Separating the effects of tree size and meristem maturation on shoot development of grafted scions of red spruce (Picea rubens Sarg.).

In order to separate the effects of size and meristem maturation on age-related changes in shoot growth behaviour, a reciprocal grafting experiment was conducted involving juvenile (J), mid-age (MA) and old-growth (OG) red spruce (Picea rubens Sarg.) trees as both scion donors and rootstock. The effects of rootstock and scion age on vegetative growth, foliar morphology and reproductive development were assessed over 7 years after grafting. Vegetative growth potential declines with meristem maturation, but the high growth potential of J meristems on J rootstock cannot be expressed when J scions are grafted on MA and OG rootstock. Branch density decreases with meristem maturation. The tendency for high J branch density is expressed across all rootstock ages, but is minimally expressed on MA rootstock where elongation of terminal shoots is significantly greater than on OG rootstock. Both needle width and leaf mass area increase with meristem maturation and increasing tree size. Reproductive competence is mainly a function of meristem maturity, and rootstock had little effect on strobilus development, although the more fecund rootstocks did promote some flowering on J scions. Developmental decline in height growth does not appear to be a function of decreased meristem vigour, but reduced terminal long shoot elongation and decreased apical control in OG trees will reduce height growth.