Soil restoration and invasive plants at the Block P Mill and tailings site, Montana

Abstract

My case study considers the Block P Mill and Tailings Site (also, Block P or the Site), a derelict lead and zinc production facility in the Little Belt Mountains of Montana. Fifteen years after environmental restoration activities concluded at this heavily contaminated site, I analyzed whether the canopy cover of plant species correlates with soil pH conditions. Specifically, I investigated whether addressing acidic conditions during mine-site restoration encouraged the presence of oxeye daisy (Leucanthemum vulgare), a non-native invasive plant species. Fieldwork in June 2019 included collection of soil pH and vegetation canopy cover data from 36 quadrats (6 m x 6 m) arrayed across this 6.6-ha restoration site. A Canonical Correspondence Analysis (CCA) of these data shows no statistically significant relationship between soil pH and overall plant species' canopy cover values. Further, linear regression analysis shows no statistically significant relationship between soil pH and the canopy cover of oxeye daisy. Therefore, it is unlikely that differences in the soil pH across the Site unevenly affect the canopy cover of plant species or unduly encourages the presence of oxeye daisy. These results are a reminder, however, that if restoration returns fertility to a previously barren and inhospitable site, the presence of novel assemblages of non-native and native plant species also may be generally encouraged, including invasive species such as oxeye daisy. I therefore conclude that the treatment of invasive plants at restoration sites like the Block P are a required managerial choice, not a philosophical imperative.