Trees - the Interceptors: an Urban Watershed Solution for Climate Change Resistance

By Oscar Westell

The sustainability of Australia’s fresh water is uncertain in the face of climate change, with the impacts of drought becoming widespread.

Urban environments rely on a matrix of impervious concrete pipes and channels to mitigate storm events, with runoff directed away from local soils. Advancements in sustainable infrastructure such as rain gardens, bioswales and modified tree pits aim to return large volumes of stormwater into urban soils through passive irrigation of amenity trees. The immediate benefits of such measures are obvious, but most stormwater heads into treatment networks due to a lack of deployment.

In the Australian context, the use of water for irrigating public assets will become less negotiable as drought events become more frequent. Considering municipalities already require substantial volumes, future deployment of green infrastructure will complement the existing grey network by reducing active irrigation of street trees while cooling urban heat islands (UHIs). Several tree-sensitive products are currently available, and we strongly recommend considering permeable materials and sympathetic practices when designing around trees.

If storm events are severe enough, the combined transport of wastewater can overflow into septic systems. Tree pit stormwater control measures (SCMs) are engineered to ease gutter flows by intercepting and partitioning water into reservoirs beneath street level, which are connected to adjacent tree pits and the surrounding soil. The water is first acquired by street trees with excess supply available to adjacent vegetation. The extent to which these SCMs benefit the environment, community and the economy overall, is likely undervalued.

Urban trees already interact with rainwater by intercepting falling precipitation and enhancing infiltration through root growth, while filtering and transpiring water to the atmosphere. We propose that cultivating the most appropriate tree species and complementary understory plants deserves consideration to further improve the functionality of SCMs, under current and future climate scenarios. The right tree planted with the right planning and protection will almost always lead to desirable outcomes.

As you would expect, less water is lost when an SCM is deployed. However, uptake of water is ultimately determined by the performance of each tree and its microbial partners. The local flow regime and residence time of water will ultimately dictate what species can be planted, as waterlogging or drought stress will arise if poorly calibrated. Big trees transpire more water compared to their establishing counterparts, highlighting the importance of mature tree retention in urban landscapes to mitigate storm flows. However, experimental tree pit SCMs in the United Kingdom, using young trees, reduced runoff from asphalt surfaces by 62%, supporting the proactive replacement of declining or poorly placed trees over mature trees.

As urban space becomes densely populated, planning and caring for trees must be a priority, particularly as growing space comes at a premium with less of it available for replanting large species. This task will be harder with climate change. A projected 3oC rise under high carbon emissions, estimated that 73% of Australian native amenity tree species would become unsuited to urban environments by 2070. Furthermore, under similar dry climate scenarios, 25% of plants at The Royal Botanic Gardens of Victoria would become unviable by 2090. Despite some unknowns, it is concerning to think that popular varieties may only be accessible to future generations in herbaria.

The following five species are suited to Melbourne’s climate predictions in 2040, 2070 and 2090. The shortlisted candidates have also been selected for their size at maturity, as planting trees under powerlines cannot be expected to stop, despite the reminders. For that reason, some larger shrubs could be given the nod.

However, not all dry land specialists will respond favourably to more water and planting exotic species will have merit where passive irrigation permits. In any case, a permeable redistribution network that prevents waterlogging and sewer overflows, while sustaining a productive urban forest, is an attractive concept.

Despite growing interest in SCM technology, challenges still arise considering the long-term viability of trees in urban areas. Problems will continue to manifest from improper nursery practices and planting methods. Severe weather events and more commonly soil compaction, improper pruning, vandalism and root damage will always be a constraint to good plant performance. The addition of SCMs creates another dimension for urban foresters to comprehend, who are already expected to maintain canopy cover, defend against pests and diseases, and the threat of more frequent significant weather events.

Where improvements to grey infrastructure are expensive or not successful at mitigating sewer malfunctions, green infrastructure systems are sustainable alternatives to control stormwater. Research is quantifying the benefits of SCMs and despite some variation, the advantages are promising. From an arboricultural perspective, incorporating SCMs into future developments is an efficient way to harvest and store runoff for opportunistic irrigation, boosting plant productivity and community services in the process.

Using permeable pavement and sympathetic construction methods around trees is the most effective way to limit construction impacts and maintain vigorous canopy cover. Novel tree pit modifications will also provide a catalyst for removing surplus energy from the UHI, by increasing shade and boosting rates of evapotranspiration. Improving water sustainability in urban areas must be a priority if future generations are to enjoy lush parks and gardens, in whatever form they eventually take.

Under any circumstance, planting the right tree in the right place today, with the right support and protection, will result in profound benefits for those enjoying its services at maturity.

This article is independent research, and thus unfunded.
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