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.