Detecting trends in diameter growth of Norway spruce on long-term forest research plots using linear mixed-effects models

The objective of the study was to introduce a simultaneous approach to extracting growth trends from diameter increment series by combining tree- and stand-level information in the framework of mixed-effects modeling. The model results are compared with those of the sequential modeling approach according to Yue et al. Can J For Res 41:1577–1589, (2011). Comprehensive data from periodically repeated tree and stand measurements on 113 Norway spruce long-term forest research stands distributed throughout southwestern Germany as well as annually resolved diameter growth data from stem analysis of 581 sample trees are used for model parameterization. Results provide clear evidence for distinct temporal variation due to environment-related effects after accounting for tree- and stand-level effects on diameter growth of Norway spruce over the past 136 years, with especially high growth levels during the last four decades. Most remarkable is the considerable increase in diameter growth rates following the 1947–1952 growth depression. From the early 1960s onwards, growth rates fluctuated on an elevated level until most recent years. Model comparison reveals that results of the sequential and simultaneous modeling approaches are similar with respect to the course of the time-specific environmental effects on growth. We conclude that the proposed simultaneous modeling approach has the advantages of extracting growth trends at a higher level of precision and being the more parsimonious modeling option.