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

Susanne Wiesner,Gregory Starr,Lindsay R Boring,Paul C Stoy,Christina L Staudhammer

doi:10.5194/bg-16-1845-2019

Abstract

Abstract. Ecosystems are open systems that exchange matter and energy with their environment. They differ in their efficiency in doing so as a result of their location on Earth, structure and disturbance, including anthropogenic legacy. Entropy has been proposed to be an effective metric to describe these differences as it relates energy use efficiencies of ecosystems to their thermodynamic environment (i.e., temperature) but has rarely been studied to understand how ecosystems with different disturbance legacies respond when confronted with environmental variability. We studied three sites in a longleaf pine ecosystem with varying levels of anthropogenic legacy and plant functional diversity, all of which were exposed to extreme drought. We quantified radiative (effrad), metabolic and overall entropy changes – as well as changes in exported to imported entropy (effflux) in response to drought disturbance and environmental variability using 24 total years of eddy covariance data (8 years per site). We show that structural and functional characteristics contribute to differences in energy use efficiencies at the three study sites. Our results demonstrate that ecosystem function during drought is modulated by decreased absorbed solar energy and variation in the partitioning of energy and entropy exports owing to differences in site enhanced vegetation index and/or soil water content. Low effrad and metabolic entropy as well as slow adjustment of effflux at the anthropogenically altered site prolonged its recovery from drought by approximately 1 year. In contrast, stands with greater plant functional diversity (i.e., the ones that included both C3 and C4 species) adjusted their entropy exports when faced with drought, which accelerated their recovery. Our study provides a path forward for using entropy to determine ecosystem function across different global ecosystems.

Highlights

Ecosystems utilize resources, such as solar radiation, nutrients and water, to maintain a state far from thermodynamic equilibrium (Amthor, 2010; Beer et al, 2009; Finzi et al, 2007; Thomas et al, 2016)
Sm was greater during summer months at all sites with no significant differences between the mesic and xeric sites from February through August but significantly lower at the intermediate site compared to the xeric site for all months (Fig. 7h, Table S5)
Higher vapor pressure deficit (VPD) significantly increased Sm similar to the model of negative metabolic energy (NEEe); slopes were more similar among the sites (Fig. 7k)

Summary

Introduction

Ecosystems utilize resources, such as solar radiation, nutrients and water, to maintain a state far from thermodynamic equilibrium (Amthor, 2010; Beer et al, 2009; Finzi et al, 2007; Thomas et al, 2016). Understanding ecosystem resource use efficiency is crucial, as anthropogenic and climate-induced changes around the globe continue to alter ecosystem structure and function (Haddeland et al, 2014; Porter et al, 2012; Reinmann and Hutyra, 2016; Thom et al, 2017). The terms LE, H and G represent energy exports through latent heat, sensible heat and ground heat fluxes, respectively; and M is an energy storage term comprised of changes in biomass accumulation through metabolic processes (Holdaway et al, 2010). S. Wiesner et al.: Quantifying energy use efficiency via entropy production tion of a steady state over longer periods and because M is much smaller in magnitude compared to other fluxes. M imposes a control on energy fluxes, like Rn, LE and H , through changes in leaf area and reflective properties, as well as through active biotic control in response to changes in environmental variables (i.e., stomata opening and closing due to water availability, Hammerle et al, 2008)

Methods

Results

Conclusion