• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Projecting Changes in Dwarf Mistletoe Infestation

Information on mistletoe life history, ballistic dispersal, and patterns of contagion was integrated into a conceptual model of mistletoe epidemiology and incorporated with simulation models of host growth, survival, and responses to infection and treatment. These models were evaluated for sensitivity and validity compared to benchmark stands. The combined mistletoe and host models were developed as practical tools managers could use to assess the potential, decades-scale consequences of various silvicultural treatments on indicators of forest condition. This work improved upon previous tools with the capability to represent stands of the complex composition, structure, and spatial arrangement of trees and mistletoe as often encountered in western forests. Both the model and monitoring confirmed the under-appreciated expectation that under certain conditions, an initially small amount of mistletoe can spread and intensify to a serious infestation with severe impacts.

This research demonstrates that the complex epidemiology of dwarf mistletoe can be practically reduced to a model representation of elements for life history, ballistic dispersal, and patterns of host and pathogen contagion. The growth, development, reproduction, and health of mistletoe plants are simulated with a few, simple relations (e.g., exposure to light) and adequately quantified with an index commonly used in field surveys, the Dwarf Mistletoe Rating (DMR). The physics of the ballistics for thousands of mistletoe seeds can be spatially modeled as a 2-dimensional grid with a 2 m resolution and probabilistic approach. Host trees occur in various distributions from random to uniform to clumped and infected trees occur in more or less distinct patches; the degree of contagion for the host and for the pathogen does have a noticeable impact on the spread and intensification of mistletoe. For stands inventoried with commonly used sampling procedures, contagion statistics can be easily estimated and employed for stands with distinct spatial structure. Model simulations that mimic two ponderosa pine stands monitored for five decades suggests there is acceptable correspondence between observed and projected spread and intensification of the mistletoe. One of these stands had been initially treated with sanitation by pruning or removing infected trees; there had been few escapes and these were adequately tracked in the simulation. The other stand had been left as a no-treatment comparison; over time, initially-infected trees died and the number of ingrowth trees increased. Again the simulation adequately tracked the changes in mistletoe infestation.

The indicators of forest health and productivity of interest to managers include measures such as fuel loading and distribution, tree mortality rate, and net volume growth. These can be affected by mistletoe infection and mistletoe infection changes over time. The epidemiological model described in this research is coupled with several vegetation simulation models to allow managers to assess some of the long-term consequences of alternative actions. The model has been employed for several demonstration projects and is compatible with variants of the Forest Vegetation Simulation used operationally in the National Forest System.

References

"Modelling dwarf mistletoe at three scales: life history, ballistics, and contagion" Ecological Modelling in press [DOI 10.1016/j.ecolmodel.2006.06.007];

"Host physiological condition regulates parasitic plant performance" Oecologia 146:179-189 [DOI 10.1007/s00442-005-0215-0];

"Characterizing the effects of dwarf mistletoe and other diseases for sustainable forest management" BC Journal of Ecosystems and Management 3:1-7 [http://www.forrex.org/jem/2004/vol3/no2/art6.pdf];

"Spatial statistical model for the spread of dwarf mistletoe within and between stands" USDA Forest Service Proceedings RMRS-P-25 [http://www.fs.fed.us/rm/pubs/rmrs_p025/rmrs_p025_178_188.pdf];

"A spatial statistical model for the spread of dwarf mistletoe within managed stands" Proceeding of 50th Western International Forest Disease Work Conference, 131-140.