Windthrow is a common, small-scale type of natural disturbance. With the death of a tree more sunlight can reach the forest floor allowing shrubs and herbs that can not live under the canopy of a forest to thrive. Small canopy gaps are part of the variety of habitats that occur in a natural old-growth forest.
The forest at this site has lived a long time since a major disturbance. These large Douglas-fir trees are probably over 500 years old. Much of the wood in their trunks has decayed, yet they may still persist for centuries to come. Canopy openings allow for dense growth of salal, red huckleberry, and shade-tolerant trees such as western redcedar and western hemlock.
Wanted Dead or Alive
Alive or dead, standing or fallen, large trees play an important role in the forest. These standing dead trees, called snags will eventually fall to the forest floor, creating habitat for a wide range of plants, animals, microbes, and fungi.
Wood peckers use snags for feeding sites, and they excavate their nest–holes in the soft rotting wood. Other birds, such as owls, may move into abandoned nest-holes made by wood peckers.
A pair of bald eagles has used this nest for many seasons, adding materials and making improvements each year. A large tree with strong branches is needed to support such a structure. Most eagle nests are located within one hundred metres of the shore where eagles forage. Seventy percent of all bald eagles in North America live on the British Columbia and Alaska coasts. Milner Forest, with its large old growth trees, located close to the ocean beaches, provides excellent habitat for BC’s largest birds.
After a forest fire or other disturbance, many features from the old forest remain, and help the new forest to develop. For example, this fallen tree, called a nurse log provides a good site for new trees, shrubs and ferns to grow on, while insects and decomposing fungi occupy the moist habitat under and within the rotting wood.
Forest Fire History
Before European settlers came to this area forest fires were fairly frequent. On average, a major forest fire would occur every 200 to 300 years and smaller fires were more frequent. Old growth Douglas-fir trees have thick corky bark that protects them from all but the hottest forest fires. Many of the old Douglas-fir trees in the Milner Forest have fire scars or charred bark, indicating that fires occurred here in the past but did not kill all the trees.
Many of the Pacific Northwest’s native plant species, such as salal and Oregon-grape, make excellent garden plants. The intrepid Scottish explorer-botanist David Douglas (after whom Douglas-fir is named) collected both shrub species in our coastal forests in the early 1800’s. Douglas brought them back to England, where they were an immediate hit with local gardeners. Yet these hardy shrubs with their handsome leathery evergreen foliage, attractive flowers and tasty edible fruits are not commonly used in North American gardens.
Moisture is the Key
Why do different plants grow in different part of the Milner Forest? The most critical factor is the amount of water that plants can draw from the soil. Availability of soil moisture is controlled by two factors:
- Soil texture means the size of the soil particles. The relatively large grains of a sandy soil allow water to drain away quickly, whereas the microscopic particles of clay hold moisture, making it available to plant roots.
- Water from rain or snow drains away from a hill top and collects in low areas, so slope position also controls which plants grow where.
In the drier soil salal is abundant, while sword fern is more common
in the moister soil below. Increasing moisture in the soil leads to
a change in understory plants.
Ferns and Salmonberry
Plants can grow in a range of soil moisture conditions. However most plants thrive where the amount of moisture available best suits their particular needs. Forest ecologists recognize many species of indicator plants that give them clues about the climate, the richness and moisture of the soil, and the disturbance history of a forest ecosystem.
The sword ferns you see around you are lush and abundant indicating that they have found their optimum conditions. Sword ferns found on sites that are drier or wetter than this won’t be as vigorous.
Salmonberry, an important food plant of First Nations, indicates moister conditions than sword fern. One of the first plants to flower in the spring, salmonberry blossoms are an important food source for Rufous Hummingbirds when they return to this area after the winter.
Wetlands are areas where the water table is at or near the soil surface for a significant portion of each growing season. Many plants in forests such as the Milner Woodland grow only in wetlands – plants such as skunk cabbage, red-osier dogwood, and slough sedge. Wetlands occupy only a small area of most coastal forest landscapes, but are important for maintaining the diversity of life – the biodiversity – of these areas.