Parameter testing and application of the 3PG model for Eucalyptus grandis x Urophylla in subtropical conditions in South Africa

ABSTRACT

Background:

The productivity of the coastal Zululand region, which was known as the South African breadbasket for fibre is declining. Climate-related changes are a significant factor contributing to this decline. The 3PG (Physiological Processes Predicting Growth) model was calibrated for E. grandis x E. urophylla hybrids planted in this region to quantify the effect of climate variation x site on their growth and survival. Monthly weather data for the ungauged plantations were estimated using the Random Forest (RF) supervised learning algorithm. A dataset consisting of 17 permanent sample plots (PSPs) and published parameter values for this hybrid in various regions of Brazil were utilized for parameter estimation. Using a parsimonious optimization approach, we developed a novel method called extended Root Mean Square Error (eRMSE) to select the optimal parameter set.

Result:

The new parameter set yielded accurate predictions for three key variables; quadratic stem diameter (R² = 0.85, E = 0.73), mean height (R² = 0.84, E = 0.78), and basal area (R² = 0.87, E = 0.78). Model performance at 15 independent sites allowed the comparison with three other Brazilian parameter sets for stand volume prediction at a specific age. The optimized parameter set provided a satisfactory, albeit slightly overestimated stand volume (V (m³ha^-1), R² = 0.65, E = -0.32) at the validation sites.

Conclusion:

The 3PG model can be adapted with parameter set from another region to characterize the growth of E.grandis x E.urophylla stands in South Africa.

Keywords:
Forest management; random forest; climate variation; process-based model.

HIGHLIGHTS

With local weather data, accurate estimates for ungauged plantations can be obtained. The 3PG parameters can easily be calibrated from previously published parameter set. Minimum ASW variable can be used to model growth on sites with groundwater access. 3PG model simulates tree growth dynamics in response to environmental changes.