United States Department of Agriculture

Proactive Limber Pine Conservation 

Strategy for the Greater Rocky 

Mountain National Park Area

Anna W. Schoettle, Kelly S. Burns, Christy M. Cleaver, J. Jeff Connor

Rocky Mountain

Research Station

General Technical Report

RMRS-GTR-379

Forest February 2019

Service
Schoettle, Anna W.; Burns, Kelly S.; Cleaver, Christy M.; Connor, J. Jeff. 2019. 

Proactive limber pine conservation strategy for the Greater Rocky 

Mountain National Park Area. Gen. Tech. Rep. RMRS-GTR-379. Fort 

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 81 p. 

Abstract

This proactive conservation strategy addresses the unique situation of limber pine 

in the Greater Rocky Mountain National Park Area (GRMNPA). The target area 

includes Rocky Mountain National Park and surrounding areas of northern Colorado 

and southern Wyoming. The GRMNPA is at the infection front for white pine blister 

rust (WPBR) where populations were also impacted by the recent mountain pine 

beetle epidemic and are threatened by climate change. This is the first proactive 

conservation strategy for a five-needle pine species in North America. It focuses 

on timing specific monitoring efforts and interventions to sustain healthy limber 

pine populations and ecosystems during invasion and naturalization of WPBR, 

thereby putting limber pine on a trajectory that reduces the probability of ecosystem 

impairment in the future. The high frequency of complete resistance to WPBR in 

limber pine populations in the GRMNPA is a distinctive feature of this area’s ecology. Having this information and other site-based genetic and disturbance ecology 

information before WPBR affects the populations is also unique and warranted 

the development of this proactive conservation strategy. The strategy outlines 

recommendations to promote (1) ex situ and in situ limber pine conservation and 

protection, (2) increased limber pine population size and sustained genetic diversity, 

(3) treatments to maintain durability of genetic resistance to WPBR, (4) monitoring forest health conditions for early detection of WPBR and changes in pathogen 

virulence, and (5) coordinated management actions within and among agencies. 

The recommendations apply to the GRMNPA and possibly to all of the southern 

Rockies; the approach used can be applied further. The recommendations herein 

are expected to be relevant for at least 20 years.

Keywords: Pinus flexilis, five-needle white pine, white pine blister rust, genetic 

disease resistance, gene conservation, regeneration, proactive 

management, resilience, Cronartium ribicola, Dendroctonus 

ponderosae

Cover photo: Limber pine along Lake Haiyaha (J. J. Connor, NPS).

All Rocky Mountain Research Station publications are published by U.S. Forest Service 

employees and are in the public domain and available at no cost. Even though U.S. 

Forest Service publications are not copyrighted, they are formatted according to U.S. 

Department of Agriculture standards and research findings and formatting cannot be 

altered in reprints. Altering content or formatting, including the cover and title page, is 

strictly prohibited.
Authors

Anna W. Schoettle is a Research Plant Ecophysiologist with the USDA Forest 

Service, Rocky Mountain Research Station in Fort Collins, Colorado. 

Kelly S. Burns is a Plant Pathologist with the USDA Forest Service, Forest Health 

Protection in Lakewood, Colorado.

Christy M. Cleaver was a Research Associate at Colorado State University in Fort 

Collins, Colorado, at the time of writing; she is now a Plant Pathologist with the 

USDA Forest Service, Forest Health Protection in Coeur d'Alene, Idaho.

J. Jeff Connor is a Natural Resource Specialist (retired) from the DOI National 

Park Service, Rocky Mountain National Park in Estes Park, Colorado.

Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not 

contain recommendations for their use, nor does it imply that the 

uses discussed here have been registered. All uses of pesticides 

must be registered by appropriate State and/or Federal agencies 

before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic 

animals, desirable plants, and fish or other wildlife if they are not 

handled or applied properly. Use all pesticides selectively and 

carefully. Follow recommended practices for the disposal of surplus pesticides and pesticide containers.

The use of trade or firm names in this publication is for reader information and 

does not imply endorsement by the U.S. Department of Agriculture of any product 

or service.

i

Preface

This strategy for the conservation of limber pine is applicable to the Greater Rocky 

Mountain National Park Area (GRMNPA), which includes Rocky Mountain National 

Park (RMNP) and surrounding areas of northern Colorado and southern Wyoming. 

An earlier version was prepared specifically for RMNP; however, the genetic and 

ecological characteristics of RMNP and the greater area are sufficiently similar 

that the authors have expanded the reference area. The recommendations in this 

strategy apply to this greater area and possibly to all of the southern Rockies; the 

approach we used can be applied further. The information used in this strategy is 

the most current available and includes site-based research on genetics, adaptive 

variation, regeneration dynamics, disturbance regimes, forest health condition, 

and ecological interactions of limber pine as well as extrapolation of research from 

limber pine in other parts of its distribution and other five-needle pines, where we 

thought it necessary and appropriate. We expect the recommendations herein will 

continue to be relevant for at least 20 years.

This geographic area is at the infection front for white pine blister rust (WPBR). As 

the disease continues to spread and intensify, the forest health condition of limber 

pine will change. Consequently, continued monitoring (as recommended in this 

strategy) and consultation between the National Park Service, U.S. Forest Service 

Forest Health Protection (FHP), and Rocky Mountain Research Station (RMRS), 

and local university collaborators are encouraged in order to assess and interpret 

the changes to these ecosystems over time. Given the dynamic nature of the 

landscape, the scheduled timing and intensity of specific actions recommended in 

this strategy will be adapted, reevaluated, and refined as new information becomes 

available.

This strategy is the outcome of a collaboration between RMNP, RMRS, and FHP. 

Begun in 2008 by Jeff Connor, Natural Resource Specialist for RMNP, this partnership was requested by RMNP and put into action products of the Proactive Strategy 

Research Program initiated by Schoettle in 2001, introduced in 2004 (Schoettle 

2004a) and further developed in 2007 (Schoettle and Sniezko 2007). We established a network of limber pine sites in and around RMNP for (1) protection from 

mountain pine beetle, (2) forest characterization, (3) collection of seed for gene 

conservation, (4) genetic studies including common-garden experiments, and (5) 

WPBR resistance testing. These studies were accomplished with additional collaborations with Colorado State University (Fort Collins, Colorado), U.S. Forest Service 

Dorena Genetic Resource Center (Cottage Grove, Oregon), and Mountain Studies 

Institute (Silverton, Colorado), and they complement other collaborative studies with 

Great Sand Dunes National Park and Preserve.

Other plans to manage effects of WPBR for high-elevation five-needle pines exist. 

However, most of the other strategies outline approaches and actions to restore 

function to populations and ecosystems that have already undergone high WPBRcaused mortality in whitebark pine—a species that has been shown to now warrant 

designation as endangered in the United States and Canada. These plans include:

•  Managing for Healthy White Pine Ecosystems in the United States to Reduce 

the Impacts of White Pine Blister Rust (Samman et al. 2003)

•  Whitebark Pine Restoration Strategy for the Pacific Northwest Region 

2009–2013 (Aubry et al. 2008)

•  Management Guide to Ecosystem Restoration Treatments: Whitebark Pine 

Forests of the Northern Rocky Mountains, U.S.A. (Keane and Parsons 2010)
ii

•  Whitebark Pine Strategy for the Greater Yellowstone Area (Greater Yellowstone 

Coordinating Committee Whitebark Pine Subcommittee 2011)

•  A Range-wide Restoration Strategy for Whitebark Pine (Pinus albicaulis) 

(Keane et al. 2012)

•  Strategies, Tools, and Challenges for Sustaining and Restoring High Elevation 

Five-Needle White Pine Forests in Western North America (Keane and 

Schoettle 2011)

•  Alberta Whitebark Pine Recovery Plan 2013–2018 (Alberta Whitebark and 

Limber Pine Recovery Team 2014b)

Only three restoration or conservation plans address limber pine. Many of the interventions recommended herein are shared with these restoration strategies, though 

they differ in their timing, site prioritization, and specifics. These plans are:

•  White Pine Blister Rust in the Rocky Mountain Region and Options for 

Management (Burns et al. 2008)

•  White Pines, Blister Rust, and Management in the Southwest (Conklin et al. 

2009)

•  Alberta Limber Pine Recovery Plan 2014–2019 (Alberta Whitebark and Limber 

Pine Recovery Team 2014a)

This conservation strategy addresses the unique situation of limber pine in RMNP 

and the southern Rockies. It recommends proactive conservation actions specific 

to the southern Rockies based on knowledge from the RMNP/RMRS/FHP program 

and other research that provides site-based information gathered before ecosystem impairment by WPBR. It focuses on timing the specific monitoring efforts and 

interventions to sustain healthy limber pine populations and ecosystems during 

invasion and naturalization of WPBR, thereby putting limber pine on a trajectory that 

reduces the probability of ecosystem impairment in the future. The high frequency 

of complete resistance to WPBR in limber pine populations in RMNP and surrounding areas is a unique feature of this area’s ecology. That we have this information 

and the other site-based genetic and disturbance ecology information before WPBR 

affects the populations is unique. This situation justifies developing a conservation 

strategy specific to RMNP and the greater area. 

Respectfully,

Anna W. Schoettle, Kelly S. Burns, Christy M. Cleaver, and J. Jeff Connor
iii

Executive Summary

Limber pine (Pinus flexilis) is common in northern Colorado and southern Wyoming 

and grows along the full elevation gradient from the Pawnee National Grasslands 

to the alpine treeline. It occurs on Federal Forest Service, National Park Service, 

and Bureau of Land Management (BLM) lands as well as State, county, and private 

lands and grows in designated wilderness areas. Limber pine is a keystone species that maintains ecosystem structure and function. It defines the alpine treeline, 

grows on exposed sites too harsh for other tree species, often becomes established 

on a site soon after disturbance, facilitates forest succession and the growth of 

understory species, aids in soil stabilization and snowpack retention and melt, and 

produces seeds that are a food source for birds, small mammals, and bears. Here, 

we outline a strategy for conserving limber pine that is applicable to the Greater 

Rocky Mountain National Park Area (GRMNPA), which includes Rocky Mountain 

National Park (RMNP) and surrounding areas of northern Colorado and southern 

Wyoming.

Limber pine has been designated a Species of Management Concern in RMNP, 

and maintains biodiversity in the park and greater landscape. Additionally, as longlived and wind-defiant trees, limber pines are admired for their aesthetic value and 

cultural significance throughout their distribution, including the sentinel on Knife 

Edge along Trail Ridge Road and another by Lake Haiyaha. Most of the limber pine 

within RMNP occurs in designated wilderness managed under Class I Revegetation 

Guidelines.

Throughout the GRMNPA, limber pine is declining due to the combined effects 

of recent severe droughts and the climate-exacerbated mountain pine beetle 

(MPB, Dendroctonus ponderosae) outbreak. It is also imminently threatened by 

the invasion of the nonnative pathogen Cronartium ribicola, which causes the 

lethal disease white pine blister rust (WPBR) in five-needle white pines. In North 

America, WPBR has spread through much or part of the range of eight out of nine 

white pine species, affecting community composition and function. WPBR was 

confirmed in southern Wyoming in the 1990s, in northern Colorado in 1998, and in 

RMNP in 2010. It is expected to spread, kill trees, and negatively affect biodiversity, 

ecosystem processes, and other resources, compounding the impacts of changing climate. These stressors are causing widespread mortality throughout much of 

limber pine’s range and basal area losses are projected to exceed 40 percent by 

2030. Consequently, limber pine is listed as a Special Status Species on BLM land 

in Wyoming and is recommended for endangered status in Canada. Loss of this 

keystone species would ultimately lead to cascading ecological consequences and 

loss of biodiversity in the GRMNPA. Sustaining limber pine populations cannot be 

achieved with a “no action” option.

The U.S. Forest Service Rocky Mountain Research Station (RMRS) began the 

Five-Needle Pine Proactive Strategy Research Program in 2001, with a focus 

on conservation and management of limber pine and Rocky Mountain bristlecone pine (P. aristata) in the southern Rockies. In 2008, a collaboration between 

RMNP, RMRS, and U.S. Forest Service Forest Health Protection (FHP) initiated 

an intensive effort to conserve limber pine genetic diversity in and around RMNP. 

The intensive RMNP program and the broader Proactive Strategy research provide the site-based information necessary to develop this Proactive Limber Pine 

Conservation Strategy for the GRMNPA. The objectives are to: (1) conserve genetic 

diversity and populations of limber pine throughout the GRMNPA, (2) provide 

science-based recommendations appropriate for the GRMNPA (in and outside of 

wilderness) to sustain healthy high-elevation ecosystems, and (3) encourage and 
iv

facilitate coordination among a variety of land managers to preserve the genetic 

integrity of native flora and fauna in the southern Rocky Mountains and support collaborative efforts to ensure the future health of limber pine.

Managers in the GRMNPA are in a unique and enviable position in western North 

America, as they are proactively focusing on this threatened species while the 

ecosystems are still largely in “healthy” condition (that is, before WPBR-caused 

mortality of reproductive trees). This proactive approach enables informed management to avoid and mitigate ecological impacts before they are expressed. Many 

western national parks and national forests are in a reactive mode, managing the 

species only after WPBR-caused mortality of limber pine and other five-needle 

pines. Learning from the ecological effects from losses of whitebark pine (Pinus 

albicaulis) and limber pine in the northern Rockies, RMNP and other entities have 

opted to implement proactive conservation activities in an effort to shift limber pine’s 

trajectory toward resilience and away from endangered species status. Natural regeneration processes with supplemental planting or seeding before and early in the 

spread of WPBR will increase the number of trees on the landscape and will offset 

future mortality. Early intervention will reduce the potential need for restoration later 

during naturalization of WPBR. Recent research and products from RMRS, FHP, 

and RMNP provide unique and timely area-specific knowledge that includes the 

following:

•  The ecosystems are still in the early stages of WPBR invasion, allowing time 

before severe ecological impacts are expressed.

•  Post-fire habitats in the GRMNPA are conducive to successful natural regeneration of limber pine.

•  Genetic resistance to WPBR, conferred by a single dominant resistance allele 

(Cr4), is present in native populations of limber pine in the GRMNPA at high 

frequency (~20 percent), although this resistance may be overcome by the rust 

with time (i.e., it may not be durable).

•  Genetic resistance to WPBR that is conferred by multiple genes (more likely to 

be durable) may be present in the GRMNPA at low frequencies.

•  MPB outbreaks in the GRMNPA are cyclical; dwarf mistletoe (Arceuthobium 

cyanocarpum) is common and other native pests are present.

•  Climate change is projected to shift limber pine populations to higher elevations 

in RMNP as well as intensify the effects of native pests and pathogens throughout the region. 

The Proactive Conservation Strategy recommends continued and expanded application of the current proactive management approach because it offers the best 

opportunity to prepare the landscape for greater resilience before WPBR-caused 

impacts develop and to sustain populations as the disease spreads.

The major focal areas for management activities are:

•  Promote ex situ conservation: Continue and expand efforts to collect and 

archive limber pine genetic diversity through seed collections.

•  Promote in situ conservation: Protect limber pine trees from MPB, WPBR, 

and fire to minimize mortality when and where land designations and management objectives permit.

•  Increase population size and sustain genetic diversity: Increase the number of limber pine trees on the landscape through planting or seeding, or both, 

immediately to offset future mortality and to sustain viable genetically diverse, 

self-sustaining populations.
v

•  Locate treatments to maintain durability of qualitative WPBR resistance: 

Minimize selective pressure on the rust by planting trees with a range of susceptibilities in low-WPBR-risk areas to reduce the probability of the proliferation 

of genotypes virulent to Cr4.

•  Discover, develop, and deploy local quantitative WPBR-resistant sources: 

Research quantitative (polygenic) WPBR resistance types in limber pine in the 

GRMNPA and establish a seed orchard/clone bank of these genotypes (which 

can be protected from fire and other stresses) to provide seed for future planting projects.

•  Monitor pines and rust: Monitor limber pine health and monitor for early detection of WPBR and WPBR virulence.

•  Coordinate management actions within and among agencies: Preserve 

and diversify the age- and size-class distribution of limber pines as well as all 

successional stages across the landscape through fuels treatments, vegetation 

management, and planned fire. 

The major focal area for leadership activities is:

•  Interagency coordination and cooperation for efficient use of resources 

for effective management outcomes: The U.S. Forest Service has an “all 

lands” strategic approach that emphasizes consideration of the greater landscape and that crosses administrative boundaries in management planning. 

RMNP also takes into consideration the greater landscape surrounding the 

park, referred to as the Protected Area Centered Ecosystem, when managing 

resources. Further, RMNP focuses on continued leadership in limber pine 

conservation in the southern Rockies through research, genetic resource management, and coordination with other agencies and parks. RMNP managers 

strive to continue research on limber pine ecosystems and the WPBR pathosystem and encourage infrastructure that supports management objectives 

(e.g., a seed orchard/clone bank). Maintaining viable seed sources and seed 

collections helps to fulfill the park’s obligation as an International Biosphere 

Reserve.
vi

Acknowledgments

We thank the contributors to this document including Sparkle Malone, Brian Howell, 

Jennifer Ross, Bob Cain, Sky Stephens, and William Jacobi. We also thank reviewers Ben Bobowski, Diana Tomback, Mike Battaglia, William Jacobi, Erin Borgman, 

and Amy Chambers, whose comments greatly improved the document. We also 

thank Jeff Witcosky, Richard Sniezko, Angelia Kegley, Betsy Goodrich, Jen Klutsch, 

Jonathan Coop, William Jacobi, John Popp, Mike Antolin, and Phyllis Pineda-Bovin 

for their assistance with the 

research that provided much 

of the science foundation for 

the strategy. We extend our 

thanks to the RMNP coordinators whose commitment 

made this project possible 

including Paul McLaughlin, 

Scott Esser, Erin Borgman, 

Brian Verhulst, John Mack, 

Dale Loper, and Judy Visty, 

and the crew of exceptional 

volunteers over the years.

We also recognize the 

contributions of Sheryl 

Costello, U.S. Forest Service 

Rocky Mountain Region 

Entomologist, who tragically 

lost her life doing what she 

loved—mountain climbing. 

Her high degree of professionalism and expertise 

played an important role in 

the early development of this 

strategy.
vii

Table of Contents

SECTION 1. PURPOSE AND NEED ........................................................................1

1.1. Purpose and Need .........................................................................................................................1

1.2. Federal Forest Service and Park Service Policy .........................................................................10

1.3. Objectives for Limber Pine Conservation ....................................................................................12

SECTION 2. STRATEGY FOR CONSERVATION ACTIONS .................................14

2.1. Consequences of Doing Nothing .................................................................................................14

2.2. The Time to Act Is Now ................................................................................................................15

2.3. Cornerstones of Management .....................................................................................................16

2.3.1. Support Regeneration and Increased Population Size to Maintain 

Adaptive Capacity ...............................................................................................................16

2.3.2. Manage Genetic Resistance to White Pine Blister Rust for Sustainability .........................16

2.3.3. Foster Early Awareness and Engagement to Enhance Opportunity for Success ...............17

SECTION 3. RECOMMENDED CONSERVATION ACTIONS ................................18

3.1. Promote Ex Situ Conservation ....................................................................................................19

3.2. Promote In Situ Conservation .....................................................................................................19

3.3. Increase Population Size and Sustain Genetic Diversity .............................................................20

3.4. Locate Treatments to Maintain Durability of Qualitative Resistance to White Pine 

Blister Rust ..............................................................................................................................22

3.5. Discover, Develop, and Deploy Local Quantitative White Pine Blister Rust-Resistant 

Seed Sources ..........................................................................................................................23

3.6. Monitor Pines and Rust ...............................................................................................................24

3.7. Coordinate Management Actions Within and Among Agencies ...................................................27

SECTION 4. RESOURCES TO EXECUTE THIS STRATEGY ...............................28

SECTION 5. GUIDELINES FOR MANAGEMENT OF LIMBER PINE ....................29

5.1 Limber Pine Seed Collections—Bulk Cone Collections ................................................................29

5.2 Limber Pine Seed Collections—Individual Tree Collections .........................................................31

5.3 Limber Pine Seed Storage and Database ....................................................................................33

5.4 Protecting Limber Pine from Mountain Pine Beetle (MPB) ..........................................................34

5.5 Sanitation Pruning to Remove White Pine Blister Rust (WPBR) Infections ................................36

5.6 Planting Limber Pine Seedlings ...................................................................................................38

5.7 Direct Seed Limber Pine ...............................................................................................................40

5.8 Seed Transfer / Source Guidelines ...............................................................................................41

5.9 Natural Regeneration ...................................................................................................................41

5.10 Locating Treatments and Selecting Seed Sources to Maintain Durability of 

WPBR Resistance ...................................................................................................................43

5.11 Testing for Quantitative Resistance to WPBR .............................................................................44

5.12 Monitoring—Forest Health ..........................................................................................................45

5.13 Monitoring—Early Detection of WPBR in Riparian Areas ...........................................................46

5.14 Monitoring – Rust Virulence .......................................................................................................47

5.15 Consider Limber Pine During Fuel Treatments and Planned and Unplanned Fire .....................47

5.16 Limber Pine Seed Orchard / In Situ Clone Bank ........................................................................49
viii

SECTION 6. BACKGROUND .................................................................................51

6.1. Overview ......................................................................................................................................51

6.2. Evolutionary History of Limber Pine ............................................................................................55

6.3. Genetic Diversity .........................................................................................................................55

6.4. Natural History Traits ...................................................................................................................56

6.5. Community Dynamics ..................................................................................................................56

6.6. Clark’s Nutcracker Ecology ..........................................................................................................57

6.7. Natural Regeneration Facilitated by Clark’s Nutcracker ..............................................................58

6.8. Mountain Pine Beetle Biology and Control ..................................................................................60

6.9. White Pine Blister Rust Biology and Control ...............................................................................61

6.10. Dwarf Mistletoe Biology and Control .........................................................................................64

6.11. Susceptibility to Other Biotic Damage Agents............................................................................64

6.12. Role of Fire in Limber Pine Ecology ..........................................................................................65

6.13. Sensitivity to Abiotic Damage Agents: Nitrogen Deposition and Climate Change .....................66

SECTION 7. REFERENCES ...................................................................................68

Abbreviations Used Herein

BLM United States Department of the Interior, Bureau of Land Management

d.b.h. diameter at breast height (4.5 ft, 1.37 m)

FHP USDA Forest Service, Forest Health Protection

FSM Forest Service Manual

GRMNPA Greater Rocky Mountain National Park Area

MPB mountain pine beetle (Dendroctonus ponderosae)

NPS United States Department of the Interior, National Park Service

RMNP Rocky Mountain National Park

RMRS USDA Forest Service, Rocky Mountain Research Station

ROMN Rocky Mountain Inventory and Monitoring Network

USDA United States Department of Agriculture

USFS USDA Forest Service

WPBR white pine blister rust (Cronartium ribicola)
USDA Forest Service RMRS-GTR-379. 2019. 1

SECTION 1. PURPOSE AND NEED

1.1. Purpose and Need

Limber pine (Pinus flexilis) is the only tree designated as a Species of 

Management Concern in Rocky Mountain National Park (RMNP) and is of management interest on other Federal, State, and county lands (Burns et al. 2008). It is declining in the Greater RMNP Area (GRMNPA) from the combined effects of native 

insects, drought, and climate change and is further threatened by a lethal, nonnative 

pathogen. Specifically, limber pine has succumbed to the recent mountain pine 

beetle (MPB, Dendroctonus ponderosae) outbreak and is imminently threatened by 

the nonnative fungal pathogen Cronartium ribicola, which causes white pine blister 

rust (WPBR). The combined threats of MPB, WPBR, limber pine dwarf mistletoe 

(Arceuthobium cyanocarpum), and climate change have caused widespread decline 

and mortality in limber pine. These threats have also reduced the capacity for forest 

recovery throughout a significant portion of the species’ range (Cleaver et al. 2015; 

Smith et al. 2013); a 40 percent loss in basal area of limber pine is projected by 

2030 in the absence of intervention (Krist et al. 2014). 

White pine blister rust was detected on limber pine in southern Wyoming in the 

1990s, in northern Colorado on limber pine in 1998 (Johnson and Jacobi 2000), 

and in south-central Colorado in 2003 on limber and Rocky Mountain bristlecone 

pine (P. aristata) (Blodgett and Sullivan 2004). In 2010, WPBR was confirmed on 

limber pine in RMNP (Schoettle et al. 2011a, 2018a). The incidence and severity of 

the disease are expected to increase over time, leading to negative impacts on biodiversity, ecosystem processes, and other natural resources. White pine blister rust 

kills trees of all ages. It kills seedlings and saplings most rapidly, thereby impairing 

post-disturbance recovery, and therefore resilience of the overall forest landscape. 

The purpose of this strategy is to coordinate management objectives and implement 

proactive strategies and tools to sustain limber pine populations and ecosystems in 

the GRMNPA.

As a foundation and keystone species, especially at high elevations and in harsh 

habitats, limber pine affects ecosystem stability and processes, and it also has a 

disproportionately large influence on biodiversity relative to its abundance. Limber 

pine is the only five-needle white pine species in most of the GRMNPA and is an 

important ecosystem component across its patchy, broad distribution in western 

North America (Schoettle 2004b; Steele 1990). Limber pine occurs range-wide 

across a greater span of elevations (2,250 ft to 12,500 ft [870 m to 3,810 m]) than 

any of the other white pine species (Schoettle and Rochelle 2000; Steele 1990). 

In the GRMNPA it has a broader elevational range than any other conifer species 

(5,250 ft to 11,155 ft [1,600 m to 3,400 m]) and is the only tree species found at all 

elevations up to treeline (Goetz et al. 2010). Limber pine defines the lower treeline 

in the Pawnee National Grassland and the upper alpine treeline in RMNP. Limber 

pine is often replaced at the alpine treeline by two other five-needle pines outside 

the GRMNPA—Rocky Mountain bristlecone pine (P. aristata) to the south and 

whitebark pine (P. albicaulis) to the north—highlighting the unique physical and 

ecological conditions of the GRMNPA. 
2 USDA Forest Service RMRS-GTR-379. 2019.

Limber pine is shade-intolerant and typically occurs in open, low-density stands 

on dry, rocky, windswept sites that are free of competition from other tree species 

(Schoettle 2004b; Schoettle and Rochelle 2000). It is often the first tree species 

to regenerate after fire and facilitates forest succession (Baumeister and Callaway 

2006; Donnegan and Rebertus 1999). Additionally, limber pine aids in soil stabilization and snowpack retention and also provides a food source for squirrels, other 

rodents, bears, and birds, particularly Clark’s nutcrackers (Nucifraga columbiana), 

which benefit limber pine by providing long-distance seed dispersal (Tomback and 

Taylor 1987). The nutcracker-limber pine interaction is similar to that of whitebark 

pine—an iconic natural history story in the West. 

Limber pine is also admired for its cultural importance and significance, aesthetic 

value, and symbolism as a survivor, and it is a favorite with mountain visitors. The 

pine’s large, nutritious seeds were an important food source for Native Americans, 

and other parts of the tree were used for medicinal and ceremonial purposes 

(Moerman 1998, 2009; Tomback et al. 2011). Limber pine inhabits historically 

significant popular mountain passes of the Rocky Mountains and provides a unique 

connection with American history (Peattie 1991). One particular limber pine beside the route of the first transcontinental railroad, named “The Old Pine Tree,” is 

deemed the “most photographed object in Wyoming,” (Peattie 1991, p. 35). Limber 

pine is a long-lived species; some individuals in the GRMNPA are more than 1,000 

years old (Schoettle 2004b). Their unique, gnarled appearance signals strength and 

endurance despite fierce winds at exposed, high-elevation sites. Several iconic and 

frequently photographed stands of limber pine exist in RMNP, including the stand 

on Knife Edge along Trail Ridge Road and another along Lake Haiyaha (fig. 1). 

Figure 1—Limber pine along Lake Haiyaha (J. J. Connor, NPS).
USDA Forest Service RMRS-GTR-379. 2019. 3

In the GRMNPA, limber pine occurs on nearly 130,000 ac (52,609 ha), yet most 

of the area is suitable for the species (fig. 2). The majority of limber pine within 

the GRMNPA occurs on lands administered by the U.S. Forest Service (USFS) (53 

percent), followed by the National Park Service (NPS) (14 percent), Bureau of Land 

Management (BLM) (3 percent), Colorado State Forest Service (3 percent), and 

Wyoming Department of Forestry (0.5 percent). Limber pine occurs on about 5,500 

ac (2,225 ha) in the park and much of the forested eastern front of RMNP is also 

potential limber pine habitat. A large portion of limber pine’s distribution within the 

GRMNPA occurs on privately owned lands (26 percent). 

The native and nonnative biological disturbance agents, combined with a warming climate, are threatening limber pine in the GRMNPA and throughout much of 

its distribution. Of the 13 pine species attacked by MPB, limber pine is a preferred 

host (Cerezke 1995). Mountain pine beetles, native to western North America, have 

caused widespread mortality of pines throughout much of its range (Chapman et al. 

2012; Raffa et al. 2008; Smith et al. 2015). In Colorado, 3.4 million ac (1.4 million 

ha) have been affected since the start of the most recent outbreak in 1996 (USFS 

Rocky Mountain Region 2014). Increased average monthly temperatures have 

favored MPB in this latest epidemic (Chapman et al. 2012), and climate change 

increases the likelihood of expansion of MPB into higher elevations in the Rocky 

Mountains, where outbreaks historically have been rare (Raffa et al. 2008).

In the GRMNPA, the effects of MPB from 1996 to 2015 are evident throughout 

the distribution of pine hosts (figs. 3 and 4). On evaluated plots in RMNP, only 55 

percent of limber pine trees were categorized as healthy (Kelly Burns, personal 

communication 2013). The effects of the MPB-caused mortality of limber pine in 

the GRMNPA are not uniform; the proportion of a plot with MPB activity ranged 

from 33 to 96 percent in subareas of the GRMNPA (average of 63 percent) (Cleaver 

et al. 2015). The percentage of stems killed by MPB also varied from 6 to 32 

percent within infested plots (Cleaver et al. 2015). In northern Colorado (excluding RMNP), MPB was active in 71 percent of the plots with an average incidence 

of 14 percent (as of 2011) (Cleaver et al. 2015), similar to the incidence in RMNP 

(average incidence of 19 percent; range of 0–49 percent). Impacts to limber pine 

include mortality of mature, cone-bearing trees—some of which may have genetic 

resistance to WPBR—and possible loss of genetic diversity. Stressors such as 

drought, disease, and insects (limber pine engraver [Ips woodi], and twig beetles 

[Pityophthorus spp. and Pityogenes spp.]) (Millar et al. 2007; USFS 2011a,b) may 

lead to increased susceptibility to bark beetles. 

Assessments in 2011 and 2012 of more than 22,500 limber pine trees in central 

Colorado, Wyoming, and southern Montana indicated that only half were categorized as healthy, another quarter were declining or dying, and the remaining quarter 

were dead primarily due to MPB, WPBR, and dwarf mistletoe (Cleaver et al. 2015; 

Cleaver et al. 2017). Incidence of WPBR on limber pine was 26 percent, incidence 

of dwarf mistletoe was 9 percent, and 18 percent were killed by bark beetles 

(Cleaver et al. 2015). Additionally, in subalpine forests of the Colorado Front 

Range, drought and warmer temperatures are increasing the rate of limber pine mortality, even in the absence of bark beetles (Smith et al. 2015). 
4 USDA Forest Service RMRS-GTR-379. 2019.

Figure 2—Current limber pine distribution (purple) and suitable limber pine habitat (gray) in the GRMNPA. Limber pine suitability 

was determined based on climate (temperature and precipitation) thresholds. Limber pine research plot locations throughout the 

Front Range were used to create a subsample of the PRISM 30-year normals (1981-2010; http://prism.nacse.org/normals/) products. 

Thresholds for minimum and maximum mean annual temperature, and mean annual precipitation were identified for sample locations.

Values greater than or less than the thresholds were classified as non-suitable (white).
USDA Forest Service RMRS-GTR-379. 2019. 5

Figure 3—Map showing concentrated areas of limber pine mortality (tan) caused by mountain pine beetle based on U.S. Forest 

Service Aerial Detection Surveys from 1996-2015. Limber pine distribution is indicated by purple. Forest health monitoring plots are 

indicated by black circles.
6 USDA Forest Service RMRS-GTR-379. 2019.

Figure 4—Map showing concentrated areas of mortality of all pines (tan) caused by mountain pine beetle based on U.S. 

Forest Service Aerial Detection Surveys from 1996-2015. Limber pine distribution is indicated by purple.
USDA Forest Service RMRS-GTR-379. 2019. 7

Over the entire GRMNPA, dwarf mistletoe is present in about 30 percent of the 

evaluated plots at an average incidence of almost 9 percent (calculated from data 

from Cleaver et al. 2015). In northern Colorado, dwarf mistletoe was found on 

limber pine in 33 percent of evaluated plots in 2011 (outside of RMNP) with an 

incidence of 7 percent (Cleaver et al. 2015). Dwarf mistletoe is a greater problem 

within RMNP, as it was present in 60 percent of evaluated study areas in 2009 

and has an overall incidence of 12 percent (arithmetic mean) (Klutsch et al. 2011). 

Before the most recent MPB epidemic, the native limber pine dwarf mistletoe 

was the most damaging agent of limber pine after WPBR range-wide (Taylor and 

Mathiasen 1999). This species of dwarf mistletoe is especially virulent and has 

caused extensive mortality in limber, whitebark, Great Basin bristlecone (Pinus 

longaeva), and Rocky Mountain bristlecone pine (Pinus aristata) (Mathiasen and 

Hawksworth 1988). Dwarf mistletoe infections cause decreased growth, vigor, and 

seed production and increased mortality and may render the host susceptible to 

other diseases or insects, including MPB and Ips beetles (Geils et al. 2002; Taylor 

and Mathiasen 1999). Seedlings are susceptible to infection and rapid mortality, and 

severe infections in mature trees can reduce cone and seed production and quality 

(Geils et al. 2002).

As WPBR continues to spread, limber pine will undergo extensive mortality in 

the GRMNPA and throughout the southern Rocky Mountains (figs. 3 and 5). White 

pine blister rust has caused widespread mortality of trees in other populations, such 

as eastern white pine (Pinus strobus), western white pine (Pinus monticola), sugar 

pine (Pinus lambertiana), and high-elevation white pine species, including northern 

populations of whitebark pine and limber pine (Geils et al. 2010; Tomback and 

Achuff 2010; Tomback et al. 2011). Major economic and ecological losses have 

ensued and continue in affected areas. Western white pine, a once-prized timber species, is now sparsely dispersed and currently occupies only 5 percent of its historical 

area (Man 2013). In addition, whitebark pine, a foundation species, is declining 

across much of its range (Tomback and Achuff 2010). Populations of all WPBRsusceptible species risk extirpation as the pathogen continues to expand. Attempts 

to eradicate the disease or control it directly have failed (Maloy 1997). The U.S. 

Fish and Wildlife Service (2011) determined in 2011 that whitebark pine warranted 

Endangered Species status protection under the Endangered Species Act, and 

Canada designated whitebark pine as endangered in 2012 under the Species at Risk 

Act (Government of Canada 2012). Assessment of limber pine by the Committee 

on the Status of Endangered Wildlife in Canada determined it to be endangered 

in Canada, and the species was recommended for legal listing in November 2014 

(Government of Canada 2014); formal proposal for the national listing as endangered is pending. The pine has been listed as endangered in Alberta for several years 

(Government of Alberta 2014). Limber pine in Wyoming, where it is more common 

than in any other State, is on the Bureau of Land Management Wyoming’s Special 

Status Species list (Bureau of Land Management Wyoming 2013). The same factors 

that led to endangered status for whitebark pine and limber pine elsewhere threaten 

the GRMNPA and southern Rocky Mountains five-needle pine ecosystems.

It is only a matter of time until WPBR becomes common within currently uninfested portions of the GRMNPA. Since its introductions to western North America 

in the early 1900s, WPBR continues to spread and intensify within the ranges of 
8 USDA Forest Service RMRS-GTR-379. 2019.

susceptible and highly vulnerable hosts, including limber pine (fig. 6) (Blodgett et 

al. 2005; Burns 2006; Burns et al. 2011; Cleaver 2014; Kearns and Jacobi 2007; 

Maloney 2011; Smith and Hoffman 2001; Smith et al. 2013). White pine blister 

rust causes cankers that lead to a loss of vigor and ultimately death and also affects 

reproduction and regeneration by killing cone-bearing branches and seedlings. 

Within the GRMNPA, WPBR is most common in the Laramie Range, being present in ~84 percent of evaluated plots with an average incidence of ~24 percent. It 

occurs in 58 percent of the plots in northern Colorado (excluding RMNP) with an 

incidence of 8 percent, and 18 percent of the plots in the Medicine Bow Mountains 

with an incidence of 2 percent (Cleaver et al. 2015). WPBR has been found in 

only one area of RMNP. Several infected trees were identified in 2010 and a larger 

infestation of about 65 trees was discovered in 2017; cankers were removed through 

sanitation pruning. Infections are sparse in areas south of the park, yet new infections continue to be found.

Figure 5—Map of the distribution of high elevation five-needle pines and the 

current (as of 2015) WPBR infection fronts in western North America. Note that 

not all stands of five-needle pines behind the infection front are infested. 
USDA Forest Service RMRS-GTR-379. 2019. 9

Figure 6—Map of WPBR presence and absence in the GRMNPA. New infestations are also noted as of 2018.
10 USDA Forest Service RMRS-GTR-379. 2019.

Fifty-three percent of the limber pine habitat in Colorado is at risk for annual 

WPBR infection (Kearns et al. 2014); remaining habitat may also become invaded 

but may not support annual infections. Important factors found to positively influence WPBR risk (potential distribution) include density of limber pine, density 

of Ribes inereme, presence of ponderosa pine (Pinus ponderosa), stream density, 

relative humidity in May and September, maximum temperature in May, and mean 

May precipitation (Cleaver 2014; Kearns et al. 2014). Riparian areas have both the 

micrometeorological conditions and the abundance of alternate hosts that permit 

WPBR infection and sporulation, including cool, humid conditions, and Ribes 

inerme; we developed risk maps (figs. 2 and 7) to identify these potential areas for 

early detection of WPBR in the GRMNPA. 

Further background information related to limber pine health and condition 

in general and specifically in the GRMNPA can be found in Section 6 of this 

document.

1.2. Federal Forest Service and Park Service Policy

The U.S. Forest Service is obligated by law and regulations, such as Executive 

Order 13112, to respond to invasive species that threaten terrestrial and aquatic 

resources of the National Forest System and to collaborate with Federal, State, 

and local partners to address invasive species that can spread from adjacent lands. 

The USFS policy for invasive species management and research has recently been 

updated by direction provided in Forest Service Manual (FSM) 2900 and FSMs 

3400 and 4000. The most recent USFS Strategic Plan (2015, p. 11) highlights the 

goal to “sustain our nation’s forests and grasslands” under which it articulated the 

subgoals to “develop and apply detection, prediction, prevention, mitigation, treatment, restoration, and climate adaptation methods, technologies, and strategies for 

addressing disturbances such as wildfire, human uses, invasive species, insects, extreme weather events (e.g., storms), and changing climatic conditions;” “collaborate 

with other Federal agencies, State agencies, private landowners, communities, and 

American Indian tribes to improve the health and resilience of the land;” and “coordinate inventory, monitoring, and assessment activities across all lands to improve 

our adaptive management of natural resources.” 

The USFS National Strategic Framework for Invasive Species Management 

(2013) presents the agency’s approach to addressing invasive species. The framework acknowledges that “proactive management to increase the resilience of 

threatened ecosystems to impacts by pending invasions might also be more cost 

effective than later restoration of degraded ecosystems,” (p. 11) and states that “the 

Forest Service will optimize efficacy and cost effectiveness of treatment by timing 

interventions to maximize forest health benefits,” (p. 10). 

The NPS is guided in its decisions about resources by the requirements of the 

1916 Organic Act, which states the agency’s purpose: “to promote and regulate the 

use of national parks in conformance with their fundamental purpose, which is to 

conserve the scenery and the natural and historic objects and the wildlife therein 

and to provide for the enjoyment of the same in such manner and by such means as 

would leave them unimpaired for the enjoyment of future generations,” (p. 535). 
USDA Forest Service RMRS-GTR-379. 2019. 11

Figure 7—Map showing current distribution of limber pine (yellow) and areas with high risk of WPBR establishment in 

the GRMNPA (blue). High-risk areas were delineated by buffering streams or water bodies to a change of 33 ft [10 m] in 

elevation (rather than Euclidean distance).
12 USDA Forest Service RMRS-GTR-379. 2019.

This authority was further clarified in the National Parks and Recreation Act of 

1978: “Congress declares that...these areas, though distinct in character, are united...

into one national park system.... The authorization of activities shall be construed 

and the protection, management, and administration of these areas shall be conducted in light of the high public value and integrity of the National Park System 

and shall not be exercised in derogation of the values and purposes for which these 

various areas have been established, except as may have been or shall be directly 

and specifically provided by Congress,” (p. 166).

The requirements placed on the NPS by the Organic Act and other environmental laws mandate that resources be passed on to future generations “unimpaired” 

(NPS 2006a). Great strides can be made to increase the probability of conserving 

limber pine and sustaining the healthy network of interacting elements that make 

up the high-elevation ecosystems in the park with the timely implementation of this 

Proactive Limber Pine Conservation Strategy. Continuing to take action early in the 

disease invasion and as WPBR continues to spread will enhance the resilience of the 

limber pine populations in the park for future generations. 

As a designated International Biosphere Reserve since 1977, RMNP is also tasked 

with conservation of biological resources, including preservation of native flora 

and fauna. Preserving ecological integrity “in the context of ecological restoration 

refers to the reestablishment, on a disturbed site, of an ecological community that 

has a high degree (1:1 in the absolute case) of ecological integrity with the community that would have been present on the site had the disturbance not occurred,” 

(NPS 2006b, pp. 8-9). Ecological integrity should be maintained and restored to 

the best of current knowledge and ability. The Proactive Limber Pine Conservation 

Strategy will guide the park toward preservation and conservation of limber pine 

and its communities by maintaining genetic diversity, population size, and ecological processes while increasing genetic resistance to WPBR during the unavoidable 

invasion of WPBR. Sustaining viable limber pine populations will position the 

park to become a seed resource for surrounding areas and fulfill the intent of the 

International Biosphere Reserve.

The Wilderness Act of 1964 applies to RMNP and USFS lands. Most of RMNP 

was designated as wilderness under the Omnibus Public Lands Management Act 

of 2009 (National Park Service 2014). Under the 1964 Act, Federal managers of 

RMNP, USFS, and other Class I wilderness areas are obligated to preserve the natural and wilderness character of the land by restricting trammeling by humans. 

1.3. Objectives for Limber Pine Conservation

Controlling the effects of infestation of invasive exotic species within the 

GRMNPA is an ongoing challenge for land managers. Invasive exotic species 

threaten natural and cultural resources (Burns et al. 2008; NPS 2003, 2006b). White 

pine blister rust threatens natural resource integrity and the ecological function and 

biodiversity of limber pine ecosystems. Because RMNP management goals include 

control of the establishment and perpetuation of nonnative species, management 

of WPBR is warranted in the Park; similar goals to sustain ecosystem health apply 
USDA Forest Service RMRS-GTR-379. 2019. 13

to USFS lands (USFS 2013). Unfortunately, direct control of WPBR (i.e., eradication) is not possible (Geils et al. 2010; Schwandt et al. 2010). However, the effect 

of WPBR can be mitigated with strategies that promote host and forest resilience 

before and during WPBR establishment. 

The long-term impact from WPBR, MPB, dwarf mistletoe, changing climatic 

conditions, and other damaging agents does not favor this species of management 

concern or the ecosystem processes and biodiversity that limber pine supports. We 

have learned from whitebark pine ecosystems in the northern Rockies that slowgrowing, high-elevation, five-needle white pines, including limber pine, are not 

sustainable in the presence of WPBR without intervention (Barringer et al. 2012; 

Fiedler and McKinney 2014; McKinney et al. 2009; Smith et al. 2008). Though 

other restoration plans exist, mostly for affected whitebark pine ecosystems, this 

conservation strategy addresses the unique situation of limber pine in the southern 

Rockies. It recommends proactive conservation actions specific to the southern 

Rockies founded on site-based scientific information of the ecosystems before they 

are impaired by WPBR. It focuses on timing specific monitoring efforts and interventions to sustain healthy limber pine populations and ecosystems during invasion 

and naturalization of WPBR, thereby increasing the probability of putting limber 

pine on a trajectory that does not lead to ecosystem impairment in the future. The 

unique genetic composition of limber pine populations in the GRMNPA (relative to 

resistance to WPBR) and the other site-based disturbance ecology information are 

features unique to this area, and the fact that we have this information before the 

populations are greatly impacted by WPBR provides an opportunity to develop a 

targeted conservation strategy specific to the GRMNPA.

The overall objectives of this conservation strategy for the GRMNPA are to (1) 

conserve the genetic diversity of limber pine and to maintain the capacity for adaptation in the face of abiotic and biotic change, and (2) guide the implementation 

of strategies appropriate for the GRMNPA to promote self-sustaining montane and 

subalpine limber pine ecosystems in the presence of WPBR.

This strategy recommends that the land management agencies continue to play an 

important role within the southern Rocky Mountains to ensure the future presence 

of limber pine by initiating concerted, coordinated, and comprehensive research, 

management, and monitoring efforts. In addition, as a designated International 

Biosphere Reserve, RMNP should continue to serve as a limber pine seed source for 

areas within the surrounding GRMNPA. RMNP is expected to “manage the natural 

and cultural resources of the National Park Service to increase resilience in the face 

of climate change and other stressors,” (NPS 2012a) and when “confronted with 

continuous and dynamic change and the goal of preserving ecological integrity, NPS 

management strategies, must be expanded to encompass a geographic scope beyond 

park boundaries to larger landscapes and to consider longer time horizons. Specific 

tactics include improving the representation of unique ecosystem types within the 

National Park System, prioritizing the protection of habitat that may serve as climate refugia, ensuring the maintenance of critical migration and dispersal corridors 

and strengthening the resilience of park ecosystems,” (NPS 2012b).
14 USDA Forest Service RMRS-GTR-379. 2019.

SECTION 2. STRATEGY FOR CONSERVATION 

ACTIONS

2.1. Consequences of Doing Nothing

The ecological consequences of no management have been documented in whitebark pine stands severely impacted by WPBR in the northern Rocky Mountains 

(Burns et al. 2008). Most of these stands have been exposed to WPBR for over 50 

years, resulting in severe changes in ecosystem function and biodiversity (Ellison et 

al. 2005; Tomback and Kendall 2001). Without management intervention, WPBR 

will spread and intensify throughout the range of limber pine, as it has in other parts 

of Wyoming, Montana, and southern Alberta, resulting in extensive mortality (Smith 

et al. 2013; Cleaver et al. 2015; Cleaver et al. 2017), unsustainable limber pine 

populations, and cascading ecological impacts. 

Ecological impacts include a decline in seed production, which disrupts the 

interaction with Clark’s nutcracker, in turn leading to reduced regeneration (e.g., 

McKinney et al. 2009) and loss of a food source for many small birds and mammals 

and even bears (Burns et al. 2008). Reduced regeneration will slow successional 

processes, including post-disturbance recovery and species distribution due to the 

lack of facilitation by limber pine (Baumeister and Callaway 2006; Rebertus et 

al. 1991). Other ecological effects due to tree mortality include changes in forest 

structure, composition, and hydrology and the loss of soil and snow retention, 

biodiversity, and wildlife habitat (Tomback and Achuff 2010). If limber pine dies on 

harsh sites where it is the only tree species, these areas may become treeless, vulnerable to erosion, and less able to capture and retain snow (Schoettle 2004b). Pressure 

by WPBR may result in a shift in the stress-tolerance traits that allow limber pine to 

survive in harsh environments, effectively contracting its fundamental and realized 

niches (Schoettle 2004b; Vogan and Schoettle 2015). 

Should land managers within the GRMNPA choose the no action option and allow WPBR to become established, extensive restoration activities will be required 

in the future with highly uncertain outcomes. Most land managers to the north of 

GRMNPA have unwittingly found themselves with only “reactive” restoration approaches available to them (Keane et al. 2012). Consequently, most WPBR management actions have met with inconsistent and sometimes poor success (Keane and 

Parsons 2010). The GRMNPA is at the WPBR infection front and therefore managers have the option to implement proactive management to mitigate future impacts. 

Mortality of seed-producing trees, combined with the added impacts of upslope 

migration due to climate change and inertia of postglacial dynamics, may render 

limber pine unable to support upper-elevational range expansion. Even limber pine 

with complete WPBR resistance may be at risk if the WPBR pathogen evolves and 

overcomes the resistance. Experience in the five-needle pine ecosystems of the 

northern Rocky Mountains and the listing of whitebark pine as endangered (precluded) under the Endangered Species Act suggest that choosing a no action option 

early in the invasion will not conserve limber pine in the GRMNPA, which faces the 

same challenges.
USDA Forest Service RMRS-GTR-379. 2019. 15

2.2. The Time to Act Is Now

The GRMNPA is in the enviable position of having some time to implement this 

biologically sound and efficient conservation strategy to respond to the challenges 

facing limber pine before the resource is impaired and in crisis. Our recommendation is to extend and expand the current proactive management approaches because 

they offer the greatest opportunity to sustain limber pine and landscape biodiversity. 

Early intervention will reduce the need for restoration in the future. Proactive intervention recommends conservation of genetic diversity of limber pine ecosystems 

and initiating actions to increase their resilience before WPBR invasion or impacts 

to maintain ecosystem processes (Schoettle and Sniezko 2007) (fig. 8). The southern 

Rocky Mountains currently afford this opportunity, even though it has long been 

lost for the heavily impacted areas of the northern Rockies. 

The Proactive Limber Pine Conservation Strategy builds on what was learned 

from past experience and what we are continuing to learn about site-specific preimpact ecology within the GRMNPA. The strategy recommends timely management 

for limber pine to avoid the magnitude of impacts that we see for whitebark pine 

ecosystems in the northern Rocky Mountains. In that area, the damage to and losses 

of whitebark pine from WPBR are so great that many whitebark pine communities 

are nonfunctional and the populations endangered (e.g., Fiedler and McKinney 

2014; Smith et al. 2008). The U.S. Fish and Wildlife Service has downgraded the 

urgency of the listing of whitebark pine under the Endangered Species Act due to 

recent activity, suggesting that proactive efforts such as those recommended in this 

strategy may also help to avoid listing limber pine in the future.

Figure 8—Proactive versus restoration strategy flow diagram (adapted from Keane and Schoettle 2011).
16 USDA Forest Service RMRS-GTR-379. 2019.

Because most of the GRMNPA ecosystems are still healthy, natural processes are 

key to this conservation strategy. Natural regeneration combined with planting or 

direct seeding or both can increase the population size to offset future mortality due 

to natural selection as WPBR continues to spread (Schoettle and Sniezko 2007). 

Diversifying age and size classes on the landscape promotes WPBR resistance 

selection in young trees while larger, mature trees facilitate ecosystem function and 

services (Schoettle and Sniezko 2007). When the landscape contains a sufficient 

frequency of WPBR-resistant trees combined with sufficient regeneration, the 

probability of long-term population sustainability is increased (Field et al. 2012; 

Schoettle et al. 2012).

2.3. Cornerstones of Management

The three cornerstones of proactive management—with the aim of conserving 

limber pine genetic diversity and creating sustainable, resilient populations—are the 

following. 

2.3.1. Support Regeneration and Increased Population Size to Maintain 

Adaptive Capacity

Increasing the population size of limber pine via planting and direct seeding 

will buffer the effects of future high mortality and support a diverse mosaic of age 

classes on the landscape. The extent of the land area of suitable habitat supporting 

limber pine, as well as the density of regeneration, should be increased. Limber 

pine’s life history strategy does not promote rapid adaptation to nonnative stresses, 

such as WPBR, with which it did not coevolve; thus, natural processes alone may 

not support viable populations in the presence of these multiple interacting stressors 

without management support to increase population sizes (Field et al. 2012). 

2.3.2. Manage Genetic Resistance to White Pine Blister Rust for 

Sustainability

Genetic resistance to WPBR has been detected in all of the North American fiveneedle pines (Sniezko et al. 2011) and is an essential component of intervention and 

restoration strategies (Keane and Schoettle 2011; Schoettle and Sniezko 2007). With 

infection by WPBR, susceptible individuals are killed, and those trees with genetic 

resistance survive. Ultimately, the long-term prognosis of a population impacted by 

this disease depends on the frequency of resistance in the population, the size of the 

population, and the balance between regeneration and mortality (Field et al. 2012). 

The greater population size and frequency of resistance, the greater the probability 

there will be enough surviving trees to sustain the population (Schoettle et al. 2012). 

Limber pine exhibits both complete (qualitative) (confirmed in the GRMNPA) 

and partial (quantitative) resistance to WPBR (Schoettle et al. 2011b; Schoettle et 

al. 2014). Although complete resistance may provide immunity, it could eventually be overcome by a more virulent race of the pathogen (Kinloch et al. 2004; 

Schoettle et al. 2014). Complete resistance to WPBR is conferred by a dominant 

allele for resistance, the Cr4 allele in limber pine, which is recognized by the rust 

fungus (avcr4 allele) and prevents the fungus from sporulating on the pine host. As 

a result, Cr4 places strong selective pressure on the rust, and rust genotypes that 
USDA Forest Service RMRS-GTR-379. 2019. 17

infect susceptible pines are the only ones that complete their life cycle and maintain 

the population. However, when trees carrying the Cr4 allele become more common 

(through mortality of susceptible trees or selective planting of Cr4 trees), a mutation 

in the rust that enables the fungus to overcome Cr4 (vcr4 allele)—and, therefore, 

sporulate on a Cr4 tree—can rapidly increase in frequency in the rust population. 

Greater vcr4 frequency may occur, especially if the rust population is isolated with 

low gene flow from other populations. This evolution of virulence reduces the 

durability—and therefore the sustained effectiveness—of Cr4 resistance. Currently, 

there is no evidence for virulence (vcr4) in the rust populations in the GRMNPA 

or elsewhere. The only method at this time to detect virulence is through detection 

of successful sporulation on a known Cr4 tree; we recommend monitoring for this 

occurrence, as discussed next. But virulence has evolved to overcome complete 

resistance in western white pine and sugar pine (Kinloch and Dupper 2002) and is 

expected to arise eventually for Cr4 (Schoettle et al. 2014).

Conditions conducive to rapid proliferation of a mutation that leads to virulence 

include high frequencies of complete resistance, local presence of the susceptible 

alternate hosts for the rust, a habitat that restricts gene flow into the area from other 

rust sources, and conditions that support frequent (annual) infection events (Burdon 

and Thrall 2008). For C. ribicola and limber pine in the GRMNPA, high-risk areas 

include wind-protected riparian areas with Ribes and nearby limber pine trees, 

which describes the habitat in which WPBR was first found in RMNP (just east 

of the Beaver Ponds area; Schoettle et al. 2018a) and in northern Colorado (Sand 

Creek area; Johnson and Jacobi 2000). To manage for sustained durability of Cr4

for as long as possible, plantings of limber pine enriched in Cr4 genotypes should 

be avoided in and near riparian areas.

Traits that confer partial, or quantitative, resistance to WPBR, such as bark 

reaction, needle shed, or slow canker growth, reduce the number or severity of 

infections, enabling the tree to survive with the disease (Sniezko et al. 2004). This 

type of resistance is likely controlled by multiple genes and has been found at 

low frequency in limber pine from the southern Rockies (Schoettle et al. 2011b). 

Quantitative resistance is more durable than qualitative resistance and every effort 

should be made to detect and then increase the frequency of this type of resistance 

in the GRMNPA. The early engagement in limber pine management and the high 

frequency of complete resistance in the native limber pine populations provide time 

to research quantitative resistance, if initiated immediately (the progeny test takes 

5–7 years). The presence of complete resistance to WPBR in the GRMNPA and the 

likelihood of partial resistance in local limber pine populations provide reason for 

optimism that the naturally occurring local genetic diversity can supply the material 

needed to manage the effects of WPBR in the GRMNPA if deployed early. 

2.3.3. Foster Early Awareness and Engagement to Enhance Opportunity for 

Success

Colorado and the southern Rocky Mountains are part of only a few remaining 

areas that contain limber pine populations not yet impacted by WPBR. There is 

evidence that the public is in favor of early intervention (Bond et al. 2011), and this 

support should be capitalized on with community and volunteer involvement in the 

implementation of this conservation strategy.
18 USDA Forest Service RMRS-GTR-379. 2019.

SECTION 3. RECOMMENDED CONSERVATION 

ACTIONS

The GRMNPA ecosystems are at risk due to the introduction of the nonnative 

pathogen that causes the lethal disease WPBR and its interactions with other native 

stresses and climate change. Managers of the GRMNPA cannot rely on past forest 

conditions to provide the information and targets needed to maintain forest sustainably into the future with these stressors. The complexities of changing climate, 

insects and diseases, changing land use patterns, and other influences will create 

new unique environmental conditions and landscape patterns that need to be considered. Management strategies that build in flexibility, reversibility, and alternative 

pathways are more likely to succeed in an uncertain future (Means 2011; Millar et 

al. 2007).

The GRMNPA is in a unique position to manage limber pine because of the following suite of conditions:

•  The ecosystems are still in the early stages of WPBR invasion, and therefore ecological 

processes are still functioning and have not yet been impaired.

•  Postfire habitats in the GRMNPA are conducive to successful natural regeneration of 

limber pine.

•  Genetic resistance to WPBR, conferred by a single dominant allele for resistance 

(Cr4), is present in native populations of limber pine in the GRMNPA at high frequency; however, this type of resistance could be overcome by the rust if a virulent 

strain evolves.

•  Genetic resistance to WPBR that is conferred by multiple genes, which is less likely to 

be overcome by the rust, may be present in the GRMNPA at low frequencies.

•  Climate change is projected to shift limber pine populations to higher elevations as 

well as intensify the impacts of other native pests and pathogens.

Consequently, the recommended management activities for this conservation 

strategy are as follows:

•  Promote ex situ conservation: Continue and expand efforts to collect and archive 

limber pine genetic diversity through bulked and individual-tree seed collections.

•  Promote in situ conservation: Protect limber pine trees from MPB, WPBR, and fire 

to minimize mortality when and where land designations and management objectives 

permit.

•  Increase population size and sustain genetic diversity: Increase the number of limber pine trees across the landscape through planting or seeding, or both, and planned 

and unplanned fire management as soon as possible to offset future mortality and to 

sustain viable genetically diverse, self-sustaining populations.

•  Locate treatments to maintain durability of qualitative resistance to WPBR: 

Minimize selective pressure on the rust by planting trees with a range of susceptibilities 

in low-WPBR-risk areas to reduce the probability of the proliferation of rust genotypes 

virulent to Cr4.

•  Discover, develop, and deploy local quantitative WPBR-resistant sources: Research 

quantitative (polygenic) WPBR resistance types in limber pine in the GRMNPA and 

establish a seed orchard/clone bank of these genotypes to provide material for future 

planting projects in the GRMNPA.

•  Monitor pines and rust: Monitor limber pine health, WPBR incidence, and WPBR 

virulence at the landscape, stand, and tree scales.
USDA Forest Service RMRS-GTR-379. 2019. 19

•  Coordinate management actions within and among agencies: Preserve and diversify 

the age- and size-class distribution of limber pines as well as early-to-late successional 

stages across the landscape through fuels treatments, vegetation management, and fire.

The major focal area for leadership activities is to:

•  Continue coordinated limber pine conservation in the southern Rockies through 

research, genetic resource management, and interagency cooperation: Continue to 

support research on limber pine ecosystems and the WPBR pathosystem and encourage 

infrastructure that supports management objectives. RMNP should pay special attention 

to maintaining viable seed sources and seed collections to further achieve its obligation 

as an International Biosphere Reserve. 

The recommendations described in the following subsections provide justification 

for the guidelines for limber pine management described in Section 5.

3.1. Promote Ex Situ Conservation

Continue and expand efforts to collect bulked and individual-tree seed 

collections from throughout the GRMNPA to archive genetic diversity, and 

to use for propagation and outplanting, further scientific study, and WPBR 

partial resistance testing. 

Because WPBR has not yet caused mortality of mature limber pine trees in much 

of the GRMNPA, the limber pine populations still represent historical genetic diversity and therefore their full adaptive capacity. This genetic diversity should be preserved before WPBR mortality has the chance to cause a genetic bottleneck, the loss 

of rare alleles (Kim et al. 2003), and the loss of stress-tolerance traits (Vogan and 

Schoettle 2015). Some collections have been made, but additional collections are 

needed at other sites (see Section 5, guidelines 1 and 2). It is estimated that limber 

pine seed can retain viability for at least 10 years, and possibility much longer, when 

stored at –0.4 to –4 °F (–18 to –20 °C). Tree and bulked seed collections should 

continue to be made during good cone production years at least every 8–10 years to 

ensure a viable collection. The sites for collections should be stratified by elevation 

and habitat type (when possible) and include areas targeted for planting or seed 

sowing to best supply the needed sources to conform to the seed transfer guidelines. 

Each seed accession should be retained for at least 20 years or longer (pending the 

results of seed viability assessments) for possible genetic studies to test for genetic 

changes in the populations over time (see Section 5, guideline 3). We expect collection of seeds to pose no conflict with wilderness character or values.

3.2. Promote In Situ Conservation 

Protect limber pine trees from MPB, WPBR, and fire to minimize limber pine 

mortality when and where land designations and management objectives 

permit.

Maintaining a viable natural seed source is essential for population sustainability 

and recovery. Verbenone should be used each year to protect any limber pine tree 

that has undergone rust resistance testing (i.e., known genotype individuals), or is 

intended for use in resistance testing, and also to protect additional high-value and 

at-risk trees (recommend >15 trees per site; see Section 5, guideline 4). Carbaryl 

is preferred and more effective; however, in remote areas, verbenone is easier to 
20 USDA Forest Service RMRS-GTR-379. 2019.

deploy. Within RMNP, all limber pine in Class III areas should be classified as 

high-value trees, and chemical insecticides used in accordance with the established 

protocols developed under the 2005 Bark Beetle Management Plan (NPS 2005a). 

There are no chemical treatments effective against WPBR; however, we recommend sanitation pruning (excision of cankered branches) to extend the lifespan of an 

infected tree (Burns et al. 2008; Crump et al. 2011; Jacobi et al. 2017; see Section 

5, guideline 5). It is imperative to tag the treated tree so it is not mistaken as cankerfree (and therefore putatively genetically resistant to WPBR) in future monitoring 

efforts. Sanitation pruning efforts should be targeted in riparian areas in late spring 

when the C. ribicola aecia are most clearly visible. 

Limber pine trees of all ages should be preserved during any fuels treatments 

implemented in the GRMNPA. In addition, fuels treatments in other forest types 

that surround these limber pine areas or in areas that would facilitate fire spreading 

into the area (i.e., where fire starts in a low-elevation area and moves up, as have 

several recent wildfires) should be implemented. A landscape-level assessment to 

optimize placement of fuels treatment is needed. 

When there are planned wildland fires, scratch lines, fire lines, and removing 

duff and vegetation around trees can be used to protect individual trees or a stand of 

limber pines (see Section 5, guideline 15). Trees whose progeny have been tested 

for WPBR resistance are especially important to protect. During unplanned events, 

locations of limber pine trees as resources at risk should be provided to fire management teams to consider for protection to the highest degree possible. These areas 

and the genetically tested trees within RMNP are identified in the park’s FIRE database as resource values at risk; that way, when fire incident managers come onsite 

from other areas, they know to protect them. Similar identification should be made 

for tested trees on other public lands. 

3.3. Increase Population Size and Sustain Genetic Diversity

Increase the number of limber pine trees across the landscape through 

planting or direct seeding, or both, and planned and unplanned fire management as soon as possible to offset future mortality to sustain viable genetically diverse, self-sustaining populations.

Increasing the number of limber pines in a population by planting, direct seeding, 

and facilitating natural regeneration will help offset mortality and reduced regeneration caused by MPB, WPBR, and dwarf mistletoe and reduce the probability of population extirpation (Field et al. 2012). Seed sources for seedling planting and seed 

sowing should contain a mix of bulked seed collections from a range of elevations 

similar to that of the target site to provide locally adapted genotypes and enough 

genetic diversity to support adaptation to climate warming (see Section 5, guidelines 

6–8). Until trees with quantitative (partial) resistance are identified, we recommend 

using bulked seedlots. We do not recommend using seeds only from the trees with 

complete resistance to WPBR (Cr4) because of concerns about rapid proliferation 

of rust genotypes virulent to Cr4 that could arise via mutation (see next subsection). 
USDA Forest Service RMRS-GTR-379. 2019. 21

Suitable target planting and seeding sites include recent burns and stands opened 

or expected to be opened by overstory mortality (e.g., due to MPB or spruce beetle) 

or disturbed by road work or other mechanical treatment. We recommend avoiding 

planting or seeding in areas with high presence of dwarf mistletoe unless the overstory is removed to prevent infection of the young trees. If dwarf mistletoe presence 

is moderate to low, the infected overstory needs to be removed before limber pine 

saplings reach 3 ft (0.9 m) tall or 10 years old, whichever is first (Worrall 2014). 

Consultation with FHP for management recommendations in areas with dwarf mistletoe is recommended. Microsites selected for planting and direct seeding should 

mimic those found in naturally regenerating stands (Coop and Schoettle 2009) to 

maintain a natural spatial arrangement and make the management treatment less 

conspicuous to forest visitors. Planting success is greatest when seedlings or seeds 

are planted near objects in open or semi-open stands (Casper et al. 2016; Smith et 

al. 2011).

In addition to the aforementioned site characteristics for limber pine planting, 

seeding, and management, we recommend targeting areas with low rust risk (figs. 2 

and 7) and areas with low fire hazard for greatest success. Planting in high-WPBRrisk locations may be futile and may increase selective pressure for the proliferation 

of virulence in the rust (see subsection 3.4). Riparian areas are conducive to WPBR 

infection because they have the high humidity and cool temperatures optimal for 

spore development, release, transport, and germination, and Ribes species, which 

serve as the best alternate hosts for C. ribicola. Riparian areas are associated with 

high WPBR risk so we recommend avoiding these areas for planting or seeding (see 

subsection 3.4). To define these areas more accurately, further study is needed on 

the distribution of suitable alternate WPBR hosts and environmental conditions.

Conditions are favorable for limber pine germination in the GRMNPA, though 

establishment may be curtailed by competition from other tree and plant species 

(Stohlgren et al. 1998) and reduced seed sources due to overstory mortality (MPB, 

WPBR). Manipulations of forest cover through planned or unplanned fire or other 

disturbance may result in limber pine establishment if a nearby healthy seed source 

and good site conditions are present (see Section 5, guideline 9).

Planting seedlings generates a much higher long-term establishment rate than 

sowing seed (72-percent and 11-percent, respectively; Smith et al. 2011); however, 

direct seeding is still a viable option (see Section 5, guideline 7). Because many 

seeds are lost to predation, direct seeding with valuable WPBR-resistant seed is not 

recommended. When seed stock with quantitative WPBR resistance is developed, 

planting seedlings will be desirable for a more successful outcome. However, the 

immediate goal is to increase the number of limber pines on the landscape, so direct 

seeding with bulked seedlots is a good option that is affordable and nonintrusive to 

wilderness values. Furthermore, gene frequencies in a bulked seedlot are similar as 

those for natural regeneration, making this approach compatible with Class I wilderness values. Direct seeding below the soil surface ensures a higher success rate than 

surface-sown seeds, which are largely taken by rodents (McCaughey 1990). Loss to 

rodents of sown seeds can also be high; more than 50 percent loss of whitebark pine 

seed has been reported (Pansing 2014). In a limber pine planting study, 43 percent 
22 USDA Forest Service RMRS-GTR-379. 2019.

of limber pine seeds planted at a depth of 1.2 in (3 cm) germinated by the second 

year and seedlings from 11 percent of sown seed survived after 3 years (Smith et al. 

2011). Similarly, other whitebark pine seed sowing trials have reported 13 percent 

germination (Schwandt et al. 2011) and preliminary results indicate survival of up to 

35 percent (Arno 2015). 

Target density for artificial regeneration should supplement natural regeneration, 

and therefore increase seedling densities to offset future mortality from WPBR, 

dwarf mistletoe, bark beetles, or other damage agents. Average density of postfire 

natural regeneration (trees <4 in [10 cm] d.b.h.) of limber pine after the Ouzel Fire 

was 194 stems ac-1 (480 stems ha-1) (Coop and Schoettle 2009). Postfire limber 

pine regeneration density measurements by Shankman and Daly (1988) were 117 

stems ac-1 (288 stems ha-1) at Sundance, 118 stems ac-1 (292 stems ha-1) at Rollins 

Pass, and 206 stems ac-1 (508 stems ha-1) at Niwot Ridge. Natural regeneration 

for trees <54 in [137 cm]) in height in existing stands of limber pine in northern 

Colorado to southern Montana, including stands impacted by WPBR, MPB, and 

dwarf mistletoe, ranged from 0 to 783 stems/ac-1 (1,935 stems ha-1) and averaged 

57 stems ac-1 (141 stems ha-1) (Cleaver 2014). In Boulder County, where WPBR 

is in the early stages of infection, mean limber pine regeneration density in existing stands was 79 stems ac-1 (194 stems ha-1) and ranged from 0 to 412 stems 

ac-1 (1,019 stems ha-1) (Cleaver 2014). Natural limber pine regeneration for trees 

less than 54 in (137 cm) in height in existing stands along the northern Colorado 

Front Range, including 17 sites in RMNP, averaged 199 stems ac-1 (492 stems 

ha-1) (Klutsch et al. 2011). Overall, we propose to use artificial regeneration to 

supplement and double the average natural regeneration density of 200 stems ac-1. 

Assuming 50 percent pilfering and 11 percent seedling establishment rate from 

direct seeding, it would require a direct seeding rate of 3,636 seeds ac-1 (8,985 

seeds ha-1) and assuming 72 percent of planted seedlings successfully establish it 

would require a planting rate of 278 seedlings ac-1 (685 seedlings ha-1) to yield the 

artificial regeneration supplemental target establishment density of 200 stems ac-1

(494 stems ha-1) and an average total seedling density (natural plus artificial regeneration) of approximately 400 stems ac-1 (988 stems ha-1).

3.4. Locate Treatments to Maintain Durability of Qualitative 

Resistance to White Pine Blister Rust

Select seed sources and locate treatment units to minimize selective pressure on the rust. This will lower the probability of the proliferation of rust 

genotypes virulent to Cr4, should they evolve via mutation, and may reduce 

the buildup of WPBR spore loads.

Every effort should be made to slow the proliferation of rust virulence (i.e., an 

evolved mechanism whereby a mutation enables the rust to overcome Cr4 resistance) to provide additional time to detect and deploy quantitative resistance in the 

GRMNPA (see Section 5, guideline 10). Wind-protected riparian areas with Ribes

and nearby Cr4 limber pine are particularly conducive to the rapid proliferation 

of virulence should it arise. To manage for sustained durability of Cr4, plantings 

of limber pine enriched in Cr4 genotypes should be avoided in and near riparian 

areas early in the invasion timeline (figs. 2 and 7). We recommend planting or 
USDA Forest Service RMRS-GTR-379. 2019. 23

direct seeding of limber pine using only bulked seedlots and planting or seeding no 

closer than 33 ft (10 m) in elevation from an active stream bed. Repeated seeding or 

planting of bulked seedlots (with both resistant and susceptible genotypes) is recommended as mortality increases. In addition, sanitation pruning should be done in 

these areas to prolong the life of the trees and to potentially reduce inoculum densities. The efficacy of pruning should be reevaluated with FHP and RMRS annually. 

3.5. Discover, Develop, and Deploy Local Quantitative White Pine 

Blister Rust-Resistant Seed Sources

Research quantitative (polygenic) WPBR resistance types in limber pine in 

the GRMNPA and establish a seed orchard/clone bank of these genotypes 

to provide seed for future planting projects in and around the park.

Whereas Cr4 resistance can sustain nearly a fifth of the original limber pine 

population for a limited number of rust generations, research to identify, build, 

and deploy seed stocks with the more durable quantitative WPBR resistance in 

the GRMNPA should begin immediately (see Section 5, guidelines 2 and 11). To 

sustain limber pine populations into the future, the frequency of trees with quantitative resistance must be increased to well above the suspected current frequency 

of ~1 percent in southern Rockies limber pine populations (Schoettle and Sniezko 

unpublished data on file at RMRS, Fort Collins, Colorado; Schoettle et al. 2011b). 

Individual tree seed collections can be tested for WPBR resistance using established 

artificial inoculation protocols at USFS facilities. Testing can only take place 5–7 

years after the seed is collected, so we recommend commencing studies as soon as 

possible for timely results. The USFS operates several artificial inoculation facilities 

to test for resistance to WPBR; however, none of them are in the southern Rockies. 

If funds for testing are not available, families could be prescreened at field planting 

sites in areas with WPBR (e.g. the Southern Rockies Rust Resistance Trial site in 

south Wyoming) to identify those families that appear to have partial resistance for 

additional testing at the artificial inoculation testing facilities. This would reduce the 

number of families, and therefore the cost, that need to be tested under controlled 

conditions. Our past research on rust resistance of limber pine in the GRMNPA was 

done at the USDA Forest Service Dorena Genetic Resource Center (Cottage Grove, 

Oregon). Use of molecular methods to detect resistance to WPBR is encouraged 

when they become available.

After the inoculation test, the seedlings that express traits associated with quantitative resistance should be grafted to root stock and established in a seed orchard/

clone bank (see Section 5, guideline 16) to mature and provide a source of seed 

(frequency of the traits can be determined by testing the first-generation progeny). 

Until then, field collected seed can be used to support planting or seeding projects. 

Scion from the seed trees that bore the resistant progeny could also be grafted for 

the clone bank. Quantitative resistance has been identified in limber pine from the 

southern Rockies, including some families from the GRMNPA; no seed sources 

from RMNP have been tested (Schoettle and Sniezko unpublished data). We recommend that clones (grafts) from these resistant genotypes (provided by RMRS and 

Great Sand Dunes National Park and Preserve) be immediately established in a 

RMNP clone bank to fulfill the park’s mandate to be a source of genetic diversity to 
24 USDA Forest Service RMRS-GTR-379. 2019.

promote sustained forest health in the park and surrounding areas. Additional sites 

for clone banks should also be considered. Other clones from the future tests can be 

added later. 

3.6. Monitor Pines and Rust

Monitor limber pine health, WPBR occurrence, and WPBR virulence at the 

landscape, stand, and tree scales.

The existing monitoring network should be enhanced and expanded throughout 

the GRMNPA. Ten of the 17 research areas and 39 long-term monitoring plots 

are a good start for forest health monitoring at the landscape and stand scales. 

Establishing long-term monitoring plots in the remaining seven research areas 

within RMNP and in areas outside of the park that have not been surveyed extensively (e.g., the Arapaho and Routt National Forests) should be a priority in the 

near future (figs. 3 and 9). A remeasurement schedule of 5 years would be adequate 

to detect changes (forest health monitoring; see Section 5, guideline 12). For early 

WPBR detection and study of invasion dynamics, additional plots should be established in riparian areas to identify infection centers and areas where management for 

durability of resistance measures should be exercised (figs. 2 and 10) (early detection monitoring; see Section 5, guideline 13) as have been established in RMNP 

(Cleaver et al. 2017). In addition, those forest trees inferred to have Cr4 resistance 

by WPBR resistance progeny testing, should be inspected each spring for signs and 

symptoms of WPBR as monitors for the evolution of virulence to Cr4 (vcr4) in the 

rust (virulence monitoring; see Section 5, guideline 14). As soon as virulence is 

detected or suspected (i.e., a known Cr4 tree is found with signs or symptoms of 

WPBR), FHP and researchers should be notified and consulted. 

The Rocky Mountain Inventory and Monitoring Network (ROMN) is focused 

on long-term ecological monitoring in six parks including RMNP. Integrating 

five-needle pine monitoring with the network’s monitoring programs would benefit 

the monitoring systems already in place; a regionwide database is currently being 

established as a collaborative effort between ROMN and FHP. Streamlining with 

the goals of ROMN and implementing early-detection WPBR monitoring allow for 

early warning of altered conditions so proper and cost-effective management decisions may be employed (Fancy et al. 2009). 
USDA Forest Service RMRS-GTR-379. 2019. 25

Figure 9—Map showing limber pine seed collection and research sites in the GRMNPA (Schoettle et al. 2011a, Schoettle et al. 

2013). Sites were stratified by elevation to sample the ecological and genetic variation of limber pine in the area.
26 USDA Forest Service RMRS-GTR-379. 2019.

Figure 10—Map showing wilderness, current distribution of limber pine stratified by elevation, research sites, and forest 

health monitoring plots in the GRMNPA.
USDA Forest Service RMRS-GTR-379. 2019. 27

3.7. Coordinate Management Actions Within and Among 

Agencies

Preserve and diversify the age- and size-class distribution of limber pines as 

well as early-to-late successional stages across the landscape through fuels 

treatments, vegetation management, and planned fire.

Using fire to benefit the resource and incorporating limber pine objectives and 

management when applicable will help sustain limber pine on the landscape. Fire 

regimes vary across the distribution of limber pine, with harsh, rocky sites receiving 

fire less often. In more productive areas, fire can be beneficial to limber pine by 

reducing competition and by providing a seedbed for Clark’s nutcracker seed caching and habitat for planting or direct seeding through the creation of openings in the 

forest canopy (Coop and Schoettle 2009; see Section 5, guidelines 9 and 15). While 

occasional wildfires can be beneficial for limber pine regeneration, large, unplanned 

wildfires may threaten valuable stands of limber pine. RMNP fire managers have 

been notified of locations of high-value limber pine and other resources at risk 

(long-term monitoring plots, known WPBR-resistant trees) so that they may be considered for fire protection; similar notification should be made to other public land 

managers. Management activities that increase resilience of other tree species on the 

landscape can help lower MPB hazard for lodgepole, ponderosa, and limber pine. 

Management activities that reduce fire hazard in ponderosa pine and Douglas-fir 

(Pseudotsuga menziesii) forest types can also limit fire spread into higher elevations 

where limber pine grows.
28 USDA Forest Service RMRS-GTR-379. 2019.

SECTION 4. RESOURCES TO EXECUTE THIS 

STRATEGY

Implementing the recommendations of this strategy will require administration 

and management to coordinate efforts, consult with local experts, secure resources 

to achieve the goals, work across agencies and with different divisions within an 

agency (i.e., vegetation, fire, wilderness, and the Learning Center staffs), provide 

oversight for clone bank planting and maintenance, and compile information to 

assess and communicate current conditions and outcomes. RMNP has established a 

Limber Pine Coordinator position for at least 5 years. Because the GRMNPA is at 

the infection front for WPBR, limber pine forest conditions will be very dynamic 

during the next 10–30 years, and close consultation among agencies and experts 

will aid in interpreting those changes and adapting projections and management. 

FHP is a particularly good resource as it has responsibility for assisting with forest 

health management across all land ownerships. Close coordination with local FHP 

professionals can help with funding, focusing, and facilitating this work. RMRS 

also shares a cross-border mandate and can provide advice on the latest research 

findings and research opportunities to assist managers in their limber pine conservation efforts. The USFS Regional Geneticist for the Rocky Mountain Region is 

another resource to consult. 
USDA Forest Service RMRS-GTR-379. 2019. 29

SECTION 5. GUIDELINES FOR MANAGEMENT OF 

LIMBER PINE

5.1 Limber Pine Seed Collections—Bulk Cone Collections

Purpose of activities:

•  Archive genetic diversity.

•  Create a seed reserve for:

○ WPBR resistance screening and research.

○ Propagation and seedling outplanting.

○ Direct seeding.

Timing of activity:

•  At least every 8–10 years during good cone production years.

•  Limber pine produces cones every 3–5 years; 2008 and 2011 were the best cone years 

over the period 2008–2015. Collecting early in the masting cycle will result in the best 

seed collections.

Activity (i.e., collection) priorities: 

•  Accessible stands stratified across the GRMNPA (can be co-located with the individual 

tree collections) (Section 5, guideline 2).

•  Stands that fulfill the recommended geographic distribution of seed sources (see USFS 

Handbook FSH 2409.26f R-1 (Seed Handbook) and consult with Regional USFS 

Geneticist).

•  Stands threatened with extirpation, i.e., with a high incidence of WPBR, low limber 

pine stem density, high levels of mortality (any cause), or any combination of those 

factors (Cleaver et al. 2015).

Description of activity:

•  Site selection criteria: 

○ Stratify sites by elevation, habitat type (when possible), and areas targeted for 

planting or seed sowing to best supply the needed sources and to conform to seed 

transfer guidelines (Section 5, guideline 8). 

○ Collect from 2–3 sites for each 328 ft [100 m] elevation band (i.e., 9,190–9,518 

ft, 9,518-9,846 ft [2,801–2,900 m, 2,901–3,000 m]) spaced across the GRMNPA.

•  Tree selection criteria:

○ Select healthy single-stemmed trees or multi-stemmed clumps with cones. When 

sampling from clumps, make sure to collect from only one stem. Be careful to 

maintain a 100 ft. [30 m] spacing between all sampled trees. 

○ Avoid collecting from trees that are heavily infested with dwarf mistletoe.

•  Follow guidelines for bulk cone collections (see Appendix in Schoettle et al. 2011a)

○ Collect an equal number of cones from each of 20+ trees per site (40+ trees is 

better). To do this, assess the amount of cone production on site—if it is light, 

collect 2 cones per tree; if it is good, collect 4 cones per tree; and if it is excellent, 

collect 6 cones per tree.

○ Select cones from high in the crown.
30 USDA Forest Service RMRS-GTR-379. 2019.

○ Do not include trees with individual seed collections. 

○ Verify seed ripeness prior to collection (see Appendix in Schoettle et al. 2011a). 

○ Collect only fully formed green cones (no small dry brown ones—they have 

aborted) (fig. 11).

○ Only collect cones cut from the tree; do not collect cones from the ground. 

○ Keep the cones out of direct sun (especially in the back of a vehicle). Move them 

to a dry, cool place with good ventilation between bags as soon as possible.

○ Seed can be extracted when cones have opened. See Section 5, guideline 3, 

Limber Pine Seed Storage and Database.

•  Data Collection:

○ Enter the number of trees sampled and the number of cones per tree on the SEED 

TREE form (Schoettle et al. 2011a, Appendix).

○ Complete the SITE SURVEY form to provide associated stand condition and 

forest health information at the time of the collection (Schoettle et al. 2011a, 

Appendix).

Figure 11—Healthy limber pine cone cluster that is close to mature for collection 

(A.W. Schoettle, USFS).
USDA Forest Service RMRS-GTR-379. 2019. 31

5.2 Limber Pine Seed Collections—Individual Tree Collections

Purpose of activity:

•  Archive genetic diversity.

•  Create a seed reserve for:

○ Further scientific study.

○ WPBR partial resistance testing.

○ Outplanting.

Timing of activity: 

•  At least every 8–10 years during good cone production years.

•  Limber pine produces cones every 3–5 years; 2008 and 2011 were the best cone years 

from 2008–2015. Collecting early in the masting cycle will result in the best seed 

collections.

Activity (i.e., collection) priorities:

•  Accessible stands stratified across the GRMNPA. Include healthy and declining stands 

stratified spatially and by elevation.

•  Stands at risk for extirpation, i.e., those with a high incidence of WPBR, low limber 

pine stem density, high levels of mortality, or any combination of those factors (Cleaver 

et al. 2015).

•  Phenotypically resistant trees and trees with confirmed resistance.

Description of activity:

•  Site selection criteria: 

○ Stratify sites by elevation, habitat type (when possible), and areas targeted for 

planting or seed sowing to best supply the needed sources and to conform to seed 

transfer guidelines (see Section 5, guideline 8). 

•  Tree selection criteria: 

○ Select healthy trees with cones. Single-stemmed trees are preferred over multistemmed clumps because clumps may consist of multiple individuals; for the 

genetic analyses, we have to be absolutely certain of the mother tree of the seeds. 

If it is unavoidable to select a multi-stemmed clump, then tag just one stem and 

be absolutely certain that all the cones are from just that stem. If you are not 100 

percent certain that the cone you cut was from the sample tree, do not include it. 

○ If WPBR is on site, select 8–9 phenotypically resistant (no or few visible cankers) 

seed trees and 1–2 susceptible (cankered) seed trees. If there is no WPBR on site, 

select 10 healthy, mature trees. 

○ Avoid collecting from trees that are heavily infested with dwarf mistletoe. 

•  Follow the guidelines for individual tree collections (see Appendix in Schoettle et al. 

2011a)

○ Maintain a 200 ft. (61 m) spacing between sample trees. 

○ Place a numbered tag on the north side of each tree and collect global positioning 

system (GPS) coordinates. Use site name and tag number on all bags of cones 

from that tree. 

○ Verify seed maturity before collecting cones (see Appendix in Schoettle et al. 

2011a).

○ Collect 20–40 cones from each tree from as high in the crown as possible 

(fig. 12). 
32 USDA Forest Service RMRS-GTR-379. 2019.

○ Collect only fully formed green cones (no small dry brown ones—they have 

aborted).

○ Keep the cones out of direct sun (especially in the back of a vehicle). Move them 

to a dry cool place with good ventilation between bags as soon as possible.

○ Seed can be extracted when cones have opened. See Section 5, guideline 3, 

Limber Pine Seed Storage and Database.

•  Data Collection:

○ Enter each sampled tree and the number of cones per tree on the SEED TREE 

form (see Appendix in Schoettle et al. 2011a). Note if WPBR is on site and 

sample tree is infected.

○ Complete the SITE SURVEY form to provide associated stand condition and forest health information at the time of the collection (see Appendix in Schoettle et 

al. 2011a).

○ Update the FIRE database (RMNP) or similar National Forest database with the 

location of sampled trees.

Figure 12—Using a pole pruner to collect cones from a limber pine tree in RMNP (A.W. Schoettle, USFS).
USDA Forest Service RMRS-GTR-379. 2019. 33

5.3 Limber Pine Seed Storage and Database

Purpose:

•  Preservation of genetic diversity of the species before WPBR-imposed bottleneck.

•  For RMNP, which is an International Biosphere Reserve, maintain seed collection as a 

seed reserve for other management units.

•  Outplanting.

Priorities:

•  Individual tree seed collections

•  Bulk seed collections

Description and guidelines:

•  Seed Storage

○ Use USFS plant-nursery facilities to store seed, or alternatively, use private 

facilities.

○ Cleaned seed (i.e., seed cleaned of all twig, needle, wing, and cone debris) 

should be stored in impermeable bags sealed to exclude as much free airspace 

as possible. Bags should be labelled with collection date, sample name, species, 

latitude, longitude, and elevation and stored in a walk-in or chest freezer at -4 °F 

(-20 °C). Seeds retain viability when stored at -0.4 °F to -4 °F (-18 °C to -20 °C) 

for at least 10 years. High-quality, large chest freezers or a walk-in freezer will be 

needed. A generator backup or alarm system is warranted.

○ Conduct an initial seed germination test of each seed lot, if possible.

○ Each seed collection accession should be retained for at least 20 years for possible genetic studies to test for genetic changes in the populations over time. 

Assess viability through germination tests (with 60-day, 35.6 °F [2 °C] stratification) before deciding to discard.

○ Do not discard individual tree seed collections without prior consultation with 

RMRS and FHP. 

○ Do not combine seed lots.

○ Add to seed collection during good cone production years.

•  Seed Database

○ If seed is not being archived by a USFS plant nursery or managed by the 

Regional Geneticist, maintain a database (or Excel spreadsheet) for all seed collections. Record the date of collection, full site name, site abbreviation, waypoint 

name (tree tag number), latitude, longitude, elevation, datum, number of trees 

in the sample, type of collection (individual vs. bulk), WPBR resistance testing 

(yes or no) and date, phenotype of seed tree in the field (individual only; WPBR 

infected or not infected), WPBR on site (yes or no), mass (g) of sample, uses 

(see below), and notes. Site data requested on the datasheet should also be added. 

For “uses” add a column each time seed is sampled from the lot, and identify the 

project for which it was used. If percent germination is determined, note it with 

the date of the test. 
34 USDA Forest Service RMRS-GTR-379. 2019.

5.4 Protecting Limber Pine from Mountain Pine Beetle (MPB) 

Purpose of treatment:

•  To preserve seed sources and minimize limber pine mortality due to MPB. 

Timing of treatment:

•  Consult with FHP annually to assess treatment needs and to optimize treatment time 

based on treatment type and the expected timing of beetle flights.

•  Spray or verbenone additional seed trees, including all trees sampled for individual-tree 

seed collections, if there are 3 years of increasing MBP populations.1

Treatment priorities:

•  Trees confirmed to have complete WPBR resistance (Cr4) (virulence monitors—see 

Section 3.6).

•  All trees that have undergone or are planned to undergo WPBR resistance testing (protecting the trees with known genetic susceptibility is also important as a control seed 

source for research studies and resistance trials). 

•  Cone-producing trees.

•  High-value trees and at risk trees.

•  Sites with reasonable access.

Description of treatments:

Carbaryl is a registered, preventive insecticide spray that is very effective for protecting 

uninfested trees from bark beetle attack for 1–2 years. A licensed and insured applicator is 

required and treatments can only be done on accessible sites and more than 50 feet from 

surface water. 

Verbenone is an anti-aggregation pheromone that sends a ‘no vacancy’ signal to bark 

beetles. Its effectiveness in protecting limber pine has not been adequately studied, but it has 

shown success for protecting individual whitebark pines from beetle attacks (Kegley and 

Gibson 2004). Verbenone comes in 7.5-gram pouches that are easy to apply (figs. 13a,b) and 

fairly inexpensive. It is recommended for inaccessible sites, sites near surface water, or in 

areas where spraying insecticides is not an option. The guideline for individual tree protection is to apply one 7.5- gram pouch before the beetle flight (late May or early June) and add 

another 7.5-gram pouch later in the summer (late July or early August). Using verbenone for 

stand protection has had more variable results (Bentz et al. 2005). In a recent study, 40 highdose verbenone pouches (with a verbenone emission rate up to 50 mg/d per pouch) per acre 

significantly reduced the number of MPB attacks compared to untreated stands. However, 

some mortality in large diameter trees still occurred. Consult with FHP for the most current 

treatment standards. 

When treatment is complete, update the FIRE database (RMNP) or similar National 

Forest database with the location of treated trees.

1 MPB populations have returned to endemic levels in the greater RMNP Area as of 2016. In the future, 

a good indicator that another epidemic is beginning would be if beetle populations increase for 3 consecutive years in and around the tree or stand needing protection. 
USDA Forest Service RMRS-GTR-379. 2019. 35

Figure 13a—Verbenone pouches in RMNP 

(A.W. Schoettle, USFS).

Figure 13b—Stapling verbenone pouches to a limber 

pine seed tree in RMNP (A.W. Schoettle, USFS).
36 USDA Forest Service RMRS-GTR-379. 2019.

5.5 Sanitation Pruning to Remove White Pine Blister Rust 

(WPBR) Infections 

Purpose of treatment:

•  Prolong the life of existing, high-value trees.

•  Attempt to reduce local C. ribicola spore load early in the invasion.

Timing of treatment:

•  Late spring (mid-May to early June) when C. ribicola aecia are most visible.

•  Yearly when paired with early detection monitoring, every 5 years for sites with 

incidence >20% (total host trees/infected trees), and every 10 years on sites where 

incidence is <20%.

•  Consult with FHP annually on when to stop pruning at early detection sites, but a general guideline is to continue pruning until WPBR incidence reaches 30 percent on the 

site.

Treatment priorities:

•  Riparian areas (33 ft [10 m] in elevation relative to active stream bed) for early detection.

•  High-value trees in campgrounds, administrative sites, or points of interest. 

•  Do not treat tagged seed trees or tree samples for cones or rust resistance testing. 

•  Areas that are unique or have ecological or aesthetic value.

Description of treatment:

•  In early detection areas, collect GPS coordinates (latitude and longitude), tag trees at the 

base using oval tag numbers (WPBR-#) specifically designated for infected and pruned 

trees (so the tree is not mistaken as canker-free and therefore putatively genetically resistant to WPBR), and maintain a database of treated trees. Do not prune tagged seed trees 

at research sites. Notify FHP as soon as possible when new infestations are found.

•  Only prune branch cankers that are more than 6 in (15 cm) from the main stem using 

extension pole side-cutting pruners (Crump et al. 2011) (fig. 14). 

•  Stem cankers that have spread less than halfway around the stem’s circumference 

and branch cankers within 6 in (15 cm) of the main stem can be scribed (Schnepf and 

Schwandt 2006) (figs. 15a,b).

•  Do not prune a tree if more than 60 percent of the crown needs to be removed or if it 

has a stem canker that extends more than halfway around the stem’s circumference. 

•  Pruning is not recommended on sites with moderate to high disease incidence 

(>30 percent) on highly susceptible trees or short trees (<20 ft [6.1 m]) with more 

than 3 branch cankers, medium height trees (20.1–30 ft [6.13–9.1 m]) with more than 

6 branch cankers, and tall trees (>30.5 ft [10 m]) with more than 10 branch cankers 

(Jacobi et al. 2017). Ten cankers on tall trees is three times the mean canker count in 

this area (Kearns 2005, Cleaver et al. 2015), so such trees are likely very susceptible or 

the environment is highly conducive for future infections. 

•  Care should be taken to minimize damage to trees during treatment. Leaving the branch 

collar intact will promote a smaller wound. When cutting branches larger than 1.5 in 

(4 cm) in diameter, make an undercut to prevent bark from stripping as the branch is 

severed.

•  Pruned branches can be left on the ground and tools do not need to be sanitized 

between trees. After pruning, cut side branches off so the infected branch lies flat on 

the ground and cover the canker with a layer of duff, soil, moss, or other material to 

prevent spore movement.

•  See Schnepf and Schwandt (2006), Burns et al. (2008), Crump et al. (2011), and Jacobi 

et al. (2017).
USDA Forest Service RMRS-GTR-379. 2019. 37

Figure 14—WPBR-infected branch that should be pruned (K.S. Burns, USFS). 

Figure 15a—After pruning a branch with a WPBR canker, 

potentially invaded tissue on the main stem is removed 

(scribed) with the goal of removing the entire infection (sanitation) (K.S. Burns, USFS).

Figure 15b—Scribing treatment to contain the growth of 

C. ribicola and prevent its spread around the main stem of 

the tree (K.S. Burns, USFS). 
38 USDA Forest Service RMRS-GTR-379. 2019.

5.6 Planting Limber Pine Seedlings 

Purpose of treatment:

•  Increase the number of limber pine trees on the landscape to offset mortality by WPBR.

•  Increase the number of seedlings with partial resistance to WPBR.

•  Restore or regenerate, or both, limber pine where they have been reduced by natural 

and anthropogenic agents. 

•  Assure the future of limber pine on the landscape.

•  Provide for future seed sources for areas where local sources have been diminished.

Treatment priorities:

•  Recent burned stands in limber pine habitat.

•  Stands with low age class diversity.

•  Sites with easy access.

•  Stands with overstory mortality or expected mortality.

•  Campground, recreational, and administrative sites.

•  Sites with low fire and WBPR risk.

Description of treatment:

Planting guidelines:

•  Plan early to plant before limber pine populations have declined. Planted seedlings can 

successfully establish in the understory (Casper et al. 2016).

•  Avoid planting in areas with high dwarf mistletoe without first pruning dwarf mistletoe 

infected branches from the nearby limber pines in the overstory.

•  Avoid planting within 33 ft (10 m) in elevation from active stream bed. See WPBR risk 

map (fig. 7).

•  Use a mix of bulked seed collections from a range of elevations similar to that of the 

target site to provide the locally adapted genotypes and enough genetic diversity to support adaptation to climate warming (see Section 5, guideline 8, Seed Transfer / Source 

Guidelines).

•  Use bulked seeds for establishing seedlings (until seeds sources with quantitative resistance are identified). 

•  Plant 2-0 seedlings (fig. 16).

•  Plant in the fall (Smith et al. 2011).

•  Use seed from the clone bank for seedlings with WPBR partial resistance when 

available.

•  Assess the frequency of genetically resistant seedling within your seed lot and adjust 

your planting density accordingly. Frequency of resistance can be estimated from 

previous tests of component lots or a new rust resistance test of a subsample of the 

planting stock should conducted.

•  Anticipate mortality when determining density and long-term goal. Plant 278 seedlings 

acre-1 (685 seedlings ha-1) for a projected artificial regeneration establishment density 

of 200 trees acre-1 (494 trees ha-1) to supplement natural regeneration.

•  Repeated planting may be needed with bulk lots that contain susceptible genotypes.

•  Microsite selection for planting should mimic those found in naturally regenerating 

stands (see Coop and Schoettle 2009) to maintain a natural spatial arrangement and 

promote establishment. 

•  Planting success is greatest when seedlings are planted near objects (on the north or 

west side), in the presence of vegetation, and in open or semi-open stands (Casper et al. 

2016, Smith et al. 2011).
USDA Forest Service RMRS-GTR-379. 2019. 39

Figure 16—Planted limber pine seedling. Tagging is only required when coordinated with research 

assessments (C. Jensen, USFS).
40 USDA Forest Service RMRS-GTR-379. 2019.

5.7 Direct Seed Limber Pine

Purpose of treatment:

•  Increase number of limber pine trees on the landscape to offset current and future high 

mortality.

•  Increase the frequency of WPBR-resistant trees on the landscape.

•  Assure the future of limber pine on the landscape.

•  Restore or regenerate, or both, limber pine where they have been reduced by natural 

and anthropogenic agents. 

•  Provide seed sources for areas where local seeds have been diminished.

Treatment priorities:

•  Remote areas where planting seedlings may not be feasible.

•  Wilderness or other areas with poor access or management restrictions.

•  Recently burned stands in limber pine habitat, especially within 10 years if natural limber pine regeneration has not established. However, earlier seeding is best to augment 

natural regeneration, promote resilience, and offset high mortality by WPBR. 

•  Sites with low fire and WBPR risk.

Guidelines for direct seeding:

•  Seed sources for sowing should contain a mix of bulked seed collections from a range 

of elevations similar to that of the target site to provide locally adapted genotypes and 

enough genetic diversity to support adaptive capacity to climate warming (see Section 

5, guideline 8—Seed Transfer / Source Guidelines).

•  Use bulked seed lots.

•  Sow in the fall (Smith et al. 2011).

•  Assess the frequency of genetically resistant seedling within your seed lot and adjust 

your planting density accordingly. Frequency of resistance can be estimated from 

previous tests of component lots or a new rust resistance test of a subsample of the 

planting stock should conducted.

•  Seeds should be sown 0.8–1.6 in (2–4 cm) deep (McCaughey 1990) in clusters of 3 

seeds—do not surface-sow (because they will be lost to predation on the surface).

•  Target sowing 3,636 seeds acre-1 (8,985 seeds ha-1) from bulked seed lots for a target 

establishment density of 200 trees acre-1 (494 trees ha-1).

•  Repeated seeding may be needed to keep susceptible genotypes on the landscape to 

delay evolution of virulence to complete resistance.

Where to plant seeds:

•  Target seeding sites include recent burns, stands opened or expected to be opened by 

overstory mortality, or stands disturbed by road work or other mechanical treatment in 

the areas of potential limber pine habitat.

•  Avoid seeding in areas with dwarf mistletoe without first pruning dwarf mistletoe 

infected branches from the nearby overstory. 

•  Avoid sowing within 33 ft (10 m) in elevation from active stream bed. See WPBR risk 

map (fig. 7).

•  Microsite selection should mimic those found in naturally regenerating stands (Coop 

and Schoettle 2009) to maintain a natural spatial arrangement. Success is greatest when 

seeds are planted near objects and in the presence of vegetation in open or semi-open 

stands (Casper et al. 2016, Smith et al. 2011).
USDA Forest Service RMRS-GTR-379. 2019. 41

5.8 Seed Transfer / Source Guidelines

Purpose:

•  Seed movement guidelines ensure seed is adapted to the growing conditions of the 

target planting area. 

Priorities:

•  Consider climate change. 

Seed transfer guidelines:

•  The GRMNPA is one seed zone that is stratified by elevation (Mahalovich 2006; 

Borgman et al. 2015).

•  For a given target site elevation, seed sources should be a mix of bulked seed lots from 

sources down to 656 ft (200 m) below the target site to sources up to 656 ft (200 m) 

above the target site elevation; this will provide enough diversity to provide adaptive 

capacity for climate change.

•  Restrict upward or downward movement of seed to 656 ft (200 m).

Building seed stock from multiple elevations while considering climate change: 

•  Conservation of the genetic diversity of limber pine in the GRMNPA requires conservation of genotypes at all elevations.

•  To include enough diversity, we recommend that at least 2–3 bulked samples be collected from each 328 ft (100 m) elevation band.

Other considerations:

•  RMNP can serve as a seed source for northern Colorado and southern Wyoming, 

but it is not recommended that the seed be moved to more southerly sites outside the 

GRMNPA (Borgman et al. 2015).

•  Movement beyond these recommendations may be warranted if the material has quantitative resistance to WPBR. 

5.9 Natural Regeneration

Purpose:

•  To offset the current or future increase in limber pine mortality, good natural regeneration and supplemental planting or direct seeding will be required (fig. 17).

•  Maintain genetic diversity.

•  Diversify size and age class structure and successional stages across the landscape.

•  Decrease costs of regeneration.

Priorities:

•  Any area with canopy disturbance within approximately 7.5 miles (12 km) of a limber 

pine seed source; this is the average distance that the Clark’s nutcracker transports and 

caches seeds. 

•  Ideally, this should be done before WPBR has impacted local limber pine seed sources 

through tree mortality.

•  Local extinctions can occur if stands of limber pine decline and seed sources are not 

close enough to provide adequate regeneration; in these cases, planting or seeding will 

be needed.
42 USDA Forest Service RMRS-GTR-379. 2019.

Description and guidelines:

•  Retain seed-producing trees in a variety of areas to ensure nutcrackers have a source of 

seeds to cache.

•  At least 20–50 cone-bearing trees per acre are needed to be considered a seed source 

(Keane et al. 2012).

•  Nutcrackers will cache seed in openings or closed canopies, steep slopes, needle litter, 

rocks, and tree trunks (Lorenz et al. 2008).

•  Creation of forest openings is a suggested management option to increase nutcracker 

caching habitat; however, adequate seed source, nutcracker population, and good site 

conditions must exist (Keane and Parsons 2010). 

•  Manipulations of forest cover through planned or unplanned fire or other disturbance 

may result in limber pine establishment if there is a nearby healthy seed source and 

good site conditions. 

•  Monitoring for natural regeneration should take place 5 years post-disturbance and the 

amount of augmentation with planting or direct seeding required can be determined 10 

years post-disturbance. Early augmentation with planting or seeding with stock that has 

partial resistance to WPBR is recommended. 

•  Natural regeneration capacity is reduced by WPBR, MPB, dwarf mistletoe, seed predation, drought, fire suppression, and other insects and diseases; prioritize planting or 

direct seeding in these areas. 

•  Any habitat that is suitable for natural regeneration should also be supplemented with 

direct seeding or seedling planting. 

•  Conditions are favorable for limber pine germination in the GRMNPA, though establishment may be curtailed by competition by other tree species (Stohlgren et al. 1998) 

and reduced seed sources due to overstory mortality (MPB, WPBR) or damage (dwarf 

mistletoe, WPBR).

•  Clearing competing vegetation from established seedlings and sapling can help survival.

Figure 17—New limber pine germinant (A.W. Schoettle, USFS).
USDA Forest Service RMRS-GTR-379. 2019. 43

Figure 18—White pine blister rust on limber pine (A.W. Schoettle, USFS).

5.10 Locating Treatments and Selecting Seed Sources to 

Maintain Durability of WPBR Resistance

Purpose:

•  Position treatments and select seed sources to minimize selective pressure on the rust, 

to lower the probability of the proliferation of rust genotypes that can overcome (i.e., 

become virulent to) Cr4, and to minimize the build-up of WPBR spore loads (fig. 18).

Description and guidelines:

•  Bulked seed lots should be used throughout the GRMNPA until seed sources with 

quantitative resistance are available.

•  Plantings of limber pine should be avoided in and near riparian areas (no closer than 33 

ft [10 m] elevation of the stream-bed channel).

•  Sanitation pruning (Section 5, guideline 5) should be done in riparian areas until 

WPBR incidence is ≥ 30 percent in the area or until advised to stop by FHP, to reduce 

the build-up of the local rust population.
44 USDA Forest Service RMRS-GTR-379. 2019.

5.11 Testing for Quantitative Resistance to WPBR

Purpose:

•  Research quantitative (polygenic, partial) WPBR resistance types in limber pine in the 

GRMNPA and establish a seed orchard/clone bank of these genotypes to provide seed 

for future planting projects. Use field collected seed from trees tested to reveal partial 

resistance traits until seed orchard or clone banks mature.

•  Quantitative resistance is less likely to be overcome by the WPBR pathogen over time 

and is more durable than qualitative complete (Cr4) resistance.

•  Partial resistance is likely low in the GRMNPA limber pine populations and needs to be 

increased to sustain limber pine population health.

Timing:

•  Quantitative resistance studies should be initiated as soon as possible, as tests take 5–7 

years after the seed is available.

•  The goal is to plant seedlings or seed with partial resistance established in the field 

before Cr4 is overcome.

Description and guidelines:

•  Existing individual tree seed lots may be used (consult with RMRS to select appropriate lots and make arrangements for testing). Additional selections and collections of 

healthy seed trees in stands that have WPBR-infected and declining trees may be useful 

to identify putatively resistant seed trees. 

•  Consider seedling field plantings in areas of high rust (e.g. Southern Rockies Rust 

Resistance Trial site on the Medicine Bow NF) to prescreen lots for possible resistance 

and prioritize families for further testing used controlled inoculation technology.

•  The USFS has several facilities for testing for quantitative resistance to WPBR using 

controlled inoculation technologies. The Dorena Genetic Resource Center has experience with limber pine (fig. 19).

•  Pricing will vary and is currently below $2,000 per individual-tree seed lot. 

Figure 19—Artificial inoculation process at the Dorena Genetic Resource Center 

(A.W. Schoettle, USFS).
USDA Forest Service RMRS-GTR-379. 2019. 45

Figure 20—Data collection during the installation of a 

forest health plot (K.S. Burns, USFS).

5.12 Monitoring—Forest Health

Purpose of treatment:

•  Monitor limber pine health at the landscape, stand, and tree scales.

•  Enhance and expand the existing monitoring network throughout the GRMNPA.

Timing of treatment:

•  Remeasure monitoring network every 5 years to detect changes.

Treatment priorities:

•  Established monitoring network.

Description of treatment:

•  Follow protocols and recommendations in Cleaver et al. 2017.

•  Within RMNP, the 17 research areas stratified across elevations plus the 13 forest 

health monitoring plots are a good start; additional areas should be added.

•  Ten of the FHP plots within the Park were installed within research areas; monumented 

plots should also be installed in the remaining 7 research areas (fig. 20).

•  Thirty-three FHP plots were installed within the GRNMPA outside of RMNP in 2006, 

and were remeasured in 2011 and 2016 (K.S. Burns, personal communication).

•  Use the Whitebark Pine Monitoring protocol (Greater Yellowstone Coordinating 

Committee Whitebark Pine Subcommittee 2011).

•  Increase the number of plots. 

•  Stratify by distance from stream and elevation.

•  Include riparian areas for early detection of WPBR (see Section 5, guideline 13).
46 USDA Forest Service RMRS-GTR-379. 2019.

5.13 Monitoring—Early Detection of WPBR in Riparian Areas

Purpose of treatment:

•  Early detection of WPBR.

•  Enhance and expand existing monitoring network into areas at high risk for WPBR.

Timing of treatment: 

•  Annual late-spring monitoring in high hazard areas with follow-up sanitation pruning if 

infections are found.

•  Fall monitoring of Ribes to assess rust production and risk.

Treatment priorities:

•  Riparian area plots to identify infection centers.

•  Established monitoring network.

Description of treatment:

•  Follow protocols and recommendations in Cleaver et al. 2017.

•  Within RMNP, an assessment transect has been established in each of six riparian areas 

for early WPBR detection monitoring (Cleaver et al. 2017). 

•  Within the GRMNPA outside of RMNP, establish similar transects in riparian areas.

•  Assess health of limber pines in late spring (last week of May and first 2 weeks of 

June) when C. ribicola aecia are most visible (fig. 18).

•  Assess alternate hosts for signs of infection in the fall (fig. 21). 

•  Mitigate WPBR infections on limber pine as necessary by sanitation pruning (see 

Section 5, guideline 5).

Figure 21—C. ribicola fruiting on the underside of a Ribes leaf (A.W. Schoettle, USFS).
USDA Forest Service RMRS-GTR-379. 2019. 47

5.14 Monitoring – Rust Virulence 

Purpose of treatment:

•  Monitor for WPBR virulence at the tree scale.

•  Look for evidence of WPBR disease on known Cr4 trees.

Timing of treatment:

•  Annual monitoring of Cr4 trees for cankers in the spring (late May to early June).

Treatment priorities:

•  Trees inferred to have Cr4 resistance by WPBR resistance testing.

Description of treatment:

•  Follow protocols and recommendations in Cleaver et al. 2017.

•  Each tree that has been determined to carry the Cr4 allele through rust resistance testing will serve as a “virulence monitor”. Consult with FHP and RMRS for the most 

current listing and location of Cr4 trees.

•  If the disease is detected on one of these trees, it may indicate that the rust has evolved 

virulence and the Cr4 allele will no longer provide immunity.

•  As soon as virulence is detected or suspected, FHP and RMRS should be notified.

5.15 Consider Limber Pine During Fuel Treatments and Planned 

and Unplanned Fire

Purpose of treatment:

•  Minimize limber pine mortality due to fuels treatments, and planned and unplanned fire 

(fig. 22).

•  Provide habitat for planting or direct seeding to increasing the number of limber pine 

trees on the landscape.

•  Stimulate natural regeneration via manipulations of forest cover through planned or 

unplanned fire or other disturbance.

Treatment priorities:

•  Work with fire management to identify areas where fire would be detrimental (i.e., result in loss of seed bearing limber pine) or beneficial to limber management goals (i.e., 

create areas where fire is near a seed source).

•  Protect limber pine with known resistance genotypes from fire (i.e., those trees whose 

progeny have been though rust resistance testing).

Description of treatment:

Planned and unplanned fire:

•  List trees with known resistance genotypes on the valued resource list for fire 

protection.

•  Retain limber pine during fuels treatments.

•  Post-fire planting or seeding limber pine.

•  Creation of forest openings is a suggested management option to increase nutcracker 

caching habitat; however, adequate seed source, nutcracker population, and good site 

conditions must exist (Keane and Parsons 2010) (fig. 23). 

•  Fire suppression considerations: protect seed sources by letting fire burn near but not in 

limber pine stands.

•  Diversify size and age classes and successional stages across the landscape.
48 USDA Forest Service RMRS-GTR-379. 2019.

Figure 23—Evidence of Clark’s nutcracker foraging on limber pine (A.W. Schoettle, USFS).

Figure 22—Forest fire in a subalpine forest within the GRMNPA (U.S. Forest Service).
USDA Forest Service RMRS-GTR-379. 2019. 49

Fuels treatments or harvesting for other objectives: 

•  Do not cut limber pine during fuels treatments or other harvesting operations, unless 

necessary.

•  Consider planting or seeding with limber pine.

•  Avoid damaging residual live limber pine during slash burning. 

5.16 Limber Pine Seed Orchard/In Situ Clone Bank

Purpose:

•  Establish a seed orchard/clone bank of quantitative (polygenic) WPBR resistance genotypes to provide seed for future planting projects within the GRNMPA.

•  Produce improved seed for outplanting.

•  Increasing the frequency of partial resistance on the landscape will significantly increase the sustainability of limber pine populations in the presence of WPBR.

Priorities:

•  Clones (grafts) from quantitative (polygenic) WPBR resistance types should be established as soon as possible in a GRMNPA clone bank (fig. 24).

•  A clone bank will help fulfill RMNP’s mandate to be a source of genetic diversity to 

promote sustained forest health in the park and greater surrounding areas.

Figure 24—Successful limber pine graft (A.W. Schoettle, USFS).
50 USDA Forest Service RMRS-GTR-379. 2019.

Description and guidelines:

•  Great Sand Dunes National Park and Preserve and RMRS conducted a test of the 

partial resistance of 74 limber pine families from the southern Rockies at the Dorena 

Genetic Resource Center. Surviving seedlings that showed evidence of partial 

resistance are being grafted and will be available for a planting in a clone bank in 

2017–2018. If additional studies are conducted in the GRMNPA, additional grafts 

could be added to the planting. 

•  Orchard-derived seeds will be valuable and are thus not recommended for direct seeding. Field collected seed from trees that revealed resistance traits in testing should be 

used to support plantings until orchards mature.

•  Planting seedlings derived from the clone bank or forest trees, or both, with quantitative resistant to WPBR should begin as soon as possible. Collaborate with researchers 

and tree improvement professionals to explore hormone treatments to accelerate flowering and cone production.

Specification for clone bank/seed orchard:

•  Accessible for protection from fire and MPB and within the range for potential supplemental watering. 

•  At least 1–2 acres (0.4–0.8 ha) and able to support 200–500 clones. 

•  A gentle slope (<15%) will allow for operation of machinery.

•  Source of water for irrigation (e.g., nearby creek, holding tank, accessible by water 

truck).

•  Good drainage.

•  No residual overstory.

•  Site preparation will be required during start up to remove shrubs, herbs, weeds, and 

grass.

•  Low rust risk/hazard site preferred.

•  Tall fencing required.

•  Single ownership, not crossing administrative boundaries.

Potential locations of a clone bank/seed orchard in RMNP: 

Hidden Valley Picnic Area—Located where the former parking lot was, adjacent 

to the road heading into the picnic area. There is limber pine in the area. Hidden 

Valley Creek is nearby and this site is not far from the Beaver Pond limber pine 

site. This site is near where WPBR was confirmed, so it may not be a desirable site; 

however, it is a higher elevation site. A new WPBR infestation was identified in this 

area in 2017 (K.S. Burns and B. Verhulst, personal communication). 

Glacier Basin Campground—Located in the southeast corner of the campground, 

the Group Loop area, or near the Borrow Pit area. This site is near Glacier Creek, so 

the plan should specify the estimated distance required away from a stream. There 

should be enough distance away from the creek. This is a higher elevation site. 

There is limber pine in the area.

Tortilla Flats—Located in the vicinity of the Fire Office and Hotshot Dorm. 

Beaver Brook is in the area but a location could be found far enough away. Lower 

elevation site near headquarters. Currently no limber pine in the area.

Greenhouse/Nursery/Maintenance Area—This area may be suitable with respect 

to low WPBR risk; it is, however, a lower elevation site. 
USDA Forest Service RMRS-GTR-379. 2019. 51

SECTION 6. BACKGROUND

6.1. Overview

Rocky Mountain National Park, established in 1915, is located in north-central 

Colorado and encompasses about 265,828 ac (107,577 ha). The park lies within 

Larimer, Boulder, and Grand Counties and is bordered by the towns of Estes Park, 

Allenspark, and Glen Haven on the east and Grand Lake on the west. The park 

is surrounded by Federal, State, local, and privately owned lands. With RMNP at 

its core, the GRMNPA includes ecosystems within and around the Brush Creek/

Hayden, Laramie, and Parks Ranger Districts of the Medicine Bow-Routt National 

Forest and the entire Arapaho-Roosevelt National Forest.

Ninety-six percent of the limber pine in RMNP occurs within designated wilderness areas. The majority of land within RMNP, which is mostly backcountry wilderness, has been given permanent protection since 2009, when 249,339 ac (100,904 

ha; 94 percent of the total park acreage) were designated as wilderness under the 

Omnibus Public Lands Management Act of 2009 (NPS 2014) (fig. 10). The designation offered a unique opportunity to provide protection of ecosystems and biodiversity extending beyond park boundaries, as four wilderness areas (Indian Peaks, 

Comanche Peaks, Neota, and Never Summer) (fig. 25) and three national forests 

(Routt, Roosevelt, and Arapaho) surround RMNP. In contrast, only about 10 percent 

of the limber pine population occurs within designated wilderness in the GRMNPA. 

Management activities within the wilderness must honor the integrity of the 

Wilderness Act of 1964 by preserving wilderness character. Certain management 

activities in wilderness areas are permitted when exotic invasive species threaten 

native species. According to the NPS Wilderness Policy, “management actions, 

including the restoration of extirpated native species, the alteration of natural fire 

regimes, the control of invasive alien species, the management of endangered species, and the protection of air and water quality, should be attempted only when the 

knowledge and tools exist to accomplish clearly articulated goals,” (NPS 2006a, p. 

83). Management of the wilderness areas surrounding RMNP is guided by the USFS 

Manual for Wilderness Management, Management of Insect and Diseases (FSM 

2324.1), which states that control of insect or plant disease outbreaks is permitted 

when “it is necessary to prevent unacceptable damage to resources on adjacent lands 

or an unnatural loss to the wilderness resource due to exotic pests,” (USFS 2007).

Type and magnitude of visitor use vary throughout the park and create differences 

in disturbance type and quantity, which in turn requires different management strategies. Therefore, RMNP has divided areas of land within the park using a disturbance classification system with three categories: Natural Areas (Class I), High Use 

Areas (Class II), and Heavily Impacted Areas (Class III). Each of these is defined 

in the RMNP Vegetation Restoration Management Plan Version 2 (NPS 2006b). 

Here we will also use this system and will consider Class I to include all designated 

wilderness within RMNP, Class II to include non-wilderness wildlands such as areas 

within 200 ft (60 m) of park roads, and Class III to include administrative areas. 
52 USDA Forest Service RMRS-GTR-379. 2019.

Figure 25—Map showing designated wilderness within the GRMNPA.
USDA Forest Service RMRS-GTR-379. 2019. 53

Rocky Mountain National Park, in collaboration with the USFS, established a 

program in 2008 to begin limber pine conservation in the park and to build the science foundation to address specific knowledge deficiencies that restricted efficient 

and effective management for limber pine conservation. This effort is built on the 

RMRS Five-Needle Pine Proactive Strategy Research Program begun in 2001. The 

science provides guidance for (1) assessing ecosystem health, (2) developing management recommendations, and (3) reducing the uncertainty of possible outcomes 

of interventions and consequences of inaction. The goal is to conserve limber pine 

and promote self-sustaining five-needle pine ecosystems in the presence of WPBR 

and other disturbances using available tools and methods that are compatible with 

land-use designations. 

The information network to develop a sound conservation program must include 

(1) forest health evaluations, (2) data describing genetic structure and disease resistance, and (3) evaluation of population structure and dynamics (fig. 26) (Schoettle 

2004a; Schoettle et al. 2011a). Sustaining population resilience requires maintaining recovery capacity after disturbance and sufficient genetic diversity to support 

adaptation over time. Therefore, conservation approaches must be considered within 

an evolutionary perspective (Schoettle et al. 2012). Tree longevity is not enough 

for multigenerational sustainability; sustainability depends on an intact regeneration cycle, sufficient tree population size, and, in the presence of WPBR, increased 

disease resistance to support survival and recovery capacity (fig. 27) (Schoettle and 

Sniezko 2007, Schoettle et al. 2018b). The speed at which this cycle completes a 

round (i.e., generation time or maturation duration) and the intensity of a selective 

pressure (mortality) dictates the rate of potential adaptive change given sufficient 

genetic diversity. If the selection is too great and the maturation time is too long, the 

number of individuals will decline and the population will be unsustainable (Field 

et al. 2012). Each stage in the regeneration cycle must be present and in balance, 

along with functional community landscape structure (i.e., seed dispersal via Clark’s 

nutcracker), for maximum resilience.

RMNP and RMRS identified 17 limber pine research sites in the park (plus one 

that was abandoned due to hazardous cliffs) and 35 sites outside the park to serve as 

the sampling framework for the site-based research on which to lay the foundation 

for this conservation strategy (fig. 9) (Schoettle et al. 2011a, 2013). This crossboundary network of sites was stratified by elevation to capture the full breadth of 

limber pine habitats in the greater geographic area. These sites were the focus of 

seed collections for conservation, WPBR resistance testing, and common garden 

studies as well as deployment of verbenone (which repels MPB attack) and regeneration and forest health assessments. To date, seed has been collected from 53 sites 

within the GRMNPA (18 in the park, 35 outside the park) (fig. 9). In addition, longterm forest health monitoring plots, independent of this network, were installed in 

29 limber pine stands throughout the GRMNPA in 2006 and in 10 of the 17 research 

sites within the park in 2013 (fig. 10). The Proactive Limber Pine Conservation 

Strategy is an extension of this program. 
54 USDA Forest Service RMRS-GTR-379. 2019.

Figure 26—Diagram showing disciplines critical for providing the science foundation for the 

Proactive Strategy aimed at managing high elevation five-needle pine ecosystems threatened 

by WPBR (adapted from Schoettle et al. 2011a).

Figure 27—Forest regeneration cycle and damage agents (adapted from Schoettle and Sniezko 

2007). White pine blister rust (WPBR) affects the regeneration cycle at all points (brown text), while 

mountain pine beetle (MPB) only impacts larger mature trees, and cone and seed insects only 

impact seed production. Dwarf mistletoe can infect trees of all ages. Climate change may directly 

impact insect populations and seedling establishment and tree growth and may indirectly affect 

other interactions within the cycle. 
USDA Forest Service RMRS-GTR-379. 2019. 55

6.2. Evolutionary History of Limber Pine

The biogeographic history of a species contributes context to management and 

restoration options (Noss 1999). During the last glacial maximum (18,000 ± 4,000 

years ago), evidence indicates there were at least four limber pine glacial refugia 

including western Kansas and Nebraska (Wells and Stewart 1987), the Great Basin, 

Fremont County in Colorado, and Bighorn County in Wyoming (Mitton et al. 

2000). In addition, macrofossil evidence suggests that limber pine persisted during 

glaciation at low elevations near the eastern base of the Rocky Mountains south 

of Pikes Peak (Wells 1976). Today in the Front Range, limber pine is most commonly found above 7,875 ft (2,400 m) (Wells 1965); however, a few anomalous 

populations occur on escarpments of the High Plains with the pine persisting as 

low as 5,250 ft (1,600 m) around 37 mi (60 km) east of Cheyenne, Wyoming, and 

Fort Collins, Colorado. The remarkable populations of limber pine on the Pawnee 

National Grassland are widely disjunct and extremely local, and may be relicts of 

Plainswide Pleistocene populations (Wells and Stewart 1987). They are also possibly the populations most immediately vulnerable to climate change (Means 2011). 

As the glaciers receded, the mountains were progressively colonized from the lowelevation refugia with the aid of seed dispersal by Clark’s nutcrackers (Mitton et al. 

2000). During the early Holocene (approximately 11,500 years ago), limber pine 

had expanded to elevations of 8,200 ft (2,500 m) (Anderson et al. 2000). Its upslope 

migration has continued since the late Holocene and the end of the little ice age 

(1890) and is projected to continue (Monahan et al. 2013). 

6.3. Genetic Diversity

The genetic diversity of a species and its distribution across the landscape, along 

with regeneration, dispersal, and disturbance dynamics, are important components 

of the species’ adaptive capacity to novel stresses (see Aitken et al. 2008). Across its 

range, limber pine is considered a moderate generalist with broad tolerances to abiotic stresses. Genetic variation can be accessed from (1) neutral markers that reflect 

migration and genetic drift dynamics; (2) common garden studies that grow trees 

from different geographic sources in a common environment or set of environments 

such that differences among sources in growth, phenology, cold hardiness, or other 

adaptive traits are inferred as attributable to adaptive genetic differentiation among 

sources; and (3) genomic markers such as single nucleotide polymorphisms that 

reflect variation in adaptive traits (Menon et al. 2018). In addition, some inferences 

can be made from in situ studies of variation in traits among trees naturally growing 

in different environments; however, it is not possible to ascribe any differences to 

genetics versus the environment. 

In the southern Rocky Mountains, a common garden study showed slight differentiation among populations from New Mexico and southern Colorado compared 

to those from northern Colorado and southern Wyoming (Borgman et al. 2015), suggesting that seed sources with a makeup similar to those from the GRMNPA extend 

from latitudes of about 40° N to 42° N in this region. In this area, neutral genetic 

marker studies (Latta and Mitton 1997; Schuster and Mitton 2000), in situ studies 

(Schoettle and Rochelle 2000; Schuster et al. 1989), and common garden studies 

(Reinhardt et al. 2011) have all detected genetic differences in limber pine from 
56 USDA Forest Service RMRS-GTR-379. 2019.

different elevations. Local adaptation to conditions along the elevation gradient is 

common in tree species and may result from strong local selection, differences in 

phenology, or limited gene flow among limber pine populations. As a result of this 

pattern of genetic structure, seed sources for planting or sowing projects should be 

from elevations slightly above and below that of the target treatment site to provide 

sufficient genetic diversity for adaptation to climate change yet avoid potential maladaptation (see Section 5, guideline 8). 

6.4. Natural History Traits

The same life history traits that enable limber pine to tolerate natural stresses 

also hinder rapid adaptive change to novel stresses. Limber pine is tolerant of cold 

and drought, but also has a slow growth rate (Schoettle and Rochelle 2000), long 

lifespan (>1,000 years), delayed maturity (>50 years), and a protracted regeneration 

dynamic (Coop and Schoettle 2009); it is not a prolific seed producer. This species’ 

evolutionary strategy does not position it favorably for rapid adaptation to novel 

stresses, such as WPBR, with which it did not coevolve, and the interactions with 

intensifying native stresses. The combination of rapid mortality, impaired recovery, 

slow maturation, and long generation time may not support viable populations in the 

presence of these multiple interacting stressors in the future without management 

intervention to increase the number of individuals and the frequency of genetic 

resistance to WPBR (Field et al. 2012).

6.5. Community Dynamics

Limber pine is often the first species to colonize an area after fire (Donnegan and 

Rebertus 1999) and is capable of doing so in suitable habitat within its elevational 

range (see fig. 2). Clark’s nutcrackers can cache seed across the undisturbed landscape as well as in the central areas of large burns where wind-dispersed seeds of 

other conifer species are scarce (Tomback et al. 1990). The germination of multiple 

seeds from one cache results in a cluster of seedlings that are often related, as the 

cache is frequently a product of a seed harvest by a nutcracker from a single or several parent trees (Carsey and Tomback 1994). The clustered distribution of seedlings 

facilitates successful establishment of limber pine (Donnegan and Rebertus 1999). 

Often, more than one seedling in a cluster survives, and together they form a multigenet tree with stems contiguous or fused at the base. However, as trees mature, the 

clustered distribution may reduce the reproductive output (Feldman et al. 1999) and 

lifespan of individuals (Donnegan and Rebertus 1999) compared to trees growing 

singly. 

The dynamic of stands containing limber pine depends on the site. Limber pine 

forms self-regenerating stands on dry rocky sites and at treeline, but tends to be limited to early successional stages on more mesic sites, where they are often replaced 

by more shade-tolerant species (Schoettle 2004b). Dry sites can be occupied by 

limber pine at any elevation within the species’ range and are often windswept and 

accumulate little snow. Limber pine dominates xeric sites, not because they provide 

the optimal physical environment for limber pine growth (Lepper 1974; Schoettle 

and Rochelle 2000), but because the conditions are not suitable for the growth of 

other species and therefore competition is minimal. Competition is likely to be 
USDA Forest Service RMRS-GTR-379. 2019. 57

the largest limitation defining the realized niche of limber pine and the location of 

sustainable limber pine stands (Schoettle 2004b). On dry sites, limber pines have 

been reported to live more than 1,500 years in Colorado (Schuster et al. 1995) and 

more than 2,000 years in Nevada and California (Lanner 1984). The stands tend 

to be open and support continual recruitment of limber pine (Knowles and Grant 

1983; Stohlgren and Bachand 1997). Sexual maturity may take more than 50 years 

(Schoettle 2004b), and open-grown limber pine produces good cone crops. 

The frequency of mast years is usually every 3 to 6 years. On harsh sites, limber 

pine stands persist not only because of the extreme longevity of some individuals, 

the lack of competing tree species, and sustained regeneration (Coop and Schoettle 

2009), but also because catastrophic disturbance (i.e., wildfire) is rare on dry, rocky 

sites. While rocky ridges and dry slopes are the most visible habitat occupied by 

limber pine, scattered occurrence of limber pine throughout the forested region of 

the GRMNPA is typical (Marr 1977; Schoettle and Rochelle 2000). On the more 

mesic sites, limber pine’s early post-disturbance dominance succeeds over time 

to other conifer species (Rebertus et al. 1991). Limber pine acts as a nurse tree, mitigating the harsh open environment after disturbances and facilitating the establishment of Engelmann spruce (Picea englemannii) and subalpine fir (Abies lasiocarpa) 

(Donnegan and Rebertus 1999; Rebertus et al. 1991). Such facilitation accelerates 

limber pine’s mortality due to the close proximity of, and competition by, the succeeding species. Limber pine also facilitate establishment of other conifers above 

tree line during the formation of tree islands in the krummholz zone (Schoettle and 

Richards, unpublished data on file at RMRS, Fort Collins, Colorado). Limber pine 

regeneration occurs frequently throughout all stand types along the elevation gradient (Brown and Schoettle 2008), yet successful establishment in late successional 

stands on mesic sites is rare (Stohlgren et al. 1998).

Seral limber pine is more likely to maintain apical dominance and retain an erect 

forest tree form and is suspected to produce fewer cones per tree than those trees 

on more open and drier sites (Lepper 1974). Seed yields for limber pine can also be 

reduced by some of the same cone and seed insects that affect co-occurring conifer 

species (Hedlin et al. 1980; Schoettle and Negrón 2001) as well as by WPBR infection, bark beetle attack, and dwarf mistletoe infection. Due to the lower seed yields 

in successional stands, it is unclear what proportion of seed from these sites is consumed by animals versus dispersed and cached. Therefore, the relative contribution 

of progeny from seral compared to persistent limber pine stands to the recolonization of nearby disturbances is unknown (Schoettle 2004b). 

6.6. Clark’s Nutcracker Ecology

Clark’s nutcracker occupies coniferous forests of western North America, particularly forests dominated by pine species producing large, wingless seeds (Tomback 

1998). Whitebark pine is a primary food source and obligate mutualist (Tomback 

and Linhart 1990) of Clark’s nutcracker, but across its range the Clark’s nutcracker 

also consumes seeds from other pines. In the southern Rocky Mountains, nutcrackers harvest seeds from limber and southwestern white pine (Pinus strobiformis) 

(Samano and Tomback 2003) and also Colorado pinyon (Pinus edulis), ponderosa, 
58 USDA Forest Service RMRS-GTR-379. 2019.

and Rocky Mountain bristlecone (Tomback 1998) pine. Limber pine is considered 

a mutualist of Clark’s nutcracker (Lanner 1980), and seed dispersal patterns influence distribution, postdisturbance establishment, and population genetics (Tomback 

2001). The dependence of all these pines to varying degrees on seed dispersal by 

nutcrackers makes the bird an important connecting “mobile link” (Tomback and 

Kendall 2001) essential to the conservation of limber pine. 

The morphological and behavioral adaptations of Clark’s nutcracker allow for 

year-round access to their primary food source (Tomback 1978). Nutcrackers extract seeds from cones with a long, pointed bill. They can carry 50 or more seeds, 

depending on seed size, in a sublingual pouch. They use an extraordinary spatial 

memory to retrieve cached seeds from thousands of locations for up to a year 

(Tomback 1978; Vander Wall and Hutchins 1983), using objects such as rocks, trees, 

or logs for visual reference (Kamil and Balda 1985; Tomback 1980, 2001). About 

three to five seeds are stored per cache (Tomback 2001) at depths of about 0.8–1.2 

in (2–3 cm) (Lanner and Vander Wall 1980; Tomback 1982). Clark’s nutcrackers 

cache limber pine seeds ranging in distance from within 0.3 mi (0.5 km) to several 

miles away from the seed tree (Lanner and Vander Wall 1980). Typical seed caching 

distances by Clark’s nutcrackers for other pines, mainly whitebark pine, range from 

a few feet to a few miles (Hutchins and Lanner 1982; Lorenz et al. 2011; Tomback 

1978, 2001; Vander Wall and Balda 1977), with a maximum dispersal distance of 

20 mi (32 km) (Lorenz et al. 2011). Tomback and Taylor (1987) found that colorbanded nutcrackers in RMNP often traveled back and forth over the Continental 

Divide looking for tourist handouts within the course of a day—a distance of about 

9 mi (14 km). Site types for seed caching include exposed, south-facing, steep 

slopes; areas with an open canopy or terrain; and disturbed sites, especially burned 

areas (Tomback 1978). 

Limber pine across most of its range conforms to a meta-population structure 

whereby a regional population comprises many local small populations and a subset 

of these local populations become extinct over time primarily from fire and advancing succession (Hanski 1999; Webster and Johnson 2000). Long-distance seed 

dispersal by Clark’s nutcrackers, moving seeds from the larger and more productive 

stands, is essential for maintaining this population structure, through recolonization 

of disturbed sites (Brown and Schoettle 2008) and the founding of new populations 

in suitable areas.

6.7. Natural Regeneration Facilitated by Clark’s Nutcracker

Research with whitebark pine suggests that the number of seeds cached by a 

single Clark’s nutcracker per season is prodigious (35,000 to 98,000); given adequate cone production, nutcrackers cache enough seeds to feed themselves and their 

young and to account for losses to rodents (Hutchins and Lanner 1982; Tomback 

1982). Roughly 45 percent of whitebark pine caches are retrieved (Tomback 1982). 

Working in the Cascade Range with whitebark pine, Lorenz et al. (2011) suggested 

that only about 15 percent of seeds are cached in habitat suitable for successful 

germination, but this merits further study. If the seeds are viable and not lost to predation, they may germinate given adequate conditions of temperature and moisture 

(Tomback 1982). This suggests that more than 2,000 seeds per bird may potentially 
USDA Forest Service RMRS-GTR-379. 2019. 59

germinate, and the seedling survival rate for cached seeds is relatively successful at 

more than 55 percent through the first year, diminishing to 25 percent by the fourth 

year (Tomback 1982). All these studies are based on whitebark pine, a species with 

indehiscent (closed at maturity) cones that depends on Clark’s nutcracker for primary seed dispersal. These estimates may underestimate limber pine seed available 

for regeneration because the cones of limber pine open readily at maturity, allowing 

for dispersal of the wingless (or near-wingless) seeds via gravity, fierce winds (for 

short distances; Anna Schoettle, personal observation 2004), or rodents (Tomback et 

al. 2005); however, these dispersal mechanisms are less reliable than that provided 

by Clark’s nutcracker. 

Capacity for natural regeneration of limber pine is reduced by WPBR, MPB, 

dwarf mistletoe, seed predation, drought, and fire suppression (McCaughey and 

Tomback 2001), all of which affect seed availability and therefore dispersal by 

nutcrackers (McKinney et al. 2009). Opportunity for natural dispersal of WPBRresistant limber pine may diminish if mortality is high enough to no longer support 

stand visitation by nutcrackers (McKinney and Tomback 2007; McKinney et al. 

2009). Ultimately, a reduction in the carrying capacity of nutcracker populations by 

mortality of five-needle white pines will exacerbate the decline in limber and other 

five-needle pine species (McKinney et al. 2009). For whitebark pine ecosystems, a 

target of 405 cones ac-1 (1,000 cones ha-1) is recommended to maintain a high probability of Clark’s nutcracker visitation and seed dispersal, and an estimated basal 

area of 8.7 ft2 ac-1 (2 m2 ha-1) (Barringer et al. 2012) to 21.8 ft2 ac-1 (5 m2 ha-1) 

(McKinney et al. 2009) is suggested to produce that amount. Clark’s nutcrackers are 

not likely to visit a stand with fewer than 53 cones ac-1 (130 cones ha-1) (McKinney 

et al. 2009). However, Barringer et al. (2012) found that there was some probability 

of nutcracker stand visitation at even very low cone densities. Lorenz and Sullivan 

(2010) did not find a correlation between cone density and probability of seed 

dispersal in whitebark pine. Critical limber pine production levels for nutcracker 

visitation are unknown.

Postfire natural regeneration was especially successful after the 1978 Ouzel Fire 

in RMNP, with limber pine regeneration occurring within the burn interior and far 

from seed sources (Coop and Schoettle 2009). Other studies have indicated postfire 

dominance of limber pine on the Colorado Front Range and within the GRMNPA, 

including Sundance, Boulder Brook, and Longs Peak burns (Huckaby 1991; 

Rebertus et al. 1991; Shankman and Daly 1988). Successful natural regeneration 

of limber pine is highly dependent on caching by Clark’s nutcracker, and not all 

Colorado Front Range postfire sites have been successfully colonized by limber 

pine, even when limber pine previously dominated the site, adequate seed sources 

were available nearby, and appropriate environmental conditions existed (Huckaby 

1991; Shankman and Daly 1988). 

Limber pine begins to establish postfire within the first decade (Huckaby 1991; 

Shankman and Daly 1988). Rebertus et al. (1991) examined limber pine on three 

northern Colorado sites and indicated that time until initial colonization was inconclusive but that peak colonization took several decades. Coop and Schoettle (2009) 

also reported sustained periods of limber pine establishment after stand-replacing 

fire such that areas are still accumulating new recruits more than 30 years after the 
60 USDA Forest Service RMRS-GTR-379. 2019.

disturbance. Likewise, limber pine seedling establishment was successful in small 

disturbances (i.e., tree fall) within an otherwise intact forest (Brown and Schoettle 

2008). Others evaluating whitebark pine regeneration recorded germination as early 

as 2 to 5 years (Tomback 1986; Tomback et al. 1993), whereas others found 5 years 

was not long enough and recommend at least 10 years (Keane and Parsons 2010). 

Regeneration can be delayed during periods of drought or in areas with seasonal 

drought.

It may take several decades until postfire tree density reaches that of surrounding 

unburned stands (Coop and Schoettle 2009; Klutsch et al. 2015), and colonization 

of limber pine occurs more slowly with increasing elevation and on xeric sites 

(Donnegan and Rebertus 1999; Shankman and Daly 1988). In the upper subalpine, 

reports indicate that it may take up to 40 years for whitebark pine to establish after 

a disturbance (Arno and Hoff 1990). Thus, enhancing natural regeneration of limber 

pine through planting and direct seeding as soon as possible will yield the greatest 

long-term benefit (Coop and Schoettle 2009).

Creation of forest openings is a suggested management option to increase nutcracker caching habitat; however, the efficacy of this technique will be limited if 

seed availability is reduced because of high stand mortality from WPBR and MPB, 

nutcracker retrieval of seed, and poor site conditions (Keane and Parsons 2010). 

6.8. Mountain Pine Beetle Biology and Control

Mountain pine beetle is native to western North America, and limber pine is a 

preferred host (Cerezke 1995). Mountain pine beetle larvae develop under the bark 

of susceptible pines, and typically complete one life cycle per year (Safranyik and 

Carroll 2006). Initially, swarms of beetles attack a tree, and once the tree’s defense 

system is overcome, the tree dies within a year. The cause of death is the combination of the excavation of galleries by adults and larvae and a mutualistic fungus 

carried by the beetle; both synergistically kill the tree (Hubbard et al. 2013; Solheim 

and Krokene 1998). Under endemic conditions, MPB prefers stressed pines, 

typically smaller in diameter, whereas larger diameter pines are preferred during 

epidemics (Safranyik and Carroll 2006). Studies in lodgepole pine (Pinus contorta) 

indicate that MPB attacks trees greater than 8 in (20.3 cm) diameter at breast height 

(d.b.h.) (Gibson et al. 2009); saplings and seedlings are not attacked. The endemicepidemic cycle is largely driven by population size of MPB, host availability, 

and climatic conditions (Safranyik and Carroll 2006). Observations suggest that 

endemic populations of MPB may remain more active and continue to contribute 

to mortality in limber pine stands (Robert Cain, Regional Entomologist, R2 Forest 

Service, Forest Health Protection, personal communication 2015). Temperature 

thresholds of beetle life stages limit the distribution of MPB on a landscape of suitable hosts. The previous MPB outbreak in lodgepole pine in the GRMNPA occurred 

during the early 1980s; it ended abruptly following lethally cold October temperatures in 1983 and 1984 (Chapman et al. 2012; Lessard et al. 1987; Malm 2009). The 

intensity and broad elevational range of the most recent epidemic is attributed to 

recent climate warming (Raffa et al. 2008). Dense, even-aged stand conditions and 

drought in the early 2000s are thought to have increased the susceptibility of trees 

to attack, along with warmer winter temperatures; in fact, in contrast to previous 
USDA Forest Service RMRS-GTR-379. 2019. 61

outbreaks, trees were attacked up to alpine treeline (Logan and Powell 2001). With 

continued climate warming, we can expect an increase in MPB epidemic periodicity, intensity, and elevational range.

It is common for secondary bark beetles, such as engraver beetles (Ips sp.) and 

twig beetles (Pityophthorus and Pityogenes spp.), to increase to very high population levels during a mountain pine beetle epidemic, and then cause significant tree 

mortality as the mountain pine beetle epidemic subsides (Evenden and Gibson 

1940; Witcosky 2017). In such an instance, an outbreak of secondary bark beetles 

may occur over 2 to 3 years, causing significant additional tree mortality, and then 

decline abruptly.

Direct control of MPB can be achieved with timely spraying of the tree bole 

with a registered insecticide such as Carbaryl (1-naphthyl methylcarbamate). Antiaggregation pheromones (semiochemicals, e.g., verbenone) can offer moderate 

control for individual trees (Kegley and Gibson 2004). The Arapaho, Roosevelt, 

and Medicine Bow National Forests have utilized both techniques to protect select 

high-value limber pine trees and populations with confirmed WPBR resistance. 

Since 2008, RMNP has deployed verbenone to protect 10–15 limber pine seed 

trees dispersed throughout the 17 research sites within the park. Two 7.5-g (0.3-oz) 

packets are deployed, one in early summer (May/June) and the other later (July/

August) to ensure a continuous elution rate. Of the 275 trees treated with verbenone 

from 2008 to 2012 in RMNP, 41 trees were unsuccessfully attacked (20 percent) 

and 5 died from MPB attack (2 percent). Because beetle pressure was not controlled 

among sites, these data cannot definitively address the efficacy of verbenone treatment. However, the very low mortality for treated trees as compared to mortality of 

19 percent of limber pine >5 in (12.7 cm) d.b.h. in RMNP during the recent MPB 

epidemic suggests that verbenone may have helped deter lethal MPB attacks on 

treated trees. 

6.9. White Pine Blister Rust Biology and Control

Cronartium ribicola, the pathogen that causes WPBR, is native to Asia (Hunt 

2003). It was accidentally introduced into eastern North America numerous times in 

the early 1900s on infected seedlings from Europe, and it is suspected that there was 

only a single introduction into western North America (Brar et al. 2015; Hunt 2009). 

Despite extensive spread, the pathogen populations in eastern and western North 

America are still distinct and show no evidence of gene flow across the Great Plains 

(Hamelin et al. 2000). 

The complex life cycle of C. ribicola requires five spore stages, three of which 

are infective, and both a primary host (five-needle white pines, subgenus Strobus) 

and an alternate host (Ribes, Castilleja, and Pedicularis spp.) (McDonald et al. 

2006; Zambino et al. 2006) (fig. 28). Cronartium ribicola requires living host tissue for survival. The needles of the primary white pine host may become infected 

during conditions of cool temperatures and high relative humidity in fall and 

exhibit yellow spots the following spring. In late spring of the next year, orange 

aeciospores, the durable, thick-walled fungal spores produced on the five-needle 

pine host, are wind-blown several hundred miles to infect the alternate hosts 
62 USDA Forest Service RMRS-GTR-379. 2019.

(Frank et al. 2008). Urediniospores are produced on the leaves of the alternate host 

midsummer and can magnify WPBR infection in nearby alternate host species. 

Impacts to alternate host species (all deciduous) are minimal because the infection 

generally remains in the leaves and these are typically shed in fall (Stewart and 

Rankin 1914). Produced in late summer on the alternate hosts after 48 hours of 100 

percent relative humidity and temperatures ≤68 °F (20 °C), fragile, thin-walled basidiospores are dispersed typically less than a thousand feet to five-needle pine hosts 

(Van Arsdel et al. 1956; Zambino 2010). 

Once the host is infected, the fungus colonizes the inner bark of the pine, expanding through the branch and toward the main bole; it causes tissue death (cankers) 

that may result in decreased vigor, branch death, topkill, or entire-tree mortality. 

White pine blister rust can ultimately cause a reduction in canopy and cone-bearing 

branches and can infect and kill seedlings. Time from infection to mortality in 

Figure 28—Life cycle of Cronartium ribicola. Species other than Ribes can also serve as the alternate host 

(W.R. Jacobi, Colorado State University).
USDA Forest Service RMRS-GTR-379. 2019. 63

mature trees may be years to decades depending on infection location and tree size 

(Geils et al. 2010). There is no evidence for tree-to-tree transmission of this disease.

Important factors found to positively influence tree damage or mortality expected 

as a result of WPBR include: years infested, percentage of large trees, density of 

Ribes inerme, relative humidity in June, higher September daily minimum temperature, and mean May precipitation (Cleaver 2014; Kearns et al. 2014). Spread and 

intensification of WPBR are increased during years of favorable weather, resulting in a wave year phenomenon (Kinloch 2003). Despite complex meteorological requirements for sporulation and infection, there are no limitations for WPBR 

where white pines and susceptible Ribes species interact (Kinloch 2003). Species 

of all WPBR alternate hosts—Ribes, Castilleja, and Pedicularis—are common in 

the GRMNPA, including R. cereum, R. inerme, R. montigenum, P. groenlandica, 

C. rhexifolia, and C. occidentalis. Both R. inerme and R. montigenum are Class B 

alternate hosts of WPBR, expressing intermediate levels of incidence, whereas R. 

cereum is a Class D alternate host—thought to be less significant in WPBR spread 

(Van Arsdel and Geils 2004). However, because of the extensive coverage of R. cereum in the GRMNPA, it may have a greater role in the epidemiology of the disease 

than its ranking suggests. 

Van Arsdel (1972) described slope bases, narrow valleys, and small openings 

in the overstory as the topographic positions and stand structure types with the 

micrometeorological conditions most suitable for WPBR infection. Riparian areas 

are conducive to the micrometeorological conditions and abundance of alternate 

hosts that permit WPBR infection and sporulation, including cool, humid conditions 

and presence of Ribes inerme; therefore, we developed risk maps (figs. 2 and 7) to 

identify potential areas for early detection of WPBR.

Efforts to control WPBR have limited success. Determining white pine resistance 

to WPBR and establishing rust-resistance breeding programs have been the most 

promising, but time-consuming, options (Burns et al. 2008). Ribes eradication was 

not successful in the western United States; chemical controls are not practical on 

a forest scale, and thinning was found to increase WPBR infection by exposing 

more foliage to inoculum (Burns et al. 2008; Maloy 1997; Schwandt et al. 1994). 

Preventive and sanitation pruning (i.e., removal of infected branches) can prolong 

the life of infected trees but is labor-intensive and therefore feasible only early in 

the infection process or for high-value trees; it is too expensive for application at a 

forest scale (Burns et al. 2008; Crump et al. 2011, Jacobi et al. 2017). While sanitation pruning does not contribute to an increase in the overall frequency of disease 

resistance in the forest and therefore resilience, it may retain high-value trees and 

enable tree survival to provide other ecosystem services (e.g. slope stabilization or 

diet for wildlife). 

Natural genetic resistance (lower susceptibility) to WPBR colonization is present 

in each of the North American white pine species, including limber pine (King et al. 

2010). Assessments of resistance are made with progeny tests and artificial inoculations with C. ribicola (see Sniezko et al. 2011). Limber pine in the GRMNPA has a 

high frequency of complete resistance conferred by a dominant resistance allele—

Cr4 (Schoettle et al. 2014). Complete resistance, or major gene resistance (MGR), 
64 USDA Forest Service RMRS-GTR-379. 2019.

provides immunity; however, the rust can evolve and overcome it. Partial or quantitative resistance is polygenic and thus is suspected to be more durable over time, but 

it occurs at low frequencies. Partial resistance slows the progression of the fungus, 

enabling trees to mature and even reproduce when infected. Quantitative resistance 

appears to occur in limber pine in the southern Rockies (Schoettle and Sniezko, 

unpublished data on file at RMRS, Fort Collins, Colorado); 84 families total with 

43 families across 8 sites within the GRMNPA have been tested (no RMNP seed 

sources were included). A balance of genetic resistance and the number of individuals to offset mortality within the pine populations is needed to sustain populations in 

the presence of the rust (Schoettle et al. 2012). 

6.10. Dwarf Mistletoe Biology and Control

Dwarf mistletoe is a native, leafless, parasitic plant that extracts both water and 

nutrients from its host. It disseminates short distances (33 ft [10 m] on average), 

by forceful ejection of sticky seeds to nearby branches and occasionally by longdistance transport by birds and mammals (Hawksworth and Wiens 1996). The 

many species of dwarf mistletoe are generally host specific, but can cross hosts on 

occasion. Due to the long life cycle of dwarf mistletoe (4–5 years), lateral tree-totree spread occurs at a rate of about 1.5 to 2 ft (0.5 to 0.6 m) per year in even-aged 

stands and more rapidly in uneven-aged stands as seeds readily fall down onto the 

understory trees and branches (Taylor and Mathiasen 1999). Once a tree becomes 

infected, survival time depends on tree size and severity of infection. Heavily infected trees of less than 9 in (23 cm) d.b.h. may die within 7–17 years, while larger 

trees may die within 10–25 years (Hawksworth and Geils 1990). The mortality rate 

of infected trees may be increased by additional stress factors including drought 

(Page 1981; Smith 1983) and bark beetles (Kenaley et al. 2006; McCambridge et al. 

1982; Millar et al. 2007). Heavily infected stands (e.g., the Boulder Brook stand in 

RMNP) may no longer be contributing to regeneration because seed production is 

suppressed, and any established seedlings are likely to become infected and die. 

Limber pine dwarf mistletoe is common in the GRMNPA, and the most effective 

control is pruning infected branches or removal of all infected trees by cutting or 

fire. Pruning delays mortality in high-value trees with moderate infections that are 

concentrated within the lower half of the crown (Hawksworth and Johnson 1993). 

Ethephon, the generic name for trade product Florel® (Lawn and Garden Products, 

Fresno, California), is an ethylene-releasing plant growth regulator that causes 

mistletoe shoots to abscise but does not kill the parasite’s roots, and therefore needs 

to be reapplied every few years. This could be an option for advanced regeneration 

and high-value trees but is not feasible or recommended at a landscape scale.

6.11. Susceptibility to Other Biotic Damage Agents

Limber pine is susceptible to other insects, diseases, and damaging agents. 

Common insects include the limber pine engraver (Ips woodi) and twig beetles 

(Pityophthorus spp. and Pityogenes spp.); both attack and can cause mortality in 

stressed and weakened trees (Witcosky 2017). Several other insects have been 

reported on limber pine including ponderosa pine budworm (Choristoneura lambertiana), woolly aphid (Pineus coloradensis), and multiple cone and seed insects 
USDA Forest Service RMRS-GTR-379. 2019. 65

(Schoettle and Negron 2001; Steele 1990) that can reduce seed yields. Diseases 

affecting limber pine, but less commonly in Colorado, include root diseases (causal 

agents: Armillaria spp. and Phaeolus schweinitzii); wood-decay fungi (Fomitopsis 

pinicola and Porodaedalea pini); and foliar diseases such as brown felt blight 

(Neopeckia coulteri), a common snow mold of conifers (Steele 1990), and Bifusella

species (reported in Colorado and Wyoming; Kelly Burns, USFS, personal communication 2015). Dothistroma needle blight (Dothistroma septospora) has not yet 

been found in Colorado (Taylor and Walla 1999). Additionally, limber pine trees are 

susceptible to wild animal damage, particularly elk (Cervus elaphus) antler rubbing 

and porcupine (Erethizon dorsatum) damage (Steele 1990). 

6.12. Role of Fire in Limber Pine Ecology

The presettlement role of fire in limber pine forests remains the subject of considerable uncertainty. Limber pine probably was exposed to a wide range of fire 

regimes across gradients of site productivity and connectivity of fuels and flammable landscapes. In dense stands and more continuous forests, stand history reconstructions provide evidence for infrequent, high-severity fires. Limber pine can be 

dispersed long distances by Clark’s nutcrackers and, in the high-elevation subalpine 

forests of the northern Colorado Front Range, it is an early colonist of extensive, 

high-severity burns. The degree to which high-severity fire was typical in limber 

pine forests is unclear. 

Following fire, regeneration dynamics take from decades to centuries (Brown and 

Schoettle 2008). Where open stands border grassy openings, limber pine trees often 

exhibit fire scars indicative of fairly frequent but low-intensity fire. Because of the 

great ages attained by limber pine, they potentially offer very long fire history reconstructions in such settings. Whether or not fire suppression has led to declines in 

limber pine—through successional shifts to shade-tolerant competitors or shifts to a 

stand-replacing fire regime—remains an open question that deserves further inquiry. 

In any case, reestablishing presettlement fire regimes, whatever they were, may 

not be as important as determining appropriate disturbance regimes given current 

conditions and management objectives. In the face of the WPBR and MPB threats 

and uncertain consequences of climate change, fire management (both prevention 

and application) can be a tool to promote resilient landscapes (Coop and Schoettle 

2011). Appropriate fire management may be used to conserve valuable stands, promote regeneration and diversify age-class structures and alter the balance between 

this species and its competitors (Schoettle and Sniezko 2007). 

Limber pine appears to benefit from infrequent stand-replacing fires that permit 

multiple successional stages across the landscape (Brown and Schoettle 2008; Coop 

and Schoettle 2011). In the Colorado Front Range, limber pine is often an early 

seral species following high-severity fire, forms self-replacing stands on xeric sites, 

and succeeds to spruce and fir (Abies spp.) on mesic sites (Donnegan and Rebertus 

1999; Peet 1981; Rebertus et al. 1991; Veblen 1986). Ribes species, alternate hosts 

of WPBR, are also early successional species that can establish a site after disturbance, including fire, thus contributing to WPBR hazard (Coop and Schoettle 2009; 

Zambino 2010). Limber pines have thin bark, which makes them highly susceptible 

to fire; however, typical open, low-density stands do not provide excessive fuels, 
66 USDA Forest Service RMRS-GTR-379. 2019.

allowing for the survival of large trees (Steele 1990). Stand-replacing fires can 

lead to extinction of local populations of limber pine (Webster and Johnson 2000). 

Summer drought, which is projected to intensify in the western United States, 

contributes to increased frequency of large wildfires (Westerling et al. 2006). Fire 

season length, as well as frequency and severity of unplanned wildfires, is expected 

to increase with climate change, resulting in an overall increase of area burned by 

wildfire (McWethy et al. 2010). Protecting limber pine from fire when possible 

maintains genetic diversity and allows retention of cone-bearing and WPBRresistant trees. 

6.13. Sensitivity to Abiotic Damage Agents: Nitrogen Deposition 

and Climate Change

The GRMNPA, located near the growing population of the northern Colorado 

Front Range communities, is vulnerable to the damaging effects of anthropogenic 

nitrogen pollution (vehicle or industrial emissions and agricultural sources) deposited in rain or snow or as dry particles (NPS 2005b). Critical thresholds of nitrogen 

deposition have been recorded in RMNP and are altering ecosystem structure and 

function (Baron et al. 2000). High-elevation ecosystems are the most vulnerable because high-elevation plants are adapted to a nitrogen-limited environment, and short 

growing seasons limit time for plant nitrogen uptake. Additionally, the rocky, shallow soils lack adequate chemical buffering against the acidifying effects of excess 

nitrogen (Colorado Department of Public Health and Environment 2014). High foliar nitrogen levels increase susceptibility of pines and Ribes to WPBR (McDonald 

and Dekker-Robertson 1998; Van Arsdel 1972; Zambino 2010). Other potential 

consequences to limber pine and conifer ecosystems include loss of biodiversity; 

increased potential for insect and disease outbreaks; shift in growth phenology, 

increasing susceptibility to spring cold damage; and a decrease in general health and 

resilience to climate change (Fenn et al. 2003; Schoettle 1999). 

Regional warming has already contributed to increasing mortality in western 

U.S. trees, including limber pine (Smith et al. 2015; van Mantgem and Stephenson 

2007). Most general circulation models suggest a substantial reduction in suitable 

range for limber pine by 2090 (Crookston and Rehfeldt 2015). Changing climate, 

including increasing temperatures and altered snowpack and precipitation patterns, 

impact limber pine growth and can affect interaction with associated insects and 

pathogens. In the southern Rocky Mountains, temperatures have risen 0.9–1.8 °F 

(0.5–1 °C) over the last 30 years, with higher elevations warming more quickly in 

some areas (McWethy et al. 2010). A projected annual mean temperature increase of 

3.6 °F (2 °C) is expected by 2050, with more precipitation occurring as rain instead 

of snow (McWethy et al. 2010). In RMNP, the bioclimatic envelope for limber pine 

is projected to expand upward in elevation and become fragmented (Monahan et al. 

2013). Consequently, under a warmer climate it is also likely that populations may 

move upslope in the GRMNPA. 

Climate change-caused warming may increase the likelihood of new, larger, or 

more frequent native and nonnative insect and disease outbreaks (McWethy et al. 

2010). The drought of the early 2000s has been associated with the MPB epidemic 
USDA Forest Service RMRS-GTR-379. 2019. 67

in the southern Rocky Mountains (Chapman et al. 2012). Increased sustained warming is also expected to expand, in both latitude and elevation, suitable habitat for 

MPB (Bentz et al. 2010). Warming conditions may also increase the occurrence of 

cone and seed insects that currently are less common at higher elevations (Schoettle 

and Negrón 2001). Predicting how climate change will affect WPBR and dwarf 

mistletoe is less certain (Kliejunas 2011). Warmer and drier conditions in the future 

may reduce the frequency of WPBR infection (which requires cool, wet conditions; 

Sturrock et al. 2011); warmer temperatures could expand the range of dwarf mistletoe because its distribution is limited by cold temperatures (Hawksworth and Wiens 

1996). Drought stress in limber pine, an effect of climate change, may reduce host 

defenses, resulting in greater tree mortality (Millar et al. 2007). 
68 USDA Forest Service RMRS-GTR-379. 2019.

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