CLIMATE AND BIODIVERSITY: THE DECLINE OF YELLOW CEDAR IN BRITISH COLUMBIA

by

Claire Wooton and Brian Klinkenberg
Lab for Advanced Spatial Analysis (LASA)

Department of Geography, University of British Columbia

For over two decades, the phenomenon of yellow-cedar decline has perplexed researchers. Yellow-cedar (Chamaecyparis nootkatensis) (D. Don) Spach), which ranges from southern Oregon to Prince William Sound, Alaska, was known to be declining on over 200,000 ha of undisturbed forest in southeast Alaska (Snyder et al. 2008). During an aerial survey in 2004, numerous large areas of dead and dying yellow-cedar were found in coastal locations in B.C., and the nature of the dieback was found to be consistent with the phenomenon in southeast Alaska (Hennon et al. 2005).

Research into the decline of this long-lived species began in the early 1980s and a sequence of symptoms was identified. The initial symptom was determined to be fine root death, followed by death of small-diameter roots (Hennon et al. 2006) (PDF). As the roots start to die, the trees develop thin off-colour crowns, and necrotic lesions spread from larger roots up the bole (Hennon et al. 2006). The natural resistance of yellow cedar heartwood to decay allows dead trees to remain standing for 80 to 100 years after death. By examining the standing snags it was possible to establish that the decline of yellow-cedars began in about 1880-1900 (Hennon & Shaw, 1997).

Investigations initially focused on finding a biotic cause of the decline, but one by one the suspected agents were ruled out (Hennon et al. 1990). Attention then shifted to abiotic factors potentially associated with the decline, and an association with wet, poorly drained soils was found. However, the relation with soil drainage was inconsistent, with limited decline occurring on wet sites at higher elevations (Hennon et al. 2006). Air and soil temperature were determined to be stronger risk factors than poorly drained soils (D'Amore & Hennon, 2006).

These clues led researchers to propose a new, complex hypothesis to explain yellow-cedar decline. According to Hennon et al. (2006), saturated soils create open, exposed canopies that experience soil warming early in the spring. This warming triggers the yellow-cedars to lose their cold tolerance, making them more susceptible to freezing injury. Snow appears to protect yellow-cedar against this freezing injury by preventing soil warming. However, the end of the Little Ice Age, which coincided with the onset of decline, has led to a reduction in snowpack at lower elevations (Hennon et al. 2006). This shift in climate may represent the environmental trigger responsible for the decline and suggests that the dieback may expand if warming trends continue (Hennon et al. 2006). 

In our research, we addressed the cause of yellow cedar die-off in British Columbia through a combination of remote sensing and GIS techniques, analysing the correlation between the distribution of decline and the environmental predictors.  Using a stratified sampling approach (selecting sample points only in yellow-cedar stands as identifed in a forest cover dataset), the presence/absence of decline at each point was determined using aerial photograph interpretation. The strength of relations between the distribution of decline and the various environmental predictors was then determined using appropriate regression techniques.  The results showed that low elevation sites close to the coast, which are more exposed and have more variation in elevation, are more likely to show evidence of decline.  This supports the proposed associations in the current decline hypothesis.  

Knowledge of what determines the spatial pattern of decline in this species will aid in prediction of areas of decline, and is important knowledge for the conservation and management of yellow-cedar.   The desire to conserve species diversity means that a management strategy incorporating the influence of a warming climate is required.  Ultimately, this research may provide insight into the devastating effects that climate change can have on a forest ecosystem. 

References

D'Amore, D. and Hennon, P.E. 2006. Evaluation of soil saturation, soil chemistry, and early spring soil and air temperatures as risk factors in yellow-cedar decline. Global Change Biology. 12: 524-545

Hennon, P.E., D'Amore, D., Wittwer, D., Johnson, A., Schaberg, P., Hawley, G., Beier, C., Sink, S. and 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(2): 427-450.

Hennon, P.E., D'Amore, D., Zeglen, S. and Grainger, M. 2005. Yellow-cedar decline in the North Coast Forest District of British Columbia. Res. Note RN-549. Portland, OR: U.S. Dep. Agric., Pacific Northwest Research Station. pp.20.

Hennon, P.E. and Shaw, C.G. III. 1997. The enigma of yellow-cedar decline - What is killing these long-lived, defensive trees? Journal of Forestry. 95(12): 4-10.

Snyder, C., Schultz, M.E. and Lundquist, J. (Compilers) 2008. Forest health conditions in Alaska - 2007: a forest health protection report. Gen. Tech. Rep. R10-PR-18. Anchorage, AL: U.S. Dep. Agric., Forest Service, Alaska Region.

Wooton, Claire Evelyn.  2010.  A landscape level analysis of yellow-cedar decline in coastal British Columbia.  Master of Science Thesis, Department of Geography, University of British Columbia.