Locally heated dormant cambium can re-initiate cell production independently of new shoot growth in deciduous conifers (Larix kaempferi)

Abstract

The periodicity of cambial cell division accounts for the formation of growth rings in trees. Cambial reactivation controls earlywood width and annual ring width, thereby affecting wood quantity and quality. However, despite the fact that cambial reactivation has been one of the most commonly reported features in cambium studies, we have not yet elucidated what directly triggers it in spring in deciduous conifers.Here we recorded responses of winter dormant cambium of deciduous conifers (Larix kaempferi) to artificial heating once (between February and March 1999) in a cooler area and three times (between December 1995 and March 1996) in a warmer area. Stem surfaces were locally warmed (19°C–24°C) for 2–4 weeks at breast height in the cooler area and at breast height and the crown base in the warmer area. The localization of storage starch surrounding the cambium during the heating periods was observed to assess the growth potential of the dormant cambium under localized heating.To resume cell division, dormant cambium in the heated portions often required heating for 2 weeks in the warmer area, whereas that in the cooler area required 3 weeks of heating. In locally heated stem portions, phloem storage tissues surrounding the cambium were rich in starch; however, cell growth of the reactivated cambium was slow, and cambial derivatives differentiated into phloem rather than xylem during periods of winter cambial dormancy. By contrast, sustained cambial activity and xylem differentiation were observed in stem portions warmed even after natural cambial reactivation occurred in non-heated stem portions.The results suggest that in deciduous conifer trees, cambial dormancy is imposed by low air temperature during mid-late winter, and a rise in temperature directly triggers cambial reactivation independently of new shoot growth in spring. However, continued cell division in the reactivated cambium and xylem differentiation of cambial derivatives might require certain factors, which originate from elongating shoots, besides starch stored in phloem.