example 1:

QDomelement func1()
{
QDomDocument doc("func1");
QDomElement elem = doc.createElement("elem");
elem.setAttribute("value", "test");
printf("elem's doc name in func1: %s\n", qPrintable(doc.name()));

return elem;
}

void func2()
{
QDomElement elem = func1();

if(elem.isNull())
return;

elem.ownerDocument().toString(4); // here application crashed
}

void func3()
{
QDomElement elem = func1();
// printf("elem's doc name in func3: %s\n", qPrintable(elem.ownerDocument().name()));
// commented out because it will cause application crash

QDomDocument doc("func3");

// outputs `func3`, but must be `func2`
printf("elem's doc name in func3: %s\n", qPrintable(elem.ownerDocument().name()));
}

I thought that internal XML-tree object will be destroyed only if all links to it are removed, but in my example I have one link - QDomElement object `elem` which returned (copied) from func1 to func2. By C++ rules in func1 XML-tree must have 2 links (`doc` and `elem` objects), at return time it must have 3 links (created copy of elem), after return it must have 1 link (`doc` and `elem` in func1 were destroyed, but `elem` in func2 or func3 is still exists and links to XML-tree).

Try out my examples.
May be I didn't understood QtXml DOM mechanism - I don't know.