| Abstrakt: |
Pine plantations are the dominant specie currently planted within the South African commercial forestry industry. Improvements in bioeconomy markets for dissolving wood pulp products have seen an expansion in fast-growing Eucalyptus plantations due to their higher productivity rates and better pulping properties than pine. This has raised concerns regarding the expansion of Eucalyptus plantations and how they will affect water resources as they have been reported to have higher transpiration (T) and total evaporation rates (ET) than pine. We compared T (mm), diameter at breast height (DBH, cm) and leaf area index (LAI) of an eight-year-old Eucalyptus grandis x Eucalyptus nitens clonal hybrid (GN) with twenty-year-old Pinus elliottii. Transpiration was measured for two consecutive seasons (2019’ 20 and 2020’ 21) using a heat ratio sap-flow method. The ET was calculated using published values of soil evaporation and rainfall canopy interception to quantify the impact of each species on water resources. In 2019’ 20 season, annual T for P. elliottii exceeded GN by 28 %, while 2020’ 21 season showed no significant differences. This was associated with 17 and 21 % greater LAI for P. elliottii than GN in 2019’ 20 and 2020’ 21 season, respectively. Dq increments were statistically similar (p > 0.05) in 2019’ 20 season, whereas the 2020’ 21 season produced significant differences (p < 0.05). Transpiration for P. elliottii showed a strong (R2 > 0.70) linear relationship with solar radiation, LAI and shallow soil matric potential, while GN had a good (R2 > 0.70) relationship with solar radiation only. The soil water potential was very low at the GN site, indicating that the site was water stressed, with trees competing for water as soon as it becomes available to sustain T, causing a rapid soil water depletion after rainfall, while P. elliottii used water more gradually. P. elliottii estimated ET was 18 % greater than GN in 2019’ 2020, with no significant differences in 2020’ 21 season. Results from this study indicated that on water limited sites, T and ET between GN and P. elliottii may not be different, however, in subtropical regions, GN T and ET have the potential to exceed P. elliottii, causing soil water depletion. Long-term total soil water balance studies in the same region would be beneficial to understand the impact of long-term commercial forestry on water resources. [ABSTRACT FROM AUTHOR] |
