Shrub growth chronologies
Annual shoot length growth was measured as distances between WMS for multiple branches of each of the 25 sampled shrubs. This was done for a total 764 branches, 10-65 per shrub. Shoot length growth series of individual branches were cross-dated using the sampling year of shoots with complete and green tips, by plotting and visually comparing similarities in growth patterns between branch series of shrub individuals, and the dates of branching. Older branch series, without any overlap with other series from the same shrub were cross-dated with dated branch series from other shrubs. This mainly concerned some series dating prior to the 1960s (cf. Fig. S2; ).
Individual branch series of C. tetragona generally exhibit growth trends with increasing growth rates during the initial years of growth, approx. during the first ten years in this study. Shoot length growth rates stabilize thereafter, before they start to decline when reaching ages of approx. 50 years . This may result in an overall positive trend in the mean site chronology (Fig. 2) and mean shrub chronologies (Fig. S2). Three different standardization or detrending methods were compared for this study (Table S1, Fig. S3) to remove such growth trends: 1) standardization with a horizontal line through the mean , 2) Regional Curve Standardization , and 3) signal-free multiple RCS-standardization . With the first method, each individual branch series was divided by the related branch mean. In RCS-detrending age-related growth trends are in principle removed, while lower-frequency climatic trends are retained , with an age-related growth curve. In SF-MRCS detrending, multiple RCS curves are calculated for different groups to prevent potential bias resulting from differing growth rates of slow and fast growing individuals . Here, RCS-curves were calculated for 11 groups (each with n>50) of branch series ranging from the fastest growing shortest series, to the slowest growing longest series. SF-MRCS detrending was executed in CRUST . RCS detrending and detrending with a horizontal line was performed in R version 4.0.3 with the ‘dplR’ package .
All methods resulted in a lowered first-order autocorrelation (Table S1). RCS-detrending retained both short-term trends and the overall positive trend in site-level mean shoot length growth. In contrast, SF-MRCS detrending resulted in the removal of most trends. Detrending with a horizontal line through the mean had an intermediate effect (Figs 2 and S3). Both RCS- and SF-MRCS detrending reduced the mean correlation between individual branch series (rbar), signal-to-noise ratio, and expressed population signal (EPS; Table S1), and these methods may thus have removed some of the common climatic signal present within the series. Therefore, the series standardized with a horizontal line were used in subsequent analyses.