Branch initiation and mortality frequencies
Branch initiation and mortality frequencies were measured to test for
links with climate. After cross-dating, in case the oldest part of a
branch was intact with visible WMS, the year its initiation was recorded
as the first calendar year of growth. This was possible for 717 of the
764 branches measured. In addition, the last year of growth was
determined for 96 branches which had an intact dead tip, with dark-brown
senesced primordial leaves. The first branch initiated in 1893. However,
as for many years during the period 1893-1928 no branch initiations were
recorded, branch initiation frequency analysis was restricted to the
period 1929-2013. The oldest dead branch dated, died in 1966 (last year
of growth) or before the growing season of 1967. Only four dead branches
could be dated that had died before 1987/1988. Hence, branch mortality
frequency analysis covered the period 1987-2013. Both frequency series
contained trends caused by either low number of old branches and a bias
towards analysis of longer branch series, i.e. relatively disregarding
the shortest branches (cf. Fig. 3a), or by senescence and organic
breakdown of dead branch tips, which easily break off, and a consequent
quick decline of intact dead branches over time (cf. Fig. 3b).
Therefore, both series were detrended with a cubic smoothing spline 2/3
of the series length and a 50 percent frequency cut-off (Fig. 3ab).
Both the branch initiation and mortality series as well as the climate
data were normalized by subtraction of the mean, followed by division
with the standard deviation, before testing for possible links. For the
latter Pearson’s correlation coefficients were calculated. This was done
between standardized branch initiation frequency and seasonal climate
(mean temperatures and precipitation sums) from early summer prior to
branch initiation to late summer of the year of initiation. In addition,
correlation coefficients were calculated between standardized branch
mortality frequency and seasonal climate from spring of the last year of
growth to late winter after the last year of growth. The same
definitions of seasons were used as for the climate-growth models (see
above).