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1. Components of the CO ₂ Exchange in a Southeastern USA Salt Marsh

   https://doi.org/10.34133/olar.0077  

   Morris, James T; Whiting, Gary J (December 2024, Ocean-Land-Atmosphere Research)

   Different CO₂exchange pathways were monitored for a year in short- and tall-formSpartina alternifloragrasses in a southeastern USA salt marsh at North Inlet, South Carolina. The tall form of grass growing close to a creek under favorable conditions reached a higher standing biomass than the short form of grass growing in the interior marsh. However, the photosynthetic parameters of both forms of grass were equivalent. The tall canopy had greater net canopy production, 973 versus 571 g C m⁻²year⁻¹, canopy growth, 700 versus 131 g C m⁻²year⁻¹, and canopy respiration, 792 versus 225 g C m⁻²year⁻¹, but lower sediment respiration, 251 versus 392 g C m⁻²year⁻¹. In a single growing season, tall-canopy biomass increased to intercept all the available solar radiation, which limits gross photosynthesis. Total respiration increased during the growing season in proportion to live biomass to a level that limited net production. Theoretically, the difference between net canopy production and canopy growth is carbon allocated to belowground growth and respiration. However, the computation of belowground production by this method was unrealistically low. This is important because carbon sequestration is proportional to belowground production and accounts for most of the vertical elevation gain of the marsh surface. Based on the allometry of standing live biomass, alternative estimates of belowground production were 927 and 193 g C m⁻²year⁻¹in creekbank and interior marshes, which would yield gains in surface elevation of 0.2 and 0.04 cm/year, respectively. 

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2. Restoration and resilience to sea level rise of a salt marsh affected by dieback events

   https://doi.org/10.1002/ecs2.4467  

   Rolando, J L; Hodges, M; Garcia, K D; Krueger, G; Williams, N; Carr, J; Robinson, J; George, A; Morris, J; Kostka, J E (April 2023, Ecosphere)

   Abstract The frequency of salt marsh dieback events has increased over the last 25 years with unknown consequences to the resilience of the ecosystem to accelerated sea level rise (SLR). Salt marsh ecosystems impacted by sudden vegetation dieback events were previously thought to recover naturally within a few months to years. In this study, we used a 13‐year collection of remotely sensed imagery to provide evidence that approximately 14% of total marsh area has not revegetated 10 years after a dieback event in Charleston, SC. Dieback onset coincided with a severe drought in 2012, as indicated by the Palmer drought stress index. A second dieback event occurred in 2016 after a historic flood influenced by Hurricane Joaquin in 2015. Unvegetated zones reached nearly 30% of the total marsh area in 2017. We used a light detection and ranging‐derived digital elevation model to determine that most affected areas were associated with lower elevation zones in the interior of the marsh. Further, restoration by grass planting was effective, with pilot‐scale restored plots having greater aboveground biomass than reference sites after two years of transplanting. A positive outcome indicated that the stressors that caused the dieback are no longer present. Despite that, many affected areas have not recovered naturally, even though they are located within the typical elevation range of healthy marshes. A mechanistic modeling approach was used to assess the effects of vegetation dieback on salt marsh resilience to SLR. Predictions indicate that a highly productive restored marsh (2000 g m⁻² year⁻¹) would persist at a moderate SLR rate of 60 cm in 100 years, whereas a nonrestored mudflat would lose all its elevation capital after 100 years. Thus, rapid restoration of marsh dieback is critical to avoid further degradation. Also, failure to incorporate the increasing frequency and intensity of extreme climatic events that trigger irreversible marsh diebacks underestimates salt marsh vulnerability to climate change. Finally, at an elevated SLR rate of 122 cm in 100 years, which is most likely an extreme climate change scenario, even highly productive ecosystems augmented by sediment placement would not keep pace with SLR. Thus, climate change mitigation actions are also urgently needed to preserve present‐day marsh ecosystems. 

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3. Early warning signs of salt marsh drowning indicated by widespread vulnerability from declining belowground plant biomass

   https://doi.org/10.1073/pnas.2425501122  

   Runion, Kyle D; Alber, Merryl; Mishra, Deepak R; Lever, Mark A; Hladik, Christine M; O’Connell, Jessica L (July 2025, Proceedings of the National Academy of Sciences)

   Salt marshes provide valuable ecosystem services but are vulnerable to drowning with accelerated sea-level rise (SLR). Marsh belowground biomass (BGB) production helps avoid drowning by building marsh surface elevation. Reductions in BGB can serve as an early warning sign of marsh deterioration, as they often precede decreases in aboveground biomass (AGB). However, landscape-scale BGB assessments to predict broad trends in marsh deterioration have not been previously available. We applied the Belowground Ecosystem Resiliency Model (BERM) to assess standing stocks and trends in both BGB and AGB over the past decade (2014–2023) across US Georgia coastSpartina alternifloramarshes (691 km²). Over this time period, BGB and AGB averaged 841 ± 323 and 221 ± 14 g m⁻², respectively, but showed opposite trends. BGB decreased on average by 0.94% per year and over most of the marsh area (72%), while AGB increased on average by 0.66% per year and showed a net increase across most of the marsh area (88%). This disconnect suggests that AGB is not a good indicator of marsh resilience, and we highlight two areas with similar AGB but different BGB. Inundation intensity, an important predictor of BGB, rose through time and was negatively related to BGB. SLR trends suggest continuing increases in inundation, which will result in further declines in BGB followed by widespread marsh drowning. Landscape BGB assessments are a valuable tool to identify ecosystem vulnerability and proactively manage salt marshes and the services they provide under rising sea levels. 

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4. Estimating Aboveground Biomass and Its Spatial Distribution in Coastal Wetlands Utilizing Planet Multispectral Imagery

   https://doi.org/10.3390/rs11172020  

   Miller, Gwen J.; Morris, James T.; Wang, Cuizhen (September 2019, Remote Sensing)

   Coastal salt marshes are biologically productive ecosystems that generate and sequester significant quantities of organic matter. Plant biomass varies spatially within a salt marsh and it is tedious and often logistically impractical to quantify biomass from field measurements across an entire landscape. Satellite data are useful for estimating aboveground biomass, however, high-resolution data are needed to resolve the spatial details within a salt marsh. This study used 3-m resolution multispectral data provided by Planet to estimate aboveground biomass within two salt marshes, North Inlet-Winyah Bay (North Inlet) National Estuary Research Reserve, and Plum Island Ecosystems (PIE) Long-Term Ecological Research site. The Akaike information criterion analysis was performed to test the fidelity of several alternative models. A combination of the modified soil vegetation index 2 (MSAVI2) and the visible difference vegetation index (VDVI) gave the best fit to the square root-normalized biomass data collected in the field at North Inlet (Willmott’s index of agreement d = 0.74, RMSE = 223.38 g/m2, AICw = 0.3848). An acceptable model was not found among all models tested for PIE data, possibly because the sample size at PIE was too small, samples were collected over a limited vertical range, in a different season, and from areas with variable canopy architecture. For North Inlet, a model-derived landscape scale biomass map showed differences in biomass density among sites, years, and showed a robust relationship between elevation and biomass. The growth curve established in this study is particularly useful as an input for biogeomorphic models of marsh development. This study showed that, used in an appropriate model with calibration, Planet data are suitable for computing and mapping aboveground biomass at high resolution on a landscape scale, which is needed to better understand spatial and temporal trends in salt marsh primary production. 

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5. Small rainfall events increase belowground production in Chihuahuan Desert grassland

   https://doi.org/10.1002/ecy.70206  

   Collins, Scott L; Brown, Renée F (September 2025, Ecology)

   Abstract Dryland productivity is highly sensitive to precipitation variability, and models predict that rainfall variability will increase in the future. Numerous studies have documented the relationship between productivity and precipitation, but most focus on aboveground production (ANPP), while the effects on belowground production (BNPP) remain poorly understood. Furthermore, previous research suggests that ANPP and BNPP are uncoupled within ecosystems, but the degree to which rainfall variability affects the interplay between aboveground and belowground production is unknown. We conducted a long‐term rainfall manipulation experiment in Chihuahuan Desert grassland to investigate how the size and frequency of growing season rain events affected BNPP and its relationship to ANPP. Experimental plots received either 12 small‐frequent rain events or 3 large‐infrequent events during the monsoon season for a total of 60 mm of added rainfall per treatment per year. All plots, including three controls, received ambient rainfall throughout the year. Total BNPP ranged from a low of 94.7 ± 38.2 g m²year⁻¹under ambient conditions to a high of 183.7 ± 44.6 g m²year⁻¹under the small‐frequent rainfall treatment. Total BNPP was highest under small‐frequent rain events, and there was no difference in BNPP between 0–15 and 15–30 cm soil depths in either rainfall treatment. ANPP and BNPP were uncorrelated within rainfall treatments, but weakly positively correlated across all plots and years. Our results contribute to a growing body of research on the importance of small rain events in drylands and provide further evidence regarding the weak coupling between aboveground and belowground processes. 

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