Invasion of Noxious Weed Species

“Invasive weed species” – it doesn’t sound very threatening, does it? But, these invaders have devastating effects on U.S. Wildlife. The 2.5 million acres of forest and 11,000 miles of road in the Idaho Panhandle National Forest (IPNF) are overrun with invasive weed species every year. State and Federal Agencies try to manage the roads lined with invasive weeds in order to provide access for forest use to the public. Employees monitor the spread of the weeds that overrun roadsides and outcompete native plants. Invasive weed species are alien species whose introduction does or is likely to cause environmental harm. The Coeur d’Alene River District in northern Idaho spends $25.50 per acre of road spraying roadsides with herbicides to inhibit the growth and spread of invasive plant species. If the Forest Service were to spray every road throughout the IPNF, they would be spending over 1.5 million dollars a year on herbicide treatment. If a different method of management for invasive weeds were possible, the money spent spraying invasive weeds could be used to improve other areas of the forest.   

Managing invasive weed species is incredibly important due to the detrimental effects on the environment and the toxicity the plants have to native animals. Spotted knapweed and St. Johnswort contain toxic chemicals that can be harmful to animals in high doses. For example, animals that eat St. Johnswort can experience intense sunburns, eye irritation, skin sloughing, and inflammation of the skin. These noxious weeds also impact wildlife habitat, deplete soil and water resources, and reduce plant and animal diversity. The Forest Service attempts to manage these weeds with the herbicide spray however, spraying roads with herbicides influences not only the invasive weeds, but also damages native species. Spray crews attempt to minimize spraying herbicides on native plant species, however weather and water conditions can lead to an unintended impact on the native plants that are growing with the noxious, invasive species. If there was a more efficient way to help control the spread of invasive weed species, the Forest Service would not have to use herbicides as frequently as they do now.

I studied an alternative way of reducing the abundance of invasive weed species by collecting data on forest canopy coverage and invasive weed species presence. Canopy coverage is the percentage of the forest floor covered by the vertical projection of the tree crowns. We researched the idea that if a roadside had high canopy coverage, there would be fewer invasive weed species along those roads. If we could keep roads narrower, we could minimize the herbicides needed to manage invasive species in the future.

Roads in the IPNF in the Coeur d’Alene River watershed were selected for reserach. 610 areas were recorded. The roads all had assorted fauna and differed in roadside canopy coverage, elevation, herbicide treatment and fire history. Each point was found using GPS coordinates and every plant species within a one meter² plot was recorded along with the canopy coverage of 
the tree crowns directly 
overhead the plots.

After we collected all the data, we compared the forest canopy coverage percentage and the difference invasive weed species present in the plots. Of the thirteen invasive weed species we searched for, we consistently found seven of them along the roadsides: St. Johnswort, oxeye daisy, orange hawkweed, meadow hawkweed, spotted knapweed, common tansy, and common mullein. We found only two of the seven plants we studied showed statistically significant relationships between canopy coverage’s and invasive weed species percentage. Although many of the different weeds showed a negative sloping trend when weed presence was compared to canopy coverage, St. Johnswort and oxeye daisy’s presence decreased in a statistically significant percentage along roadsides as the canopy coverage increased. Overall, when weed presence was combined and compared to the canopy coverage, there was a statistically significant difference between the percentage of canopy coverage and the percentage of invasive weed species. We were able to determine that as forest canopy coverage increases, the presence of invasive weed species will decrease.
With this information, we can help the forest service make better choices when clearing and managing roadsides. If roads were kept narrower, around 10 feet across, there would be a smaller presence of invasive weed species. This, in turn, would allow the forest service to delegate less time to spraying the roadsides with harmful herbicides that are killing native species as well as invasive species.