Few participants provided comments on the decision support framework, as the key focus of the exercise was to rate a list of indicators. Those that did comment questioned the terminology. Definitions of words such as: Goals, objectives, and values, have different implications within different fields of study. The discipline of Forestry often uses “criteria and indicators” to develop and measure scenarios. Social science research often discusses “values”. Following the feedback, the decision support framework was modified slightly.
3.2. Comments
Participant comments are presented in the following section. They are presented by indicator in the order that all participants ranked the indicators. For each indicator, the rationale statement sent to participants is provided, as well as the revised statement following the Delphi, to give a better understanding of each indicator.
3.2.1. Urban Tree Diversity
Rationale statement provided in Round One: A diverse urban forest increases the ability of the forest to withstand change. Trees should be of diverse ages, species, genera, and families in order to ensure the forest can adapt to future climate change scenarios.
Revised rationale statement following Delphi: A diverse urban forest increases the ability of the forest to withstand change, which is of key importance to the long-term stability of the forest. Diversity is also influential on psycho-social outcomes of urban forests. Trees should be of diverse sizes, ages, species, genera, and families in order to ensure the forest can adapt to future climate change scenarios. Public opinion about desirable tree sizes/types/forms can help inform the creation of a diverse urban forest. This opinion will vary between communities.
In the participant comments, “urban tree diversity” was connected to climate change adaptation, ecosystem service provision, long term planning, decision making, psycho-social outcomes, cultural values, forest stability, and forest resilience. Participants commented that this was a useful indicator linked to many different aspects of urban forest planning. Good diversity is important to long term stability, resiliency, and is an important base for a range of ecosystem services. One participant noted that it “should be balanced with species selection for cohesive and consistent streetscapes” (Participant One). Another noted that companion plants should be included in the diversity discussion.
3.2.2. Physical Access to Nature
Rationale statement provided in Round One: Access to nature has been tied to increased recreation, particularly walking, which is beneficial to both physical and mental health of residents. It could also promote more sustainable commuting, as residents are more likely to walk, jog, or cycle to work along aesthetically pleasing routes. Ensuring equal access to nature for all residents within a community promotes greater equality.
Revised rationale statement following Delphi: Access to nature has been tied to increased recreation, particularly walking, which is beneficial to both physical and mental health of residents. “Play in nature” is very important for people to gain connection to nature and urban forests. It could also promote more sustainable commuting, as residents are more likely to walk, jog, or cycle to work along aesthetically pleasing routes. Ensuring equal access to nature for all residents within a community promotes greater equality. Urban forests should also provide a diversity of potential uses.
Participants commented that “physical access to nature” was an important indicator linking urban forests to human health. They noted that this indicator could start to address issues such as equitable distribution of green spaces and improved population health. Safety, structure, size, and accessibility of greenspace were noted as important considerations when using physical access as a measure of urban forest success. It was also noted that the type of community (dense urban vs. sprawling suburban) could change the way this indicator was measured.
3.2.3. Canopy Cover
Rationale statement provided in Round One: Canopy cover is a very common metric used to evaluate a city’s urban forest. It is relatively easy to measure and communicate with the general public.
Revised rationale statement following Delphi: “Canopy cover” is a very common metric used to evaluate a city’s urban forest. It is relatively easy to measure and communicate with the general public and a good starting point for quantifying a community’s urban forest. “Canopy volume” estimates total leaf area of a tree’s canopy, which provides more information about a tree’s overall ecosystem service provision. Communities with sufficient means are encouraged to measure “canopy volume”, as well as “canopy cover”.
“Canopy cover” is an important indicator that is becoming commonly used. It was highly rated by both academics and practitioners. The ease of using and communicating this indicator was noted by participants as reasons to continue using “canopy cover” as an urban forest indicator. Another was the ability to communicate long-term trends using “canopy cover”. One participant noted the use of “canopy cover” in communicating the “borderless nature of the urban forest to the public and decision-makers” (Participant One). Some participants noted that with emerging three-dimensional technologies, such as LiDAR, canopy cover could eventually be measured in the third dimension as canopy volume, which would capture a more robust measurement of the urban forest in a community. One participant noted that “while canopy cover is an important measure, it does not tell as complete a story as canopy volume does in terms of the overall ecosystem services that trees provide” (Participant Two).
3.2.4. Stormwater Control
Rationale statement provided in Round One: Trees filter and infiltrate storm water, cleaning and moderating the amount of water running into engineered systems. If designed and planned with this function in mind, urban forests can provide both cost savings and enhanced environmental benefits for urban areas.
The rationale statement was not revised, as participants generally agreed with the statement provided.
“Stormwater control” was rated as an important indicator for measuring urban forests. Participants commented that this indicator was “widely recognized as one of the most important services provided by the urban forest” (Participant Three). Another mentioned that it was “an extremely powerful tool for raising support for green infrastructure” (Participant One). There was disagreement about the ease of measuring this indicator, though many participants noted that tools, such as iTree measure the stormwater control benefits of urban forests.
3.2.5. Visual Access to Nature
Rationale statement provided in Round One: There is growing evidence that access to nature, even when viewed through a window, is beneficial to well being. Increasing visual access to nature for community residents could work towards lowering stress levels and improving mental wellness.
The rationale statement was not revised, as participants generally agreed with the statement provided.
Participants commented that “visual access to nature” was an important indicator for connecting populations to nature. It was ranked highly by most participants, but one participant gave it a low importance rating on the scale. There was concern that the actual content of the view, and what constitutes “natural” would be difficult to define. There was consensus that natural should include trees, not just anything that is green, because trees “give us a vital third dimension (height) to our experience and interaction with them” (Participant Four). This indicator had the most comments and suggestions about possible measurement techniques.
3.2.6. Habitat Provision
Rationale statement provided in Round One: Urban forests can help protect biodiversity and provide habitat for urban flora and fauna. Different types of urban forest provide different amounts and quality of habitat. For example, a naturalized park would likely support more urban nature than a concrete planter holding a non-native species.
Revised rationale statement following Delphi: Urban forests can help protect biodiversity and provide habitat for urban flora and fauna. Quantity and quality of habitat varies in urban forests. For example, a large park with diverse trees and understory plantings would likely support more urban nature than a concrete planter holding a non-native species.
While some participants noted this was an important indicator, most were concerned about the difficulty of defining “habitat” and measuring success. One participant noted that, “it is a mistake to concentrate just on native species, particularly in the light of the pests and diseases issue and the changes that have already contributed to the urban heat island—changes that are only going to get worse. We need a resilient urban forest, comprising many species, as they have many roles to fulfill as well as ‘nature’” (Participant Five). Other participants also linked this indicator to the “urban forest diversity” indicator.
3.2.7. Air Quality Improvement
Rationale statement provided in Round One: Research shows that the presence of trees is generally beneficial for the air quality and human health in an area. Trees absorb a variety of air pollutants in varying amounts, depending on a number of characteristics such as species, age, location, tree health, etc.
The rationale statement was not revised because this indicator was dropped from the list after the first round.
There was less consensus on the merits of measuring air quality improvement benefits of urban forests. Some participants noted that, “the air pollution reduction benefits of urban trees are under debate” (Participant Three). It was agreed that poor urban air quality is an important health concern, but that measuring the urban forests’ contribution to this was difficult and would likely be relatively small. A few participants noted that pollen and VOC production would have to be taken into account when measuring a forest’s net air quality improvement. It was also noted that this concept was not well understood or appreciated by the general public.
3.2.8. Available Growing Space
Rationale statement provided in Round One: The amount of available growing space indicates the potential of a community to increase and maintain their urban forest. Without space and suitable soil, the urban forest will be difficult to expand and manage.
Revised rationale statement following Delphi: The amount of available growing space indicates the ability of a community to increase and maintain their urban forest. Without sufficient soil volume and quality, the urban forest will be difficult to expand and manage. Focusing efforts on providing adequate space for trees, and planting the correct tree in the available space, will result in substantially reduced costs associated with maintenance (pruning) and infrastructure damage caused by trunk buttress flare and root expansion. The challenge is for communities to prioritize and plan for adequate growing space and soil volumes well in advance.
“Available growing space” was connected to tree health, tree canopy size, planning, and permeability. Participants commented that “space” should be measured in three dimensions, because adequate soil volume was of key importance to forest health. Along with property value benefits, this indicator generated the most comments, with every participant writing an opinion on the challenges in using this indicator. Many noted that it was an important indicator or priority during development of a new project or area, and that is was often overlooked. Participants also noted the need to consider public versus private landownership when looking at “available growing space”.
3.2.9. Greenhouse Gas Sequestration and Storage
Rationale statement provided in Round One: Greenhouse gas sequestration and storage measures the amount of carbon dioxide absorbed and stored by trees and within the soil of urban forests. Carbon dioxide is the most abundant greenhouse gas derived from fossil fuels. The amount of carbon absorbed and stored reflects the contribution of urban forests to mitigating climate change. This indicator proposes bundling the two values to convey the full impact of trees on carbon mitigation.
The rationale statement was not revised, as participants generally agreed with the statement provided.
While climate change was acknowledged as an important concern, there was less consensus about the amount of carbon sequestered and stored by urban forests, and the value urban citizens place on this issue. Some participants suggested that other greenhouse gases, such as ozone and methane, be included. Others commented that inclusion of other components, such as urban soils, was important. One participant suggested that “the only reason to make carbon calculations might be to choose species and spacings wisely, and to convey to the public that trees are exceptional agents of climate-change mitigation” (Participant Four). Another participant noted that the indicator “can be a part of a good case for advocacy for funding urban forest initiatives” (Participant Six).
3.2.10. Energy Conservation
Rationale statement provided in Round One: Energy conservation measures the contribution of the urban forest to reducing a community’s energy use. This could be a reduction in building energy use through shading during hot summers.
The rationale statement was not revised because this indicator was dropped from the list after the first round.
Participants appreciated the energy conservation benefits of urban forests, but there was debate about how the scale of measuring this indicator might fit with the other urban forest indicators. Interventions and detailed measurements would make more sense on a site-specific scale, not at the scale of the entire urban forest. As one participant noted, “while I rate it low, it’s possible that the public would really appreciate this as an indicator. It’s one of the few indicators where the individual can feel they’re making a measurable difference” (Participant Seven). A few different participants noted this personal benefit. It was also noted that the type of energy savings would differ globally, and that design opportunities would be regionally based.
3.2.11. Property Value Benefits
Rationale statement provided in Round One: Trees can contribute to an individual’s economic well-being if they increase property value. Economic indicators such as this can be easy to communicate with residents.
The rationale statement was not revised because this indicator was dropped from the list after the first round.
Property value was included as an indicator for two reasons. The first is that it provided a clear economic indicator missing from the list. The second reason is to acknowledge the history of research surrounding urban forests and property value [
1]. While difficult to rationalize as an enabling indicator, it could provide some guidance to designers about tree planting locations to promote more equitable distribution of urban forest resources.
“Property value benefits” was another indicator where scale was an issue noted by participants. At a finer scale, issues, such as tree maintenance, hazard trees, and other undesirable characteristics, would be important considerations. Many pointed to ethical concerns about an indicator that favors property owners, and single-family residences specifically. As one participant pointed out “higher prices mean that certain parts of the population are ‘frozen out’ and have less access to the urban forest” (Participant Three). Another pointed out that those living in high-rise towers would see little property value benefits in adjacent urban forests.
3.2.12. Tree Risk
Rationale statement provided in Round One: Often residents’ concerns about urban forests stem from fears of potential damage to people, structures or utilities due to tree or limb fall during storms or from disease. Urban forests with lower risk due to healthy trees planted in appropriate locations might be more compatible with community members. Tree age by species should be considered when factoring risk for trees.
The rationale statement was not revised because this indicator was dropped from the list after the first round.
“Tree risk” was ranked as the least important indicator in the first round. While participants noted that tree risk can be a large part of the urban forestry discourse, and was important from a management perspective, they pointed out that perception of risk and actual risk were often at odds. One participant pointed out that it was time consuming to measure, and “would be more trouble and cost than it would be worth” (Participant Eight). One participant argued that it is equally important to communicate the negative side of urban forestry, so the “tree risk” indicator helps achieve this.