treeview issue with + and -

hi,
I am showing record in treeview format using asp .net treeview control for web application.
Note: + and - is  treenode expand event  and tree1,tree1.1,tree2,tree2.1,tree2.2 are treenode change event. this we know generally.
for example:
+ tree1
     - tree1.1
     -tree1.2
+ tree2
      -tree2.1
      -tree2.2

so, i took my data from data base and formed in treeview format. till now everything works fine.
the problem is that: when i click + on tree1 family , it expands it's child---> here also no issue.
and secondly    when i click + on tree2 family , it expands it's child----> here the first family (ie tree1) should collapse automatically. right? but it is not doing. that is the issue.
Note: here when you click + and -  , no loading or postpack. just e ...