Posted by: oikosasa | June 18, 2013

Fish eye view of forest canopies

 “In discussing the peculiar type of refraction which occurs when light from the sky enters the surface of still water, it seems of interest to ascertain how the external world appears to the fish.” With these words renowned physicist R.W. Wood, Professor of Experimental Physics at Johns Hopkins University and proud owner of one of the firsthome aquariums, started his 1906 article “Fish-Eye Views and Vision Underwater“. The article was set to offer a scientifically based description of how a fish might view the world outside his glass tank.

Fish eye 1Even with all his intellectual curiosity and intuitiveness, Prof. Wood probably could not have imagined that decades later a modern descendant of the water camera he had once designed would be balanced on tripods in forests around the world. Nor could he have envisioned that it would soon become the standard field instrument for characterization of canopy structure and light regimes of forest ecosystems.

But let’s take a step back to understand how the fish got to view the forest.

The phenomenon Prof. Wood exploited in his experiment is governed by Snell’s law. Dating back to the 17th century, Snell’s law also known as Snell’s window is a phenomenon by which an observer looking up from beneath the water sees a perfectly circular image of the entire above-water hemisphere—from horizon to horizon. This is caused by refraction, light bending as it travels from one medium (air) to another (water). As argued by Prof. Wood, the cone of light entering the fish’s eye has an aperture of about 96°, but the rays within it came originally from a cone of 180°. This is the same phenomenon by which a fisheye lens (or hemispherical lens) is able to reach far to the sides of a scene and pull in the visual information of the entire hemisphere onto a plane.

The first practicable methods of hemispherical photographs were developed in 1924 shortly after Dr. R. Hill developed the first fisheye camera for cloud survey records and formation studies. During mid the 50s two ecologists, G.C. Evans and D.E. Coombe from the Botany School of the University of Cambridge, learned that one of these ingenious fisheye cameras had survived the war. Shortly after, they were standing under the dense shade of a small tree of Napoleona vogelii situated in Oil Palm bush near Ibadan, Nigeria. Of course – taking hemispherical photographs.

In 2007 another camera equipped with a fisheye lens was pointing upward to the sky in the canopy of yet another tropical forest, this time in Taita Hills South-East Kenya, where Alemu Gonsamo and colleagues from the University of Helsinki were conducting an extensive measurement campaign for the remnant cloud forest fragments.  Gonsamo and colleagues did not have to face many of the technological shortcomings Evans and Coombe were confronted with just half a century earlier. At that time hemispherical photograph analysis required tedious manual overlaying of sky quadrants and superimposing the track of the sun. With the advent of personal computers and with the replacement of film cameras by digital cameras, researchers are now able to develop digital analysis techniques (link here) and today various commercial and non-commercial software programs have become available for rapid hemispherical photograph processing and analysis. Yet many fundamental issues remain to be resolved.

The resulting hemispherical photographs serve as a permanent record of the canopy geometry, which can be precisely used to characterize canopy structure and light regimes. Canopy structural parameters are critical to adequately represent vegetated ecosystems for purposes ranging from primary productivity, climate change studies, water and carbon exchanges, and radiation extinction. However, as observed by Gonsamo and co-authors, standardization in the definitions of the fractional canopy cover and openness parameters has fallen short, leading to confusion of terms and concepts even in standard text books, making the comparison of historic measures futile.

Fish eye2

In the Oikos Early View paper Measuring fractional forest canopy element cover and openness–definitions and methodologies revisited Alemu Gonsamo and colleagues take an exciting tour, reviewing concepts, polishing up definitions, and presenting new methodologies to obtain large scale fractional canopy element cover and openness measures using photographs with a fisheye view perspective. Finally, in their Oikos paper, Gonsamo and colleagues argue that hemispherical photography is less time, labour and resource intensive, as compared to the traditional point based measuring techniques of canopy element cover and openness. This included measurements in topographically complex terrains.


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