Can forest structural diversity be a response to anthropogenic stress? A case study in old-growth fir Abies alba Mill. stands

Key message From 1973 to 1991, Polish SO 2 emissions above 3250 Gg/year resulted in a decline of fir Abies alba Mill. After stresses connected with these emissions, five main diameter at breast height (DBH) structural types were described. This type of heterogeneous forest structure is supposed to increase forest resistance and resilience to abiotic, biotic and anthropogenic disturbances. Context The analyses of forest structure are important under the current scenario of global change, since heterogeneous structures allow for better responses to disturbances. Forests with more complex structures should present greater vitality. Aims The main hypotheses were as follows: (1) the temporal changes of atmospheric SO 2 emissions caused (a) the abrupt changes in the tree DBH radial increment and (b) the death of fir trees; and (2) atmospheric SO 2 emissions and related fir decline and recovery processes ultimately result in the development of stands characterised by diverse DBH structures. Methods Radial growth trends of 49 and 215 fir trees in the older and younger generations, respectively, and 85 dead fir trees were evaluated. Using data collected in 32 stands, the DBH structural types were identified, and the shapes of these types were illustrated. Results The structural diversification of forest patches may influence forest resistance and resilience to disturbances; five main structural types were identified: ML1 and ML2 represent DBH distributions of multi-layered stands, and OS, TS1 and TS2 represent DBH distributions of one- and two-storied stands. Conclusion Structural diversity of forests was a response to SO 2 emissions; fir trees had the ability to increase their radial growth, although there were still high SO 2 emissions.