Patterns of growth compensation in eastern white pine (Pinus strobus L.): the influence of herbivory intensity and competitive environments.

To investigate complex growth compensation patterns, white pine (Pinus strobus L.) seedlings were clipped to simulate different herbivory levels. Seedlings were growing with different understory competition levels (created through monthly weeding vs no brush control) under a range of overstory canopy closures. Compensation patterns varied for the different growth and size measures. After one growing season, seedlings did not fully compensate for lost biomass regardless of the competitive environments of the seedlings. Although relative height growth was stimulated by light intensity clipping (20-40% of last-year shoots removed), relative diameter growth, total biomass, and biomass growth of seedlings declined sharply with increasing clipping intensity. Likewise, all growth parameters declined with increasing interspecific competition. Results showed that seedlings in highly competitive environments showed smaller growth loss due to clipping than those in competition-free environments, presumably because seedlings experiencing high interspecific competition devoted more energy to maintaining apical dominance and a balanced shoot-root ratio. While competition from canopy trees altered compensatory patterns, competition from understory vegetation only altered the magnitude, but not the patterns, of compensatory growth. We suggest that compensatory growth follows a complex pattern that will vary with the parameters measured, competitive conditions, and clipping intensities. Our results support the assertion that overcompensation may be an adaptation to competitive ability, rather than a response to herbivory itself.