How do woodrats typically cause damage to plants?

Woodrats are known for their behavior of chewing the bark off seedlings and small trees, which is a significant way they cause damage to plants. This bark-chewing behavior exposes the inner cambium layer, which is crucial for the plant's nutrient and water transport. When this layer is compromised, it can lead to stunted growth, disease, or even death of the plant.

In contrast, while woodrats may dig up roots or feed on leaves, these behaviors do not typically result in the same level of severity in terms of plant damage as bark stripping. Digging up roots can disrupt the plant, but it is the bark damage that directly impacts the plant's health and longevity. The option regarding seed spreading does not accurately reflect a damaging interaction but rather describes a neutral or potentially beneficial ecological role. Thus, chewing bark is the primary and most detrimental action that woodrats take against plants, making it the correct answer.