Field experiments show dump soil briefly boosts non-native plant growth

Field experiments near Churchill, Manitoba, found that soil moved from human-disturbed dump sites increased the growth of three non-native plants in tundra and boreal forest, with the strongest effects occurring where the imported soil was placed 20 centimetres deep. The benefit did not guarantee lasting establishment: transplanted plants generally declined over time, and soil origin did not significantly improve survival in the multi-year common-garden experiment. The results indicate that transported dump soil can provide non-native species with more than a means of dispersal, temporarily creating growing conditions that favour their establishment in otherwise resistant subarctic habitats.

The study examined common plantain, Plantago major; common toadflax, Linaria vulgaris; and common dandelion, Taraxacum officinale. These perennial species were introduced from Europe, have occurred around Churchill for more than 60 years and persist in the study area through seeds or, particularly for toadflax, rhizome fragments. Researchers established two experiments in 2021 and 2023 to examine the overall effect of translocated dump soil rather than trying to separate its potentially overlapping influences, including nutrient enrichment, disturbance and the transport of plant propagules.

Churchill, at 58.5 degrees north, has long received seeds and other plant material from southern agricultural areas. A railway has connected the town’s wharf and grain elevator with southern farming regions since 1931, carrying grain that contained contaminants cleaned from shipments before export. Material from these shipments was disposed of in several dumps near town. Some sites were also used for local manure disposal and may have received household or garden waste. Their soils are visibly darker than surrounding soils, suggesting greater organic content and previous nutrient enrichment, and some have become dominated by persistent non-native plants. Soil from these areas is regularly moved for activities including road construction, power-line maintenance and gardening.

In the first experiment, a spatially replicated common garden was established in August 2021 near the Churchill Northern Studies Centre, about 20 kilometres east of town. The site had sparse vegetation on stony marine sediment and was considered representative of disturbed ground vulnerable to soil dumping. Researchers created thirty 25-by-25-centimetre plots. Half were disturbed with a rake to a depth of five centimetres while retaining their original soil; in the other half, the original soil was removed to that depth and replaced with homogenized soil collected from three dumpsites at the Port of Churchill.

One transplant of each species was placed in every plot: a shoot with five centimetres of rhizome for toadflax and a complete rosette with roots for plantain and dandelion. The plots were monitored through three growing seasons from 2022 to 2024, with survival, plant dimensions and ramet or leaf counts recorded for each species. Soil origin did not significantly affect the survival of any species, although survival declined significantly over time for all three. No toadflax or dandelion in control plots survived beyond 2022. By 2024, only three transplants remained: one plantain in dump soil, one plantain in control soil and one toadflax in dump soil, limiting later comparisons.

Dandelion was the only species whose physical size showed a clear initial response in the common garden. One year after transplanting, its leaves averaged 7.6 centimetres in dump soil and 4.9 centimetres in control soil, a statistically significant difference. The number of leaves subsequently decreased from averages of 8.7 and 7.0 in dump and control soil in 2022 to 5.0 in dump soil in 2023. Plantain leaf numbers also declined over time, while its leaves tended to shorten, although that change was only marginally non-significant. Toadflax height and ramet number did not differ by soil origin. Too few plants survived for meaningful biomass or flowering comparisons, so the experiment could not establish whether dump soil altered those outcomes across the full three-year period.

The second experiment, established in August 2023, tested soil depth across two roadside habitats. Sixteen locations were placed near the research centre, with eight along Palsa Road beside lichen-heath tundra and eight along Twin Lakes Road beside white-spruce-dominated boreal forest. Each location contained three 25-by-25-centimetre plots: one received five centimetres of dump soil, one received 20 centimetres and a control was disturbed to five centimetres while retaining its original soil. Transplants of the same three species were added to every site, and researchers separately recorded seedlings emerging from the seed bank in the imported material.

Plants in the deepest treatment generally grew larger. Toadflax in 20-centimetre dump soil reached average heights of 19.2 centimetres in tundra and 16.2 centimetres in boreal forest, compared with 3.1 and 4.1 centimetres in the respective controls. It also produced more ramets: 9.1 on average in tundra and 7.8 in forest, versus 2.8 and 4.7 in control soil. Plantain had significantly longer leaves in dump soil, with 20-centimetre treatments averaging 7.3 centimetres in tundra and 11.5 centimetres in forest, compared with 5.2 and 4.6 centimetres in controls. Dandelion also had longer leaves and more leaves in dump soil. Habitat effects were generally not significant, although plantain leaves tended to be slightly longer in the forest.

The growth advantage was also reflected in dry biomass after one growing season. Toadflax in 20-centimetre dump soil averaged 2,586 milligrams in tundra and 1,494 milligrams in forest, compared with 235 and 200 milligrams in controls. Plantain averaged 434 milligrams in tundra and 578 milligrams in forest in deep dump soil, versus 172 and 137 milligrams in controls. Dandelion weighed 884 milligrams in tundra and 1,504 milligrams in forest in the deepest treatment, compared with 317 and 151 milligrams in controls. Soil depth significantly affected biomass for all three species, and the models showed a general trend toward greater production with deeper soil. Inconsistent, non-significant habitat effects meant that neither tundra nor boreal forest was consistently more productive for every species.

Improved growth did not consistently translate into improved survival. In the roadside experiment, neither soil depth nor habitat significantly affected the survival of toadflax or plantain, while dandelion survival was higher in boreal forest than in tundra. Seedlings of five non-native species emerged from the imported soil: shepherd’s purse, hemp-nettle, plantain, dandelion and field pennycress. Neither treatment depth nor habitat produced a statistically significant difference in emergence, and the analysis of the number of germinated seedlings was not significant for either factor. The researchers noted that successful germination generally occurs at or near the soil surface, so deeper layers may contain seeds while offering a lower chance of emergence. The experiment recorded germinated seedlings but did not assess ungerminated seeds.

The authors interpret the growth response primarily as a likely consequence of the nutrient-enriched nature of dumpsite soils, although their experiment was designed to measure the overall effect of translocated soil rather than isolate individual causes. Nutrient-poor subarctic environments may benefit plants adapted to productive disturbed habitats, while repeated roadside disturbance can also alter soil properties and microbial communities. The short-lived nature of the response remains central to the findings. Despite increased growth in favourable soil, transplanted non-native plants generally failed to persist, suggesting that ameliorated below-ground conditions alone were insufficient to overcome local environmental constraints. The experiments involved small soil volumes, while the authors have observed multi-year persistence at sites where truckloads of soil were deposited; they did not experimentally compare those larger movements with the small treatments used here.

Climate change may interact with this temporary establishment advantage. High-latitude environments are warming faster than the global average, and previous studies cited by the authors found that warming can improve the performance of non-native plants in northern ecosystems. Dandelion has also been reported as more abundant in warmer microsites in Churchill. The present experiments did not test climate warming directly, but their results identify human movement of soil as a local disturbance that can give temperate-origin species a measurable head start in tundra and boreal forest. The supported conclusion is therefore narrower than a prediction of inevitable invasion: dump soil can briefly improve growth and allow plants to emerge in previously uninvaded habitats, yet persistence depends on conditions and the scale of disturbance, which remain important uncertainties.

Subject of Research: Effects of anthropogenically modified dump soil on non-native plant growth in subarctic ecosystems

Article Title: Anthropogenically-modified soil increases the performance of non-native plants in a subarctic ecosystem

Article References: Zhang, V. M., & Kotanen, P. M. (2025). Anthropogenically-modified soil increases the performance of non-native plants in a subarctic ecosystem. NeoBiota, 104, 59-72. https://doi.org/10.3897/neobiota.104.160626

Image Credits: AI Generated

DOI: 10.3897/neobiota.104.160626

Keywords: non-native plants, dump soil, soil transport, plant invasion, Churchill, subarctic ecology, tundra, boreal forest, plant establishment, field experiments, Anthropogenically-modified, soil