When a tree falls in a forest, it can serve as excellent habitat for many saproxylic species, which require dead wood for food or habitat, and create an advantageous recruitment site for tree seedlings. These fallen trees are known as nurse logs and provide the ecological function of recycling nutrients within a forest. Once these dead trees reach the forest floor, they are introduced to far more life inside them than that of when they were alive. Fungi, microbes, and insects begin to colonize the log, breaking down the cambium and heartwood. Mosses will eventually cover the surface of a log and retain moisture, softening the bark and creating a microsite suitable for seed germination. This decomposing wood will create a spongy soil known as humus. These new growing conditions satisfy an ecological niche that many tree species recognize will give an upper hand when establishing themselves amongst the understory.
White pine (Pinus strobus) and red maple (Acer rubrum) seedlings establishing atop nurse logs. | White Pine Trail
Large trees that fall within a forest will tear up and disturb the soil and create gaps in the canopy, allowing more sunlight to reach the forest floor. These gaps will provide the right growing conditions to release seed banks and initiate regeneration in the understory. Many species will begin competing for this new growing space. Seedlings that germinate on a nurse log give themselves the advantage of an elevated growing site above their competition and are better protected from pathogens and predation.
The buttress roots of a mature white pine growing around the remnants of an old stump. | Lady Slipper Trail.
Once these seedlings become established, their roots will reach through or around the decaying log and into mineral soil. Over time insects, fungi and small mammals will break down the remaining wood of the log, exposing the roots of the established tree. These remaining prop root structures are referred to as “flying buttresses” which can serve as dens for a variety of mammals seeking shelter.
Two remaining northern white cedars (Thuga occedentalis) grown on a nurse log that has decomposed.
| Lady Slipper Trail
Evidence of nurse logs can be seen through the arrangement of past generations of trees. Flying buttresses and trees that are grown in a line on elevated mounds provide a glimpse into the legacy of a nurse log. Next time you are walking the trails of Hirundo, keep an eye out for nurse logs and share in an appreciation for the unique role they satisfy in forest succession.