Canopy level photosynthesis under conditions of diffuse and direct sunlight and its implications for light use efficiency by plants under future climates

Estimating the carbon fixed by plants through photosynthesis as part of the global carbon cycle gives biologists a key role in preparing us to meet the challenge of a changing climate. One particular puzzle for researchers to solve is how the efficiency with which plant canopies use light changes with shifts in incoming radiation from direct under clear skies towards diffuse sunlight under hazy or cloudy skies. To address this question, we will model how predicted increases in cloudiness, e.g. over Finland, will affect radiative transfer and consequently change net primary productivity through plant canopies. This question is also particularly prescient with respect to proposed geoengineering solutions to mitigate climate change that involve reducing and diffusing net incoming solar radiation by injecting sulphate aerosols into the atmosphere. As the challenge of meeting emissions’ targets for greenhouse gases becomes increasingly difficult, such geoengineering solutions are being seriously considered, but will be very risky without a better understanding of how the changes in incoming solar radiation and shift between direct and diffuse radiation can affect plant productivity. To improve estimates of canopy-level light use efficiency we will measure the efficiency of photosynthesis in forests under a mixture of light conditions. The mechanisms will be identified by which photosynthesis differs under diffuse and direct irradiance dependent on leaf structure, pigmentation and optical properties. These data will be used to improve models of primary productivity in altered sunlight conditions under climate change, and increase our capacity to forecast how aerosol pollution and geoengineering with aerosols could affect plant production. This approach also lets us test how canopy structure can be manipulated to increase agricultural productivity for a given environment. By reaching a better understanding of the physiological feedbacks generated by fluctuating light conditions within the leaf and across the plant canopy, we can hope to select crop plants that will grow in canopies that more efficiently use the available light in controlled plant production environments. Knowledge of how canopy photosynthesis responds to changes in the irradiance and composition of sunlight allows us to input realistic values for carbon capture by ecosystems into those models that are the mainstay of Earth System Modelling and upon which environmental policies are based.

We imagine clear sunny days are better than hazy days for plant photosynthesis. In fact because sunlight is diffused by clouds and aerosol pollutants, they cause less light to be available at the top of plant canopies but more light to penetrate deeper. This produces more-even photosynthesis through forest or crop canopies, improving their light use efficiency. But the broader implications of changing patterns of sunlight under fluctuating sky conditions are not clear. Only by monitoring the changes in sunlight and photosynthesis in canopies in nature will we be able to improve estimates of canopy photosynthesis under diffuse sunlight in models of ecosystem functioning. Such models let us test the consequences of proposed geoengineering solutions to climate change e.g. injecting aerosols into the atmosphere to reduce warming. Understanding the role of diffuse sunlight in canopies is also useful to improve efficiency of crop production in greenhouses and fields under climate change.