Climate‐induced yellow‐cedar decline on the island archipelago of Haida Gwaii

Abstract

The global rise in temperature and associated changes in climate have led to decline of forests around the globe, across multiple species and ecosystems. Yellow-cedar (Callitropsis nootkatensis) decline is one of the most severe in North America. We found abundant evidence of tree decline and mortality on Haida Gwaii across multiple watersheds and over a range of elevations. This decline on Haida Gwaii parallels the broader yellow-cedar decline in terms of spatial distribution, symptoms, magnitude, and timing. However, Haida Gwaii has a more temperate climate and ephemeral snowpack than declining yellowcedar forests in Alaska where the link to climate was first uncovered. Given these important differences, we investigated several possible drivers both at the local and at the regional scale, using population demography, dendrochronology, and daily weather data. We explored stand dynamics as a driver and tested the known link to climate. Our 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. Alternatively, the magnitude and timing of the decline are consistent with well-documented long-term directional trends in regional climate. Onset of basal area increment decline and mortality have been accumulating over time, with increased rates since the 1980s. Our sites were located at the edge of the expected range of mortality, and we found only four thaw-freeze events over the past~80 yrs. However, superposed epoch analysis using daily weather data revealed that mortality and onset of decline events were associated with warmer winter conditions, consistent with the drivers from Alaska. Rather than isolated extreme thawfreeze events, warmer winter temperatures on Haida Gwaii may mean less cold hardening throughout the winter, which 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.

Contrasting Impacts of Climate Warming on Coastal Old-Growth Tree Species Reveal an Early Warning of Forest Decline

“…The sensitivity of yellow-cedar to warm dry winters in recent decades is consistent with other studies showing increased vulnerability to late-winter thaw-freeze events in declining forests (Beier et al, 2008;Comeau et al, 2019Comeau et al, , 2021. However, this study is the first to detect this response in an apparently healthy oldgrowth forest, in which no trees showed visual signs of decline, such as discolored foliage and thinning crowns.…”

“…However, this study is the first to detect this response in an apparently healthy oldgrowth forest, in which no trees showed visual signs of decline, such as discolored foliage and thinning crowns. When comparing variation in individual ring-width indices from 1900 to 2015, we found some yellow-cedar suffered intermittent growth reductions after 1950 that may be consistent with thaw-freeze events and provide an additional early warning sign of decline (Cailleret et al, 2017;Comeau et al, 2021). We recommend careful monitoring of forest health to determine if canopy dieback or tree mortality of yellow-cedar results in future as winter climate continues to warm.…”

“…However, the relatively high elevation, closed canopy, and well-drained soils of the forest we studied likely buffered yellow-cedar trees from severe damage by thaw-freeze events in the twentieth century. Snowpack is more persistent and melts later in the spring at high elevations, so the yellow-cedar trees studied growing at midelevation (c. 400 masl) on Haida Gwaii were better protected from frost damage than forests closer to sea level (Buma et al, 2017;Comeau et al, 2021), with poorly drained soils (Comeau et al, 2019), where much of the decline on Haida Gwaii has been found.…”

Multiple divergent patterns in yellow-cedar growth driven by anthropogenic climate change

“…In previous work within stands with visual evidence of yellow-cedar decline on Haida Gwaii, we have shown that yellow-cedars that have died or are visually declining grow alongside those with healthy crowns (Comeau et al 2019(Comeau et al , 2021. However, there are discrepancies between internal tree-ring and external crown symptoms in many trees (Comeau et al 2021). Yellow-cedars exhibit both positive and negative responses to climatic variation and change (Comeau et al 2019(Comeau et al , 2021.…”

“…On the island archipelago of Haida Gwaii, located 70 km west of the mainland, yellowcedar decline was unexpected due to the moderating effect of the Pacific Ocean yielding an ephemeral snowpack, but mild winter temperatures and few frosts (Banner et al 2014). However, many stands of yellow-cedar on Haida Gwaii now exhibit moderate-to highseverity decline, in which an average of 70% of trees are dying or dead (Comeau et al 2021). Given the relatively mild climate on Haida Gwaii, we hypothesize the mechanism driving yellow-cedar decline may differ from that characterized in forests on the mainland.…”