Canopy Structure and Environment

Light

The reduction in light with canopy depth is one of the most obvious and profound effects of the forest overstory on associated understory communities. Studies of understory plants often focus on light as a major resource limitation; forest floors typically receive less than 5% of full sunlight, and often closer to 1% (e.g. Chazdon and Fetcher 1984, Hadley 2000). More than 80% of the high-energy shortwave radiation is typically absorbed by canopy leaves; the remainder is transmitted downward through the canopy or reflected back up (Parker 1995). Longwave radiation, in contrast, is nearly all transmitted or reflected.

The spectral quality of light as well as its total abundance is greatly altered as it passes through a canopy, with major reductions in the blue, red and far-red wavelengths. A large proportion of the light harvested by understory plants comes from sunflecks, which are very brief and localized but spectrally similar to direct sunlight (Chazdon and Pearcy 1991).

Figure 1. Horizontal light map of a mixed-conifer forest plot, generated by the spatially explicit light model tRAYci. From Gersonde and O'hara (2001), http://www.cnr.berkeley.edu/~gersonde/light_poster.html.

 

Figure 2. Vertical light map of a mixed-conifer forest plot, generated by the spatially explicit light model tRAYci. From Gersonde and O'hara (2001), http://www.cnr.berkeley.edu/~gersonde/light_poster.html. Color coding is the same as in Figure 2.

Light quality and quantity are closely related to canopy structure, and as a result may vary over horizontal and vertical space as well as through time. Figures 1 and 2 illustrate typical horizontal and vertical light variation respecively, taken from a mixed-conifer forest in the Sierra Nevadas (Gersonde and O'hara 2001). Horizontal light variation arises from large-scale changes in stand composition and density (e.g. Kato and Komiyama 2002), with the formation of canopy gaps playing an influential role (e.g. Canham 1988). Light availability along the vertical axis features a gradual extinction with canopy depth (Figure 3), although the shape of any particular extinction curve is determined by local foliage distribution; Figure 2 is a good example of how much light profiles can vary at a local scale. Short-term temporal variation is most notable in deciduous forests due to seasonal leaf loss (Kato and Komiyama 2002), while long-term trends may be linked to successional changes in forest type.

Figure 3. Vertical light transmittance profiles based on direct versus remote measurements. Data were collected from an old-growth Douglas fir forest in the Cascade mountain range in Washington, in the western United States. "Field transmittance" was determined using a Li-Cor quantum sensor mounted on the platform of a tower crane. "SLICER transmittance" is based on remote sensing data from a Scanning Lidar Imager of Canopies (SLICER) laser altimeter. Image is from Parker et al. (2001).

Light distribution and availability is fundamental to many forest ecosystem processes, and accurate models and measurements are therefore of great importance to forest ecology. Light is not easy to measure, however. Not only are its vertical gradients and horizontal patterns complex and variable, but measurement techniques are often unwieldy (i.e., crane- or balloon-mounted quantum sensors for vertical profiles; Figure 4), expensive (i.e., remote sensing techniques) or dificult to use (i.e., hemispheric photography). Nevertheless, substantial progress has been made in the past few decades, and models of light dynamics continue to become more precise, accurate, and obtainable.

Figure 4. Canopy Crane at the Wind River research facility, Washington state. From: http://depts.washington.edu/wrccrf/