Your Woods, Part Two
As much as forty percent of the Island’s forests are composed of red alder, so, if your property has woods, there’s a strong chance it contains a stand of alder. Knowing something about the state of this tree species on the Island might help you manage it—maybe starting soon.
Alnus rubra is known as a pioneer species because it’s often the first tree to sprout in forests disturbed by some large event, such as a windstorm or clearcut logging. Alder’s lifespan is short compared to that of other Northwest forest trees—around 50 to 75 years under average circumstances. During that span, it grows vigorously, improving the soil by capturing nitrogen from the air and fixing it in the ground. When alders die and eventually fall, they rot quickly, contributing a moist humus that further improves the soil. In these two ways, alder is a perfect pioneer, preparing the way for the long-term forest trees, the conifers, that come next as natural forest succession proceeds.
It seems accurate to say that most of the alder on Vashon are at the end of their life cycle. Driving around the Island, you can see many alder dead on the ground or dying slowly from the top down, a condition with the gruesome name of standing morbidity. Gruesomeness aside, this end-stage and death of alder is perfectly natural.
So why would you need to step in to manage an alder stand if things are taking their natural course there? Two inter-related reasons why: because of the relatively uniform age of the alders on the Island and because of the non-native plants that accompanied Euro-American settlement of this place.
As mentioned in the previous post about your woods, the disturbance behind most current forests on the Island was widespread logging in the early to middle of the twentieth century. Because that disturbance occurred within a narrow window of time, the alder in forests that grew up post-disturbance are approximately all the same age. This means that, since they’re now mostly at the end of their lifespan, they’re dying roughly all at the same time, also now.
If there’s a sizable stand of alders on your property, as they die together and fall over, a large area can open up quickly. When it does, non-native plants introduced by Euro-American settlers can invade and out-compete any native conifers trying to get established as part of forest succession. The transition from alders to conifers has probably been interrupted. Instead of a stand of new conifer seedlings, you end up with a rough patch of invasives.
The challenge facing forest landowners with stands of alder, then, is preventing this interruption, and soon, before more of the alder die and open up more space for invasives. The easiest prevention lies in assisted succession—humans stepping in to plant the conifer seedlings that would have voluntarily established themselves under natural conditions.
The next post will cover tree-planting as part of assisted succession.
