Nurse Logs: Nurturers of the Forest

When a tree falls in a forest… it can become a nurse log!

In cool climates with plenty of rainfall, nurse logs are a common phenomenon. Nurse logs are stumps or logs of dead cedar, Douglas fir, Sitka spruce, or hemlock trees that become garden beds for new life. They are typically covered with organisms like moss, ferns, fungi, seedlings, and other microscopic life. Nurse logs can actually contain up to five times more living matter than they did when they were alive as trees!

When one of these trees falls and dies, insects, microorganisms, and fungi first start to break down the tissues of the wood and bark. This includes molecules like cellulose and lignin, which give trees their structural support. When the tree is broken down, gaps and holes are created that become filled with nitrogen-rich soil called hummus.

In forests, competition between plants for resources can be fierce, but nurse logs provide nutrients, moisture, space, and shelter that gives new life a chance to survive. Sometimes large enough nurse logs sit above the undergrowth and provide seedlings with rare access to light, as well. It’s recently been hypothesized that nurse logs may even protect new seedlings from soil-borne pathogens that would otherwise threaten them on the forest floor.

Mosses, lichens, mushrooms, small plants, and often other trees grow from these logs and stumps, but the life they support doesn’t end there. Small animals like squirrels and birds benefit from nurse logs as well. They use nurse logs as a place to rest, roost, or forage. In turn, animals enrich the soil and provide fertilization for seeds and sprouts with their waste products.

A typical rule of thumb is that the years a tree was alive will match the time it takes for it to decompose, which can be hundreds of years or more! This slow process creates habitat that lasts a long time, giving young organisms plenty of time to become established. Eventually, the log will be entirely broken down and only the life that grew from it will remain.

Nurse logs are an incredible example of renewal and regeneration that takes place in healthy forests. In Washington state, for example, it’s been found that as many as 90% of the population of Sitka spruce and western hemlock grew from nurse logs. And when these trees die, they may become nurse logs and support new life, all over again!

Nurse logs

Picture Source: americanforests.com