Islands have always been special cases for biogeography. The insularity of islands produces extremes—gigantism (moas that were 14 feet tall and weta, grasshopper-like insects almost 4 inches long, are examples in NZ), tameness (New Zealand’s parrots in particular are naïve to mammals), and abrupt transitions over short distances where you can climb from desert grassland to alpine in hundreds of feet rather than thousands. Our whirlwind tour of the North Island has allowed us to see some of these ecological transitions quickly enough that you can appreciate the wonder that early European explorers must have felt when they found these islands and the challenges that must have occupied the first Maoris as they transitioned from tropical islands to the more temperate Aotearoa.
When we visited Northland, I snuck away for an afternoon to take some photos of a remnant patch of kauri trees (Agathis australis). Kauris occupied much of subtropics in NZ including Northland, Auckland, and the Coromandel peninsula. This species was an island giant. They grow on low fertility, podzolized soils that are clay rich but nutrient poor. They can live over 1000 years and the largest have trunks with a diameter larger than 2 meters. Tane mahuta, the largest living kauri (you know it is a large tree when it’s been named) is 4-meters in diameter, and some stumps are >7 meters in diameter. Europeans logged most of the largest trees for shipbuilding and timber. Now the few remaining trees are endangered because of a water mold that attack its roots and can be moved by people who love these threatened giants and inadvertently move soil on their hiking boots from one location to another.
Kauri forests are interesting in part because of their rarity. While once widespread, now people must seek them out to find them. Here is a tour of a short hike that I took in the Puketi Forest just outside of Kerikeri and the Bay of Islands. While this forest doesn’t have the named giants found on the west coast of Northland, the grove that I hiked to was still quite impressive.
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You’ll see that the trunk of the full grown Kauri is a grayish brown. The color and texture of the bark, coupled with the size of the mature individuals, makes them easy to identify. They are broad-leaved conifers (such a strange conception for a westerner so influenced by our pine conifers). The female cones take three years to mature and produce seed. While young kauri have the classic conical shape seen so often in North America, once they reach the canopy, the lower branches are shed, producing the tall straight trunk that made them such valuable timber.
By the time kauri are 700-800 years old they will typically be 90-130 feet (30-40 m) tall and have a basal diameter of over 3 feet (1-m). Currently there are a few giants remaining, such as Tāne Mahuta (the god of the forest) with diameters over 12 feet (4-m). Unbelievably, there are kauri stumps more than 7-m in diameter.
One thing that is obvious on this tour is the structural complexity of the kauri forest, including the layering of the canopy, the diversity of woody and herbaceous plants, and the presence–or possibly domination–by epiphytic plants and lianas. Tree ferns are found across much of New Zealand and are obviously abundant here. The ground is dominated by kauri grass (Astelia trinerva) and liane kiekie (Freycinetia banksii).