Your picture shows that althogh the trees are tall, there are gaps between trees, and between branches. Also the leaves are not dense. Under this condition, blockage of RF signal will not be big enough to reduce your range.
Install a better antenna at the same location as the small magnetic antenna, and I am sure you range will increase considerably compred to current range.
I agree with @foxhunter that instead of spending money on 60cm antenna straight away, try a negligible cost diy antenna first, mounted at same location where you have mounted the magnetic antenna.
(1) QUICK SPIDER - No Soldering, No Connector
(2) EASY ANTENNA #2 : CANTENNA