White pines, blister rust, and management in the Southwest

White pines in New Mexico and Arizona are threatened by the invasive disease white pine blister rust, Cronartium ribicola. Blister rust is already causing severe damage to a large population of southwestern white pine in the Sacramento Mountains of southern New Mexico. Recent detection in northern and western New Mexico suggests that a major expansion of the disease is likely over the next several years. Although little can be done to control blister rust in most forest situations, managers can address this long-term threat in a responsive and prudent manner. White pines, which include southwestern white, limber, and bristlecone pine, are found in most forested ranges of the Southwest. They most often occur as minor components in mixed conifer forests; several thousand acres are classified as white pine cover type. White pines have value for biodiversity, wildlife, aesthetics, and commercial timber. Although all North American white pine are highly susceptible to blister rust, there is evidence that low levels of genetic resistance occur in many populations. Resistance has already been found in several trees in the Sacramento Mountains, and seed from additional parent trees is being tested. Blister rust can be expected to impact white pines throughout most of the Southwest in coming decades. Nonetheless, some sites are more prone to blister rust than others. Even where conditions are especially favorable for blister rust, some trees may be resistant, providing a seed source for natural selection and eventual recovery of a population. However, near to complete extirpation of white pines may occur in some areas. On low hazard sites, infection rates and mortality are expected to be relatively low. These sites serve as important genetic refugia for white pines. Maintaining and promoting genetic diversity among white pines should be a key management objective, and a statement to this effect should be included in Forest Plans. We suggest that white pines be given a high species preference in silvicultural prescriptions. This simple, cost-effective strategy, by encouraging a diverse gene pool, would help insure the long-term survival of these unique trees. Eventually, seed from large numbers of known resistant trees could become the basis for a planting program to supplement natural populations.