Many forest types contain kauri. These have different features, reflecting soils, terrain, and temperature. Kauri forest is structurally complex, often with 20 different tree species per hectare. In mature stands, kauri trees emerge above a canopy of smaller conifers and broadleaved trees. Up to nine other conifers, including rimu (Dacrydium cupressinum), mountain tōtara (Podocarpus hallii), miro (Prumnopitys ferruginea), and further south, hard beech (Nothofagus truncata) can grow alongside kauri.
Kauri forest is similar to other sub-tropical forests. Some trees have buttressed trunks, and many are festooned with lianes (woody twining plants) and perching epiphytic plants. There are tree ferns, nīkau palms and orchids. Kauri forest contains the sole New Zealand representatives of some tropical Pacific families, such as pūriri (Vitex lucens), taraire (Beilschmiedia tarairi) and kohekohe (Dysoxylum spectabile).
The lower, open tiers of the forest contain small trees and shrubs, many of which are found only in kauri forest. Ground cover is predominantly kauri grass (a native lily, Astelia trinerva), a sedge-like plant (Gahnia xanthocarpa) and the liane kiekie (Freycinetia banksii).
In the far north, extensive areas of podzolised soils can be accompanied by compacted layers that develop in poorly drained sub-soil. These areas now support only stunted scrub species like mānuka and kānuka. Known as ‘gumlands’ because of the large deposits of fossil kauri gum once found in them, they are a legacy of former kauri forest.
Podzolised kauri soils
Wherever kauri grow, their mark is left in the soil. Kauri forest flourishes on a wide range of podzolised soils – poorly drained, strongly weathered, acidic clay soils, out of which heavy rainfall has leached most of the nutrients needed for plant growth. The presence of kauri intensifies this process so that long after the forest has disappeared, nutrient-poor areas in the soil show where individual trees once grew.
Competition with other trees
Adaptation to nutrient-poor soils is one way that slow-growing conifers like kauri can compete successfully with faster-growing broadleaved trees. Kauri forest tends to occupy the poorest soils, with broadleaf forest on more fertile soils. Because kauri can tolerate a range of soil moisture conditions they will grow on sites as varied as drought-prone ridge tops, podzolised sand dunes, and waterlogged rolling terrain (such as Waipoua Forest).
Kauri contribute to the low fertility of podzolised soils by producing a deep layer of litter that is slow to decay because it includes woody components such as bark, branchlets and cones, as well as kauri gum. Leaves, normally a large component of litter, are retained on the tree for up to 15 years.
This litter eventually develops into a highly acidic humus that is slow to release nutrients. It is another strategy by which kauri edge out competing broadleaf species.
The nutrients locked up in kauri litter include up to four times the nitrogen stored in the litter of most other forest types. Some of this is contributed by nitrogen-fixing bacteria that live in the surface layers of kauri litter. All this stored nitrogen is relatively inaccessible to plants – including kauri.
How then can kauri achieve their massive size and grow vigorously for hundreds of years? Although kauri do show signs of nitrogen deficiency (and respond to nitrogen fertilisers), they are highly efficient nitrogen users. With the help of specialist fungi (called mycorrhizas), kauri forest produces nearly twice the amount of wood, bark and cones for a given amount of nitrogen than that achieved by most other forest types. Kauri is the ultimate recycler.