Photoperiod plays a dominant and irreplaceable role in triggering secondary growth resumption REPLY

In their Letter, Elmendorf and Ettinger (1) question thedominant role of photoperiod in driving secondarygrowth resumption (hereafter referred to as xylem formationonset) of the Northern Hemisphere conifers, recentlyreported by Huang et al. (2). Their opinions aregrounded on the following three aspects, including 1)the seasonality of the photoperiod, 2) the dependenceof the predictor variables (e.g., photoperiod, forcing, andchilling) on the response variable (the date of onset ofxylem formation, day of the year [DOY]), and 3) the limitedvalue of the obtained models for interannual forecasting.We think they bring up an interesting issue thatdeserves further discussion and clarification.