Int J Biometeorol (2004) 48:192–201 DOI 10.1007/s00484-004-0201-y ORIGINAL ARTICLE T. Holst · S. Hauser · A. Kirchgäßner · A. Matzarakis · H. Mayer · D. Schindler Measuring and modelling plant area index in beech stands Received: 14 January 2003 / Accepted: 25 November 2003 / Published online: 29 January 2004  ISB 2004 Abstract For some beech (Fagus sylvatica L.) stands with different stand densities the plant area index (PAI) was measured by means of a Licor LAI-2000 plant canopy analyser. The stands are located on the slopes of a valley in south-west Germany and had been treated by different types of silvicultural management (heavy shel- terwood felling, light shelterwood felling, control plot). The analyser was used (a) to investigate the light conditions on plots of the same thinning regime, (b) to quantify the differences between the different treatments and (c) to obtain absolute values of PAI for interdisci- plinary research. PAI was measured at three different phenological stages (leafless, leaf-unfolding and fully leafed season in 2000) and was found to be about 5.2 for the fully developed canopy on the control plots, 3.2 on the light fellings and about 2.0 for the heavy fellings. In the leafless period PAI was between 1.1 (control) and 0.4 (heavy felling). Measurements made in summer 2000 and summer 2002 were compared, and showed an increase of PAI, especially on the thinned plots. Measurements of photosynthetically active radiation (PAR) above and below the canopy in combination with measured PAI were used to apply Beer’s Law of radiation extinction to calculate the extinction coefficient k for different sky conditions and for the different growing seasons on the control plots. The extinction coefficient k for the beech stands was found to be between 0.99 and 1.39 in the leafless period, 0.62 to 0.91 during leaf unfolding and between 0.68 and 0.83 in the fully leafed period. Using PAR measurements and the k values obtained, the annual cycle of PAI was modelled inverting Beer’s Law. Keywords Plant area index · LAI-2000 · Fagus sylvatica L. · Photosynthetically active radiation · Beer’s Law Introduction The canopy of a forest has a significant impact on the forest’s microclimate. The crown layer has a strong influence on the radiation regime within a stand (e.g. Geiger et al. 1995; Baldocchi et al. 1984a, b; Hutchison and Matt 1977). In the canopy layer, part of the incident short-wave radiation is used for photosynthesis, while some radiation is transmitted through the leaves or reflected by the canopy. Since short-wave radiation is the most important energy input, this has a strong impact on air and soil temperatures, humidity levels and evap- otranspiration, defining the growing conditions for the understory and the forest stand itself (e.g. Grimmond et al. 2000; Morecroft et al. 1998; Geiger et al. 1995; van Eimern and Riedinger 1986). Therefore information about the structure of the canopy of a forest is not only necessary to address micrometeorological questions, but also for the analysis of structures and processes in other disciplines like forestry, biology or soil science. A widely used method to describe the amount of foliage is the plant area index (PAI) when referring to all light blocking elements (stems, twigs, leaves), or the leaf area index (LAI) when accounting for leaves only (López- Serrano et al. 2000; Meir et al. 2000; Nackaerts et al. 2000; Ross et al. 2000; Levy and Jarvis 1999). Though PAI and LAI are dimensionless they can be thought of as m 2 (one-sided) foliage per m 2 ground area. For the determination of a stand’s PAI or LAI, there are direct and indirect techniques: while harvesting of the whole canopy or some samples of the vegetation is destructive and laborious, taking samples of litter is non- T. Holst ( ) ) · A. Kirchgäßner · A. Matzarakis · H. Mayer · D. Schindler Meteorological Institute, University of Freiburg, Werderring 10, 79085 Freiburg, Germany e-mail: thomas.holst@meteo.uni-freiburg.de Tel.: +49-761-2033590 Fax: +49-761-2033586 S. Hauser Institute for Forest Growth, University of Freiburg, Tennenbacherstraße 4, 79085 Freiburg, Germany Present address: A. Kirchgäßner, Meteorological Institute, University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany