Structural complexity and productivity
In literature, there is a hypothesis as to why forests with high complexity might grow so well: Jørgensen (1992) argued that mature and complex ecosystems, including forests, are able to capture more exergy (usable energy) than immature ecosystems with low complexity. In a study incorporating forests across the eastern United States, it was shown that the fraction of photosynthetically absorbed radiation increases with canopy structural complexity of the stands (Atkins et al. 2018a). Accordingly, when analyzing monocultures and mixed stands, Forrester et al. (2018) found that the light absorption of stands increases as canopy volume increases. The light capturing rate is hypothesized to flatten out when maturity is reached (see Jørgensen 1992) and, in case of a fully stocked mature and unmanaged forest, it likely oscillates around a high level in absence of major disturbances. Since maturity of a forest ecosystem usually corresponds to a larger leaf area per unit ground area, it is not surprising that this measure was shown to relate closely to forest productivity (e.g. Bolstad et al. 2001). However, stand growth depends more directly on light absorption than on leaf area (Binkley et al. 2013). Thus, a given photosynthetically active surface area distributed over a larger vertical extent (multi-layered forest) results not only in a higher structural complexity of the stand when compared to the same leaf area being located in a thin canopy layer (single-layered forest), but possibly also in a higher light absorption and consequently a higher productivity (e.g. Juchheim et al. 2017). Since such multi-layeredness results in more stable microclimate conditions (Ehbrecht et al. 2019), heat loads in the canopy are lower than in single layered stands due to higher exergy uptake. A strongly vertically distributed light absorption and reduced heat loads would also reduce heat stress, and enable efficient photosynthesis which is thermosensitive process (e.g. Wang et al. 2008). In fact, Weigel et al. (2022) stated that it is likely that direct and negative effects of heat on leaf physiology are often underestimated.