An approach to assessing site index changes of Norway spruce based on spatially and temporally disjunct measurement series

•Method for checking and quantifying site index changes based on rates of change in site index (ROCs).•Applicable for databases with temporally and spatially disjunct measurement series.•Checking and dating site index changes by OLS-CUSUM type empirical fluctuation process test.•Combination of forward and backward ROCs models to assess site index changes in annual resolution.•Norway spruce, Southwest Germany (1872–2010): site indices have increased since the mid 1950s.