| Summary: | As the rate and scale of human activities increase throughout the world, the structure and function of Earth systems are consequently altered. Human-induced direct and indirect perturbations, such as changes in atmospheric temperature or the burning or logging of vegetation, alter the thermodynamic environment in which ecosystems operate. Yet, the ecosystem-level vegetation response is coupled to its thermodynamic regime, and changes therein are still relatively unknown. Thus, a framework for characterizing and understanding the self-organization of ecosystem vegetation from the thermodynamic perspective is needed to understand its emergent response to natural and human-induced perturbations. The goals of this thesis are to (i) develop a thermodynamic framework to characterize the existence of emergent vegetation structure at any given location, and (ii) utilize this framework to gain insight into the thermodynamic response of ecosystem behavior to direct alteration of vegetation structure through human activities. Vegetation structure, which refers to the number and type of plant functional groups comprising an ecosystem, is the result of self-organization, or the spontaneous emergence of order from random fluctuations. By treating ecosystems as open thermodynamic systems, we use a multi-layer canopy-root-soil model to calculate their thermodynamic properties -- such as energy, entropy, and work -- for field sites across various climates, vegetation structures, and disturbance regimes. We first ask the question: Why do ecosystems exhibit a prevalence of vegetation structure consisting of multiple functional groups? In other words, does the coexistence of multiple functional groups provide a thermodynamic advantage over the individual functional groups that each ecosystem comprises. From this work, we conclude that ecosystems self-organize towards the multiple functional group vegetation structure due to greater fluxes of entropy, work, and work efficiency. Together, these characteristics comprise the concept of thermodynamic advantage. Since multiple functional groups do not exist everywhere in nature, we study and analyze the thermodynamic basis for the existence of ecosystems with a single functional group vegetation structure -- in particular, the region beyond the treeline in alpine and Arctic ecosystems. We therefore ask the question: Since the existence of multiple vegetation groups provides a thermodynamic advantage, is the existence of only a single functional group a result of a thermodynamic limitation? This analysis using counterfactual scenarios comprising of hypothetical trees existing beyond the treeline identifies two conditions of thermodynamic infeasibility. We find that the existence of trees beyond the treeline would result in negative work, and in some cases, net leaf carbon loss from the ecosystem, both comprising a thermodynamic infeasibility condition. Based on these two components, we conclude that an ecosystem will self-organize towards the most advantageous vegetation structure made possible by thermodynamic feasibility. These concepts of thermodynamic feasibility and thermodynamic advantage are then applied to study ecosystems perturbed by human activities through logging and fire. Findings indicate that a forest that is consistently logged is held in a sub-optimal state with lower fluxes of entropy and work efficiency than an undisturbed forest, meaning that human activities prevent the ecosystem from reaching its most thermodynamically advantageous vegetation structure. However, for controlled burns on a tallgrass prairie the advantageous vegetation structure is dependent on the frequency of the burn. Overall, logging events force forests into a disadvantageous vegetation structure while the frequency of burn events determines and reinforces the resulting vegetation structure. This thesis develops a novel framework for analyzing ecosystems as thermodynamic systems driven by thermodynamic feasibility and thermodynamic advantage. Further, by characterizing the behavior of vegetation upon direct alterations to its structure, this work provides a foundation for understanding and predicting the thermodynamic response of vegetation structure to emergent climate scenarios that could impact the thermodynamic environment in which ecosystems operate.
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