Unfortunately, there are no « clean » way to limit the maximum number of characters (or more generally apply constraints) in edit mode of a tree, unlike textbox elements.

Here is the trick : the oninput event of a tree is only fired when you are editing :

Short tree example (limiting max number of characters to 10):

<tree hidecolumnpicker="true" minwidth="100px" height="300" editable="true" flex="1" 
 oninput="event.originalTarget.value = event.originalTarget.value.substring(0,10);">
  <treecols>
    <treecol primary="true" flex="1"/>
  </treecols>
  <treechildren context="">
    <treeitem>
      <treerow>
        <treecell label="joe@somewhere.com"/>
      </treerow>
    </treeitem>
    <treeitem>
      <treerow>
        <treecell label="mel@whereever.com"/>
      </treerow>
    </treeitem>
  </treechildren>
</tree>