the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Sep 2022
Comparative water-use by fast-growing E. grandis x E. nitens clonal hybrid and Pinus elliottii near the Two Streams Research Catchment, South Africa
Nkosinathi David Kaptein
Alistair David Clulow
Michele Lynn Toucher
Colin S. Everson
Steven Brian Dovey
Ilaria Germishuizen
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.
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Nkosinathi David Kaptein et al.
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2022-650', Jacob Crous, 16 Sep 2022
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
Citation: https://doi.org/10.5194/egusphere-2022-650-CC1 -
AC1: 'Reply on CC1', Nkosinathi Kaptein, 02 Oct 2022
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.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC1
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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
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)
Citation: https://doi.org/10.5194/egusphere-2022-650-RC1 -
CC2: 'Reply on RC1', Nkosinathi Kaptein, 02 Feb 2023
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.
Citation: https://doi.org/10.5194/egusphere-2022-650-CC2 -
AC2: 'Reply on RC1', Nkosinathi Kaptein, 06 Mar 2023
Authors would like to thank Miriam Coenders-Gerrits for the insightful comments that have added a valuable contribution to the manuscript. Authors response to her comments are indicated below:
- “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)”. 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.
- 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? We have modified this to read, ‘the contribution of fog could be significant’. The word precipitation should be changed to rainfall. The rainfall has been measured accurately, although rainfall can be highly variable, we believe the rainfall measurement represent the actual rainfall accurately. The accuracy of the rainfall results would impact the interception because the interception is based on the rainfall. We used a Texas instruments raingauge at a standard height with a backup raingauge in case of failure.
- “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” Authors agree with Miriam and this will be incorporated in the manuscript.
- “Equations 3 +4: I would make this liniear. No need (and reason) for polynomial”. The best regression co-efficient was found when both equations were polynomial, which is the reason why they were made polynomial, however if the reviewer would prefer linear then that would be acceptable to the authors as the difference in the model fit is small.
- “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”. Authors agree, and will keep this section for qualitative comments.
- “L187: unit of Is should be MJ/m2/d. Also check fig 3b”. Authors have noted this comment and adjustment will be made throughout the manuscript.
- “I would recommend to write all parameters in the text in italic and make use of subscript (e.g., Tair => Tair)”. Authors are in agreement and will modify the document throughout.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC2 -
AC3: 'Reply on RC1', Nkosinathi Kaptein, 06 Mar 2023
Authors would like to thank Miriam Coenders-Gerrits for the insightful comments that have added a valuable contribution to the manuscript. Authors responses to comments are indicated below in bold writing:
- “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)”. 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.
- 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? We have modified this to read, ‘the contribution of fog could be significant’. The word precipitation should be changed to rainfall. The rainfall has been measured accurate, although rainfall can be highly variable, we believe the rainfall measurement represent the actual rainfall accurately. The accuracy of the rainfall results would impact the interception because the interception is based on the rainfall. We used a Texas instruments raingauge at a standard height with a backup raingauge in case of failure.
- “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” Authors agree with Miriam and this will be incorporated in the manuscript.
- “Equations 3 +4: I would make this liniear. No need (and reason) for polynomial”. The best regression co-efficient was found when both equations were polynomial, which is the reason why they were made polynomial, however if the reviewer would prefer linear then that would be acceptable to the authors as the difference in the model fit is small.
- “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”. Authors agree, and will keep this section for qualitative comments.
- “L187: unit of Is should be MJ/m2/d. Also check fig 3b”. Authors have noted this comment and adjustment will be made throughout the manuscript.
- “I would recommend to write all parameters in the text in italic and make use of subscript (e.g., Tair => Tair)”. Authors are in agreement and will modify the document throughout.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC3
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RC2: 'Comment on egusphere-2022-650', David Scott, 16 Feb 2023
An interesting paper, representing a lot of difficult work, on a topic that receives little detailed investigation, despite its significance. Thus I consider the work worthwhile and worthy of publication, provided that several points are attended to in revision. Whether these represent minor or major revision is a matter of opinion.
1. the comparison between eucalypts and pine is relevant, but there is not enough hard data of make a definitive determination of their relative effects. I think that the title and abstract should soften the emphasis on this comparative water use. Interesting data is presented on the comparative transpiration, but the other elements of total water use are poorly quantifiable, so the final comparison of ET by the two species is little better than speculative. Specifically, the crops are very different ages and thus a direct comparison is not convincing (at least not without a lot more convincing information on why a comparison is reasonable. Secondly, the soil water component is poorly defined, and the soil & interception figures are estimated from such general sources (of dubious applicability) that the errors on those estimates make the overall estimates speculative. I think these are serious flaws, but I think the information on direct measurements in the paper are still important and useful. But the authors should not attempt to close the water balance except in a broad and speculative way.
2. My annotated version of the manuscript contains several parts where the language or expression is lacking and can be improved.
3. Section 2.5 on the soil water content left me puzzled, and I think that it and the later references to the soil water store, are wrong. Soil wetness is presumably measured with some sort of TDR instrument, that give a volumetric wetness at the 3 depths (this should be stated more clearly in the text - I don't know what a CS616 sensor is). (a) These volumetric wetness figures could be used to estimate a depth of water in the profille, which is a measure of the absolute amount of water available to the trees (PAW). It doesn't make sense to me to go through some sort of model to estimate the approximate soil water potentials that these wetness figures would relate to (section 3.2). If the objective is to consider plant available water, then it is best to use volumetric wetness, and convert these to an estimate of a depth of water in the soil profile at a each particular point in time.
(b) With the deepest probe at 60 cm (properly 0.6 m in SI units), there is a large uncertainty about how much water is still available to the trees at depths below say 0.75 m? The estimates of soil water are therefore incomplete, and only give an indication of the real situation. The information on the response of the trees to increase water availability immediately after rain, or in the dry season, are interesting and useful, but it needs to be acknowledged that the trees may have access to water in parts of the profile that have not been monitored. {Consider Peter Dye's measurements of eucalypts under stress - 1996, Tree Physiology, 16: 233-238, which showed water was being used to 8 m below the surface and beyond}
(c) Related to the point in (b), more should be said about the soil profiles, and this should include materials below the conventional soil profile and include the substrate beneath, from which water may be drawn. Is it hard rock, or is it decomposing and permeable (as well as accessible to tree roots?)
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AC4: 'Reply on RC2', Nkosinathi Kaptein, 06 Mar 2023
Authors would like to thank David Scott for his valuable comments to the manuscript. Response to his comments by authors is below in bold writing:
- the comparison between eucalypts and pine is relevant, but there is not enough hard data of make a definitive determination of their relative effects. We agree and have therefore not been definitive in our statements regarding their relative water use. We have recommended that further measurements and hard data for comparison be collected.
- I think that the title and abstract should soften the emphasis on this comparative water use. We will soften the emphasis on this comparison in the title and water use. We could take out the word ‘comparative’ from the title, so it reads “Water-use by fast-growing Eucalyptus grandis x E. nitens clonal hybrid and Pinus elliottii near the Two Streams Research Catchment, South Africa”.
- Interesting data is presented on the comparative transpiration, but the other elements of total water use are poorly quantifiable, so the final comparison of ET by the two species is little better than speculative. Specifically, the crops are very different ages and thus a direct comparison is not convincing (at least not without a lot more convincing information on why a comparison is reasonable). Noted and agreed. More data would be convincing but we felt this was a good starting point and acknowledge the speculation although the estimations of soil ET and rainfall interception were based on results from numerous studies in the literature. The trees are at different ages but comparable in terms of their stage in their growth cycle and therefore we felt warrant comparison. We agree and recommend more comparative work and data collection. Measurements of ET in particular would be beneficial but obviously quite costly.
- Secondly, the soil water component is poorly defined, and the soil & interception figures are estimated from such general sources (of dubious applicability) that the errors on those estimates make the overall estimates speculative. I think these are serious flaws, but I think the information on direct measurements in the paper are still important and useful. But the authors should not attempt to close the water balance except in a broad and speculative way. We will focus on the results of transpiration and limit the results of the estimated water balance at each site to a broad and speculative discussion.
- My annotated version of the manuscript contains several parts where the language or expression is lacking and can be improved. Authors thank David Scott for his corrections in the document. We will address those sections and improve the expression.
- Section 2.5 on the soil water content left me puzzled, and I think that it and the later references to the soil water store, are wrong. Soil wetness is presumably measured with some sort of TDR instrument, that give a volumetric wetness at the 3 depths (this should be stated more clearly in the text - I don't know what a CS616 sensor is). Yes, the soil water content was measured with a TDR probe. A clear definition of soil water measurement measuring equipment will be included in the manuscript as well as the conversion to soil water potential
6.1 These volumetric wetness figures could be used to estimate a depth of water in the profille, which is a measure of the absolute amount of water available to the trees (PAW). It doesn't make sense to me to go through some sort of model to estimate the approximate soil water potentials that these wetness figures would relate to (section 3.2). If the objective is to consider plant available water, then it is best to use volumetric wetness, and convert these to an estimate of a depth of water in the soil profile at a each particular point in time.
We used the Hyprop system to measure the soil retention curve of the soils and were able to then convert the volumetric water content using a robust method into soil water potential. We felt it would be useful to present soil water retention as this shows the dryness of the soil quite well, independent of particle size distribution. If the reviewer would prefer us to show it as an absolute depth of water in the soil we are able to do that.
6.2 With the deepest probe at 60 cm (properly 0.6 m in SI units), there is a large uncertainty about how much water is still available to the trees at depths below say 0.75 m? The estimates of soil water are therefore incomplete, and only give an indication of the real situation. The information on the response of the trees to increase water availability immediately after rain, or in the dry season, are interesting and useful, but it needs to be acknowledged that the trees may have access to water in parts of the profile that have not been monitored. {Consider Peter Dye's measurements of eucalypts under stress - 1996, Tree Physiology, 16: 233-238, which showed water was being used to 8 m below the surface and beyond}. Root experiments conducted in commercial forest stands (Laclau et al. 2001, Fabiao et al. 1995) reported that majority of fine roots are located at the soil surface (top one meter of soil) decreasing with increase in soil depth. However, authors are aware that pine and Eucalyptus trees develop a dirmophic root system (deep taproot and shallow fine roots), with a tap root as deep as the height of a trees, to increase chances of sourcing water. We therefore agree we David Scott that trees in our study probably sourced water stored deep in the soil profile and authors will indicate this possibility in the manuscript. We will acknowledge that the trees may have access to water in deeper parts of the profile that are not monitored.
6.3 Related to the point in (b), more should be said about the soil profiles, and this should include materials below the conventional soil profile and include the substrate beneath, from which water may be drawn. Is it hard rock, or is it decomposing and permeable (as well as accessible to tree roots?). Authors agree with David and details of the soil profile will be included where available. The substrate is unconsolidated material with rock only at about 40 m.
References
Fabiao A, Steen E, Katterer T, Ribeiro C, Araujo C. 1995. Development of root biomass in an Eucalyptus globulus plantation under different water and nutrient regime. Plant and Soil 168: 215-223.
Laclau JP, Arnaud M, Bouillet JP, Ranger J. 2001. Spatial distribution of Eucalyptus roots in a deep sand soil in the Congo: relationships with the ability of the stand to take up water and nutrients. Tree Physiology 21: 129-136.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC4
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AC4: 'Reply on RC2', Nkosinathi Kaptein, 06 Mar 2023
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2022-650', Jacob Crous, 16 Sep 2022
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
Citation: https://doi.org/10.5194/egusphere-2022-650-CC1 -
AC1: 'Reply on CC1', Nkosinathi Kaptein, 02 Oct 2022
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.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC1
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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
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)
Citation: https://doi.org/10.5194/egusphere-2022-650-RC1 -
CC2: 'Reply on RC1', Nkosinathi Kaptein, 02 Feb 2023
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.
Citation: https://doi.org/10.5194/egusphere-2022-650-CC2 -
AC2: 'Reply on RC1', Nkosinathi Kaptein, 06 Mar 2023
Authors would like to thank Miriam Coenders-Gerrits for the insightful comments that have added a valuable contribution to the manuscript. Authors response to her comments are indicated below:
- “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)”. 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.
- 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? We have modified this to read, ‘the contribution of fog could be significant’. The word precipitation should be changed to rainfall. The rainfall has been measured accurately, although rainfall can be highly variable, we believe the rainfall measurement represent the actual rainfall accurately. The accuracy of the rainfall results would impact the interception because the interception is based on the rainfall. We used a Texas instruments raingauge at a standard height with a backup raingauge in case of failure.
- “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” Authors agree with Miriam and this will be incorporated in the manuscript.
- “Equations 3 +4: I would make this liniear. No need (and reason) for polynomial”. The best regression co-efficient was found when both equations were polynomial, which is the reason why they were made polynomial, however if the reviewer would prefer linear then that would be acceptable to the authors as the difference in the model fit is small.
- “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”. Authors agree, and will keep this section for qualitative comments.
- “L187: unit of Is should be MJ/m2/d. Also check fig 3b”. Authors have noted this comment and adjustment will be made throughout the manuscript.
- “I would recommend to write all parameters in the text in italic and make use of subscript (e.g., Tair => Tair)”. Authors are in agreement and will modify the document throughout.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC2 -
AC3: 'Reply on RC1', Nkosinathi Kaptein, 06 Mar 2023
Authors would like to thank Miriam Coenders-Gerrits for the insightful comments that have added a valuable contribution to the manuscript. Authors responses to comments are indicated below in bold writing:
- “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)”. 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.
- 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? We have modified this to read, ‘the contribution of fog could be significant’. The word precipitation should be changed to rainfall. The rainfall has been measured accurate, although rainfall can be highly variable, we believe the rainfall measurement represent the actual rainfall accurately. The accuracy of the rainfall results would impact the interception because the interception is based on the rainfall. We used a Texas instruments raingauge at a standard height with a backup raingauge in case of failure.
- “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” Authors agree with Miriam and this will be incorporated in the manuscript.
- “Equations 3 +4: I would make this liniear. No need (and reason) for polynomial”. The best regression co-efficient was found when both equations were polynomial, which is the reason why they were made polynomial, however if the reviewer would prefer linear then that would be acceptable to the authors as the difference in the model fit is small.
- “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”. Authors agree, and will keep this section for qualitative comments.
- “L187: unit of Is should be MJ/m2/d. Also check fig 3b”. Authors have noted this comment and adjustment will be made throughout the manuscript.
- “I would recommend to write all parameters in the text in italic and make use of subscript (e.g., Tair => Tair)”. Authors are in agreement and will modify the document throughout.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC3
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RC2: 'Comment on egusphere-2022-650', David Scott, 16 Feb 2023
An interesting paper, representing a lot of difficult work, on a topic that receives little detailed investigation, despite its significance. Thus I consider the work worthwhile and worthy of publication, provided that several points are attended to in revision. Whether these represent minor or major revision is a matter of opinion.
1. the comparison between eucalypts and pine is relevant, but there is not enough hard data of make a definitive determination of their relative effects. I think that the title and abstract should soften the emphasis on this comparative water use. Interesting data is presented on the comparative transpiration, but the other elements of total water use are poorly quantifiable, so the final comparison of ET by the two species is little better than speculative. Specifically, the crops are very different ages and thus a direct comparison is not convincing (at least not without a lot more convincing information on why a comparison is reasonable. Secondly, the soil water component is poorly defined, and the soil & interception figures are estimated from such general sources (of dubious applicability) that the errors on those estimates make the overall estimates speculative. I think these are serious flaws, but I think the information on direct measurements in the paper are still important and useful. But the authors should not attempt to close the water balance except in a broad and speculative way.
2. My annotated version of the manuscript contains several parts where the language or expression is lacking and can be improved.
3. Section 2.5 on the soil water content left me puzzled, and I think that it and the later references to the soil water store, are wrong. Soil wetness is presumably measured with some sort of TDR instrument, that give a volumetric wetness at the 3 depths (this should be stated more clearly in the text - I don't know what a CS616 sensor is). (a) These volumetric wetness figures could be used to estimate a depth of water in the profille, which is a measure of the absolute amount of water available to the trees (PAW). It doesn't make sense to me to go through some sort of model to estimate the approximate soil water potentials that these wetness figures would relate to (section 3.2). If the objective is to consider plant available water, then it is best to use volumetric wetness, and convert these to an estimate of a depth of water in the soil profile at a each particular point in time.
(b) With the deepest probe at 60 cm (properly 0.6 m in SI units), there is a large uncertainty about how much water is still available to the trees at depths below say 0.75 m? The estimates of soil water are therefore incomplete, and only give an indication of the real situation. The information on the response of the trees to increase water availability immediately after rain, or in the dry season, are interesting and useful, but it needs to be acknowledged that the trees may have access to water in parts of the profile that have not been monitored. {Consider Peter Dye's measurements of eucalypts under stress - 1996, Tree Physiology, 16: 233-238, which showed water was being used to 8 m below the surface and beyond}
(c) Related to the point in (b), more should be said about the soil profiles, and this should include materials below the conventional soil profile and include the substrate beneath, from which water may be drawn. Is it hard rock, or is it decomposing and permeable (as well as accessible to tree roots?)
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AC4: 'Reply on RC2', Nkosinathi Kaptein, 06 Mar 2023
Authors would like to thank David Scott for his valuable comments to the manuscript. Response to his comments by authors is below in bold writing:
- the comparison between eucalypts and pine is relevant, but there is not enough hard data of make a definitive determination of their relative effects. We agree and have therefore not been definitive in our statements regarding their relative water use. We have recommended that further measurements and hard data for comparison be collected.
- I think that the title and abstract should soften the emphasis on this comparative water use. We will soften the emphasis on this comparison in the title and water use. We could take out the word ‘comparative’ from the title, so it reads “Water-use by fast-growing Eucalyptus grandis x E. nitens clonal hybrid and Pinus elliottii near the Two Streams Research Catchment, South Africa”.
- Interesting data is presented on the comparative transpiration, but the other elements of total water use are poorly quantifiable, so the final comparison of ET by the two species is little better than speculative. Specifically, the crops are very different ages and thus a direct comparison is not convincing (at least not without a lot more convincing information on why a comparison is reasonable). Noted and agreed. More data would be convincing but we felt this was a good starting point and acknowledge the speculation although the estimations of soil ET and rainfall interception were based on results from numerous studies in the literature. The trees are at different ages but comparable in terms of their stage in their growth cycle and therefore we felt warrant comparison. We agree and recommend more comparative work and data collection. Measurements of ET in particular would be beneficial but obviously quite costly.
- Secondly, the soil water component is poorly defined, and the soil & interception figures are estimated from such general sources (of dubious applicability) that the errors on those estimates make the overall estimates speculative. I think these are serious flaws, but I think the information on direct measurements in the paper are still important and useful. But the authors should not attempt to close the water balance except in a broad and speculative way. We will focus on the results of transpiration and limit the results of the estimated water balance at each site to a broad and speculative discussion.
- My annotated version of the manuscript contains several parts where the language or expression is lacking and can be improved. Authors thank David Scott for his corrections in the document. We will address those sections and improve the expression.
- Section 2.5 on the soil water content left me puzzled, and I think that it and the later references to the soil water store, are wrong. Soil wetness is presumably measured with some sort of TDR instrument, that give a volumetric wetness at the 3 depths (this should be stated more clearly in the text - I don't know what a CS616 sensor is). Yes, the soil water content was measured with a TDR probe. A clear definition of soil water measurement measuring equipment will be included in the manuscript as well as the conversion to soil water potential
6.1 These volumetric wetness figures could be used to estimate a depth of water in the profille, which is a measure of the absolute amount of water available to the trees (PAW). It doesn't make sense to me to go through some sort of model to estimate the approximate soil water potentials that these wetness figures would relate to (section 3.2). If the objective is to consider plant available water, then it is best to use volumetric wetness, and convert these to an estimate of a depth of water in the soil profile at a each particular point in time.
We used the Hyprop system to measure the soil retention curve of the soils and were able to then convert the volumetric water content using a robust method into soil water potential. We felt it would be useful to present soil water retention as this shows the dryness of the soil quite well, independent of particle size distribution. If the reviewer would prefer us to show it as an absolute depth of water in the soil we are able to do that.
6.2 With the deepest probe at 60 cm (properly 0.6 m in SI units), there is a large uncertainty about how much water is still available to the trees at depths below say 0.75 m? The estimates of soil water are therefore incomplete, and only give an indication of the real situation. The information on the response of the trees to increase water availability immediately after rain, or in the dry season, are interesting and useful, but it needs to be acknowledged that the trees may have access to water in parts of the profile that have not been monitored. {Consider Peter Dye's measurements of eucalypts under stress - 1996, Tree Physiology, 16: 233-238, which showed water was being used to 8 m below the surface and beyond}. Root experiments conducted in commercial forest stands (Laclau et al. 2001, Fabiao et al. 1995) reported that majority of fine roots are located at the soil surface (top one meter of soil) decreasing with increase in soil depth. However, authors are aware that pine and Eucalyptus trees develop a dirmophic root system (deep taproot and shallow fine roots), with a tap root as deep as the height of a trees, to increase chances of sourcing water. We therefore agree we David Scott that trees in our study probably sourced water stored deep in the soil profile and authors will indicate this possibility in the manuscript. We will acknowledge that the trees may have access to water in deeper parts of the profile that are not monitored.
6.3 Related to the point in (b), more should be said about the soil profiles, and this should include materials below the conventional soil profile and include the substrate beneath, from which water may be drawn. Is it hard rock, or is it decomposing and permeable (as well as accessible to tree roots?). Authors agree with David and details of the soil profile will be included where available. The substrate is unconsolidated material with rock only at about 40 m.
References
Fabiao A, Steen E, Katterer T, Ribeiro C, Araujo C. 1995. Development of root biomass in an Eucalyptus globulus plantation under different water and nutrient regime. Plant and Soil 168: 215-223.
Laclau JP, Arnaud M, Bouillet JP, Ranger J. 2001. Spatial distribution of Eucalyptus roots in a deep sand soil in the Congo: relationships with the ability of the stand to take up water and nutrients. Tree Physiology 21: 129-136.
Citation: https://doi.org/10.5194/egusphere-2022-650-AC4
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AC4: 'Reply on RC2', Nkosinathi Kaptein, 06 Mar 2023
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