Layer cake: on canopy gaps, leaf area, and photosynthesis

By now you are familiar with PhenoCams. However, a few extraordinary cameras exist in the network. In general, PhenoCams track seasonality and the structure of the vegetation in a top-down fashion, mostly looking down onto the the top of the canopy or vegetation. Yet, some of the cameras have a different vantage point. Instead of looking down on the canopy, they look up through it!

Below you see a composite image made of images from the upward-looking Harvard Forest Barn Tower camera. The top half of the composite was taken at the beginning of May while the bottom half was taken at the end of May, after leaf out. This split image not only shows leaf unfolding throughout the month of May, it also illustrates how this affects other canopy properties during spring.

One of these canopy properties is the transparency or openness of the canopy, which is determined, in part, by gaps in the canopy. In the top half of the composite, the canopy is more or less wide open and the sky can be seen fairly easily. In the bottom half, after leaf out, very little of the sky is still visible, because all the space is taken up by emerging leaves crowding out the sky.

Looking upward through the canopy, this lack of sky illustrates how different trees in the canopy optimally use all available light for photosynthesis. Although not all leaves will be sitting at the same height in the canopy, and some might be shaded by others at times, the transparency of the canopy is roughly proportional to the height of the trees and the density of the vegetation: its “leaf area”. More formally, we can represent the canopy according to the Beer–Lambert–Bouguer law which describes the attenuation of light through a medium — in this case, a canopy. The attenuation of light within our images is therefore directly coupled to a particular leaf area.

Leaf area, or how many leaves there are for a given surface area, provides an idea of how many layers there are in the layer cake which is a forest canopy. Leaf area is also related to the productivity — or how much carbon dioxide these leaves can capture from the air and are transformed into sugars. Intuitively, one can deduce that a higher leaf area should yield a higher productivity. Or, the more layers to the canopy cake the better a forest potentially performs.

In short, measuring the transparency of the canopy provides us with additional information on canopy development and structural characteristics which can be related more directly to photosynthesis and are not provided by the downward looking PhenoCams.


About Koen Hufkens

I’m a research associate at Harvard University where my time is divided between modelling plant phenology across various biomes in response to climate change, being the lab geek and intrepid open source hardware and software developer.
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