Invasive tree disease disrupts pine/bird mutualism

Distributions of whitebark pine, Clark's nutcracker and McKinney's study sites in the Rocky Mountains.

Many trees with large seeds rely on vertebrate seed predators to disperse their seeds. The whitebark pine, a key subalpine species, has coevolved with the Clark’s nutcracker into a tight mutualism.  In their paper in the April Ecological Applications, Shawn McKinney, a post-doc at the University of Montana, and his colleagues studied a natural disruption to this mutualism: an invasive tree fungus.

Clark’s nutcrackers feed on whitebark seeds throughout the summer. Beginning in late summer, the nutcrackers extract ripe whitebark pine seeds from pinecones and bury them in the ground for use as food during the winter and spring. Although the nutcrackers harvest and cache seeds of other pines, whitebark pines depend nearly exclusively on nutcrackers for seed dispersal.

White pine blister rust, a disease caused by a tree pathogen that kills cone-bearing branches, has been spreading through the subalpine forests and now occurs in almost all whitebark pine populations. According to the researchers, mortality from blister rust reaches 90 percent or higher in some populations in the northern Rocky Mountains.

For the trees, this disease can spell disaster: When whitebark pine seeds are scarce, the nutcrackers often leave subalpine forests in search of food at lower altitudes. Decimated by disease and left without dispersers to spread what few seeds they produce, whietbark pines can dwindle rapidly. McKinney and his colleagues quantified forest conditions and ecological interactions between nutcrackers and whitebark pines in three Rocky Mountain ecosystems that differed in levels of rust infection and mortality. They estimated that within each hectare, a threshold level of 1000 pinecones, or more than a 5-meter square area of trees, is needed for a high likelihood of seed dispersal by nutcrackers.

The researchers say that their estimates can be used by forest managers to assess when and where to plant new stands of whitebark pines to encourage nutcrackers to stick around. They say the estimates will be especially useful in Montana, where pinecone production and tree density fell below threshold levels.

McKinney, S., Fiedler, C., & Tomback, D. (2009). Invasive pathogen threatens bird–pine mutualism: implications for sustaining a high-elevation ecosystem Ecological Applications, 19 (3), 597-607 DOI: 10.1890/08-0151.1