“Before death, the leaves display magnificent beauty, and then fall to the earth to serve a greater purpose.”

Chick tracts, " Why Should I? http://www.chick.com/reading/tracts/1079/1079_01.asp" (2012)

Addressing German soldiers departing for the front in WWI (August 1914), as quoted in The Chanak Affair (1969) by David Walder, p. 21
1910s
Variant: You men will be home when the leaves fall.

Source: The Wealth of Nations (1776), Book V, Chapter II, Part I, p. 891.
Context: Lands for the purposes of pleasure and magnificence, parks, gardens, public walks, &c. possessions which are every where considered as causes of expence, not as sources of revenue, seem to be the only lands which, in a great and civilized monarchy, ought to belong the crown.

G. P. R. James Henry Masterton (1832; repr. London: Richard Bentley, 1837) p. 297
Misattributed

—And the Moon Be Still as Bright (1948)
The Martian Chronicles (1950)

In an interview in Artforum, Nov. 83
Interviews

Calvero's answer to Terry's question: "What is there to fight for?" in Limelight (1952)
Context: Life is a beautiful, magnificent thing, even to a jellyfish. … The trouble is you won't fight. You've given up. But there's something just as inevitable as death. And that's life. Think of the power of the universe — turning the Earth, growing the trees. That's the same power within you — if you'll only have the courage and the will to use it.

Podcast Series 1 Episode 2
On Sex