Analyzing Word Choice: 'Crooned' in a Poem

Read the following excerpt from a poem:

"The gnarled trees crooned softly as the dusk settled over the tranquil lake, their shadows intertwining like whispered secrets. The water mirrored the last rays of sunlight, shimmering like scattered diamonds on a velvet cloth."

Based on the word choice in this excerpt, what is the primary effect of the author’s use of the word 'crooned'?