If there are very few trees or smaller trees or no close by trees on the other side of the big tree that is blocking the signal , then put the antenna on the other side of the big tree. It might help to put a antenna lower to the ground and shoot under the big tree. Or move the antenna to the side of the big tree. To go higher , here is a fold over tower , can do all antenna work standing on the ground ,
http://www.rohnnet.com/towers-foldover. Here is tall tripod tower that is mounted on the roof ,
http://www.ronard.com/Tripods%200703/4712.html