garlic mustard blues

I wish I could talk more about birds in this space, but what with the snow (we ended up with “only” 4.5 inches) and cold, migration is pretty much at a standstill. I’m left to cope with one of my spring routines: destroying Garlic Mustard (Alliaria petiolata) in my study plot.

My field site is full of invasive species. Garlic Mustard is the most annoying and conspicuous. Here are the dirty facts:

• A European native first reported in the U.S. in New York in 1868; now most abundant in northeast and Midwest, but satellite populations elsewhere.
• A self- or cross-pollinating biennial which produces an average of 350 seeds per plant, but up to 7900 seeds in robust individuals; <5% of germinating plants survive to flower.
• Seeds remain dormant from 8 (south) to 20 (north) months; seed bank remains viable for 5 years.
• Tolerates full sun to full shade, moist floodplains to dry, sandy sites, and a wide variety of soils (but intolerant of acidic soils).
• Disturbed areas are most susceptible to invasion, where it can dominate within 10 years; however a study (Biological Invasions 1:169-179, 1999) in Illinois found a mean rate of spread at 5.4 m year in high-quality, undisturbed forest, with annual means increasing from 9.0 m in year one to 31.8 m in year eight.

The impact of Garlic Mustard monopolization is mostly anecdotal at this time, although there is evidence that sites dominated by it have low native herbaceous species richness. Garlic Mustard produces several phytotoxic chemicals that may inhibit mycorrhizal activity, and therefore growth of native species. It may alter habitat suitability for salamander, insect, and earthworm communities (although ironically, in northern North America, most earthworms, isopods, and some other leaf-litter denizens are also non-native). It definitely interferes with reproduction of two native butterflies, the Mustard White (Pieris napi) and the West Virginia white (P.virginiensis) which lay their eggs on the Garlic Mustard, where their larva cannot complete development.

My beef with Garlic Mustard is that it so thickly blankets the forest floor, at a height of about three feet, that it must somehow impact ground-foraging birds such as thrushes (click on the photo for a look at what most of the forest here looks like by June). It seems to me that it has got to be difficult for birds to efficiently explore the forest floor and scratch in the litter when this plant has taken over. If there are negative impacts of Garlic Mustard infestation on forest floor invertebrates, then prey items may be reduced. Finally, a bird foraging in Garlic Mustard would have little ability to monitor for predators in the tall, dense stands, which may deter them from foraging in Garlic Mustard stands. Nobody is currently studying these impacts, as far as I know, but I am investigating it…any collaborators out there for a full-blown study?

Whatever its impacts, Garlic Mustard is not welcome in my study plot, which I try to maintain in a steady state of succession and floristic make-up. That way, I minimize any bias in my current work that might be introduced via changing vegetation, and preserve a control plot for that future study!