Clonal variation in basal area growth patterns during stand development in hybrid poplar

Two-year periodic basal area increments (PBAI) of short-rotation Populus stands were followed through a 6-year rotation in monoclonal plots at three sites to assess variation in response patterns among 20 Populus trichocarpa Torr. & Gray × Populus deltoides Bartr. clones. Data were analyzed using a split-plot model for repeated measurements with the main effect of time and the time × clone interaction partitioned into their linear and quadratic orthogonal components. The form of the mean clone growth response was mainly quadratic with PBAI culminating during the second period at 145–160% of the rotation mean. Increments fell to 95–116% of the rotation mean during the following period. Response variation occurred mainly in the degree of linearity at two sites with clones varying in the rate of growth deceleration during the third period, while quadratic response variation was of prime importance at the third site. Changes in basal area levels among the topmost clones over the last stage of stand development were associated with the divergence in the linear and quadratic responses. Selection on the basis of interaction contrasts of periodic growth from monoclonal plot trials is discussed in terms of sustaining yield improvements and maintaining adaptation to short rotation strategies.