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1. Quantifying energy use efficiency via entropy production: a case study from longleaf pine ecosystems

   https://doi.org/10.5194/bg-16-1845-2019  

   Wiesner, Susanne; Staudhammer, Christina L.; Stoy, Paul C.; Boring, Lindsay R.; Starr, Gregory (January 2019, Biogeosciences)

   Abstract. Ecosystems are open systems that exchange matter and energy with theirenvironment. They differ in their efficiency in doing so as a result of theirlocation on Earth, structure and disturbance, including anthropogenic legacy.Entropy has been proposed to be an effective metric to describe thesedifferences as it relates energy use efficiencies of ecosystems to theirthermodynamic environment (i.e., temperature) but has rarely been studied tounderstand how ecosystems with different disturbance legacies respond whenconfronted with environmental variability. We studied three sites in alongleaf pine ecosystem with varying levels of anthropogenic legacy and plantfunctional diversity, all of which were exposed to extreme drought. Wequantified radiative (eff_rad), metabolic and overall entropychanges – as well as changes in exported to imported entropy(eff_flux) in response to drought disturbance and environmentalvariability using 24 total years of eddy covariance data (8 years per site).We show that structural and functional characteristics contribute todifferences in energy use efficiencies at the three study sites. Our resultsdemonstrate that ecosystem function during drought is modulated by decreasedabsorbed solar energy and variation in the partitioning of energy and entropyexports owing to differences in site enhanced vegetation index and/or soilwater content. Low eff_rad and metabolic entropy as well as slowadjustment of eff_flux at the anthropogenically altered siteprolonged its recovery from drought by approximately 1 year. In contrast,stands with greater plant functional diversity (i.e., the ones that includedboth C₃ and C₄ species) adjusted their entropy exports when facedwith drought, which accelerated their recovery. Our study provides a pathforward for using entropy to determine ecosystem function across differentglobal ecosystems. 

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2. Using Metabolic Energy Density Metrics to Understand Differences in Ecosystem Function During Drought

   https://doi.org/10.1029/2019JG005335  

   Wiesner, Susanne; Stoy, Paul C.; Staudhammer, Christina L.; Starr, Gregory (February 2020, Journal of Geophysical Research: Biogeosciences)

   Abstract Terrestrial ecosystems obtain energy in the form of carbon‐containing molecules. Quantifying energy acquisition and dissipation throughout an ecosystem may be useful for describing their resistance and resilience to disturbances. Three longleaf pine savannas with different vegetation composition—a result of variation in soil moisture and land use legacy—were used as a case study to test energy‐based metrics of ecosystem metabolic function. Available energy from gross ecosystem exchange of CO₂and its dissipation into metabolic energy density (E_M) and energy storage were used to identify differences in drought recovery over an 8‐year period. Sites with higher plant functional diversity in the understory stored more energy and lowered their E_Mby ~20% when adapting to drought. In contrast, the site with greater abundance of woody understory and overstory species relied on stored energy twice as often as the more diverse sites. The absence of native understory species, due to anthropogenic legacy, prolonged ecosystem‐scale drought recovery by 1 year. This study provides the tools to understand differences in site metabolic energy dynamics and has the potential to identify site characteristics that indicate greater vulnerability to disturbances. Metabolic energy density can be applied to any global ecosystem and provides a first step to describe coupled carbon and energy allocation in ecosystems, which may be used to further refine ecological theory and its management implications. 

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3. Modeling the physiology of the aquatic temnospondyl <i>Archegosaurusdecheni</i> from the early Permian of Germany

   https://doi.org/10.5194/fr-20-105-2017  

   Witzmann, Florian; Brainerd, Elizabeth (January 2017, Fossil Record)

   Abstract. Physiological aspects like heat balance, gas exchange, osmoregulation, and digestion of the early Permian aquatic temnospondyl Archegosaurus decheni, which lived in a tropical freshwater lake, are assessed based on osteological correlates of physiologically relevant soft-tissue organs and by physiological estimations analogous to air-breathing fishes. Body mass (M) of an adult Archegosaurus with an overall body length of more than 1m is estimated as 7kg using graphic double integration. Standard metabolic rate (SMR) at 20°C (12kJh⁻¹) and active metabolic rate (AMR) at 25°C (47kJh⁻¹) were estimated according to the interspecific allometry of metabolic rate (measured as oxygen consumption) of all fish (V_O2 = 4. 8M^0. 88) and form the basis for most of the subsequent estimations. Archegosaurus is interpreted as a facultative air breather that got O₂ from the internal gills at rest in well-aerated water but relied on its lungs for O₂ uptake in times of activity and hypoxia. The bulk of CO₂ was always eliminated via the gills. Our estimations suggest that if Archegosaurus did not have gills and released 100% CO₂ from its lungs, it would have to breathe much more frequently to release enough CO₂ relative to the lung ventilation required for just O₂ uptake. Estimations of absorption and assimilation in the digestive tract of Archegosaurus suggest that an adult had to eat about six middle-sized specimens of the acanthodian fish Acanthodes (ca. 8cm body length) per day to meet its energy demands. Archegosaurus is regarded as an ammonotelic animal that excreted ammonia (NH₃) directly to the water through the gills and the skin, and these diffusional routes dominated nitrogen excretion by the kidneys as urine. Osmotic influx of water through the gills had to be compensated for by production of dilute, hypoosmotic urine by the kidneys. Whereas Archegosaurus has long been regarded as a salamander-like animal, there is evidence that its physiology was more fish- than tetrapod-like in many respects. 

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4. Heterogeneity promotes resilience in restored prairie: Implications for the environmental heterogeneity hypothesis

   https://doi.org/10.1002/eap.3006  

   Wojciechowski, Ashley_A; Blair, John_M; Collins, Scott_L; Baer, Sara_G (July 2024, Ecological Applications)

   Abstract Enhancing resilience in formerly degraded ecosystems is an important goal of restoration ecology. However, evidence for the recovery of resilience and its underlying mechanisms require long‐term experiments and comparison with reference ecosystems. We used data from an experimental prairie restoration that featured long‐term soil heterogeneity manipulations and data from two long‐term experiments located in a comparable remnant (reference) prairie to (1) quantify the recovery of ecosystem functioning (i.e., productivity) relative to remnant prairie, (2) compare the resilience of restored and remnant prairies to a natural drought, and (3) test whether soil heterogeneity enhances resilience of restored prairie. We compared sensitivity and legacy effects between prairie types (remnant and restored) and among four prairie sites that included two remnant prairie sites and prairie restored under homogeneous and heterogeneous soil conditions. We measured sensitivity and resilience as the proportional change in aboveground net primary productivity (ANPP) during and following drought (sensitivity and legacy effects, respectively) relative to average ANPP based on 4 pre‐drought years (2014–2017). In nondrought years, total ANPP was similar between remnant and restored prairie, but remnant prairie had higher grass productivity and lower forb productivity compared with restored prairie. These ANPP patterns generally persisted during drought. The sensitivity of total ANPP to drought was similar between restored and remnant prairie, but grasses in the restored prairie were more sensitive to drought. Post‐drought legacy effects were more positive in the restored prairie, and we attributed this to the more positive and less variable legacy response of forb ANPP in the restored prairie, especially in the heterogeneous soil treatment. Our results suggest that productivity recovers in restored prairie and exhibits similar sensitivity to drought as in remnant prairie. Furthermore, creating heterogeneity promotes forb productivity and enhances restored prairie resilience to drought. 

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5. Dynamics and controls of heterotrophic prokaryotic production in the western tropical South Pacific Ocean: links with diazotrophic and photosynthetic activity

   https://doi.org/10.5194/bg-15-2669-2018  

   Van Wambeke, France; Gimenez, Audrey; Duhamel, Solange; Dupouy, Cécile; Lefevre, Dominique; Pujo-Pay, Mireille; Moutin, Thierry (January 2018, Biogeosciences)

   Abstract. Heterotrophic prokaryotic production (BP) was studied in the western tropical South Pacific (WTSP) using the leucine technique, revealing spatial and temporal variability within the region. Integrated over the euphotic zone, BP ranged from 58 to 120mg Cm⁻²d⁻¹ within the Melanesian Archipelago, and from 31 to 50mg Cm⁻²d⁻¹ within the western subtropical gyre. The collapse of a bloom was followed during 6 days in the south of Vanuatu using a Lagrangian sampling strategy. During this period, rapid evolution was observed in the three main parameters influencing the metabolic state: BP, primary production (PP) and bacterial growth efficiency. With N₂ fixation being one of the most important fluxes fueling new production, we explored relationships between BP, PP and N₂ fixation rates over the WTSP. The contribution of N₂ fixation rates to bacterial nitrogen demand ranged from 3 to 81%. BP variability was better explained by the variability of N₂ fixation rates than by that of PP in surface waters of the Melanesian Archipelago, which were characterized by N-depleted layers and low DIP turnover times (T_DIP&lt;100h). This is consistent with the fact that nitrogen was often one of the main factors controlling BP on short timescales, as shown using enrichment experiments, followed by dissolved inorganic phosphate (DIP) near the surface and labile organic carbon deeper in the euphotic zone. However, BP was more significantly correlated with PP, but not with N₂ fixation rates where DIP was more available (T_DIP&gt;100h), deeper in the Melanesian Archipelago, or within the entire euphotic zone in the subtropical gyre. The bacterial carbon demand to gross primary production ratio ranged from 0.75 to 3.1. These values are discussed in the framework of various assumptions and conversion factors used to estimate this ratio, including the methodological errors, the daily variability of BP, the bacterial growth efficiency and one bias so far not considered: the ability for Prochlorococcus to assimilate leucine in the dark. 

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