| Summary: | The effect of laminated (Phellinus) root rot in coastal second growth Douglas-fir ecosystems of southeastern Vancouver Island was investigated in 139 ecosystems centered around 215 growth and yield permanent sample plots (PSP's). The impact of Phellinus root rot on stand productivity was estimated from PSP records and a percent basal area reduction (%BAR) parameter estimate determined using variable-radius plot sampling. The methods involved ecological site characterization to Biogeoclimatic Ecosystem Classification system units, sampling of root rot incidence and severity, and calculation of root rot damage intensity, sampling of old growth stand conditions (stand density and species composition), determination of second growth stand origin, and summaries of second growth stand conditions and growth and yield from the 30 to 35yr long PSP records. Phellinus root rot was present in 87% of the 139 sample survey units, while the incidence in the PSP's was 37%. Root rot damage intensity varied significantly between the CDFmm and CWHxm subzones, respectively 5.94% and 11.11%. Generally, root rot did not vary much between subzone variants, plant alliances and associations and site associations. The highest root rot intensity was in the mesic conditions and tailed off in the drier and fresher soil moisture regimes. Only stand density (stems/ha) of old growth Douglas-fir and western hemlock were significantly and negatively correlated with Phellinus root rot intensity. It appears that the nearly two-times greater density of old growth Douglas-fir in the CWHxm contributed to nearly double the damage intensity, as compared to the CDFmm subzone. Trends in the old growth species composition indicated a natural, host-pathogen dynamic equilibrium is part of the ecosystem. That is, Phellinus kills-out susceptible, shade intolerant pioneer seral species by creating infection center gaps, making way for less-susceptible, shade tolerant mid-late seral species. In doing so, the inoculum is reduced but rarely eliminated, therefore keeping it in equilibrium with its hosts. Elevation (m asl), slope (%), mineral soil pH and coarse fragment content(% by volume) varied substantially between subzones, and likely play some role in the disease incidence and damage intensity. No other ecological site or stand attributes were correlated to damage intensity. Second growth stand attributes (stand density (stems/ha), relative density and species composition) had similar distribution patterns to damage intensity, although no meaningful relationships were determined in multiple linear regression models. Second growth less- to non-susceptible host species composition was found to increase by about4% in Phellinus infected conditions (both in temporary variable-radius %BAR sampling and PSP's) compared to a 0.2% increase in healthy conditions, strongly suggesting Phellinus acts as a biogenic successional agent that induces species shifts as part of the host-pathogen equilibrium. Root rot damage intensities were highest in second growth stands of wildfire origin, followed by logged-only, then logged-and-slash burned stands. Although stand origin class ages varied significantly (they were positively related to the damage intensity) there was an indication that logging and logging-and-slash burning could have played a role in reducing the inoculum levels particularly through fire. Ecosystem hazard and Phellinus risk were approximated for ecosystem units on the basis of root rot incidence and intensity estimates. The site association tax on model (climax species potential) was found to accurately estimate Phellinus risk when the host Phellinus susceptibility patterns are considered in light of the postulated host-pathogen dynamic equilibrium. Within a set of comparable PSP's, yield reductions ranged up to 30% depending on stand age and damage intensity. Yield models estimated volume reductions to range between 4.97 - 8.86% at age 80, or site height 35 m, depending on ecological and age10 stand density stratifications. Percent basal area reductions ranged from 8.25 - 9.99%, depending on the species and diameter class stratification of the estimate parameter. Management at the stand and forest level must consider the pervasive nature of Phellinus root rot. The significant ecological role the disease appears to play in inducing a species shift, has direct implications on the use of less-susceptible species in control and management strategies. Stand yield impacts appear to be substantial enough to warrant treatment where timber production objectives are primary.
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