From the NERDTree help-file:
:NERDTree [ | ] *: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)