The timing of cambial reactivation plays an important role in the control of both the quantity and the quality of wood. The effect of localized heating on cambial reactivation in the main stem of a deciduous hardwood hybrid poplar (Populus sieboldii × P. grandidentata) was investigated. Electric heating tape (20–22 °C) was wrapped at one side of the main stem of cloned hybrid poplar trees at breast height in winter. Small blocks were collected from both heated and non-heated control portions of the stem for sequential observations of cambial activity and for studies of the localization of storage starch around the cambium from dormancy to reactivation by light microscopy. Cell division in phloem began earlier than cambial reactivation in locally heated portions of stems. Moreover, the cambial reactivation induced by localized heating occurred earlier than natural cambial reactivation. In heated stems, well-developed secondary xylem was produced that had almost the same structure as the natural xylem. When cambial reactivation was induced by heating, the buds of trees had not yet burst, indicating that there was no close temporal relationship between bud burst and cambial reactivation. In heated stems, the amount of storage starch decreased near the cambium upon reactivation of the cambium. After cambial reactivation, storage starch disappeared completely. Storage starch appeared again, near the cambium, during xylem differentiation in heated stems. The results suggest that, in deciduous diffuse-porous hardwood poplar growing in a temperate zone, the temperature in the stem is a limiting factor for reactivation of phloem and cambium. An increase in temperature might induce the conversion of storage starch to sucrose for the activation of cambial cell division and secondary xylem. Localized heating in poplar stems provides a useful experimental system for studies of cambial biology.

Several studies have demonstrated that localized heating of tree stems induces localized cambial reactivation. We analyzed by light microscopy the effects of early spring increases in ambient temperature in 2005 and 2007 on the timing of cambial reactivation and xylem differentiation in stems of two trees of a cloned deciduous hardwood hybrid poplar (Populus sieboldii Miquel. × P. grandidentata Michx.) growing under natural conditions. Meteorological data at the study site showed that temperatures in late winter and early spring differed markedly between 2005 and 2007, with trends toward higher temperatures starting around April 3 in 2005 and around March 20 in 2007. Cambial reactivation occurred about 17 days earlier in 2007 than in 2005. The cumulative daily maximum temperature in excess of 15 °C (maximum daily temperatures minus 15 °C) in late winter and early spring before cambial reactivation was defined as the cambial reactivation index (CRI₁₅). Cambial reactivation, which began when the minimum temperature rose above 0 °C, occurred when the CRI₁₅ was 93 and 96 °C in 2005 and 2007, respectively. The differentiation of secondary xylem started earlier in 2007 than in 2005. On May 27, we found a wider current-year band of xylem and a higher frequency of small-diameter vessel elements in 2007 than in 2005. We propose that the timing of cambial reactivation is controlled by air temperature and that earlier cambial reactivation induces earlier differentiation of xylem in hybrid poplar under natural conditions. Our results indicate that the CRI might be a useful indicator of the timing of cambial reactivation.