9. A Comparative Review of Forest Dynamics
201
age structure. The greatest extent of old-growth forest now occurs in the
perhumid zone in British Columbia and Alaska and in relatively small
fragments in southern British Columbia, Washington , and Oregon.
Commercial forestry plantations were established from the 1930s
through the 1960s. Problems with poor seed selection and poor regeneration techniques plagued early attempts at forest plantations. Genetic
improvements, better planting, and nursery techniques improved results
from the 1970s to the present. Intensive forestry , including thinning,
fertilization , and pest control, have only been implemented in the past
two decades, and still exist primarily on private industrial land in Oregon
and Washington. Throughout the seasonal rainforest zone Pseudotsuga
has been planted even along the coast, leading to a less species rich forest
than would occur after natural disturbance. Natural forest regeneration is
relied upon rather than planted nursery stock for the great majority of
lands within the perhumid rainforest region.
The mild wet climate in the rainforest zone allows for a rapid establishment of forest stands following logging or fire. Tsuga regeneration is
usually rapid with significant survival from the previous stand (Alaback &
Tappeiner, 1991; Harris, 1974). Herbs and shrubs resprout and dominate
sites 5 and 15 years after logging , respectively. By stand age 20 to 30
years most understory plants and all tree seedlings are shaded out. In the
perhumid zone on productive sites this depauperate stem exclusion stage
(sensu Oliver, 1981) may persist for 100 years or more (Alaback, 1982).
By stand age 150 to 250 years, the understory reinitiation stage develops ,
first with shade-tolerant shrubs like Vaccinium , then eventually with
evergreen herbs or short shrubs such as Comus canadensis , Rubus pedatus
and Tiarella trifoliata. In two to four centuries, mature forests develop
with a rich diversity of understory species and overstory canopy structure.
The conversion of old-growth to young second-growth vegetation thus
results in a significant decline in diversity and habitat value for deer and
other wildlife species (Alaback, 1982; Hanley, Robbins , & Spalinger,
1989; Wallmo & Schoen, 1980). In the Pacific Northwest this pattern is
less dramatic because of the less dense canopies of Pseudotsuga and
greater solar radiation intensities. Understory plants colonize more
rapidly during the aggradation stages of forest development and ruderal
species playa much larger role in the initial stages of the succession than
in the perhumid zone (Bailey & Poulton, 1968; Schoonmaker & McKee
1988; Spies & Franklin 1989).
The Effects of Recent Climate Variation on Disturbance Regimes
and Forest Dynamics
The effects of recent (i.e. , <1000 years B.P.) climate vanations in
southern Chile and Argentina are intimately interrelated with the effects
201
age structure. The greatest extent of old-growth forest now occurs in the
perhumid zone in British Columbia and Alaska and in relatively small
fragments in southern British Columbia, Washington , and Oregon.
Commercial forestry plantations were established from the 1930s
through the 1960s. Problems with poor seed selection and poor regeneration techniques plagued early attempts at forest plantations. Genetic
improvements, better planting, and nursery techniques improved results
from the 1970s to the present. Intensive forestry , including thinning,
fertilization , and pest control, have only been implemented in the past
two decades, and still exist primarily on private industrial land in Oregon
and Washington. Throughout the seasonal rainforest zone Pseudotsuga
has been planted even along the coast, leading to a less species rich forest
than would occur after natural disturbance. Natural forest regeneration is
relied upon rather than planted nursery stock for the great majority of
lands within the perhumid rainforest region.
The mild wet climate in the rainforest zone allows for a rapid establishment of forest stands following logging or fire. Tsuga regeneration is
usually rapid with significant survival from the previous stand (Alaback &
Tappeiner, 1991; Harris, 1974). Herbs and shrubs resprout and dominate
sites 5 and 15 years after logging , respectively. By stand age 20 to 30
years most understory plants and all tree seedlings are shaded out. In the
perhumid zone on productive sites this depauperate stem exclusion stage
(sensu Oliver, 1981) may persist for 100 years or more (Alaback, 1982).
By stand age 150 to 250 years, the understory reinitiation stage develops ,
first with shade-tolerant shrubs like Vaccinium , then eventually with
evergreen herbs or short shrubs such as Comus canadensis , Rubus pedatus
and Tiarella trifoliata. In two to four centuries, mature forests develop
with a rich diversity of understory species and overstory canopy structure.
The conversion of old-growth to young second-growth vegetation thus
results in a significant decline in diversity and habitat value for deer and
other wildlife species (Alaback, 1982; Hanley, Robbins , & Spalinger,
1989; Wallmo & Schoen, 1980). In the Pacific Northwest this pattern is
less dramatic because of the less dense canopies of Pseudotsuga and
greater solar radiation intensities. Understory plants colonize more
rapidly during the aggradation stages of forest development and ruderal
species playa much larger role in the initial stages of the succession than
in the perhumid zone (Bailey & Poulton, 1968; Schoonmaker & McKee
1988; Spies & Franklin 1989).
The Effects of Recent Climate Variation on Disturbance Regimes
and Forest Dynamics
The effects of recent (i.e. , <1000 years B.P.) climate vanations in
southern Chile and Argentina are intimately interrelated with the effects