgrass is valleys and low slopes with recently disturbed areas, but it will also grow on steep rocky hillsides. The low water requirements for buffelgrass emergence and its ability to capture more water resources than native plants give buffelgrass a highly competitive advantage in arid environments. Buffelgrass also seems to be able to adapt more rapidly to environmental changes than native species.
Buffelgrass (Pennisetum ciliare) in the Preserve presents a fire hazard and dominates native desert plants wherever it grows. Photograph by Steve Jones.
Fountain grass in Arizona prefers sandy soils in desert riparian areas and washes, but is also found in grasslands, canyons, rocky slopes, roadsides, and disturbed sites. Like buffelgrass, it is an apomictic, wind dispersed, perennial bunchgrass. However, unlike buffelgrass, fountain grass does not spread vegetatively.
Buffelgrass and fountain grass are highly productive and present a significant fire hazard. Both grasses dramatically increase the fuel load for fire consumption compared to native communities, and therefore increase the threat of wildfire. The threat is particularly acute in the Sonoran Desert, where the ecosystem is not fire adapted. This means that when a desert fire destroys native plants, reestablishment is slow. Buffelgrass and fountain grass, on the other hand, are fire adapted, and readily reestablish themselves after fire. Several studies have clearly documented an increase in their abundance after fire. The cycle of invasive grass species increasing fire load and frequency change ecosystem conditions in a feedback loop that leads to invasive grass dominance. This loop has been dubbed a grass fire cycle.
Fountain grass (Pennisetum setaceum) will establish itself in recently disturbed ground outside the Preserve and then move into the Preserve to displace native plant species. Photograph by Steve Jones.
Even in the absence of a fire threat, both nonnative species have been shown to displace native species and reduce plant biodiversity where they invade. Thus, native plant communities invaded by buffelgrass demonstrate less native species richness. Buffelgrass has been implicated in the die-off of the native woody species, the yellow palo verde, due to the consumption of shallow water by these bunchgrasses. Finally, in a study of 14 sites in Saguaro National Park near Tucson, Arizona, soil nutrients such as carbon and nitrogen were elevated twofold in sites with buffelgrass compared to sites without buffelgrass. Nitrogen has been shown to have an even more negative effect on desert native plant species in the Sonoran Desert than buffelgrass itself. Buffelgrass is on the Arizona noxious weed list.
Like buffelgrass, fountain grass competes successfully with native species for shallow water. Its presence contributes to increased fire frequency, and its buried seeds germinate after fire. Fountain grass is listed as a noxious weed in Hawaii and Nevada, but although research indicates that fountain grass will continue to thrive in Arizona’s climate, it is not yet listed on Arizona’s noxious weed list.
Over the past three years, the McDowell Sonoran Conservancy Field Institute has been surveying and mapping nonnative plant species in the Preserve in order to better understand their distribution, and to better understand how to manage these grasses. Over the same time period, the Field Institute developed an Ecological Resource Plan (ERP) with the City of Scottsdale. The ERP serves to prioritize conservation needs, including nonnatives, in the Preserve. In Fall 2015, the Field Institute initiated a study group of staff and stewards to search peer reviewed literature and determine the best practices for removing and preventing reestablishment of buffelgrass and fountain grass. Given what was learned in other environments, and the rapid spread of these grasses in the Preserve, control of these two perennial grasses was recommended as a high priority for conservation of the Preserve.
As a result of studying the scientific literature, the study group worked together to develop a nonnative species strategic plan for the Preserve and surrounding areas. Strategic plans, a common approach in integrated pest management, help managers prioritize actions given limited resources. The study group reviewed plans from Australia, Tucson, and Texas, and developed a strategy that combined removal of dispersed populations, and monitoring of the grasses; designed a field study to test different removal techniques in the Preserve; and prioritized surveys for the Preserve perimeter and along major roads near the Preserve.
Buffelgrass (Pennisetum ciliare) flowers quickly even after a light rain. Each pound of buffelgrass can produce 260,000 seeds that are very successful at germinating. Photograph by Steve Jones.
Although there has been substantial research on buffelgrass and fountain grass, the group found no published papers that compared the efficacy of various removal treatments on controlling the nonnative plants with the treatment effects on native vegetation over a long period. Thus, the group developed an experiment to compare common removal methods in a field study to determine which treatment can best control the invasive species and protect native plant species at the lowest cost. The experimental treatments will be a combination of pulling, mowing, and herbicide repeated annually. We are seeking funding to launch the experiment.
There was one positive finding from the literature search. Given the longevity and robustness of native seeds in the soil, native plant species can dominate post treatment vegetation after the removal of buffelgrass. Ultimately, the removal treatment results will be integrated into removals elsewhere on the Preserve and regionally, so that managers and volunteer teams throughout the Valley know the most effective and relevant integrated weed management approaches.
The Field Institute is undertaking the daunting effort of nonnative grass removal because the Preserve is one of the most biologically diverse open-space areas in the Phoenix Valley. It hosts over 378 species of plants, 200 species of vertebrate animals, and countless invertebrates. The Preserve is also home to six species of animals listed as Species of Concern by the U.S. Fish and Wildlife Service. Those riches alone make the Preserve worth conserving. When you also add on the magnificent views and the trails for hiking, biking, and horseback riding, you have a remarkable place where it is justifiable to line up the volunteers and take a stand against the invaders. This work will greatly improve our capacity to protect this sensitive Sonoran Desert habitat for the long term.
Fountain grass (Pennisetum setaceum) produces lots of seeds that help it to invade natural habitats where its dense stands of grass will successfully compete with native species, displacing the native plants. Photograph by S Jones.
For Additional Information read “Your ornamental fountain grass is a wildlife danger” or visit the McDowell Sonoran Conservancy website.
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