Canopy Structure

Arielle Cooley and Michael Wolosin

Canopy research is a relatively new science. Ecologists necessarily focus on things that they can reach, and for a long time that included very little above head height. Canopies possessed mystique but also a bothersome degree of inaccessibility.

The first published canopy data (Allee 1926) comprise a series of physical measurements made in a tropical rainforest in Panama. In 1926, and for several decades following, the mere feat of getting into the canopy with sufficient equipment to measure something was notable in itself. A steel tower, erected in a Ugandan forest in the 1950s, and the first aerial walkway ten years later, in Malaysia, foreshadowed current methods of access but remained isolated novelties for quite some time (Sutton 2001).

Figure 1. Canopy walkways. Walkways are one of the few means of providing researcher access to canopies. This one is from http://www.treemail.nl/kronendak/overview.htm

In the 1970s and 1980s canopy research expanded rapidly. Towers and walkways proliferated. New methods were developed, such as fogging and Single Rope Technique (Sutton 2001). All of the available methods, however, were laborious and costly and often dangerous. Many publications focused on the achievement of accessing the canopy, and observational studies with small sample sizes and little or no replication predominated.

Figure 2. Canopy access via hot air balloon. From the University of Victoria Entymology Homepage, http://web.uvic.ca/~canopy/index.htm .

Technological developments in the 1990s permitted increasingly detailed and sophisticated analyses of canopy composition and structure. Variation in horizontal, vertical and temporal dimensions were explored using remote sensing, computer modelling, research towers within forests, and improved ground-based tools for measuring relevant ecological variables. As a result, we now have a working knowledge of many important aspects of canopy structure.

In the long run, however, canopy structure in and of itself will hold little interest for most ecologists. Rather, it is of interest because it is the living template for many of the most important processes in forest ecology. These processes span the range of ecological sub-disciplines. Physiological ecologists may think about light interception, photosynthesis, and the heat balance of a leaf; population biologists might view the canopy as habitat for a species of interest; a community ecologist may ask if complex structure in the canopy is necessary to maintain diversity; a landscape ecologist might model patches of canopy with varying degrees of connectivity; and an ecosystem ecologist might ask how the structure of the canopy affects carbon exchange with the atmosphere.

Regardless of the ecological scale of study, there are some key questions a researcher should ask herself when investigating canopy structure (Table 1 from Bongers 2001):

	• What do you want to know about structure (sensu latu)? Structure as a subject to describe, or structure as a correlate for other aspects, e.g., as a basis for epiphyte studies?
	• At what spatial scale do you want to work (e.g., at the local scale of a tree, or at a regional scale of a forested landscape)?
	• What do you define as components making up the structure (e.g., leaves, flowers, individuals, forest patches)?
	• Is the spatial 3-D location of the elements important?
	• Is canopy structure conceived as the occupied or as the empty space in the forest canopy (e.g., the empty space is available for colonisation)
	• At which levels of organisation are the components determined (e.g., leaves at the level of individuals, species, life forms, forest patch, forest)?
	• Do you want to follow structure over time (canopy dynamics)?
	• Do you depart from physical structure of from functional structure?
	• Do you conceive canopy structure as habitat for e.g., smaller plants or animals?

Here we present an overview some important principles of current canopy research, and consider in more detail a few of the ecological processes that have thus far been studied in the context of canopy structure.