Mary E. Wilkins compares Candace's expression to seeing the silhouette of a tree during a forest fire just before it collapses.
After Candace was dismissed from the choir at church and replaced with Alma, she raged. She sang over Alma's solos and refused to stop even when the preacher asked. She fought with her nephew and generally had a bad attitude about everything. However, she becomes very ill and in her last days, she seeks to make amends with people.
Alma comes to sing to her. Wilkins writes that Alma stops singing and looks at Candace and it's like getting a "secondary glimpse of the old shape of a forest tree through the smoke and flame of the transfiguring fire the instant before it falls." Candace was a strong and steady person who was an important member of the community; she's like a great, ancient tree in the forest. But her time is coming to an end. Alma sees that—the last glimpse of that greatness—as Candace prepares to die.
It's likely that Wilkins meant Alma was seeing the strength and power of Candace in the last moment before it disappeared. She was looking at something that was about to be burned away even though perhaps once it seemed immovable. Alma is getting one last look at something strong and mighty in the instant before it's gone.