Gardening with Gael - Kauri has ‘relationship’ with the soil

Box and I have been given some beautiful trees to plant in memory of our parents. This weekend, with the rumour of impending rain, we decided to plant the kauri. News that New Zealand’s oldest kauri Tane Mahuta appears to be suffering from the drought gave added significance to the planting. Apparently the Department of Conservation has poured 40,000 litres of water around Tane Mahuta in an effort to replenish the moisture content and alleviate any stress the tree is suffering from. The rain this week is very welcome.

We stood beneath the trees on the edge of the bush looking up for a gap in the canopy for our tree to grow into. A juvenile kauri is a beautiful pyramidal tree which can take more than 50 years before the mature ‘crown’ begins to appear. We found the perfect place near the drive where we can regularly see it nestled in a group of totara and rimu. Mingimingi and hange hange grow as an under storey and the ground has plenty of rotting leaves and decomposing vegetation necessary to feed the fine hairs of the roots which are near the surface. In spite of the bush litter, Box said the ground is very hard and dry. Kauri can withstand a variety of soil types. Several sites I Googled had the following information about the growth  and nitrogenous cycles of the kauri. This is from the Kauri Coast Information site:

“One of the defining aspects of this tree's unique niche is its relationship with the soil below. Much like podocarps, it feeds in the organic litter near the surface of the soil through fine root hairs. This layer of the soil is composed of organic matter derived from falling leaves and branches as well as dead trees, and is constantly undergoing decomposition…Although its root system is very shallow, it also has several downwardly directed peg roots which anchor it firmly in the soil. Such a solid foundation is necessary for a tree the size of a kauri to avoid blowing over, especially during storms and cyclones.

The litter left by kauri is much more acidic than most trees, and as it decays similarly acidic compounds are liberated. In a process known as leaching, these acidic molecules pass through the soil layers with the help of rainfall, and release other nutrients trapped in clay such as nitrogen and phosphorus. This leaves these important nutrients unavailable to other trees, as they are washed down into deeper layers. This process is known as podsolization, and changes the soil colour to a dull grey. For a single tree, this leaves an area of leached soil beneath known as a cup podsol. This leaching process is important for kauri's survival as it competes with other species for space.

Leaf litter and other decaying parts of a kauri decompose much slower than most other species. Besides its acidity, the plant also bears substances such as waxes and phenols that are harmful to microorganisms. This results in a large buildup of litter around the base of a mature tree in which its own roots feed. These feeding roots also house a symbiotic fungi known as mycorrhiza which increase the plant's efficiency in taking up nutrients. In this mutualistic relationship, the fungus derives its own nutrition from the roots. In its interactions with the soil, kauri is thus able to starve its competitors of much needed nutrients and compete with much younger lineages.”