I’ve seen most of the trees (at least in Canada) change color in autumn to an orangey, I’ve heard it has to do with better absorption of sunlight waves that change during this time of the year.
But why they begin to change from top to bottom? I would think the inner ones or lowest ones would change first since they have less access to sunlight than the top ones.
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The leaves are generally to fragile to survive winter so it’s better for a tree to reabsorb what it can from them and cut them off as daily sunlight dimishes in autumn, the top ones are the hardest to get sap to, that’s probably why they are the first to go.
They absorb the chlorophyll into the cambium to store during winter. Also in the leaves are carotenoids. They reflect in the orange and red range, in lower amounts than chlorophyll. As the chlorophyll gets absorbed, leaf tissue turns from green to yellow to orange and finally red. In this case the upper leaves are absorbing the chlorophyll more than the lower leaves.
Look at a few trees an hour or so before sunset. The leaves the sunlight hits at that time will be the first to change.
The changing of the color is the leaves dying off and losing their chlorophyll to photosynthesize as the tree is going dormant in the branches and moving sap down to the roots. It goes top to bottom because the top is farthest from the roots.
The leaves change colour because the tree is reabsorbing the green chlorophyll.
Chlorophyll is hard to produce for a tree, so instead of having to make it new in spring it just stores it in the trunk so it can safely lose the leaves over autumn and winter.
The remaining colours in the leaves are what the actual tissue in the leaf looks like without the chlorophyll saturating every cell.
The reason it tends to happen from one direction (top to bottom, side to side, etc) is due to a number of factors. Sap flow, majority light facing, etc.
Different trees do this differently. The birches outside my house get random yellow streaks that add up with time. Being exposed to a low temperature seems to kickstart the process. If it has to do with light and shadow when it comes to maples, it could be that being cold while being exposed to sunlight starts the process earlier.
Lots of answers that don’t address your specific question. It’s temperature – it’s cooler and less wind-protected at the periphery of branches, higher off the ground, and adjacent to open spaces like roads.
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