This discussion will address the negative impacts of invasive plant species on soil resources throughout the western United States. Spotted knapweed (Centaurea maculosa) and Cheatgrass (Bromus tectorum) are examples of invasive and noxious forbs and grasses that continue to spread rapidly, with devastating effects on the ecological and economic health and function of western landscapes and waterways.
Invasive plants did not evolve in North America, and were introduced during European settlement across the United States. Invasive plants are highly competitive and displace native vegetation. They are toxic to some fauna and flora, may not be palatable for native fauna, and often change soil properties to their advantage. They add substantial costs to agricultural production and land rehabilitation projects, and reduce ecological, real estate, and recreational values.
Centaurea maculosa (Cm), aka spotted knapweed, is a native of eastern Europe. Cm covers over 8 million acres nationwide, with 5 million acres in Montana. The species spread across western states at an average rate of increase of 27% per year since the 1920’s. Cm is allelopathic, and exudes an acidifying compound that is toxic to most range vegetation. It can overwhelm native vegetation in 10 to 40 years. CM is not palatable for most herbivorous fauna, thus reducing wildlife and livestock forage. Cm has a deep and narrow tap-root, and is most competitive for soil water during periods of severe drought. Most bunchgrasses are rhizomatous or sod forming, and are effective at binding soil. Small plot scale studies demonstrated that Cm displacement of native bunchgrass increased surface runoff by 56%, and sediment yield by 192% (Lacy, et. al., 1989). The economic impact in Montana from decreased crop production and suppression efforts costs over $42 million dollars annually.
Bromus tectorum (Bt), aka cheatgrass or downy brome, is a native of southern Europe. It currently occupies over 100 million acres in western states. Approximately 10 million acres of Idaho and Utah are infested by Bt monocultures. Bt has fine and shallow roots (<20 cm.), and is extremely competitive for surface soil moisture. It matures and produces seed before native grasses have fully emerged. Bt greatly reduces overall production of organic matter and mycorrhizal fungi, and increases soil nitrogen concentrations to levels detrimental to native grasses. Bt infestations greatly increase the frequency of wildfire. This regular fire effect increases soil hydrophobicity, denudes existing vegetation, which increases surface water runoff, soil erosion, and sediment yields. Plot and hillslope scale studies demonstrate that frequent burning of Bt increases erosion rates by factors of 2 to 40 over small plot scales, and more than 100-fold over large plots to hillslope scales (Williams, et. al. 2014). Increased fire frequency also inhibits native plant recovery and succession, which favors the aggressive expansion of Bt into native range. In the Pacific Northwest, Bt can reduce winter wheat production and winter rye seed yield up to 30% through displacement and crop contamination. Bt is thought to be one of the most ecologically destructive plant species in North America.
John Whittingham, CPESC, PWS, IECA MSC Montana Representative Owner, Basic Biological Services LLC, Dillon. Mr. Whittingham requests readers provide reference to quantitative research on soil erosion and invasive plant species. johnw@basicbiologicalservices.com