NERDTree Load Particular Directory Automatically

I have the following code in my vimrc file

cd ~/documents
map <F2> :NERDTreeToggle<CR>
" open Nerd Tree in folder of file in active buffer
map <Leader>nt :NERDTree %:p:h<CR>

The cd command is not NerdTree specific. It just changes the working directory when Vim starts to something which for me is sensible.

From the NERDTree help-file: :NERDTree [<start-directory> | <bookmark>] *:NERDTree* Opens a fresh NERD tree. The root of the tree depends on the argument given. There are 3 cases: If no argument is given, the current directory will be used. If a directory is given, that will be used. If a bookmark name is given, the corresponding directory will be used. For example: > :NERDTree /home/marty/vim7/src :NERDTree foo (foo is the name of a bookmark)