Safe from the eyes of owls and other predators, mice and their rodent relatives dug and digested vulnerable turf and plant stems beneath the snow. The tastiest saplings suffered most. Poplars, cherries, apples, and even dogwood and conifers had their bark efficiently stripped for consumption. The outer bark of trees both young old protects a layer of cells that is responsible for transporting sap, water, and nutrients to and from the plant's upper reaches. These living cells are reasonably tasty, so their attraction to over-wintering rodents is not surprising.
Rodents chew away a complete ring around plant stems, effectively girdling saplings.
When the layer of sap-transport cells is cut off from delivering nutrients from the roots to the branches, the plant starves above the point of injury. In nature, trees and saplings compromised in this way represent a form of vegetation thinning. Plant life that happens to survive this drastic type of selection will have more room to grow and more available sunlight as a result of their neighbours' demise. So this simple form of selection is beneficial for natural systems. Trees are very resilient.