garlic mustard changes forests

A recent paper [1]  in the open-access journal PLoS Biology paints an increasingly ugly picture of the impacts of the non-native biennial Garlic Mustard (Alliaria petiolata).  This study verifies how Garlic Mustard inhibits the growth of understory plants, including the seedlings of canopy trees. Through chemicals it emits, Garlic Mustard disturbs and inhibits the growth of mycorrhizal fungi, symbiotic fungi that provide nutrients and act as extensions of vascular plant root systems.  Many plants are highly dependent upon this relationship, especially species of late-successional forests.

Experiments showed that native trees grown in soils that had been invaded by Garlic Mustard had impaired growth and less colonization by beneficial fungi -- similar to levels in sterilized soil.  The implications are sobering: repression of regeneration of native canopy trees while favoring weedy plants that have lower dependence on mycorrhizal fungi.

My study site is severely invaded by Garlic Mustard. We are also at the epicenter of the infestation of the Emerald Ash Borer, which has killed all the mature ash trees at my site; ash trees made up a significant portion of  the canopy trees here.  Now it would seem that areas opened up by the loss of the ash trees will not regenerate native plants and trees because the whole place is already heavily infested by Garlic Mustard. Further, the developing open forest mosaic is increasingly appealing to White-tailed Deer.  They have already done their part to eliminate many native plants and aided in the spread of invasive species.  This is the ecological equivalent of the perfect storm.

As an urban ecologist, I will find documenting these profound changes interesting but depressing. This is not a situation you want to see for yourself if you value native biodiversity in our forests! I urge all of you (who live in invaded areas of North America) to educate yourself about this aggressive weed. Pull it when you see it, join or organize Garlic Mustard pulls at your local parks, and spread the word. Resources below the fold.

• Garlic Mustard profile from the National Park Service -- excellent photos and biological summary.
• Garlic Mustard profile from Michigan State -- additional helpful photos of first-year seedlings.
• Garlic Mustard management options -- with excellent details on how it spreads and how management differs with different levels of infestation.\
• Garlic Mustard summary from Invasive Plants.org, with biocontrol research (scroll down)

[1] Stinson, K. A., S. A. Campbell, J. R. Powell, B. E. Wolfe, R. M. Callaway, G. C. Thelen, S. G. Hallett, D. Prati, and J. N. Klironomos. 2006.  Invasive plant surpresses the growth of native tree seedlings by disrupting belowground mutalisms.  PLoS Biology (open access, full text link above is free).

Photos:
Jody Shimp, Illinois Department of Natural Resources, invasives.org
Chris Evans, Univ. Georgia, invasives.org

Comments

[Patrick]

It's such a shame. We spent some time pulling it at a local migration trap here in NJ last week. In your research, have you found any info on what to do with the GM once you pull it? I fear bringing it to the local dump for fear that it will somehow spread from there to other places.

[Nuthatch]

First, my preferred method in my small plot is to spray the tiny seedlings with Round Up early in the season, since nothing else has yet sprouted. I've had excellent success killing the seedlings and having all the native plants come up afterwards with no harm done. Seed back persists for 5 years, and invariably more seeds get dispersed, so this is no doubt an annual task.

Disposing of mature plants is more of a problem. They can stay alive long enough to set seed if they are pulled while in flower and just tossed into piles. Despite the fact that our landfills don't need more volume, it is recommended that the plants are bagged and sent away to be buried. The state of Wisconsin even made an exception to the "no yard waste to landfills" rule for invasive plants like Garlic Mustard.

Or, you can eat it or donate it to a food bank, which is what the Michigan Nature Association does every year. Here are few recipes to get you started!

[Nuthatch]

I should have also linked to the Nature Conservancy's element stewardship abstract with its abundant references.

[John]

A lot of the woods in D.C. are full of garlic mustard. Possibly the worst is Roosevelt Island, which has a large deer population for its size. Well, that and some of the small and undeveloped wooded parks. There is something pretty about garlic mustard infested woods with their high canopies and clear interior vistas, but also something disconcerting - you know that is not the way a healthy forest should look. The same goes for places infested with barberry and similar invasives.

[Ontario Wanderer]

Given that seeds last for at least 5 years, one of the local conservation area people suggested that perhaps the best way to eliminate garlic mustard was to just remove the flowers. That being said, nothing seems to work very well. At present, on our small 35 acres of property, we are pulling out what we find and burning it in the burn barrel. In the local conservation area and the Royal Botanical Garden wild areas, the garlic mustard is taking over.

[Ontario Wanderer]

OK, one more follow up re Garlic Mustard from my visit to the not-so-wild parts of the Royal Botanical Gardens yesterday. I noticed that 1. Garlic Mustard that had been pulled and let lay on the ground had picked up it's growing tip and continued to bloom even though the roots were not in the ground and 2. the cut Garlic Mustard had put up blooming tips that were only 1.5 inches tall. It is a survivor.

[Nuthatch]

Yeah, it's pretty bulletproof. I am amazed, too, at the amount of seed one struggling 3-inch plant can dump. It's really a lesson in getting it before the infestation becomes discouraging and seemingly insurmountable. You can keep it at bay if you catch it early!

[Roger B.]

Over here in the UK, garlic mustard is indigenous. It rarely forms large stands in woodland. It's usually found along woodland edges and hedgerows (one of its many folk names is "Jack-by-the-hedge"). It's regarded as innocuous... or even desirable because it's an important food plant for the orange-tip butterfly.

There's a useful autoecological account for garlic mustard in "Comparative Plant Ecology" by Grime, Hodgson and Hunt. I can scan it if you like.

[Gwyn]

I discovered a large area of it at one of my favorite spots the other day. Not having bags, I didn't pull at that time, but I will.

Why has it become so prevalent all of a sudden? I haven't seen it in this spot until last week. I realize being careless about not cleaning off shoes in an infested area is part of the problem, but have conditions somehow contributed to the recent infestations?

[Nuthatch]

Gwyn, yes, the seeds are carried on shoes and animal hooves; deer spread it around on our site, for instance, bringing it right to the kind of open areas that it prefers. It likes disturbed sites, which are certainly increasing. This species was introduced in the U.S. nearly 150 years ago, but like some invaders, it just takes awhile to gain momentum. In my original post I note that even in undisturbed forests it spreads at an increasing rate, from 9 meters a year early on to 31 meters by year 8. Another reason to pull early and often!