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Title: Radiative transfer modeling of direct and diffuse sunlight in a Siberian pine forest
Authors: Alton, P. B.
North, P.
Kaduk, Jörg
Los, Sietse O.
First Published: 8-Dec-2005
Publisher: American Geophysical Union (AGU)
Citation: Journal of Geophysical Research, 2005, 110, D23209.
Abstract: We have expanded the Monte Carlo, ray-tracing model FLIGHT in order to simulate photosynthesis within three-dimensional, heterogeneous tree canopies. In contrast to the simple radiative transfer schemes adopted in many land-surface models (e.g., the Big Leaf approximation), our simulation calculates explicitly the leaf irradiance at different heights within the canopy and thus produces an accurate scale-up in photosynthesis from leaf to canopy level. We also account for both diffuse and direct sunlight. For a Siberian stand of Scots pine Pinus sylvestris, FLIGHT predicts observed carbon assimilation, across the full range of sky radiance, with an r.m.s. error of 12%. Our main findings for this sparse canopy, using both measurements and model, are as follows: (1) Observationally, we detect a light-use efficiency (LUE) increase of only ≤10% for the canopy when the proportion of diffuse sky radiance is 75% rather than 25%. The corresponding enhancement predicted by our simulations is 10–20%. With such small increases in LUE, our site will not assimilate more carbon on overcast days compared to seasonally equivalent sunny days; (2) the scale-up in photosynthesis from top-leaf to canopy is less than unity. The Big Leaf approximation, based on Beer's law and light-acclimated leaf nitrogen, overpredicts this scale-up by ≥60% for low sky radiance (≤500 μmolPAR m−2 s−1); (3) when leaf nitrogen is distributed so as to maximize canopy photosynthesis, the increase in the canopy carbon assimilation, compared with a uniform nitrogen distribution, is small (≅4%). Maximum assimilation occurs when the vertical gradient of leaf nitrogen is slightly shallower than that of the light profile.
DOI Link: 10.1029/2005JD006060
ISSN: 0148-0227
Type: Article
Rights: This paper was published as Journal of Geophysical Research, 2005, 110, D23209. Copyright © 2005 American Geophysical Union. It is also available from the publisher's website at Doi: 10.1029/2005JD006060
Appears in Collections:Published Articles, Dept. of Geography

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