Mountain pine beetle outbreaks in high elevation whitebark pine forests: the effects of tree host species and blister rust infection severity on beetle productivity

In recent years, the mountain pine beetle (MPB, Dendroctonus ponderosae) has caused dramatic levels of mortality of whitebark pine (Pinus albicaulis). This keystone and foundation tree species is also being killed by the exotic fungus, Cronartium ribicola, which causes the disease white pine blister rust (WPBR). This study examines MPB productivity in whitebark pine compared to that in lodgepole pine, and also in whitebark pine of varying WPBR infection severities to determine if either of these host factors contributes to the current, dramatic MPB outbreaks in whitebark pine. To evaluate host tree effects on MPB, I tracked attack densities, emergence rates, size and sex ratio of MPB from lodgepole pine, and from healthy and WPBR-infected whitebark pine. Beetle emergence rate was higher from whitebark pine. I found no differences in beetle size between lodgepole and whitebark pine. The three populations I tracked declined precipitously during the period of study. This decline was likely caused by a cold snap in October 2009 as indicated by combined phenology/cold tolerance model results. MPB attack density was lowest on the most severely WPBR-infected trees, but emergence rates and size of beetles from these trees were greatest. Low attack rates in severely infected whitebark pine may indicate that these trees have lower defensive capabilities, while the greater emergence rates from these trees are likely due to low intraspecific competition afforded by low attack rates. Given that highly infected whitebark pine support high MPB productivity, these trees could support rapid MPB population growth when environmental conditions are favorable. It appears that whitebark pine is a better host for MPB than lodgepole pine, but whitebark pine of varying WPBR infection severity do not differ from each other in terms of beetle productivity. Therefore, the extensive MPB outbreaks in whitebark pine likely reflect the combination of both whitebark pine’s superior host quality, as well as the favorable conditions for MPB development created by a warming climate in high elevation forests

Tree-Ring Evidence of Climate and Environmental Change, Beartooth Mountains, Wyoming, U.S.A.

Long-lived, subalpine tree species like whitebark pine and Engelmann spruce may eventually cease to exist due to the combination of climate change and exacerbated native and invasive biological threats. While this loss would have dire consequences for mountain ecosystems, it would also result in the irreversible loss of valuable climatological and ecological data preserved in the growth rings of these trees. The purpose of this dissertation research was to develop extended whitebark pine and Engelmann spruce tree-ring chronologies for use in regional analyses of climate and disturbance, and more importantly to demonstrate the potential of these tree species and the need for increased tree-ring based work. From a high-elevation site in the Beartooth Mountains of Wyoming, I collected hundreds of samples from both living and remnant whitebark pine and Engelmann spruce, and used dendrochronological methods to develop two millennial-length tree-ring chronologies for these species. Using information gained from these chronologies, I examined evidence of climate and environmental change. First, I reconstructed the millennial-length history of a subalpine “ghost” forest. I determined that massive whitebark pine and Engelmann spruce trees had established and lived in the Beartooth Mountains during the warm and dry Medieval Warm Period but perished during the colder Little Ice Age. Next, I evaluated climate drivers of tree growth and implications for ongoing climate change. I discerned the key climatic factors controlling whitebark pine and Engelmann spruce growth, but also discovered that tree responses to climate may be changing. Finally, I investigated the influence of volcanic cooling events in the Beartooth Mountains. I discovered frost and narrow rings associated with climate-changing volcanic eruptions and contributed important information to our understanding of volcanic cooling. I hope that my work may serve as a guide for future efforts to collect and analyze data from whitebark pine and other species that are, and may be, threatened with extinction. I also call for increased contribution of tree-ring data, from whitebark pine especially, but also from other subalpine species, to accessible archives, such as the International Tree-Ring Data Bank (ITRDB), which will make tree-ring data available for current and future scientists

Ecological Process and the Blister Rust Epidemic: Cone Production, Cone Predation, and Seed Dispersal in Whitebark Pine (Pinus albicaulis)

Whitebark pine (Pinus albicaulis), a high elevation foundation species, is experiencing population declines throughout the northern part of its range. The introduced fungal pathogen, Cronartium ribicola (white pine blister rust), infects whitebark pine and kills cone-bearing branches and trees. Blister rust has spread nearly rangewide and damage and mortality are highest in the northwest US and southwest Canada. Mortality caused by mountain pine beetle (Dendroctonus ponderosae) population upsurges, and successional replacement and loss of regeneration opportunities from fire suppression, are also impacting some whitebark pine populations. Within this dissertation, I present three manuscripts that address the impact of whitebark pine\u27s decline on species interactions and ecological processes within subalpine forests. Research was conducted in three ecosystems in the Rocky Mountains USA that are distinct in whitebark pine health conditions (rust infection and mortality) and abundance. In the first manuscript, I explore how the relationship between whitebark pine and Clark\u27s Nutcracker (Nucifraga columbiana), its primary seed disperser, is being affected by whitebark\u27s decline. Nutcrackers were less likely to use and disperse seeds from forests where cone production is below a threshold. In the second manuscript, I describe habitat use of whitebark pine forests by red squirrels (Tamiasciurus hudsonicus). Squirrel residency and impact of cone predation increased with decreasing whitebark pine abundance. The third manuscript focuses on the tree-level ecological process, predispersal cone survival, as a function of coarse scale whitebark pine abundance. Surviving trees in high mortality forests were found to have a lower rate and higher variability of cone survival, suggesting that the putative levels of rust-resistance in surviving trees of high mortality forests may not be passed on to future generations. At the ecosystem level, the Northern Divide had the highest levels of rust infection and tree mortality and lowest nutcracker interaction and regeneration levels; the Greater Yellowstone had the lowest infection and mortality levels and nutcrackers were present and dispersing seeds at all research sites in all years, while the Bitterroot Mountains were intermediate in these comparisons. These findings provide important components for developing a long-term strategy to conserve and restore whitebark pine ecosystems in the Rocky Mountains

Efficacy of SPLAT® Verb for Protecting Individual Pinus contorta, Pinus ponderosa, and Pinus lambertiana from Mortality Attributed to Dendroctonus ponderosae

We evaluated the efficacy of a newly registered formulation of (-)-verbenone for protecting individual lodgepole pines, Pinus contorta Dougl. ex Loud., ponderosa pines, P. ponderosa Dougl. ex Laws., and sugar pines, P. lambertiana Dougl., from mortality attributed to mounatin pine beetle, Dendroctonus ponderosae Hopkins

GROWTH RESPONSE OF WHITEBARK PINE (PINUS ALBICAULIS) REGENERATION TO THINNING AND PRESCRIBED BURN RELEASE TREATMENTS

Whitebark pine (Pinus albicaulis Engelm.) plays a prominent role throughout high-elevation ecosystems in the northern Rocky Mountains. It is an important food source for many birds and mammals, as well as a major player in high-elevation watershed maintenance, both slowing snowmelt and stabilizing soils. Whitebark pine is vanishing from the landscape due to three main factors – white pine blister rust (Cronartium ribicola) invasions, mountain pine beetle (Dendroctonus ponderosae) outbreaks, and successional replacement by more shade-tolerant tree species historically controlled by wildfire. In the past century, human activity such as fire suppression has altered these systems, potentially causing dramatic changes to the landscape. Managers now are implementing a variety of treatments across the landscape to encourage whitebark pine regeneration and survival. The objective of this study was to determine how whitebark pine regeneration (less than 9 inches diameter at breast height) responds to selective thinning and prescribed burn treatments, otherwise known as release treatments, intended to cause an increase in annual growth. I examined the growth ratio (GR) obtained from tree cores and destructive sampling at four sites in Montana and Idaho treated in the late 1990s. Overall, the average annual radial growth rates of the trees in treated areas was greater than that of trees in control areas. Specifically, there were significant increases in the GR in the two sites that were both thinned and later burned. All sites showed high variability in the GR of individual trees; however, there was greater variability in the annual growth rates of trees in treated areas than in trees from the control areas. I also mapped the height to age relationship of a subsample of the trees to examine how the vertical growth profile changed after treatment. Results suggest that whitebark pine regeneration can respond to thin and burn release treatments and that managers may see positive results in other areas that are treated similarly

Time series analysis of limber pine (Pinus flexilis) health in the U.S. Rocky Mountains in response to white pine blister rust (Cronartium ribicola) and bark beetles

2018 Summer.Includes bibliographical references.From 2004-2007, 106 permanent limber pine monitoring plots were established and measured throughout the U.S. Rocky Mountains (MT, WY, CO) to characterize health trends in response to white pine blister rust (WPBR) and bark beetles (including mountain pine beetle, "MPB", and Ips spp., "Ips") over time. These plots were subsequently measured in 2011-2013 and again in 2016-17 to form a time series analysis of limber pine health. Data were gathered on 8,206 monumented trees (4,176 limber pine) and included measurements on various stand, ground cover, and landscape characteristics over the three time intervals. The overall percentage of live trees infected with WPBR was 29.4% in 2004-07 and 25.7% in 2016-17, with incidence decreasing in parts of Wyoming (Pole Mountain, Laramie Peak), increasing in southern Colorado (Sangre de Cristo Mountains), and stable in other subregions. However, of limber pines that were healthy during the first measurement, 22.2% were declining/dying and 21.1% had died by the end of the study period due to WPBR and/or bark beetle damages. Due to this, it is likely that new WPBR infections are occurring as the large number of live, infected trees dying during the survey may have masked newly infected trees in incidence calculations. In heavily WPBR-infected areas such as Pole Mountain, Wyoming, 65% of live trees were infected (in 2004-07), and of trees that began the study as healthy, 23% were declining or dying and 38% had died by the end of the study period (2016-17). Additionally, WPBR severity increased significantly from the beginning of the study with 4 previously uninfected sites gaining WPBR infections, 29 sites advancing to 'moderately infected' and 5 sites becoming 'heavily infected'. The overall average number of cankers per tree (3.5) was stable, but the number of infected limber pine with a canker in the lower 1/3 of the stem (18%) increased significantly (+4.2%, P = 0.001). When examining all limber pine in the study, 8%, 3% and 3% were killed by MPB/Ips., WPBR, and combined effects of these agents, respectively. Of the 887 live, but declining or dying limber pine, 52% had WPBR infections and 38% had damage from twig beetles (Pityophthorus spp., Pityogenes spp.) in 2016-17. Though all sites had ≥ 20% limber pine composition, 34% of sites had no limber pine regeneration and 7% had no regeneration of any tree species over the entirety of the study period. The results of this time series indicate that limber pine populations in the U.S. Rocky Mountains are declining due to effects from WPBR and MPB/Ips. Long-term surveys capture the effects of these damage agents on native tree populations and provide critical guidance for future management and restoration of these ecologically valuable species. Limber pine is at risk due to the various biotic and abiotic agents threatening their health. Thus, future directions involve restorative management practices for highly impacted areas where limber pine is a climax species and proactive management for healthy limber stands to promote resilience to likely damage agents. In highly impacted areas (WPBR incidence, mortality, or bark beetle damage on >50% of trees and low limber pine density and regeneration), where limber pine co-exists with other tree species, it may be favorable to allow the natural succession of other tree species to become dominant. However in xeric, harsh sites where limber pine is a climax species, these highly impacted areas are at-risk for losing all tree cover and should be considered for protective and restorative planting strategies. As natural resistance to WPBR occurs on the landscape, genetic screening and protection of mature limber pine carrying either complete or partial resistance to the pathogen should be pursued to preserve this genetic diversity. A priority should be to protect resistant against bark beetles and fire using established management practices. Additionally, seed-sourcing from resistant trees can allow for resistant progeny to be out-planted into high priority areas, thus buffering stands at risk for high WPBR mortality. Moreover management plans that promote diversification of age and diameter classes within stands can provide resilience against pest and pathogen attacks, as bark beetles vary in diameter preference and WPBR infections tend to cause higher mortality in smaller diameter trees. Lastly in healthy limber pine stands, proactive management of pest impacts to promote stand resilience is recommended as in Schoettle & Sniezko (2007) in order to preserve these healthy populations

GLOBAL CHANGE AND WHITEBARK PINE: RESTORATION, REFUGIA, AND ALPINE TREELINES

Whitebark pine is a major component of subalpine forests in western North America. The species occupies harsh high-mountain sites up to treeline, where it is often the dominant species. The species is ecologically important but is also a valuable species for studying the dynamics of alpine treelines. However, whitebark pine has experienced significant mortality in recent decades from mountain pine beetle outbreaks and white pine blister rust. This kind of rapid environmental change presents significant challenges to our understanding and management of the dynamics of ecological communities. On one hand, the effects of climate change on forest ecosystems could provide unique opportunities to study how species, populations, communities, and ecosystems respond to large-scale disturbance. On the other, prediction of future ecosystem behaviors and associated management decisions are complicated by a current lack of understanding of long-term dynamics. Managers are responding to indirect effects of climate change by expanding restoration activities into previously unmanaged, and often poorly understood, forest ecosystems. In this dissertation I investigated three aspects of whitebark pine ecology and conservation: 1) the ecological responses of whitebark pine stands to restoration treatment, 2) the potential of treeline habitats as refugia from mountain pine beetle attack, 3) and the climate-related processes that control growth form at treeline