Nkosinathi David Kaptein

We measured the tree transpiration of 9-year-old, Eucalyptus grandis x Eucalyptus urophylla clonal hybrid (GU) trees in the commercial forestry area of northern KwaZulu-Natal, South Africa. Transpiration was measured using the heat ratio method over two consecutive hydrological years (2019/20 and 2020/21) and up-scaled to a stand level. Leaf area index (LAI), quadratic mean diameter, and soil water content (SWC) were measured over the same period using an LAI 2200 plant canopy analyser, manual dendrometers and CS616 sensors, respectively. The depth to groundwater was estimated to be approx. 28 m, using a borehole next to our study site. Results showed that transpiration followed a seasonal pattern, with daily mean of 2.3 mm‧tree⁻¹‧day⁻¹ (range: 0.18 to 4.55 mm‧tree⁻¹‧day⁻¹) and 3.3 mm‧tree⁻¹‧day⁻¹ (range: 0.06 to 6.6 mm‧tree⁻¹‧day⁻¹) for 2019/20 and 2020/21, respectively. Annual GU transpiration was higher than that found by international studies under similar conditions, but was within the same transpiration range as Eucalyptus genotypes in the KwaMbonambi area. Plantation water productivity, calculated as a ratio of stand volume to transpiration, was higher than for other published studies, which was attributed to a very high productive potential of the study site. Multiple regression using the random forests predictive model indicated that solar radiation, SWC and air temperature highly influence transpiration. There is a high possibility that our GU tree rooting system extracted water in the unsaturated zone during the dry season. Due to the use of short-term results in this study, the impact of GU on water resources could not be quantified; however, previous long-term paired catchment studies in South Africa concluded that Eucalyptus has a negative impact on water resources. Further research is suggested with long-term measurements of transpiration and total evaporation and an isotope study to confirm the use of water by GU trees in the unsaturated zone.

Pine plantations are the dominant species currently planted within the South African commercial forestry industry. Improvements in bio-economy 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 water use (quantified using transpiration rates) than pine. We measured transpiration rates (mm yr⁻¹), diameter at breast height (quantified as quadratic mean diameter, D_q, m) and leaf area index of an 8-year-old Eucalyptus grandis × Eucalyptus nitens clonal hybrid (GN) and a 20-year-old Pinus elliottii. Transpiration rates were measured for two consecutive hydrological years (2019/20 and 2020/21) using a heat ratio sap-flow method, calibrated against a lysimeter. In the 2019/20 year, annual transpiration for P. elliottii exceeded GN by 28 %, while for the 2020/21 hydrological year, there was no significant difference between the transpiration of the two species, despite a 17 % and 21 % greater leaf area index for P. elliottii than GN in 2019/20 and 2020/21 measurement years respectively. Quadratic mean diameter increments were statistically similar (p > 0.05) in 2019/20, whereas the 2020/21 year produced significant differences (pGN trees, while the soil profile was depleted more gradually at the P. elliottii site. As a result, trees at the GN site appeared to be water stressed (reduced stem diameters and transpiration), suggesting that there was limited access to alternative water source (such as groundwater). The study concluded that previous long-term paired catchment studies indicate that eucalypts use more water than pine; however, periods of soil water stress and reduced transpiration observed in this study must be accommodated in hydrological models. Long-term total soil water balance studies are recommended in the same region to understand the long-term impact of commercial plantations on water resources.

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.

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.

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.

Global demand for forest products is ever-increasing, creating competition for water between downstream water users and commercial forest producers. Tree production should therefore aim at the effe...

Traditional timer-based systems for irrigation management, which are more commonly used in commercial nurseries in South Africa, are not ideal as they may not irrigate seedlings efficiently. A sensor-based irrigation system is presented as an alternative, as this can provide several benefits to nurseries and nursery-grown seedlings. Small-sized soil water sensors that could fit in small-volume nursery containers (25 to 100 mL), and could be integrated into an automated irrigation system, are reviewed. Several experiments have been conducted internationally to measure soil water status of small-volume containers in soilless substrates, and a large body of knowledge is now available. In this review, we describe the principles of several currently commercially available sensors that can be adapted to this purpose, giving advantages and disadvantages of each type. We conclude that a sensor-based irrigation system has great potential to address the challenges associated with irrigation scheduling, while improving water usage in most nurseries.

The combination of granular matrix sensors with a Hobo 4 analogue channel datalogger provides a relatively inexpensive continuous soil water monitoring system. However, the datalogger excites all 4 channels concomitantly as it reads the sensor measurement of each channel in sequence. This results in localised electrolysis causing measurement bias error in Channels 2, 3 and 4. To evaluate this channel bias, Watermark granular matrix sensors were connected to Hobo 4 channel dataloggers to measure electrical conductance of the soil. This study formed part of a larger study aimed at understanding water use by Eucalyptus plantations at different soil depths. The measured soil conductivity was calibrated against the gravimetric method in soil derived from Natal Group Sandstone and Dwyka Tillite that occur in southern KwaZulu-Natal. The channels of a Hobo 4 channel datalogger were successfully calibrated against the gravimetric method for both soil types (R2 > 0.92). The voltage measurements of each channel increased in the order 1 < 2 < 3 < 4 for both soil types at a soil water content range of 12 to 44% and 6 to 46% for Dwyka Tillite and Natal Group Sandstone soils, respectively. Channel measurements were similar at soil water content ranges below 12 and 6% for tillite and sandstone soils, respectively. The study showed that Channels 2, 3 and 4 of the Hobo 4 channel datalogger are affected by electrolysis. If this analogue datalogger is used with these types of sensors, these channels need to be calibrated back to Channel 1.

Efficient nursery irrigation is a key factor for the production of high-quality forestry planting stock. Conventional timer-based irrigation systems used by most forestry nurseries have some drawbacks that, if not properly managed, can lead to over-irrigation and water wastage, leaching of nutrients and contribute to increased incidences of pests and/or diseases. A practical method to accurately measure media water content and control irrigation more carefully could prevent some of these issues. To evaluate the potential use of an automated irrigation system, commercially available Decagon EC-5 dielectric soil water content sensors were laboratory- calibrated against the gravimetric method using coir/pine bark/vermiculite, coir/perlite and pine bark media to measure water content of nursery seedling containers. Using Eucalyptus dunnii seedlings raised in polystyrene seedling containers to test the irrigation system, a datalogger was programmed to control irrigation for low (0.22–0.26 m3 m−3), medium (0.26...