Pattern and process in old-growth temperate rainforests of southern British Columbia
Creators: name: Arsenault, André
Publisher: name: University of British Columbia
Year: 2009

description: I examined the effects of natural and anthropogenic disturbances on the patterns of species composition, tree regeneration, and forest architecture in old-growth forests from southern coastal British Columbia. This study was conducted in the submontane portion of the Coastal Western Hemlock Zone in the Vancouver watersheds, Pacific Spirit Park, and Clayoquot Sound. In the greater Vancouver area, old forests (250 yr) exhibited greater structural and compositional heterogeneity than young (31-60 yr) and mature (61-80 yr) forests. Size class distributions of living and dead standing trees in the three age groups suggested both qualitative and quantitative differences in regeneration and mortality processes. The canonical correlation between structure and composition was high (Rc = 0.84) but a substantial amount of total variation remained unexplained by the analysis. PCA (principal component analysis) axis 1 of species composition separated the lower elevation (warmer/drier) mature forests from the higher elevation (cooler/wetter) young and old forests. PCA axis 1 of structure separated the young and mature forests from the old forests. Stand history reconstructions in the Capilano watershed and Clayoquot Sound indicated that frequent small-scale disturbances (0.1-0.2 treefalls/year) and relatively slow growth rates explain the relatively open character and complex architecture of old-growth cedar-hemlock forests. All three study plots exhibited reverse-J tree size distributions considered indicative of climax or steady-state conditions; however, age structures showed important differences. One plot originated from a fire 300 yr ago while the other two were over 1000 years old and showed no signs of catastrophic disturbances. Gap-phase dynamics may influence patterns of tree regeneration if small-scale disturbance events are relatively close in space and time. However, the spatial pattern of understory trees was not significantly correlated with canopy structure. Thus, the role of gap-phase dynamics appears to be more important as a release mechanism for suppressed trees already established in the understory. The presence of standing water had a strong influence on the spatial pattern of understory plant communities. In addition, bryophyte species composition was related to stand structure and dynamics as evidenced by a succession gradient on wood. Except for Douglas-fir, which appears to require large scale disturbances for its regeneration, all other tree species examined are well adapted to a range of disturbances. This indicates that coastal silviculture could use a diversity of cutting methods analogous to natural disturbances instead of relying solely on clearcutting. The estimated forest turnover time varied between 375 and 1096 yr, based on the present proportion of area in canopy gaps (39%), and the estimated time to fill canopy gaps. Tree ring signatures revealed 208 yr-326 yr are needed for small disturbances to have a noticeable effect on tree growth for an area equivalent to the size of the study plots (1/2 ha). At the landscape level, the main catastrophic disturbance in the Capilano watershed before logging was fire. Large scale fires were associated with warm south facing slopes and occurred at low frequencies (ca 600 years) . The fire regime also coincided with periods of low sunspot activity. Logging has significantly changed the ecology of disturbance and patch dynamics in the Capilano watershed. A slow recovery process following the impact from human disturbances and a trend towards unprecedented levels of forest fragmentation supports concerns regarding conservation in our coastal forests.