Divergent patterns in yellow-cedar growth driven by anthropogenic climate change
The global rise in temperature and associated changes in climate have led to decline of forests around the globe. A particularly severe example of this is yellow-cedar (Callitropsis nootkatensis) decline along the coast of British Columbia and Alaska, where anthropogenic climate change has led to reduced insulating snowpack, leaving yellow-cedar roots vulnerable to thaw-freeze events, resulting in freezing damage to roots and water stress during the subsequent growing season. Yellow-cedar decline is complex, with the potential for freezing injury over multiple years and damage that can accumulate over time. We used tree rings to uncover the relationship between yellow-cedar decline and climate. Declining stands included healthy trees and trees at various stages of decline, from long dead to currently declining. A traditional single site-level chronology would have masked these complex signals. Instead, we conducted a principal component analysis and identified four distinct growth patterns, with divergent responses to climate, among yellow-cedars within the same stands, across all sites. We found three distinct periods of an onset of decline (1960s, 1990s, 2000s). 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. Our research highlights the need to consider multiple signals within a site that would be obscured by a single site-level chronology.