the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Comparative water-use by fast-growing E. grandis x E. nitens clonal hybrid and Pinus elliottii near the Two Streams Research Catchment, South Africa
- 1Discipline of Agrometeorology, University of KwaZulu-Natal, Pietermaritzburg, 3209, South Africa
- 2Centre for Water Resources Research, University of KwaZulu-Natal, Pietermaritzburg, 3209, South Africa
- 3Grasslands-Forests-Wetlands Node, South African Environmental Observation Network, Pietermaritzburg, 3201, South Africa
- 4Shaw Research Institute, Sappi Forests, Howick, 3290, South Africa
- 5Institute for Commercial Forestry Research, Scottsville, 3201, South Africa
- 6Department Plant and Soil Sciences, University of Pretoria, Pretoria, South Africa
- 1Discipline of Agrometeorology, University of KwaZulu-Natal, Pietermaritzburg, 3209, South Africa
- 2Centre for Water Resources Research, University of KwaZulu-Natal, Pietermaritzburg, 3209, South Africa
- 3Grasslands-Forests-Wetlands Node, South African Environmental Observation Network, Pietermaritzburg, 3201, South Africa
- 4Shaw Research Institute, Sappi Forests, Howick, 3290, South Africa
- 5Institute for Commercial Forestry Research, Scottsville, 3201, South Africa
- 6Department Plant and Soil Sciences, University of Pretoria, Pretoria, South Africa
Abstract. 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.
Nkosinathi David Kaptein et al.
Status: open (extended)
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CC1: 'Comment on egusphere-2022-650', Jacob Crous, 16 Sep 2022
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Information on water use of plantation trees in South Africa is very limit and this paper contributes important scientific data. I totally agree with the approach to compare the different genotypes at the same stage of development as the two genera are grown at different rotation lengths.
I am concerned about comments with regard to “subtropical regions” in the paper (lines 31, 338 and 350). Statements are made that in subtropical regions water tables can be shallow and that water is not limiting in these regions. However, no references are provided to motivate these statements or to indicate where these areas occur, or which climate classification system is referred to. If I compare the Kőppen-Geiger climate map for South Africa (Wikimedia repository) to maps showing storage coefficient of aquifers, current precipitation, depth to water level (Dennis and Dennis 2012), I do not observe strong correlations between climate regions and aquifer parameters. Thus, in my opinion, due to high variability of environmental variables, some areas within subtropical regions in South Africa the water table can also be deep, or soils can be relatively shallow and water availability can also be limiting tree growth and thus have an impact on tree water use.
It is hypothesised that the water use of Eucalyptus has the potential to exceed that of pine trees in the subtropical region (lines 339 and 351). In my opinion, the scientific evidence provided (GN T exceeded that of P. elliottii for a short period after a high rainfall event, line 351) is lacking to make this claim and is pure speculation. Short-term responses from the reported study (in a temperate region - Cwb Kőppen-Geiger climate zone) should not be extrapolated to potential long-term responses in other areas (subtropical regions – Cfa Kőppen-Geiger climate zone) that might differ markedly from the study site (difference in rainfall quantity, annual rainfall distribution, temperature, soils, soil water storage, depth to water level, different genotypes, etc.). Furthermore, Eucalyptus grandis x E. nitens hybrids are not climatically suited to be established in the subtropical region as E. nitens, one of the hybrid partners, is a cold-tolerant eucalypt that require MAT of below 15.5°C (Herbert, 2000).
References:
Dennis, I. and Dennis, R., 2012. Climate change vulnerability index for South African aquifers. Water SA, 38(3), pp.417-426.
Herbert M., 2000: Eucalypt and Wattle Species. In: Owen, D. (ed.), South African Forestry Handbook. Southern African Institute of Forestry, Menlo park.
Wikimedia Commons, the free media repository: https://commons.wikimedia.org/wiki/File:Koppen-Geiger_Map_ZAF_present.svg
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AC1: 'Reply on CC1', Nkosinathi Kaptein, 02 Oct 2022
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Dr Jacob Crous is thanked for his review and comments, and his contribution to the content of the paper. Authors agree with Dr Crous that the use of a word “subtropical regions” is not accurate in the context of the findings of this paper. Authors suggest substituting “subtropical regions” with “regions where soil water is not limiting”. Authors further agree with Dr Crous that in our study, there is no solid scientific proof supporting our statement that Eucalyptus water use has a potential to exceed pine in regions where soil water is not limiting”. Eucalyptus and pine comparative water use studies reported that eucalypts are not profligate consumers of water than pine (Myers et al. 1995, White et al. 2021). Authors will therefore adjust this statement in the manuscript and further clarify that water use results could be influenced by climate, soils, experimental methodologies etc, as stated by Dr Crous.
References:
White, D. A., Silberstein, R. P., Contreras, F. B., Quiroga, J. J., Meason, D. F., Palma, J. H. N., de Arellano, P. R.: Growth, water use, and water use efficiency of Eucalyptus globulus and Pinus radiata plantations compared with natural stands of Roble-Hualo Forest in the coastal mountains of central Chile, For. Ecol. Manage., 501, 119675-119676, https://doi.org/10.1016/j.foreco.2021.119676, 2021.
Myers, B. J., Theiveyanathan, S., O’Brien, N. D. O., Bond, W.J. 1996. Growth and water use of Eucalyptus grandis and Pinus radiata plantations irrigated with effluent. Tree physiology 16: 211-219.
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AC1: 'Reply on CC1', Nkosinathi Kaptein, 02 Oct 2022
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RC1: 'Comment on egusphere-2022-650', Miriam Coenders-Gerrits, 29 Jan 2023
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With pleasure I read the manuscript of Kaptein et al. It deals with an important question whether the newly planted GN use more water then Pinus elliottii. The authors compared two sites and equiped several trees with sapflow sensors to quantify the transpiration. As the HPV-system has many limitations in case one wants to know the stand transpiration, the authors did an attempt to quantify the relation between the sapflow measurements and the transpiration rates determined via a lysimeter. My compliments for doing this!
Overall, I like the study very much. It is relevant, well written and easy to read. I only have the following comments, which can improve the manuscript:
- you conclude that the transpiration is GN is lower than in Pinus due to the lack of water. Of course this is true: transpiration reduces as water becomes more limited (fig 5). However, I wonder if this very low matrix potential is not caused by the high transpiration rates of GN before the study period? So that the GN depleted already the soil? (feedback mechanism).
- The contribution of fog. In line 70 you say that fog precipitation is significant. How does this affect your results? How reliable are your precipitation observations? And how would this affect your interception results?
- Units/dimensions: I think the authors should do a carefull check on the units. In my view transpiration is a flux, thus having a time dimension. Therefore, most of the numbers given in the paper should have the unit mm/day or mm/month or mm/year (instead of mm). Some quick examples: L19, 151 (liter), 183 (mm/year), 213, 214, 215, 242, 265, 285 (mm/year), fig 3d (mm/day) and caption, fig 5 (rainfall mm/day), fig 5 (T=mm/day), fig 6 (rainfall mm/day), fig 8 (rainfall mm/day) and caption, fig 11 (mm/month) plus caption
- Equations 3 +4: I would make this liniear. No need (and reason) for polynomial.
- Section 4.3: I would keep this section for qualitative as you did not measure interception and soil evaporation. Especially, since you have fog the interception could be very high.
- L187: unit of Is should be MJ/m2/d. Also check fig 3b.
- I would recommend to write all parameters in the text in italic and make use of subscript (e.g., Tair => Tair)
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CC2: 'Reply on RC1', Nkosinathi Kaptein, 02 Feb 2023
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Authors would like to thank Mirian Coenders-Gerrits for insightful comments that add valuable contribution to the manuscript. On point number 1, possibility that GN trees depleted soil water before the study period, authors agree with this possibility, as literature states that transpiration rates are generally high in Eucalyptus early in the rotation and decrease as trees mature. With our GN trees close to maturity during the study period, it is possible that high rates of transpiration depleted soil water greater than recharge before the study period.
Nkosinathi David Kaptein et al.
Nkosinathi David Kaptein et al.
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