Measuring Canopy Structure

Indirect methods

With indirect methods, we can bypass the access problems by making our observations from the ground or from above. The simplest indirect measurements are based on using our eyes, and include the use of such simple tools as a spherical crown densiometer <insert image of densiometer>. Technically, indirect methods include such techniques such as looking at the canopy through binoculars, measuring the height of a tree with a clinometer, or counting the number of sunflecks that have penetrated the canopy to a particular area of ground. (see Campbell and Norman 1989)

However, technology has been the driving force for the development of a vast array of more complex indirect methods.

One of the earliest remote sensing methods, the camera, was applied to forestry within a few decades in an attempt to capture the tree canopy. In 1887, a German forester "boarded an airship (balloon) equipped with the newest photographic apparatus. At an altitude at which he could see the entire area… he exposed his plates." (From a history in Philipson 1997).

Technological advancements since these have led to a vast array of remote sensing instruments used to map forest canopy structures. Whatever the method, the usefulness to canopy structure research will be determined by several questions:
· Are data are gathered from above the canopy or from below?
· Is the radiation energy actively emitted by the instrument or is background radiation passively recorded?
· What range of the electromagnetic spectrum is being recorded?

Indirect methods allow for the rapid acquisition of vast quantities of data, and allow the researcher to increase the spatial scale of study dramatically. For example, aerial stereo-photogrammetry can provide a three dimensional reconstruction of the canopy using simple geometric measurements, allowing reconstruction of heights. Laser altimetry, or lidar remote sensing also allows canopy height determination, but using very different technology and providing a very different type of data. There is a major catch, however. All indirect methods require the coupling of data with an appropriate radiative transfer theory so that structural characteristics can be estimated by "inverting" the data (Norman and Campbell 1989).