Invasive Plant Impacts, Monitoring, and Restoration 1

Thursday, October 20 at 1:00-2:40 pm, Fir Room

*note alternate instance of this session – Friday at 3pm

Session Description: Invasive plants are a major threat to native plant biodiversity in California. This session highlights cutting-edge ecological research on invasive plants and shares reports on habitat restoration projects incorporating invasive plant control.

Session Chairs: Jutta Burger and Doug Johnson (California Invasive Plant Council, Berkeley, CA, USA) 

7.1 Atmospheric nitrogen deposition is an acute, but underappreciated threat to imperiled California flora

Stuart B. Weiss (Creekside Science, Menlo Park, CA, USA)

Smog is nitrogen fertilizer. Emissions of nitrogen oxides and ammonia from vehicles, combustion, fertilizers, livestock, and other sources deposit onto downwind ecosystems over years and decades. The scientific community recognizes atmospheric nitrogen (N) deposition as a major threat to biodiversity. The rich flora of California, including more than 225 taxa listed as Threatened or Endangered, is at risk from many factors including habitat conversion, fragmentation, and climate change, but N-deposition threats have largely flown under the conservation radar. N-deposition stimulates growth of introduced annual grasses that crowd out native wildflowers and enhance fire cycles; annual grasses are consistently raised as a major stewardship issue. Exposure of imperiled flora is quantified by overlaying NADP Total Deposition maps onto plant taxa in the CNDDB. More than 60% of listed taxa, and 44% of more than 1,400 rare but unlisted taxa, are exposed to >5kg-N ha-1 yr-1, and many occupy sensitive habitats such as serpentine, other nutrient-poor soils, vernal pools, coastal sage scrub, deserts, and grasslands. N-deposition directly impacts people as well. For example, fires now carry across the deserts and more annual grass produces more pollen allergies, in addition to direct impacts of air pollution on health. Total N-deposition has decreased since 2002, primarily driven by strict regulation of NOx emissions. But ammonia emissions, which are not regulated, have increased, especially in urban areas where catalytic converters on vehicles produce large quantities of ammonia. Excessive N-deposition will continue into future decades. N-deposition should be routinely incorporated into environmental review, conservation planning, and mitigation. Project specific mitigations in several counties, and a regional HCP/NCCP in Santa Clara County provide models for addressing N-deposition impacts, especially maintaining moderate grazing regimes to crop excessive grass growth.

 7.2 Using seed traits as predictive tools for assessing invasiveness

Marina LaForgia (University of California, Davis, Davis, CA, USA), Carmen Ebel (University of Oregon, Eugene, OR, USA), Pierre-Olivier Cheptou (Center for Functional and Evolutionary Ecology, Montpellier, France), Lauren Hallett (University of Oregon, Eugene, OR, USA) and Jenny Gremer (University of California, Davis, Davis, CA, USA)

While many California native annual species are seed-limited, invasive annual species excel at seed production. Seeds of invasive annuals take advantage of multiple dispersal vectors, including humans, to explore new territory and establish new populations. Critically, many of these species also maintain seed banks. While a healthy native seed bank is critical to native persistence, a dense invasive seed bank makes it more difficult to move the system towards a desirable state. Despite their importance, little is known about what types of species maintain seed banks and how the presence of a seed bank alters invasion dynamics and invasiveness. We are exploring how understudied seed traits are linked to seed-banking behavior in 150 native annual and 50 invasive annual species across California. To then link these traits to post-germination invasiveness, we are conducting a large-scale field experiment assessing the competitive ability of 20 native and invasive annual species. So far, this work has revealed a trade-off between colonization and seed survival that can be predicted from easily measured seed traits and also suggests that invaders display different seed strategies from their co-occurring natives. Further analyses of experimental data will clarify how these strategies are linked to competitive behavior and invasiveness.

7.3 The hybrid Spartina invasion of San Francisco Bay

Debra Ayres (CNPS El Dorado Chapter, University of California, Davis (retired), Placerville, CA, USA)

Spartina invasions of tidal wetlands worldwide are due to Spartina alterniflora and its ancient and modern interspecific hybrids. The rapid invasion of San Francisco Bay was due to hybridization between the native species, S. foliosa, and S. alterniflora that was introduced from the U.S. east coast to San Francisco Bay in the 1970s. From 2000 and continuing today, non-native Spartina control throughout the SF estuary has cost the California Coastal Conservancy $10s millions. In genetic studies we found a highly variable swarm of hybrids parented by both species. In field studies we found hybrids produced 2x the florets of the native species, seed production was 1.5x higher, and viable pollen production was 8x higher. Seed production was 2x (native) or 3.5x (hybrids) higher in a wet year compared to a drier year. Seedling recruitment was also episodic and related to rainfall the previous year. Some advanced generation hybrids had overcome inbreeding depression and were highly self-fertile. This resulted in surprisingly local colonization (<200 m) with a few self-fertile plants dominating the cohorts of recruiting seedlings, producing fine-scale and temporal genetic structure. To examine the interplay of seedling recruitment, pollen swamping, and vegetative growth on invasion rate of S. alterniflora x foliosa hybrids we evaluated these factors in a mathematical model. In our model, we found that seedling recruitment drove hybrid Spartina spread, and studies showed that seedling recruitment was episodic in response to climatic events and dependent on safe-sites where seed was trapped and tidal waters were muted. Plant factors that promote spread in Spartina introductions are self-fertility and factors, like hybridity, that increase seedling production. Environmental factors that promote invasion are a lack of competition on open mudflats, climatic conditions that promote flowering, seed production and seedling establishment, and plentiful seedling regeneration sites. Understanding these factors can bring about successful control efforts.

7.4 Progress in tidal marsh restoration by the San Francisco Estuary Invasive Spartina Project (ISP), a regionally coordinated partner-led effort to restore tidal marsh and mudflat habitats after invasion

Jeanne Hammond (Olofson Environmental, Inc., Oakland, CA, USA), Brian Ort (Olofson Environmental, Inc., Oakland, CA, USA)

In the 1970s, an East Coast cordgrass, Spartina alterniflora, was introduced into San Francisco Bay and promptly hybridized with native Pacific cordgrass Spartina foliosa. Tall, dense, and fast-growing, the hybrids outcompete other marsh species, overrunning wetlands and colonizing mudflats. Infestation reduces habitat for many species of shorebirds but also provides cover for secretive marsh birds, including the endangered California Ridgway’s Rail (RIRA). In 2000, the State Coastal Conservancy established the San Francisco Estuary Invasive Spartina Project (ISP), a regionally coordinated project tasked with eradicating invasive Spartina species from the Estuary and restoring native habitat. The project has reduced hybrid Spartina from 805 net acres Bay-wide in 2005 to 22.5 in 2021, a reduction of over 97%. Since 2011, the ISP has also actively enhanced tidal marsh habitat at more than 40 Bay locations to benefit RIRA and other wildlife. A key component of rail habitat that is under-represented in tidal marshes is cover from predators during high tides, including extreme high tides (king tides). Rail mortality is highest during winter, especially during extreme high tides that coincide with winter storms. This problem will worsen with climate-change-induced sea level rise and increases in storm severity. To date, the ISP and partners have constructed 82 “high-tide refuge islands” in 16 tidal marshes to provide cover during extreme high tides. Project partners have also planted over 530,000 native plants, aiming to establish dense patches of taller vegetation to benefit nesting, foraging, and roosting RIRA and other wildlife. The overarching goal of the ISP continues to be restoration of native tidal marsh – habitat that is critical for wildlife and acts as a buffer against sea level rise for Bay shoreline communities.