Black spruce (Picea marianna) architecture

Genetically determined rules underlying black spruce (Picea mariana (Mill.) BSP) morphological development were revealed by using tree architecture analysis. Black spruce develops according to Rauh's model. Its relatively simple and well-hierarchized architecture comprises four categories of axes (A 1 -A 4 ) and three distinct branching levels. First- and second-order axes (the trunk and branches) are monopodial, orthotropic structures with rhythmic growth and branching, and an unlimited life-span (or only limited by tree longevity). They are mainly devoted to spatial exploration and occupation. Axes A 3 and A 4 are monopodial, plagiotropic structures with a limited life-span, which are devoted to assimilation and reproduction. Female cones develop in terminal position on A 3, whereas male reproductive structures develop in terminal or lateral position on A 4 . Reiteration (or duplication of axes) capacity is an important feature of the architecture of black spruce. Adaptive reiteration processes allow trees to renew their foliage and to reproduce vegetatively through layering. Traumatic reiteration processes tend to restore the tree's original architecture by replacing injured axes or parts of axes. In both cases, replicas originate from dormant buds located at the bottom of growth units (deferred or proleptic reiteration) or from growing axes (immediate or sylleptic reiteration).