Inside many birch trees is dark heartwood. This wood is contrasted with the white wood on the outside. Heartwood isn't necessarily weaker than the outside wood, but the water content is far less. If a sled member has a little heartwood and the rest is white wood, it will tend to warp strongly towards the white wood when it dries, shrinking more on that side than the heartwood side. When women peel birch trees to make birch baskets, the tree isn't killed.
Old timers say the tree reacts by producing much more heartwood.
It is often easy to tell how much heartwood is present in a tree by noticing the black flecks on the bark. They look like woodpecker holes. The more black flecks there are, the more heartwood there is. Tough fibers Trees are like people. Some are tough and some aren't.