Temperature and water-level effects on greenhouse gas fluxes from black ash (Fraxinus nigra) wetland soils in the Upper Great Lakes region, USA
Introduction
In the northern Great Lakes region (Upper Michigan, Wisconsin, and northern Minnesota), many forests have significant components of ash (Fraxinus spp.) in the overstory, which are especially vulnerable to changes in composition and succession sequences following emerald ash borer (EAB) infestation (MacFarlane and Meyer, 2005). Since its discovery in the United States in 2002, EAB has spread quickly across the country, killing ash forests within 6 years of infestation (Herms and McCullough, 2014; Knight et al., 2013; Smitley et al., 2008). The invasion of EAB and large-scale ash dieback can decrease litter input, increase amounts of large woody debris, and cause canopy gap formation; cascading effects that may lead to large-scale changes in ecosystem structure, food web interactions, and altered biogeochemical cycling (Kolka et al., 2018). Forested black ash (F. nigra) wetlands are especially prone to ecosystem-scale changes because ash makes up 40–100% of the canopy (Looney et al., 2015; Van Grinsven et al., 2017), they are already experiencing ash dieback (Palik et al., 2012, Palik et al., 2011), and transpiration by ash trees is a major control on hydrology (Telander et al., 2015). Ash mortality in these ecosystems causes the water table to rise, potentially transitioning a forested ecosystem to an open marsh (Slesak et al., 2014; Diamond et al., 2018). This groundwater sensitivity to transpiration makes forested ash wetlands especially vulnerable to altered ecosystem function following the invasion of EAB.
Changes in the canopy, understory vegetation communities, water-level, and soil temperature may lead to altered biogeochemical cycling and potential nutrient loss from black ash wetlands to the atmosphere and down-stream ecosystems following EAB invasion. The gaseous fluxes of carbon (as CO2 and CH4) and nitrogen (N2O) as greenhouse gasses are measurable indicators of how changes in ecosystem dynamics will alter nutrient cycling and ecosystem function. Greenhouse gas fluxes are controlled by the abundance and productivity of microbial populations in the soil, which are regulated by several factors including temperature, organic matter, and the availability of electron donors and acceptors needed to complete the oxidation-reduction reactions of metabolic processes under aerobic and anaerobic conditions (Altshuler et al., 2019; Ebrahimi and Or, 2016; Hou et al., 2000). The availability of electron donors can also limit nutrient cycling reactions in the soil and therefore gas production (Wang et al., 1993). For example, denitrification requires a soluble carbon electron donor and nitrification uses ammonium as an electron donor. A noticeable shift in Eh and gas fluxes will only occur if soil microbes have an abundant food source (soil organic matter), abundant electron donors and acceptors to complete metabolic reactions, and an environment warm enough to be biologically active. Given the above, it is necessary to understand how soil temperature and degree of soil saturation affect gas fluxes in black ash wetlands with different hydroperiods and soil composition (i.e., mineral vs. organic soils).
Van Grinsven et al. (2018) measured soil CO2 and CH4 fluxes in black ash wetlands with simulated EAB disturbance in Upper Michigan. The disturbance resulted in significantly higher CO2 fluxes, largely because the disturbed sites had elevated soil temperatures due to increased solar radiation with canopy loss. The average CH4 fluxes in disturbed sites were not significantly different from the control sites, but larger CH4 fluxes did occur more frequently in disturbed sites. Overall, changes in soil temperature and moisture regulated the soil carbon fluxes, producing significant treatment effects. In northern Minnesota, simulated EAB disturbance in black ash wetlands resulted in warmer air temperatures, but soil gas fluxes were not evaluated (Slesak et al., 2014). The differences in hydrology and soil composition between the black ash wetlands studied in Upper Michigan, and those in other regions of the Great Lakes states will likely result in different gaseous carbon flux responses following EAB induced ash dieback.
Field experiments simulating EAB-induced ash mortality in black ash wetlands in northern Minnesota and Upper Michigan have measured changes in water table, understory plant communities, soil temperature, and nutrient cycling (Davis et al., 2017; Looney et al., 2015; Slesak et al., 2014; Telander et al., 2015; Van Grinsven et al., 2017, Van Grinsven et al., 2018). Other studies and literature reviews suggest EAB will cause changes in ash foliar chemistry, invertebrate communities, and trophic interactions that will cascade through entire ecosystems (Chen et al., 2011; Nisbet et al., 2015; Perry and Herms, 2016; Youngquist et al., 2017). However, many hypothesized impacts of emerald ash borer on forested wetlands are still uncertain, and there is an urgent need to understand the cascading impacts of EAB on ecosystem processes such as greenhouse gaseous efflux. We used a soil core incubation experiment that mimicked potential future hydrologic and temperature regimes to investigate the interactions among water-level, soil temperature, soil oxidation-reduction potential (redox), and greenhouse gas (CO2, CH4, and N2O) fluxes in black ash wetland soils in the Great Lakes region to determine if: (1) warmer soil temperatures will result in greater greenhouse gas fluxes, (2) higher water levels will increase greenhouse gas fluxes, and if so, (3) there is an interaction with soil temperature that amplifies the water table response, and (4) the response differs between mineral and peat soils. This study builds on previous research studying a critical system of feedbacks facing the imminent threat of emerald ash borer invasion and a cascade of ecosystem-scale changes (Davis et al., 2017; Looney et al., 2015; Slesak et al., 2014; Van Grinsven et al., 2018, Van Grinsven et al., 2017).
Section snippets
Study area
Intact soil cores were collected from black ash wetlands with mineral and organic, peat soils in northern Minnesota and Upper Michigan, respectively. The northern Minnesota site with mineral soils was an undisturbed area adjacent to treatment plots presented in the Slesak et al. (2014) and Looney et al. (2015) studies. The study site is located in the Chippewa National Forest in northern Minnesota's Itasca County (N47.5°, W94°) and has a continental climate with a mean growing season (May–Oct.)
Gas flux response
In soils from both sites, gas fluxes were generally greatest in the 20 °C treatment, and fluxes were commonly near zero in the 10 °C treatment (Fig. 2). Temporal trends were most apparent in the saturated treatment at 20 °C, where fluxes increased over the course of the incubation experiment. As the experiment progressed, the gas fluxes became more variable as indicated by increasing standard error in each measurement period, especially in the 20 °C treatments and for CH4 fluxes. Significant
Discussion
Forested black ash wetlands in the Great Lakes region are especially threatened by the impending invasion of EAB because the ash-dominated canopies control the local groundwater levels via transpiration (Telander et al., 2015) and loss of ash from the canopy will result in higher water table levels and warmer air temperatures (Slesak et al., 2014; Van Grinsven et al., 2017). This laboratory experiment shows that black ash wetlands in northern Minnesota and Upper Michigan with mineral and peat
Acknowledgements
This research was funded in part by the Minnesota Forest Resources Council, USDA Forest Service Northern Research Station, and the University of Minnesota Department of Forest Resources. We gratefully acknowledge Anne Gapinski, Joseph Shannon, Mitchell Slater, and David and Patricia Toczydlowski for assistance with fieldwork, and Nathan Aspelin, Douglas Brinkman, Cindy Buschena, Michael Dolan, and the USDA USFS Northern Research Station for conducting and assisting with laboratory analyses.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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