Xylem Hydraulic Traits Time Series

For investigating the inter-annual variability of xylem hydraulic traits, we developed chronologies of D_H, K_S, and V_X for the three studied species following standard dendrochronological procedures. We observed an increasing trend in hydraulic traits' time series (Figure S2). It has been shown that D_H systematically increases with plant size, including plant height and correlated stem diameter in tropical woody species (Olson and Rosell, 2013) as well as on a global scale (Olson et al., 2014). To avoid any bias of increasing vessel size with increasing stem diameter, we removed age or size related trends from each hydraulic trait time series before chronology development. We used a cubic smoothing spline function with a 50% frequency response at 10 years for detrending, because we aimed at removing multi-decadal variability from our rather short (< 100 years) time series, and to retain high-frequency signals related to inter-annual climate variability. Detrended time series were then obtained by dividing the observed values by the fitted values (Briffa and Jones, 1990). The detrended time series of nine trees of each of the three species were averaged by a bi-weight robust mean to build the chronologies. The chronologies were build using the “chron” function in the Dendrochronology Program Library in R (dplR) (Bunn, 2008, 2010) within the R statistical programming environment (R Development Core Team, 2016). The robustness of the chronologies were assessed using standard statistics commonly used in dendrochronology including the mean inter-series correlation between trees (rbar.bt) (Briffa and Jones, 1990), the expressed population signal (EPS) (Wigley et al., 1984; Buras, 2017) and mean sensitivity (MS) (Briffa and Jones, 1990).

Intra-annual variability of wood anatomical parameters is also a research focus particularly in temperate regions. Such investigations require splitting of a ring into several sections. However, the number of the generally rather large vessels is very low in our studied species particularly in T. ciliata and L. speciosa (Figure ​(Figure3).3). Splitting a ring into different sections might further reduce the vessel number per section due to splitting across some vessels. Particularly in narrow rings, sectioning is not feasible due to the very low number or even complete absence of vessels in some sections which does not provide adequate replication for statistical analysis. Hence, we refrained from intra-annual analysis of vessel features and instead considered the complete annual rings.