Photosynthetically Active Radiation (PAR)

Before any energy can begin flowing through an ecosystem, a producer first has to capture usable sunlight — and only a narrow slice of the full solar spectrum is actually useful for this. Photosynthetically Active Radiation (PAR) is the amount of light available for photosynthesis, restricted to wavelengths between roughly 400 and 700 nanometres. PAR is not a constant, steady supply: cloud cover, shade from trees, dust and haze in the air, the season, latitude, and how many hours of daylight are available all cause it to fluctuate continuously, and plants generally make the most efficient use of the blue and red ends of this band. Of the total sunlight that reaches the top of the atmosphere, roughly 34% is reflected straight back into space, another 10% is absorbed by ozone, water vapour and other atmospheric gases, and only the remaining 56% actually reaches the Earth's surface. Even of that 56% that does arrive, green plants manage to capture only about 2–10% of it for photosynthesis — the rest is simply dissipated as heat. PAR itself is measured with silicon photovoltaic detectors tuned specifically to the 400–700 nm band, reported in millimoles per square metre per second; it falls to zero at night and can peak at 2000–3000 millimoles/m²/second around midday in summer. The chapter also introduces a related but separate idea — the different 'colours' of carbon storage (green carbon in the living biosphe …