Nicely done! Trees' leaves don't actually 'change' colour... what we see as colour changes are actually the result of shutting down chlorophyll production (what makes leaves green). There are carotenoids and other isomeric hydrocarbons in the leaves that account for the yellow, orange and red pigments. Now trees and other green plants use chlorophyll and carbon dioxide to make food (sugar), and since they give off oxygen in the process it's very good for us that they do! However, in most temperate climates the winters are too cold for the leaves to survive so the trees create a 'bud' to protect the new growth over the winter and use a chemical process to sever the leaves from the branches. Of course those leaves fall on the ground, decompose and provide nutrients back to the tree.
Now there's a balance between shutting down chlorophyll production too soon (not making enough food) and waiting too long (leaves literally die on the tree), and there's a lot of variance between species and things, but essentially they rely on temperature changes and photoperiod (the amound of daylight vs. night). However, if a tree is stressed from lack of light or lack of water, disease, or other factors, they can shut down early, putting themselves essentially into a hibernative state.
More or less.
Enough dendrology lessons for one day!
Mike.
