Tree light capture and spatial variability of understory light increase with species mixing and tree size heterogeneity

Abstract

Mixed and multi-layered forest ecosystems are sometimes more productive than monospecific and single-layered ones. It has been suggested that trees of different species and sizes occupy complementary positions in space which would act as a mechanism to increase canopy light interception and wood production. However, greater canopy light interception reduces the average amount and variability of transmitted radiation offering fewer opportunities for all species to regenerate and to maintain forest heterogeneity in the long run. We investigated whether increasing overstory heterogeneity indeed results in greater canopy light interception and lower variability in transmittance. We modeled the three-dimensional structure of forest stands with three typical forest structures, 10 mixtures of four tree species, and three different basal areas. We used the forest light interception model SAMSARALIGHT and performed three-way analyses of covariance to analyze the effects of the three varied components of forest heterogeneity. We found no evidence that increasing structural heterogeneity increases canopy light interception. However, the light interception by mixed canopies was greater than the weighted average of light interception by the corresponding pure canopies. Variability in transmittance increased in some cases with compositional heterogeneity and, to a lesser extent, with tree size inequalities. The advantage of heterogeneous forests is in opportunities for natural regeneration as well as in opportunities to enhance canopy light interception.

How tree species identity and diversity affect light transmittance to the understory in mature temperate forests

“…This is most likely due to the complementary shrub layer that was high in plots with a rather open canopy and lower under closed canopies. In conclusion, we do find indication for a higher light interception at the tree canopy level if tree diversity increases (Jucker et al., ; Ligot et al., ), but the complementary shrub layer effectively homogenizes light transmittance to the forest floor across species combinations.…”

“…Two different studies using a computer model of broadleaf and coniferous trees found a complementarity effect. Light interception in mixed stands was intermediate between the interception values in monocultures but higher than expected if the effects would be purely additive (Forrester et al, 2017;Ligot et al, 2016). The three mentioned studies focus on forests in the stem exclusion phase; however, evidence for higher light interception in more diverse forests from field studies in mature forests in the understory re-initiation stage is lacking.…”

“…Several studies found that mixed stands had a denser canopy than monocultures because of complementary crown architecture and plasticity (Jucker, Bouriaud, & Coomes, ; Pretzsch, ; Sapijanskas, Paquette, Potvin, Kunert, & Loreau, ; Williams, Paquette, Cavender‐Bares, Messier, & Reich, ). Such increased canopy packing allows the trees to preempt the light resource more effectively and leads to lower light availability below the canopy (Forrester et al., ; Ligot et al., ). This is in accordance with the more general prediction of functional biodiversity research that more diverse systems use resources more efficiently due to complementarity between species (Forrester, ; Loreau & Hector, ; Tilman, ).…”

“…Old-growth forests with natural tree fall dynamics often show a high quantity and spatial heterogeneity of light transmittance (Canham, Finzi, Pacala, & Burbank, 1994;Tinya & Ódor, 2016), but only few of the temperate forests are actually in the old-growth stage (Bengtsson, Nilsson, Franc, & Menozzi, 2000;Hannah, Carr, & Lankerani, 1995). The more prevalent mature forests in the understory re-initiation stage generally have continuous closed canopies and light is expected to be primarily determined by species composition under similar stand basal area (Canham et al, 1994;Ligot, Ameztegui, Courbaud, Coll, & Kneeshaw, 2016). For these systems, it is therefore important to understand how the tree community composition (here identity and diversity) affects the light availability at the forest floor.…”

High rates of primary production in structurally complex forests

“…Positive correlations among site-level structural complexity traits, vegetation area index, and species diversity indices support the hypothesis (Gough et al 2016) that canopies require the crown architectural variety supplied by a diverse plant community and, additionally, a critical mass of vegetation with which to build complex structure. Multilayered, heterogeneously arranged canopies absorb more light (Ligot et al 2016) and they contain a complement of sun and shade leaves functioning optimally under a range of light conditions. This finding is congruent with theory and modeling asserting that structurally complex forests composed of species and individuals possessing a diversity of leaf physiological traits are more optimally organized to acquire and use light to assimilate carbon (Anten 2016).…”