Fragile fragments
I revisit my favourite childhood forest (8 minute read)
On Sunday, I will share one more holiday Turnstone, then next Friday (the 24th of January) I will be returning to gene technology, with an article about Douglas fir trees. Based on your feedback, more people would find it useful to receive my in-depth articles about science on a Friday, so that there is time over the weekend to read them. These articles will remain free, because I believe that the science behind important issues should be accessible to everyone.
I will be sharing more of my writing about special places and my observations of nature on Sundays. Most of this writing will be for my paying subscribers, because your support is a huge help to me right now when there’s very little science work around. I want to thank you so much for supporting my work.
I used to think that Smith’s Bush, on Auckland’s North Shore, was the perfect forest. I wasn’t energetic as a child, so walking up and down hills always seemed a lot of effort. Smith’s Bush, though, was on the flat. It was easy to walk through it and the trees were so tall. I especially admired the grove of huge pūriri trees, their branches heavy with perching kahakaha plants. This area is known as Cathedral Grove, and it truly deserves its name. It’s a place of reverence, despite the sound of the motorway in the background.
It's impossible to talk about Smith’s Bush without reference to the motorway. As a child, my favourite part of the motorway was the stretch which ran through the middle of Smith’s Bush. It was one of the most beautiful roads in Auckland, with tall kahikatea trees growing either side. I never thought about the fact that the area must have once been very different. Before the motorway, Smith’s Bush was one piece of forest.
As much as I loved, and still love, Smith’s Bush, today it breaks my heart. It’s a miracle that it survives, but I fear for it.
Exactly why it was preserved, when almost all of the forest in the area was milled for timber in the mid to late 1800s, is unclear. Perhaps the lovely grove of pūriri inspired the same sense of awe in Auckland’s early European settlers as I felt as a child. In the early part of the 20th century, the forest was owned by James and Catherine Smith, and it’s their name it now carries. In the 1940s, it was threatened with subdivision, but the Auckland Botanical Society and Auckland Museum, and particularly botanist Lucy Cranwell, ensured that it was protected as a reserve. Sadly, this protection proved to be only partial. In the 1950s, it was cut in two by the construction of the northern motorway. Ten years later, the motorway was widened, destroying more of the forest.
Running the motorway through the forest didn’t only destroy the area which was cut down and sealed over. It created two new edges. In a natural environment, and edge is often an area of greater diversity, where species from two different habitats meet. But in an urban forest fragment, an edge is an area of vulnerability. Firstly, it is an entry point for weeds. Many urban forest fragments are encircled by tradescantia, which creeps inexorably from the edges to the centre, smothering seedlings as it grows. Because it spreads from the movement of fragments and not seeds, it grows from where it has been dumped, or where it has been present in a garden.
It isn’t only weeds which enter the forest from the edges. So, too, does the wind. The main canopy tree in Smith’s Bush is the kahikatea, New Zealand’s tallest tree. Māori refer to a stand of kahikatea trees to represent the strength which comes from unity, acknowledging the way the trees grow together, with roots and branches intertwined. Without their neighbours, though, kahikatea trees are vulnerable. They haven’t developed the strength to cope with the wind.
It won’t take long for the weakest trees to fall and stronger shrubs or small trees to grow along the edges, so the wind is a short-term threat. But there are other changes, too, and these can never be reversed. Up to half of the rain which falls on a forest is intercepted by leaves before it reaches the forest floor, and evaporates back into the atmosphere. Of the rain which reaches the forest floor, some is intercepted by the dense layer of fallen leaves and also evaporates back into the atmosphere. Then, of the rain which enters the soil, up to half is absorbed by the roots of plants. This is eventually released from the leaves of these plants, as part of a process called transpiration (here’s a video explaining the process).
In comparison, when rain falls on a city, it lands on hard surfaces. While some evaporates, much of it is channelled into drains which empty into streams and carry the water away. This is much worse for us, because it means that streams are much more subject to extreme fluctuations and floods (something I’ve written about before). However, it also affects the surrounding atmosphere. Deep inside a forest, it is much more humid than outside the forest. Some plants simply don’t grow if it isn’t humid enough.
When a forest has a lot of edge, all the humidity created by the trees and other plants in the forest evaporates away. Many of the soft mosses and ferns which carpet tree trunks and the ground can’t survive. The forest simply isn’t the same, something I notice every time I walk in a small fragment of forest now. The tree ferns, in particular, often have naked trunks, with no other plants growing on them. In healthy forest, there might be half a dozen other ferns growing on a single trunk.
The loss of humidity affects the forest floor as well. Instead of a thick carpet of moss, ferns and leaf litter, I remember Smith’s Bush having a scattering of dried leaves over compacted ground. Partly this would have been due to the dry atmosphere, but it was also because Smith’s Bush never had much in the way of a track. I used to love wandering around, but so did all the visitors, and it didn’t do the forest any favours. At some point, maybe around 25 years ago, a boardwalk was built, so that people could walk through without trampling the forest floor. That’s one thing I can see which has improved there – the undergrowth is much denser than I remember it from my childhood.
Despite this improvement, there’s much that is missing from Smith’s Bush. Fragmentation affects animals as well as plants. I don’t know, for example, whether the giant green pūriri moth would survive in Smith’s Bush. It spends its early life in the leaf litter, feeding on fungi, before boring a hole in a tree trunk (not only pūriri, but many other native hardwood trees as well). There, it continues to feed, usually for 2-3 years before forming its cocoon. When it emerges as an adult, it survives only a few days. There are suitable trees in Smith’s Bush, but I don’t know whether the leaf litter is thick and healthy enough to sustain the young caterpillars.
As well as insects, there are also many native birds which can’t survive in fragments. For example, kōkako require a minimum of around 2000 hectares and it’s thought that North Island brown kiwi require an area of around 10,000 hectares of suitable habitat to sustain a population. For some types of kiwi, the areas are even larger. Even though there was little intact forest left in Wellington 100 years ago, fragments such as Khandallah Park, Wilton Bush and Huntleigh Park are now connected by regenerating forest. It’s enough to allow kiwi to be reintroduced, and other native birds such as kākā and kārearea (falcon) to breed within the city. Perhaps, one day, the forest will have recovered enough to support kōkako.
Smith’s Bush, though, doesn’t stand a chance. Nearby, there are sports fields, a golf club and a lot of suburban housing. It does adjoin a small wetland, but that is also split in half by the motorway. It’s never going to be part of a wider network. It will always stand alone.
I visit Smith’s Bush sometimes when I’m in Auckland, and I’m encouraged by the healthy regeneration I see. I’m sure that the boardwalk has made a real difference. On the whole, the weed problems have been kept under control. But on my most recent visit, I saw something which made my heart drop. It was clear that the forest had suffered some catastrophe.
Smith’s Bush has three species of tree in the canopy – pūriri, kahikatea and taraire. The first two I have mentioned already, and the third is another tree I love. It has large, dimpled leaves and fruit around an inch long, which means its largely dependent on kererū for dispersal. Like the pūriri, its relatives are largely tropical. When I walked around Smith’s Bush just prior to Christmas late last year, I noticed something about the taraire trees. Every single one was dead, apart from one tree which had lost all its leaves but was sprouting back. These were big trees too, not a few saplings.
Everywhere I looked, I could see trees with nothing but dead leaves. What could have happened?
When I was back home, I did some searching and found the answer. The trees had died during the exceptionally wet summer of 2023. I’m not sure whether they drowned, unable to absorb any oxygen through their roots, or whether they were killed by fungi favoured by the weather. Either way, it was gut-wrenching. All of those huge trees, all dead.
Although the summer of 2023 was exceptional, it won’t be the first time that unusual weather conditions will have affected an area of forest. A storm, a flood, an unseasonal frost, any of these can kill trees, and disturbance by extreme weather has always been a part of the New Zealand environment. When Auckland’s North Shore was mostly forest, a patch of taraire dying would have been no more than a local setback. But for an already fragile fragment, the loss of so many canopy trees is not a setback, it’s a body blow.
You write science beautifully. Thank you. How could I resist today’s title!
In the image of a fungus growing on a tree trunk it appears to be growing out of a hole. Is the hole the exit hole of a pūriri moth?