Main goal of the paper is to present a new concept for stress in trees – eustress, which the authors define as a set of years with low stem growth, caused by unfavorable ecological impacts. The developed methodology for identification and assessment of eustress (SPPAM application) is demonstrated by analysis of 325 beech chronologies from 10 locations in Europe for the period 1904 - 2007. Three groups of locations are distinguished on the base of average tree ring width of the model sequences. In order to find eustress years the tree ring width sequences are approximated with polynomials of mainly 7th degree and R2 in the range 0.55 - 0.89. Then the index (It) is computed for every year as a ratio between measured and approximated value. Years where It is lower than a threshold value, the authors categorize as stress ones. The obtained inverse correlation between It and altitude is described by a linear regression and a power function. An inverse linear regression between It and the negative deviation of precipitation from the climate norms (dP) also exists for 60% of the studied locations. The specified in the study climatic and biological climatic year for a certain location and year, show significant differences that best occur in small and large altitudes. Common to 50% of the studied locations are 25 years: 1910-1914, 1918-1921, 1928-1929, 1946-1949 etc. The predominance of warm and dry years in total number of adverse years is established, except for the locations at high altitude. The eustress effect of wet and cold years on beech trees is significant, especially for the locations at high altitude. A four grade scale for assessment of frequency, duration and negative deviation of eustress periods is proposed. Five groups of locations are formed due to the comparative analysis of these characteristics. Tree of them have a potential to buffer the negative influences, while two of them need further investigations on the eustress etymology. If the sampled trees from a certain location have stress periods, which are long, frequently appearing and with large reduction in the growth, the probability of degradation, successive changes in species composition or the reduction of occupied area of forest community, exists. The capability of the approach allows manifestation of reactive functional types of trees (PFTs), which may support the vegetation modelling. The groups of trees with similar behavior, connected to the climatic types of years, and with similar characteristics of eustress periods, could be recognized and treated as a kind of PFTs.

Dendrochronology, Eustress, Climatic Type of Year, Reactive Functional Type of Tree, SPPAM Program, Fagus silvatica L.