Effects of Yellow Cypress Decline on Coastal Temperate Rainforest Climate Interactions
Gavin McNicol, University of Illinois Chicago (Principal Investigator)
Max Berkelhammer, University of Illinois Chicago (Co-Investigator)
Benjamin Gaglioti, University of Alaska Fairbanks (Co-Investigator)
Hélène Genet, University of Alaska Fairbanks (Co-Investigator)
The N. Pacific coastal temperate rainforest in Southeast Alaska (SEAK) stores vast quantities of organic carbon in its plants and soils, due to high forest integrity, a cool and very wet climate, and limited disturbance by fire and logging. However, the relative abundance of the ecologically and culturally significant tree species, Callitropsis nootkatensis (yellow cypress, or yellow-cedar), is in decline due to tree mortality across at least 2,800 km2 of the SEAK rainforest, with more than 70% mortality of mature trees in affected areas. The cause of tree mortality has been linked to loss of late-winter snow cover, which exposes yellow cypress’s shallow roots to frost injury. Yellow cypress decline is predicted to become more prevalent across SEAK as more of the region crosses a winter snow-to-rain precipitation threshold in the coming decades. Despite the large range of these observed and predicted SEAK yellow cypress declines, we still don’t know whether decline will affect the amount of organic carbon and moisture stored in plants and soils, and if it does, whether these changes will accelerate or dampen future climate changes in the coastal temperate rainforest.
In our study, with permission of land owners and managers, we will revisit after 14 years a gradient of yellow cypress decline on the western coast of Chichagof Island, SEAK. This land is federally protected within the W. Chichagof-Yakobi Wilderness area of the Tongass National Forest and is within the unceded territories of the Sheet’ká ?wáan (Sitka Tribe of Alaska) on Lingít Aaní (Tlingit lands). Study plots will be spread evenly between two decline classes: live, and recent mortality, dominated by trees that died within the last ~20 years. Each plot has detailed tree inventory data collected in 2011-2012 and forest structure and composition will be re-surveyed using the same methods. At a subset of plots within each of the two decline classes, we will quantify the amount of carbon stored in plants and the soil, including roots, and quantify rates of decomposition over three years using bags of common leaf material placed on the surface of, or buried within, the soil. At new plots near Fick Cove, a sheltered inlet on the east coast of Chichagof Island, we will set up a research grade weather station and a soil and tree stem sensor array to measure radiation, temperature, and moisture conditions in the air, soil, and trees, respectively. We will analyze data from the yellow cypress decline plot network using statistical methods to describe and explain patterns in carbon and moisture variation, and to attribute how much of this variation is caused by yellow cypress decline. We hypothesize that increased temperatures and decreased evaporation rates in the forest canopy as well as increased decomposition within the forest floor will cause a reduction in carbon stored in plants and soils over the first few decades of yellow cypress decline, leading to net losses of forest carbon.
The observational and experimental field data generated by this study will help the broader scientific community, land owners and managers, and the general public to better anticipate the future climate and ecosystem conditions of the N. Pacific coastal temperate rainforest. Data on carbon stocks, decomposition rates, and local temperature and moisture changes associated with yellow cypress decline will be used to iteratively test and refine new tree mortality processes in the Dynamic Organic Soil version of the Terrestrial Ecosystem Model. Community connections will be maintained throughout the project via outreach activities facilitated by the Sitka Sound Science Center, the Alaska Coastal Rainforest Center and the US Forest Service Pacific Northwest Research Station in Juneau, and the University of Alaska. Through our U.S. Department of Energy (DOE) collaborators at the Pacific Northwest and Lawrence Berkeley National Laboratories, we will roadmap how to integrate our temperate rainforest model improvements into the land surface module of DOE’s Energy Exascale Earth System Model (E3SM).
