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.