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.
If rodents have not nibbled too deeply or have not completely encircled the stalk, certain cells will still survive that possess the capacity to generate new tissue. These new tissues will eventually bridge the wound, allowing fluids to flow to and from the branches. A valuable tree that has been completely girdled requires a surgically implanted conduit or bridge of living cells to conduct water and nutrients across the gap. This surgical technique is called bridge-grafting. The living layer of cells located under a tree's bark contains three distinct cell types.