Heartwood by R.J. Cannon

It was the starving time again. A farmer led two children into deep snow and through the naked forest. And left them alone below a leafless poplar.

Later, the boy, shivering in the cold, gave his sister a seed he had found by the light of a sympathetic moon. She bit down and felt herself lengthening, her breaking bones stretching long and thin. Toes reaching into the ground, fingers sharpening into green needles, seeking light. She wrapped her bark around him as she grew.

Evergreen, they called her later, and listened for the beating heart within, even through darkest winter.