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In one of our study plots in Rosedale Valley, 90% of the woody stems were identified as Norway maple, one of the most invasive species in North America. (Read more about this species from our previous article: Norway Maple (Acer platanoides): when street trees break bad.)
On the other hand, we have also found some big old oaks that are producing large amount of acorns, but we haven’t seen too many young oaks in the ravines. Why aren’t they regenerating? Is there any other reason besides the squirrels and Norway maples? Our team is starting seed forecasting on these oaks. Fall is about to start, the oaks are ready to help us revitalize the ravines, and vice versa.
We finished surveying Burke Brook Ravine (our control plot) last week and it had one very distinctive feature, the presence of conifers and one conifer in particular the Eastern White Pine, Pinus strobus. The Eastern White Pine is the provincial tree of Ontario and for good reason, the tree helped to drive the early economic engine of the province but in order to understand why we must first look at the ecology of the species. Pinus strobus is the tallest tree in eastern North America and is fire resistant which is why mature trees have no branches on the lower trunk. It is one of the most easily recognizable Pine spps as it is the only one with bunches of five soft needles.
When settlers first arrived in Ontario, White and Red Pine were one of the most common species, so White Pines were tall, straight, fire resistant and abundant which made them the perfect lumber for the British Navy because they made perfect ship masts and when Napoleon cut off their timber supply in the Baltic ports, the White Pines of Ontario became an very valuable resource indeed. White pines were heavily logged from the 18th century into the early 20th century and the impacts are still being felt today. In the Haliburton forest region of Northwestern ON, White Pine (Pinus strobes L.) was reduced from 11.5% in 1863 to less than 1% in 1999 due to logging and fire suppression while shade tolerant species like the American beech (Fagus grandifolia) and Sugar Maple (Acer saccharum), have increased from 34.5% in 1863 to 62.8% in 1999. This shift in forest composition in Haliburton forest follows a greater trend in the Great Lakes-St Lawrence forest within Ontario in the last century where logging in conifer stands has allowed for the regeneration of shade-tolerant hardwoods and it is a trend that continues in the Toronto Ravines.
How do trees talk to each other? Canadian forest ecologist Suzanne Simard will tell you how trees in a forest are connected through the massive underground network – the mycorrhiza:
Norway Maple (Acer platanoides) in indigenous not only to its namesake country but to much of Eastern Europe and down to the Caucasus, it was introduced to North America in 1756 were it became a popular street tree in North America because it is a fast growing, hardy species that can withstand city pollution. In Toronto Norway Maple was widely planted to replace the elm trees that were decimated by the spread of Dutch Elm Disease in the the 60s (80% of elms in Toronto were lost). While the Dutch Elm Disease highlighted the dangers of planting street tree monocultures and the devastating effects of invasive pests, Norway Maple is now being recognized as a highly invasive species that out competes native tree species and decreases the biodiversity of our urban forests.
Norway Maples are shade tolerant, they grow very fast and they produce large seed crops; these traits have allowed them to escape the confines of the streets and make their way into Toronto’s parks and ravines. They also leaf out earlier and their leave stay on later in the fall, suck up water leaving behind dry soil and have a very shallow root system whcih can increase soil erosion. We know Norway Maple are in our ravines because a study was commissioned in 1977 to gather ecological data in four ravines and they found that Norway Maple accounted for 10% of the canopy cover; last year a member of our team began to resurvey those ravines and found that Norway Maple now accounted for 40% of the canopy cover. A 30% increase in less then 40 years is a major ecological shift as it comes at the expense of native species like oaks, as and beech, which decreases biodiversity in the ravine systems. The very traits that made it a good street have allowed Norway Maples to spread into one of our most important and fragile ecosystems; we must be vigilante in monitoring and protecting our ravines.
Come out to the Trinity Bellwoods Farmer’s Market tomorrow, Tuesday July 5th from 3pm to 7pm at Dundas St W and Crawford St. We will be there with a booth to present our study and to engage the public in ecological research. The City of Toronto will also be there to take suggestions from the pubic to guide their Ravine Strategy. Come out, have a chat with us and tell the City you want ecology included in the Ravine Strategy!
We have been out in the ravines setting up our research plots, we are recreating the plots that were set up in 1977 in order to resurvey the canopy cover by identifying all of the trees over 3 diameters at breast height and the under story vegetation. This will give us a quantitative baseline of tree species composition and allow us to track changes in the ravine. Because the plots are large, 80×50 meters we are setting up 10×10 grids within each plot in order for our resurvey to be manageable and have been lucky enough to be able to use a Trimble GPS system. Using a precise GPS system allows us to accurately plot the grids very quickly, we were able to do four plots in an afternoon, which would have taken us twice as long with a measuring tape and compass, though a compass will always come in handy when you are out in the field.