Ecological dynamics
These sites are only found within the rain shadow of the Puget Trough. They are relatively cool, moist sites on gentle to steep slopes. The historic plant climax community (HPCP) is dominated by western hemlock with a minority of Douglas-fir and western redcedar.
Wind storms, insects and diseases (particularly root- and butt-rots) are the main disturbance agent on these sites, causing mostly small patch-sized disruptions. If the opening created is large enough, Douglas-fir would likely seed in. Smaller gaps would be dominated by one or more of the many shrub species.
Cutting practices could easily shift these forests to a Douglas-fir dominated community, but this would have to be maintained by steady management. Without intervention, shade tolerant hemlock would infill and eventually replace the majority of the Douglas-fir, moving back to the HPCP
Community 1.1
Western hemlock - Douglas-fir/Cascade Oregongrape
PC 1.1 (HCPC) – western hemlock – Douglas-fir/Cascade Oregongrape
Structure: multistory with small gap dynamics.
Western hemlock, a highly shade-tolerant species, dominates this plant community. Douglas-fir, even though it is intermediate in tolerance, will persist in the HCPC due to its longevity. Armillaria root rot can be a locally common disease which will kill young hemlocks and Douglas-firs and weaken older trees, leaving them susceptible to windthrow and insect attacks. The small openings created by the death of one or two trees allow sunlight into the understory, benefiting shrubs and forbs and releasing advanced regeneration. Western hemlock is also prone to other root rots and pockets of dead and dying trees can occasionally extend for several acres, allowing enough sunlight for the regeneration of less shade tolerant species.
Forest overstory. Typically dominated by western hemlock, Douglas-fir is usually present and occasionally codominant. Western redcedar, Sitka spruce and red alder may or may not be present as minor components.
Forest understory. The understory is often sparse due to the dense canopy of western hemlock. Where there are holes in the canopy, shrub species dominate.
Community 1.2
Western hemlock – Douglas-fir/Cascade Oregongrape – prickly currant
Structure: mosaic of mature overstory with regenerating openings. PC 1.2 retains much of the structure of PC 1.1 but also contains moderate sized areas of regeneration and/or shrub communities. These openings can be created by large pockets of disease, windthrow or uneven-aged harvesting (group selection or shelterwood with reserves). Depending on the seed sources present, the seedling patches may contain hemlock, western redcedar or Douglas- fir as well as baldhip rose, trailing blackberry or prickly currant. With no further management, some patches may be dominated by shrubs for several years, until the overstory canopy closes.
Community 1.3
Douglas-fir – western hemlock/Cascade Oregongrape – baldhip rose/western
PC 1.3 is forestland in regeneration; species composition depends on the natural seed sources present and the intensity of management. Western hemlock is a prolific seed producer and this often leads to dense, single-species stands. Douglas-fir has faster height growth than hemlock, however, and if seedlings become established at the same time as hemlock it will persist in the stand. Western hemlock stands can benefit from precommercial thinning as early as 10- to 15-years old; this would release the remaining trees, allowing for increased growth rates. If the stand is dense and not thinned, intermediate and suppressed individuals will eventually die due to strong competition. Shrubs and forbs in this plant community include baldhip rose, dull Oregongrape, brackenfern and stinging nettle. As the overstory canopy closes, however, the understory will decline.
Community 1.4
Western hemlock – Douglas fir/Cascade Oregongrape – baldhip rose
Structure: dense single story with sparse understory. PC 1.4 continues the ‘stem exclusion’ phase of forest development. The canopy has fully closed, leading to competition for water and nutrients. The stand appears relatively uniform with little vertical diversity and the understory is often very sparse due to the lack of sunlight.
Community 1.5
Maturing forest Western hemlock – Douglas-fir/Cascade Oregongrape – baldhip rose
Structure: single story with scattered openings. PC 1.5 is maturing forest. Although the canopy remains generally closed, the stand is starting to differentiate vertically. The death of scattered individuals allows more sunlight to reach the forest floor, benefiting understory species and advanced regeneration. Pockets of baldhip rose, trailing blackberry, prickly currant and western swordfern can all be found in this plant community.
Pathway 1.1a
Community 1.1 to 1.2
This pathway represents a larger disturbance (several acres). Large openings can be caused by disease, windthrow or group selection harvest. Full sunlight would benefit Douglas-fir and alder seedlings as well as hemlocks and may initially lead to a more diverse forest. If a seed source is present, these gaps may become dominated by shrubs such as baldhip rose and prickly currant. This community could persist for many years until the canopy gradually closes again.
Pathway 1.1b
Community 1.1 to 1.3
This is the pathway of a major disturbance. Historically this would have been a large-scale windthrow event or, more rarely, a stand-replacing fire; in modern times a block harvest, with or without a post-harvest prescribed fire would initiate this pathway. Regeneration can be either natural or planted.
Pathway 1.2a
Community 1.2 to 1.1
This pathway represents growth over time with no further management or major disturbance. The areas of regeneration pass through the typical stand phases – competitive exclusion, maturation, understory re-initiation – until they resemble the old-growth structure of the HCPC.
Pathway 1.3b
Community 1.3 to 1.4
This is the pathway of no active management. Overtime, less competitive trees will die out but the effect will not be as immediate as with thinning.
Pathway 1.3a
Community 1.3 to 1.5
This pathway indicates management and growth over time. Precommercial thinning early on allows trees more room to grow, increasing potential yield, and may also improve overall wind firmness in the future.
Pathway 1.4a
Community 1.4 to 1.5
This pathway represents growth over time, with or without management. The stand could carry a commercial thinning at this point, which would reduce competition and allow for increased growth. If not thinned, the competition will lead to the ongoing mortality of suppressed trees.
Pathway 1.5a
Community 1.5 to 1.1
This is the pathway of growth over time with no major disturbances or further management. The diversity of understory species increases as the stand begins to regain the horizontal and vertical diversity characteristic of old-growth forests.
Pathway 1.5b
Community 1.5 to 1.3
This pathway represents intensive management focusing on wood products. Block harvest, with or without a post-harvest prescribed fire, followed by either planting or natural seeding will reestablish a young stand.
