Passive experimental warming decouples air and sediment temperatures in a salt marsh

Abstract

Open top chambers (OTCs) are a commonly used passive warming technique in experimental warming studies. OTCs have been shown to be effective in multiple types of terrestrial systems, but their utility in wetland environments remains uncertain. The objective of this work was to evaluate the effectiveness of using OTCs to warm a temperate salt marsh across diurnal and seasonal cycles. We found that OTCs are effective at warming air temperatures on the marsh, with average air temperatures 1.6 ± 0.007°C and 1.1 ± 0.006°C warmer within the high and low marsh, respectively over a 16‐month period. In contrast, OTCs were ineffective at warming sediments, especially during the day. In fact, sediment temperatures within the OTC were cooler during the day relative to ambient conditions. Such daytime warming of air, but cooling of sediments relative to ambient conditions resulted in a significant decoupling of above and belowground temperatures in the marsh (r = −0.99 and −0.82 on low and high marsh, respectively). Our data indicate that shading by OTCs was responsible for the daytime sediment cooling relative to ambient conditions during most of the year, as incoming solar radiation was reduced by 30% within OTCs. Wet sediments require more energy to heat than the air due to their higher specific heat capacity. Thus, reductions in radiation by OTCs prevented effective warming of sediments, but still allowed for the warming of air. In turn, we conclude that OTCs are not an effective method to experimentally warm tidal marsh sediments.

Asynchronous nitrogen supply and demand produce nonlinear plant allocation responses to warming and elevated CO2

“…Although in some ecosystems passive chambers are able to raise soil temperature by 1 to 1.5°C during the summer (30), they do not yield consistent temperature differentials throughout the day, can decrease in effectiveness over time, and are effective only during the growing season (29,30). A recent wetland experiment reported that passive chambers also decoupled aboveground and belowground responses by warming the air while cooling the soil (48). These experimental designs are unlikely to capture the asynchronous, nonlinear patterns that we observed in SMARTX, which arose from the balance of different warming-response curves for plant versus soil microorganisms.…”

Higher Temperature Sensitivity of Ecosystem Respiration in Low Marsh Compared to High Elevation Marsh Ecosystems

“…To do this, we place open top chambers at half the plots in early June 2014 (three in high marsh and three in low marsh), a month before our study commenced. However, as we documented previously (Carey et al, 2018), our experimental warming techniques were unsuccessful at warming the marsh. As such, the data we present here includes data from all plots.…”

Design and Assessment of a Novel Approach for Ecosystem Warming Experiments in High‐Energy Tidal Wetlands

“…The feedback‐controlled warming applied at MERIT effectively heats soils in a high‐energy tidal wetland. This approach is a clear improvement over OTC‐only experiments where minimal or diurnally unstable soil warming occurs (Carey et al., 2018). Feedback‐controlled belowground warming systems have previously been used in a handful of experiments, mainly in terrestrial ecosystems (Peterjohn et al., 1993; Pries et al., 2017; Reich et al., 2020; Rich et al., 2015).…”

“…Only two examples exist in wetland ecosystems (Hanson et al., 2017; Noyce et al., 2019), and these are both situated in organic soils and non‐tidal or micro‐tidal systems. Passive open‐top chamber (OTC) experiments in the field have limited success in warming salt marsh ecosystems (Carey et al., 2018). Issues with OTC experiments include strong diurnal patterning with effective daytime warming and no warming or even temperature reduction at night (Carey et al., 2018; Gedan & Bertness, 2010), low effective warming during the summer as compared to other parts of the year, and only modest or no belowground warming (Zhong et al., 2013).…”

“…Passive open‐top chamber (OTC) experiments in the field have limited success in warming salt marsh ecosystems (Carey et al., 2018). Issues with OTC experiments include strong diurnal patterning with effective daytime warming and no warming or even temperature reduction at night (Carey et al., 2018; Gedan & Bertness, 2010), low effective warming during the summer as compared to other parts of the year, and only modest or no belowground warming (Zhong et al., 2013). Passive aboveground warming experiments are often chosen for remote locations but are disadvantageous because key ecosystem processes like decomposition occur belowground.…”