Temporal size-dependent growth responses within density-stressed black spruce stands: Competition processes and budworm effects

Abstract

This study assessed temporal size-dependent growth response patterns within 18 density-stressed black spruce ( Picea mariana (Mill.) B.S.P.) stands situated on medium quality sites in central insular Newfoundland. The assessment employed a power function to analyze the relationship between mean annual stem volume increment and cumulative size. Parameters of the power functions were interpreted in terms of: (1) their expression of alternative modes of competitive interaction, according to potential interactions by either resource depletion or resource pre-emption; and (2) temporal size-dependent responses relating to an epidemic spruce budworm ( Choristoneura fumiferana Clem.) infestation that occurred within the study area during the late 1970s. The results derived from the relationships did not allow alternative modes of competitive interaction to be distinguished due to the low levels of statistical power associated with parameter estimates. However, the analysis did reveal the effects of budworm defoliation on competitive dynamics within the sampled stands. Five distinct phases of response to budworm were revealed as follows: (1) competition-induced, size-proportional, growth rate declines among all-sized individuals during the pre-defoliation period (<1976); (2) budworm-induced, size-proportional, rapid growth rate declines among all-sized individuals during the period of severe defoliation (1976–1979); (3) delayed recovery among smaller-sized individuals during the post-defoliation period (1980–1982); (4) a recovery period (1983–1985) involving increasing competitive asymmetry, during which time the growth rate of larger-sized individuals increased at a greater relative rate than those of smaller-sized individuals; and (5) a period in which competitive asymmetry remained relatively stable (>1985). These patterns suggest that the principal effect of budworm defoliation was to increase competitive asymmetry within the sampled stands, thereby increasing the likelihood of episodic competition-induced mortality events.