By Dave Demers
Many welcomed this cooler summer just now behind us, one that a dry spring had seemed to render unlikely. Despite this relief, our native western red cedars and grand firs, as well as a lot of our street trees, showed major signs of distress, taking a less than attractive fall attire in June or simply defoliating before any water bag can be brought out to the rescue.
This made for a worrisome look throughout the city, one that has me, a seasoned gardener turned park commissioner, wondering if the park board and city are ready to keep the reality of a changing climate from thwarting our own aggressive efforts in preserving and growing our urban canopy.
Phenological indications of deep-seated problems started piling up early this past spring: prescient whiffs of burned caramel from Kadsura trees*, abnormally large crops of samaras on Japanese maples, top die-back on cedars, and stunted yearly growth on species otherwise known for their vigour—all signs that troubles were brewing. (* Such a scent usually briefly accompanies its fall foliage.)
As summer rolled in, subtle hints turned into evidence. European hornbeams scorched to a russet-y brown everywhere downtown, blocks after blocks of mid-size deciduous trees drying up along Broadway, and again along Cambie just south of Vancouver City Hall, along too much of Main Street, and so on.
Maples of all sizes, flowering dogwoods, even some Persian ironwoods and Eastern redbuds, newly planted or well-established, throughout the city, showed serious discontent. With each tree faltering, failing, our shared quality of living takes a hit, and our common purse a blow.
What’s at stake?
From a conservation perspective, one could point to 1888 as the birth of urban forestry in Vancouver: the official creation of Stanley Park. (See the Urban Forest Strategy for more information.)
The foresight underpinning this creation and the ensuing dedication to actively preserving, planting, and nurturing, has led to a world-class canopy that makes Vancouver the envy of many. While recent land development has chewed at this leafy heritage, our civic government has by now managed to stop and reverse this degradation. (Canopy coverage in 1995: 20 percent; 2013: 18 percent; in 2015: 19 percent.)
Indeed, as per the goals set in the Greenest City Action plan, 150,000 trees will be planted by the end of 2020. By 2050, 22 percent canopy coverage is targeted, which would represent an ambitious growth of three percent. Of an estimated 450,000 existing trees, about 140,000 are found on our streets. And finally, let’s note that plum and cherry trees make up 28 percent of current canopy, maples 24 percent, and hornbeams 3 percent, three genera not particularly resistant to prolonged droughts. Indeed, loads of maples are suffering this year, second only to hornbeams.
A fatigue built over the years
Growing trees on our streets is, to start with, quite a feat. A large, modern city like Vancouver is a most unnatural setting for plants, native or not. Such a city is one big heat island of asphalt and manufactured soils, where steel and concrete cover up most alluvial depots, moraines, and bedrock.
In this setting, our trees are imposed growing conditions stretched to the limits of any functionality, conditions that don’t allow most natural phenomena such as self-regeneration, population migration, and most symbiotic relationships.
When the lens of climate change is applied to this situation, I see street trees as the canary in the gold mine: the first elements of our city that could potentially be nudged and then pushed over the edge by an environment in turmoil.
The summers of 2015, 2017, and 2018 all turned out drier and hotter than historical averages. As smoky skies and strict water restrictions disturbed our lives, our trees suffered quietly.
It is important to realize that the return of the rain in the fall does not simply reset the health of a tree. Prolonged water shortages may take a long-lasting toll on trees, a toll that may be carried from one year into the next and accumulate until enough "normalcy" allows for a potential recovery.
Among various survival strategies, a tree often reacts to droughts by abandoning some of its canopy and root system—the underground part of a tree represents 20 to 40 percent of its entire biomass. This, in turns impacts its ability to produce and store "food", stay hydrated, stand solid against storms, fence off pathogens, et cetera.
Seeing so many of our street trees turning brown despite the summer rains, I point a finger at these repeated summer droughts for inducing this built-in fatigue and this alarming rate of failure. It's worth noting that in May and June, Vancouver received just short of half of the usual rainfall.
That these damages are just now clearly made visible to us, after a few years of insidious build-up, is a serious warning of what’s to come.
When climate change hits home
The dire predictions of sea-level rise, heat waves, and global disruption may still sound like remote threats many of us don't fully compute. Changes roll in slowly until…the ordinariness of our lives is suddenly interrupted and turns ugly, as per some of our streets this past summer.
To make it all more tangible, let’s dig into some of the findings of a report called Climate Projections for Metro Vancouver. According to this report, by 2050, annual rainfall average should increase by a modest five percent yet precipitation patterns are expected to change dramatically: 20 percent less rainfall in the summer but a serious increase for the fall, during which more storm events are expected.
During the same time frame, our climate should warm by an average of three degrees. That should result in a doubling of days above 25 C and in a 60 percent decrease in the number of frost days. Seeing how badly a few consecutive years of "abnormal" conditions impacted our urban forest, I can only imagine the difficulties ahead of us.
While climate change may seem much slower than most other aspects of our modern lives, it is going at a full gallop when compared to the slow pace of establishing a full, comprehensive urban canopy.
Consider the early life of an average street tree. From seed to nursery field: two or three years.
From nursery field to our street: another five to 10 years. From freshly urbanized juveniles to full canopy above our heads: another 15 to 20 years. For a total seed-to-full canopy: 25 years-plus.
The financial, health, social, and ecological value of such a forest is hard to establish, and I would argue, priceless. And in this case, we can hardly buy time.
So what now?
The park board's planning and forestry teams are some of the best that could be assembled, with passion and expertise to undertake the most daunting of challenges, I’m sure. In the case of our urban forest, we are already doing a lot, with well-identified opportunities for improvement.
Of those, let’s consider taking a leap away from grey to green infrastructures. Permeable sidewalks and structural soils, curb cuts, detention tanks, and rain gardens are all measures that would allow us to capture and retain as much of the rainfall as possible and offer improved growing conditions.
Remember, we are foreseeing not a diminution of annual rainfall as much as a disruption in regular precipitation patterns. (This consideration was at the centre of a recent park board decision in relation to sewers separation.)
Given that a continuous canopy has much great resilience than fewer, isolated trees, we could reconduct our tree planting targets (+150,000 trees by 2030 maybe?) and create further incentives for homeowners to plant abundantly.
Other opportunities would aim at quickly adjusting our traditional ways in anticipation to a changed climate; shifting the nature of trees to be planted toward more resilient taxa, in close collaboration with local growers; and increasing our preparedness and overall capacity for active irrigation, as we unhook our water trucks from ever more in-demand potable water sources.
Looking at the trees just outside our windows today can help us better plan for the forest of tomorrow. With growing conditions already getting harsher and expected to worsen for decades, urban foresters, engineers, and policymakers need to change their ways and up their game without any further delays.
Dave Demers is a Vancouver Greens park commissioner.