Influence of decay fungi on species composition and size class structure in mature Picea glauca × engelmannii and Abies lasiocarpa in sub-boreal forests of central British Columbia

Abstract

Disturbance patterns in the sub-boreal spruce forests of central British Columbia have long been thought to result from frequent stand-initiating fires. However, recent evidence suggests that fires in the wetter areas of this region are infrequent (>500 years) and the uneven-aged stand structures have been shown to be self-maintaining in the absence of fire. The importance of decay fungi as agents of gap-formation and facilitators of uneven-aged stand structure was investigated. Three plots on each of two recently clearcut sites were established. Each stump in the plots was stem mapped and the species and diameter recorded. Decay at the stump top was recorded as white or brown rot, and the area occupied by decay was measured. Spruce stumps dominated the larger diameter classes but had less butt rot than sub-alpine fir stumps. Decay fungi contribute to stem breakage in living trees with heart rot, and as saprots of dead trees. Therefore, the results suggest that decay fungi play an important role in removing sub-alpine fir trees from the canopy of these wetter sub-boreal ecosystems, and in enabling spruce recruitment. The type of decay observed in sub-alpine fir suggests that breakage is predominantly due to white rot fungi causing heart rot, such as Echinodontium tinctorium and Stereum sanguinolentum. In spruce, brown rot fungi, which are predominantly saprot or wound-entry decay fungi, became more common in larger spruce, and may result from wounding as sub-alpine fir fall from the canopy. Spatial analysis indicated the stumps with stem decay were clumped. However, this is more likely due to strong clumping of the host, rather than spreading of butt rot pathogens through the roots.