Long-term changes in a temperate forest impacted by climate change

Background/Question/Methods

Pervasive forest mortality events due to climate change have been observed on all six plant covered continents in recent years, and such forest diebacks are expected to increase in the coming decades as a result of increasing temperatures. Yellow-cedar (Callitropsis nootkatensis) has been dying off since the late 1800s in southeast Alaska, with intensifying rates observed in the latter part of the 20^th century. Current research identifies climate change as a key driver; reduced snowpack makes these trees vulnerable to sudden freeze-thaw events. Death of yellow-cedar can have consequences on the suite of ecosystem services these forests provide but equally important are the changes in these forest structure and understory community composition that emerge in response to decline. We conducted vegetation surveys at 50 sites along the outer coast of southeast Alaska across a chronosequence of yellow-cedar mortality to study the process of forest development in forests affected by decline. We examined (1) changes in sapling abundance and community structure of conifer species in the overstory; (2) 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. 

Results/Conclusions

Yellow-cedar sapling occurrence and abundance decreased in forests experiencing extensive mortality.  We observed yellow-cedar dominated forests shifting to western hemlock (Tsuga heterophylla) dominated forests, with co-occurring species mountain hemlock (Tsuga mertensiana), Sitka spruce (Picea sitchensis), and shore pine (Pinus contorta). Vascular plant species richness and functional group diversity in the understory increased. The community composition of understory plants changed dynamically over time such that bryophytes became relatively less abundant; grasses responded early to mortality by increasing in relative abundance whereas shrubs, including forage species Alaska blueberry (Vaccinium alaskaense), increased in relative abundance and volume in latter stages of stand development. Our results suggest that once forests become affected by decline, yellow-cedar becomes significantly less likely to regenerate. We found that a species dieback can dynamically rearrange the overstory and understory plant communities over time, emphasizing the importance of understanding community interactions when assessing the impacts of climate change on biodiversity and other ecosystem services and adapting forest management to a changing climate.