Introduction
The structural complexity of a forest can be defined as all dimensional,
architectural, and distributional patterns of plant individuals and
their organs in a given space at a given point in time (Seidel et al.
2020). Today, it is possible to quantify the complexity of forest
structures objectively, holistically, and efficiently based on light
detection and ranging (LiDAR) technology (e.g. Ehbrecht et al. 2021,
Heidenreich and Seidel 2022). Structural complexity in general was shown
to have beneficial effects on various ecosystem functions and services
provided by forests (Lindenmayer et al. 2000, Knoke and Seifert 2008,
D’Amato et al. 2011, Neill and Puettmann 2013). Therefore, managing
forests for increased structural complexity and for its maintenance, as
well as acknowledging them as ‘complex adaptive systems’ (Holland 1992,
Gell-Man and Lloyd 1996, Levin 2003) has become a paradigm in modern
silviculture in many countries (Messier et al. 2013). In fact, Möller
(1922) who developed the idea of a ‘continuous cover’ forest management
had predicted such beneficial effects already 100 years ago (Ammer
2021). Interestingly, there is solid quantitative evidence that, all
others things being equal, primary forests possess (on average) a
greater structural complexity than managed forests. This was shown for
temperate coniferous forests (e.g. Seidel et al. 2016), temperate
deciduous forests (e.g. Stiers et al. 2018, 2020), tropical forests
(Camaretta et al. 2021), and also boreal forests (Kuuluvainen et al.
1996), but on different absolute levels of structural complexity. Some
forests have been explicitly managed for high vertical and horizontal as
well compositional heterogeneity over decades and provide a largely
uneven-aged structure (Heliwell 1997, Stiers et al. 2020). Such stands
can possess high structural complexity, approaching that observed for
primary forests (Stiers et al. 2020). Beside management, the absolute
level of above-ground structural complexity is determined by the abiotic
environment. For example, it was shown that structural complexity of
primary forests on global scale strongly increases with the available
water (Ehbrecht et al. 2021). Therefore, the global pattern in forest
complexity clearly peaks in the tropics, drops drastically to the
subtropics, rises strongly towards the temperate zone and finally
flattens out at the polar regions (Ehbrecht et al. 2021). Water
availability and sufficient time to spatially arrange plant tissue
without major disturbance can therefore be described as critical factors
for a high structural complexity in forest systems.
While forest management aims at a specific stand structure according to
the management goal, the question remains why natural forests seem to
gravitate towards maximum structural complexity, at least aboveground.
To answer this, it might be helpful to see forests through the lens of
thermodynamic theory.