Leaf Area Index (LAI) is a dimensionless quantity that characterizes the canopy of an ecosystem. It was defined by Watson (1947) as the total one-sided area of leaf tissue per unit ground surface area:
LAI is related with a series of significant ecosystem parameters, such as radiation extinction and the microclimatic conditions inside and below the canopy. More importantly, LAI determines the critical area that gas exchanges are performed between the canopy and the atmosphere. It is therefore a key component of biogeochemical cycles in ecosystems. Process-based ecosystem simulations, such as canopy photosynthesis models, use LAI as input parameter in order to scale-up from leaf to canopy level simulations. Large-scale mapping and monitoring of LAI is crucial in order to produce regional or even global CO2 flux estimates that could help climate change research.
LAI presents intense spatial and temporal variations that are difficult to be accurately determined. There are two kinds of methods in order to measure LAI in the field. The direct or semi-direct methods are usually based on leaf sampling and measurement of the leaf area of the samples. They are time-consuming, destructive in evergreen species and present many practical difficulties. The most common direct method is to collect leaves in traps distributed below the canopy during leaf fall, a method that can be implemented only on deciduous stands. The indirect methods are non-destructive, quick and convenient. This kind of LAI measurements are based on the transmission of radiation through the canopy, making use of the radiative transfer theory (Ross 1981).
The Laboratory of Botany implements various methods in LAI field measurements using linear PAR sensor analyzers (Photo 1), fish-eye photography (Photo 2) and litter traps (Photo 3).
Watson, D.J. (1947). Comparative physiological studies on the growth of field crops: I. Variation in net assimilation rate and leaf area between species and varieties, and within and between years. Annals of Botany, 11, 41-76.
Ross, J. (1981). The Radiation Regime and Architecture of Plant Stands. Hague: Dr. W. Junk Publishers.
Photo 1. Measuring LAI in a Quercus sp. forest with an AccuPAR LP-80 (Decagon Devices) ceptometer.
Photo 2. Measuring LAI in a Pinus nigra forest with fish-eye photography.
Photo 3. Measuring LAI in a Querqus sp. forest with litter traps.