Short-term changes in Pinus strobus sapling height/diameter ratios following partial release: Testing the acclimative stem-form development hypothesis

Following release, mature trees can acclimatively shift aboveground growth allocation from height to diameter at breast height (DBH) growth to adjust their stem form (reducing height/diameter ratio (HDR)) to better withstand increased wind stress. The purpose of this study was to determine if, over a 6-year period, 7- to 12-year-old eastern white pines (Pinus strobus L.) also responded to different levels of partial release with (i) stem-form adjustments (reduced HDR) through growth-allocation shifts from height to DBH growth and (ii) lower HDR values (shorter heights) than for unreleased trees of the same DBH ("acclimative stem-form development hypothesis"). Over the 6-year postrelease period, juvenile white pine seedlings exhibited unsynchronized height and DBH growth response patterns that depended on their prerelease height growth. Height growth of faster growing white pines was temporarily reduced. Concurrently, DBH growth was enhanced with increasing release intensity. HDR reductions followed a fairly narrow and predictable trajectory, but HDR values of released trees were not lower than those of unreleased trees of the same DBH after 6 years. Juvenile white pine seedlings appear to maintain a balance among aboveground tree parts to enhance future tree stability, which may be an adaptive trait for moderately shade-tolerant species.