RAVINE INVENTORY

Ecologist Katherine Baird details native plants, invasive species and human disturbances in Wilket Creek Ravine

PHOTOS: KATHERINE BAIRD

Equipped with a cruiser vest, forestry tools and high-accuracy GPS and tablet, I set out to sample Wilket Creek Ravine’s natural areas. The field surveys conducted in late summer 2020 are the first highly detailed natural cover inventories in the TBG expansion area. Initiated to assess the condition of the ravine and threats facing it, these findings will inform future stewardship plans to conserve and enhance the ecological integrity of Wilket Creek Ravine.

I applied Vegetation Sampling Protocol (VSP*) to sample 38 predefined 400 m² sampling plots within the TBG expansion area. All plant species were identified and recorded for strata: groundcover, shrub, sub-canopy and canopy. Tree diameters and heights were also recorded, along with measurements of deadwood, tree regeneration, canopy closure, community type, and plot surface and disturbance information. This field data is essential to provide a baseline assessment of natural cover quality, and can be used to derive indices of forest composition, structure, and function such as carbon storage or species diversity. Plots were marked for long-term monitoring, to measure change over time and assess the success of future restoration efforts. This expands on the work the City of Toronto initiated with VSP surveys throughout its natural areas in the summers of 2019 and 2020, meaning TBG surveys relate to the broader context of the City’s ravine system.

New England aster

The portion of the Wilket Creek Ravine designated an Environmentally Significant Area (ESA) contains impressive mature forest. Large native trees including sugar maple, red oak, and American beech, and conifers Eastern hemlock and white pine, tower over the ground flora – the largest recorded, a 94.4 cm diameter red oak. Native plants uncommon to Toronto included great blue lobelia and buttonbush in riparian (riverside) areas. A rare vegetation community within Toronto – Yellow Birch Mineral Deciduous Swamp – was also inventoried, containing wetland species swamp aster, sensitive fern, dwarf raspberry and others. In total, 146 native plant species were recorded across all plots, which included upland forest, riparian forest and wetlands.

Bumblebee on a buttonbush.

While in the field, I observed monarchs, ruby-throated hummingbirds, bumble bees, wasps and metallic green sweat bees nectaring on native wildflowers. I also spotted a hairy woodpecker foraging for insects on standing deadwood and a juvenile red-tailed hawk hunting rodents. Although surveys focused on measuring forest structure and composition, these attributes directly relate to ecological services like wildlife habitat.

Native Spotted jewelweed

Even the highest quality sites, however, are not free from human disturbance, including invasive plants, soil erosion, compaction and organic dumping. These impacts had measureable effects such as reduced ground flora, in some cases less than one per cent native groundcover, and poor regeneration of native trees like red oak. Native plants often require leaf litter or deadwood to establish, which were sparse in many areas of the ravine.

Hairy woodpecker

Surveys also noted a significant change in a portion of the ravine severely impacted by invasive Emerald Ash Borer. Once dominated by white ash, the forest east of the TBG Teaching Garden is now an open canopy with only a few trees remaining. This major loss of forest canopy also increases the risk for invasive plant species.

Invasive plants are a major threat to Wilket Creek Ravine, as they are across most settled areas. They out-compete and displace native species, reduce biodiversity, alter ecosystems contributing to issues like erosion, and negatively impact wildlife that rely on native plants for food or shelter. Flora inventories identified 131 nonnative plant species, comprising 31 per cent of the total cover. The most abundant invasive species included Norway maple, English ivy, common buckthorn, winter creeper, Japanese butterbur, five-leaved aralia, horse chestnut, European euonymus and coltsfoot. Invasive plants occurring at more than half the plots sampled, included common buckthorn, bittersweet nightshade, Norway maple, garlic mustard, dog strangling vine, wood avens, multiflora rose and European euonymus.

Coring invasive Norway maple

Invasive trees were also mapped in forested areas, totalling 466 trees, with common buckthorn and Norway maple comprising almost 80 per cent of these alone. The size distribution of Norway maples revealed that in addition to mature trees as large as 57 cm diameter, there are numerous seedlings and saplings, indicating regeneration. The concern over Norway maple is relevant to much of Toronto, as a four-fold increase in canopy cover was noted in nearby Park Drive Ravine from 1977 to 2015 (Dong 2015**). This motivated a research collaboration with U of T Forestry focused on Norway maple at TBG. A subset of Norway maples was cored along with other measurements taken, to assess their age or duration of invasion and impacts.

Japanese butterbur

While most of the invasive species recorded are well-known, others like Japanese butterbur are unusual. Japanese butterbur was found at three locations, covering a sizable area and encroaching onto riparian and wetland habitat. Presumably escaped from the Edwards Garden’s pond, this plant has large ‘elephant-ear’ leaves and spreads through rhizomes. It has pushed out beneficial native species like spotted jewelweed, with very few native species growing beneath its dense shade. Located at the edge of eroding stream banks, it poses high risk for downstream spread. Still a fairly isolated invasion, this presents the opportunity to remove these plants before they becomes a wider spread issue. This garden escape also reveals how decisions made within the surrounding TBG gardens (and even our own gardens) impact sensitive ecological features.

Different parts of the ravine are not equally impacted by disturbance, and differ in terms of forest condition. Areas with few invasive species and high ecological value are a priority to protect, demonstrating one way this field data helps prioritize management actions.

This project was supported by the Ontario Trillium Foundation. Research was conducted in partnership with the Forests in Settled & Urban Landscapes research lab at the Institute of Forestry and Conservation at Daniels, University of Toronto.

To learn more about the project or get involved, contact me at ecologist@torontobotanicalgarden.ca.

Ecologist Katherine Baird sampling in the ravine.

FOOTNOTES

*Puric-Mladenovic, D., & Kenny, W. A. (2015). The VSP Field Inventory and Monitoring Pocket Guide. Faculty of Forestry, University of Toronto. **Dong, A. (2015). Ecological Integrity in the Park Drive Ravine; 1977 to 2015. Faculty of Forestry, University of Toronto.