A Famous Invader

It sounds like a dark riddle: what famous invader disrupts developing frog embryos, prevents meadow rues from germinating, and is allied with invasive earthworms?  If you think something so destructive is found only in sci-fi movies, then you’d be surprised to learn it’s an enemy that volunteers have been battling at Coldwater: buckthorn.

Common buckthorn leaves
All of the green shrubs  shown above are buckthorn.
Buckthorn is fierce competition for many plants at Coldwater because it is so good at many of the very normal things plants do. Its dense foliage soaks up so much sun that plants underneath it don’t get enough sunlight to survive. Its berries are irresistible to birds, and like kids eating candy, they gobble them up and distribute evidence of their foraging widely, thus spreading the seeds. Buckthorn bushes also display a zombie-like determination to live after being cut down. If you remove the trunk and fail to apply herbicide before leaving the plant to die, the stump will send up many new shoots and continue to thrive.

Western Chorus Frog
Western Chorus Frog eggs are affected by buckthorn.
Buckthorn has many effects on nearby living organisms that are unusual for plants. For instance, buckthorn excretes high levels of a chemical called emodin during leaf out, coinciding with activity from early-breeding amphibians. The timing of this is particularly devastating to frogs because this chemical, which is released from all parts of the plant, has shown toxic effects on amphibian embryos, including disrupting their development and preventing them from hatching.

Meadow Rue in bloom

Buckthorn leaf litter also has a dramatic effect on the growth of beautiful forest plants called meadow rues because it poisons the soil. When seeds from a species of meadow rue were placed in soils under buckthorn’s leaf litter, the percentage of seeds that germinated was reduced by more than 10%. The meadow rue seeds that could grow in these conditions germinated nearly a week later than seeds growing under maple leaves. This makes it harder for the meadow rue to survive because other plants get about a week’s head start, giving them the advantage as they compete for water, nutrients, and sun, making it harder for meadow rues to survive.

Meadow Rue leaves are delicate and beautiful

In addition to inhibiting the development of native frogs and flowering plants, buckthorn hurts the forest by facilitating the growth of invasive earth worm populations. No earthworms are native to Minnesota, but they are in our soils from when people transplanted European plants in European soils, complete with European earthworms, and from fisherman releasing unused worms intended as bait into the soils. Minnesota’s plants evolved to conditions in which leaf litter decomposes slowly. Slowly decaying litter is the growing medium of choice for many understory plants and flowers, and the ideal home for many microbes, insects and invertebrates that make up the base of the woodland food chain. Buckthorn leaves are preferred by earthworm populations and they provide such a good source of food for them that their populations are much higher in the soils surrounding buckthorn plants. Where there are more earthworms, there is less leaf litter that is needed by many plants and animals to grow, so their populations decline.

At Coldwater Spring, the National Park Service and Mississippi River Fund, together with help from thousands of volunteers, have cleared old growth buckthorn from 29 acres over the past 3 years. Though we will continue to deal with sprouting seedlings for years to come, we hope that these efforts will improve the oak savanna and woodland habitats at this important historical and cultural place. Though we will probably never completely rid the world of buckthorn, restoring natural spaces for native plants to survive will help create places for birds, frogs, and people to thrive.

Volunteers have removed buckthorn from many areas in our National Park

Written as a joint effort between Maria DeLaundreau and Anna Waugh.