There are many reasons why green plants, and especially trees, are important in the city.
Explore the expandable sections below to find out more! |
Trees are amazing! Every day, a 40 foot tree takes in 50 gallons of dissolved nutrients from the soil, raises it to its topmost leaves, converts it into 10 pounds of carbohydrates, and releases about 60 cubic feet of pure oxygen into the air! |
1. Trees provide oxygen
As they use sunlight to make sugars to meet their own energy requirements, plant leaves absorb sunlight and take in carbon dioxide (CO2) through tiny pores in their leaves, called stomata. In a complicated series of reactions (photosynthesis), tiny organelles within the leaf cells (chloroplasts) use carbon dioxide, water (H2O) and the energy to manufacture sugars (C6 H12 O6), and release oxygen (O2). The plant then uses the sugars for the energy and building blocks to make the materials it needs for growth and reproduction. Some of the oxygen is used in those processes (including in respiration, which breaks down the sugars to provide energy in a form that can be used by the cells and releases some CO2 back into the air). However, most of the oxygen escapes from the leaves through the stomata and into the air. (This is why it is said plants give out O2 at night: even in the absence of sunlight the plant still needs energy, so respiration continues, although photosynthesis is stopped.)
Resources
Resources
- A short description of the chemical processes of photosynthesis, with links to more information: https://www.nationalgeographic.org/encyclopedia/photosynthesis/
- A short description of the process of photosynthesis, with explanation of terms: https://www.khanacademy.org/science/high-school-biology/hs-energy-andtransport/hs-photosynthesis/a/hs-photosynthesis-review
2. Trees combat the climate emergency
Carbon dioxide is a gas that enters the atmosphere when people burn fossil fuels to power cars and factories. Too much carbon dioxide is a problem because it is one of the gases that cause the greenhouse effect. The gas acts like a blanket in the atmosphere around the earth, trapping heat close to the surface. This extra heat affects the overall climate on Earth, raising the global temperature and causing other changes to natural systems. This triggers disasters like droughts, fires, and floods that negatively impact life - including people! The disasters are getting worse, so we are calling the situation a climate emergency. Emergencies need to be addressed quickly! One way to address the climate emergency is by planting trees. As they photosynthesize, trees help to control the amount of CO2 in the atmosphere by absorbing it and storing it in their wood. Less CO2 in the atmosphere means a cooler climate - a good thing for us and everything else that calls this planet home.
Resources
https://www.acs.org/content/acs/en/climatescience/climatesciencenarratives.html
- A brief description of the mechanism of the greenhouse effect:https://www.britannica.com/science/greenhouse-effect
- More detailed descriptions of the the greenhouse effect:
https://www.acs.org/content/acs/en/climatescience/climatesciencenarratives.html
- How trees can remove carbon from the atmosphere better than human technology: https://www.climatecentral.org/news/scientists-say-nature-is-better-at-carbon-farming-16329
3. Trees purify the air
Trees, especially street trees, help trap dust from the air, on their leaves and in the crevices of their bark.
Trees are also able to absorb through their leaves some of the harmful chemicals that are produced by modern human activity, like sulphur dioxide, nitrogen dioxide and volatile organic compounds. They also trap liquid particles from the air in the crevices in their bark. However they are also harmed by some of those chemicals, especially by ozone (O3), which is produced by cars and (especially electrical) machinery.
Resources
Trees are also able to absorb through their leaves some of the harmful chemicals that are produced by modern human activity, like sulphur dioxide, nitrogen dioxide and volatile organic compounds. They also trap liquid particles from the air in the crevices in their bark. However they are also harmed by some of those chemicals, especially by ozone (O3), which is produced by cars and (especially electrical) machinery.
Resources
- How trees help clean the air: https://edu.rsc.org/feature/how-do-trees-clean-our-air/4010864.article
- Lay-person’s description of a study which looked at tree’s absorption of a particular group of air pollutants called oxygenated volatile organic compounds (oVOCs): https://www.enn.com/articles/41915-air-pollution-control-by-trees
- All about ground level ozone pollution: https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics
- Trees can take up nitrogen directly from the air: https://www.nature.com/articles/news.2008.1046
4. TREES Reduce the "heat island" effect
During both respiration and photosynthesis, trees emit water vapour from the pores in their leaves. The water requires heat (energy) to evaporate, and this is taken from the air. This has the effect of cooling the air. If in sufficient quantity (as in tropical regions and even here in parts of Canada in the summer), the water vapour forms the clouds that usually gather in the afternoon, which give rise to the regular afternoon rain showers that are a feature of some tropical areas.
The shade trees provide also reduces the amount of heat absorbed by a city’s surfaces, making shaded places measurably cooler. This is especially important because the planet is warming as part of the climate emergency.
Resources
The shade trees provide also reduces the amount of heat absorbed by a city’s surfaces, making shaded places measurably cooler. This is especially important because the planet is warming as part of the climate emergency.
Resources
- Cities are heat islands:
https://www.epa.gov/heatislands/learn-about-heat-islands
5. TREES help slow the wind
High-speed air flow increases the rate of evaporation from surfaces, and thus causes a cooling effect by removing heat from both the air and the surfaces. (Heat is required to convert liquid water into vapour.) This is particularly noticeable in winter (when we call it wind chill), but it affects plants by drying the leaves, often faster than they can suck up water from the soil. High speed air flow also dries the surface of the soil, and can thus cause increased soil erosion, as it did on the prairies in the ‘dust bowl’ years.
Even individual street trees help break up the flow, and therefore reduce the speed of wind blowing along a street. A tree canopy forms a ‘floor’ above which the wind flows, so that an area with a number of trees is generally calmer than an open space.
Resources
Even individual street trees help break up the flow, and therefore reduce the speed of wind blowing along a street. A tree canopy forms a ‘floor’ above which the wind flows, so that an area with a number of trees is generally calmer than an open space.
Resources
- How trees mitigate wind effects in cities: https://www.sciencedaily.com/releases/2017/07/170726091513.htm
6. TREES Anchor the soil
Soil erosion is the washing, or blowing, away of soil. It happens when soil is dried to dust, as on the prairies in the dust bowl years, and on slopes as surface water runs downhill, carrying soil particles with it. These particles often finish up in streams, where they cause turbidity (cloudiness), which impedes the ability of many fish and smaller creatures to feed. Eventually the particles settle to the bottom in slower-moving parts of the stream, covering the stones and gravel, which is the spawning area for some fish, and also provides refuge for small creatures and growing surfaces for algae and other organisms.
The tree canopy slows the fall of rain to the ground, reducing the impact of the drops and lessening the resulting breakup of the soil surface. Some of the water is channelled down the branches and trunk more slowly to where it can soak into the soil. Tree roots reinforce the soil, thus helping to hold the soil in place. Where the roots are at the surface, they slow the run-off and provide pockets where the water can pool and seep into the soil. The presence of fallen leaves on the surface also absorbs some of the impact of the raindrops, and further slows the movement of water over the soil by holding some of it in their folds.
Resources
The tree canopy slows the fall of rain to the ground, reducing the impact of the drops and lessening the resulting breakup of the soil surface. Some of the water is channelled down the branches and trunk more slowly to where it can soak into the soil. Tree roots reinforce the soil, thus helping to hold the soil in place. Where the roots are at the surface, they slow the run-off and provide pockets where the water can pool and seep into the soil. The presence of fallen leaves on the surface also absorbs some of the impact of the raindrops, and further slows the movement of water over the soil by holding some of it in their folds.
Resources
- How trees can help prevent soil erosion: https://sciencing.com/how-can-trees-help-prevent-soil-erosion-12620837.html
- The effects of forest management on soil erosion: https://forest.moscowfsl.wsu.edu/smp/docs/docs/Elliot_1-57444-100-0.html
7. TREES HELP maintain the water balance of soil
Perhaps the largest effect of trees on the soil is due to the water absorbed by the tree. The whole of an area under a tree and especially the ‘drip zone’ (where water drips from the edge of the canopy) is filled close to the surface with ‘feeder roots’. Feeder roots are tiny rootlets covered with root hairs (thin-walled cells in the epidermis (skin) of the root which protrude out into the soil). These do the job of absorbing water for the tree (number 1 in diagram above).
The roots take up the enormous amounts of water which the tree transpires (carries to the leaves for photosynthesis). It is this water that then evaporates from the surfaces of the leaves through their stomata (pores), returning water vapour to the air. Under normal conditions, this prevents the soil from becoming waterlogged, but if excessive, can also keep the soil too dry for understorey plants (shrubs, small trees and herbs) to flourish.
The water is moved into the xylem of the root. This is the tissue responsible for carrying water and dissolved nutrients, and consists of dead cells with relatively thick, strong walls which are joined into continuous hollow tubes which extend all the way up the trunk and through the branches and twigs into the leaves. (Xylem is the woody part of the tree trunk (number 2 in diagram). The layer immediately under the protective bark is the phloem, living cells which transport sugars from the leaves for storage in the roots and other parts of the tree, like nuts or fruit.) The mechanism that moves the water is not conclusively proven, but seems to be suction pressure (also known as TATC, Transpiration - Adhesion - Tension - Cohesion), which is driven entirely by the evaporation of water from the leaf surface (number 3 in diagram). This is aided by the narrow width of the individual xylem tubes, and the properties of water molecules themselves, which together allow the water molecules to cohere - stick to one another - and adhere - stick to the walls of the xylem vessels - and so remain connected in a continuous column up the tube. This amazing mechanism allows water to reach the leaves of even the tallest trees.
Resources
The roots take up the enormous amounts of water which the tree transpires (carries to the leaves for photosynthesis). It is this water that then evaporates from the surfaces of the leaves through their stomata (pores), returning water vapour to the air. Under normal conditions, this prevents the soil from becoming waterlogged, but if excessive, can also keep the soil too dry for understorey plants (shrubs, small trees and herbs) to flourish.
The water is moved into the xylem of the root. This is the tissue responsible for carrying water and dissolved nutrients, and consists of dead cells with relatively thick, strong walls which are joined into continuous hollow tubes which extend all the way up the trunk and through the branches and twigs into the leaves. (Xylem is the woody part of the tree trunk (number 2 in diagram). The layer immediately under the protective bark is the phloem, living cells which transport sugars from the leaves for storage in the roots and other parts of the tree, like nuts or fruit.) The mechanism that moves the water is not conclusively proven, but seems to be suction pressure (also known as TATC, Transpiration - Adhesion - Tension - Cohesion), which is driven entirely by the evaporation of water from the leaf surface (number 3 in diagram). This is aided by the narrow width of the individual xylem tubes, and the properties of water molecules themselves, which together allow the water molecules to cohere - stick to one another - and adhere - stick to the walls of the xylem vessels - and so remain connected in a continuous column up the tube. This amazing mechanism allows water to reach the leaves of even the tallest trees.
Resources
- A detailed scientific description of water uptake by plants: https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037/
- Transpiration in plants: https://passel2.unl.edu/view/lesson/c242ac4fbaaf/3
8. TREES HELP MAINTAIN THE fertility OF SOIL
Soil is made up of mineral particles from the rock from which it originated, humus (decayed organic matter) and billions of microscopic organisms - bacteria, fungi and others. The organisms, both microscopic ones and larger invertebrates (like earthworms and millipedes), convert the humus into food for themselves, and in the process, release soluble chemicals into the soil that plants need for their growth and development. (These include the nitrogen, phosphorus and potassium compounds that are found in bags of fertilizer.) By shedding their leaves, trees return the chemicals to the soil to be turned into humus and recycled.
Some trees also harbour, or facilitate the growth of specialized fungi which in turn provide the chemicals it requires to the tree. Sometimes these fungi form great mats of mycorrhiza (threads of fungus) within the soil, and sometimes produce large reproductive structures above (mushrooms, puffballs) or below the soil surface.. Such a fungus grows truffles in the oak/hickory forests of northern Italy and southern France. The fungus absorbs sugars (which it cannot manufacture itself) from the tree rootlet, and in return gives the roots mineral nutrients and water, which a fungus absorbs from the soil more efficiently than the tree. The fungal threads also carry messages from tree to tree, so that an older tree can direct resources to its offspring.
Resources
Some trees also harbour, or facilitate the growth of specialized fungi which in turn provide the chemicals it requires to the tree. Sometimes these fungi form great mats of mycorrhiza (threads of fungus) within the soil, and sometimes produce large reproductive structures above (mushrooms, puffballs) or below the soil surface.. Such a fungus grows truffles in the oak/hickory forests of northern Italy and southern France. The fungus absorbs sugars (which it cannot manufacture itself) from the tree rootlet, and in return gives the roots mineral nutrients and water, which a fungus absorbs from the soil more efficiently than the tree. The fungal threads also carry messages from tree to tree, so that an older tree can direct resources to its offspring.
Resources
- About truffles: https://www.discovermagazine.com/planet-earth/the-biology-of-truffles
- The importance of mycorrhizal fungi in forest health: https://www.cbc.ca/radio/quirks/may-1-lightning-cleans-the-atmosphere-a-142-year-and-counting-experiment-and-more-1.6007496/a-pioneering-forest-researcher-s-memoir-describes-finding-the-mother-tree-1.6007500
9. TREES provide food and shelter for birds and other animals
As part of the base of the food web, trees provide for most of the birds and other animals that live with us in the city. |
A food chain shows the feeding pattern of a small number of organisms. For example: grass → rabbit → fox
The grass plants are the producers, the rabbits are primary consumers, and the fox is a secondary consumer. The total mass of organisms at each trophic level (feeding level), or biomass, is less than at the level below.
The grass plants are the producers, the rabbits are primary consumers, and the fox is a secondary consumer. The total mass of organisms at each trophic level (feeding level), or biomass, is less than at the level below.
But in real ecosystems the relationships among the living creatures are very complex in terms of who eats whom or what. They can be illustrated as layers of a food pyramid, with the numbers of organisms at each level smaller than the one below to reflect the reduced biomass and energy content.
But when mapping the feeding relationships of a larger number of creatures, it is clear that the situation is much more like a web, with some creatures acting simply as herbivores (plant-eaters), some as carnivores (meat-eaters), but many feed on both plant and animal matter and some on decayed material too (omnivores).
Plants are at the base of most terrestrial food webs, and trees are an important part of the food webs in the city. Insects (principally) suck the sap from the trees, or lay their eggs in the bark or wood so that the larval stages can burrow into and feed on the softer, sugar-carrying phloem layer just under the bark. Carpenter ants are among those that burrow deeper into the wood, using it for both food and shelter. Other small creatures burrow within the leaves and feed on the juices inside the cells, or gnaw holes in the leaves. Spiders and other invertebrates feed on these plant-eating creatures; many birds (e.g. the smaller woodpeckers) are part of both the second and third level, feeding on both the smaller animals and plants themselves (seeds, fruit (nuts, berries) and flowers).
Trees also provide shelter for not only the birds, raccoons and squirrels that nest in the branches or in hollows, but also for the millions of tiny invertebrates that shelter in and on them; in bark crevices, on leaves and branches and among the roots.
Resources
https://www.nature.com/scitable/knowledge/library/food-web-concept-and-applications-84077181/
Trees also provide shelter for not only the birds, raccoons and squirrels that nest in the branches or in hollows, but also for the millions of tiny invertebrates that shelter in and on them; in bark crevices, on leaves and branches and among the roots.
Resources
- Food webs and chains: https://www.nationalgeographic.org/encyclopedia/food-web/
- Food webs and chains in more detail:
https://www.nature.com/scitable/knowledge/library/food-web-concept-and-applications-84077181/
10. TREES PROVIDE green space which is beneficial to our health
It has become increasingly clear in recent years that access to forest and other green space is important for human health. In some European countries, Japan, and now in Canada, doctors are issuing ‘forest prescriptions’ which require the patient to spend a certain amount of time in green spaces, to help reduce anxiety, stress and the conditions that are aggravated by those factors, such as heart disease and diabetes. What gives rise to the benefits experienced by spending time in a forest is not entirely clear, but one hypothesis suggests that chemicals given off by trees are at least partly responsible.
Researchers at the University of Toronto Faculty of Forestry found that higher tree cover and diverse species composition was correlated with the academic performance of primary school students in Toronto. If a student went to a school with higher tree cover on its grounds, it was a significant positive predictor of that student's performance, and especially so in schools that faced external challenges to learning. This suggests that planting trees on school grounds improves the mental focus of young students.
Resources
https://www.spph.ubc.ca/parks-big-and-small-needed-for-public-health/
Researchers at the University of Toronto Faculty of Forestry found that higher tree cover and diverse species composition was correlated with the academic performance of primary school students in Toronto. If a student went to a school with higher tree cover on its grounds, it was a significant positive predictor of that student's performance, and especially so in schools that faced external challenges to learning. This suggests that planting trees on school grounds improves the mental focus of young students.
Resources
- Tree cover and species composition effects on academic performance of primary school students article: https://doi.org/10.1371/journal.pone.0193254
- Studies are beginning to tease out how green spaces affect our physiology, mood, focus and creativity:
https://www.spph.ubc.ca/parks-big-and-small-needed-for-public-health/
- Summary of many studies on the benefits of green space and how such spaces might be developed and promoted: https://www.canada.ca/en/public-health/services/reports-publications/health-promotion-chronic-disease-prevention-canada-research-policy-practice/vol-39-no-4-2019/climate-change-health-green-space-co-benefits.html
11. TREES PROVIDE us with many products that improve our lives
Modern city dwellers are most familiar with wood products for building, for furniture and firewood and with nuts and fruit as useful products provided to humans by trees. But Indigenous people used trees for far more; cedar Cupressus spp.), just for one example, provided wood for totem poles, masks, building longhouses and canoes, and for carving paddles, spears and fishing gear; roots were dried and braided for ropes; bark provided an anti-inflammatory tea and could be made into armour, tinder for torches and fires, and its fibres became thread for stitching clothing and weaving into hats and baskets; the soft inner bark was used for baby diapers. Sheets of birch bark (Betula papyrifera) were fashioned into canoes, but also into baskets and containers for water, and to cover roofs. Many trees provided medical benefits, including the Pacific yew (Taus brevifolia), trembling aspen (Populus tremuloides) and cascara (Rhamnus purshiana).
Resources
Resources
- How the North-west Coast Indigenous people harvested and used the bark of cedar trees: https://www.ictinc.ca/blog/indigenous-culturally-modified-trees
- Traditional uses of medicinal plants in the boreal forest areas of Canada: https://ethnobiomed.biomedcentral.com/articles/10.1186/1746-4269-8-7