Re: Why are leaves on most trees larger on bottom/smaller on top?

Leaves on a large plant like a tree are in very different environments when 
on the top or the bottom.  Those on the top are exposed to direct sunlight 
and moving air, which increase the rate at which water is lost from the 
leaves.  Smaller leaves lose less water but in the bright light can still 
capture a lot of sunlight.  Leaves on the bottom are shaded by the tree 
crown above, and their environment is shady and moister in their protected 
position.  Leaves here are bigger to capture more of the partially filtered 
sunlight coming through the leaves above.  They are also thinner, but the 
water loss is not so great in the moister, stiller air.  These are 
refered to as sun and shade leaves.  Trees that produce noticeably 
different sun and shade leaves have are plastic, they have a 
leaf development that changes in different environments.

You made an interesting observation about leaf sizes that prompted this 
question, but do you really know the leaves are bigger on the bottom?  
Did you just compare one or two?  To find out for certain a scientist would 
measure a lot of leaves in exactly the same way from both locations and 
compare the average size difference.

You also ask about differences in leaf color on different parts of a tree. 
Similarly leaves developing under different conditions can show differences 
in pigmentation because of differences in light or other factors.