Sunflecks are brief increases in

solar irradiance Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ( ...

that occur in understories of an ecosystem when sunlight is able to reach the ground directly. They are created as moving leaves or branches periodically open gaps in the

canopy Canopy may refer to: Plants * Canopy (biology), aboveground portion of plant community or crop (including forests) * Canopy (grape), aboveground portion of grapes Religion and ceremonies * Baldachin or canopy of state, typically placed over an a ...

, or as the sun's position shifts during the day. Although each sunfleck lasts only seconds or minutes, they can deliver more than 80% of the

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

s reaching understory leaves and account for up to 35% of daily

carbon fixation Biological carbon fixation, or сarbon assimilation, is the Biological process, process by which living organisms convert Total inorganic carbon, inorganic carbon (particularly carbon dioxide, ) to Organic compound, organic compounds. These o ...

. This makes them an essential energy source for shade plants. How much energy an individual sunfleck supplies depends on its duration, size, shape and the intensity of

photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more ...

(PAR), which in turn is set by canopy structure and solar angle. The frequency and brightness of sunflecks vary widely within and among ecosystems, but generally decline as tree height and

leaf area index Leaf area index (LAI) is a dimensionless quantity that characterizes plant Canopy (forest), canopies. It is defined as the one-sided green leaf area per unit ground surface area (''LAI = leaf area / ground area, m2 / m2'') in Broad-leaved tree, bro ...

rise. The systematic study of sunflecks began in the 1920s, when botanists working in Panamanian rainforests first quantified their carbon value. Progress was slow until portable canopy-light recorders and standardised gap-analysis methods became available in the early 1970s; the field then expanded rapidly once fast CO₂/H₂O analysers in the 1980s made second-by-second

gas exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a b ...

measurements practical. These advances revealed that transient light is processed differently from steady light: when a fleck arrives,

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

accelerates only as quickly as

rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the light-independent (or "dark") part of photosynthesis, including the carbon fixation by wh ...

can be activated and

stomata In botany, a stoma (: stomata, from Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange between the internal air spa ...

can open, processes that together create an "induction lag". Conversely, the biochemical machinery shuts down more slowly once shade returns, meaning some carbon is fixed after the fleck ends, though the net gain remains small in deep shade. Comparative work indicates that shade-tolerant trees often gain induction more quickly, and lose it more slowly, than early- successional species, yet large cross-taxon surveys find no simple link between shade tolerance and induction speed. Sunfleck use is therefore context dependent, shaped by leaf age, canopy position and even the time of day.

Abiotic stress Abiotic stress is the negative impact of non-living factors on the living organisms in a specific environment. The non-living variable must influence the environment beyond its normal range of variation to adversely affect the population performan ...

es further modulate the outcome: high leaf temperatures, water deficits and sudden PAR spikes can all diminish the realised benefit, while drought restricts stomatal conductance more in shade leaves than sun leaves, stretching the lag. Despite these complexities, models that ignore induction typically over-estimate daily understory photosynthesis by up to about 40%, demonstrating the need for dynamic light response routines. Research is extending these insights to entire canopies and to global change scenarios, where shifts in tree height,

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, parti ...

density and storm frequency could alter fleck patterns; resolving such issues remains a major frontier for canopy-carbon modelling.

References

{{Reflist, refs= {{Cite journal , last1=Chazdon , first1=R.L. , author-link=Robin Chazdon , last2=Pearcy , first2=R.W. , year=1991, title=The importance of sunflecks for forest understory plants , journal=BioScience , volume=41 , issue=11 , pages=760–766 , doi=10.2307/1311725 , jstor=1311725 {{cite journal , last1=Pearcy , first1=R W , last2=Way , first2=D A , year=2012 , title=Two decades of sunfleck research: looking back to move forward , journal=Tree Physiology , volume=32 , issue=9 , pages=1059–1061 , doi=10.1093/treephys/tps084, pmid=22977203 Forest ecology