URBAN FORESTRY: PUGWASH REPORT TO CANADA’S MINISTER OF ENVIRONMENT

By Metta Spencer, 29 June 2023
Speakers: Hashem Akbari, Bill Bhaneja, Robin Collins, Eric Davies, Bjorn Embren, Joyce Hostyn, John Liu, Peter Meincke, Lorien Nesbitt, David Price, Heather Schibli, Stephen Sheppard, John Stone.
        Note: This is not a research report in same sense as a journal article. It is a report about the conversations among a number of experts whose ideas were occasionally incompatible. In such cases, this reporter did not attempt to decide who was right and therefore whose views should be advanced. Instead, I simply presented my most faithful summary of what was said, without appraising who was right or wrong. All disagreements expressed by the participants when later reviewing this report are represented in footnotes. Fortunately, the panelists reached generally compatible conclusions, enabling me to propose a recommendation at the end to which they all evidently concur.
 
Biodiversity and Community
        The Pugwash Group joined with Project Save the world in a series of recorded video discussions with various forestry experts. Our goal was to review the possible benefits and disadvantages of increasing the tree canopies of Canadian urban areas.
        Early in our discussions we heard John Liu describe his own career, beginning as a filmmaker documenting the restoration of the degraded loess plateau in China through the organized efforts of local communities. He shared his insights about how collective work on a reforestation project can influence ecological evolution^[1] as well as build a greater sense of community among the inhabitants.
        In evolutionary succession,^[2] says Liu, there is more biodiversity, more biomass, more accumulated organic matter. This continually renews the oxygenated atmosphere, and the freshwater system and the soil fertility. If you take something out of this functional system, it seeks equilibrium, but at a lower rate. And if you continuously do that, then you’ve reversed evolutionary succession. You lose the regulation of temperature, the hydrological cycle and the climate.^[3]
        But Liu filmed the restoration of fertility by million or so people working with their hands and shovels in China. This experience has prompted him to create ecological camps for people around the world, which are self-organizing and self-governing.
        Liu founded the first camp in the US in response to the devastating fire in Paradise, California, which killed 85 people. He shared his experience of restoring degraded landscapes in China and Africa, which made the demoralized local inhabitants happier and more satisfied. Their restoration work improved the resilience of the landscapes while uplifting their spirits. This led Liu to create similar camps in other countries, including Somalia and Syria, which address food insecurity.^[4]
        Liu’s first urban camp, “The Birdhouse,” was in Hollywood, where fruit trees come into fruit at the same time.^[5] The members gathered the fruit from the backyards of movie stars and took it to a women’s shelter downtown where people were eating more potato chips than fresh food. Next the group concentrated on recycling gray water. Today it’s 10 degrees cooler in the Birdhouse than in other parts of Los Angeles.^[6]
        Some of the panelists questioned the effectiveness of urban trees in reducing carbon emissions. Liu says that biodiversity and organic soil material augment carbon sequestration. He emphasizes that ecosystem disruptions go beyond carbon disequilibrium, with the moisture content of soils also being crucial. Liu creates multi-dimensional symbiotic systems. Trees sequester carbon, but an additional question is whether moisture is retained in the soil as it should be. Soil is the second largest sink of carbon, and the first sink is oceans.[7]
        With a billion people added to the planet every 12 years[8], humans are destroying the ecosystem. Liu admonishes us to shift our purposes from going shopping to restoring ecological function. He says it is possible to create dark, fertile soils in 17 days with human consciousness about how these systems work[9]. But if people are ignorant of such things, they won’t fix the problem.

Where Trees Belong and Don’t Belong
        There are suitable and unsuitable locations for trees, and every project hoping to increase the total number of trees on the planet should be careful about where they are to be planted. In 2019 a group of scientists at a Swiss University, ETH-Zurich, published the results of their study, which sought to estimate how many trees there are on the planet and how many more could be accommodated to reduce global warming. Tom Crowther’s project reported that there are now three trillion trees and that there is room for one trillion more – an addition that would greatly improve the global warming problem.[10]
        The researchers even published a map showing where it is reasonable to expect to locate the additional trees. They subtracted urban areas, existing forests, and farms from the potential land, on the theory that not many trees can, or should, be planted in those places. What land was left for future forestry included a considerable portion of the Arctic.[11] If they were just predicting, rather than promoting, such locations for growth, they might be considered correct, but a reader would have assumed[12] that they were recommending in the report that the Arctic be planted with more trees.
        That would be most unfortunate for, on balance, Arctic trees have a net-warming effect on the soil, speeding up the melting of the permafrost.[13] Nevertheless, it is true that the treeline is moving northward in this age of global warming. A British writer, Ben Rawlence, published a book, Treeline, in which he traveled through seven of the Arctic countries,[14] observing the effects of the increasing forests. He had already been a guest on Project Save the World’s forum before the Pugwash series began. (See https://tosavetheworld.ca/episode-422-arctic-trees). Although the current report is about cities, not Arctic wilderness, it is important to point out that trees are not advantageous in the far north and should not be encouraged to grow there. The better question is how to stop them from spreading.[15]
        More relevant to our Pugwash report is the regrettable fact that the Crowther group omitted farmland and urban areas from their projected location of new forests. This too is a mistake.[16] Many trees could be grown around the edges of crop fields and some pastures and crops can even benefit from sharing space with trees. Also, trees can be planted along country roads., where it is easier to maintain them than in remote locations. [17]
        Planting trees should certainly be encouraged in cities and town, as all our guest speakers agreed. For twenty years or more, these new trees will not have much effect on global warming by removing carbon from the atmosphere, but they will cool the cities in other ways and will confer other amenities on city dwellers. The present report will emphasize those other beneficial effects of urban forestry.[18]
        Trees can have a better life expectancy if planted where there are human beings to water and maintain them.[19] In California, when they are harvesting an area, every tree removed is replaced with about 10 seedlings. Each seedling costs only a few cents to plant,[20] and if nine out of 10 die but only one grows to the maturity, they is enough to sustain that forest indefinitely.
        So far, Canada lacks an effective system for integrating urban forest plans and community engagement. A better-engaged community would promote such solutions as de-paving yards [21]and reclaiming street spaces.
        With the impending advent of electric taxis, there will be little need for parking spaces, which can then be used for tree planting.[22] These taxis will be much less expensive than maintaining private vehicles, and they do not park at all but simply pick up and drop off passengers before moving on. Within a few years the streets will be crowded but almost all parking spots will be empty.[23] Governments should already be preparing for these future changes and enacting regulations requiring that the vast parking spots be converted into gardens with trees.[24]
        Some space taken by parallel parking could be utilized for planting trees, especially where there are bulge-outs for traffic calming. Although larger trees are more beneficial in the long run, it’s also possible to grow small or medium-sized trees in small spots along the street.
        It is essential to choose the species of tree carefully when planting in public spaces to minimize the cost of maintaining the trees, which are inevitably far more expensive than trees in faraway forests. Drought-resistant trees are the most suitable for urban planting. Some cities are developing guidelines for selecting tree species that will have a long lifespan and high adaptability.
        The cooling effect of trees is primarily due to transpiration – water circulation[25] – while the shade from tree canopies can lower the surface temperature of the soil. The Canadian West Coast’s humid climate differs from the drier conditions in, say, Alberta. Thus, the role of tree cover should always be considered in relation to the city’s climate and conditions.[26]

Trees and Human Health
        Increasing a neighborhood’s tree cover can potentially decrease the mortality rate in a neighborhood. Numerous scientific studies have investigated the relationship between tree cover and human well-being, including mortality rates. There are four probable explanations:

1. Air Quality Improvement: Trees help mitigate air pollution by absorbing pollutants such as carbon dioxide, ozone, nitrogen dioxide, and particulate matter. By reducing the concentration of these pollutants, tree cover can improve air quality, which in turn can positively affect respiratory health and reduce mortality rates associated with respiratory diseases.
2. Temperature Regulation: Trees provide shade and help cool the surrounding environment through evapotranspiration, which is the process of water evaporation from leaves. By lowering ambient temperatures, tree cover can mitigate the impact of heatwaves and reduce mortality rates related to heat-related illnesses, particularly among vulnerable populations.
3. Mental Health and Stress Reduction: Access to nature, including green spaces with trees, has been linked to improved mental health outcomes. Exposure to natural environments has been shown to reduce stress, anxiety, and depression, which indirectly contributes to overall well-being and potentially influences mortality rates.
4. Physical Activity and Social Cohesion: Tree-lined streets and green spaces encourage physical activity and outdoor recreational opportunities, such as walking, jogging, or cycling. Engaging in regular physical activity promotes cardiovascular health, reduces the risk of chronic diseases, and positively impacts mortality rates. Additionally, green spaces can serve as gathering places, fostering social connections and community cohesion, which can contribute to overall well-being.

 
Costs and Benefits of Urban Trees
        Yet one hears frequent objections to tree planting, complaints that often stem from poor choices in planting sites and poor tree management. Ideally the community is involved in planting and caring for the trees where they live, particularly in low-income areas.
        In a Toronto initiative, children collect and grow tree seeds in cans. But urban trees struggle because urban soil conditions often lack necessary nutrients. Developers often remove the original, nutrient-rich soil, replacing it with sand.[27] The mortality rate for newly planted trees in such conditions can be up to 50% in the first 10-15 years.
        Western red cedar trees are notably failing, possibly due to drought or loss of canopy space. ‘Gator bags’ are designed to provide long-duration watering for newly planted trees. Placed around the base of trees, they gradually release water to prevent excessive runoff.[28] But urban trees face many challenges, including leaf blowers, which pollute and remove essential nutrients and moisture retention. It is better to let the leaves stay in place where they fall, so they can decompose naturally and improve the soil.[29]
        David Price suggests converting entire city blocks into small urban forests to provide a self-contained ecosystem that supports wildlife,[30] absorbs water, and allows leaves to decompose naturally, benefiting the soil. Such clusters of trees can cool down the city and provide carbon benefits due to transpiration.
        If run-down areas are converted into small urban forests, economies of scale could reduce the cost per tree. Redevelopers often ignore the tree protection bylaws. Citizens need to be educated about the benefits of trees, including reduced energy bills and improved health. Old urban trees also need to be maintained as they provide a buffer before young trees can develop a full ecosystem. Any CO2 that is sequestered will eventually be released as the tree decays. This reminds us of the need to protect old-growth trees, whether in urban or rural forests.[31]
      The cost of maintaining a tree in an urban environment is 100 times greater than in wilderness, and therefore, the promotion of trees should be based on overall quality of living in an urban setting. Some trees absorb ozone.[32] But trees need constant maintenance. There are issues with fire. Tree roots can damage the foundation of the buildings, and trees have limited lifetime, after which they have to be removed.[33] Each area has different sets of costs. The transpiration of trees cools the area wonderfully, especially in dry climates, and shade trees even reduce the cost of air conditioning in the adjacent buildings. The inhabitants can have an air conditioner but use it only a couple of days per year if they have shade trees.[34]
      In a forest, when a tree is harvested it would bring in close to $2000 to $3,000 per tree. In an urban setting, harvesting the same tree is worth zero, because it would take $10,000 for particular equipment to come in to safely remove it.[35] So, harvesting the trees in most urban environments would not be economical. Instead, the trees are desirable for their effect on the overall quality of living. The urban benefits can include the mitigation of other climate change risks such as flooding, as well as carbon sequestration, cooling, and promoting biodiversity.[36]
Private and Public Trees
        There are three domains of the urban forest to consider: parks, streetscapes, and private yards. Each has different potentials and challenges in terms of contributing to canopy cover. The optimal level of canopy is around 40% to achieve neighborhood cooling effects.[37] It is more productive now to focus on private spaces and the promotion of tree planting among residents.
    In a typical city, about 75% of the land is owned by private citizens, who are responsible for maintaining the trees. The other 25% of land is owned by the city government, which is financed by the same people. So urban forestry has to be providing rather immediate or mid-term solutions to the needs of the people. Nevertheless, citizens generally seem reluctant to increase tree coverage in cities; they don’t understand the benefits that will accrue from it.[38]
Heat Islands
        Heat islands are cities that have higher temperatures than the surrounding suburban areas.[39] Urban areas, with their dark and impermeable surfaces, absorb more of the sun’s energy, leading to increased temperatures. The cumulative effect of larger areas of dark surfaces[40] can raise temperatures by a few degrees Celsius. Reducing urban heat is crucial not only for the well-being of city residents but also for mitigating climate change impacts.
        Hashem Akbari notes that the lack of vegetation in cities and their darker surfaces are the two main causes of heat islands. Trees can reduce community temperature by shading buildings,[41] transpiring, and absorbing dust particles. They also provide shade for pedestrians, clean the air, and absorb ozone.
        Additionally, trees affect wind patterns, making evergreen trees particularly effective in reducing heating energy during the winter.[42] However, there are costs, including root damage to building foundations and the need for eventual removal. The decision to plant trees should be made on a case-by-case basis, taking into account the benefits and costs in each specific environment.[43]
        Some researchers promote an increase of 50% in urban canopy, which currently covers 24% of urban landscapes. The potential benefits of increased tree cover include a reduction in electricity use for cooling and heating, resulting in reduced carbon dioxide emissions. However, trees in certain areas may actually hinder cooling due to the albedo effect.[44]
        There are various ways to cool cities, including changing the color and permeability of surfaces. Dark, impermeable surfaces can be transformed into lighter-colored and more permeable alternatives, such as light-colored roofs and pavements. The use of energy is also relevant, making a transition to more energy-efficient solutions, such as electric vehicles, important.
        Trees provide additional amenities beyond the cooling effect, making direct comparisons with other measures challenging. The placement of trees is also discussed, noting their interactions with building energy consumption through shading, evapotranspiration, and windbreak effects.
        Collins asked Akbari to compare the importance of insulation and heat pumps in building efficiency and energy consumption. Akbari explained that insulation slows down the escape of heat from inside a building during winter, reducing heating energy consumption. However, during summer, excessive insulation can hinder the natural escape of heat, requiring the use of air conditioning. He emphasizes the need to optimize insulation based on the annual energy use of the building, which includes both heating and cooling requirements.
        Heat pumps use the reverse process of an air conditioner and can provide more heat energy than the electricity they consume.[45] Akbari recommends using heat pumps instead of resistance heaters for heating buildings, as they are more efficient. However, he notes that the cost of electricity and the initial cost of the heat pump should be considered to justify the switch from resistance heaters to heat pumps. In Canada, where the cost of electricity is relatively low, Akbari considers using heat pumps a favorable option.
        There are several ways of cooling pavements, such as the use of light-colored aggregates in asphalt concrete, chemical-based binders and resins, and porous pavements that allow water infiltration and promote cooler surface temperatures. The selection of cool pavement technologies should be based on local conditions, cost-effectiveness, and the specific requirements of the area.
        Choose the right trees. While there is no perfect tree that stays mature and trouble-free forever, selecting appropriate species is crucial. Removing mature trees can be costly, so consider that too. The selection of trees should involve individual preferences, the improvement of energy efficiency, and the overall aesthetic appeal of urban environments.
        The challenge is to optimize building efficiency through insulation and heat pumps, cool pavements, and careful selection of trees.[46]
Miyawaki Forests
Heather Schibli is one of the guest speakers who specializes in planting Miyawaki forests. This is a system developed by a Japanese botanist to recreate ancient forests with only indigenous trees. These forests are densely grown combinations of native plants that can be created even in small city plots. The Canadian government has pledged to plant two billion trees in the next 10 years. It offers free batches of 50,000 trees to organizations. Schibli obtains batches and divides them up, giving some to any group that intends to create a Miyawaki forest.
        Joyce Hostyn creates Miyawaki forests in Kingston, Ontario, where she has been planting three such forests per year, including one at an addiction treatment center and one near a prison farm. The clients came out and planted trees, and so did the high school students, building a sense of community. Hostyn’s minimum is about 100 square meters, but even smaller ones are possible.
        Miyawaki forests plant trees close together in layers according to their final canopy height so they do not compete much for light. Such trees grow upward more than outward, which gives the impression that they are growing fast, though actually they are not. There are three to five woody stems per square meter, preferably of climax species, such as sugar maple and American beech hemlock, which are slower growing but stronger.[47] They can survive in the shade for the first several decades, while they wait for a gap to open up in the forest canopy.^[48]
The Stockholm Solution
        In contrast to the community-based system of planting that Liu, Hostyn, and Schibli favor, some cities use highly mechanical processes for changing their urban landscaping, specially along city streets. Stockholm, Sweden is a leader in developing a new, highly effective form of urban forestry. Bjorn Embren led Stockholm’s development of an extensive tree planting project. He was inspired by research from the University of Hanover, which found that the best root development for trees occurred in open stone material.
        The Stockholm project used large stones (macadam)[49] to create tree planting beds with high porosity that could support any load, such as buses and trucks. These beds allow for efficient gas exchange and water infiltration, making it ideal for tree growth. To capture rainwater for the trees, they built wells (or cisterns) that collected water from roofs, sidewalks, and streets, reducing pressure on the sewer system. This innovative approach to urban forestry has been adopted by other cities in Scandinavia and has been praised for its benefits in counteracting flooding and promoting healthier tree growth in urban environments.
        Bjorn Embren described his innovative planting method in a forum. He used biochar to enhance soil quality. Initially, the city used a mix of compacted stone and soil to support tree growth, but the introduction of biochar provided exceptional results. Biochar improved the water and nutrient retention capacity of the soil, which allowed trees to grow faster and healthier than ever before.
        The success of this method was partially due to the use of water bags with nutrients that provided a constant supply of both water and nutrients to the trees. It was discovered that the biochar could store excess water and nutrients, ensuring that the trees never experienced shortages.
        In the Bloomberg Philanthropies competition for future cities, this innovative use of biochar earned them second place. They used the prize money to build their first biochar machine, which processed garden waste from Stockholm residents into biochar. This process engaged citizens in the fight against climate change.
        Using biochar helped with stormwater infiltration and soil quality, and cleaned polluted water from roads before it entered nearby lakes,[50] such as Lake Mälaren, which provides drinking water for Stockholm residents. This technique offers multiple environmental benefits and highlights the importance of innovative methods in addressing urban challenges.
        While urban tree planting may not greatly reduce CO2 levels in the atmosphere, it provides valuable services such as air purification and water pollution control.
        The participants discuss the effectiveness of structural soil for tree growth in urban environments. Structural soil is a medium that can be compacted to pavement design and installation requirements while permitting root growth. It is a mixture of gap-graded gravels and soil. It provides an integrated, root penetrable, high strength pavement system that shifts design away from individual tree pits. 
        One panelist remarked that structural soil works well initially, but over time, soil volume may be lost.[51] Bjorn Embren disagreed, claiming that trees in urban areas can survive for a long time without additional nutrients. Structural soil allows for better water infiltration and gas exchange, essential for tree survival in urban settings.
        Structural soil uses larger fractions of material, such as recycled concrete and local materials,[52] to keep the ground open and prevent compaction. In addition, biochar can be added to soil to improve its quality.
        Research on regenerative farming is teaching us that soil should not be disturbed, for this can cause CO2 to escape. However, oxygen must still reach the roots of plants, so it is water infiltration that helps oxygen reach the roots without disturbing the soil.[53] Crushing stones or using soil with biochar can also improve its quality.
        In cities, heavy construction machinery often compacts the soil, making it difficult for trees to grow. Structural soil can help alleviate this issue. Over the past 20 years, Stockholm has planted around 40,000 trees using this method.
        Bjorn uses a mixture of biochar, compost, and stones to create a more resilient and water-absorbing planting bed for trees. He explained that the typical construction for planting beds is one meter deep, with the first 60-80 centimeters being the planting bed. Connecting the planting beds of multiple trees along a block, allows the root systems to travel from one side of the block to the other, thus providing access to water and nutrients. This method also helps to increase the surface area for mycorrhizae, leading to healthier trees.
        The limiting factor for tree growth in Stockholm is usually water availability, but once every 5-10 years, there may be too much water. In such situations, it is important to select tree species that can withstand a lot of water and use biochar and compost in the planting mix.
        Bjorn advocates the use of open stone material mixtures to help protect against flooding in cities. He agrees that involving local communities in tree planting and maintenance could help improve tree survival rates. Making individuals or families responsible for the care of specific trees could lead to better tree care and foster a sense of community.[54]
Recommendations
        This report reflects the discussions carried out among fifteen persons who appeared on eight one-hour-long forums, discussing various aspects of urban forestry. We all agreed from the outset that planting trees is not the most effective way of reducing greenhouse gas from the atmosphere.[55] If new city trees have that effect significantly, it will be about twenty years after they are planted.[56]
        Nevertheless, there are many other benefits to be gained, even in terms of global warming,[57] from increasing the canopy coverage of Canadian cities. The chief effect is that trees can directly cool “heat island” cities with their direct shading and transpiration in the summer and can even reduce the heating and cooling bills when strategically placed near buildings. They reduce flooding and sewer problems, and even reduce human mortality rates. Some urban trees provide fruit and nuts to their neighbors. Moreover, encouraging neighbors to take responsibility for a tree on their street will build community spirit.[58] Especially school children can benefit by collecting tree seeds, growing them in their classrooms, transplanting them in their neighborhood, then tending and watering them.
        However, most of the care of urban forests must be done by professional workers using heavy equipment that can replace pavements and build structures to accommodate large trees without compacting the soil.[59] We can learn much by emulating Stockholm’s innovations, as indeed most of the other Nordic cities have already done.
        Canada is going to plant two billion trees within the next decade.[60] Most of them will be located far from our homes or our farms. But a significant portion of those trees should be planted in urban areas and along country roads, where people can tend them and develop their own emotional and social well-being in the process.
        The panelists all seemed to concur with the proposal that Canada’s ministry of environment[61] spearhead a nation-wide campaign, in partnership with all interested municipalities, to provide equipment, instructional programs, and millions of trees for a grand program of urban forestry over the next ten years.

END NOTES:
[1] David Price writes: What does this mean? In a strictly scientific sense, it would mean a natural process that takes thousands of years. What I think you mean is “development of human interactions that improve long-term ecosystem sustainability”, or words to that effect.
[2] David Price writes: This too is suspect! Maybe the word “evolutionary” is redundant. “Succession” in an ecological context means the process by which an ecosystem changes over time, due to disturbances or natural mortality (e.g., as long-lived shade-tolerant species replace early sun-loving short-lived “pioneer” species).
[3] David Price writes: These are motherhood statements that would never make it in peer-reviewed scientific literature. I won’t say it is complete nonsense, but it is close. 
[4] David Price writes: Rewrite as: “in other countries, where food insecurity is a big problem, such as Somalia and Syria.”
[5] David Price writes: Delete! Change following sentence to “…gathered fruit from trees in the backyards…”
[6] David Price writes: Who says this? How was the temperature measured? It sounds physically impossible – unless the area was 10 F cooler before anything was done of course!
[7] David Price writes: Soil moisture is essential for plant growth. If it doesn’t rain and there is no irrigation, the plants will dry out the soil and eventually die (depending on their adaptation – cacti for example survive a lot longer). It is completely untrue to say “soil is the second largest sink of carbon.” Arguably soils are one of the largest carbon stores, globally, but they are invariably weak C sinks. Of course the size of the store and the strength of the sink vary enormously with location – (i.e., with regional climates). Also debatable whether oceans are number one. Currently, ocean and terrestrial C sinks are approximately equal (about 3 Gt C per year each).  Here are a couple of reasonably reliable sources of info. _https://www.researchgate.net/figure/Global-Carbon-Stocks-in-Vegetation-and-Soil-Carbon-Pools-Down-to-a-Depth-of-3-Meter_fig1_331087537 __https://www.fs.usda.gov/ccrc/topics/global-carbon#:~:text=The%20amount%20of%20carbon%20stored,millions%20of%20years%20(2). __https://en.wikipedia.org/wiki/Carbon_sink__
[8] David Price writes: NOT TRUE!!
[9] David Price writes: It will take a lot more than human consciousness to do anything! The “17 days” also beggars belief. Dr Liu may say these things but a lot of it is nonsense – or he is being misinterpreted!
[10] David Price writes: The phrasing I used in my review of 1 Trillion Trees, was as follows, and I think still the right language: “The authors of the original trillion tree proposal issued three corrections to their study (Bastin et al. 2019), which included a statement that they were wrong to claim “tree restoration is the most effective solution to climate change to date” (Bastin et al. 2020: online). Since then, even Crowther has walked back some of his original arguments (Greenfield 2021). Pearce makes clear at the start of his book that if we want a trillion more trees on our planet—and he believes we should—a large-scale, global project to plant them is entirely unnecessary. Instead, he argues for a primarily naturalized process of tree regeneration…”
[11] Robin Collins writes: We should acknowledge and distinguish Arctic from flavours of the Arctic, including the sub-Arctic, and the differing definitions, which include the tree line, the Arctic circle, etc.
[12] David Price writes: Better to write this as “Some readers might have assumed….”
[13] David Price writes: This is presumably a reference to the albedo effect. It’s not strictly true. First it assumes that ice and summer snow cover would have melted (otherwise trees could not establish, and permafrost melting would be slowed). But in the interim, the dark surfaces exposed, initially devoid of vegetation, will have a lower summer albedo than some vegetation cover. Second, deciduous trees are generally more reflective than dark evergreen conifers. Many researchers have assumed trees, either planted or establishing naturally in the arctic would be conifers – but we know that would not happen naturally for hundreds, or even thousands, of years.
[14] David Price writes: Northward movement of Canada’s “treeline” is not that rapid. There are things happening for sure. But it’s not a crisis. I don’t know the “official” average rate of northward expansion, but I guess it can only be 100 meters per year, at most. The rate of climate change at these latitudes will far exceed the rate at which conifers can spread, and may render the place so inhospitable that they won’t survive anyway. Deciduous broadleaved spp. like aspen, willows, birches will be able to keep up.
[15] In the general scheme of things this is an irrelevant question! Large-scale applications of Round-up, or ignition from helicopters, might work. But we probably don’t want to go there.
[16] Robin Collins writes: I don’t think this is a good way to portray it. Crowther, Bastin et al were looking at something different — possible places in the large scale of things that humans don’t already use. They were not suggesting urban or farming areas couldn’t be enhanced also (look at, and I would refer to, the NCS people, like Drever et al. who do focus on this aspect as MOST IMPORTANT elements for climate impacts, greater than trees they argue. )
[17] David Price writes: And where they are more prone to vandalism and the effects of traffic (though increasing use of EVs will be beneficial!)
[18] Peter Meincke writes: I think you should mention the danger of damage from fallen trees during violent storms. 
[19] David Price writes: While this can be true, the SCALE is entirely different. What can be planted near cities and tended by citizens may be more likely to survive, but the mass planting will take place where humans will not normally spend a lot of time returning to and, given the rates of survival (which are pretty good generally), they don’t need to return. As H Akbari noted, the cost of planting in cities can be massive as compared to boreal etc forestry costs. “Pennies to plant” is very deceptive language.
[20] David Price writes: I’d check this. Cost of raising tree seedlings in Canada is more than “a few cents” and that doesn’t include planting costs. Also seedlings (saplings) for urban locations typically must be bigger, which means they take more time and resources to grow and plant – so exponentially more expensive than seedlings grown for forest planting.
[21] David Price writes: In areas where water shortages are not a concern
[22] David Price writes: That’s a big leap of faith! By “impending” you must mean “over the next 20 to 50 years”?
[23] Peter Meincke writes: I think you should mention the danger of damage from fallen trees during violent storms. Also, I would leave out using land made available by a reduced demand for parking because people will use special taxis instead of their own vehicle. It may well happen but it leaves an opening for dispute at this stage.
[24] David Price writes: Dream on! They’d be much better off supporting a program to triage the resilience of Canada’s forests to climatic injury.
[25] David Price writes: It is EVAPORATION from leaves that cools – due to sensible heat being converted to latent heat of vaporization.
[26] Robin Collins writes: (let’s include this descriptor that David uses here)
[27] David Price writes: Where is the evidence for this statement? It seems to be true that high quality soil is often removed by the developer – who sells it to landscaping supply companies who can then sell it back to the new owners of the buildings (perhaps at other developments). But I’ve never heard of it being replaced by sand. If it gets replaced at all it is likely to be poor quality “fill” collected from other locations. In any case, while this sounds like a conspiracy theory, there are probably benefits in removing the quality topsoil before building starts – as otherwise it would become contaminated and compacted etc. etc.
[28] David Price writes: It ensures the tree has a supply that lasts for more than a couple of days which is all you’d get from conventional watering. But I see your point: it means the water is used more frugally and not wasted watering ground that doesn’t have any roots.
[29] David Price writes: This is a generalization for sure. We have a lot of trees around and on our property and we mow the leaves in the fall, but we cannot let them all lie there or they would saturate and kill the other plants. Mind you, we haven’t done the experiment; we did notice that the leaves unmowed are mostly back in the spring and it ain’t pretty. But wildflower gardens do not equal: let the leaves lie, and they cannot be mowed in those gardens, so they get bagged and removed, or at least lots of them.
[30] David Price writes: Well, I suggested that in areas where buildings were to be demolished over entire city blocks for development reasons, perhaps in underprivileged areas, creating urban “forests” is an option that municipal government might consider. Of course, this would have to be balanced against the need for more housing etc.
[31] David Price writes: Many species selected for use as urban trees will be fast-growing in their early years – which means they will generally have relatively short natural lifespans. Urban old-growth is a real rarity!
[32] David Price writes: Again, a ref would be useful. Not so much about this but whether “absorbing ozone” is harmful to the tree. In general, it is, but some species may be significantly more tolerant than others.
[33] David Price writes: Yes! Generally when or before they die.
[34] David Price writes: More likely they will use A/C a lot more if they have it, but local tree cover will reduce the load and hence lower overall cooling costs.
[35] David Price write: Perhaps in extreme cases, but in general the cost of removal would be much less. And the wood is often cut into firewood billets and can be sold as such (though some maybe extremely valued for use as one of a kind table tops etc.). If tree removal was that expensive, there’d be a lot of dead city trees falling, and people getting sued every year! _I found this link: _https://homestars.com/cost-guides/tree-removal-cost/
[36] David Price writes: Here is an interesting analysis for you: AFA estimated US$57,000 for a single urban tree over 50 years! https://extension.usu.edu/forestry/trees-cities-towns/urban-forestry/what-is-a-tree-worth#:~:text=They%20have%20found%20that%20a,a%20tree%20value%20of%20%2457%2C151
[37] David Price writes: Where did this come from? 100% cover would give the maximum cooling benefit, if there’s enough water, but that is clearly impractical.
[38] David Price writes: More like many people just don’t care!
[39] David Price writes: Better to say “Inner city areas can form heat islands, which are generally warmer than surrounding suburban areas.”
[40] David Price writes: And dry impermeable surfaces.
[41] David Price writes: Shading will only be significant in low-rise residential neighbourhoods – not in downtown areas with lots of highrise buildings.
[42] David Price writes: Highly debatable – and in any case changing wind patterns doesn’t change air temperature – the overall benefits in reducing heat loss from a leaky building are going to be minimal. Much better to spend money on plugging the leaks and adding insulation.
[43] David Price writes: In cold climates the benefits of trees to reduce heating costs will be greatly outweighed by the benefits of better building envelopes. In hot climates, the benefits of trees for cooling could outweigh the benefits (and costs) of better insulation and/or of cooling using A/C. This still requires adequate moisture supplies to keep trees well-watered and alive.
[44] David Price writes: Assuming fossil energy is used to generate power.
[45] David Price writes: No! The process is identical. But the process is used in reverse. Heat pumps will produce more heat than would occur if the electricity was used in a resistive heater, like electric baseboard heaters. (That is why they are called heat pumps!) BUT, if the fluid from which heat is being extracted by the compressor is too cold, the heat pump may not be able to extract sufficient heat for heating the building. (And conversely, if the fluid into which excess heat is being dissipated is too warm, it will not provide sufficient cooling.)
[46] David Price writes: No. Building efficiency can (must?) be “optimized” without cool pavements and trees. Cool pavements and cooling trees are additional possibilities aimed at reducing energy consumption of the already optimized buildings. My point here is that almost invariably (as in I cannot think of a situation where it won’t be true), making buildings energy efficient is the number one priority. After that, using trees and other infrastructure to moderate local climate/weather extremes may be able to further reduce building energy consumption.
[47] Peter Meincke sent this news item as his comment: “Urban trees have gotten a good bit of attention — and funding — lately, from everyone from President Joe Biden to Amazon founder Jeff Bezos. The benefits of trees in cities, including their ability to cool neighborhoods, are well-documented. If anything, the urgency to expand urban green spaces in cities has only grown after a summer plagued by extreme heat, which experts say will likely be part of the new normal as climate change worsens. 
    “But as city leaders work to fulfill their promises to increase tree canopy, certain questions remain. For example, do different types of green spaces provide different cooling benefits? The Natural Areas Conservancy set out last summer to answer to that question. 
    “The study is based on land-temperature data from satellites and air-temperature data from sensors placed on trees in Seattle; Minneapolis-St. Paul; New York; Baltimore; Chicago; Miami; Houston; St. Louis; Indianapolis; Billings, Montana; Austin, Texas; and Tampa-Hillsborough County, Florida. These 12 urban areas are all part of the Forests in Cities network, a national coalition of urban forest professionals. 
    “Forested natural areas — characterized by multiple layers of plants and trees of different ages — were 3 to 9 degrees F lower than the average citywide temperature, depending on forest and city type. The coolest type of forest was conifer, with the only studied example Seattle. Forests with wetter conditions, such as forested wetlands and mangrove forests, were also particularly cool.
    “Landscaped areas, such as bare soil, lawns and street trees, generally had less cooling benefits. In Billings, for example, a forest was over 14 degrees F cooler than a landscaped location at 6 p.m. on Sept. 3, 2022.
    “Healthy forests were generally cooler than degraded forests during the afternoon, with less temperature variation throughout the day.
    “’Understanding the relationship between forest condition and cooling impacts is important,’ the study says. ‘Urban forests face many threats including fragmentation and increased pressures from problem species.’
    “Despite these benefits, the study said urban natural areas are “underfunded and unprotected, leaving them imperiled in cities across the country.” They may be viewed as “weedy” compared with landscaped parks and street trees; cities in the study allocated an average of 4% of park budgets to caring for forests, even though they make up a majority of city parkland.
 
    “We view natural areas as a less well understood but critically important piece of the entire urban forest,” Natural Areas Conservancy Executive Director Sarah Charlop-Powers said Tuesday on the The Brian Lehrer Show. “We view this work as an important puzzle piece in solving the really large and thorny challenge of extreme heat in New York City and in cities across the country.”

• Google’s free tree canopy tool now covers hundreds more cities. Here’s how early adopters are using it. By Ysabelle Kempe • April 13, 2023
• Bezos fund commits $400M for urban green space, with initial $50M for 5 cities By Ysabelle Kempe • July 20, 2023
• ‘Dead trees all over the city’: Where tree-planting efforts go wrong By Ysabelle Kempe • June 27, 2023.

To Meincke, Metta Spencer replied: “I think it fits with the recommendation of David Price, who would like whole city blocks made into forests. Of course, we should do that wherever possible — and also do the street trees and parks. It’s not either-or but and, and, and, and…. I notice that they talk about the density and staggered layers of the canopy as important. That’s the reason I kept harping on the value of Miyawaki forests, which can be very tiny but dense and multi-layered.”
David Price replied, “I think this is interesting but it’s really not surprising and to some extent it is misleading. 
“What the researchers have found (though I admit I am guessing at this point) is that when
there are a lot of trees bunched together (as in a small urban forest or a city block), then,
all other factors being equal, the local temperature depression in the middle of that patch
of trees, due to evaporative cooling, is greater than if the same trees were spaced out over
a much larger area. But if we assume all trees have adequate irrigation and are able to support 
the same total leaf area, then the average temperature depression (over time and expressed
relative to the same larger total ground area covered) would be the same. 

“The basic physics behind this is that latent heat of evaporation (e.g., measured in kilojoules per 
kg of water at a given temperature) cannot be changed: if water is provided to the roots, and
the total leaf area on all the trees is the same, then for a given set of conditions (air temperature,
humidity and solar radiation), the same amount of water will evaporate and the same amount of
heat energy will be converted to water vapour — which is what causes the cooling. (Energy cannot 
be created or destroyed but only transformed.)  

“However, the amount of water evaporated from spaced out trees, per square meter of foliage,
might actually be greater over the short term — due to horizontal heat advection, sometimes 
called “the oasis effect”. Individual trees also don’t benefit from shading by their neighbours so 
they get maximum exposure to solar radiative heating. So long as they are watered, this would 
cause more cooling over the short term. But without that additional watering, the trees would 
become water-deficient more rapidly and then “shut down” and, if not watered or relieved from 
the stress, start to die. 
“So it is likely that there will be some ecological benefits of establishing urban trees in larger extended patches (and Miyawaki forests might be a great example). “A carefully maintained patch would likely provide better and/or more sustained cooling benefits. E.g., mutual shading and sheltering of trees and understory plants would give them greater resilience to extreme heat, and to the effects of pollutants from traffic and other urban sources. So the viability of the tree cover to cool the extended area over longer periods could be greater (I am speculating, but I think it makes sense
“Moreover, the utility of these spaces to the local human population would be greater. One need only think of how people generally enjoy urban park areas, with trees, grass cover and ponds or lakes etc. at least partially because they are generally cooler on hot summer days than the surrounding dry surfaces covered in asphalt or concrete. For sure, landscaped parks won’t be as cool as the middle of an extensive tree-covered area, because they generally don’t have as much leaf area (relative to ground area).