Martin Dodge, of Balclutha Primary School, asks :-

Why do cones grow in clusters on the pinus radiata branch? How many clusters develop each year?

Matt Larcome, a botanist at the University of Otago, responded.

Pinus radiata is one of the moist widely grown forestry plantation trees in the world. It is used to produce timber for building and paper. However, it is very rare in the wild. It grows along the coast of California and is actually at risk of extinction in its natural habitat.

Pine trees are part of a really old group of plants called conifers, which have been around for about 350 million years. Unlike flowering plants like roses or cherry trees, conifers have cones instead of flowers. There are two types of cones, pollen cones and seed cones, and it’s the seed cones that you see on the trees in clusters.

Each year the branches of a pine trees grow from the tips and the cones (both pollen and seed) grow in a ring around the branch just below this growing tip (red arrows above). The pollen cones fall off each year, but the seed cone takes two years for the seeds to fully develop and once mature the can stay on the tree for up to 20 years.

So the reason pine cones grow in clusters is that they form at the growing tip and each cluster represents the cones from a single year. This means that the cones closest to the tip of the branches are the youngest and the ones closest to the trunk are the oldest. The reason the cones stay on the tree so long is because of a special adaptation to fire, where the cones only open and release the seed after fire. This is so if the adult tree is killed by the fire, the seed will produce new trees to replace it.

As for how many clusters develop each year, this is a tricky question! Seed cones (the ones you see) develop towards the top of the canopy. Most branches in the canopy will develop cones so the bigger the tree, the more branches and the more cones that develop each year.

You can find out more about Pinus radiata and the development of seed and pollen cones at then pi/Pinus_radiata.

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