Plants are open, irreversible, non-equilibrium systems interacting continuously with their environment through mass and energy exchanges. As such, they are suitable for thermodynamic analyses and can be considered energy converters, transforming solar energy into chemical energy to sustain cellular metabolism and promote growth.

This study presents a novel thermodynamic model based on mass conservation and the First and Second Laws of Thermodynamics, capable of calculating the efficiency of ecological systems.

The approach centers on determining exergy inflows and outflows from a defined control volume. Exergy represents the quality of energy, quantifying a system’s thermodynamic distance from equilibrium with its environment, and providing a combined qualitative and quantitative assessment of ecosystem energy content. Initially adopted in ecology as eco-exergy, it relates total exergy to total biomass, enabling analysis of ecological structure, function, survival capacity, energy-use efficiency, and regulatory interactions. However, eco-exergy departs from traditional thermodynamics by not adequately representing the actual work capacity of ecological systems.

To address this limitation, this research defines the exergy of a Mediterranean forest ecosystem (Palo Laziale, Rome) over a five-year period (2018-2022), within the context of climate change. A simplified exergy model, treating the canopy as a single "big leaf," was applied using quantified CO₂ and H₂O fluxes combined with local climatic data.

The exergy analysis highlighted solar radiation as the dominant incoming energy source, followed by energy from liquid water, with minimal contribution from assimilated CO₂. Outgoing exergy was mainly directed toward biomass formation and water diffusion, while considerable amounts were lost due to spontaneous irreversible processes. Exergy efficiency peaked in 2021 (0.0197%), coinciding with warmer mean temperatures (18.8°C) and moderate precipitation (639.6 mm), contrasting with lower efficiency years characterized by non-limiting conditions. The model enabled the determination of dynamic exergy efficiency over time, providing useful insights into energy resource management which can support the development of effective management practices and policies.