Development and urban intensification add to transport related greenhouse gas emissions by increased traffic volumes and contribute to rises in urban heating and storm water runoff by increased amounts of hard impermeable surface.
Production of oxygen
Trees use light energy from the sun to convert carbon dioxide and water into glucose and oxygen. This process is called photosynthesis. Oxygen is released into the atmosphere through the leaves.
Reduction of air pollution and greenhouse gas emissions
Through the process of photosynthesis trees remove carbon dioxide, nitrous oxides, sulphur dioxide, carbon monoxide and ozone from the atmosphere. Trees remove carbon from the atmosphere and store it in their leaves, branches, roots and trunks.
Trees also act as filters to remove particulate matter from the atmosphere.Particulates that are greater than 2.5 microns and smaller than 10 microns in diameter (PM2.5) are easily inhaled and cause damage to the heart and lungs.
Trees can remove particulate matter from the atmosphere by capturing it on their leaves, contributing to the quality of the environment and public health.
Cooling hard surfaces and waterways
Large urban areas often have higher temperatures than their surrounding rural areas. This is known as the urban heat island effect and is a recognised problem worldwide. Urban heat islands result from a complex built environment with a high density of human activities.
Trees cool the city and restrict unwanted weed growth in waterways by intercepting ultra violet rays and transpiring water from their leaves.
Stormwater management and erosion control
Trees manage stormwater ﬂows through their canopies and root systems. The canopies intercept rainfall, and root systems act like sponges by soaking up water which is then taken up to be used by the tree during its growing processes. The bigger and healthier the “sponge” the more stormwater is managed.
Healthy trees help reduce the nitrogen, phosphorus and heavy metal content in stormwater. Numerous studies have been undertaken to evaluate the potential for using street trees as elements of a stormwater system.
Trees also reduce erosion by protecting soils from the impact of heavy rain, binding soil on slopes and river and stream margins, and up taking and processing excess ground moisture.
Habitat for flora and fauna and increased biodiversity
Trees provide shelter and food for a variety of birds and small animals through their flowers, fruits, leaves, buds, and woody parts. Bacteria and fungi invade the trees causing decay pockets which create nesting sites for some birds and small animals, including native bats.
Decomposing leaves, twigs and non-woody roots increase soil fertility and structure for soil borne organisms including bacteria, fungi, protozoa, nematodes, arthropods and earthworms.
Trees also provide habitats for other plant life. This includes parasitic plants that live directly off the tree by feeding from it or epiphytes which use the tree as support.
Trees can also create a protective environment that allows the growth of plants that would otherwise not be there e.g. shade dwelling or frost tender plants. Conversely some trees can also discourage other trees and plants from establishing in close proximity by creating too much shade, using up available nutrients or they may have allopathic qualities.
Trees contribute to overall plant biodiversity as well as indigenous and endemic biodiversity.