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I was wondering whether it would be feasible to have a tree that at times may have light green, at other times dark green leaves.
Why would a tree evolve to change the shade of its leaves?
Yes, I am aware of the effects of autumn, but the colour of the leaves change as a side-effect of the leaf dying. I do not want dead leaves, and I would like the change of colour to have its own purpose.
The leaves do not actually need to change colors.
Leaves have a "top" that faces the sun as well as a "bottom" that is in the shade. The plant cells on the top of a leaf are very dense with chloroform so they will have a dark green color while the cells on the underside have very little chloroform so the leaf will be a lighter green or grey-green color on that side.
Also consider that some plants are heliotropic (or phototropic) meaning that they twist to keep their leaves and/or flowers facing the sun. Sunflowers are the best known example of this.
Plants that open and close their leaves/flowers in response to darkness are called nyctinastic. Other plants will fold or close their leaves in response to other stimuli such as some types of mimosa that will close it's leaves when touched so that it is less attractive to grazing herbivores.
Combine the above properties and you have trees that will appear to change color based on which side of the leaves is visible to the viewer.
For example:
If the native climate for the trees is hot and dry then the trees might only open the leaves to get sunlight during morning and evening when the sun is out but temperatures are low. At night when there is no sunlight and during mid-afternoon when the temps reach their peaks the leaves will fold closed to prevent moisture loss. These trees would appear dark green at morning and evening but at night and midday they would appear to be a silvery grey-green color that tends to reflect light.
At extreme latitudes (polar regions) where days can be short with the sun staying low on the horizon, trees might evolve to keep the topside of the leaves pointed at the sun to maximize the chloroform's exposure to sunlight. These trees would appear dark green when viewed from the sunny side but a much lighter green when viewed from the other.
If the area has bad wind storms then the trees may have evolved to close the leaves whenever the wind is strong enough to make the branches sway, protecting the leaves from being damaged or stripped off by the wind. These trees would also close their leaves and appear to change color if a person or animal were climbing the tree and caused a branch to sway. There could also be an interesting visual wave of color across a forest in response to a low flying aircraft disturbing the branches.
Tree with delicate leaves might close the leaves similar to the mimosa I mentioned above.
Even evergreens undergo seasonal colour change. This is normally secondary as a result of new spring growth.
Below is an example of this for a Yew tree
What you see is that the new growth is a much lighter colour than the old leaves further down the branch. This leads to the tree appearing a much lighter colour in the spring and darker through the rest of the year.
You can also have brighter coloured trees as a result of variegated foliage.
What this normally means is that rather than being a single colour, the leaves have a range of shades, normally across each leaf rather than regional on the tree.
There aren't all that many trees that display this particular feature, it's more common in smaller species, bright colours are less common than a couple of different shades of green. For some reason it appears particularly common in ivy (at least to me).
https://www.ornamental-trees.co.uk/cornus-florida-rainbow-tree-p795
