DETERMINING ENVIRONMENTAL AND PHYSIOLOGICAL CAUSES OF WESTERN REDCEDAR DECLINE AND MORTALITY

Western redcedar (Thuja plicata) is a culturally important tree to native peoples of the Pacific Northwest, with many traditional uses, and is called the Tree of Life by the Salish. Western redcedar is also an important timbers species, with a higher value per volume than any other species grown in the region, although challenging to produce with typical NW silvicultural practices. Widespread canopy dieback and mortality of western redcedar has been observed across the Pacific Northwest, from the Willamette Valley through the Puget Sound region and north into British Columbia, and east to the Northern Rockies of Idaho. Both wildlands and horticultural plantings in parks and urban areas have been affected. No insect or pathogen has been found associated with the dieback and mortality, but recent high temperatures and associated increases in vapor pressure deficit (atmospheric demand for water) may be an important driver. Working with colleagues across the Northwest interested in this problem, our lab will investigate how environmental conditions interact with physiological function to cause western redcedar mortality and dieback. We plan to use various approaches, including field research and greenhouse experiments, retrospective tree-ring assessment of live/dead and healthy/unhealthy trees, investigation of physiological function in the field and with experiments, and comparative measurement of co-occurring unaffected trees. Through understanding what appears to be causing the decline of this species, we aim to determine better prediction of global change effects on Northwest forest species, and develop management approaches for mitigating these stresses.

The influence of physiological stress from drought
on pine defenses against bark beetle attack

Drought-induced forest die-off is a global phenomenon with far-reaching impacts. In the western US, tree death from drought, higher temperatures, and bark beetle outbreaks now exceeds growth in our forests. Despite increases in the frequency and severity of drought-related insect outbreaks, factors influencing a tree’s susceptibility to insect herbivores, such as the presence of chemical defenses, have received little attention. In particular, it is unclear how drought causes trees to shift resources toward or away from the chemical defenses that deter insect attacks and the subsequent trade-offs that exist with other important plant functions. This study will focus on how piñon pine trees allocate their carbon resources toward defense and other physiological processes under increasing drought stress to avoid death by drought or bark beetle attack.

Most work to date on drought-related tree death has focused on understanding the coupled roles of carbon starvation and hydraulic failure, but drought is oftentimes accompanied by insect outbreaks that cause or contribute to tree mortality. Research has yet to determine when trees cease investment in effective chemical defenses against biotic attack along the continuum of drought stress, and how these shifts in carbon availability simultaneously impact other plant physiological processes. This study seeks a mechanistic understanding of how drought stress affects the interactions among tree hydraulic function, carbohydrate availability, and chemical defense. Using both greenhouse experiments and field drought manipulations this research aims to identify mechanisms responsible for shifts in piñon pine allocation of recently fixed carbon at the level of individual compounds with known impacts on bark beetle behavior. This approach will also allow identification trade-offs involved in the synthesis of defense compounds at different drought severities while also advancing our fundamental understanding of tree physiology and whole tree C budgets. By providing a comprehensive understanding of the effects of drought-induced physiological stress on mechanisms determining defense against bark beetles, a new, more complete framework for assessing mechanisms of tree mortality will be developed.

Physiological Thresholds for drought-induced
tree mortality

Widespread tree mortality from drought, increased temperatures, and tree pests and pathogens in response to climate change has the potential to reshape forest ecosystems and alter biosphere-earth system feedbacks, including terrestrial C sink strength. Projection of these changes could be improved through a mechanistic understanding of the physiological process of tree death from drought. Experimental and observational research spanning multiple life stages from seedling to tree across a wide variety of tree species is needed to push the field beyond the false dichotomy between carbon starvation and hydraulic failure. Most recent research on how trees die from drought has not been able to distinguish the symptoms of dying from the actual causes of death. This research aims to identify the physiological risk factors for tree mortality by experimentally testing for lethal thresholds across a range of physiological drought stress. The applications of this research include building better predictive models of tree mortality that can better inform projections of vegetation change, including forest die-off, and the associated effects on regional and global carbon budgets.

The role of drought in fire risk from eastern redcedar

Invasion and expansion of eastern redcedar (Juniperus viriginia) is the greatest land management challenge facing states in the Great Plains and Midwest US. Woody encroachment from this species causes economic losses through reduction of forage for grazing, alters hydrological flows to negatively affect water resources, increases allergenic pollen counts, degrades wildlife habitat, and increases the risk of catastrophic wildfire. In Oklahoma, eastern redcedar threatens conversion of much of the state from grassland to woodland over the next 10-20 years. Eastern redcedar is considered a fire-intolerant species but it has a dynamic relationship with fire. The frequent low-intensity fires that were typical in Oklahoma prior to Euro-American settlement severely restricted the range of eastern redcedar, as its seedlings and saplings are very vulnerable to fire. Larger eastern redcedar trees are much more resistant to fire, especially during ideal conditions for prescribed fire when foliar moisture content is high. However, eastern redcedar is a highly drought-tolerant tree species that can survive relatively low tissue water content. During drought, when foliar moisture is low, eastern redcedar becomes much more easily combusted, posing a risk to life and property during wildfire. Recent research has found a threshold in fire behavior at 60% foliar moisture, and below this threshold time to ignition rapidly declines and flame height rapidly increases with declining foliar moisture. Our research aims to determine how drought influences eastern redcedar foliar moisture to better assess the effect of this tree on wildfire risk across Oklahoma. This study uses a combination of field observations and greenhouse experiments to determine just how much drought stress it takes to increase fire risk from eastern redcedar.

The influence of climate and forest management
on loblolly pine defenses against bark beetles
across the southeast US

Warmer and drier winters have the potential to increase the severity of bark beetle outbreaks in loblolly pine (Pinus taeda) plantations and cause billions of dollars in damage. Loblolly pine is planted on over 16 million ha across the southeastern US, making it the most important timber species in the most important timber producing region of the US. Across western North America, warmer winters in recent decades have led to widespread and ongoing bark beetle outbreak and tree mortality that has affected over 60 million hectares. While eastern forests have been spared this level of destruction, worsening outbreaks of bark beetles in loblolly pine plantations have the potential to cause billions of dollars in damage. The primary defense mechanism against bark beetles in loblolly and other pines is production of viscous resin which physically pushes out attacking beetles and is also toxic to them. Resin production and the extensiveness of the resin duct network are good predictors of tree survival from beetle attacks and both vary with environmental conditions. This research aims to assess the variability in defensive resin ducts across the range of loblolly pine, and determine resin duct responses to drought and management.

Environmental Ecology of
the eastern prairie-forest ecotone (former project)

The eastern prairie-forest ecotone is a prominent feature of the North American continent driven by gradients in temperature, moisture, and disturbance by fire. Yet the future position of this boundary between biomes under climate change is uncertain. Some tree species, such as eastern redcedar appear to be responding to these changes by widespread expansion, yet inverse temperature-growth relationships in many other tree species suggest a potential for increased forest stress with climate change. Widespread mortality of oaks across Oklahoma associated with drought, higher temperatures and infection with hypoxylon canker has peaked in recent years and may be associated with forest density, as influenced by management regimes. Our research addressesed past, present and future drivers of ecosystem change in the Crosstimbers region of Oklahoma, and the potential for management to foster mitigation and adaptation to future disturbances.

henry.adams [at] WSU.edu

WSU Environmental Ecology Lab
Dr. Henry D. Adams
230 Dairy Road
Pullman, WA 99164