Another week, another decades-old tree is on the chopping block. This time a Norfolk pine in Ellerslie is being removed to make way for a car port. Residents believe the tree is unsafe. It’s all too common that people are worried about the perceived dangers of trees but there are plenty of benefits that are often forgotten in the rush the remove a ‘nuisance’ or ‘dangerous’ tree.
Ellery McNaughton already lamented the loss of trees at her urban study sites in February. So what good are trees?
1) Trees capture and store carbon. Through the process of photosynthesis, trees take up CO2 from the atmosphere and store carbon in their roots, stem, branches and leaves. The bigger the tree, the more carbon it stores as approximately 50% of biomass is carbon so that huge Norfolk pine is likely to store tonnes of carbon in wood and it will take decades for that carbon to be recaptured but a replacement tree.
2) Trees reduce air temperature. Trees cool things down in two ways. First, they obviously provide shade. Second, they lose water through their leaves through the process of transpiration. As water is lost from the surface of the leaf, evaporative cooling takes place. Trees are helpful in reducing the urban heat island effect, counterbalancing the warming effects of sealed roads, driveways and roofs.
3) Trees modulate the water cycle. Water taken up from the soil by roots travels up the trunk and then exits the leaves, returning to the atmosphere as transpiration. This process slowly removes water from the soil so when it rains, water can soak into the soil instead of becoming runoff and causing floods. Trees also act as a giant umbrella, catching water in their leaves. We call this interception and because tree canopies are complex, they can store huge amounts of water on the surfaces of leaves and branches. In a closed kauri forest, up to 44% of incoming rainfall across the year is captured in this way and returns to the atmosphere as evaporation when the sun comes out. This is hugely helpful in preventing floods!
4) Trees bind the soil, preventing erosion. This is particularly important in steep terrain where fast-moving water is more likely to cause slips, especially during heavy rain events.
5) Trees enhance biodiversity. Trees provide food and homes for birds, invertebrates, reptiles and other plants.
6) Trees provide colour. Without trees, out landscapes become dull and grey. Trees provide greens of leaves but also reds, yellows, whites and oranges when they flower and fruit.
We know that trees improve property values because leafy areas are seen as being more affluent. While asthetics are important, there are clearly so many other good reasons to love trees. Surely it’s time to value are trees for the wonderful services they provide!
The million trees programme is a great way to rebuild forest but we also need to preserve what we already have with better tree protection.
Dr Cate Macinnis-Ng is a Senior Lecturer and Rutherford Discovery Fellow, School of Biological Sciences, University of Auckland. She is a plant ecophysiologist and ecohydrologist working on plant-climate interactions.