I have what I believe to be a silver dollar eucalyptus in my backyard.  The entire top half of the leaves (about 2-3 ft.) looks very yellow and much thinner than the bottom half.  When it first started sprouting up last spring they were a beautiful yellow/red color.  They never amounted to much and still looks the same.  There was a little new growth this year, but it looks so sickly on the top.  Could I be over/under watering?  Watering incorrectly?  Or do I need to remove some of the bottom?  All of my other trees are doing amazing, so I'm assuming there's something to this tree I'm not getting.