Mild speculation and questionable returns
There is a tree that I drive past on my way to work. I call it my tree, but that’s a stretch and calling it a tree isn’t much better. A green blob would be a more accurate description – a blob maybe 4m tall, khaki in colour with a crusty beige brown trunk. Hardly a tree. So unremarkable is this blob, that I drove past it for almost five years before I even noticed it was there. Then one day all of its leaves fell off. Then some weeks later they all grew back. Suddenly the blob was interesting.
The blob is a cork oak, Quercus suber. They are not terribly common in my part of the world – apart from the obvious, they’re where cork comes from, I’ve not had much to do with them. They are an evergreen species, drought tolerant, don’t mind getting hot, and do surprisingly well in low-nutrient soils. As wholesome as harvesting cork goes you don’t get that many corks per tree per harvest. Due to cork taint (a poor seal) or the potential thereof, it’s an expensive and time-consuming risk, hence there are not many wine bottles being sealed with corks any more.
As I said, I didn’t notice this tree until it had lost its leaves and they and grown back. Because of this, I stopped and had a look. The new leaves looked like new leaves – basically, the tree was bursting forth into spring growth. Except it wasn’t spring, it was mid-summer. The gardeners had been and everything was clean – crime-scene clues had been removed. All I could really tell was that the new growth seemed remarkably healthy. That was that, and off I went.
Nothing much seemed to happen with my tree for the next few years. We had a series of very mild winters which is nice if don’t like the cold, but even nicer if you’re an insect. The year of leaf-minor and every other bug and plant beastie was upon us. The blob got it bad and was overrun with sap-sucking, leaf-blistering, energy-zapping bugs. It went from khaki to golden brown. Then normal winters resumed and the following spring it came out in a flush of new growth. It’s an evergreen tree, so even though the new growth looked good, the majority of its retained leaves were still brown and blistered. It looked better, but not great. Summer was wet, warm, and pretty much bug-free. I forgot about my tree, it continued to grow, then within the space of a couple of weeks all of its leaves fell off and it grew a new set. Suddenly the blob was interesting again.
It was almost as if my tree did some basic accounting; it ran a cost-benefit ratio and decided to cut its losses and start again. But it couldn’t have – trees don’t study accounting. To be honest, I’ve not actually studied accounting, and I have no first-hand knowledge of who or what attends accountancy class. I suspect it’s unlikely that there are rooms dotted with trees and shrubbery scratching away – but there are online options, so maybe? Silliness aside, the mind does not boggle, because if it did the next logical possibility is that trees and shrubbery frequent casinos and gambling halls. The risk of intentionally losing everything (shedding all of your leaves) would only be a good idea if the environment in which these new leaves were to grow was going to be ideal and remain ideal. But how could it predict the future?
There are plenty of higher organisms that seize the moment and bet on the future. When the conditions are good, aphids for example don’t bother wasting time and effort mating. They literally bud a new sister from an existing sister (viviparous birth). A female kangaroo can put an embryo on hold for years at a time. She can mate, stash the fertilised egg in her pouch and re-activate it when boy kangaroos are in short supply. But how does it know when to do this? There are species of frogs, that take this to the next level. When there are too many of one gender, some of them will change sex. There are actually several animals that can do this including clownfish… which does make you wonder if there was a hidden meaning behind Finding Nemo?
Survival in the animal world is wonderfully complex.
When it comes to plants we know that trees lose their leaves as a survival technique. Come autumn deciduous trees lose their leaves due to water availability – winter arrives the ground freezes and/or water becomes less or unavailable for the plant. They drop their leaves. The same reasons trigger leaf fall in so-called semi-deciduous plants (dry-deciduousness). This happens in areas where there isn’t really a winter, and the plant may or may not drop its leaves every year. Conventionally or seasonally deciduous trees can also shed their leaves in the middle of summer during drought conditions. As natural processes go and in the natural environment this is a relatively low-risk activity – leaves fall under the tree, protect the soil, rot down, and nutrients are recycled. But in the built environment, it’s maybe not a good idea – we tend to clean away the leaves, expose the soil and break the cycle.
When it comes to my tree, I don’t think water availability was the cause, and I’m confident gender reassignment had nothing to do with it either.
We know that leaves don’t live forever. The older a leaf gets the less efficient it becomes at photosynthesizing. Not only because the leaf’s position on the outside of the canopy has been overtaken by newer younger models, but because its photoreceptors start to break down (UV damage). An old leaf, with half of its surface area covered in brown blisters, wouldn’t be returning much compared to what the tree was investing in it. Photosynthesis is an active process, if the leaf isn’t producing, if it isn’t coming through with the goods, it makes sense to shed it. Out with the old, in with the new. But why did my tree shed all of its new leaves as well? I don’t know.
When it comes to questions of biology, the answer often appears once you have removed known or probable possibilities. You might not know what it is, but you do know what it’s not. When it comes to my tree, I am comfortable that I have removed known, probable, and even questionable possibilities. But no answer has appeared. I like the idea that my tree decided to cut its losses and start again – I don’t think it did that, but I don’t think we should underestimate our trees either.