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. x 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 degrees C (maximum daily temperatures minus 15 degrees C) in late winter and early spring before cambial reactivation was defined as the cambial reactivation index (CRI(15)). Cambial reactivation, which began when the minimum temperature rose above 0 degrees C, occurred when the CRI(15) was 93 and 96 degrees 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.

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We attempted to observe differences in vessel element anatomy, and physiological and morphological traits of leaves in Fagus crenata seedlings originated from seven different provenances grown under the uniform environmental conditions. We also investigated the relationships between the anatomical characteristics of the vessel elements and physiological plus morphological traits of leaves in Fagus crenata seedlings. To carry out the experiments, Fagus crenata samples were prepared from Chichibu Research Forest of Tokyo University. For anatomical studies of the vessel elements, vessel number per mm 2 , average vessel area, and the percentage of vessel area from the pith to the bark side were measured. We also measured transpiration rate, stomatal conductance, leaf area, leaf thickness, leaf dry mass per unit leaf area and leaf density for foliar studies. The anatomical characteristics of the vessel elements as well as the physiological and morphological traits of leaves were noticeably different among provenances. In addition, we found significant correlations between the foliar characteristics not only with vessel number per mm 2 , but also with vessel area percentage and sum of the (vessel area) 2 , theoretical hydraulic conductivity, from the pith to the bark side. Therefore, we concluded that variations in physiological and morphological traits of leaves in response to provenance variation were in correspondence with vessel number per mm 2 as a parameter that determines the total vessel area and consequent water hydraulic conductance in Fagus crenata as a diffuse porous hardwood.

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