May - Kauri - Native Plant of the Month
Updated: May 18
Kauri is the giant of our northern forests, a ‘keystone’ species that supports entire ecosystems, and an iconic tree to all New Zealanders. Despite their ecological and cultural importance, the future of kauri is uncertain; historical exploitation degraded their populations, and present-day survivors are now put at serious risk by kauri dieback disease.
Kauri are in the 65 million-year-old genus Agathis, which is part of the ancient conifer family Araucariaceae, a group once widespread during the Jurassic and Cretaceous periods. Today, this incredible family of trees is largely restricted to the Southern Hemisphere. Our species is endemic, meaning that it is entirely unique to New Zealand, and has been here for millions of years. Kauri are one of our most long-lived tree species, with our largest living tree, Tāne Mahuta, estimated to be around 2,000 years old. Whilst not quite the tallest native tree compared to kahikatea, kauri can grow to a still-impressive 52 metres tall and have the largest trunk girth of any native - rivalling Californian Sequoias at over 5 metres in diameter. Try hugging that!
A kauri tree carries male and female cones. The male cone, which is narrower, produces pollen and the female (round) cone bears seeds. The male cone ripens to dark brown in about a year, while female cones take three years to mature after fertilisation, at which time they are about 7 centimetres in diameter. When fully ripe they disintegrate, scattering their seeds to the wind. Kauri seeds look a little like scales and often cover the forest floor near kauri trees.
Kauri grow from sea level up to 600 metres in elevation, and once covered over a million hectares in the Coromandel, Auckland and Northland. Today they can be found in smaller patches across these areas, covering only about 7,500 hectares (only 1% of their original range). They do not naturally occur south of the Coromandel region, but can be spread and grown by humans south of this border, and with the warming climate their natural range is likely to travel south, too. Kauri are considered taonga by Māori and their health is considered to be imperative to the health of the whole ngahere (forest). Māori used the timber of kauri to build waka – canoes, and kauri gum for a variety of purposes, including as an insecticide and to make torches.
The main reason that kauri are so rare today is the extensive logging practices of European colonists of the 1800’s and early 1900’s. Kauri wood is prized for its strength, beauty, slow deterioration and general usefulness, and was commonly used for building homes, boats and other structures. This meant that logging Kauri was highly profitable, and so hundreds of thousands of hectares of kauri were felled to fuel the economy and development in Aotearoa. Kauri gum was also a highly prized material for making resins and linoleum, and from 1850 to 1900, kauri gum was Auckland’s main export. This gum was mainly dug up from swamps and scrublands; areas that once had large kauri but which had since fallen and decomposed, leaving gum deposits behind. However, when deposits began running out, people attempted to cut kauri to make them seep more gum from their trunks. This sadly often killed trees. So much can be learnt from our destructive past, and today all kauri are much more highly protected.
Kauri can create deposits of slowly-decomposing leaf litter up to 2 metres deep. This litter alters the soil that they grow in, making soils around them acidic and less suitable for non-associated plant species, but more suitable for associated species that kauri often grow with. These ‘allelopathic’ properties of their leaf litter can inhibit many seedlings from growing near them - which is awesome when it comes to keeping nasty weeds out!
A ‘keystone’ species is one that has a disproportionately large effect on its natural environment relative to its abundance. The ecosystem that it inhabits and influences would otherwise collapse without or be drastically impacted by if absent. Kauri are considered a keystone for the forests they have inhabited due to their soil-altering, and thus forest-altering, abilities. Around 17 species have been identified that would seriously struggle to exist without Kauri around, as they have adapted to living on kauri or in kauri podsol (a soil type created by kauri). Examples of these dependent species are korokio (Corokia buddleioides) and the beautiful kauri greenhood orchids (Pterostylis agathicola). All the more reason to protect our majestic kauri forests!
A major threat to our dwindling Kauri forests today is kauri dieback disease. This root dieback disease is caused by the oomycete pathogen Phytophthora agathidicida, which is water and soil-borne, so the spread of even the tiniest specks of soil can spread this disease from tree-to-tree. The disease slowly prevents kauri roots from being able to absorb adequate nutrients, effectively starving the tree, and it can take a decade before a tree shows symptoms of infection. The noted incidence of kauri dieback has increased in the past decade, and even trees more than 1000 years old are not immune. You can help kauri by preventing the movement of soil between trees, such as by always scrubbing and disinfecting your boots and equipment at wash stations, washing dog paws and ensuring dogs stay on the track, never moving plant or soil material between parks or properties, and always keeping to the track when walking in kauri reserves. To learn more, visit https://www.kauriprotection.co.nz/.
While we would love to see more kauri growing in Kaipātiki, there are strict rules around growing and planting these plants yourself. We recommend buying seedlings from certified plant nurseries to ensure your trees are disease-free and healthy. Please consider how large kauri can grow, however, before planting them in your backyard! If you have kauri on your property and notice that they may be struggling with symptoms of kauri dieback, please report this to Tiakina Kauri. For more information on growing your own kauri and dealing with dieback on your property, visit https://www.kauriprotection.co.nz/national-plan/growing-kauri/.