Is light interception of understorey species facilitated by light reflection from plant neighbours?

Background: Facilitation is an ecological process that has previously been discussed largely in relation to the modification of abiotic conditions in extreme habitats. Here, we introduce the concept of reflected radiation as a potential facilitation mechanism that may raise the beneficiary’s resource level in light-limited habitats. Objectives: We examined whether plants may be able to use their spatial architecture to increase their light absorption through the use of radiation reflected by their neighbours. We tested if this interaction resulted in increased biomass or leaf area between the respective pairs of species. Methods: We determined the biomass and leaf area of eight common herb-layer species, composed of four grass and four herb species, in a beech forest. Plant three-dimensional architecture was digitised and used to simulate reflected and absorbed radiation between the species in a virtual black box. To accomplish this, a so-called light donor plant (which reflected light from its leaf surfaces) was surrounded by four receptor plants (which absorbed this reflected radiation) at a distance of 10 cm. This was repeated using every combination of the available species. We established a ray-trace simulation, including measured light reflection and transmission rates of leaves that enabled us to quantify the relative light absorption by receptor plants, expressed as brightness per leaf area, and estimated the amount of incoming photons. Furthermore, we calculated Seifan’s importance index as an indicator for the relevance of the postulated facilitation effect on plant biomass and leaf area. Results: The leaves of the species studied reflected ca. 7% of radiation between 400 and 700 nm and 18% between 540 to 650 nm of daily incident photon flux density, corresponding to the accessory pigments’ range of efficacy. An average of 0.19% of radiation reflected by a donor plant reached a receptor plant. The benefits to receptor plants varied significantly with donor identity. Conspecific donors supplied significantly larger benefits for Stellaria, Galium, Brachypodium and Hordelymus, which are rhizomatous plants with short stolons. Clones of these species may be able to increase their daily light intake through absorption of reflected light by 0.2%, 0.5–5.0%, 5.0% and 0.6–11.6%, respectively, depending on the number of donor ramets and on the distance between the donor and receptor ramets. Nonetheless, the estimated conspecific facilitation effects through light reflectance within a single clone did not translate into increased biomass. Conclusion: As light intensity decreases with distance according to Lambert’s law, a significant increase of light intake through neighbours can only be predicted for plants with a high density of donor ramets, such as clonal species having many ramets within a short distance. We suggest that in strongly light-limited habitats there may be an advantage to a clonal growth strategy due to the observed facilitation effect of absorption of reflected radiation. To evaluate the magnitude of this effect, future studies should concentrate on ‘payback times’ of leaves with low light intake to gauge the period of time necessary for one leaf to offset the necessary investment in photosynthetic light capture, calculated as cost-benefit calculation between the cost of the leaf’s production and the net carbon gain realised by it.