In the era of global warming and endangered ecosystems, we need a better understanding of how forests and vegetation at large grow.
Vegetation is fundamentally affected by light, which is essential for photosynthesis and plant ‘breathing’. Light transmission through canopy affects forest productivity. Among other things, the transmission of light, or radiation, is controlled by leaf orientation. That is why it is important to measure the leaf angle distribution.
So far, it has been a headache to measure the leaf angle distribution, particularly for trees. Jan Pisek from Tartu Observatory and his colleagues from Harvard University and Université de Montréal have developed and tested a cost-efficient method that can be used virtually by anyone: all you need is a digital camera or a phone equipped with a camera and freely available image processing software.
Jan has created a Lego stop-motion animation to describe the measuring of leaves in an attractive and easy-to-follow way (turn on the subtitles):
Transcript of the animation
Directional distribution of leaves is about how tree leaves are oriented: for example, are they rather horizontal or do they point straight upward? This is the primary parameter needed to predict light transmission through the canopy.
Namely, leaves absorb a different amount of light depending on their orientation. With the sun right up above in the sky, leaves pointing straight upward would expose a smaller area towards the sun and absorb far less light than if they were horizontal. Leaf angles also affect leaf temperature, gas exchange, and the efficiency of their water use.
The thing is that with trees we cannot easily use the leaf angle measurement methods that were originally developed and worked for crops. In the 1960s, scientists including A.A. Nichiporovich, Kees De Wit, Juhan Ross, and Tiit Nilson estimated leaf angles using clinometers and other instruments that were placed directly on the leaf.
With trees, it is not so easy, because we are here, and the leaves are there. So how we can do it? We can use cameras.
The original idea came from Oliver Sonnentag. The approach consists of using a leveled digital camera at several height levels of a canopy. Different levels can be accessed using ladders, platforms, or by climbing towers that are run and analysed by researchers, including Dennis Baldocchi of Berkeley or Timo Vesala from Helsinki.
Once up there, series of photographs of the surrounding canopy are taken at each level. Once we download the images, we inspect them for the presence of leaves oriented approximately perpendicular to the viewing direction of the camera. The leaf angles of the selected leaves are then estimated using the angle measurement tool or scripts inside a public domain image processing software called ImageJ.
Fortunately, we do not have to measure all the leaves out there. With less than 100 leaves, we can obtain a fairly accurate directional distribution of leaves at the whole tree level. Results indicate that using digital photography for measuring leaf angles can indeed deliver equivalent results to manual sampling for various ecosystems.
The camera approach offers several practical advantages over other existing methods. Firstly, the method is simple and much more affordable compared to the costs of 3-dimensional digitizers or ground-based LiDAR systems. Secondly, manual sampling with a clinometer requires direct access to the canopy; the camera method is less restrictive. Thirdly, the recorded images can be stored on a computer for later review.
The animation enjoys an international audience. For instance, it is currently used in the course, ‘Biometeorology: Plant-Ecosystem-Atmosphere Interactions’, taught at the University of California, Berkeley, USA, by Prof. Dennis Baldocchi, the world-leading scientist in the field of experimental and theoretical studies of the physical, biological, and chemical control of trace gas exchange between vegetation and the atmosphere.
Jan Pisek is currently looking for bachelor’s- and master’s-level students to participate in the project. If you would like to work in the field of remote sensing or Earth observation in general, please contact Jan at firstname.lastname@example.org. Chances are you might appear in the follow-up Lego animation as well!
Inga Külmoja is an author and the editor of the UT Blog.