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UCL Home  /  Geography  /  News & Events  /  News  /  News Archive  /  May 2013  /  Canopy structure claims cause controversy

Canopy structure claims cause controversy

Lively responses to National Academy of Science paper

Canopy structure claims cause controversy

Research involving Professor Philip Lewis and Dr. Mat Disney from UCL Geography, and led by Prof. Yuri Knyazikhin from Boston University, has provoked active debate over how sunlight reflected from vegetation is related to canopy structure and biochemistry. Understanding this relationship allows us to use satellite measurements to infer the status of vegetation on global scales and help understand feedbacks between vegetation and climate.

(See December 2012 News: Forest leaves rich in nitrogen do not help to cool the atmosphere)

Various studies have found apparent strong positive correlations between reflected sunlight and leaf nitrogen, a key indicator of plant status. However, a recent paper in the Proceedings of the National Academy of Science (PNAS) by Knyazikhin et al. (including Lewis and Disney) showed that these apparent correlations were almost certainly not due to leaf nitrogen, and could be explained by vegetation structure - variations in leaf and crown size and arrangement (1).

The paper has been hailed as an important step in extending understanding of the canopy signal (2), providing a new method for deriving leaf information from satellite measurements. This interpretation has nevertheless sparked robust debate. Some have argued that the reflectance/nitrogen correlations are real and signify an (as yet unexplained) evolutionary relationship between structure and nitrogen (3, 4).

Knyazikhin, Lewis, Disney et al. have responded (5, 6) by emphasising that existing physical theory already explains the observations very well, so that evidence to support more complex explanations will need to be much stronger.

1: Kynazikhin et al. (2012) Hyperspectral remote sensing of foliar nitrogen content. Proceedings of the National Academy of Science, 110(3), E185-E192 (PDF from http://www2.geog.ucl.ac.uk/~mdisney/papers/PNAS-2012-Knyazikhin-1210196109.pdf).

2: Ustin, S. (2012) http://www.pnas.org/content/110/3/804.full.pdf

3: Townsend et al. (2013) http://www.pnas.org/content/110/12/E1074

4: Ollinger et al. (2013) http://www.pnas.org/content/early/2013/05/10/1304176110.full.pdf+html

5: Kynazikhin, Y. Lewis, P., Disney, M. I. et al. (2013) www.pnas.org/cgi/doi/10.1073/pnas.1301247110

6:Kynazikhin, Y. Lewis, P., Disney, M. I. et al. (2013) http://www.pnas.org.libproxy.ucl.ac.uk/content/early/2013/05/10/1305930110.full.pdf+html


Image

Canopy structure on Fraser Island, Queensland


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