Tuesday, June 21, 2022

Does Garlic Mustard Eventually Decline?

One study found that it does, but it takes many years.

Garlic mustard plants with small white flowers and linear green fruits.
Garlic mustard, Alliaria petiolata, with flowers and maturing fruits in late May 2022.













Garlic mustard, the aromatic invader of forest understories and edges, has been here a long time. It first arrived in North America in the 1800s, when colonists likely carried it onshore to use as a medicine or potherb. Since then, it has spread from east to west, and now its invasive habits have landed it on many weed lists.

The lists inspire -- or require -- action, so each spring parties gather to pull out, cut off, or otherwise get rid of garlic mustard. It's easy where they've barely made inroads, but where populations are large and dense, removal takes many hours of stooping, kneeling, reaching and pulling. Then there's the additional commitment: Because garlic mustard seeds can remain viable in the soil for ten years or more, it's necessary to return year after year for monitoring and control. 

A less laborious solution would be welcome, and it may be emerging. A study led by Bernd Blossey at Cornell University found that although populations of garlic mustard initially increase, they eventually decrease (1). Is it best, then, to let nature take its course? Could garlic mustard's long residence be its downfall? Possibly, but many questions remain. 

Garlic Mustard Biology and Ecology

Garlic mustard is a biennial that produces rosettes the first year and flowering stems the second. Terminal clusters of small, white flowers bloom in April and May. Fruits are one- to two-inch long siliques -- narrow, linear pod-like structures that bear small seeds. One plant can produce hundreds to thousands of seeds. They are released in summer and germinate the following spring. 

At least on the leading edge of the population, growth is thick. Dense mats of seedlings grow into rosettes of similar cover, and after they overwinter, the plants bolt into a crowded stand of flowering stems. Fruits and seeds follow, and the cycle repeats as garlic mustard advances.

From left: A cluster of garlic mustard seedlings, a vigorous rosette, and a second-year, flowering stem.








Not every community is susceptible to a garlic mustard takeover. Although it's generally recognized as harmful, the magnitude of garlic mustard impacts depends on what else is present in the community -- which plants, animals, microbes and soils, for example, and how climate interacts with all of these. 

Vikki Rodgers, Sara Scanga and their team reviewed research since 2008 and teased out of that complexity a likely scenario for a successful garlic mustard invasion (2). First, earthworms and deer deplete populations of native plants. Less competition then gives garlic mustard an edge, and as it grows it further harms native plants through allelopathy, the release of compounds into the soil that harms other plants. Specifically, allelopathic compounds from garlic mustard disrupt the establishment of mycorrhizae, the associations between fungi and roots that help many plants absorb water and nutrients.  

Garlic mustard has the additional advantages of an extended growing season -- it begins in early spring, before most other species -- and prolific seed production. In a favorable location, all these characteristics appear to be an unbeatable combination. Time, however, could work against its dominance. 

Residence Time and Population Growth

The age of populations can also affect their staying power, and this is where Blossey's research comes in. From 2000 to 2006, he and his colleagues established 16 long term, permanent monitoring sites along garlic mustard's invasion trail, from states in the Northeast, where populations are older, to the Midwest, where they are younger. Each site was monitored for 5 to 15 years. 

At each location they set up quadrats (four-sided sampling plots) where twice a year they recorded stem or rosette density, stem height and percent cover. At the end of the study, they concluded that as residence time increases, garlic mustard populations become less able to sustain themselves. Although they are initially abundant, populations eventually decline until their growth rate falls below a level needed to maintain steady or increasing numbers. Overall, it took just over ten years to reach that point. 

According to Blossey, there are several possible reasons why this happens. Increasing residence time may help communities develop local biotic resistance, such as the buildup of parasites, diseases and insect herbivores that target garlic mustard. Garlic mustard decline was faster and greater in eastern study sites, which might be explained by regional differences in climate, soil and vegetation, three additional factors that could affect garlic mustard performance.

Blossey's team thinks that negative plant-soil feedback also plays an important role in garlic mustard decline. In a separate experiment, they found that the survival of garlic mustard rosettes was greater in soils that were not yet invaded or recently invaded, compared to soils that had "old" invasions of more than five years. In this experiment, at least, garlic mustard evidently creates or fosters soil conditions that work against its vigor in the long run. 

Call Off the Garlic Gangs?

Blossey points out that his research was possible only in areas where garlic mustard was not actively managed. Declines were observed in populations that were allowed to run their natural course and any interference could delay the development of biotic resistance, feedback mechanisms or other causes of garlic mustard's eventual decline. 

However, he also states that further long-term research is needed to answer questions that could determine if a hands-off approach is best. If garlic mustard eventually peters out, is the decline permanent, or will populations rebound? How fast can native communities recover after garlic mustard declines? Where garlic mustard is established, does it alone explain the impacts on native plants, or could other, more persistent, factors, such as deer or earthworms, contribute to the harm? 

Although not discussed in this research, questions of patience and acceptance are also important. Can people tolerate garlic mustard on their properties or in public parks, where they may be fostering or expecting to observe native communities? Are they willing to accept the advancing, dense growth of garlic mustard populations while they wait ten or more years for them to subside? Are there places where garlic mustard should be allowed to play out, and places where it shouldn't? 

The questions go on. Depending on the answers, it might be too soon to retire efforts to manage garlic mustard. More research is needed, especially long-term studies that include both land managers and research scientists. State or local laws may also have to be changed or exceptions granted to allow garlic mustard to grow unhindered. 

In the meantime, garlic mustard gangs have their work cut out for them. Some organizations offer contests and prizes for the most plants pulled. They might be the only occasions when a lot of garlic mustard is a good thing. 

References


(1) Blossey, B. et al. 2020. Residence time determines invasiveness and performance of garlic mustard (Alliaria petiolata) in North America. Ecology Letters 24(2): 327-336. https://doi.org/10.1111/ele.13649

(2) Vikki L Rodgers, Sara E Scanga, Mary Beth Kolozsvary, Danielle E Garneau, Jason S Kilgore, Laurel J Anderson, Kristine N Hopfensperger, Anna G Aguilera, Rebecca A Urban, Kevyn J Juneau, Where Is Garlic Mustard? Understanding the Ecological Context for Invasions of Alliaria petiolata, BioScience, 2022; biac012, https://doi.org/10.1093/biosci/biac012

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