Recovery after defoliation in Eucalyptus globulus saplings: respiration and growth

Despite the importance of respiration in the overall carbon balance of plants, recovery after defoliation and debudding has been largely related to changes in carbon uptake; the significance of respiration has received much less attention. Growth, biomass and leaf-level carbon balance (both photosynthesis and dark respiration at night) responses of young Eucalyptus globulus potted-saplings to debudding (B), partial defoliation (D) and combined B&D treatments were assessed over a 12-week recovery period. Light-saturated photosynthetic rates (A₁₅₀₀) were asynchronous with night respiration rates (R_dark) throughout the course of the experiment; 5 weeks after defoliation, significant increases in A₁₅₀₀ were accompanied by concomitant increases in R_dark in the B&D and B and D treatments. By week 8, while A₁₅₀₀ returned to control values, R_dark had increased, particularly in the B&D treatment. Saplings in the B and D treatments showed full recovery with growth, biomass and leaf area being similar to control saplings by week 12. In contrast, saplings in the B&D treatment appeared unable to compensate for the combined removal of all buds and 35% leaf area as evidenced by slowed height increments and reductions in total biomass of >30%. Simple modelling of whole-plant net CO₂ uptake showed that saplings in the B&D treatment fixed 20% less CO₂ than the other treatments at week 12, suggesting that recovery following this treatment and the D treatment was dependent on changes in total leaf area development and whole-tree assimilation rather than differences in assimilation or respiration per unit foliage area. Increased biomass allocation to bud in weeks 5 and 8 suggested that the pattern of refoliation after defoliation and debudding was related to changes in tree architecture from the release of latent buds.