by Danni Sinnett and Max Walters….
Trees provide many benefits to people and wildlife in urban environments. Seen as an opportunity to address climate and ecological emergencies, many cities are developing targets for increasing tree canopy cover and Bristol is no exception. Bristol’s One City Plan includes a target to double tree canopy cover by 2045.
We investigated the feasibility of Bristol’s target and explored different planting scenarios for its achievement. We asked the following questions:
- How many trees would need to be planted between 2018 and 2045 to achieve the Bristol One City Plan target to double canopy cover over this period?
- How does the timing of planting, size of trees when planted and tree mortality affect canopy cover and leaf area index over this period?
In 2018, the Forest of Avon Trust and partners assessed the benefits provided by Bristol’s trees using the i-Tree Eco software. This estimated canopy cover at 11.9%, lower than the 18.6% from the One City Plan. We used i-Tree Forecast to project future urban forest growth under different tree planting scenarios over 27 years. Because iTree Forecast requires an existing i-Tree Eco assessment we needed to model canopy cover growth from 11.9% to 37.2% (i.e. more than doubling) over the 27-year period, which is therefore a more ambitious target.
Three tree stock sizes were used, selected to be representative of the typical diameters of commercial nursery stock:
- Small: equivalent to feathered trees
- Medium: equivalent to light standards
- Large: equivalent to heavy standards.
We calculated the number of small, medium and large trees needed to achieve the target under the following scenarios:
- An equal number of trees planted each year to reflect continuous investment in tree planting
- Intermittent planting every 5 years to reflect large scale projects such as major housing developments that might deliver significant tree planting in bursts
- Planting concentrated between 2018-2027 to test the effectiveness of planting trees in the near term and allowing them to mature
- Planting concentrated between 2036-2045 to reflect, for example, shortages in funding that might necessitate more planting towards the end of the period.
These were compared against a scenario of no new planting over 27 years and planting 10,000 small, medium or large trees per year, representative of current planting levels. The impact of tree felling was not considered but two annual mortality rates were used: 0.5% chosen to represent the best-case scenario and 3% to represent typical mortality rates in urban areas.
We estimated that:
- Assuming a 0.5% annual mortality rate, planting no additional trees or planting 10,000 small, medium or large stock trees per year will not achieve the 37.2% canopy cover target but would achieve a doubling of canopy cover from a baseline of 11.9%.
- Assuming a more realistic 3% annual mortality rate, planting no trees results in the canopy cover decreasing to 11.4%, whilst planting 10,000 trees each year results in an increased canopy cover of around 4%.
- Late planting between 2036 and 2045 achieves the desired canopy cover but requires the greatest number of trees to be planted and provides between 12% and 75% less leaf area than the other scenarios, which means less air pollution removal, carbon storage and sequestration.
- Early planting between 2018 and 2027 achieves the greatest canopy cover for the smallest total number of trees, therefore requiring fewer planting locations. It also allows trees to mature providing the greatest benefits in terms of air pollutant removal, carbon storage and sequestration. However, it would require 35,000 heavy standards to be planted each year at 0.5% mortality increasing to 104,000 with 3% mortality.
- Planting the same number of trees each year for 27 years requires more total trees but would result in a more stable population over time, ensuring that tree replacement strategies are more manageable. It would require between 18,000 and 44,000 heavy standards to be planted each year.
The greater the annual mortality rate, the more trees will need to be planted to achieve the canopy cover target. Given that Bristol’s current planting is around 10,000 trees per year, planting at least 35,000 trees per year is ambitious, especially given that higher mortality rates may require a greater number to be planted, increasing costs. Whereas adopting a continuous planting regime of 18,000 trees may be more feasible. This demonstrates the importance of good stewardship and long-term maintenance to allow urban trees to reach full maturity and provide the greatest benefits.
Existing tree cover is not evenly distributed across the city, a study by Bristol City Council (2011) found that Greater Bedminster and Ashley, Easton and Lawrence Hill have the lowest levels of canopy cover in Bristol at 11% and 9% respectively. These neighbourhoods have also been identified to be some of the most deprived within the city. Although city wide targets are useful tree planting should be targeted to address inequalities across the city.
Summary of results for large tree stock equivalent to heavy standards
|Approach||Annual mortality||Canopy cover %||Total number of trees||Sequestrated Carbon/ton||Total value of air pollutant uptake|
|Current planting of 10,000 trees per year||0.5%||31.4||723,186||349,003.2||£26,933,674|
|Early planting 2018-2027||0.5%||37.3||787,282||384,941.7||£33,201,379|
|Late planting 2036-2045s||0.5%||37.3||1,154,472||339,476.5||£25,244,798|
You can read the full open access paper here: https://www.sciencedirect.com/science/article/pii/S161886672100323X.