Attaining freshwater and estuarine-water soil saturation in an ecosystem-scale coastal flooding experiment

Abstract

Coastal upland forests are facing widespread mortality as sea-level rise accelerates and precipitation and storm regimes change. The loss of coastal forests has significant implications for the coastal carbon cycle; yet, predicting mortality likelihood is difficult due to our limited understanding of disturbance impacts on coastal forests. The manipulative, ecosystem-scale Terrestrial Ecosystem Manipulation to Probe the Effects of Storm Treatments (TEMPEST) experiment addresses the potential for freshwater and estuarine-water disturbance events to alter tree function, species composition, and ecosystem processes in a deciduous coastal forest in MD, USA. The experiment uses a large-unit (2000 m2), un-replicated experimental design, with three 50 m × 40 m plots serving as control, freshwater, and estuarine-water treatments. Transient saturation (5 h) of the entire soil rooting zone (0–30 cm) across a 2000 m2 coastal forest was attained by delivering 300 m3 of water through a spatially distributed irrigation network at a rate just above the soil infiltration rate. Our water delivery approach also elevated the water table (typically ~ 2 m belowground) and achieved extensive, low-level inundation (~ 8 cm standing water). A TEMPEST simulation approximated a 15-cm rainfall event and based on historic records, was of comparable intensity to a 10-year storm for the area. This characterization was supported by showing that Hurricane Ida’s (~ 5 cm rainfall) hydrologic impacts were shorter (40% lower duration) and less expansive (80% less coverage) than those generated through experimental manipulation. Future work will apply TEMPEST treatments to evaluate coastal forest resilience to changing hydrologic disturbance regimes and identify conditions that initiate ecosystem state transitions.

Root and Microbial Soil CO2 and CH4 Fluxes Respond Differently to Seasonal and Episodic Environmental Changes in a Temperate Forest

“…Pulse inundation events drove variation in soil heterotrophic CH 4 uptake, echoing our results for CO 2 release. Soil heterotrophic CH 4 uptake was negatively related to VWC and was responsible for the transient decrease in the forest soil CH 4 sink following saturation of the entire 0-30 cm soil profile (Hopple et al, 2023). Koide et al (2010) reported similar declines in CH 4 uptake that were attributed to soil saturation from extreme precipitation creating anoxic conditions and limiting CH 4 oxidation.…”

“…In our study, the resistance of root‐and‐rhizosphere CO 2 flux to changing soil VWC buffered soil heterotrophic impacts on total CO 2 flux, as demonstrated by the relatively rapid recovery of total CO 2 flux following episodic inundation events. Soil heterotrophs may have been more sensitive to episodic change because they are in surficial litter and soil where the greatest impacts of pulse inundation events occurred (Hopple et al., 2023), while plants have deep root systems that may require larger and/or prolonged moisture pulses that penetrate the soil profile to trigger physiological responses (Ogle & Reynolds, 2004).…”

“…Each TEMPEST simulation approximated a 15-cm rainfall event. Based on historic records, this was comparable to a 10-year storm for the region (full description provided in Hopple et al, 2023). During the 10-hr precipitation simulation events, 300 m 3 of water was delivered through a spatially distributed irrigation network at a rate just above the soil infiltration rate, resulting in transient saturation (5 hr) of the entire soil rooting zone (0-30 cm) across each 2,000 m 2 treatment plot.…”

“…Both mesh sizes allowed the passage of water and solutes. The primary rooting zone is 0–30 cm and deeper root growth is limited by a thick, largely continuous clay layer at ∼50 cm (Hopple et al., 2023), so we severed roots and trenched the 0.5 m 2 subplot perimeter to a depth of at least 60 cm (Figure 1b). The removed soil was kept in depth‐specific piles on tarps (e.g., 0–10, 10–30, 30–60, and >60 cm) and added back to the trenches in reverse order to lessen disturbance impacts on the structure and hydrology of soils surrounding the subplots.…”

Ideas and perspectives: Beyond model evaluation – combining experiments and models to advance terrestrial ecosystem science

“…In a model-EME context, this allows the models to be evaluated at multiple treatment levels, circumventing the step change issues. Large, full ecosystem level experiments with sometimes groundbreaking treatments such as the TEMPEST flooding experiment (Hopple et al, 2023), are also not replicated but provide invaluable insights into ecosystem functioning.…”