By Janelle Hunt, Metro Vancouver, BC
Metro Vancouver has been using the region’s biosolids to reclaim mine tailings and rock disposal sites (RDS) for decades; however, the establishment of coniferous trees has been a challenge due to the competitive effects of understory vegetation. The Mount Polley Mine, an operating open-pit copper-gold mine located in the interior of British Columbia, has a progressive reclamation plan that is designed to re-establish forest ecosystems containing self-sustaining native vegetation by mimicking natural succession.
In 2013/2014 a trial was established on the slope of a RDS at Mount Polley Mine to evaluate various herbaceous groundcovers’ ability to promote coniferous tree establishment on parcels amended with biosolids. Twelve parcels (2 replicates of 6 treatments) were established and each parcel was covered with 20 cm of overburden and subsequently amended with biosolids at a rate of 110 dry tonnes per hectare (2 control parcels did not receive biosolids). The parcels were then seeded with either: a native grasses and forbs mix (at 5 kg/ha or 10 kg/ha), a native forbs mix, fireweed, or left unseeded. The control parcels were also left unseeded. In the spring, six months post-biosolids application, Lodgepole pine, Douglas-fir, and native deciduous tree and shrub seedlings were planted across all parcels.
After one growing season the herbaceous ground covers were assessed. The parcels seeded with grasses established well with the higher seeding rate resulting in a higher percent cover. Parcels seeded with a mixture of native forbs were dominated by Yarrow (Achillea millefolium) with some limited emergence of other species. The fireweed parcels had very little germination; therefore, the herbaceous vegetation was dominated by weedy species and the parcels appeared very similar to the biosolids amended parcels that were not seeded. In subsequent growing seasons, the grass parcels started to encroach on the adjacent parcels as the grass seed was dispersed across the site.
In 2017 (2-3 years after the plots were established), a comprehensive survival and growth assessment was completed on the conifers. The biggest factors impacting conifer survival were vegetation competition, vegetation/snow press and herbivory in the form of rodent damage (girdling). These impacts increased on parcels with a higher percentage of grasses; even light seeding rates (5 kg/ha of a native grass/forb mix) produced heavy vegetative cover. Seedling survival was highest on the control, native forbs, and fireweed parcels (Figure 2). Although survival was highest on the control parcels, tree growth was poorer than on the biosolids amended parcels and many trees were chlorotic, indicating nutrient deficiency. There was no observable difference in growth of the trees between the different understory treatments; however, surviving trees on the biosolids amended parcels grew taller, had longer leader growth and thicker root collar diameters than those in the control parcels. Health and form of the trees was impacted by the understory vegetation. On parcels seeded with the native grasses and forbs mix, the trees were generally classified as ‘poor to fair’; while on the forbs, fireweed and no seed parcels, the trees generally were ‘fair to good.’
A seeding of native forbs and non-graminoid species was found to be non-competitive with conifer species and is recommended for reclamation using biosolids amendments in order to promote biodiversity while providing vegetative cover for the site. However, plots seeded only with fireweed or not seeded provided lower competition for invasive species, so a balance is needed to promote conifer survival and discourage invasive species establishment. Results from this trial indicate that this may be achieved by seeding with a mixture of native forbs.