For those of you who aren’t familiar with buckthorn, it’s an invasive tree (some consider it to be a shrub) that is all but impossible to get rid of. It is considered “invasive” because it’s not native to North America, and because of its ability to crowd out native trees, plants and completely dominate a landscape. Buckthorn’s ability to grow fast and crowd out other plants happens for two main reasons. First, its leaves and stems are not desirable to grazing animals and second, it loves soil that is saturated with nitrogen.
Nitrogen receives a lot of public attention in rural, agricultural areas, but I’ve noticed people don’t talk about it much in the suburban areas where I work. This makes sense considering East Metro water bodies are typically more affected by phosphorus than nitrogen. However, excess nitrogen remains in many areas that were once farmed.
I hadn’t considered the land effects of nitrogen until I visited the South Washington Conservation Corridor in Woodbury as part of a volunteer event with Great River Greening. This volunteer event involved planting native plants to help the landscape develop more flowers for pollinators and a deeper root system to combat land erosion.
As I was planting little nodding onions in the old farm field, I noticed a large patch of buckthorn in a low-lying area. Our ecology guide for the day informed us that buckthorn loves this area because they are very tolerant of high nitrogen levels in the soil. Long ago, when the Southern portion of Washington County was farmed and nitrogen-rich fertilizer was used broadly, stormwater runoff brought excess nitrogen down into low-lying areas where it accumulated to toxic levels.
Most plants get sick if there’s too much nitrogen in an area because it encourages the plant to put all of its energy into growing leaves, so the energy for developing roots and flowers becomes limited. This then destabilizes the plant and makes it unable to spread seeds. Excess nitrogen also acts like a salt and dehydrates the soil, causing plants to wilt and turn brown. Buckthorn, on the other hand, tolerates high nitrogen levels and even thrives in them because it doesn’t have to compete with other plants. Buckthorn in many of our metro park areas isn’t only a nuisance: it’s an indicator of areas where there is an imbalance of nitrogen.
The relationship between invasive buckthorn and soil nitrogen reminds us that environmental problems that seem isolated and dissimilar can actually be very connected. It can be easy to treat conservation issues as single issues – this is especially true when it comes to invasive species, which are aggravating and easy to focus on but often dominate an ecosystem because of imbalances or poor health in other aspects of the system. This lesson learned at the South Washington Conservation Corridor makes me more committed than ever to supporting conservation policies. Guidelines that regulate fertilizer use and stormwater management can have substantial benefits to overall habitat. Conversely, neglecting proper management can lead to many habitat set backs, some of which we can’t even predict.
Do you have a landscape lesson you learned? I’d love to hear it. Email me at Julie@conservationminnesota.org.