Part XVII: Leaf area measurements

Leaves represent the largest proportion of the total forest canopy surface and also the main surface for physiologically active exchange with the atmosphere. Processes like photosynthetic light absorption, carbon uptake and assimilation, transpiration of water, and emission of volatile organic compounds are nearly exclusively performed via leaf surfaces, while processes like element deposition, interception of rain, evaporation, and susceptibility to wind damage are in part also dependent on the surface area of woody canopy elements. The increasing need to quantify and simulate such interactions between forest canopies and the atmosphere with models has led to a growing demand for reliable information on the surface area of leaves in the canopy. This manual part provides a guideline for measurements of leaf area index (LAI) in the framework of ICP Forests. - Numerous methods have been developed to measure LAI, including direct contact methods, passive optical methods and active remote sensing methods. This manual can only focus on a few of them that are most often used or were considered most reliable or best comparable. This second version of the manual will probably need to be updated and extended during the coming years , when more measurements and methodological comparisons are available that allow better judgement. Also the fast technological development of optical and remote sensing methods will most probably lead to necessary changes in the future. We therefore decided to add the author names of each method-related part of the manual in order to facilitate the feedback of other experts to the small team that prepared these guidelines. The most direct way to discuss items of the manual will be the discussion within the expert panel on meteorology, phenology and leaf area index.