Why do some trees go yellow and some go red in autumn?

I was pondering this question as I looked outside my office window...

Yellow and red trees

So, thanks to the wonders of the internet, I have now found answers to this question! The yellow and orange colours in leaves at this time of year are caused by pigments in the leaves that are there throughout the year. The rest of the year though, the colours are hidden by the green of the chlorophyll in the leaves - this is the stuff that trees need to make food for themselves by the process of photosynthesis.

The beautiful red colours that you see in some species of cherry and acers etc is caused by pigments called anthocyanins - these are only present in the leaf once the green colour (chlorophyll) has started to break down. Anthocyanins are thought to act as sunscreens, as they filter out the UV elements of sunlight, protecting the leaf from light energy that earlier in the year would have been absorbed by the chlorophyll.

However, like so often in science, this has ended up being a very complicated question, and there is still much research that needs to be done to answer it! The Leaf Color Bibliography website gives links to some detailed research if you have the time to read it...