Transitional climate mortality: slower warming may result in increased climate‐induced mortality in some systems

Abstract

The potential for climate change to cause mass tree mortality in forested systems by pushing environmental conditions past physiological tolerance thresholds is well documented. Less well studied is damage and mortality associated with climatic transitions, where mortality is less on either side of the transition; the shift from freezing winter conditions to thawed winter conditions in temperate and high latitudes is a clear example. “Transitional climate mortality” is sporadic, but widespread, associated with exposure to mortality during these climatic transitions, and triggered by proximal weather events like a hard freeze after a period of above freezing temperatures. Interestingly, this suggests that slower warming could result in more intensive mortality because of extended exposure to potential mortality events. The concept is tested using a well‐studied species (Callitropsis nootkatensis) on the US/Canadian Pacific coast. To identify the transitional mortality zone, statistical modeling combined with a current mortality map and bioclimatic variables was used. This process identified the −5° to 0°C zone (mean temperature of the coldest month) as particularly associated with transitional mortality. Weather station data from multiple locations were used to validate observations. Four GCMs (General Circulation Model) and two future warming scenarios (representative concentration pathway 2.6 and 8.5) were used to estimate time and spatial extent of exposure at broad scales; slower warming results in more intensive cumulative exposure than faster warming. Finally, by combining the observed mortality zone with weather station data a generalizable binomial model was tested as a means to estimate potential future mortality. This model is applicable to any system where the transitional mortality zone (e.g., temperature range) and the frequency of proximal mortality triggers are known. This type of mortality phenomena has been understudied but may be a large driver of future forest change, given the frequency of mortality events and the ubiquity of the freeze–thaw transition across temperate systems globally.

From canopy to seed: Loss of snow drives directional changes in forest composition

“…Inset plots for illustration of the numerical distribution of plots only (scale varies). For a complete range and decline map, see Buma et al () or Buma ()…”

“…Inset plots for illustration of the numerical distribution of plots only (scale varies). For a complete range and decline map, see Buma et al (2017) or Buma (2018) decades (30-year normal, 1981-2010), the five-year period prior to and including sampling year (five-year average, range 2007-2016), and the year of sampling (annual, range: 2011-2016). Annual data were used to evaluate the influence of current year conditions on the regenerating community, while data from the five-year period were used to quantify the influence of multiyear climate on canopy mortality, potential seed availability and viability, and regenerating community composition.…”

“…Little (Alaska yellow‐cedar) is experiencing extensive, climate change‐driven mortality over more than 400,000 ha and ten degrees of latitude (Buma et al, ). Regional wintertime temperatures average 0°C; thus, a slight warming results in significant snow loss (Buma, ). Subfreezing weather events following snowmelt in late winter and early spring kills fine roots of mature C. nootkatensis (Hennon et al, ; Schaberg, Hennon, D'amore, & Hawley, ), and this phenomenon has been ongoing for several decades (Beier, Sink, Hennon, D'Amore, & Juday, ; Hennon, D'Amore, Schaberg, Wittwer, & Shanley, ).…”

“…Drought-induced canopy mortality is well-established (Allen et al, 2015;Anderegg et al, 2013), and recent studies have quantified the impacts of prolonged drought (Redmond, Weisberg, Cobb, & Clifford, 2018) and postfire drought conditions (Stevens-Rumann et al, 2017;Young et al, 2019) on regeneration. However, relatively little is known about the consequences of reduced snowpack and early season frost damage on canopy mortality and regeneration response, although these phenomena may become more common as the climate warms (Woldendorp, Hill, Doran, & Ball, 2008)-leading to root mortality and nutrient loss (Decker, Wang, Waite, & Scherbatskoy, 2003), needle and bud injury (Man, Kayahara, Dang, & Rice, 2009), canopy mortality (Buma, 2018), and seedling mortality (Camarero & Gutiérrez, 2004). Regardless of the climatic stressor, widespread canopy mortality is likely to be ongoing under the more extreme conditions predicted for the future, and under such conditions, range contraction potential will be governed by regeneration success or failure.…”

Emergent freeze and fire disturbance dynamics in temperate rainforests

“…This climatological location makes CTRF snow regimes especially vulnerable to a loss of days below freezing given even minimal warming (Meehl et al . ), as illustrated by the crossing of the snow‐to‐rain threshold already reported in portions of the biome (Buma ).…”