Dwarf Mistletoe in Coastal Western Hemlock Stands

Traditional silvicultural treatment of mistletoe includes the removal of infected stands through clear cut harvest, a practice that is effective at minimising or eliminating residual infection. The quantitative impacts of management practices other than clear cut harvesting are not yet fully understood from empirical observations, and long-term data collection efforts comparing mistletoe dynamics under alternative management regimes are rare or absent. There is, however, a concern that mistletoe infestation may not be adequately controlled following partial harvest entries. For example, residual overstorey infections can potentially propagate into regenerating understorey growth.

In the absence of comprehensive knowledge, we used a simulation modelling approach to do a preliminary evaluation of potential long-term impacts in coastal western hemlock stands in BC. To accomplish this, the spatial statistical model of dwarf mistletoe spread and intensification was linked to a coastal variant of the FVS stand projection model. The model was used to explore a number of management options, including different regeneration assumptions.

The resulting simulations produced plausible projections of stand growth and infection dynamics and demonstrated the key aspects of the model dynamics - sensitivity to:

	overstorey and understorey structure;
	different management activities; and
	different regeneration assumptions.

The model generally shows a dramatic decline in DMR and in the number of infected trees after the first entry. The degree of decline depends on the regeneration assumptions: if regeneration is present prior to the entry, there is less impact on the DMR. After the thinning, the model shows a rebound of infection as the infection is transferred from the residual overstorey to the new understorey trees. As expected, scenarios with a higher residual overstorey always transmit more mistletoe than do scenarios with low residual overstorey since they contain more overstorey trees. Likewise, regeneration scenarios that incorporate western red cedar all result in lower transmission success. This reduction is achieved through the physical blocking of seeds and because the infection measures are based on summing over both western red cedar (which is never infected) and western hemlock.

For more information about simulating the effects of dwarf mistletoe in coastal western hemlock stands, see Robinson et al. (2002) [PDF - 3.2 mb].