What is the difference between datatype "const_tree" and "tree" ?

[冠人 王 via gcc]

In the function emit_side_effect_warnings, I find its inputs are "location_t loc" and "tree expr".
And, there is a function warn_if_unused_value called by emit_side_effect_warnings:
emit_side_effect_warnings(location_t loc, tree expr){    ...
    else
        warn_if_unused_value(expr,loc)
}

To call warn_if_unused_value, we use location_t loc and tree expr as inputs
In the function warn_if_unused_value, I find its inputs are "location_t loc" and "const_tree expr"
warn_if_unused_value(const_tree exp, location_t locus){    ...
}
I am confused about the difference between "tree expr" and "const_tree expr" since
if I modify the"const_tree" to "tree", I make failed.
Besides this, I try to use debug_tree function to dump the imformation of the node.
I am successful when I use it in the function emit_side_effect_warnings but
not in the function warn_if_unused_value