Some intrinsic factors affecting seed production in balsam fir

Abstract

Attempts have previously been made to correlate seed-bearing periodicity of forest-tree species with climatic factors, but intrinsic factors that may also contribute have been little studied. It is hypothesized that when seed-bearing is biennial, as in balsam fir (Abies balsamea (L.) Mill.), intrinsic factors must play a major role. Some intrinsic tree and cone-bearing-zone characters were investigated throughout a six-year period to determine their effects on seed production and seed-bearing periodicity in balsam fir.Tree-character variables expressing some aspect of tree size were positively related to cone production. The variables used explained only about 65 per cent of the cone-production variation. Some additional variation was associated with between-tree differences in mean numbers of branches per whorl and per internode and hence with characters of the cone-bearing zones.The number of cones formed was positively related to length of the cone-bearing zone and to the number of whorls, internodes or branches in the zone. The average level of the separation of the female and male zones varied little between seed years showing that the potential cone-bearing zone has short-term size stability. In the long-term, it increases with tree size to a maximum, then declines with overmaturity. The actual cone-bearing zone fluctuated in size with the quality of the seed year, as did the intensity of cone bearing on individual comparable branches.Cone-bearing intensity and cone size (number of ovuliferous scales) decreased downwards throughout the cone-bearing zone and inwards on shoots of decreasing morphological category on individual branches. These decreases were related to shoot vigour since shoot length also decreased in similar fashions. The effective cone size (number of seed -producing scales) was directly proportional to cone size on individual trees. On some trees, the largest cones had twice the seed potential of the smallest cones: they also contained, on average, larger seeds. Cone size was predictable in terms of relative height in the zone and shoot order. The proportion of the potential seed crop borne in the upper part of the zone was greater than the proportion of cones.Cones normally develop from lateral buds. These are initiated each year but development does not necessarily follow. Undeveloped lateral buds are latent, capable of vegetative development if forced. In the female zone, the numbers of both lateral and terminal (including subterminal) buds per shoot were positively related to shoot length. Shoots were shorter and buds fewer when cones were present. Shoot-length reductions accounted for the fewer terminal buds, but not entirely for the fewer lateral buds. Positionally vigorous shoots produced more lateral buds in a cone year than positionally weaker shoots of similar length in a non-cone year.Most female-zone shoots bore zero, one or two lateral buds. The proportion of non-budded shoots increased in a cone year and the number of lateral buds per shoot decreased. The decrease occurred largely in the lower part of the zone: in the vigorous upper part, lateral -bud production per shoot was uniform in cone and non- cone years. In the latter, megasporangiate buds replaced both vegetative and latent buds on vigorous shoots, and mainly latent buds on weaker shoots.The presence of megasporangiate buds appears to reduce the number of lateral buds initiated in the spring. The presence of developing strobili reduces the capacity of these buds to develop: those that do are generally distal and differentiate vegetatively. When strobili are absent, more lateral buds develop and many become megasporangiate. In poorer seed years, most buds on weaker shoots are latent.Some aspects of the physiology associated with the morphological variations which have been found to influence seed production and periodicity are discussed.