Design concepts adopted in long-term forest monitoring programs in Europe—problems for the future?

Long-term intensive monitoring in Europe is presently proceeding in more than 800 plots, where a number of investigations are carried out according to allegedly standardized protocols. While the potential of the program cannot be denied, certain aspects that are binding for data analysis may be a source of problems for future evaluation of program results. Here it is argued that: (i) current biological response indicators adopted by the program will not permit air pollution effects to be distinguished from effects due to other stressors and/or natural variation; (ii) the sampling strategy adopted to select monitoring sites does not enable European scale estimates of the status of attributes of interest or their changes; and that (iii) the sampling tactic suggested at plot level is ambiguous and cannot provide representative, unbiased estimates at plot scale. This latter point implies consequences when plot–level data are used in models, correlative studies and/or to infer cause–effect relationships.