Last updated: August 8, 2024
Article
Monitoring Team Confronts Trio of Problematic Plants
By Botanist Kelsey Songer, San Francisco Bay Area Inventory & Monitoring Network
NPS / Laura Booth
July 2024 - Characterized by unique topography and blankets of coastal fog, the coastal Bay Area national parks boast thousands of native plant species. Stunning examples of this biodiversity occur in the rolling prairies of the Marin Headlands and the redwood forests of Muir Woods in Golden Gate, all the way to the coastal dunes in Point Reyes. Across these vegetation types a drama is unfolding between introduced, invasive plants gaining ground and native species struggling to hold their own. This year, the San Francisco Bay Area Inventory & Monitoring Network’s Botany Team has been confronted by a tenacious trio of problematic species collectively referred to as “broom”.
Each summer, we fan out across the Bay Area national parks. Our goal is to monitor plant communities and track down invasive species spreading along trails and roadsides. This season, one of our focal plant communities has been coast live oak woodlands. This community, named for its typical canopy species, Quercus agrifolia, is also characterized by a variety of shrubs and herbs in the understory. As we worked through our woodland monitoring plots, we were surprised to see that some had been overtaken by broom.
© Vernon Smith / Photo 0311 3238 / 2011 / CalPhotos / CC BY-NC-ND 3.0 (left);
© Zoya Akulova / Photo 0609 1148 / 2009 / CalPhotos / CC BY-NC 3.0 (top right);
© Barry Breckling / Photo 0315 3068 / 2015 / CalPhotos / CC BY-NC-SA 3.0 (bottom right).
The trio of brooms found in our parks includes French broom (Gensita monspessulana), Scotch broom (Cytisus scoparius), and Portuguese broom (Cytisus striatus). If you’ve ever taken a road trip up the West Coast, you’ve likely seen one or more of these species coating the roadsides. They’re all closely related perennial shrubs with showy yellow flowers. “Folks often mention being reminded of pea plants,” says Golden Gate Biologist Laura Booth of when she shows habitat restoration volunteers sprigs of flowering broom, “[and] that’s right on the money.” Brooms are members of the pea family.
Originally introduced from Northern Europe and the Mediterranean as ornamental plants or for erosion control, Scotch, French, and Portuguese broom have spread rapidly in California and elsewhere. They thrive in disturbed landscapes, and their numerous long-lived seeds and ability to resprout following damage can thwart efforts to manage them. “Without the checks-and-balances of coexisting on evolutionary timescales in a plant community, these plants’ traits allow them to run rampant, effectively outcompeting the native plant community,” explains Booth.
That process was particularly evident on one of our monitoring plots where broom cover jumped from just 3% in 2016 to 45% in 2024. We expect this plot will be further overtaken by broom the next time we visit.
A look at what changed over eight years
Left image
Coast live oak transect in 2016.
Credit: NPS
Right image
The same transect eight years later, covered in broom.
Credit: NPS
As one can imagine, the dense monocultures that broom forms here threaten our incredible native plant and wildlife diversity. But brooms threaten our region in other ways too. As nitrogen fixers, they can alter soil microbial content. They’re also highly flammable and can act as ladder fuels carrying fire to the tree canopy, increasing fire hazards.
Given such serious economic and environmental impacts, land managers have been working hard to track and curtail the spread of problematic broom populations. The botany team has been monitoring the spread of broom since 2009 via the Invasive Species Early Detection Protocol. This project aims to map, assess, and promptly report occurrences of priority invasive plants along trails and roads within network parks. A large part of our early detection surveys involves following up on historical observations of invasive plants. This season, the botany team has revisited several known broom populations in Golden Gate National Recreation Area. By reassessing known occurrences of invasive plants, we can track how a population may be changing over time or determine whether treatment efforts were successful.
Our mapping work, coupled with park vegetation team efforts, help us understand where these species are spreading so that parks can take action. “Since brooms are widespread across the Bay Area, early detection for these species [helps us keep] them out of desirable habitats where they don’t yet have a foothold,” explains Booth. And being able to act fast is important, as “letting even one plant set seed results in a cascade of spatial and temporal management needs.”
Managing broom isn’t easy, often requiring multiple techniques and many years of repeat treatments. But there are some bright spots. For example, broom can be easy and fun for volunteers to remove with a weed wrench—a sort of lever that can help lift a whole huge shrub right out of the ground by its taproot. The results are also rewarding. “I love treating French broom,” says Booth, “for the sheer joy of looking back at a meadow full of [native] bunchgrasses and spring wildflowers.”