Freezing to death in a warming climate : drivers of yellow-cedar decline on Haida Gwaii

The global rise in temperature and associated changes in climate have led to decline of forests around the globe, across multiple species and ecosystems. This includes yellow-cedar (Callitropsis nootkatensis) decline, which is one of the most severe in North America. I found abundant evidence of tree decline and mortality of yellow-cedar on Haida Gwaii across multiple watersheds and over a range of elevations. The decline on Haida Gwaii parallels the broader yellow-cedar decline in terms of spatial distribution, symptoms, magnitude and timing. Nevertheless, the proposed drivers on the mainland may not adequately explain the decline on Haida Gwaii, due to the more temperate climate and lack of persistent snowpack. I investigated several possible drivers both at the local and regional scale. My results are inconsistent with stand dynamics as a driver of elevated decline and mortality. Neither increased competition, nor aging of a cohort explains the decline. Onset of basal area increment decline and mortality have been accumulating over time, with increased rates since the 1980s. Alternatively, the magnitude and timing of the decline is consistent with well-documented multi-decadal variations and long-term directional trends in regional climate. I found patterns of divergent growth and divergent response to climate among yellow-cedars at the same sites. Yellow-cedars affected by decline were decreasing in growth and negatively associated with warmer drier winter conditions. Whereas, yellow-cedars not affected by decline were increasing in growth and positively associated with warmer growing season temperatures. The limiting factors for declining trees, warm dry winter conditions, are consistent with the hypothesis from the mainland that climate warming has led to root freezing. However, compared to drivers of yellow-cedar decline on the mainland, snowpack plays a less important role on Haida Gwaii. Warming temperatures likely have a more direct effect. I propose warmer winter temperatures have led to a combination of decreased cold-hardening and earlier dehardening, in conjunction with increased frequency of thaw-freeze cycles and freeze events during otherwise milder winters. This exposes yellow-cedar’s fine roots to varying degrees of freezing damage over multiple winter thaw-freeze cycles, causing physiological stress, tree decline and eventual death.Forestry, Faculty ofGraduat