Discerning the thermodynamic feasibility of the spontaneous coexistence of multiple functional vegetation groups

Abstract

Can the Second Law of Thermodynamics explain why ecosystems naturally organize into a complex structure composed of multiple vegetation species and functional groups? Ecosystem structure, which refers to the number and type of plant functional groups, is the result of self-organization, or the spontaneous emergence of order from random fluctuations. By considering ecosystems as open thermodynamic systems, we model and study these fluctuations of throughput signatures on short timescales to determine the drivers and characteristics of ecosystem structure. This diagnostic approach allows us to use fluxes of energy and entropy to calculate an ecosystem’s estimated work and understand the thermodynamic behavior of the system. We use a multi-layer canopy-root-soil model to calculate the energy and entropy fluxes of different scenarios for field sites across various climates. At each site, scenarios comprised of native individual plant functional groups and a coexisting multi-group composition scenario including all functional groups observed at the site are compared. Ecosystem-scale calculations demonstrate that entropy fluxes and work efficiency—the work performed for the amount of radiation entering the ecosystem—are greatest in the multi-group scenario when its leaf area is significantly larger than each of its individual functional groups. Thus, we conclude that ecosystems self-organize towards the vegetation structure with the greatest outgoing entropy flux and work efficiency, resulting in the coexistence of multiple functional groups and performing the maximum amount of work within the constraints of locally available energy, water, and nutrients.

Thermodynamic basis for the demarcation of Arctic and alpine treelines

“…In classical thermodynamics, work is performed due to a transfer, or physical movement, of heat 18 . In the context of ecosystems, work performed by an ecosystem is represented by the exchange of heat with the external environment outside the ecosystem control volume 12 . Work performed by an ecosystem is, therefore, estimated as the vertical transport of heat in the form of latent and sensible heat, driven by the vertical gradient in temperature within the control volume structured by both the incoming downward shortwave and longwave radiation and the vegetation structure.…”

“…The study period for each pair of sites above and below the treeline was chosen as the longest consecutive time series of available data for both sites: 2012-2013 for the Italian Alps, 2008-2013 for the United States Rocky Mountains, and 2016-2018 for the Western Canadian Taiga-Tundra. MLCan has been validated for numerous sites across the Western Hemisphere 12,[23][24][25][26][27][28][29] . To apply MLCan to the harsh winter conditions of Arctic and alpine ecosystems, we included new parameterizations for the hydraulic and thermal properties of peat soils and photosynthesis switches for dormancy during winter.…”

“…To address this question, we use an extensively validated multi-layer 1-D physics-based ecohydrological model, ML-Can 12,[23][24][25][26][27][28][29] , consisting of 20 above-ground layers, 1 ground surface layer, and 12 below-ground layers (see Supplementary Material). This model is chosen because of its ability to capture interactions among functional groups, such as the impact of shading on understory vegetation and the resulting thermal environment within the canopy 26 .…”

“…The multi-layer canopy model, MLCan, models the mass and energy fluxes occurring within the canopy, roots, and soil system to resolve the energy and entropy fluxes, and associated work efficiencies for each ecosystem 12,[23][24][25][26] . Observed environmental data -including wind speed, air temperature, global radiation, precipitation, friction velocity, air pressure, and relative humidity -from various sources [37][38][39][40][41][42] were used as forcing for the model.…”

“…where T sur f and T air are the temperature (in Kelvin) of the soil surface and the air above the canopy, respectively, and h e is the ecosystem height, taken as the maximum canopy height in each site pair (see Table S1 in the Supplementary Material) 12 .…”