An unusual surge in the invasive Canada thistle (Cirsium arvense) has been observed in Alberta, attributed to climate change, land disturbances, and poor pasture management.
Observation by Ailsa Le May:
This summer I observed a dramatic increase over previous years in number and density of Cirsium arvense (Canada thistle or creeping thistle) (iNaturalist, n.d.) in pastures, along ditches, and within private and public green spaces. Cirsium arvense is a non-native thistle introduced from northern Europe that is now common across Alberta (Government of Alberta, n.d.). It is considered a noxious weed and is regulated under the Alberta Weed Control Act as it competes with native vegetation, spreads rapidly, reduces agricultural production and forage potential for pastures, and causes severe economic hardship (Government of Alberta, n.d.; Government of Alberta, 2017). It is especially noticeable in mid to late summer as the thistle flowers have bloomed and gone to seed. Locally, the thistles have been impacting the pasture and vegetable garden on my acreage. We have pulled weeds by hand all spring and summer, and recently mowed and swept up the cuttings in an in attempt to remove patches from the pasture.
Factors Driving the Increase of Invasive Species
Succession is the way plants develop after a disturbance, allowing opportunistic species such as noxious weeds to grow rapidly and dominate for a short time. A decreased snowpack, early snow melt, and rising temperatures have decreased soil moisture throughout Alberta, with extreme weather events (e.g., 2013 Calgary flood) promoting seed dispersal though overland flooding (Clements & Jones, 2021; Government of Canada, 2019). In Alberta, continued disturbance from anthropogenic activities such as land development, overgrazing due to poor pasture management, and climate change have all contributed to a stressed grassland ecosystem that is unable to compete against Cirsium arvense, which has a deep root system to access moisture.
References
Alberta Prairie Conservation Forum. (n.d.). Alberta’s Natural Areas. https://www.albertapcf.org/about-prairies/albertas-natural-areas
Clements, D.R., & Jones, V.L. (2021). Rapid evolution of invasive weeds under climate change: Present evidence and future research needs. Frontiers in Agronomy, 3, Article 664034. https://doi.org/10.3389/fagro.2021.664034
Government of Alberta. (n.d.). Thistle, Canada. Alberta Agriculture and Forestry. http://www.agric.gov.ab.ca/app107/loadPest?action=display&id=69
Government of Alberta. (2017). Weed Control Act. http://www.qp.alberta.ca/570.cfm?frm_isbn=9780779801220&search_by=link
Government of Canada. (2019). Canada’s changing climate report. Environment and Climate Change Canada. https://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/energy/Climate-change/pdf/CCCR_FULLREPORT-EN-FINAL.pdf
iNaturalist. (n.d.). Creeping Thistle (Cirsium arvense). Retrieved September 19, 2022. https://www.inaturalist.org/taxa/60132-Cirsium-arvense
iNaturalist