Model analysis of the importance of reiteration for branch longevity in Pseudotsuga menziesii compared with Abies grandis

Reiteration is an important process in the maintenance of tree crowns and in plant longevity. We use a geometric simulation model of branch growth to explore differences in longevity between old-growth Pseudotsuga menziesii (Mirb.) Franco and Abies grandis (D. Don ex Lamb.) Lindl. branches. Reiteration is defined through rules that reflect apical dominance relationships, and these rules are used to define shoot cluster units (SCU) on P. menziesii branches. Reiteration through epicormic production dominates growth in simulated P. menziesii branches and is shown to be a major factor that differentiates growth between P. menziesii and A. grandis. Branch growth is shown to be highly sensitive to rules for bifurcation and capacity for reiteration. The rules employed in the model that define epicormic initiation and SCU independence reveal possible physiological mechanisms through which reiteration occurs in P. menziesii. A simple morphological rule fails to simulate branch growth adequately, whereas a physiological rule through epicormic initiation after release from inhibition of a lateral axis yields realistic simulated branches. Branch growth is best simulated through a combination of physiological controls and morphological rules.