Abstract
Pervasive forest mortality is expected to increase in future decades as a result of increasing temperatures. Climate‐induced forest dieback can have consequences on ecosystem services, potentially mediated by changes in forest structure and understory community composition that emerge in response to tree death. Although many dieback events around the world have been documented in recent years, yellow‐cedar (Callitropsis nootkatensis) decline provides an opportunity to study vegetation changes occurring over the past century. Current research identifies climate‐related reductions in snow cover as a key driver of this species dieback. To examine the process of forest development post‐dieback, we conducted vegetation surveys at 50 plots along the outer coast of southeast Alaska across a chronosequence of mortality. Our main study objectives were to examine changes in seedling and sapling abundance, and community structure of conifer species in the overstory; effects of yellow‐cedar mortality on plant diversity and community composition of functional groups in the understory; and volume of key forage species for Sitka black‐tailed deer (Odocoileus hemionus sitkensis) managed throughout the region. The probability of yellow‐cedar sapling occurrence was reduced across the chronosequence. Yellow‐cedar seedling and sapling abundance also decreased. We observed a turnover from yellow‐cedar to western hemlock (Tsuga heterophylla) dominated forests. Functional plant diversity increased and the community composition of the understory changed across the chronosequence. Bryophytes became less abundant and grasses more abundant in the early stages of stand development, and shrubs increased in relative abundance in latter stages. Our results demonstrate that yellow‐cedar is significantly less likely to regenerate in forests affected by widespread mortality, and a species dieback can dynamically rearrange the plant community over time. These findings emphasize the importance of considering long‐term temporal dynamics when assessing the impacts of climate change on biodiversity and ecosystem services, and adapting forest management to a changing climate.
Highlights
Tree death is a natural part of forest dynamics (Franklin et al 1987), but increasing rates of mortality can result when climatic conditions exceed a species’ physiological threshold (Allen 2009)
A global overview of climateinduced forest mortality (Allen et al 2010) provides a detailed assessment of events driven by climatic water/heat stress since 1970; few of these documented dieback events provide opportunity to examine vegetation changes that occur over a longer time frame
We removed these two plots from the dataset, because clustering results and field observations affirmed that these plots experienced relatively higher levels of background tree death compared with other live plots but not continued mortality representative of forests affected by widespread decline
Summary
Tree death is a natural part of forest dynamics (Franklin et al 1987), but increasing rates of mortality can result when climatic conditions exceed a species’ physiological threshold (Allen 2009). Directional climate change has historically resulted in shifts in the distributions of species and ecosystems (Davis 1986), comparatively rapid shifts in tree distributions attributed OAKES ET AL. To anthropogenic climate change have been documented (Parmesan 2006, Lenoir et al 2008, Harsch et al 2009) on all six plant-covered continents (Allen and Breshears 2007, Allen et al 2010). A global overview of climateinduced forest mortality (Allen et al 2010) provides a detailed assessment of events driven by climatic water/heat stress since 1970; few of these documented dieback events provide opportunity to examine vegetation changes that occur over a longer time frame
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