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.