staff in bushes

Using the Concept of Leaf Area Index in Monitoring Vegetation

by Kent Williges

researchers in bushes
Upland Habitat biologists, Bailey Piper and Tom Carney, set up a fisheye camera to photograph the canopy strata of scrub habitat on Guana River Wildlife Management Area near St. Augustine.

During the last several months, Upland Habitat biologists have been busy testing the applicability of Leaf Area Index (LAI) theory for use in vegetation monitoring in Florida. LAI is defined as the total one‐sided area of leaf tissue per unit ground surface area. It indirectly measures biomass as a function of light availability, and it can be thought of as a method of inventorying the population of leaves in a plant community that absorb sunlight. It is typically used to characterize the plant canopy, and lends itself very well to tracking changes in the canopy biomass.  All land management activities whether they include forest stand thinning, roller chopping, applying herbicide, or application of prescribed fire essentially reduce plant biomass which results in greater light penetration through the canopy strata of a given plant community.  The light availability concept has potential to be incorporated into a standardized rapid assessment vegetation/burn monitoring protocol for evaluating habitat quality that significantly improves efficiency and repeatability by nontechnical personnel.  We are currently testing several instruments that are designed to indirectly assess the LAI of plant canopies in real time by gap fraction analysis.  The basic tenet of gap fraction analysis is that canopy leaf area can be inferred from measurements of canopy gap area (the area not covered by leaves). Gap area can be inferred from canopy photographs taken with a camera equipped with a specialized fisheye lens, or by estimation of the amount of the direct solar beam that penetrates the canopy as measured by various photometers.  Currently, monitoring methods tend to be project specific, and often vary by land management entity. This makes it difficult to compare plant community or fire data collected from different regions of the state, and to compare data collected by various public agencies or the private sector.  In addition, monitoring methods are often complex and intensive, and therefore hard to use by nontechnical personnel. These types of programs essentially rely on measuring plant community structural attributes (basal area, cover, stem density, stem height, etc.) to indirectly estimate vegetation biomass, and usually require a high level of expertise by the data collectors. Our idea is to use light penetration as a substitute for many of the structural attributes used to evaluate habitat quality in Florida plant communities, if it is found they are highly correlated. A rapid assessment method that requires fewer measurements would be more efficient, cost effective, and would be easily repeatable by multiple nontechnical personnel especially on private lands.

man in bushes
Tim Carney recording direct solar penetration with a Li-Cor LAI-2200 photometer in scrub habitat at Guana River WMA.