The Effects of Blister Rust on Post‐Fire Regeneration of Whitebark Pine: The Sundance Burn of Northern Idaho (U.S.A.)

Abstract

In the northern Rocky Mountains, whitebark pine (Pinus albicaulis) is rapidly declining as a result of previous fire exclusion policies, mountain pine beetle (Dendroctonus ponderosae) outbreaks, and white pine blister rust (Cronartium ribicola). Blister rust is potentially the most destructive agent, killing seedlings, cone‐bearing branches, and, eventually, mature trees. We examined densities of whitebark pine regeneration and the incidence and severity of blister‐rust infection of seedlings and saplings in the 25‐year‐old Sundance Burn in the Selkirk Range of northern Idaho, an area heavily infected by blister rust. We found that the mean regeneration density of whitebark pine was significantly lower than that of two other comparably aged burns in western Montana. The low density was attributed to the severe damage to the seed source on the burn perimeter, resulting from previous infestation of mountain pine beetle and blister rust. Overall, 29% of the whitebark pine regeneration in the Sundance Burn was infected by blister rust. Age and height of seedlings were important predictors of incidence of infection, and height was the most important predictor of severity of infection. Thus, as seedlings grow larger, they present a bigger target to airborne blister‐rust spores. Because of the lack of seed production in the adjacent forest and expected mortality, regeneration of whitebark pine in the Sundance Burn will be slow. In areas of northern Idaho and northwestern Montana affected by blister rust and pine beetle, prescribed fires for managing whitebark pine ecosystems should be restricted to small areas or should require plantings of rust‐resistant seedlings.