At the moment, the HL1 is configured for a particular channel and does not automatically move off that. You may get slightly better performance at range (at the cost of data rate) if you switch to a 1mhz or 2mhz channel (use the Quick Config page).
I suspect the issue here is the trees and/or the antenna positioning. You might want to have a look at this discussion: Halowlink1 high gain antenna's