Why do Christmas trees drop needles?

21 December 2017

Interview with

David Hanke, University of Cambridge

Christmas Tree

Christmas Tree with decorations

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The first thing we need to do for Christmas is put up the Christmas tree! Sadly, Chris Smith and Georgia Mills only had a plastic one but there’s nothing quite like a real Christmas tree... Even if their needles do make a bit of a mess of the carpet! It was over to plant researcher and ecologist, David Hanke, from Cambridge University to explain just why trees drop their needles. 

David - In brief, it’s an emergency crisis response to the loss of the roots, and it’s not simply because you deprive them of water. Putting the Christmas tree after you’ve chopped the roots off into a bucket of water makes absolutely no difference to it shedding the leaves.

Shedding needles is a burnt-earth panic response to a loss of contact with the roots - so the needles panic. It’s a very active energy-dependent process. New enzymes have to be made to break down the base of the needle. More usually, this happens because of pathogen or herbivore attack that separated a section of the tree or a branch from the roots. The needles contain all the nutrients that the attacker is after. There’s not much worth eating in the rest of the tree frankly, so they’re ditched in order to starve the enemy.

Georgia - Oh wow! I’m feeling quite guilty now about sending all these Christmas trees into panic mode when we put them in our house. Is there anything we can do to keep their needles on?

David - Ahh no! Nobody has ever found anything. The only thing we can do about it is selective breeding because there’s a lot of variation out there in tree populations in terms of their resistance to needle abscission as it’s called.

Chris - Can I ask a question then because you said this is an active process, David, that the tree has to make an enzyme to then literally sever the needle from the tree. So if you were to say steam treat your tree at a really high temperature with steam and kill it, would the needle then stay stuck on albeit the tree’s dead?

David - It would stay on.

Chris - That’s the answer then, you just have to steam treat your tree. You need a big steamer.

David - You need a big steamer and the whole thing might start to smell after a relatively short period of time. I would just worry what you’d do with a cooked Christmas tree. Do you know what happens to needles when you cook them?

Chris - Hugh?

Hugh - So you’re saying it doesn’t matter whether you put the tree in water or not?

David - Well, slightly. You can slow down the process to a very limited extent and really only if you’ve got one of these types that shed their needles very quickly then putting it in water will slow it down. But if you’ve got a type that has high resistance needle abscission that’s going to give you 60 days it doesn’t matter, it won’t make any difference.

Georgia - So we're obviously getting through our Christmas trees quite quickly, so what's the best way to grow a Christmas tree?

David - Very important, because Christmas trees tend to be grown on extremely poor soil, so you need a nice fungus in there that is going to be able to recover scarce mineral resources, especially phosphorus in the soil, and relay them onto your tree.

Georgia - So there’s a fungus underground sort of helping the tree grow?

David - There is indeed. And the tree is helping the fungus by supplying it with the carbohydrates that it needs for energy.

Georgia - That’s very kind of it. Does it get anything in return?

David - Yes. It gets sugars from the tree, so all its energy and carbon supplies come from the tree. They live together.

Georgia - If this fungi helps the tree nice and green, what about making sure it’s that nice Christmas tree shape we all know and love?

David - It depends what you mean by nice. This is a very subjective judgement here. If you’re German or on the continent then your nice Christmas tree shape consists of tiers of horizontal branches with a big space in between because you want to hang your decs into the spaces in between.

Until the middle of the 20th century almost all Christmas trees grew like this. Now growers reported a change in the shape progressively, but eventually we’ve sussed it out that when you mis-spray these trees with ammonium nitrate as a supply of nitrogen, you get a big increase in the level of the hormone called cytokinins. It’s the cytokines that make lots of otherwise dormant buds grow out and become branches. So you fill in those spaces with lots of branches, you end up with a bushy tree as a result and, actually, bushy trees are much favoured by the Americans because they prefer to wrap their decs around the outside of the tree. They’re very happy with the new busy trees.

Chris - Philipe is nodding in agreement, so you like a nice bushy tree?

Philipe - Yes. I need a very, I wouldn’t say leafy but spiny tree I guess. Although, I was thinking as you were talking it seems very complicated to have a Christmas tree. You need to actually grow it and you need the fungus and everything, couldn’t we just have a bunch of maple trees? Not stereotypically Canadian or anything but, what if we cut maple trees and all have Christmas tree made of maple and maple syrup at the same time?

Chris - Yeah. Why do we have to have Christmas trees David? Where’s that come from?

David - Ah. Because of  Prince Albert really. He brought them over from Germany. What you need is to bring in an evergreen. It’s the most magic of evergreens because they are still green when everything else is dead and looks awful. We actually use a lemon tree. It grows in a pot and the leaves of a lemon tree, as you will know, are one of the only citrus plants that actually has the oil, the essential lemon oil in the leaves. What you can do is trim the leaves off and you wrap them around your sea bass on Christmas eve and wrap it up with foil and cook it, and you have the most delicious Christmas eve meal. Get a lemon tree, don’t bother with a spruce tree.

 

 

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