Water-use dynamics of an alien-invaded riparian forest within the summer rainfall zone of South Africa

In South Africa the invasion of riparian forests by alien trees has the potential to affect the country’s limited water resources. Tree water-use measurements have therefore become an important component of recent hydrological studies. It is difficult for South African government initiatives, such as the Working for Water (WfW) alien clearing programme, to justify alien tree removal and implement rehabilitation unless hydrological benefits are known. The objective of this study was to investigate the water use (transpiration rates) of a selection of introduced and indigenous tree species and quantify the hydrological benefit that could be achieved through a suitable rehabilitation programme. Consequently water use within a riparian forest in the upper Mgeni catchment of KwaZulu-Natal in South Africa was monitored over a 2-year period. The site consisted of an indigenous stand of eastern mistbelt forest that had been invaded by Acacia mearnsii, Eucalyptus nitens and Solanum mauritianum. The heat ratio method of the heat pulse velocity (HPV) sap flow technique and the stem steady state (SSS) techniques were used to measure the sap flow of a selection of indigenous and introduced species. The indigenous trees at New Forest, South Africa, showed clear seasonal trends in the daily sap flow rates varying from 8 to 25 L day 1 in summer (sap flow being directly proportional to tree size). In the winter periods this was reduced to between 3 and 6 L day 1 when limited energy flux was available to drive the transpiration process. The water use in the A. mearnsii and E. grandis trees showed a slight seasonal trend, with a high flow during the winter months in contrast to the indigenous species. The water use in the understorey indicated that multi-stemmed species used up to 12 L day 1. Small alien trees (< 30 mm) A. mearnsii and S. mauritianum used up to 4 L day 1 each. The total accumulated sap flow per year for the three individual A. mearnsii and E. grandis trees was 6548 and 7405 L a 1 respectively. In contrast, the indigenous species averaged 2934 L a 1, clearly demonstrating the higher water use of the introduced species. After spatial upscaling, it was concluded that, at the current state of invasion (21% of the stand being alien species), the stand used 40% more water per unit area than if the stand were in a pristine state. If the stand were to be heavily invaded at the same stem density of the indigenous forest, a 100% increase in water use would occur over an average rainfall year.The research presented in this paper forms part of an unsolicited research project (Rehabilitation of alien invaded riparian zones and catchments using indigenous trees: an assessment of indigenous tree water use) that was initiated by the Water Research Commission (WRC) of South Africa.The Water Research Commission (WRC) of South Africa and the Department of Economic Development, Tourism and Environmental Affairs (EDTEA).https://www.hydrology-and-earth-system-sciences.netam2020Plant Production and Soil Scienc

Topo-edaphic environment and forestry plantation disturbance affect the distribution of grassland forage and non-forage resources, Maputaland, South Africa

Grasslands are integral to rural livelihoods in southern Africa, because they provide hydrological regulation services and a variety of plant resources, including livestock fodder, medicines, and food products. To ensure ongoing provision of these resources in rapidly developing rural landscapes, an understanding of the relationships between grassland species composition and ecosystem services is required. This study examines the provision of grassland forage and non-forage resources across five grassland types in relation to environmental determinants of site topography, soil conditions, and plantation-forestry disturbance. Grasslands characteristic of low-lying and fertile landscape positions were dominated by nutritious lawn grasses and therefore tended to complement rangeland practices, whereas grasslands associated with elevated areas or infertile conditions were diverse in species composition and consequently provided the majority of plant medicines, spiritual resources, fruit-beverage resources, oils, and craft materials. Secondary grassland, resulting from forestry plantation abandonment, had moderate forage potential and limited non-forage resources. Our results provide a simple framework for approaching grassland resource classification, grassland conservation and land use management on the Maputaland coastal plain.The Water Research Commission (WRC) of South Africa and the Department of Economic Development, Tourism and Environmental Affairs (EDTEA).http://www.tandfonline.com/loi/tarf202021-12-08hj2021Plant Production and Soil Scienc

Optimizing resource distribution and crop productivity in hedgerow intercropping by manipulating tree arrangement

The potential of tree arrangement in optimizing radiation and soil water distribution and crop yield of hedgerow intercropping systems was investigated using a Jatropha curcas–Pennisetum clandestinum (kikuyu) system (Ukulinga, South Africa). Treatments (1110 t ha-1) of Jatropha-only (JO), single-row Jatropha and kikuyu (SR), and double-row Jatropha and kikuyu (DR) were used. Treatments had asymmetrical radiation distribution across tree–crop (T–C) interfaces and different radiation interception by trees (JO: 27 %, SR: 11 %, DR: 8 %). Soil water varied among treatments and was asymmetrically distributed showing no consistent trend towards trees. Evapotranspiration was higher (p\0.05) in SR than DR (13–65 %) and JO (16–37 %) for most part of the season. Estimated fine tree roots distribution was symmetrical in DR but not JO and SR. Total roots in SR were concentrated in the top 0.2 m (91 %) and beneath tree row (30 %), and varied inconsistently with distance from trees. Grass yields increased farther from tree rows and were higher in SR (8.7 t ha-1 in 2006–2007; 4.3 t ha-1 in 2007–2008) than DR (by \9 %). Radiation use efficiency of kikuyu (0.14–0.44 g MJ-1) also increased with distance from trees but irradiance correlated with grass yields poorly (R2\0.32) despite high rainfall and no nutrient limitation. Considering total productivity and evapotranspiration, DR was better tree arrangement than SR. Water availability dictated T–C interactions and intercrop yield more than radiation. It was possible to optimize radiation and water distribution and intercrop growth by manipulating tree arrangement without changing density.Water Research Commission (WRC) of South Africa project (K5/1480/1/12).http://link.springer.com/journal/104572017-10-30hb2016Plant Production and Soil Scienc

Topo-edaphic environment and forest plantation disturbance explain patterns of grassland species richness, composition and structure in an agro-ecological landscape, Maputaland, South Africa

Grasslands of the Maputaland coastal plain are biologically diverse and provide a variety of ecosystems services. Yet grasslands in this region are vulnerable to continuing development by plantation forestry that provides economic benefits to local communities. In order to provide a framework for land use that maintains grassland heterogeneity in complex agro-ecological systems, this paper characterises the main relations between grassland species composition, the physical environment and forestry plantation disturbance. Grassland species composition corresponded with landscape position and soil organic carbon. Grasslands occurring in infertile, elevated landscape positions were the most diverse having a greater richness of tufted graminoids, herbaceous forbs and geoxylic suffrutices than grassland occurring in low-lying dystrophic sites, which were dominated by rhizomatous and stoloniferous graminoids. Previously afforested grasslands (i.e. secondary grassland) were species poor, lacked keystone grasses, such as Themeda triandra, were dominated by a few species of rhizomatous or stoloniferous grasses and had been colonised by pioneer or ruderal forbs. Grassland species composition affects the provision of ecosystem services so to maintain a full complement of these attributes, grassland landscapes in Maputaland should comprise intact grasslands that cover both elevated and low-lying topographic positions.https://www.tandfonline.com/loi/tarf202021-12-08hj2021Plant Production and Soil Scienc

Water-use dynamics of an alien-invaded riparian forest within the Mediterranean climate zone of the Western Cape, South Africa

In South Africa, the invasion of riparian forests by alien trees has the potential to affect the country’s limited water resources. Tree water-use measurements have therefore become an important component of recent hydrological studies. It is difficult for South African government initiatives, such as the Working for Water (WfW) alien clearing program, to justify alien tree removal and implement rehabilitation unless hydrological benefits are known. Consequently, water use within a riparian forest along the Buffeljags River in the Western Cape of South Africa was monitored over a 3-year period. The site consisted of an indigenous stand of Western Cape afrotemperate forest adjacent to a large stand of introduced Acacia mearnsii. The heat ratio method of the heat pulse velocity sap flow technique was used to measure the sap flow of a selection of indigenous species in the indigenous stand, a selection of A. mearnsii trees in the alien stand and two clusters of indigenous species within the alien stand. The indigenous trees in the alien stand at Buffeljags River showed significant intraspecific differences in the daily sap flow rates varying from 15 to 32 L day 1 in summer (sap flow being directly proportional to tree size). In winter (June), this was reduced to only 7 L day 1 when limited energy was available to drive the transpiration process. The water use in the A. mearnsii trees showed peaks in transpiration during the months of March 2012, September 2012 and February 2013. These periods had high average temperatures, rainfall and high daily vapor pressure deficits (VPDs – average of 1.26 kPa). The average daily sap flow ranged from 25 to 35 L in summer and approximately 10 L in the winter. The combined accumulated daily sap flow per year for the three Vepris lanceolata and three A. mearnsii trees was 5700 and 9200 L, respectively, clearly demonstrating the higher water use of the introduced Acacia trees during the winter months. After spatially upscaling the findings, it was concluded that, annually, the alien stand used nearly 6 times more water per unit area than the indigenous stand (585mma 1 compared to 101mma 1/. This finding indicates that there would be a gain in groundwater recharge and/or streamflow if the alien species are removed from riparian forests and rehabilitated back to their natural state.The Water Research Commission (WRC) of South Africa and the Department of Economic Development, Tourism and Environmental Affairs (EDTEA).https://www.hydrology-and-earth-system-sciences.netam2017Plant Production and Soil Scienc

Can productivity and post-pruning growth of Jatropha curcas in silvopastoral systems be regulated by manipulating tree spacing/arrangement without changing tree density?

The potential of tree spacing/arrangement to alleviate effects of interspecific competition of hedgerow intercropping systems on productivity and response to pruning of Jatropha curcas (Jatropha) was investigated using a Jatropha e Pennisetum clandestinum (kikuyu) silvopastoral system at Ukulinga (KwaZulu Natal, South Africa). Treatments were differentiated by presence/absence of kikuyu and arrangement/spacing of Jatropha trees (one, two and three tree hedgerows on either side of the alley) and had the same tree density (1110 ha 1). When high water availability and kikuyu dormancy coincided, tree growth rates did not significantly differ across treatments. Trees of the treatment without interspecific competition (tree-only) were the tallest. Even when rainfall was high, post-pruning tree height growth rate was affected by belowground (BG) interspecific competition and tree spacing/arrangement. Treatment with a single tree hedgerow between alleys had the most frequent maximum stem growth rate and was the most efficient during limited water availability. Its trees showed slow initial response to pruning due to a high tree-grass interface, followed by compensatory growth when competition for water with grass was low. Generally, length of tree-grass interfaces affected yield inversely especially as trees matured toward their maximum-yield age (4e5 years). BG competition reduced tree yield more than tree biomass, while tree spacing/arrangement did not affect tree harvest index. Manipulation of tree arrangement/spacing without changing tree density had no consistent effects on tree productivity.Water Research Commission (WRC) and National Research Foundation (NRF) of South Africa for funding the project (K5/1480/1/12).http://www.elsevier.com/locate/biombioe2016-03-31hb201

Transpiration rates from mature Eucalyptus grandis × E. nitens clonal hybrid and Pinus elliottii plantations near the Two Streams Research Catchment, South Africa

DATA AVAILABILITY : Due to the high frequency of the data used in this paper, all data with linked figures and tables have been uploaded to the central database at the Centre for Water Resources Research (CWRR) at the University of KwaZulu-Natal in Pietermaritzburg. The author, Nkosinathi David Kaptein, can be contacted for these data at [email protected] 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, Dq, 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 (p<0.05). Tree transpiration is known to be influenced by climatic variables; therefore, a random forest regression model was used to test the level of influence between tree transpiration and climatic parameters. The soil water content, solar radiation and vapour pressure deficit were found to highly influence transpiration, suggesting these variables can be used in future water-use modelling studies. The profile water content recharge was influenced by rainfall events. After rainfall and soil profile water recharge, there was a rapid depletion of soil water by the GN 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.The South African Department of Water and Sanitation through a Water Research Commission project.https://www.hydrology-and-earth-system-sciences.nethj2024Plant Production and Soil ScienceSDG-15:Life on lan

Changes in energy balance and total evaporation with age, and between two commercial forestry species in South Africa

DATA AVAILABILITY : The authors do not have permission to share data.Expansion of the area planted to eucalypts has been observed in the last two decades due to an improvement in markets for products from this tree species. This has raised concerns over the management of freshwater resources as other species are replaced by Eucalyptus, which has been shown to use more water than other commercial forestry species. The energy balance (EB) and total evaporation (ET) over Acacia mearnsii was previously monitored at the Two Streams research catchment, and the site harvested in 2018 with subsequent re-planting of E. dunnii. This presented an opportunity to measure the two-year-old E. dunnii (Edun2) EB and ET for comparison on the same site with the previously planted A. mearnsii with results from two-year-old A. mearnsii (Amear2) and six-year-old A. mearnsii (Amear6) crops. ET and EB measurements on Amear2 were obtained using a large aperture scintillometer, while eddy covariance was used for Amear6 and Edun2. Measurements were conducted in October 2007 to September 2008, October 2012 to September 2013 and October 2019 to September 2020 for Amear2, Amear6 and Edun2. The leaf area index (LAI) was measured using a LAI 2200 plant canopy analyser for all crops. The annual plantation water productivity (PWPWOOD) was calculated as a ratio of productive stand volume to ET for Amear2, Amear6 and Edun2. Results showed that latent energy fluxes dominated the EB in all crops for both summer and winter seasons, indicating a possibility that trees were not limited by plant available water in winter (dry season). The Edun2 and Amear2 annual ET was statistically (p > 0.05) similar, while ET of the younger crops (Amear2 and Edun2) was 12% greater than Amear6. High ET in Edun2 was caused by high LAI while Amear2 was caused by high transpiration per unit leaf area in young trees than in mature trees. Monthly crop factors were derived from FAO ETo and ET for all three crops, providing a convenient and transferable method of estimating ET from meteorological data over a large scale. The Edun2 PWPWOOD was greater than Amear2, while Amear6 was greater than both the young crops. This study provides insight into the total water-use by different species at different stages of growth at the same site. It is recommended that catchment water balance measurements be continued onThe Department of Water and Sanitation through Water Research Commission.http://www.elsevier.com/locate/jhydrolam2024Plant Production and Soil ScienceSDG-06:Clean water and sanitationSDG-15:Life on lan

Water use and potential hydrological implications of fast-growing Eucalyptus grandis x Eucalyptus urophylla hybrid in northern Zululand, South Africa

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.The Department of Water and Sanitation through Water Research Commission.https://www.watersa.nethj2024Plant Production and Soil ScienceSDG-12:Responsible consumption and productionSDG-15:Life on lan

A preliminary investigation of the water use efficiency of sweet sorghum for biofuel in South Africa

Sweet sorghum (Sorghum bicolor (L.) Moench) has been recognized globally as a potential biofuel crop for ethanol production. Sweet sorghum is a drought-tolerant crop that is widely adapted to different environmental growing conditions. The aim of this study was to determine the water use efficiency (utilisable yield per unit amount of water used) of drip-irrigated sweet sorghum (variety Sugargraze) under two different climatic conditions in South Africa. The sweet sorghum trials were conducted at Ukulinga research farm (University of KwaZulu-Natal, Pietermaritzburg) and Hatfield experimental farm (University of Pretoria, Pretoria), South Africa. Field trials were conducted in two successive seasons, viz., 2010/11 and 2011/12. Seasonal water use was estimated using eddy covariance and surface renewal methods. Fresh and dry aboveground biomass yield, stalk yield and stalk Brix % were measured at final harvest. Theoretical ethanol yield was calculated from fresh stalk yield and Brix %. Water use for the two growing seasons was 415 mm at Ukulinga and 398 mm at Hatfield. The ethanol water use efficiency (WUE) values for the sweet sorghum at Ukulinga were 0.27 and 0.60 L∙m-3 for 2010/11 and 2011/12 growing seasons, respectively. The ethanol WUE estimate of the sweet sorghum at Hatfield was 0.53 L∙m-3 for the 2010/11 season and 0.70 L∙m-3 for the 2011/12 growing season. WUE estimates of the sweet sorghum crop were higher for Hatfield compared to Ukulinga research farm. The results from this study showed that the WUE of sweet sorghum was sensitive to plant density. The WUE values confirm that sweet sorghum has high WUE under different climatic conditions.The research presented in this paper forms part of a solicited research project (Water use of cropping systems adapted to bio-climatic regions in South Africa and suitable for biofuel production) that was initiated by the Water Research Commission (WRC) of South Africa in Key Strategic Area on Water Utilisation in Agriculture).Water Research Commission (WRC) of South Africahttp://www.wrc.org.zaam2016Plant Production and Soil Scienc