Using phenology to guide invasive plant management

Successful restoration of degraded land often depends on well-timed interventions to control invasive species. In their recently published article, Taylor and colleagues present a case study of the effects of incorporating phenology information into invasive plant control operations at Midway Atoll National Wildlife Refuge (NWR), USA. The authors share their story below.

Towards the end of April, millions of birds, including warblers, tanagers, buntings, grosbeaks and orioles, migrate north from their southern winter homes to their breeding grounds in the temperate and arctic regions of North America. This mass movement of birds — a seasonal, natural phenomenon — happens with predictable timing each year. As described by the USA National Phenology Network, phenology is nature’s calendar. And timing is everything.

Phenology, in its simplest form, is the study of the timing of periodic and life cycle events of various organisms, such as the emergence of the first leaves during spring, the first appearance of a migratory bird species, or the date when the leaves of a deciduous tree change colour as winter approaches. However, these phenomena are very sensitive to changes in climate, especially temperature. Thus as the climate changes, some phenological events occur earlier or later which can lead to mismatches between organisms, such as asynchrony between migratory birds and spring green-up. Understanding how plant and animal phenology responds can help us to predict how species populations will change in time.

Out on Midway Atoll National Wildlife Refuge (home to the world’s largest albatross colony), we also monitored phenology — but not of periodic events such as animal migration. Instead, we studied the life cycle events of an invasive plant species. But why?

A field of grass

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A view of Midway Atoll National Wildlife Refuge—the world’s largest albatross colony and home to more than 3 birds from over 20 different species. Photo: Daniel Clark/U.S. Fish and Wildlife Service

Despite its remoteness, Midway has experienced tremendous change in the last 100 or so years, beginning with the construction of the United States’ telegraph cable across the Pacific Ocean in the early 20th century. A harbour was built and seawalls erected; when the US Navy took over the islands in 1936, barracks, runways and other facilities were built — physically transforming the atoll’s islands. 

Plant communities evolved and new species were carried to the atoll by its human residents, sometimes intentionally (for example, vegetables and ornamental plants) and sometimes accidentally. One introduced plant in particular, Golden Crownbeard (Verbesina encelioides), thrived in the sandy soils of Midway. In fact, this species proliferated so well that it covered over 60% of Midway’s land area. Golden Crownbeard grew in dense stands, which severely impacted albatross adults and chicks.

Photos: (left) Laysan Albatross (Phoebastria immutabilis) with Golden Crownbeard in the background in 2015; (centre) Botanist Kim Starr wades through a dense field of Golden Crownbeard (Verbesina encelioides) during a survey on Midway Atoll NWR in 2008; (right) detail of Golden Crownbeard flowers. Photos: Forest & Kim Starr/Starr Environmental

Over the years, Refuge staff tried numerous tactics to suppress Golden Crownbeard, including large-scale mechanical means such as mowing. Various concoctions of herbicides were also tested and applied to control this weed. It was not until several years ago that a more effective herbicide mixture was developed, a blend of glyphosate and Milestone (a pre- and post-emergence control herbicide). This mixture not only killed the Golden Crownbeard plants on which it was applied, but prevented seeds in the soil from sprouting. Most importantly, it was deemed safe for use in close proximity to wildlife. Although these efforts reduced Golden Crownbeard cover to about 1%, control efforts were incredibly time-consuming and challenging. For eradication efforts to succeed, a more sophisticated approach to control would be necessary.

Golden Crownbeard reproduces exclusively by seed, which the plants produce after advancing through vegetative and flowering phenophases. In addition, the plant is known to exhibit long periods of seed dormancy, and then rapidly respond, grow, and develop as environmental and climatic conditions become favourable. Thus for successful control and, ultimately, eradication, Golden Crownbeard should not be given the opportunity to produce and disperse ripe seeds. From an operational perspective, this requires invasive plant control technicians to return to areas previously treated in less time than it takes for a seedling to grow to maturity and drop its seeds. How much time is that?  No one really knows and this lack of knowledge hampered previous control efforts.

A single flower head of Golden Crownbeard can yield 300-350 seeds; moreover, it is uncertain how long seeds can remain dormant before germinating. Photo: Forest & Kim Starr/Starr Environmental

By partnering with the USA National Phenology Network and their citizen science program Nature’s Notebook, we documented the timing of key reproductive life cycle events of Golden Crownbeard on Midway Atoll NWR. Over the course of a year-long monitoring project, we found that it took an average of 76 days for Golden Crownbeard to transition from leaves to seed drop, although the time required varied across the year (range: 31-175 days). Accordingly, invasive plant control schedules were adjusted to re-treat infested areas every 30 days in order to control plants before they are able to disperse seeds.

By incorporating phenology information into invasive plant control operations at Midway Atoll NWR, efforts to eradicate Golden Crownbeard will have a higher chance of succeeding. Standardised, well-tested methods, such as those developed by the USA National Phenology Network (USA-NPN), provide useful tools for optimising the timing of management practices and a way to share data with anyone with access to a computer. Golden Crownbeard isn’t just a problem on Midway; this invasive species has vastly expanded its range through Europe, Asia, Africa, and Australia. We hope that the phenology we have contributed to NPN data can help to better inform management of Golden Crownbeard not just on Midway, but in other places as well.

To learn more about how monitoring phenology can guide invasive plant management, check out our article in Ecological Solutions and Evidence!

Read the full research: “Using phenology data to improve control of invasive plant species: A case study on Midway Atoll NWR” in Issue 1:1 of Ecological Solutions and Evidence.

This is the first ‘From Practice’ article published in our new open access journal, Ecological Solutions and Evidence. From Practice papers provide practitioners a venue for communicating case studies, calls for new approaches for dealing with persistent problems or perspectives on research topics relevant for management. Find out more about ESE and our other article types in our Editorial.

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