Emerging climate‐driven disturbance processes: widespread mortality associated with snow‐to‐rain transitions across 10° of latitude and half the range of a climate‐threatened conifer

Abstract

Climate change is causing rapid changes to forest disturbance regimes worldwide. While the consequences of climate change for existing disturbance processes, like fires, are relatively well studied, emerging drivers of disturbance such as snow loss and subsequent mortality are much less documented. As the climate warms, a transition from winter snow to rain in high latitudes will cause significant changes in environmental conditions such as soil temperatures, historically buffered by snow cover. The Pacific coast of North America is an excellent test case, as mean winter temperatures are currently at the snow-rain threshold and have been warming for approximately 100 years post-Little Ice Age. Increased mortality in a widespread tree species in the region has been linked to warmer winters and snow loss. Here, we present the first high-resolution range map of this climate-sensitive species, Callitropsis nootkatensis (yellow-cedar), and document the magnitude and location of observed mortality across Canada and the United States. Snow cover loss related mortality spans approximately 10° latitude (half the native range of the species) and 7% of the overall species range and appears linked to this snow-rain transition across its range. Mortality is commonly >70% of basal area in affected areas, and more common where mean winter temperatures is at or above the snow-rain threshold (>0 °C mean winter temperature). Approximately 50% of areas with a currently suitable climate for the species (<-2 °C) are expected to warm beyond that threshold by the late 21st century. Regardless of climate change scenario, little of the range which is expected to remain suitable in the future (e.g., a climatic refugia) is in currently protected landscapes (<1-9%). These results are the first documentation of this type of emerging climate disturbance and highlight the difficulties of anticipating novel disturbance processes when planning for conservation and management.

Alternative interpretation and scale-based context for “No evidence of recent (1995–2013) decrease of yellow-cedar in Alaska” (Barrett and Pattison 2017)

“…The cumulative aerial survey layer covers all yellowcedar decline mapped in southeastern Alaska since the 1980s, including older decline (standing snags that have died since about 1900) and active mortality. We believe that comparing the FIA plots with a yellow-cedar decline layer from the same period (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) would have been a more appropriate methodological approach to characterizing differences between declining and nondeclining forests, likely yielding live tree to snag ratios and basal area numbers more consistent with existing yellow-cedar research (Buma et al 2017;Hennon et al 2016). By including older mapped mortality in their analysis, Barrett and Pattison may have been assigning plots located in areas of old decline (e.g., substantial mortality in the early 1900s) that are now in recovery (see Fig.…”

“…The greatest extent of its range, as well as substantial areas of decline, lies in British Columbia, Canada; therefore, any long-term assessments of the species' population stability and predicted risk should occur at the range-wide scale (Buma et al 2017). …”

“…Multiple lines of evidence (Hennon et al 2012;Buma et al 2017;including Barrett and Pattison 2017) demonstrate that yellow-cedar decline is a widespread, yet episodic and patchy, phenomenon. Several studies, including Barrett and Pattison (2017), estimate 175 000 to 275 000 ha of declining yellow-cedar forests and significant mortality in Alaska alone, generally with >70% basal area dead (Hennon et al 2016;Barrett and Pattison 2017;Buma et al 2017;USDA Forest Service 2016).…”

“…Several studies, including Barrett and Pattison (2017), estimate 175 000 to 275 000 ha of declining yellow-cedar forests and significant mortality in Alaska alone, generally with >70% basal area dead (Hennon et al 2016;Barrett and Pattison 2017;Buma et al 2017;USDA Forest Service 2016). Snow model predictions based on future climate scenarios indicate that decline is expected to emerge in new portions of the landscape as snow cover is reduced at higher elevations and latitudes (see University of Alaska Fairbanks SNAP (http://www.snap.uaf.edu/) and Wang et al (2012) snow modeling used in Hennon et al (2016)).…”

“…Our response first outlines Barrett and Pattison's major findings and then summarizes pertinent, complementary yellowcedar research that spans the last 35 years (Shaw et al 1985;Hennon et al 1990aHennon et al , 2012Hennon et al , 2016Hennon and Shaw 1997;D'Amore et al 2009;Oakes et al 2014Oakes et al , 2015Buma et al 2017). We discuss agreement between the estimated extent of concentrated yellowcedar mortality in Barrett and Pattison (2017) and the existing body of research, clarify where the FIA data were inappropriately matched (temporally and spatially) to forest health aerial survey data, and then provide an alternative explanation of the study's results in the context of ongoing yellow-cedar distribution and decline research in southeastern Alaska.…”