text: Where are the defences of bristlecones as valid proxies in tree ring terms? While we??ve pointed out a number of possible mechanisms for fertilization in the 20th century, the most prominent theory is CO2 fertilization The usual mechanism by which CO2 fertilization is hypothesized to lead to increased ring widths is through increased water use efficiency. Water use efficiency changes are studied in a series of remarkable articles by Xiahung Feng of Dartmouth, of which Tang, Feng and Funkhouser is notable. "Plant water use efficiency" was defined by Farquhar et al 1989 (cited by Tang et al) as the ratio of net carbon fixed to the total water cost. The connection between CO2 fertilization and increased water use efficiency (W) is summarized by Tang et al. as follows: