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Last year’s Level 3 paper on ‘plant responses & animal behaviour’ (AS 90716) had a question on northern rata - rather a lovely tree; I remember that we had one on our section back in Wairoa, when I was a kid. For some reason that tree & the big totara next to it had been left when the rest of the section was cleared.. Anyway, this question began (as all the questions do) with a bit of contextual information:

Northern rata (Metrosideros robusta) is found in lowland forest throughout the North Island and near the northwest coast of the South Island. It is much more common as an epiphyte than a ground plant, and is mostly found growing on established tress such as the rimu (Dacrydium cupressinum). Both the rata and its host require plenty of light.

The rata epiphyte develops tuber-like swellings on its roots, which help with water storage.

Eventually roots from the epiphyte grow down the trunk of the supporting tree to the ground, forming a massive trunk and root system. This system replaces and eventually kills the supporting tree.

This is a really interesting relationship. When it’s young, the rata gains considerable benefits from its epiphytic lifestyle. It’s up at canopy level (not down on the ground like most seedlings) & as a result is exposed to much higher light levels than would be experienced on the forest floor. It doesn’t expend a lot of energy in growing taller, but can put that energy into producing leaves instead. (Those water-swollen roots mentioned in the original context statement are an adaptation to this lifestyle.) At this point what we’re looking at is a commensal relationship: the rata gains from its perch high in the canopy, while the rimu may not gain from it but it’s not significantly harmed, either.

But eventually things turn nasty – for the rimu, anyway. Over time there’s a structural change in the epiphyte ( is this just due to size? or age? – hopefully a better botanist than me can enlighten all of us!). Once its roots hit the ground, & it can tap into a constant groundwater supply, the rata is on the way to becoming an independent canopy tree with a hollow trunk formed by the fusion of its ground-seeking roots (just like the strangler figs we saw in Queensland last year). Ultimately the rimu dies – & rather than its death opening up a space for other seedlings to grow, it’s replaced by the rata. This fate isn’t something where natural selection might see the rimu population adapt, because the rimu is already a reproductively mature plant before the rata takes it out – it’s already reproduced & passed on its genes. Normally, as I’ve said, the open, sunlit space left by a fallen rimu would quickly see the growth of seedlings, including those of the rimu. But with the rata already a tall canopy tree (they can grow up to 25m tall), there is no sunlit opening in the forest, & so germination & growth of other seeds must wait until the rata itself comes crashing down.

However, all is not rosy for the rata – in many areas they are under severe browsing pressure from possums. Apparently it can take as little as 3 years to kill a mature tree. When a lot of trees are being hit quickly, this opens up the canopy on a fairly large scale, wih the potential for future damage due to windthrow in storms, and attack by insects & disease organisms. At the same time, it’s getting more expensive to control possums, something that’s compounded by growing public opposition to the use of 1080 for this purpose. Yet without some efficient, cost-effective means of controlling these furry pests, we stand to lose this beautiful tree, & many more besides.