It is a bit hard to see in this picture but you are able to put 16 trees in the area of one plot. This means you need to sum up the coins received for 16 trees when you compare to other crops.
Farmville does not care that in real life crowding trees like this would be a problem. Not all the tress would get the sun and circulation they need.
The only down side to crowding trees like this is that it is hard to click on an individual tree come harvest time.