Layer cake: on canopy gaps, leaf area and photosynthesis

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. However, some of the cameras have a different vantage point. Instead of looking down on the canopy, they look through it!

Below you see a composite image I made of 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.

canopy closure
A change in canopy transparency

One of these canopy properties is the transparency of the canopy, in part determined by canopy gaps. In the top half 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 and all space is taken up by emerging leaves crowding out the sky.

Looking upward through the canopy this lack of transparency 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 density of the vegetation above, it’s leaf area.

Leaf area, or how many leaves there are for a given surface area, therefore 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 CO2 these leaves can capture from the air and are transformed into sugars. Intuitively, one can deduce that overall a higher leaf area should yield a higher productivity. Or, the more layers to the canopy cake the better a forest potentially performs.