[Al Skierkiewicz]

Scott,
Sorry but work got in the way of responding. I have to weigh in on the pipe moving around. We have ours mounted on 4" galvanized pipe and it is only 10 feet high (roof mounted) to keep the patterns aligned. Mounting another dish on a tree flexes even more that a small pipe. With both the tower and the tree moving around and twisting, it is very likely that the dishes will pull out of alignment on a regular basis that will be nearly impossible to compensate with a moving dish.
A system to aim antennas real time, requires some method to determine the direction of the move and magnitude. I have seen this done with a system that "wobbles" the device so that a feedback mechanism can detect both direction and magnitude. You would not want to due that with a dish for mechanical stress. You could move the feed horn which is a much smaller mass but it may not give you the range needed.
As to trees in the fresnel zone, if they were simply into the zone and not obstructing it, the path loss would seem to overcome the phase rotation introduced by the leaves. However, I think the trees are likely obstructing the path and you have significant path loss that this dish pair cannot overcome. Just running a quick path loss calc it looks like you could easily have 80 dB loss without the trees with this dish pair. I would bet that the tress introduce a variable or between 20 and 40 dB additional loss. That eats all of your available margin. The only way to overcome path loss is to increase transmit power, receiver gain or antenna gain(size). With the antennas you are using, none of those are possible. Even going to the 620 antenna only gets you another 2dBi. I think you have to pick up another 10-20dB of gain.
For my money, a larger antenna mounted on the tower and not a pipe is preferred. I can speak to leaves being a significant attenuator at 450 MHz. Many digital broadcast stations are finding that antenna placement for our viewers need to be away from trees. Many folks who had no problem with reception of our high VHF station in the past, have issues on our Ch47 (660MHz) UHF digital station. When it rains, data loss is significant to the point of no reception at all.