The use of artificial defoliation to evaluate the impact of defoliation on young Eucalyptus dunnii in the Midlands region of KwaZulu-Natal, South Africa

This study used artificial defoliation to evaluate the impact of defoliation on young E. dunnii in the Midlands region of KwaZulu-Natal, South Africa. Juvenile foliage was manually removed at three severity levels (20%, 50%, 90%) and four seasonal timing combinations. Results indicated that 50% and 90% defoliation significantly reduced basal area and the change in tree height, mainly when defoliation occurred in both summer and winter. The findings of this novel study in South Africa suggest that defoliation greater than 50%, particularly when repeated, should be avoided in the same year to preserve growth potential. The 3-PG (Physiological Principles Predicting Growth) model, integrated with satellite-derived defoliation estimates, was used to simulate and quantify the long-term productivity losses in an 8-year-old E. dunnii trial. A modified R-based version of 3-PG (3-PG.d) accurately simulated growth in chemically protected plots but overestimated wood volume, although not significantly, in untreated, defoliated plots—likely due to limitations in remote sensing-based defoliation assessment. Sensitivity analysis confirmed that increasing defoliation intensity reduces harvest wood volume, proving that the model can be used as a defoliation risk assessment tool. Improving defoliation detection with other technologies, such as UAV-based multispectral imagery and drone imagery, could significantly enhance the model’s prediction accuracy.