I think the story in question is "Chlorophyll" by Stephen Tall, first published in the June 1976 edition of The Magazine of Fantasy & Science Fiction. The magazine can be read online at the Internet Archive here.
In view of the valley, all along its length, were high, round-topped
hills. Atop each hill a single great green Being sat and grew,
majestic and beautiful and ancient. Thus God was always with the
people. He was always in view. He was jealous God, and he demanded
obedience. To go to God was eventually, every man's fate. It was his
ultimate ecstasy. It was his final glory.
From the time he was small, Sam had helped to prepare the gifts, the
offerings, to the great green Beings on the hills. He knew full well
that all the tools that he used in his work, the hoes and the rakes,
the scythes and mattocks and chisels, all had made the pilgrimage up
the nearest hill, that they might be blessed by the green God. And he
knew that each blessing had cost lives. Men had later trudged up the
hill, gathered the goods and implements blessed for use, and brought
them soberly down.
"It is wrong," he would mutter. "It is wicked.
And," he would add darkly, "one of these days I'll prove it!"