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.