On a hot day in the city, the shade under a leafy tree can be a welcome relief from the sun. But can you imagine if you looked around and there were no trees in sight? Or the only trees you can see are either dead or dying?
Unfortunately, with the realities of our global climate crisis, our urban trees are under threat. Rising temperatures and prolonged drought are just some of the issues that our urban trees, like us, are having to face. If you live in a very green city, it might be easy to take trees for granted sometimes, but it’s important to remember that they do so much for us and our urban environments. Not only do they look beautiful, but they also cool our streets, provide habitat for other critters, improve air quality, sequester carbon, and so much more.
So how do we make sure our urban environments can remain (or become) filled with the leafy goodness of trees, and all the benefits that come with them? One part of the solution is to ensure we’re planting trees that are already resilient to the challenges posed by climate change. Two recent papers, one by Hirons and colleagues in the journal Plants, People, Planet and another by Sjoman and Watkins in the journal Urban Forestry & Urban Greening, discuss how to select climate-resilient urban trees for the future and some of the practical issues that come with this task.
How to select the “right tree for the right place”
One of the major challenges when it comes to urban forestry is weighing up which trees will be best suited to an area based on its current climate and projected future climate scenarios. In their paper, Sjoman and Watkins argue that, while most of the urban forestry literature focuses on finding the “right tree for the right place”, we must instead focus on finding the right genotypes (or ecotypes) of these right trees, so that they will already be suited (or at least better suited) to the climate scenarios forecast for that urban area.
“If the goal of urban forestry is to create long-term sustainable tree populations that develop large, prosperous trees over time, it is of the utmost importance that the plant material that is used is of the best possible fit with the target site,” Sjoman and Watkins wrote. “This means that the plant material is of a genetic origin that is ecologically suited to both the climate and growing conditions on site… Understanding how a tree’s provenance and/or ecotype interacts with important selection criteria, such as tolerance of drought or anoxia, may be fundamental to securing the most appropriate genotype for a planting site.”
Hirons and colleagues also argue for a tree-selection approach focused on plant functional traits and environmental factors. They suggest water potential at leaf turgor loss (a key trait related to drought-resistance) should be an essential criteria for urban tree selection “particularly when considering sites with paved surfaces and those subjected to higher evaporative demand as a result of the urban heat island effect”.
Are our urban forestry supply chains equipped for this?
Preparing our urban landscapes for climate change will not be an overnight fix, with both papers making points about how we need to review how tree species are selected and address any barriers for this within the current supply chains for urban forestry. Hirons and colleagues expressed that “despite the recognition that many trees in urban environments are vulnerable to a changing climate, practitioners express uncertainty about how to make robust species selection decisions for future environments”.
Sjoman and Watkins explored how much we know about the origins of our urban trees, with a focus on five of the most commonly planted trees in urban environments in central and northern Europe. Through interviews with 24 of the largest nurseries in Northern Europe, they found that a majority of nurseries did not have information about the original provenance of the tree species they supply for urban forestry, and none of the nurseries could provide information about the type of ecosystem from which their genetic material originated. Unfortunately, the authors write that this means “it is not currently possible for specifiers to select trees at an intra-specific level based on climate or ecological criteria”.
“The scarcity of information available to specifiers relating to the provenance of plant material in the nursery sector makes it almost impossible to consider genotypes with enhanced environmental tolerance within their selection decisions,” Sjoman and Watkins wrote. “As the necessity to build resilience into gardens and public green infrastructure becomes more prominent, it will become essential to equip those specifying trees with evidence-based information relating to key environmental tolerances, such as drought.”
A key role for botanic gardens and arboreta
So how can we make sure we’re finding the right trees for the right place? Well, one approach that both studies suggest is to consider the role that botanic gardens and arboreta could play in assisting with the selection and evaluation of urban tree species and genotypes that will be most suited to future climates: “Arboreta and botanic gardens are uniquely placed to support the evaluation of a range of landscape plants – including trees – because they are intended to be long-term custodians of plants, they have substantial expertise in the management of living collections, and keep precise records of plant origins”.
Hirons and colleagues emphasised that these institution’s “diligent record keeping of plant origins, phenology (e.g., flowering periods), and horticultural requirements make them exceptional resources for scientific research”. Sjoman and Watkins agreed: “Living collections in botanic gardens can provide us with extremely useful information about optimal and suboptimal growing conditions, particularly for long-lived species such as trees. The direct involvement of botanic gardens in sustainable urban forestry is needed now more than ever to amplify best practices and to engage local citizens in a community-based process.”
While Hirons and colleagues argue the importance of botanic gardens and arboreta in this process, they do also heed some caution: “a substantial limitation of evaluating species and cultivars in botanic gardens and arboreta is that there is likely to be a much wider range of genotypes growing in the wild than is represented in gardens. This is likely to bias the evaluation of species in specialist horticultural texts, particularly for taxonomic groups that are not well studied.” They conclude that “arboreta and botanic collections can play a vital role in the evaluation of plant material for urban environments, provided they are curated with scientific objectives at the forefront of management policy and are not managed purely as visitor attractions.”