A hollow tree stump at the Jurassic fossil forest of Curio Bay, New Zealand. The inside of the stump (the wood is yellow, and has come black, ‘coally’ material inside) has been filled with sandy material (grey). Could this be an in-filled fire-scar?
The Jurassic fossil forest exposed on the shore at Curio Bay, southern New Zealand, is one of several forest horizons that are visible in the cliffs. Curio Bay appears to have been in an environment where a forest-inundating flood was never far off. A flood would sweep down from the volcanic uplands, cover everything in volcanic material (mostly sandy), and a new forest would have to start again.
The wet volcanic material was full of silica, and this quickly silicified the tree stumps – preserving even the growth rings in the wood. These show that the main forest, and in fact, all of them, were rather young when they were suddenly buried. Most trees are only 50-60 years old, though a few may be around 200.
Figure 1. A hollow tree stump at the Jurassic forest at Curio Bay, New Zealand. A possible fire-scar?
Scattered all through the forest, if you look carefully, are stumps that were hollow. They are surrounded by, and filled with – the dark grey (even slightly greenish) volcanic material (see the featured image). I don’t think this is something that happened after burial. In my opinion, what we can see are stumps that were hollow at the moment the flood rushed in, and were instantly filled with sand.
So here’s the problem – if the forest was that young, why were so many trees hollow? When trees get very old, they may already have started to decay in the middle. But at least by the standards of living conifers in New Zealand, they’ve got to be pretty old, and, by that stage, very big trees as well. The hollow stumps at Curio Bay are both young, and small – we’re only talking about stumps that are 20–30 cm across.
Figure 2. A hollow tree stump at the Jurassic forest at Curio Bay, New Zealand. A possible fire-scar?
I’m wondering if the answer is – fire. In forests today, fire scars are something that can develop with the repeated passage of ground fires. Once a fire burns through the living zone, just beneath the bark, future fires can start on the easier to burn dead wood on the inside. This can quickly hollow out a living, and quite healthy tree.
Fire was clearly part of the broader Curio Bay landscape. I’ve found Jurassic charcoal up and down the Catlins coast from Curio Bay – though not as yet, from Curio Bay itself. You might think that a careful look at the stumps in the field would easily show charcoal, but it’s not that simple. Most of the Curio Bay wood has been truly ‘petrified’, that is, not only have the cell spaces in the wood filled up with silica, but then the wood itself has decayed – and been replaced by silica as well. This would make identification of charcoal, at least in the field, difficult.
Figure 3. A hollow tree stump at the Jurassic forest at Curio Bay, New Zealand. A possible fire-scar?
In the featured image, there is indeed a zone of shiny black material between the yellowish bark, and the in-fill of sand. However, it seems ‘coally’ – a response of wood to burial, rather than burning. The general environment of today- with daily inundation by sea water, growth of sea weed and so-on, hinders field observation. It would probably require careful microscopic thin-sections to get a real answer.
In the meantime, the hollow stumps are one more thing to look out for when you’re exploring Curio Bay. See if you can find some. What do you think?
