Source of variability in Chlorophyll a fluorescence of Norway spruce (Picea abies (L.) H. Karst) trees: consequences for sampling

Forest health surveys in Europe are based on visual assessment of the reduction of the density of foliage on tree crown, termed crown transparency. This attribute has been criticized because its unclear relationship with tree physiological status and the need for more objective indicators of tree health has been frequently claimed. Chlorophyll a fluorescence of tree foliage permits to evaluate plant vitality in relation to different stressors and provides information on the status of photosynthetic apparatus of foliage of individual plants and stands. The question is therefore how to obtain reliable estimates of Chorophyll a fluorescence at tree level. Here we present the results of a pilot study carried out on Norway spruce trees in Trentino (N. Italy) under two different condition: juvenile trees growing into a forest nursery, and adult trees into a real forest. The aims were (i) to evaluate sources of variability that should be accounted for when designing a sampling strategy for Chorophyll a fluorescence and (ii) to estimate the required sampling effort.