Heat-scorched Quercus palustris (foreground) next to healthy one (background) (credit: AO)
Authors: Samiya Tabassum and Manuel Esperon-Rodriguez, 26th September 2021
Urban trees play a significant role in creating healthy and liveable communities, by providing benefits such as stormwater runoff reduction, reducing air and noise pollution, providing shade and insulation, and offering habitat for fauna. These benefits can only be provided by healthy thriving trees, while failed trees quickly become a liability rather than an asset. However, failures do not always have to be doom and gloom. In fact, failures can often give us valuable clues about underlying issues that may be affecting tree success in a given area. Recording data on both success and failure of tree plantings will help to improve species selection and narrow down choice of planting sites and maintenance regimes for future plantings.
What causes failures?
Many factors can contribute to the failure of an urban tree planting. In general, these factors can be divided into two categories:
Biophysical factors – including climate change, extreme weather events, and pest and diseases.
Management factors – including poor plant selection and quality of nursery material, inadequate planting technique, insufficient site preparation, and insufficient maintenance during the establishment phase
Case study: the effect of the 2019-2020 heatwave on Western Sydney’s street trees
During the 2019-2020 summer, the western Sydney Local Government Area (LGA) of Penrith experienced abnormally high temperatures with very little rainfall. This culminated in the highest temperature ever recorded in greater Sydney, of 48.9°C on 4 January 2020. To understand the effect of this extreme weather event on the health of Penrith’s street trees, members of the Which Plant Where team conducted a visual assessment of street tree health in the weeks following that extreme temperature day. We visually assessed the health of over 5,500 street trees and found that more than 10% of all trees showed some level of canopy damage following this extreme event (Tabassum et al., 2021). When we dug a little deeper into which species were damaged the most, we found that exotic deciduous species experienced far more damage than native evergreen species. After one year we resurveyed these trees and found that most of them had recovered. However, if climate change continues on its current trajectory, then urban forests will be exposed to even more severe and frequent extreme weather events, including heatwaves and drought, which could significantly impact their ability to recover. We estimated a cost of almost $1,000,000 AUD to replace heat damaged trees in Penrith, which is an enormous cost for any council to bear.
Record, record, record!
The failures in this case study were caused by a combination of an extreme weather event and species selection. Unfortunately, ongoing climate change means that extreme events will not be once in a blue moon events but will become the new normal. In order to future proof our urban trees to be able to survive these new conditions, we need to ensure we plant climate-ready species now. This should become a priority for local governments as our research shows that more than half of species currently planted in urban areas in Australia are already at risk of heat and drought (Esperon-Rodriguez et al., 2019), and we can expect the number of species at risk to increase in coming years. The Which Plant Where selection tool allows users to find species which are likely to be able to survive climate change, based on the most current climate models. However, another crucial step in identifying climate-ready species is to collect ‘on the ground’ data on which species are succeeding as well as failing. Evidence to support selection of climate-ready trees will become more and more important as the effects of climate change increase and management techniques such as irrigation become less effective (read our blog on ‘Drought tolerators and avoiders’). Data on failures and successes not only informs local tree management plans but can also give scientists valuable information in order to look at the bigger picture.
REFERENCES
Tabassum, S., Manea, A., Ossola, A., Buyani, T., Blackham, D., Leishman, M.R., 2021. The angriest summer on record: assessing canopy damage and economic costs of an extreme climatic event. Urban Forestry and Urban Greening, 63: 127221. https://doi.org/10.1016/j.ufug.2021.127221.
Esperon‐Rodriguez, M., Power, S.A., Tjoelker, M.G., Beaumont, L.J., Burley, H., Caballero‐Rodriguez, D., Rymer, P.D., 2019. Assessing the vulnerability of Australia’s urban forests to climate extremes. Plants, People, Planet, 1(4), pp.387-397. https://doi.org/10.1002/ppp3.10064