#include "tree.h" is a path-based inclusion i.e., it is trying to look for tree.h in ./
#include <tree.h> searches in all the standard paths (e.g., /usr/include). We can add to the list of standard paths by including -I flag to gcc.
Type make -n in tree/ and you can see the list of commands we use.
One of the flags will be -I which specifies the path to tree.h.