I like plugins for plugins, and I noticed something interesting in vim-line-jump (GitHub: rargo / vim-line-jump) -- you can specify mappings based on the current buffer with an
autocmd. That means you can add NERDTree-specific mappings with something like this:
autocmd BufEnter NERD_tree_\d\+ nnoremap <buffer> <nowait> <silent> f <ESC>:silent! call LineJumpSelectForward()<cr>
The rest of the examples are for NERDTree and Tagbar so you can navigate between files more easily.