Understanding the Mechanisms of Conifer Tree Mortality from Seawater Exposure Using E3SM Learning Model (FATES-Hydro)

Authors

Junyan Ding1* (junyan.ding@pnnl.gov), Nate McDowell1,2, Yilin Fang3, Nick Ward4, Matt Kirwan5, Peter Regier4, Pat Megonigal6, Peipei Zhang7, Hongxia Zhang8, Wenzhi Wang7, Weibin Li9, Stephanie C. Pennington10, Stephanie J. Wilson6, Alice Stearns6, Vanessa Bailey1

Institutions

1Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA; 2School of Biological Sciences, Washington State University, Pullman, WA; 3Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, WA; 4Marine and Coastal Research Laboratory, Sequim, WA; 5Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA; 6Smithsonian Environmental Research Center, Edgewater, MD; 7Chinese Academy of Sciences, Chengdu, China; 8Chinese Academy of Sciences, Lanzhou, China; 9Ministry of Agriculture and Rural Affairs, Lanzhou University, Lanzhou, China; 10Joint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, MD

URLs

Abstract

Widespread tree mortality in coastal regions associated with sea level rise has been observed globally. Rising sea level is threatens coastal forests through rising porewater hypoxia and salinity. Although the physiological effects of hypoxia and salinity has been well studied, few studies have tested these mechanisms under conditions of coastal tree mortality.

Likewise, there has been very limited model development for simulations of coastal woody-plant mortality. In this study, researchers incorporate the effect of porewater hypoxia and salinity on plant physiology in an ecosystem demography model that incorporates plant hydro-dynamic processes (FATES-Hydro) to investigate the loss of conifer trees from sea water exposure.

The major model developments included: (1) osmotic reduction in soil water potential due to salinity; (2) root death and reduction in belowground conductance due to hypoxia and salinity; (3) osmotic impact on xylem water potential and conductance; and (4) photosynthetic capacity loss from reduced biochemical reaction rates.
The team conducted numerical experiments at three coastal conifer forests. One is a spruce forest in the Pacific Northwest at Beaver Creek (BC) that has experienced a dramatic increase in tidal seawater exposure since 2015 when a culvert was removed. The other two are loblolly pine forests on the Chesapeake Bay (CP) on the western shores that have experienced gradually increasing sea water exposure from sea level rise. All sites experienced significant tree mortality by 2019. The team conducted a 10–year simulation at BC (2010 to 2019) and a 30–year simulation at CP (1990 to 2019).

The conifer species at the coastal areas have safe and sensitive stomata. With gentle and slow increase of salinity (less flood) at CP, this strategy leads to less hydraulic stress, negative carbon (C) balance, and mortality is mainly caused by C starvation. The sudden and dramatic rise of soil salinity and hypoxia at BC, in contrast, causes a significant drop of soil water potential and a quick loss of xylem conductivity resulting in significant hydraulic stress. Hydraulic failure is the main cause of mortality; over time the hydraulic stress reinforces the negative C balance and promotes C starvation driven mortality. This analysis benchmarks model performance while providing testable hypotheses for future experiments.