a few years ago a friend of mine contracted a forester to do a survey of a relative's property for a conservation/stewardship tax scheme, and i spoke with him a few times. he had some very radical notions about urban forestry. for context, we live in hamilton ontario which for much of the 20th century was home to a canadian steel company and the mill dominates the harbour. the steel-making stopped years ago, now they just make coke. the yellow haze of pollution hangs over the city hemmed in by the surrounding escarpment. looking at the industrial quarter via aerial photography now its like a giant scar:
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after one sojourn to the property and retiring for drinks afterward we got to talking about land management and reclamation, and the forester said if he had the option he'd plant tree of heaven all over the site. tremendously aggressive and tolerant of both soil and air pollution, he said it would be the first stage of a generations-long project of rehabilitation in order to someday re-establish a healthy sustainable ecology. my friend and i, both working at a botanic garden devoted to conservation and wetland stewardship, were agog at his suggestions. but now, years later, i often think about what he said.
we're in a difficult position. the competing pressures of unlivable climate, biodiversity loss, and the need for urban greening have to be balanced. if we did like that forester said and mass planted tree of heaven we would successfully green a harsh environment. but it would come at the cost of biodiversity, the mass displacement of existing species evolved in this area, introduction of a monoculture (always high risk), and likely spread of an aggressive invasive species far beyond the bounds of the original planting area, and a massive labour expenditure in control of that invasive. maybe he would say thats the cost of doing business, maybe in a hundred years theyre not going to ask how it was done but how much canopy was grown. this invites much broader questions about the future which i wont go into here.
the question of invasive species also becomes complicated by the concept of lag periods. how many plants that we have introduced already which arent a problem now will become a problem in the future? not being able to know that in advance makes introduction of new species even more fraught:
Published: 08 February 2024 Invading plants remain undetected in a lag phase while they explore suitable climates
personally my views on the use of non-native plants in the landscape have softened over the years. once upon a time i would have said not to even bother with non-natives, but experience and necessity (both ecological and economic) have pushed me to a more permissive stance. we've created harsh landscapes, but plants have adapted to similar conditions. i happened to watch this recently with much interest:
an old abandoned airstrip in new york city, a big concrete oven. and after decades of neglect, a variety of plants have colonized the area. without direct intervention, an invasive-heavy mix of plants, so here he does something before he walks away: he drops some echinacea seeds in the hopes they germinate and establish and begin contributing to this small ecosystem. just like our ancestors tens of thousands of years ago before the rise of agriculture, spreading seeds deliberately along our migration routes so upon our return voyages we have desirable plants to forage from.
assisted migration and pushing the boundaries of hardiness zones, creation and exploitation of microclimates, the use of non-native plant species, they all have to be in the toolkit for us to create and sustain urban landscapes, but they must go hand in hand with observation, clear judgment, and the willingness to acknowledge and rectify mistakes
