Books photos about executed POWS June 1944

OUR DEFENDERS

These, in the day when Heaven was falling,
The hour when Earth's foundations fled ...
Their shoulders held the Sky suspended;
They stood, and Earth's foundations stay.

And these did strike for Canada;
And man and maid are free
To foil and spoil the tyrant
Beneath the greenwood tree.

With sincere good wishes.

.... Does anyone understand the symbolism of the "GREENWOOD TREE" in this poem? I don't

Thanks, Carman