Perform power transformation simultaneously for a data.frame of
raw tree-ring width series.
Usage
powt(rwl)
Arguments
rwl
a data.frame of raw tree-ring widths series, such as that
produced by read.rwl or read.fh
Value
A data.frame containing the power transformed ring width series.
Details
This procedure is a variance stabilization technique implemented after
Cook & Peters (1997): for each series a linear model is fitted on the
logs of level and spread, where level is defined as the local mean
$M_t = (R_t + R_(t-1))/2$ with
ring widths R, and spread S is the local standard deviation defined as
$S_t = |R_t - R_(t-1)|$. The
regression coefficient b from $log S = k +
b log M$ is then used for the power transform $R*_t = R_t^(1-b)$.
References
Cook, E. R. and Peters, K. (1997) Calculating unbiased
tree-ring indices for the study of climatic and environmental
change. The Holocene, 7(3), 361–370.