Leader growth and the architecture of three North American hemlocks

Height growth in hemlock (Tsuga canadensis (L.) Carr., T. heterophylla (Raf.) Sarg., T. mertensiana (Bong.) Carr.) is by rhythmic growth of a monopodial axis with continuous branch production throughout the growing season. Leader growth is plagiotropic and leader erection is a process lasting several years. Two types of events disrupt the basically monopodial nature of the axis. (1) Frequent (43%) apical meristem death shifts dominance to a nearby lateral branch in T. canadensis. (2) Weak apical control allows occasional shifts in dominance from the leader to a branch without meristem death (13 and 24% in T. heterophylla and T. canadensis, respectively, but none in T. mertensiana). These growth patterns contain elements of several tree architectural models but fit none well.