What is the effect of gnawing by spermophilus beecheyi on trees?

Gnawing by spermophilus beecheyi, commonly known as the California ground squirrel, leads to girdling of tree limbs and trunks. Girdling occurs when the bark and cambium layers of a tree are removed or damaged in a circular pattern around the trunk or branches. This interruption in the vascular system, which is responsible for the transportation of nutrients and water, can impair the tree's overall health and ultimately result in the death of affected limbs or even the entire tree. When girdling happens, the tree is unable to sustain itself properly, leading to its decline.

The other options suggest favorable outcomes for the health and growth of trees, which does not align with the consequences of squirrel gnawing. Trees affected by this behavior suffer from structural damage and compromised physiological functions, making the correct choice clear.