Influence of the disturbance regime on forest succession in the coastal western hemlock drier maritime subzone, British Columbia

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.Forestry, Faculty ofGraduat