How does Miller signify tragedy to the audience in Act 1 of All My Sons?

In the first act of All My Sons, the
lightning striking the apple tree in the Kellers' yard signifies the tragedy of the
play.  At the beginning of the play, the characters try to hide the tree from Kate who
will be especially hurt by the collapse of the tree because it is a symbol for her son
Larry.  Kate believes that Larry will one day return from the war, even though the
others secretly believe that Larry has died in battle.  In the first act, the audience
learns that the tree was planted in Larry's birth month, and now it has fallen in his
birth month.  These events symbolize Larry's death and the path that the characters must
take to accept the fact that Larry will not return.  With this, the audience is
introduced to the tragedy of the play.