Measuring and modelling plant area index in beech stands.

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 shelterwood 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 interdisciplinary 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.