Forests through the lens of thermodynamics
Already 30 years ago, scientists argued that it might be a promising endeavor to apply the principles of thermodynamics to ecosystems (Jørgensen 1990, 1992, Abel and Trevors 2006). Originally developed to describe the energy budgets in closed systems, thermodynamic theory was never utilized to help understanding tree or forest structural complexity.
The first law of thermodynamic states that in a closed system, energy cannot be created or destroyed, it can only be converted from one form to another (Clausius 1850). The second law of thermodynamics tells us that in a closed system, entropy can never be eliminated, it can only remain steady or increase (Clausius 1854). In such a system, all processes therefore result in the conversion of higher-quality energy to lower-quality energy and heat production. This change in energy quality is unidirectional (only from high to low quality) and describes a decreasing availability of exergy, which is that part of energy that can be used to do actual ‘work’ (Nielsen et al. 2020). Brillouin (1960) already described this descending quality of energy from high quality with low entropy (e.g. radiation) to low quality with high entropy (heat). From this, it is obvious that the non-living physical world, for which the laws of thermodynamics have been developed, inevitably runs into a condition of maximized entropy, with entropy being best defined as the “level of dissipation that already happened” (cf. Nielsen et al. 2020). Anything that could have happened already happened at the end of this development, all energy will have been converted to heat. However, it is known since more than 100 years (e.g. Lotka 1922) that this does not hold for living systems, like plants.
Living organisms are able to make use of external energy sources to power their own biophysical processes, basically to sustain their own life, through the creation of structures in the form of complex molecules (Lotka 1922, Fonseca et al. 2002, Ludovisi 2014). Here, mechanisms involve energy trapping, transduction and storage or immediate utilization for cellular work (Abel and Trevors 2006). Nielsen et al. (2020) stated that the key question is therefore how the thermodynamic balances are handled by living organisms to allow for the build-up of ordered and efficient structures. It was proposed that living systems exploit an energy source (e.g. light) as effectively as possible to maximize the intake of energy (here: exergy or eco-exergy; e.g. Fonseca et al. 2002) in their system to be used for their own processes and in order to create their own structures which are necessary to support the biochemical processes (sensu Lotka 1922). Thereby, the metabolism of living organisms never violates the second law of thermodynamics (Abel and Trevors 2006). Through the utilization and storage however, living systems delay the process of entropy production, often referred to as the creation of anentropy or negentropy (Schrödinger 1944, Nicolis and Prigogine 1971, Nielsen et a. 2020). This was proposed to serve as handy criteria to define what life is (cf. Nielsen et al. 2020) and earlier, to express what evolution might strive for, i.e. energy efficiency and maximized energy throughflow (Lotka 1922).
The first implication of this, when applied to forest ecosystems, is rather trivial: the energy provided by the sun can be interpreted as the fundamental source for the development of forest structures. The second one is less trivial, since one can derive from the above said that forests might actually develop complex structures to enable an efficient use of sunlight (cf. Odum and Pinkerton 1955, Schneider and Kay 1994). If complex structures allow using light as energy source as efficiently as possible on the way between the forest top and the ground floor, structural complex forest have more aboveground resources available to be invested in growth, defense, storage, adaptation or reproduction. Some evidence for this hypothesis exists, since it could be shown that net primary productivity of forests is positively correlated with structural complexity (Harthun 2017, Gough et al. 2019). For trees, a positive relationship between the structural complexity and growth efficiency (wooden tree volume to crown surface area) was discovered (Seidel et al. 2019), as well as a positive relationship between the structural complexity and the productivity of individual trees across more than 40 species (Seidel 2018, Seidel et al. 2019, Dorji et al. 2021). However, the primary productivity of the entire ecosystem (forest) is still difficult to quantify, since it is not possible to quantify belowground productivity well enough. Even for the aboveground parts of a forest, inventories usually do not consider trees smaller than 7 cm in diameter at breast height.