Mike Ellison of Quantified Tree Risk Assessment outlines the challenges of establishing, managing and assessing trees in our urban environment.
Urban arboriculture is being reshaped by two accelerating forces: climate change and the spread of exotic pests. These two factors increase the physiological and structural stresses on trees. They can reduce the lifespan of urban tree plantings and increase the likelihood of their failure. At the same time, cities are becoming increasingly reliant on trees to mitigate the urban heat-island effect. Hard surfaces like roads and buildings absorb and trap heat while tree cover is limited.
For arborists, the challenge is managing risks to and from existing trees while ensuring new plantings are resilient enough to survive in harsher future conditions. Here are some of the key implications of climate change and exotic pests for urban trees, and how high-quality planting and maintenance, particularly tree selection and below-ground design, can help establish more resilient cities.
Climate change is increasing the risk of mortality in urban trees
Urban trees are experiencing unprecedented temperatures. Heatwaves are longer and more frequent, and the urban heat-island effect raises temperatures even at night. Research indicates many tree species cannot physiologically acclimatise to elevated temperatures, meaning their thermal tolerance does not increase in line with warming conditions.
For arborists managing urban trees, this translates into more frequent canopy decline and dieback, increased water stress – especially in confined rooting environments – susceptibility to disease and insect pests, a potentially higher incidence of sudden branch drop during and after hot and dry conditions, and potentially reduced growth rates and shorter lifespans.
Heat-stressed trees might show subtle early symptoms, such as reduced leaf size and density or premature leaf senescence. Those symptoms can impact the tree’s resilience to hot, dry conditions, and that can sometimes lead to a spiral of decline.
Drought is a climate-driven stressor for trees, limiting fine-root development and the uptake of water and mineral nutrients. In urban settings, where soil volumes and conditions are often already sub-optimal and a limitation on growth, drought stress can escalate quickly. Higher temperatures can increase pest reproductive rates and reduce winter mortality. Urban heat islands may create microclimates where exotic pests that vector fungal pathogens, such as the Polyphagous Shot Hole Borer, can thrive outside their natural range.
Arborists are well placed to develop and contribute to solutions
Arborists managing urban tree populations should expect more trees to experience physiological stress and greater vulnerability to secondary pathogens that exploit drought-weakened trees.
Many urban tree populations rely heavily on a small range of tree species. This creates vulnerability to species-specific pests and disease. Arborists can play a key role in advising councils and clients on diversification strategies that reduce pest-related risk.
Trees are our most effective natural tool for mitigating urban heat. They provide shade and reduce surface temperature, and evapotranspiration has a direct cooling effect. However, these benefits depend on large, healthy, long-lived trees. Short-lived and poorly established trees contribute minimal benefits.
While the urban heat-island effect increases the need for trees, it also increases the difficulty of establishing and maintaining them. This makes high-quality planting in a sustainable rooting environment even more important.
Arborists are uniquely positioned to influence planting standards, species selection, and below-ground design, and can guide clients toward species that demonstrate:
* High thermal tolerance;
* drought resilience;
* pest and disease resistance;
* strong structural form;
* proven performance in comparable climates; and
* diversity of interest.
Species selection should consider future climate projections, not just current conditions, and this may require introducing new species and reducing reliance on species proven vulnerable to the effects of climate change. Even within a species, selecting plants of a known provenance and proven resilience in hotter, drier conditions should be a priority.
Diverse plantings reduce the likelihood of catastrophic pest outbreaks and improve overall resilience of urban tree populations, so seeking to reduce reliance on a small range of species is essential.
On the fringes of urban developments is the interface with native flora and fauna where we should not lose sight of the interdependency of those coevolved species. Even where native trees are experiencing decline, wherever possible, we should seek to retain them for the benefit of other species, particularly fungi and invertebrates, which often rely on species-specific habitats including hollows and cavities, dead wood and decay.
Sustainable below-ground growing conditions
Most urban-tree planting failures originate below ground.
To provide optimal benefits, tree planting must include adequate soil volume to allow root growth under pavements and adjacent to infrastructure. Specialised load-bearing systems should be used where necessary to enable the provision of rootable soils without compromising load bearing capacity. Soil conditions must provide water and mineral nutrients, but also drainage and aeration. Compromise on any of these elements will result in suboptimal conditions for the tree.
Passive irrigation systems (stormwater harvesting and kerb inlets such as those developed by treenet.org) can improve establishment and long-term health.
Risk management
Climate change and exotic pests are increasing some of the risks to and from urban trees, but they also highlight the importance of trees as critical urban infrastructure. For arborists, the challenge is to manage these risks while ensuring new plantings are designed and maintained to thrive in hotter, more variable climates.
By focusing on resilient species, diverse plantings, and high-quality below-ground conditions, arborists can help cities build urban forests that are safer, cooler, and more sustainable for decades to come. As an industry, we can guide urban designers towards more successful tree establishment and management solutions.
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