Mature trees in the city

Unlike people, trees can grow to become as old as thousands of years old. Additionally, unlike people, trees do not age fast, nor do they grow with all their organs intact. In fact, they recreate the materials they need for survival every year, i.e., leaves, seeds and flowers. It is rare for a tree to die of old age alone. Instead, it is the exposure to the stress of wind, disease, insects, pollution, soil erosion, soil compaction, weather and people that will most likely cause it to deteriorate and die off. Research indicates that younger forests are better at storing carbon. One reason that might explain this is that newly deforested areas are open and sunny and are easily recolonized by fast growing species. These plants are able to extract carbon from the air and incorporate it into their biomass more quickly than mature trees that must compete with more neighbours and less sunlight.

Old trees are often (but not always) larger in size. Large trees are excellent filters for urban pollutants and fine particulates. They absorb pollutant gases (such as carbon monoxide, nitrogen oxides, ozone and sulphur oxides) and filter fine particulates such as dust, dirt or smoke out of the air by trapping them on leaves and bark. Mature trees regulate water flows and play a key role in preventing floods and reducing the risk of natural disasters. A mature evergreen tree, for instance, can hold on to more than 15000 liters of water per year.

Especially large, old trees are used by many animals for nesting, resting and for places from which to hunt or capture prey. For example, some large birds of prey use snags or dead branches to get a clear view of potential prey when hunting. Also, smaller birds catch flying insects directly out of the air using the large branches high up to attack. In the end, the old tree turns into organic matter and returns to being part of the earth. But before that, it is the nutrition for many decomposers, such as polypores and some insects. In fact, fungi exist in its own kingdom apart from plants and animals and forms often a bond with the roots of trees.

Fungi is present throughout the lives of trees, first by sustaining them by bringing them nutrients and water from further away through the mycorrhiza networks under ground, and later, as the tree becomes wounded or weakens due to very old age, a polypore starts to decompose the still standing tree. Therefore, trees and fungi have a very special life long connection.

Even when the tree dies, it is still very useful for some species, such as woodpeckers that use dead trees to drill for food and to nest in cavities excavated in snags (or dead parts of living trees). Other birds use the branches of mature trees for nests. As trees grow older, their branches become large and begin to grow horizontally rather than vertically, creating attractive platforms for nest construction. Some mammals such as squirrels and raccoons use dead trees as nesting sites. Logs are useful for snakes that like to sun themselves in summer to help regulate their internal temperature as well as providing them with a place to hide, find a meal or (for some) hibernate for the winter. Other amfibians such as salamanders use rotting logs and stumps as both shelters and food sources. Without big old trees many animals would suffer or perish. Smaller trees simply cannot play all the different roles the mature trees can as they have fewer dead branches and flowers, less nectar, peeling bark and woody debris compared with large, established trees. Tree hollows that are important nesting places for birds like owls, can take over 200 years to form.

Regardless of their importance, we keep losing old trees in the cities to make way for urban expansion. Property owners and city officials are worried for damage caused by breaking trees due to storm or heavy winds. Due to lack of understanding of the vital services an old or dead tree has to offer to its surroundings, old trees are quickly removed from making way for younger trees, which are also important, but become even more important as they age.

Ecosystem services of a tree

How trees make our cities livable

Water cycle in the city

Web of life

Native and non native species

Air quality in a green city

Dr Forest at your service

Colours of the forest

Look into the future food forests