Your Woods, Part Three
If you have woods on your property and are interested in their health, you might want to learn about forest succession because a healthy forest is one moving steadily through succeeding stages of this natural process.
On a landscape such as Vashon’s, forest succession begins when some major disturbance—wildfire, blowdown, clearcut-logging—opens savannah, and a pioneer phase in forest development often sees red alder establishing itself as the dominant tree. As alder’s relatively brief lifespan ends, conifers, mostly Douglas-fir, have the opportunity to seed themselves in. The resulting rapid-growth phase—the “teenager” years of this kind of forest—eventually closes the canopy when the top-branching parts of Doug-firs touch and even penetrate one another’s growing space. (Other conifers and deciduous trees will grow in the stand, too, though they may develop less rapidly.)
The mature phase of these forests develops when natural levels of disease, accident, and genetic differences among individual trees result in deaths that open holes in the canopy, allowing in light that sponsors the development of an understory of more shade-tolerant species like Western red cedar and Western hemlock.
Much later, what is commonly called the “old-growth” phase emerges from this diversification of a stand. A rare final stage, misleadingly called forest climax, eventually may develop, though it’s highly likely a major disturbance will happen before it can do so, taking forest development back to its beginning.
Despite its oversimplification, this summary of forest succession might help you direct your forest stewardship, depending on what phase of succession describes your woods. If you have lots of pioneering alder, you can practice assisted succession to bring about the rapid-growth phase of a forest. To do so, remove the alder and plant Doug-fir. Or plant shade-tolerant conifers among the alder, and then, as the alder dies of old age, plant Doug-fir to take the alder’s place. The Doug-fir will probably grow more quickly than the shade-tolerant trees and produce a canopy, below which the latter trees will develop as an understory. The mix will come to resemble a forest in its mature phase.
If your stand of trees is already in its mature phase, you can thin selectively, being sure to leave the dominant (largest) and codominant (next largest) conifers. (See an earlier post to this blog on ecological thinning.) Then you can underplant with shade-tolerant species to create an understory. This arrangement will eventually (albeit over a long time span) start to resemble old growth.
If your woods are already a so-called “old-growth forest,” then lucky you are. Few such stands exist on the Island and typically occur as refugia—remnants of the large forests that once stood around them. These frequently occur in ravines, where it was difficult or expensive to timber during the Island’s widespread logging last century. The management regime for old-growth involves, on a simple level, monitoring for unnatural levels of disease and planting an understory in light pockets, sunnier areas on the forest floor opened by the natural deaths of large trees.
