Agathis Australis - Root Structure and Soil Interaction

Root Structure and Soil Interaction

One of the defining aspects of the kauri trees's unique ecological 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. On the other hand, broadleaf trees such as māhoe derive a good fraction of their nutrition in the deeper mineral layer of the soil. 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 prevent it blowing over in 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 more slowly than those of most other species. Besides its acidity, the plant also bears substances such as waxes and phenols, most notably tannins, 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.