Leaf Area Index - LAI

Definition

Leaf Area Index (LAI) is defined as the one sided green leaf area per unit ground area in broadleaf canopies, or as the projected needleleaf area per unit ground area in needle canopies.

Information

The interaction between vegetation surface and the atmosphere, e.g. radiation uptake, precipitation interception, energy conversion, momentum and gas exchange, is substantially determined by the vegetation surface (Monteith and Unsworth, 1990). During vegetation period of deciduous trees, total vegetation surface itself is mainly composed of leaf area, and by lesser part of twigs, branches and stem surface. Whereas during times of absent foliage (i.e. winter in temperate climates, dry season in tropical areas) woody parts determine vegetation surface area.

Various destructive and non-destructive methodologies to measure or derive LAI do exist. Non-destructive methods include hemispherical photography, sunfleck ceptometers, and other optical instruments like TRAC, LAI-2000 or LI-COR (detailed description of techniques are presented by Chen et al. 1997).

Calibration and correction factors have to be determined for all optically-based methods. Direct litterfall measurements with littertraps are commonly used as well as completely harvesting whole trees to derive appropriate values and set up fitting functions to calibrate their optical methods.

It has to be taken into account that all methods do have advantages as well as disadvantages in estimating LAI and data are not always directly comparable.Espacially older literature tends to contain higher values of LAI than later references (see Technical Memorandum of Scurlock et al. 2001). The annual course of LAI for deciduous trees peaks during the height of growing season, whereas LAI of coniferous stands vary far less over the year. Some deciduous trees keep old leaves till next seasons budding whilst other species completely shed their branches. LAI of plants, especially grasses, consists of photosynthetically active green and senescent leaves. Even though old leaves do not influence photosynthesis, they still play an important role in intercepting precipitation. Therefore, as in the case of modelling water interception, a LAI greater than zero has to be maintained throughout the year for forests and pasture species in contrast to agricultural sites, which start with a LAI of zero after ploughing. Information on this lower bound of LAI, here referred to as minimum LAI or LAImin are relatively scarce. Published LAImin for grassland species are in a range between 0.3 to 2.0. Measurements of LAI in winter times for Fagus sylvatica or assessments of area indices of branches and stems for Populus tremoluides reveal a highest LAImin of 1.1. Area index of woody parts of coniferous trees can be assumed to be around 0.5, covering a range of 0.2 to 0.9. (Breuer et al. 2002). As some eco-hydrological models need information on LAI of whole stands, a surcharge for understory and litter has to be added to the LAI of trees to obtain the integrated total LAI of a given forest stand. We therefore listed values for understory as well as shrubs and woodland so modellers can sum up the needed LAI for their purpose. On average we assume that a surcharge for understory vegetation and litter of approx. 2.0 should be added to total LAI of deciduous and coniferous forests stands. Treatments like fertilization of pasture or thinning of trees might have a strong effect on LAI. Narrow plant spacing compared to broad plant spacing in crop stands leads to rising LAI, whereas thinning in deciduous forest reduced LAI. References Breuer L, Eckhardt K, Frede H-G, 2003. Plant parameter values for models in temperate climates. Ecol Model 169, 237-293. Chen JM, Rich PM, Gower ST, Norman JM, Plummer S, 1997. Leaf area index of boreal forests: Theory, techniques, and measurements. J Geophys Res 102, 29429-29443. Monteith JL, Unsworth MH, 1990. Principles of environmental physics. Edward Arnold, London, 291 pp. Scurlock JMO, Asner GP, Gower ST, 2001. Worldwide Historical Estimates and Bibliography of Leaf Area Index, 1932-2000. ORNL Technical Memorandum TM-2001/268, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A. Links Global Leaf Area Index Data Published estimates of LAI, covering the period 1932-2000, compiled at ORNL DAAC (Oak Ridge National Laboratory Distributed Active Archive Center)