The twisted gum – an Alpine Ash in New Zealand

Whilst the exact age of the tree is unknown, a photo in Trees from Other Lands in New Zealand: Eucalypts by J.H. Simmonds (published in 1927) shows the tree was already well established during the time of publication. Based on this, I believe it is safe to say that the tree is approximately 100 years old (or young, when considered in the context of trees). How many more years it will be around is anybody’s guess, but one thing which is certain is that the tree is a feature of the Botanic Gardens and anything we can do to prolong its life here is well worth it. It is also listed as the largest specimen in the New Zealand Tree Register.

This leads nicely into the work that has recently been undertaken around the tree. Those of you who have visited the Gardens in the last few months would have seen a large area around the tree which has been mulched (I will talk more about this later in the piece). A few of you who have been fortunate enough to come to the Gardens in the last few weeks would have seen the addition of a new platform which leads up to the base of the tree. The reason for this is to prevent compaction of the rooting environment around the tree.

So what is compaction, how does it happen, and why is it a problem? Okay, so this is a big topic, which I will attempt to condense down to a single paragraph. First of all, compaction is the condensing of the soil structure due to a force being applied. This can be caused in a variety of ways — from vehicles, pedestrians, or simply the natural force of gravity. The degree of compaction which can occur is largely related to the amount of force applied and the frequency (other factors such as soil type and hydrology also play a large part in this; however, for the purpose of this article, we will be focusing on force and frequency). What happens when soil is compacted is that the soil particles are squashed together, removing the pore spaces which would ordinarily hold water and air (both of which are vital for tree root growth and development).

In the context of the Alpine Ash, this tree is literally visited by thousands of people every year, and each of those people has played a part in compacting the soil which the tree relies so much upon. Over time this has led to an early decline in the tree’s health, which is evident through some dieback within the canopy and the sparseness of leaves compared to a specimen in perfect condition. This circles back nicely to what we have done recently and why.

MULCH, MULCH and more MULCH
The first thing we did was apply a layer of mulch around the tree. This has two key purposes: firstly, it helps to retain water in the soil, and secondly, it acts like a cushion to reduce the force applied to the soil from people walking on it. The mulch furthermore provides other benefits such as adding nutrients to the soil, habitat for insects, regulating soil temperature — the list goes on. The mulch that was applied is eucalyptus mulch from other removals around the city. As this breaks down it will provide the nutrients that would naturally occur if the tree were in a eucalyptus forest environment. There is some evidence that using the same genus/species as mulch can have added benefit.

Interaction without the Compaction
So whilst the mulch will go some way to reducing the compaction of the soil caused by pedestrians, over time the mulch will be squashed and compaction of the soil below will continue. In order to prevent this while retaining the ability for people to interact with the tree (and keep the tree’s popular Instagram profile going), a platform has been built leading up to the base of the tree. By installing this platform, people can continue to walk up to the tree without compacting the soil below, resulting in a happy tree and many more photos to come. The next and final step will be to put up bollards and chain around the edge of the mulch to direct people to the platform. Since the platform has gone in, we have had positive feedback — including from a couple of world-renowned arborists, who recently visited to run a workshop. With the addition of the mulch and the platform, it is hoped that the tree’s health will improve and we will be able to enjoy the tree for another 100 years. We will be adding interpretation to the area to tell our visitors.