White Pine Blister Rust Surveys in the Sangre De Cristo and Wet Mountains of Southern Colorado

Biological Evaluation R2-06-05 White Pine Blister Rust Surveys in the Sangre de Cristo and Wet Mountains of Southern Colorado Photo: Anna Schoettle, RMRS Kelly Sullivan Burns USDA Forest Service Rocky Mountain Region February 2006 White Pine Blister Rust Surveys in the Sangre de Cristo and Wet Mountains of Southern Colorado Biological Evaluation R2-06-05 February 2006 Prepared by: __________________________________ Kelly Sullivan Burns Forest Pathologist, Lakewood Service Center (LSC), Forest Health Management Approved by: ___________________________________ Jeffrey J. Witcosky Entomologist and Leader, LSC, Forest Health Management and _________________________________ Frank J. Cross Group Leader, Forest Health Management USDA Forest Service Rocky Mountain Region Renewable Resources P. O. Box 25127 Denver, CO 80225-0127 2 INTRODUCTION White pine blister rust (WPBR), caused by the fungus Cronartium ribicola, is an exotic, invasive disease of white, stone, and foxtail pines in the subgenus Strobus. The disease was accidentally introduced into the Pacific Northwest in 1910 and since then has spread nearly throughout the range of western white pines. White pine blister rust has a complex lifecycle involving five different spore stages and an alternate host, currants and gooseberries in the genus Ribes1. Pine hosts in Region 2 include limber pine (Pinus flexilis), Rocky Mountain (RM) bristlecone pine (P. aristata), whitebark pine (P. albicaulis), and southwestern white pine (P. strobiformis). White This WPBR canker will eventually girdle pine blister rust causes cankers which result in the tree. mortality of the portion of the branch or stem above the canker. Trees weakened by blister rust become susceptible to other damaging agents such as bark beetles. White pine blister rust may significantly impact reproductive potential by killing off cone-bearing branches. Small trees are easily killed because infections generally occur close to the main stem and ultimately girdle the tree. More detailed information on WPBR biology is available on the Forest and Shade Tree Pathology website (Worrall 2006): http://www.forestpathology.org/dis_wpbr.html. Limber and RM bristlecone pine are well distributed within the Sangre de Cristo and Wet Mountains (Biery 2003). High elevation white pines serve many important ecological functions. They provide food for wildlife, stabilize slopes, and maintain cover on harsh, rugged sites where little else could grow (Schoettle 2004b). They are some of the oldest and largest pines in the Rocky Mountain Region and are especially valued because of their unique cultural and ecological characteristics. White pine blister rust may result in greatly altered and even devastated ecosystems. For example, in heavily impacted areas, reduced post-fire reforestation and reduced sustainability of bird and wildlife species may result. On sites where limber pine is the only tree species present and mortality is high, hydrologic changes and slope instability could occur. Rocky Mountain bristlecone pine is a high elevation species which grows on dry sites. The range of RM bristlecone pine is almost entirely within the state of Colorado, with populations extending south into northern New Mexico and a disjunct population in the San Francisco Peaks of northern Arizona. In the Sangre de Cristo and Wet Mountains, RM bristlecone pines are often found mixed with other species such as aspen (Populus tremuloides), Engelmann spruce (Picea engelmanni), limber pine, and/or ponderosa pine (Pinus ponderosa) (Biery 2003). They are commonly found on rocky sites, ridges and 1 Recently, researchers identified naturally occurring C. ribicola infections on non-Ribes species hosts including Pedicularis racemosa and Castilleja miniata (Zambino et al., In Press). This new discovery could dramatically affect our understanding of WPBR epidemiology. 3 open parks on southeast to west aspects from 9,000 to 12,000 ft in elevation. Bristlecone pine is common throughout the Sangre de Cristo Mountains and on the west side of the Wet Mountains. Limber pine has a broad elevational range growing from below the lower treeline up to upper treeline (Schoettle 2004). Small amounts of limber pine occur in most stands in the Sangre de Cristo and Wet Mountains, but it becomes codominant near ridges and on rocky, south aspects between 8,500 and 10,000 feet in elevation (Biery 2003). Limber pine is gradually replaced by RM bristlecone pine at higher elevations and on the westward side of mountains. Limber pine is often a minor component of stands dominated by other species and is more common in the Wet Mountains where elevations are generally lower. In the Sangre de Cristo Mountains, limber pine-dominated stands are most common in the southeastern portion of the range forming an intermediate belt between lower elevation and subalpine forests (Alington 1998). WPBR was discovered for the first time in Colorado in 1998 just below the Colorado- Wyoming border north of Red Feather Lakes (Johnson and Jacobi 2000). Presumably, the disease spread south into Colorado from Wyoming where the disease has been present for decades (Brown 1978). Other parts of Colorado had not been surveyed extensively until an Evaluation Monitoring project was initiated in 2001, Monitoring White Pine Blister Rust Spread and Establishment in the Central Rocky Mountains (J.L. Harris and J.T. Hoffman, unpublished data). Surprisingly, field crews discovered WPBR in the Sangre de Cristo and Wet Mountains of southern Colorado, nearly 200 miles from any other known infection zone. The closest outbreak area to the south is on Gallinas Peak on the Cibola National Forest in central New Mexico (Conklin 2004). Infections in southern Colorado were found primarily on limber pine, but infected RM bristlecone pines were also observed for the first time in their native range (Blodgett and Sullivan 2004a, Blodgett and Sullivan 2004b). Great Basin bristlecone pine (Pinus longaeva) is the only native North American five-needle pine that remains uninfected. The distribution of WPBR in the central Rocky Mountains is displayed in Figure 4. It is unclear how Cronartium ribicola dispersed such a long distance. Possible theories include long distance spore dispersal in upper-level air currents or via infected nursery stock (either Ribes or white pine). Research suggests that it is likely that an isolated infestation in New Mexico is the result of a single introduction by long-distance atmospheric transport from infected areas in the Sierra Nevada Mountains (Frank and Geils, unpublished data). Atmospheric conditions capable of transporting spores were analyzed for transport capacity and then compared to dates on which surface conditions occurred that were conducive to fungal infection. Results demonstrate that necessary atmospheric conditions are common and surface conditions that coincide with these atmospheric events are rare, but do occur in the vicinity of the New Mexico infestations. It is possible that a similar situation occurred in southern Colorado. A canker growth rate model was developed which uses maximum canker length as a surrogate for canker age allowing managers to estimate the duration of WPBR infestations in limber pine (Kearns 2005). Mean longitudinal growth rate of actively 4 growing branch cankers was 3.3 inches per year. In 2005, the USFS and Colorado State University scientists measured over 600 WPBR branch cankers along the Mosca Creek Trail in the Great Sand Dunes National Park and Preserve (GRSA) as part of a pruning study. Maximum canker length was 36 inches suggesting that the disease has been present in that valley since 1993. The longest canker observed in the Wet Mountains was 16 in (this study) signifying that the disease was introduced on the pine host around 1999. The potential distribution of WPBR for white pines in Colorado was modeled using data from Wyoming limber pine plots, extensive Ribes plots installed throughout Wyoming and Colorado and PRISM climate data (Kearns 2005). Only the PRISM climate data were selected using classification and regression tree (CART) analyses to model the relationship between the site and climate variables and the presence or absence of rust for the state of Colorado. Important classification variables were May relative humidity, May minimum temperature, May precipitation and August minimum temperature. Results suggest that approximately 50 percent of Colorado’s white pine stands are at risk for WPBR establishment. This includes a significant portion of the Sangre de Cristo and Wet Mountains of southern Colorado. An analysis of the model and how it can be applied locally is available on the Rocky Mountains Region’s Forest Health Management website (http://www.fs.fed.us/r2/fhm/). Objectives In light of the new infestations in southern Colorado, a more in-depth survey was initiated in 2004 to supplement the Evaluation Monitoring survey initiated in 2001. The purpose of this study was to delimit the current geographic extent and intensity of the southern Colorado infestation, to determine the incidence of the disease on RM bristlecone pine and to establish long-term survey plots in and around the Sangre de Cristo infestation to monitor distribution, rates of spread and intensification and ecological impacts of the disease. An additional objective was to create a GIS database of recent USFS (Region 2, 3 and 4) and CSU survey locations that can be used to map the current distribution of WPBR and locate areas for future surveys and monitoring. This report provides results from all USFS Forest Health Management surveys conducted in the Sangre de Cristo and Wet Mountains of Colorado between 2003 and 2005. METHODS Study Area This study was conducted in the Sangre de Cristo and Wet Mountains of south-central Colorado (Figure 2) encompassing portions of the San Carlos Ranger District of the San Isabel National Forest, the Conejos Peak Ranger District of the Rio Grande National Forest and the GRSA. The Saguache Ranger District of the Rio Grande National Forest, the Huerfano State Wildlife Area and BLM lands bordering the San Isabel National 5 Forest were visited on reconnaissance trips however no plots were installed in these areas.

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