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

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.
