In chapter 6, Paul remembers the line of old poplar trees in the meadow behind the town, and how the children, including himself, used to love them. The children would sit beneath the trees and dangle their feet into the stream that ran by, listening to the rustling of the leaves in the wind. And the children, listening to the wind and inhaling "the pure fragrance of the water," would lose themselves in daydreams.
The two qualities that Paul initially associates with the poplar trees are the love that he and the other children had for them, and also the effect which they still have on him, which is to make his "heart pause in its beating." His heart pauses, or skips a beat, because the memory either excites him or scares him—or possibly both excites and scares him at the same time.
He does subsequently associate other qualities with the poplar trees too. For example, he says that memories like and including those of the poplar trees always have the quality of stillness, and also quietness, in sharp contrast of course to the noise and chaos of the war he is currently in. He then also says that these memories awaken in him a "sorrow—a vast, inapprehensible melancholy." This is because he is sad that he can no longer enjoy, as he once could, the poplar trees, and the stream, at least not outside of his memories.