Oh, that's easy. And I would assume you'd like to reset the 4 forms to blank as well?
If so, it would be better if you'd give each of your <form>s a unique name or id.
I'll assume you gave them unique ids (preferable over names) of "joistForm1", "joistForm2", etc.
var opts = this.options;
for ( var i = 0; i < opts.length; ++i )
var tblID = opts[i].value;
document.getElementById(tblID).style.display = ( opts[i].selected ? "block" : "none" );
// reset the species choice:
document.getElementById("species").selectedIndex = 0;
// reset all 4 forms:
for ( var fnum = 1; fnum <= 4; ++fnum )
document.getElementById("joistForm" + fnum).reset();
Untested, but I think it should work.