40 YEARS LATER…
A novel study on a gravel pit rehabilitated four decades ago sheds light on the need for native species and ongoing maintenance
By Warren Heeley
In 2018, The Ontario Aggregate Resources Corporation (TOARC) partnered with the University of Guelph Arboretum to study a 40-year-old rehabilitated gravel pit on the university’s Arboretum land. The findings, published this past March, were revealing.
Inventories of the woody plant species showed that 53 of the 90 species originally planted had survived and 41 of these species showed signs of self-generation and expansion beyond the original planting area. A number of the large trees originally planted had grown well and made up the current upper canopy in the gravel pit.
In terms of soil samples collected from the pit area and Victoria Woods, an old-growth forest beside it, researchers discovered that “higher values of key plant nutrients (phosphorous and potassium) exist at the gravel pit compared to Victoria Woods.” While appearing positive at the outset, this could be the result of a more balanced plant community in the Woods that is consuming all of these nutrients leaving very little in the soil samples.
Somewhat discouraging was that invasive species made up the top species found in the pit; however, this was also understandable given that some were included in the original plantings.
“In certain cases, the invasive species were planted as a part of the original rehab work as they would have been considered best practices at that time,” states Danielle Solondz, program manager at TOARC. “Other species simply invaded the site. The study determined that the top five species found in the pit were invasive, but it should be noted that these species are common in any ecosystem if not removed manually.”
To deal with the prominence of invasive species, the study determined that rehabilitation sites may need ongoing maintenance. “In addition to planting native species, there may need to be an approach used from the beginning of the rehab process to keep out or remove invasive species,” says Solondz.
The study also highlighted that maintenance approaches for such sites may need to be reassessed on a regular basis and that 40 years is simply too long to leave a site without maintenance. As Solondz emphasizes, “TOARC and producers can set a site up for success, but land stewardship is a key part of creating valuable ecosystems.”
LENGTHY PROJECT
Founded in 1970 on nearly 400 acres, the Arboretum encompasses many land types, including a section that contained a legacy gravel pit. It was the rehabilitation of that pit, conducted between 1976 and 1978, that the TOARC research project examined. The original rehabilitation included the planting of more than 90 woody plant species throughout the pit area, which were left to naturalize over the next 40 years.
To determine the results of that rehabilitation, the Arboretum approached TOARC to support a joint research study, with TOARC providing funding and inhouse support with their rehabilitation expertise. As the trustee of the Ontario Aggregate Resources Trust, TOARC is responsible for rehabilitating more than 8,200 legacy pits and quarries in Ontario.
The primary goals of the study were: to discover if the biodiversity of the site improved, to apply the results to future rehab practices, and to see how the site was integrating into the Guelph area urban sprawl that grew over the 40-year period. It aimed to answer three key questions:
What are the major changes that have occurred in terms of woody plant survival, abundance and colonization over the last 40 years as the site has naturalized?
How can the site continue to improve in terms of integrating it into the wider habitat matrix for the benefit of native biodiversity?
What findings can be taken away that could apply to similar locations?
THE APPROACH
To determine the answers to the study’s key questions, the team required records of the rehabilitation work going back to the 1970s, including planting maps and notes made by the original rehab team. By accessing archived documents, they were able to develop a geographic information system (GIS) that included planting lists and original planting maps and locations in the pit.
Work started in January 2021, and by August the research team had collected data on the current state of the woody plant inventory. In addition, they recorded assessments of soil and vegetation. They faced several challenges, however, including the fact that soil conditions in many cases were either not kept or were lost in the historic records, and the same number of each plant species was not always planted, which complicated the process of systematic comparisons.
As well, the original planting had included a number of woody plant species that would not be included in present-day rehabilitation because of their tendency to become invasive. As experts now know, the planting of native species is critical to sustainable rehabilitation projects. That said, even with native species, it is difficult to restrict invasive woody plant seeds from being carried into the site by wind or other means.
Another consideration derived from the study is the acceptability of woody plants to provide potential benefits to wildlife, pollinators and other such species through flowering, fruit bearing and habitats. This highlights the need for diversity in the plants used in the rehab process to increase the site’s resiliency to harsh environmental conditions. “A wider ecological approach allows practitioners to capitalize on potential species–dependent interactions, such as insect pollinated woody plants, or the use of cavity‐forming tree species to help support birds and small mammals in the future,” states the study report.
Diversity in restoration brings up a recurring question also raised in the report, namely how many species of woody plants should be used to meet appropriate diversity levels? There is no definitive answer to this as the field is still relatively new, states the report. However, it cites a 2001 report by Loreau et al that states: “A line of thought in the field of biodiversity and ecosystem functioning is that biodiversity begets biodiversity, thus it could be predicted that maximizing the diversity of suitable plants on any given site should maximize the opportunities for the diversity of animals, fungi, and other plants facilitating the site along its chosen trajectory.”
The report also supports moving to a more future-focused strategy by recognizing that rehabilitation is an evolving process and sites need to be visited and looked at regularly. “Revisiting sites over time encourages continued ecosystem functioning and mitigates the effects of invasive plant species, which is often more effective given that disturbed areas are generally more vulnerable,” it states.
As such, it’s important to understand that the long-term success of the rehabilitation process depends on the commitment of the landowner. To ensure a successful restoration, the owner of the land must provide ongoing land stewardship and care for the ecosystem.
MAIN TAKEAWAY
Without question, the TOARC and University of Guelph research study raises the bar on best practices for the rehabilitation of pits and quarries. In the near future, for instance, its results will be compared against results from a similar site that used a passive yet more regular approach to maintenance and dealing with the invasive species challenge.
Most importantly, however, the study shows that rehabilitation does work, even when initially conducted with somewhat dated methodology in terms of plant species.
“The most important takeaway from the Arboretum project is the opportunity to review a site in this detail, 40 years after it’s been rehabilitated, and see such positive results,” states Solondz. “The results concerning invasive species and regular maintenance on sites is part of the learning experience that helps the industry continue on the right path.”
To access a copy of the gravel pit research study, go to https://toarc.com/legacy-pits-quarriesmaap/research-funding/research-publications/ and click on the link under “A 40-year Research and Demonstration Project.” The woody plant inventory and the soil samples are seen by the research team as “an important metric for comparison of the site in the future.”