Climate Change and Yellow-Cedar Decline
Yellow-cedar is an ecologically, economically, and culturally important tree species that has experienced dramatic mortality in Alaska and nearby British Columbia for about 100 years. The extensive mortality, referred to as yellow-cedar decline, has now been documented on over 200,000 hectares and is not associated with any biotic factor (insect or disease) as a causal agent of decline. However, the onset of yellow-cedar decline coincided with the beginning of a period of climatic warming after the Little Ice Age (ca 1850), and the distribution of decline parallels milder winter temperature isoclines in the region. One consequence of milder winter temperatures is a change in the pattern of insulative snow cover. Because ambient winter temperatures often hover around freezing, and precipitation levels are commonly heavy, even small alterations in temperature can lead to precipitation dominated by rainfall, reducing the depth and duration of snowpack. Reductions in protective snow cover could increase possibilities for soil freezing, which would be especially damaging to roots where the depth of rooting is limited by soil water saturation.
We are testing the hypothesis that reduced snowpack and associated increased amounts of soil freezing are resulting in increased root freezing injury and decline among yellow-cedar trees. For example, our measurements of the cold tolerance of mature trees in the field highlighted two patterns in freezing injury susceptibility that are consistent with field reports of yellow-cedar decline: 1) that yellow-cedar trees dehardened more and sooner in the spring than co-occurring western hemlock, which made yellow-cedar more vulnerable to freezing injury at this time, and 2) that trees in low- and mid-elevation sites (where decline is evident) were more vulnerable to freezing injury than trees in high elevation sites. In a separate study we measured the freezing tolerance and injury expression of yellow-cedar seedlings exposed to different levels of simulated snow cover during winter or spring. On all dates sampled (winter through spring) roots of seedlings from all treatments were only tolerant to about -5°C – a level well above the soil temperature recorded in the exposed treatment soils. As a result of this limited root cold tolerance, roots of seedlings with no simulated snow protection had significantly higher relative electrolyte leakage (REL – an indicator of injury) throughout the winter and early spring than seedlings in simulated snow treatments. Seedlings lacking simulated snow protection also had significantly higher foliar REL values and greater visual foliar injury than seedlings from the other treatments starting early spring. All seedlings in the treatment lacking snow protection eventually had 100% fine root damage and died. The progression of initial root damage followed by foliar browning and mortality after the onset of warming conditions is consistent with reports of yellow-cedar decline symptom development in the field.
More recent work suggests that yellow-cedar has a shallower roots than other co-occurring tree species. Shallow roots could help yellow-cedar better take up nitrogen in the spring, but also would causes yellow-cedar to be more vulnerable to root freezing injury relative to other species. An article by our research group summarizing the causes of yellow-cedar decline was the cover story in the journal BioScience in February 2012. This paper also outlines a dynamic conservation strategy for yellow-cedar in the North Pacific Rainforest.
Verification that climate change-induced reductions in snowpack do increase root freezing injury and incite decline of yellow-cedar will equip policy makers with valuable evidence of the consequences of climate change on the health and productivity of forest systems. Furthermore, identification of the cause of yellow-cedar decline should lead to adaptive management strategies (e.g., altered range suggestions, silvicultural strategies and/or genetic selection programs) to either reduce the likelihood of freezing injury or bolster species cold tolerance to survive periodic stress events. Through a combination of policy and management alternatives, the numerous products and ecosystem services provided by yellow-cedar can be safeguarded for future generations.
Hennon, Paul E.; D'Amore, David V.; Schaberg, Paul G.; Wittwer, Dustin T.; Shanley, Colin S. 2012. Shifting climate, altered niche, and a dynamic conservation strategy for yellow-cedar in the North Pacific coastal rainforest. BioScience. 62: 147-158.
Schaberg, Paul G.; D'Amore, David V.; Hennon, Paul E.; Halman, Joshua M.; Hawley, Gary J.2011. Do limited cold tolerance and shallow depth of roots contribute to yellow-cedar decline? Forest Ecology and Management. 262: 2142-2150.
D'Amore, David V.; Hennon, Paul E.; Schaberg, Paul G.; Hawley, Gary J. 2009. Adaptation to exploit nitrate in surface soils predisposes yellow-cedar to climate-induced decline while enhancing the survival of western redcedar: a new hypothesis. Forest Ecology and Management. 258: 2261-2268.
Schaberg, P.G.; Hennon, P.E.; D’Amore, D.V.; Hawley, G.J. 2008. Influence of simulated snow cover on the cold tolerance and freezing injury of yellow-cedar seedlings. Global Change Biology 14:1-12.
Hennon, P.; D’Amore, D.; Wittwer, D.; Johnson, A.; Schaberg, P.; Hawley, G.; Beier, C.; Sink, S.; Juday, G. 2006. Climate Warming, Reduced Snow, and Freezing Injury Could Explain the Demise of Yellow-cedar in Southeast Alaska, USA. World Resource Review 18: 427-450.
Schaberg, P.G.; Hennon, P.E.; D’Amore, D.V.; Hawley, G.J.; Borer, C.H. 2005. Seasonal differences in the cold tolerance of yellow-cedar and western hemlock trees at a site affected by yellow-cedar decline. Can. J. For. Res. 35:2065-2070.
- Paul Schaberg, US Forest Service - Northern Research Station, Research Plant Physiologist
- Paul Hennon, US Forest Service, Pacific Northwest Research Station
- Dave D’Amore, US Forest Service, Pacific Northwest Research Station
- Gary Hawley, The University of Vermont – Senior Researcher
Last Modified: 02/06/2013