It is autumn and the leaves have already left their trees on all birch and other trees that has leaves, except one. This birch has a spotlight pointed to the tree and it is on 24/7. I have recognized that this birch have lost almost no leaves. And if we take a closer look at this birch, the back side of the birch, which is faced away from the spotlight have lost a noticeable amount more leaves than the front side which is directly hit by the light from the spotlight.

My conclusion is that having a lightsource pointing to a leafy tree will postpone the time from when it loses its leaves, relative to trees that does not have a lightsource pointerd at them. But I have no idea why this is happening. I do not know anything about trees, leaves, chloryphyll or the reason why trees drops their leaves in the autumn.

If any one is a expert in this field and might know why this is happening, or just have a hypothesis of why this is happening please provide it since I find this really interesting and I can't find anything on Google about this phenomenon.

You can make your anwer as complicated as you wish, I will understand it even if I might need to Google some of the things in the answer :).


  • $\begingroup$ Hi, first of all i higly recomment you to take a tour in our help center. Belive me, this provide you with lots of information about this site including valueble price. About your question if i understod it right you would like know if light source can post pone the deatf of leaves on tree. I higly doubt it. First off all the light source is very dangerous and tricky energy source. So i think it rather be vice versa. $\endgroup$
    – L.Diago
    Oct 22, 2018 at 9:48
  • $\begingroup$ Yes, you have understood my question right. The thing is that I have had this spotlight pointed at many different trees under many years, and I always get the exact same result. So I have started to see a pattern. Lets say you have 2 trees, X and Y. If i point the lightsource at X, that tree will lose its leaves much later than other trees of the same species, while Y will lose its leaves same time as all the others trees. If I then switch the lightsource to Y another year, Y will lose its leaves much later than the other trees and X will lose its leaves the same time as the other trees, $\endgroup$
    – Jacce
    Oct 22, 2018 at 14:20
  • 1
    $\begingroup$ Don't really have time to write out a proper answer, but the basics is: tree's (as well as many other organisms/animals) are able to measure length of a day using light sensitive proteins combined with circadian/rhythmically expressed regulators. This allows trees (etc) to properly regulate physiological functions - like losing leaves - according to the seasons. Since tree/plants are immobile targeting with a big spotlight will confuse their light sensors and disrupt this system (more than for animals which can/will move away at some point). $\endgroup$
    – Nicolai
    Oct 23, 2018 at 14:38
  • $\begingroup$ The leaves drop because of the sharp drop in temp. your 'keeping a spot[heat]light on them 24/7'' ,isn't allowing the tree to 'feel' the drop in temp. Therefore keeping it's leaves longer[postponing falltime] .Trees appear to be sensitive to temp.change. I dont believe trees are 'Photosensitive' that applies to cannabis. $\endgroup$
    – Lennie T
    Jan 1, 2019 at 3:09

1 Answer 1


It's to do with the chemistry taking place in the system, and then in turn the balance of energy resources.

Leaves contain chlorophyll, a molecule that traps light energy. It is large molecule containing a long carbon chain and a magnesium atom surrounded by 4 nitrogen atoms. In terms of a plants energy resources it is an 'expensive' molecule, but it is worth it as it is important to the plant.

The conversion of light energy by plants, using chlorophyll, is called photosynthesis. The chemistry of photosynthesis using chlorophyll is complex, but essentially Carbon Dioxide (CO2) from the air is combined with Water (H2O) from the roots to produce sugar and oxygen. Excess sugar is transported to the roots and stored as starch, and some sugar is used to keep the living cells throughout the plant alive. They need energy and this gained through respiration. Again, the actual chemistry is quite complex, but simply put the cells take in Oxygen from the air and 'burn' the sugar to release energy, this gives off Carbon Dioxide and Water. (There is plenty of resources and explanation on the actual mechanism - search for "ATP and ADP in plants")

Notice that it starts with carbon dioxide and water and ends with carbon dioxide and water, the energy is held within the chemical bonds.

When there is plenty of light (and water) the rate of photosynthesis, and therefore the rate of energy production, is high. The plant is producing excess energy, and the leaves are 'worth' the energy needed to keep them alive and functioning. Note that temperature also has an influence. The warmer the temperature (to a point) the greater the rate of photosynthesis.

So, in temperate climates with markedly different day lengths and temperatures between the summer and winter seasons some plants have evolved to be deciduous and shed their leaves in the autumn. The point at which this will start to happen will be when the day length shortens, and the temperature drops, and so when the amount of energy that the leaves are able to produce starts to balance the amount of energy being used to maintain the leaf.

If a plant has an artificial light source then the leaves will continue to be able to produce more energy than they use and so will be retained for longer.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .