To Plant or Not to Plant: Three Phases in the Growth of Forests

In my last blog, I left off questioning if it makes sense to plant new trees under the canopy of forests, since the existing trees accumulate mass at an ever-increasing rate. I think part of the answer lies in the stages of forest growth in terms of three broad phases of overall biomass accumulation.

Phase One – Biomass Accumulation in a Young Forest

After a major forest disturbance opens the canopy, a new generation of trees grows rapidly and steadily accumulates biomass with each passing year. Depending on the type of forest, phase one can last up to 200 years. Later in this phase the forest begins to thin naturally, but the ever-increasing rate of biomass accumulation in dominant trees more than makes up for the death and decomposition of smaller trees. Examples of phase one forests in the Puget Lowlands include thick patches of Red Alder in sites that have been cleared and graded.

Phase Two – Biomass Decline in a Mature Forest

Phase one ends when a significant number of large trees begins to die and total forest biomass begins to decline. Even though the remaining large trees continue to grow faster, the amount of biomass they add is less than the amount lost due to the death of their peers. Depending on the type of forest, phase two may last for another 200 years. During this phase, new openings in the canopy allow another generation of trees to become established, setting the stage for phase three. Many of our patches of urban forests are in phase two decline, especially some Douglas Fir/Western Hemlock forests in which many of the trees are succumbing to root rot, and aging Red Alder/Bigleaf Maple forests in which the trees are reaching the end of their life spans.

Phase Three – Biomass Equilibrium in the Old Growth Forest  

Eventually an equilibrium is established in which the overall amount of biomass lost through the death of the remaining large trees is matched by that accumulated in the growth of the next generation. The forest at this point is characterized by a mix of tree species and tree ages and can be called “old-growth.” Phase three can last for thousands of years until another major disturbance starts the process anew. Almost no phase three forests still exist in the Puget Lowlands.

To Plant or Not to Plant?

In the context of these three phases, does it make sense to plant trees under the canopy of existing urban forests?

• No, during phase one, when a new generation of trees is growing close together, and biomass is accumulating steadily.
• No, during phase three, when the very few patches of old growth forests left in our urban areas should be disturbed as little as possible.
• Yes, during phase two, especially if there are not enough new trees of preferred species sprouting naturally.

Since many of the forests in the Puget Lowlands are in phase two, there would appear to be viable opportunities for planting trees, but deciding what to plant is another question altogether.