Sources of errors in measurements of PAR

Ross, Juhan; Sulev, Madis

doi:10.1016/s0168-1923(99)00144-6

Cited by 143 publications

(78 citation statements)

References 44 publications

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“…The January to November mean value (0.41) of PAR inc. /S inc. is quite close to the corresponding values found by Szeicz (1974), Ross (1975), Stanhill and Fuchs (1977), Baldocchi et al (1984), Ross and Sulev (2000) and Möttus et al (2001). The September mean value (0.14) of PAR′ inc. /S′ inc. is much lower compared with the value (0.24) found by Baldocchi et al (1984) for the Oak Ridge forest at a similar latitude (although with taller trees (17 m) and a higher LAI (4.9)), indicating that the depletion of the global solar radiation in the PAR region is more intensive in the Greek oak forest.…”

Section: Absorptivitysupporting

confidence: 87%

Optical properties affecting the radiant energy of an oak forest

Liakatas

Proutsos

Alexandris

2002

Meteorological Applications

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The amount of solar radiation absorbed, reflected or transmitted by vegetation depends on the radiation distribution and the canopy properties. This study of the optical properties of an oak forest under foliated and non‐foliated conditions is attempted in order to better understand radiation use and exchange. Canopy absorptivity, reflectivity and transmittivity to global solar and photosynthetically active radiation (S and PAR, respectively) are considered. To estimate the forest optical properties, radiation measurements above and below the canopy were taken in the middle of a 0.97 km2 natural, slightly sloped, 50‐year‐old, oak forest in the Northern Peloponese, Greece. It was found that the greatest part of the incident S is absorbed by the foliated or non‐foliated canopy. In the first case, absorptivity is close to 0.7, with reflectivity and transmittivity sharing the remaining portion. Before leaf appearance, although reflectivity may be only a little lower, transmittivity is considerably higher, allowing more than 40% of the incoming radiation to reach the ground. PAR is absorbed to an even higher degree by the green elements of the canopy, allowing only a 5% radiation loss, via transmission, since canopy reflectivity is practically zero. The ground contribution to the canopy short‐wave radiation balance is small during the dense foliation period, but it becomes considerable during the leafless period. Especially when considering the balance in the PAR region, the ground has no effect. Copyright © 2002 Royal Meteorological Society.

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Section: Absorptivitysupporting

confidence: 87%

Optical properties affecting the radiant energy of an oak forest

Liakatas

Proutsos

Alexandris

2002

Meteorological Applications

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“…where q is a conversion factor between photosynthetically active radiation and total solar radiation, approximated as 0.5 [97].…”

Section: Discussionmentioning

confidence: 99%

“…For the hourly estimates of the canopy resistance, r c , we used the same sequence of equations [95][96][97][98][99][100][101] that was used for instantaneous calculations. Calculated E Tr values from both daily and hourly data were then averaged over the entire year.…”

Section: Discussionmentioning

confidence: 99%

Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?

Kirschbaum

McMillan

2018

Curr Forestry Rep

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Plant transpiration is a key component of the terrestrial water cycle, and it is important to understand whether rates are likely to increase or decrease in the future. Plant transpiration rates are affected by biophysical factors, such as air temperature, vapour pressure deficits and net radiation, and by plant factors, such as canopy leaf area and stomatal conductance. Under future climate change, global temperature increases, and associated increases in vapour pressure deficits, will act to increase canopy transpiration rates. Increasing atmospheric CO 2 concentrations, however, is likely to lead to some reduction in stomatal conductance, which will reduce canopy transpiration rates. The objective of the present paper was to quantitatively compare the importance of these opposing driving forces. First, we reviewed the existing literature and list a large range of observations of the extent of decreasing stomatal conductance with increasing CO 2 concentrations. We considered observations ranging from short-term laboratory-based experiments with plants grown under different CO 2 concentrations to studies of plants exposed to the naturally increasing atmospheric CO 2 concentrations. Using these empirical observations of plant responses, and a set of well-tested biophysical relationships, we then estimated the net effect of the opposing influences of warming and CO 2 concentration on transpiration rates. As specific cases studies, we explored expected changes in greater detail for six specific representative locations, covering the range from tropical to boreal forests. For most locations investigated, we calculated reductions in daily transpiration rates over the twenty-first century that became stronger under higher atmospheric CO 2 concentrations. It showed that the effect of CO 2 -induced reduction of stomatal conductance would have a stronger transpiration-depressing effect than the stimulatory effect of future warming. For currently cold regions, global warming would, however, lengthen the growing seasons so that annual sums of transpiration could increase in those regions despite reductions in daily transpiration rates over the summer months.

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“…The technology and the bandwidth of the HOBO UA-002 is similar to that of the LI-COR LI-200SA Pyranometer Sensor with a 150-1200-nm wavelength range. Ross and Sulev (2000) compared the LI-200SA data with the more accurate light irradiance measurements obtained by the LI-COR LI-190SA Quantum Sensor, an accurate silicon photodiode covered with a visible bandpass interference filter and a coloured glass filter. They found that the LI-200SA was underestimating light irradiance by up to 20% in high-LAI forests (including reference data from a rainforest understory).…”

Section: Ground Measurement Of the Micro-environmentmentioning

confidence: 99%