| Summary: | The Coastal Western Hemlock (CWH) is an important biogeoclimatic zone in British
Columbia. Studies on forest stand dynamics in this zone have been quite limited, and have
generally focussed on moist, old-growth ecosystems. This lack of study of serai forest
development may have resulted partly from the belief that the major stand development work
had already been done. Although several studies provide a contextual framework for
understanding successional dynamics in these forests, substantially more work is required on
the dynamics of natural mature stands, on the responses of stands to different types and sizes
of disturbance, on processes associated with gap dynamics, and many other phenomena if we
are to understand the disturbance ecology of these forests, and how to design management
systems that will achieve specific successional outcomes.
The aims of this study were to: (i) describe the stand structure characteristics of mature
forest stands following two types of disturbance: wildfire and clearcut logging with
slashburning; (ii) compare and contrast the acclimation of planted seedlings of two conifer
species with different light ecologies: Douglas-fir and western hemlock; (iii) test if differences
in the growth performance of planted Douglas-fir and western hemlock seedlings could be
explained, to some extent, by differences in their leaf pigment concentrations or pigment
ratios along a gradient of natural light; (iv) adapt and calibrate a spatially-explicit forest gap
model for use in coastal, coniferous forests of southern British Columbia (CWHdm
subzone); (v) use the calibrated model to study the effects of various disturbance regimes on
forest successional dynamics in these forests.
An investigation of the stand structure characteristics of mature forest stands following
two types of disturbance (wildfire and clearcut logging with slashburning) indicated that: (i) the
three main species considered, Douglas-fir, western hemlock, and western redcedar, were all
pioneers after disturbance (either fire or clearcutting); (ii) their representation in the main
canopy depended on the type of disturbance, with more Douglas-fir on fire-origin sites and
more western hemlock on clearcut-origin sites; (iii) Douglas-fir generally dominated its contemporaries in height and size, while western hemlock and western redcedar were
abundant in the lower main canopy and in the subcanopy layers; (iv) western hemlock was the
most important component of the forest understory and in canopy gaps, whereas western
redcedar was barely present and Douglas-fir totally absent.
A field experiment designed to compare mortality, growth, leaf morphological and
physiological acclimation of two conifer species of contrasting shade tolerances, Douglas-fir
and western hemlock indicated that: (i) after two growing seasons Douglas-fir mortality
occurred mainly at relative light intensity (RLI) below 20%, while western hemlock mortality
was evenly distributed along the light gradient; (ii) height, diameter, and biomass of the planted
seedlings increased with increasing light for both species, but at different rates, and maximum
biomass accumulation always occurred in the open; (iii) Douglas-fir allocated more resources
to stem biomass than western hemlock, which accumulated more foliage biomass; (iv)
increases in specific leaf area for Douglas-fir seedlings occurred at RLI < 0.4 and red:far red
(R:FR) ratio < 0.6, which appear to be the minimal optimum light levels for growth; (v)
western hemlock seedlings adjusted their leaf morphology in a more regular pattern than
Douglas-fir, and changes were less pronounced at low light levels; (vi) Douglas-fir's net
photosynthetic rates per unit leaf area along the light gradient were found to be twice as high
compared to western hemlock; (vii) the chlorophyll a:b ratio increased more strongly with
increasing light than any other pigment or pigment ratio, especially for western hemlock
seedlings; (viii) the VAZ/Chl b ratio was also found to be strongly related to light for both
species, while the lutein/VAZ ratio was inversely related to light only in western hemlock
seedlings.
Model simulations of forest successional dynamics suggested that: (i) repeated, smallscale
disturbances such as light windstorms or small patch harvesting can accelerate the rate of
tree species replacement by accelerating forest succession; (ii) large-scale disturbances such as
infrequent severe fires set back succession to an earlier serai stage dominated by Douglas-fir;
(iii) clearcutting without Douglas-fir planting accelerates forest succession towards a western hemlock/western redcedar forest; (iv) on mesic sites in the CWHdm subzone, Douglas-fir
will not dominate stand dynamics indefinitely even though it persists in the canopy, unless
there are infrequent, severe fire disturbances; (v) the creation of small openings through
partial harvesting did not allow shade intolerant species to reestablish and dominate the forest
stand dynamics. It was concluded that successional dynamics and composition of these forests
are largely a function of the initial competitive interactions and the shade tolerance of each
species involved.
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