Contribution of understorey vegetation to evapotranspiration partitioning in apple orchards under Mediterranean climatic conditions in South Africa

Orchard evapotranspiration (ET) is a complex flux which has been the subject of many studies. It often includes transpiration from the trees, cover crops and weeds, evaporation from the soil, mulches and other orchard artefacts. In this study we investigated the contribution of the orchard floor evaporative fluxes to whole orchard ET focusing on the transpiration dynamics of understorey vegetation which is currently not well known. Data on the partitioning of ET into its constituent components were collected in apple (Malus Domestica Bork) orchards with varying fractional canopy cover. The study orchards were in the prime apple growing regions in South Africa. The orchards were planted to the Golden Delicious/Reinders and the red cultivars (i.e. Cripps’ Pink/ Royal Gala/Fuji). Tree transpiration was quantified using the heat ratio method and the thermal dissipation sap flow techniques. Understorey transpiration was measured at selected intervals using micro stem heat balance sap flow gauges calibrated against infrared gas analyser readings. Orchard ET was measured using an open path eddy covariance system while the microclimate, radiation interception, and soil evaporation were also monitored. Orchard floor evaporative fluxes accounted for as much as 80% of the measured ET in young orchards with dense understorey vegetation that covered most of the orchard floor

Water use of selected cover crop species commonly grown in South African fruit orchards and their response to drought stress

Cover crops are widely planted in orchards for a variety of reasons. These include suppressing soil erosion, nutrient cycling, phytosanitary purposes, general orchard aesthetics etc. However, there is need to balance these benefits against use of scarce resources such as water and nutrients. Currently no information exists on how different cover crop species use water in orchards and how they cope with drought stress. The aim of this study was therefore to compare the transpiration dynamics of various cover crop types in order to identify species with conservative water use rates. Studied species included: 1) two exotic legumes i.e. Lupine (Lupinus albus L.), and Common vetch (Vicia sativa), 2) three exotic grasses i.e. Tall fescue (Festuca arundinacea), Rye grass (Lolium perenne), and Kikuyu grass (Pennisetum clandestium) and; 3) grasses that are indigenous to sub-Saharan Africa i.e. African Lovegrass (Eragrostis capensis) and Rhodes grass (Chloris gayana). The crops were planted in pots under controlled greenhouse conditions. Transpiration rates were quantified using miniature stem heat balance sap flow gauges and by manual weighing. Drought stress was imposed by withholding irrigation at selected intervals and the responses were quantified through changes in the water relations of the plants. The study showed that exotic legumes had the highest daily water use which peaked at about 2.4 L per square metre of leaf area per day, followed by exotic grasses at 1.5–2.0 L/m2/d. The indigenous grasses used the least water ranging from 0.8 to 1.2 L/m2/d. The indigenous grasses largely displayed an isohydric response to drought stress by maintaining their leaf water status with increasing soil water deficit. The exotic species, on the other hand, showed risk taking behaviour (anisohydry) wherein both the transpiration and leaf water status decreased sharply as drought stress increased. Consequently, some exotic species failed to recover when stress was relieved. From a water use perspective, this study demonstrates that indigenous grass species are more appropriate as cover crops in South African orchards because of their low transpiration rates and the ability to cope with extended periods of water deficit. © 2021 Elsevier Lt

Characterising the water use and hydraulic properties of riparian tree invasions: A case study of Populus canescens in South Africa

Invasive alien plants (IAPs) pose a serious threat to the already limited water resources in dry countries like South Africa which are facing increasing water shortages. Much of South Africa is expected to get drier in future due to climate change. In addition, the future climatic conditions are also predicted to accelerate the rate at which IAPs will spread, due to favourable growing conditions, further disrupting the provision of goods and services. Previous studies on tree water use in South Africa focused on commercial forests of introduced genera mainly Pinus, Eucalyptus, and Acacia. This study sought to expand these observations by quantifying water use and its drivers in riparian Grey Poplar (Populus canescens) invasions in the Berg River catchment of South Africa. Whole tree hydraulic resistance ranged from ~ 1.4 MPa·h·g-1 for large trees to ~14.3 MPa·h·g-1 for the small ones. These resistances are higher than those found for poplars in temperate climates, suggesting substantial hydraulic constraints to transpiration. Daily peak transpiration varied from 5 to 6 L·tree-1 in small trees (~9.2 cm DBH) to between 35 and 40 L·tree-1 for large trees (~24 cm DBH). Stand-level transpiration peaked at ~4 mm·d-1 in summer (Jan– Feb). However, the annual total transpiration was only 338 mm due to the deciduous nature of the species and also the high hydraulic resistance in the transpiration stream. Daily transpiration was strongly correlated to solar radiation (R2 > 0.81) while the air vapour pressure deficit (VPD) constrained transpiration at high VPD values. We conclude that the water use of the poplar invasions is significantly lower than that of other riparian invasions. The impact of these invasions on the water resources is therefore likely quite low, warranting less priority in alien plant clearing operations aimed at salvaging water

Using sap flow data to assess variations in water use and water status of apple orchards of varying age groups in the Western Cape Province of South Africa

No accurate quantitative information currently exists on how water use of apple (Malus domestica) orchards varies from planting to full-bearing age, leading to poor irrigation and water allocation decision making. This study sought to address this knowledge gap by investigating how the water use and tree water status vary with canopy cover, cultivar, and climatic conditions in 12 orchards growing in prime apple-producing regions in South Africa. The orchards were planted to the Golden Delicious/Golden Delicious Reinders cultivars which are widely planted in South Africa and the Cripps’ Pink/Cripps’ Red/Rosy Glow which are high-value lateseason cultivars. The performance of two transpiration reduction coefficients, one based on sap flow (Ksf) and the other based on soil water depletion (Ks) (FAO approach) were evaluated against the midday stem water potential (MSWP) in all the orchards. While canopy cover had a clear effect on the whole-tree sap flow rates, there were no significant differences in the transpiration per unit leaf area among the cultivars. The daily average sap flux density under unstressed conditions was highest (~284 cm3∙cm–2) in the medium canopy cover orchards (30–44% fractional cover), followed by the mature orchards (~226 cm3∙cm–2), and was lowest in the young orchards (~137 cm3∙cm–2). Canopy cover rather than growing season length had a greater effect on seasonal total water use. Peak daily orchard transpiration ranged from 1.7 mm for young Golden Delicious Reinders trees to 5.0 mm in mature Golden Delicious trees that were maintained with  large canopies to reduce sunburn damage to the fruit. For the red cultivars, the peak daily transpiration ranged from 2.0 to 3.9 mm, and the mature trees were maintained with less dense canopies to facilitate the development of the red fruit colour. The less dense canopies on the red cultivars had water-saving benefits since the seasonal total transpiration was lower relative to the Golden Delicious cultivar. The sap flow derived stress  coefficient was strongly correlated to the MSWP (R2 ~ 0.60–0.97) in all the orchards while Ks was not able to detect plant stress due to over-irrigation. Keywords: cultivar canopy cover transpiration reduction xylem water potentia

Water use dynamics of young and mature apple trees planted in South African orchards: a case study of the Golden Delicious and Cripps' Pink cultivars

Apple orchards have previously been bearing 60–80 t ha−1 at most. However in recent years yield has increased to more than 100 t ha−1. There is need to understand the water requirements of the high yielding orchards, given that high crop loads are associated with high water use rates. The aim of this study was to quantify the water requirements of young and mature unstressed apple orchards. We also assess the impact of climate variables on transpiration rates. Data was collected in 4 orchards in the Western Cape Province. The orchards comprised young non-bearing (< 3 years) and mature trees planted to the Golden Delicious and Cripps' Pink cultivars, all under micro-sprinkler irrigation. Transpiration by the trees was measured using heat pulse velocity sap flow sensors hourly throughout the growing season (October–June). Weather was monitored using an automatic weather station. Tree transpiration was linearly related to the solar radiation, but the relationship with the vapour pressure deficit (VPD) was non-linear. There were no significant differences (p>0.05) in the sapflux density of the Golden Delicious and Cripp's Pink cultivars. This suggests that these two cultivars have similar water use characteristics. Mature orchards transpired between 6000 to 8000 m3 ha−1 season−1 while non-bearing orchards used between 2000 to 3000 m3 ha−1 season−1

Using sap flow data to assess variations in water use and water status of apple orchards of varying age groups in the Western Cape Province of South Africa

No accurate quantitative information currently exists on how water use of apple (Malus domestica) orchards varies from planting to full-bearing age, leading to poor irrigation and water allocation decision making. This study sought to address this knowledge gap by investigating how the water use and tree water status vary with canopy cover, cultivar, and climatic conditions in 12 orchards growing in prime apple-producing regions in South Africa. The orchards were planted to the Golden Delicious/Golden Delicious Reinders cultivars which are widely planted in South Africa and the Cripps’ Pink/Cripps’ Red/Rosy Glow which are high-value late-season cultivars. The performance of two transpiration reduction coefficients, one based on sap flow (Ksf) and the other based on soil water depletion (Ks) (FAO approach) were evaluated against the midday stem water potential (MSWP) in all the orchards. While canopy cover had a clear effect on the whole-tree sap flow rates, there were no significant differences in the transpiration per unit leaf area among the cultivars. The daily average sap flux density under unstressed conditions was highest (~284 cm3∙cm-2) in the medium canopy cover orchards (30–44% fractional cover), followed by the mature orchards (~226 cm3∙cm-2), and was lowest in the young orchards (~137 cm3∙cm-2). Canopy cover rather than growing season length had a greater effect on seasonal total water use. Peak daily orchard transpiration ranged from 1.7 mm for young Golden Delicious Reinders trees to 5.0 mm in mature Golden Delicious trees that were maintained with large canopies to reduce sunburn damage to the fruit. For the red cultivars, the peak daily transpiration ranged from 2.0 to 3.9 mm, and the mature trees were maintained with less dense canopies to facilitate the development of the red fruit colour. The less dense canopies on the red cultivars had water-saving benefits since the seasonal total transpiration was lower relative to the Golden Delicious cultivar. The sap flow derived stress coefficient was strongly correlated to the MSWP (R2 ~ 0.60–0.97) in all the orchards while Ks was not able to detect plant stress due to over-irrigation

Estimating the water requirements of high yielding and young apple orchards in the winter rainfall areas of South Africa using a dual source evapotranspiration model

Exceptionally high yielding (>100 t ha−1) apple orchards (Malus domestica Borkh.) are becoming common in South Africa and elsewhere in the world. However, no accurate quantitative information currently exists on the water requirements of these orchards. Information is also sparse on the water use of young apple orchards. This paucity of data may cause inaccurate irrigation scheduling and water allocation decisions, leading to inefficient use of often limited water resources. The aim of this study was therefore to investigate the dynamics of water use in eight apple orchards in South Africa planted to Golden Delicious and the red cultivars i.e. Cripps’ Pink, Cripps’ Red and Rosy Glow in order to understand how canopy cover and crop load influence orchard water use. Four of the orchards were young (3–4 years after planting) and non-bearing, while the other four were mature high yielding orchards. Transpiration was monitored using sap flow sensors while orchard evapotranspiration (ET) was measured during selected periods using eddy covariance systems. Scaling up of ET to seasonal water use was done using a modified Shuttleworth and Wallace model that incorporated variable canopy and soil surface resistances. This model provided reasonable estimates in both mature and young orchards. The average yield in the two mature ‘Cripps’ Pink’ was ∼110 t ha−1 compared to ∼88 t ha−1 in the ‘Golden Delicious’ orchards. However, average transpiration (Oct-Jun) was ∼638 mm for the ‘Cripps’ Pink’ and ∼778 mm in the ‘Golden Delicious’ orchards. The peak leaf area index was ∼2.6 and ∼ 3.3 for the mature ‘Cripps’ Pink and ‘Golden Delicious’ orchards. So, canopy cover rather than crop load was the main driver of orchard water use. Transpiration by the young orchards ranged from 130 to 270 mm. The predicted seasonal total ET varied from ∼900 to 1100 mm in the mature orchards and it was ∼500 mm in the young orchards. Orchard floor evaporation accounted for ∼18 to 36% of ET in mature orchards depending on canopy cover and this increased to more than 60% in young orchards.The Water Research Commission of South Africa (Project no WRC K5 2398//4), the South African Apples and Pears Producers Association and the South African Parliamentary Grant to the CSIR on Water Security (Project number ECHS043).http://www.elsevier.com/locate/agwat2019-09-30hj2018Plant Production and Soil Scienc