Actions in forestry are characterized by the principles of sustainability and long-term considerations. In spite of the sometimes expressed opinion that forestry solely focuses on the productive function of forests, maintaining the multiple functions of forests has a long tradition – at least in Central Europe. Sustainability, long-term planning and multiple functions of forests are the framework for inventorying and monitoring forests.
Long-term monitoring of forests is not a new concept: more than a hundred years ago the first long-term observation plots were installed in Switzerland. Today forest monitoring has three instruments available: surveys, experiments and case studies. Surveys and experiments render statistical inference. The scope of the statistically verified results does not necessarily cover the total forest population, but depends on the underlying objective of the experiment and the sampling frame. Case studies in a statistical sense are more or less intensive observations of isolated cases, but are not planned. They may lead to important understanding of forest ecosystems, but the evidence of case studies has to be verified by experiments or surveys.
The Swiss National Forest Inventory (NFI) is a sample based survey and renders statistically sound results for all Swiss forests. The second Swiss NFI is based on the first NFI. In planning the statistical design of the second Swiss NFI some requirements had to be taken into account. The sampling methods, especially those for assessing changes, have to be applicable for the entire conditions found in Swiss forests and for all attributes of interest. The design has to be open to regional surveys as well as for sampling on successive occasions. As the budget for the second NFI had been cut by 3 Mil. Swiss francs, the cost-efficiency of the survey had to be improved. That means the survey costs had to be reduced to meet the specified level of precision. By applying two-phase sampling for stratification and by intensifying the aerial photo interpretation, the number of field plots could be reduced by almost fifty percent.
The development of methods has changed the Swiss NFI towards an instrument for long-term monitoring of Swiss forests. The results are not only of national importance, but cover many of the international commitments for monitoring the sustainability of forests.