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

Abstract

Climate change is altering the conditions for tree recruitment, growth, and survival, and impacting forest community composition. Across southeast Alaska, USA, and British Columbia, Canada, Callitropsis nootkatensis (Alaska yellow‐cedar) is experiencing extensive climate change‐induced canopy mortality due to fine‐root death during soil freezing events following warmer winters and the loss of insulating snowpack. Here, we examine the effects of ongoing, climate‐driven canopy mortality on forest community composition and identify potential shifts in stand trajectories due to the loss of a single canopy species. We sampled canopy and regenerating forest communities across the extent of C. nootkatensis decline in southeast Alaska to quantify the effects of climate, community, and stand‐level drivers on C. nootkatensis canopy mortality and regeneration as well as postdecline regenerating community composition. Across the plot network, C. nootkatensis exhibited significantly higher mortality than co‐occurring conifers across all size classes and locations. Regenerating community composition was highly variable but closely related to the severity of C. nootkatensis mortality. Callitropsis nootkatensis canopy mortality was correlated with winter temperatures and precipitation as well as local soil drainage, with regenerating community composition and C. nootkatensis regeneration abundances best explained by available seed source. In areas of high C. nootkatensis mortality, C. nootkatensis regeneration was low and replaced by Tsuga . Our study suggests that climate‐induced forest mortality is driving alternate successional pathways in forests where C. nootkatensis was once a major component. These pathways are likely to lead to long‐term shifts in forest community composition and stand dynamics. Our analysis fills a critical knowledge gap on forest ecosystem response and rearrangement following the climate‐driven decline of a single species, providing new insight into stand dynamics in a changing climate. As tree species across the globe are increasingly stressed by climate change‐induced alteration of suitable habitat, identifying the autecological factors contributing to successful regeneration, or lack thereof, will provide key insight into forest resilience and persistence on the landscape.

Contrasting root and shoot cold tolerance of a conifer with indeterminate growth

“…A greater proportion of populations in decline occur on gentle slopes while healthy populations are more frequently found on steeper slopes (24°-40°) (Wooton & Klinkenberg, 2011). Soil saturation could be a factor on the flatter areas, creating stands with open canopies and shallow roots (Hennon et al, 2016) and shifts in community composition and stand dynamics can be expected (Bisbing et al 2019).…”

“…A number of studies have reported that yellow-cedar mortality is higher in open-canopy forests at low elevations with typically wet soils, or on shallow soils over bedrock (Hennon et al 1990, Schaberg et al 2005 (Comeau et al, 2019). As climate change results in this loss of mature, seed-D r a f t producing trees of a particular species, restructuring of forest communities over large regions may be observed (Bisbing et al 2019).…”

Climate-Mediated Changes to Linked Terrestrial and Marine Ecosystems across the Northeast Pacific Coastal Temperate Rainforest Margin

“…More frequent freeze–thaw events associated with warming winters can alter couplings between plant nutrient uptake, microbial communities, and nutrient cycling that ultimately lead to increased soil N and the potential for P leaching (Wipf et al 2015 ). Freeze–thaw events associated with transitions from snow to rain are additionally driving sudden shifts in forest community composition (e.g., yellow cedar Callitropsis nootkatensis replacement by western hemlock Tsuga heterophylla ; Oakes et al 2014 , Bisbing et al 2019 ). These shifts can in turn influence soil N cycling rates (Bisbing and D'Amore 2018 ) although total nutrient stocks may change more slowly.…”

Is the modern-day dieback of yellow-cedar unprecedented?

“…1880) and expansion (ca. 1925) of YC dieback throughout Southeast Alaska and British Columbia (Hennon et al 1990a;D'Amore and Hennon 2006;Stan et al 2011;Wooton and Klinkenberg 2011;Oakes et al 2014;Hennon et al 2016;Bisbing et al 2019;Comeau et al 2019).…”