Update: this blog post has been adapted and published as a climate guide in Planet Detroit. Check it out!
What kind of tree should I plant in my yard?
The answer: it depends.
This is a common question I hear from family and friends who know I studied something tree-related in school (but not arboriculture, I remind them). In natural areas, a given tree will succeed in certain places because of many interrelated and interacting factors such as climate, soil, topography, competition, herbivory, dispersal, and chance events. In highly managed areas, such as a yard, garden, or easement, where humans have more influence on tree selection, any (actual) certified arborist would tell you that the above environmental factors – and more! – must still be considered for a tree to succeed.
Often, the first conditions they will advise you to inventory are related to the planting site; namely, climate, soil texture, sun exposure, and space availability for tree growth. City dwellers have additional details to sort out, like if the tree will be planted near electrical wires – where branches will need to be pruned – or sidewalks – where tree species with aggressive roots may pose infrastructure problems, and where exposure to road salt or cars may in turn pose risks to the tree. Urban residents may also need to find out whether or not their municipality has placed restrictions on planting any species that would be considered messy (some fruiting trees), smelly (ex. female gingko trees), disease-susceptible (elm or ash), weedy (ex. tree-of-heaven), or overly representative in the tree canopy (ex. Norway maple in Hamtramck). The ecologically minded may want to select a tree that would maximize habitat for countless insects and birds. A design-oriented planter might suggest options based on foliage texture and color. And of course, an urban planner may want to consider if nearby residents even want the trees at all. Many websites run by extensions, arboretums, and various tree organizations offer great resources* to aid in answering these questions.
But one additional factor that I’ve been wondering about lately is climate change.
Which trees have the best chance at tolerating a rapidly changing urban climate in the next twenty, fifty, one hundred years?
The Fourth National Climate Assessment projects that Midwestern cities will experience hotter summers, with more frequent and intense rain events. Drought and flooding are two seemingly opposed phenomena that humans and trees are increasingly forced to deal with, sometimes in the same year.
When humans discuss how we will adapt to these changes, tree planting is often brought up as an important strategy. Tree roots soak up water from soil, reducing runoff; tree canopies reduce provide shade and evapotranspirative cooling in the summer; and while alive, tree wood sequesters carbon. But what about the trees themselves – will they be able to survive and thrive in the face of such drastic change?
Because I love trees, and planting them, and learning, I consulted the research. The research answered: it depends.
The first helpful resource I came across was a report by the Huron River Watershed Council (HRWC) and the Great Lakes Integrated Sciences and Assessments Center (GLISA), titled “Climate Resilient Communities: Review of climate impacts to tree species of the Huron River watershed.” Though Detroit is not in the Huron River watershed, it’s close enough for this to be a good starting point. The report synthesizes climate model research for common species native to the watershed, assigning each species a positive, negative, or neutral favorability score relative to the future climate. (This analysis was extended from an original thirteen to thirty species, with fact sheets for each on the HRCW website).
The predicted winners? Blackgum, black oak, box elder, bur oak, chinkapin oak, eastern redbud, flowering dogwood, hackberry, hickory species, honeylocust, Kentucky coffeetree, sassafras, serviceberry, tuliptree, and white oak. Reasons for success vary. Boxelder may benefit from increased flooding, while the three oaks mentioned (all fire-adapted) may benefit from interactions between climate and fire, and invade once-wet areas as they dry out. Hickories may stay afloat because “they already exhibit wide adaptability to warmer, southern climates.”
Neutral favorability scores were assigned to American basswood, American elm, Eastern hopbornbeam, red maple, swamp white oak, and sycamore. The drying of wetter areas may benefit species like red maple and basswood, while in other scenarios, species distribution will be limited by factors such as drought (swamp white oak), fire (red maple, sycamore), increasing temperatures (basswood), inundation (sycamore). The report recommended against American elm as a street tree due to Dutch elm disease.
The predicted losers were American beech, American hornbeam, black cherry, black spruce, Eastern white pine, Northern red oak, paper birch, sugar maple, and tamarack. Some, such as sugar maple, paper birch, red oak, and Eastern white pine, would lose out due to requiring colder temperatures. Tamarack and black spruce are found in bog habitats, which will themselves be sensitive to climate change. American beech is predicted to decline due its firm intolerance of drought, while black cherry may be hurt by flooding or insect damages.
A second helpful study from 2021, titled “Vulnerability of street trees in Upper Midwest cities to climate change,” by Leslie A. Brandt and fellow researchers, developed Adaptive Capacity scores for one hundred and seventy-eight tree species and cultivars found in rights-of-ways of fourteen cities in Michigan, Minnesota and Wisconsin (including Detroit). The authors defined a species’ adaptive capacity as a “function if its ability to withstand disturbances such as drought and flooding and its biological adaptations that allow it to survive in a variety of growing conditions in an urban environment.” Scores were based on extensive literature review, as well as input from horticulture industry experts, and were applied to low and high future climate scenarios in three future time periods. The vulnerability of a tree factored in adaptive capacity, as well as exposure (projected shifts in frost and heat maps) and sensitivity (whether the planting zone was suitable or not).
In a low emissions scenario (“rapid reduction of greenhouse gases”), the Brandt et al. (2021) study predicted the majority of Detroit’s trees will be of the low or low-moderate vulnerability category. Under a high emissions scenario (“business as usual”), over half of Detroit’s trees would be in the moderate to moderate-high vulnerability category. Until industrialized nations around the world radically reduce fossil fuel output, the second scenario is looking more likely. A positive spin – commonly-occurring trees considered to have high adaptive capacity and low to low-moderate climate vulnerability in the two emission scenarios, across all the fourteen cities, include Norway maple and Gingko; many in this category agreed with HRWC’s positive favorability scores, like hackberry, Kentucky coffeetree and bur oak. In contrast to the HRWC report, this paper rated white oak as having low adaptive capacity and moderate to moderate-high vulnerability (perhaps due to the urban context?). Common street trees rated with medium adaptive capacity include honeylocust, sugar, silver and red maples, London planetree, callery pear, sycamore, Siberian elm, Northern catalpa, and pin oak.
Both reports discussed the difficulties of balancing multiple, complex factors at once in their scoring systems. Predicting species’ response to an unpredictable world is tricky business, but an arguably better approach than assuming that our current practices will hold up just fine. In general, “species habitat is moving north and east in response to climate change.” (HRWC/GLISA). How will the trees keep up with habitat shifts? Tree migration happens naturally – it is how the Great Lakes states eventually became reforested after the most recent glaciers receded 14,000 years ago – but it takes a lot of time, and with environmental change happening at such an accelerated rate, we don’t have time to spare. As forest managers are experimenting with assisted migration – the intentional stocking of more southern-ranging trees to aid in their movement towards future suitable habitat – so, too, should urban foresters.
Notably, Brandt et al. (2021) list uncommon species with high adaptive capacity that may be worth planting more in cities of the upper Midwest. Native species included blackgum, chinkapin oak, and downy serviceberry. Several non-native species and cultivars were also listed, including smoketree, and disease-resistant elm varieties. This brings up an interesting dilmena – if cities are to become more important habitat corridors in a biodiversity-challenged future climate, selecting native trees may provide valuable habitat for the highly specialized insects that depend on them (and the birds that eat those insects); however, if the priority is to optimize overall ecosystem services in cities, then selecting non-native species may be preferred, even if they host few native insects, or only generalist insects.
There is one more factor worth mentioning in tree selection, which climate change will definitely affect – human health impacts. Vibrant Cities Lab outlines some of these interactions, some considered beneficial to human health, such as urban forests mellowing out extreme heat via canopy cover, while others are considered may be harmful, such as increased precripitation accelerating plant pathogens and mold allergens. Several studies have shown that pollen season is increasing with climate change, which may cause female dioecious trees to be favored in future plantings.
I personally believe the net benefits of urban trees outweigh the drawbacks. Uncertainty should not stop us from planting trees in cities, nor should it freeze us in indecision. Using the best available science, some reasonably well-informed selections can be made. If Metro Detroit has learned anything from Dutch elm disease or emerald ash borer, we should at the least aim for planting a diversity of trees. Whether we will like it or not, time will show us what works.
*Other resources for tree selection/planting in Michigan
https://www.releafmichigan.org/tree-planting-resources.html
https://www.greeningofdetroit.com/caring-for-trees
https://www.canr.msu.edu/home_gardening/trees-shrubs/
https://www.michigan.gov/dnr/managing-resources/forestry/urban
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