Stand stability in the Swiss National Forest Inventory: assessment technique, reproducibility and relevance

Stand stability is an important criterion for evaluating the condition of f orests, but it is also quite difficult to assess. The objective of this stu dy was to evaluate the assessment technique of stand stability applied in t he Swiss National Forest Inventory (NFI). The Swiss NFI recorded various tr ee, stand and site attributes on sample plots at two inventory occasions, b etween 1983 and 1985 and between 1993 and 1995. The NFI defined the attribu te "stand stability" as the probability that severe damage will occur to th e considered stand within a fixed interval of time. So, the assessment of s tand stability in NFI can be considered a risk evaluation. The assessment t echnique is based on evaluations performed by experts following specific as sessment guidelines. First, the risk factors listed in the guidelines were rated regarding the potential hazard to the stand and then, the listed stan d attributes were rated considering the potential resistance of the stand. Finally, the attribute "stand stability" was derived from these rates. Sinc e the rating of both risk factors and stand attributes relies on the opinio n of experts, this technique runs a risk of resulting in a rather subjectiv e evaluation. This circumstance affects the interpretation of stand stabili ty and in particular, makes the verification of changes of stand stability very uncertain. An examination of the assessment technique, thus, is especi ally relevant as regards the attribute "stand stability". The study was focused on the reproducibility and relevance of this attribut e. Ln particular, it investigated the concordance between stand stability a s evaluated at the first inventory occasion and the damage occurred afterwa rds, between the first and the second occasion. It also explored whether st and stability can be related to more objective and reproducible variables. The variables providing the best explanation of stand stability were select ed by means of regression models. As regards reproducibility, the evaluations of stand stability resulting fr om two independent surveys did not differ significantly, but stand stabilit y proved to be an unprecise variable. On the other hand, stand stability as evaluated at the first inventory occasion resulted to accord with the leve l of damage occurred afterwards to the stands. Some of the variables assess ed in NFI were related to the attribute stand stability, but only weak corr elations were found. The results suggest that the attribute stand stability can be considered a valid indicator for the risk of damage, but should be used and interpreted only on certain conditions. (C) 2001 Elsevier Science B.V. All rights reserved.