Browsing by Author "Sibiya, Gabigabi"
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- ItemPredicting growth and future yield in Eucalyptus grandis x urophylla stands using the CABALA process-based model(Stellenbosch : Stellenbosch University, 2021-03) Sibiya, Gabigabi; Drew, David M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.ENGLISH ABSTRACT: The aims of this research were to (1) partly parameterise and initialise and (2) test the CABALA process-based model for hybrid Eucalyptus grandis x urophylla clones in the Zululand region of South Africa. To achieve these aims, detailed data were obtained from a set of 18 sites at which permanent sample plots had been monitored for seven or more years. In July 2018, a data acquisition campaign was undertaken and diameter at breast height (DBH), and the heights of all trees at all sites were measured, soil samples taken, and leaf area index estimated using a LP80- ceptometer from all 18 sites. Destructive biomass measurements and collection of biomass subsamples were done at a subset of five sites in order to determine allometric parameters. Leaf area index was also obtained for all 18 compartments using the MODIS product provided by NASA. A set of experiments in nursery and laboratory environments were also undertaken to determine minimum pre-dawn leaf water potential, and the relationship between leaf nitrogen content and specific leaf area. Partial parameterisation of 17 parameters of the model for Eucalyptus grandis x urophylla was done by building on the existing E. globulus parameter set. This parameter set was altered using parameter data from published literature, allometric parameters obtained from field measurements and destructive sampling, using parameters obtained from experiments executed in the nursery and glasshouse, and by undertaking a limited calibrating/optimisation exercise. The model was run using two sources of weather data: (1) long-term mean weather data (1949-1999) obtained from South African Atlas of Climatology and Agro- Hydrology weather data (2) and daily weather data (2008-2018) obtained from a variety of weather stations from South African Sugarcane Research Institute (SASRI) and Mondi, VitalWeather Systems and the South African Weather Services (SAWS). The long-term mean weather data represented conditions in which there was no drought whereas the period 2008 – 2018 was one in which the region experienced a record-breaking period of drought. Hence these two datasets provided an interesting contrast. The output from CABALA predictions were compared to observed stand volume, mean diameter at breast height, and mean height for the 18 study sites. Overall, when the long-term mean monthly weather data were used, CABALA overestimated stand mean diameter at breast height, mean height, and stand volume. When the daily weather data measured during the period when the trees actually grew (2008 – 2018) was used, the model gave better estimations of mean diameter at breast height but tended to underestimate mean height and stand volume, especially on higher productivity sites that generally received high mean annual precipitation. The leaf area index estimated by CABALA was compared to estimates from the MODIS LAI product and while CABALA overestimated leaf area index in many instances, it predicted the leaf area index drop trend observed during the drought period 2014-2016 as soon as the daily weather conditions were introduced. The model with its modified parameter set underestimated mortality in all sites irrespective of weather conditions.