Esper et al  divides trees into "linear" and "nonlinear" trees depending on their growth – a classificaiton that is idiosyncratic to this publication as far as I can tell. Esper at al.  provides a citation to a publication "in press" that supposedly explains this, but I can’t locate any explanation in the publication. Perhaps I’m missing something in plain sight, but I don’t think so.
Esper et al  says:
To build RCS chronologies from the whole data set that contains different sites and species, we analyzed the growth levels and trends of the individual ring-width series after aligning them by cambial age and classifying them into two groups: one with age trends that have a weakly “linear” form (443 series) and one with age trends that are more “nonlinear” (762 series) (8). The data sets were divided this way because differences in growth levels and slopes can bias resulting RCS chronologies (21). For example, according to their mean age trends (8), the young nonlinear trees grow 2 to 3 times faster than the linear trees up until 200 years of age.
The references are :
8. Supplementary details are available on Science Online at www.sciencemag.org/cgi/content/full/295/5563/2250/DC1.
21. J. Esper, E. R. Cook, K. Peters, Tree-Ring Res., in press.
I have been unable to identify anything that corresponds exactly to citation (21), but I presume that this is Esper, Cook , Krusic, Peters and Schweingruber, Tree Ring Res. . online here
http://www.wsl.ch/staff/jan.esper/publications/TRR_2003.pdf , which I’ve been discussing in connection with Gotland.
As evidenced by most posts on Gotland, it’s pretty obvious that I’ve been reading this article carefully. The pdf doesn’t search unfortunately. Visually I can find no reference to the linear/non-linear distinction. Thus, this key distinction remains unaccounted for: another frustrating Hockey Team publication in Science.