A new study by Camilo Ordóñez and colleagues with the ambitious title “Quantifying the importance of urban trees to people and nature through tree removal experiments” has been published in People and Nature. Rather than sneak into parks at night with axes and a can-do attitude, the scientists took advantage of scheduled tree removals in Melbourne parks and streets to conduct Before-After-Control-Impact (BACI) experiments measuring ecological and social changes. Their results show that tree removal is a matter of context, with some tree removals having a much more significant impact than others.
To carry out the urban tree removal experiments, the researchers collaborated closely with local city councils and tree managers who had scheduled tree removals and were willing to partner in the research. Before the councils cut down any trees, the scientists surveyed birds, mammals, and invertebrates in the parks and streets. They also interviewed many park and street users about their perceptions. City crews then removed the trees. After waiting several months for communities to adjust, the researchers returned to conduct all the ecological and social surveys again in the same places. The experiment was repeated across multiple parks and streets, allowing comparisons of areas where trees were removed versus nearby similar areas with no removals. Strict protocols ensured the survey methods were identical before and after. This research design allowed the scientists to isolate the impacts of losing mature urban trees.
One of the major ecological impacts observed in the study was a substantial decline in species dependent on the mature trees. One of the sites the experiment looked at was a park in Melbourne, Australia, where around 30 large elm trees over 50 years old were cut down. In surveys before and after the removals, the researchers found that sightings of nectar-feeding birds like lorikeets and rainbow lorikeets dropped by 62%. Possum sightings also decreased by 63% after the trees were removed. The scientists attribute these significant decreases to the loss of critical habitat features the large elms provided, especially food sources like nectar and foliage, as well as nesting hollows and shelters. The findings demonstrate that mature trees in urban parks support specialized wildlife adapted to foraging and roosting in them. When these large old trees are lost, dependent animal populations can rapidly decline.
The study also found that invertebrate predation increased substantially in the park after tree removal, indicating higher predator activity. The researchers measured predation rates using plasticine caterpillars placed on branches and twigs before and after tree removal. The fake caterpillars were scored for bite marks from predator species like birds after being deployed for seven days. Invertebrate predation increased by 82% following removal of the mature trees. The authors think this could be a consequence of the hunting area for predators shrinking. Ordóñez and colleagues write:
The increase in invertebrate predation could be explained by an ecological refuge effect, meaning predation being concentrated on the reduced amount of habitat that remains after a significant portion of the habitat is lost, leading to an increase in attacks to invertebrates. However, it is unclear whether these changes will be sustained long term, and these changes may be context dependent.Ordóñez et al. 2023.
The study demonstrated the social impacts of losing mature urban trees. The researchers surveyed over 500 park users before and after tree removal about how important the park and its trees were to them. They found that after cutting down the large old elms, people valued the park less, with its average importance rating decreasing from 4.3 to 4.1 on a 5-point scale. The perceived importance of the trees remaining in the park also declined significantly after tree removal, dropping from an average of 4.6 to 4.4. These results suggest that the presence and abundance of mature trees strongly contribute to the importance people assign to urban natural areas. When many large trees are lost, it negatively impacts people’s connections to and valuing of the site. The findings highlight that mature urban trees shape the experience of a space in ways that matter to city residents.
The experiment also revealed a notable change in people’s attitudes toward urban trees after mature tree loss. The researchers asked park users whether they would prefer to remove more trees, leave the trees as is, or plant more trees. Following the tree removals, there was a significant increase at the impact site in the proportion of people who wanted to plant more trees, shifting from an average response of 2.2 to 2.7 on a 1-3 scale of preference. Ordóñez and colleagues write:
While the observed changes of perceived importance were small, this confirms findings from other studies that have observed changes in perceived preference for urban places with or without trees (Arnberger et al., 2017), and points towards the contribution that trees make to the importance of urban nature sites: having more trees makes these sites more important to people. Moreover, the observed changes in tree planting attitudes were both significant and meaningful, reflecting results from other studies that report changes in tree attitudes after urban tree loss or removal (Heimlich et al., 2008; Hunter, 2011). However, while such changes can be immediate and generally positive (i.e. people preferring more trees in a site; more people wanting to plant more trees), it is unclear whether these changes will be sustained long term, and these changes may be context dependent.Ordóñez et al. 2023.
In contrast to the park experiment, the study found minimal ecological or social changes when fewer and smaller trees were removed from a residential street site. Only ten younger eucalyptus trees were cut down at this location compared to thirty large mature elms in the park. Surveys before and after the street tree removal detected no significant differences in bird diversity or abundance, possum sightings, invertebrate predation, or people’s ratings of importance and attitudes. The researchers conclude that the effects of losing urban trees may depend on tree size, age, abundance and overall biomass removed.
The study shows that tree planting isn’t a simple matter of adding or subtracting amenities in an area. There appears to be a threshold where removing relatively few small trees does not impact wildlife populations or human perceptions. But when many large, old trees are lost, the reductions in habitat resources and people’s connections to the space are measurable. So, mature tree removals have clearer social-ecological consequences. The authors conclude:
Urban tree removal experiments provide an opportunity to quantify the changes in people and nature after loss of social–ecological function in urban nature sites. People and nature responses to urban tree removal are very likely to be contextual. Just as removing urban trees sometimes does not result in measurable biodiversity or human changes, adding urban trees may not automatically result in more ecological and social benefits. This study provides an important step in bringing people and nature impacts of tree removal into conceptual understandings of impact/changes, with important implications for planning and management practice.Ordóñez et al. 2023.
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Ordóñez, C., Threlfall, C.G., Kendal, D., Baumann, J., Sonkkila, C., Hochuli, D.F., van der Ree, R., Fuller, R.A., Davern, M., Herzog, K., English, A. and Livesley, S.J. (2023) “Quantifying the importance of urban trees to people and nature through tree removal experiments,” People and Nature, 5(4), pp. 1316–1335. Available at: https://doi.org/10.1002/pan3.10509.