The adverse impacts of alien plant invasions on water flows have been a prime motivation for South Africa’s Working for Water Programme. The approach used in this study builds on a previous national ...assessment in 1998 by incorporating factors that limit plant water-use, information from recent research and improved flow reduction models. The total reduction in flows is estimated to be 1 444 million m3•yr-1 or 2.9% of the naturalised mean annual runoff (MAR), less than half of the 3 300 million m3•yr-1 estimated in 1998. Two main factors account for this difference: (a) a decrease in the estimated unit-area flow reduction to 970 m3•ha-1•yr-1 compared with 1 900 m3•ha-1•yr-1 estimated in 1998, largely due to the new model being based on more representative reduction factors; and (b) the updated estimate of the condensed invaded area of 1.50 million ha (previously 1.76 million ha), although the taxa mapped for this assessment only accounted for 1.00 million of the 1.76 million ha reported in 1998. Reductions due to invasions in Lesotho are estimated to be about 161 million m3•yr-1 and those in Swaziland about 193 million m3•yr-1. The taxon with the greatest estimated impact was wattles (Acacia mearnsii, A. dealbata, A. decurrens) with 34.0% of the total reductions, followed by Pinus species (19.3%) and Eucalyptus species (15.8%). The revised estimate is considered on the low side largely because the extent and impacts of riparian invasions have been underestimated. If the current estimates that 4-6% of Acacia mearnsii, Eucalyptus, Populus and Salix invasions are riparian, are adjusted to a more representative 20%, 50%, 80% and 80%, respectively, the total reductions increase by nearly 70% to around 2 444 million m3•yr-1. Producing these estimates involved a number of assumptions and extrapolations, and further research is needed to provide more robust estimates of the impacts.
The potential of the country's numerous indigenous tree species to address challenges facing the commercial forestry industry in South Africa is under-explored. Relevant issues include the rising ...demand for timber and non-timber forest products due to population and economic growth, minimal available land for expanding traditional commercial forestry operations, and known streamflow reduction impacts associated with introduced plantation tree species. However, little is known about the water use and corresponding growth rates of indigenous tree species, and consequently their potential as an alternative form of forestry and sustainable resource use. In this study, the water use, growth rates and resultant water-use efficiency of Vachellia kosiensis (Acacia kosiensis) (dune sweet thorn) were quantified within a mature stand of these trees situated on rehabilitated dune mining land in the Richards Bay area of South Africa. Hourly sap flow rates were measured over a two-year period in five trees, and tree heights and stem circumferences were recorded periodically throughout the monitoring period, to derive biomass increments. Rates of growth and water use were used to calculate water-use efficiency, defined as mass of utilisable (stem) wood produced per unit of water transpired. Results were compared with similarly sampled data for introduced plantation species, including Eucalyptus, Pinus and Casuarina. Results showed that the indigenous V. kosiensis trees used less than half the water used by introduced plantation species. The growth rates of individual V. kosiensis trees were unable to compete with introduced species; however, their higher tree density nevertheless yielded a mean annual increment of 10.3 m
3
ha
−1
y
−1
(7.2 t ha
−1
y
−1
). Furthermore, their correspondingly low water-use rates indicated that the indigenous trees had similar biophysical water-use efficiency values compared with genetically improved introduced tree species and highlighted their potential as an attractive land-use option in appropriate locations within water-constrained or dryland areas.
Societal Impact Statement
Daunting global challenges of climate change and biodiversity loss may seem overwhelming. However, gardeners have a secret weapon—gardens, balconies, indoor planting, yards ...and allotments are mini‐ecosystems that offer opportunities to counter perceptions of helplessness, inadequacy and resultant inaction by using those spaces to ‘Do what we can, with what we have, where we are’. Minimising gardening ‘footprints’ to mitigate harmful impacts, whilst maximising gardening ‘handprints’ to enhance benefits, is readily achievable. With this in mind, the Royal Horticultural Society is leading research into environmental horticulture for gardens, and benefits for individual wellbeing.
Summary
This article presents an integrated and applied research approach to the unique and multi‐disciplinary area of science referred to here as environmental horticulture. It does this by: (a) providing an institutional perspective (The Royal Horticultural Society) on a research approach for this particular area, emphasising why domestic and community gardens are important in the context of global environmental threats; (b) presenting four primary research focus areas and project examples; and (c) highlighting interdisciplinary linkages, future research needs, public engagement/knowledge sharing opportunities, and ‘Green Skills’ development in the area of environmental horticulture. Research focus areas discussed are: (1) responding to the changing climate (adaptation, mitigation and resilience solutions in gardens); (2) ‘plants for purpose’ (harnessing the potential of horticultural plant diversity, and gardening, to help regulate environmental conditions); (3) sustainability and climate risk reduction through effective and efficient resource management (reduction, re‐use, recycling and repurposing); and (4) gardening and cultivated plant choice for human health and wellbeing. We argue that a key research priority is improving our understanding of the linkages and interactions between soil, water, plants, weather and people. These crucial linkages affect above and below ground processes, for both outdoor and indoor plants. They impact the effectiveness with which water and nutrient cycling takes place, the extent to which ecosystem services may be delivered, and the resultant capacity of gardens and gardening to provide environmental and human health benefits.
Daunting global challenges of climate change and biodiversity loss may seem overwhelming. However, gardeners have a secret weapon—gardens, balconies, indoor planting, yards and allotments are mini‐ecosystems that offer opportunities to counter perceptions of helplessness, inadequacy and resultant inaction by using those spaces to ‘Do what we can, with what we have, where we are’. Minimising gardening ‘footprints’ to mitigate harmful impacts, whilst maximising gardening ‘handprints’ to enhance benefits, is readily achievable. With this in mind, the Royal Horticultural Society is leading research into environmental horticulture for gardens, and benefits for individual wellbeing.
Urban hedgerows can act as barriers to roadside particulate air pollution, but details on methodologies to quantify pollutant capture, most efficient species to use, and practical planning advice are ...still evolving. We aimed to compare three widely used approaches to quantify particulate accumulation and deposition, and to ascertain the most cost-effective and robust approach for the rapid screening of various types of hedges. Secondly, using the most efficient methodology, we screened the summertime deposition of particulates on roadside hedges in Reading (UK), not just on species with differing leaf surface characteristics, but also along a transect of the hedge depth. Finally, we also compared particles’ capture by hedge leaf surfaces in locations with different traffic intensities, to try and ascertain the extent of reduction of particles’ concentration in various hedge types and urban locations. Results suggest that the gravimetric determination of particulate capture was most rapid and cost-effective, while being least technically demanding. We confirmed that hairy and more complex leaves captured most particulates, particularly in the >10 μm range. However, species choice only had a significant impact on the extent of capture on major roads, where the pollutant concentrations were highest. Furthermore, only hedge depths in excess of 2 m were found to noticeably reduce the concentration of fine particles in species with less capacity for particulates’ capture. Findings complement the growing body of knowledge to guide urban and landscape planners in choosing the most appropriate species to mitigate air quality in various urban contexts.
•Pecans should not be grouped under stone fruit in FAO-56.•A six stage crop growth curve should be applied to pecan trees.•Pecan reference Kc values should be adjusted for climate and canopy ...cover.•Thermal time is not the sole determinant of canopy development.•Visual observations of crop growth stages are recommended.
Mature pecans use large quantities of water and therefore the accurate estimation of water use or evapotranspiration (ET) of pecan orchards is critical for judicious irrigation water management and planning. Measuring ET under all possible combinations of climate and management practices is not possible, and as a result, models are used to estimate ET. Empirical modelling approaches are more widely adopted than the more complex mechanistic models, as they are more easily parameterized, but they are not always easily transferred across a wide range of growing conditions, making local evaluation and validation essential. This study evaluated existing crop coefficient models in a mature pecan orchard for three seasons in a semi-arid subtropical climate. Whilst the generic FAO-56 approach, using parameters provided for stone fruit performed reasonably well on a seasonal basis, accurate monthly estimates of ET were not achieved throughout the season. A closer analysis of data from the current study and a previous study in New Mexico, revealed that a six stage crop coefficient curve should be considered for pecans, together with higher mid-season crop coefficient (Kc) values for mature orchards. More accurate estimates of monthly ET for mature pecan orchards were obtained when reference Kc (Kc-ref) values for a well-managed mature pecan orchard in New Mexico were adjusted for local conditions of climate, using a growing degree day—Kc relationship and canopy cover. The adjustment for climate should, however, be used with caution. A comparison between seasons at Cullinan and with New Mexico suggests that whilst thermal time is likely to predict the start of leaf fall, it is unlikely to accurately predict canopy development at the start of the season. As a result it is suggested that in future a crop growth curve based on visual observations of phenological stages is developed.
Forests and trees are key to solving water availability problems in the face of climate change and to achieving the United Nations Sustainable Development Goals. A recent global assessment of forest ...and water science posed the question: How do forests matter for water? Here we synthesize science from that assessment, which shows that forests and water are an integrated system. We assert that forests, from the tops of their canopies to the base of the soils in which trees are rooted, must be considered a key component in the complex temporal and spatial dimensions of the hydrologic cycle. While it is clear that forests influence both downstream and downwind water availability, their actual impact depends on where they are located and their processes affected by natural and anthropogenic conditions. A holistic approach is needed to manage the connections between forests, water and people in the face of current governance systems that often ignore these connections. We need policy interventions that will lead to forestation strategies that decrease the dangerous rate of loss in forest cover and that-where appropriate-increase the gain in forest cover. We need collective interventions that will integrate transboundary forest and water management to ensure sustainability of water supplies at local, national and continental scales. The United Nations should continue to show leadership by providing forums in which interventions can be discussed, negotiated and monitored, and national governments must collaborate to sustainably manage forests to ensure secure water supplies and equitable and sustainable outcomes.
► Afforestation in South African catchments is limited due to water scarcity. ► A challenge is to increase production within water-resource constraints. ► Biophysical and economic measures of ...water-use efficiency (WUE) were tested. ► Biophysical WUE was higher for introduced tree species due to fast growth rates. ► Indigenous species had good economic WUE due to high output prices.
Water resources in many catchments in South Africa (SA) are over committed and water is projected to become scarcer. The impacts of plantation forestry on water resources in SA are well known and legislation limits further afforestation. Nevertheless demands for wood continue to grow. A challenge therefore is to increase the production of forest products within water constraints. This paper presents research into the economic and biophysical efficiencies with which indigenous and introduced tree-production systems in SA use water to produce harvestable biomass. Its purpose is to better inform resource allocations. Key findings are that: introduced plantations are more efficient at using water to produce harvestable biomass than indigenous species; the lower water-use efficiencies of indigenous species are due to slow growth rates and not high water-use rates; and the performance of indigenous forests improves when using the economic return per unit of water used – using the residual imputation approach to value the water – because of their lower production costs and higher product prices. Introduced plantations make up the majority of afforested land and total outputs in SA, however, therefore innovative mechanisms are needed to overcome barriers preventing the financing of indigenous forests. Possible financing mechanisms include the UN CDM and REDD programmes and tax breaks for superannuation funds.
•2 Years of sap flow and total evaporation data from apple trees are presented.•Water use, irrigation and yield data are used to determine apple water footprints.•Novel partitioning of Kc values into ...‘blue’ and ‘green’ WF components.•Field data substantially improves accuracy of WF calculations & facilitates extrapolation.•Results show a water footprint of 187–237 m³ per tonne of apples produced.
Field scale quantifications of the water footprints (WF) of crops, based on actual measurements, provide valuable and detailed information for on-farm water use management. However, watershed-based WF assessments are more appropriate for large-scale water resources management beyond the farm boundaries. In this study, blue, green and grey WF information, using the Water Footprint Network approach up to farm gate level, was determined for an apple (Malus pumila) orchard growing under Mediterranean climate conditions in South Africa. WFblue and WFgreen were determined through measurements of transpiration, total evaporation, rainfall, irrigation and other operational water uses, and WFgrey was calculated from fertilizer applications. Combined field-scale blue/green/grey water footprint data were extrapolated to watershed scale by means of representative monthly FAO-56 type reference potential evaporation (ETo) values and crop coefficients derived from the field scale observations. Resultant water use values were converted to a volumetric equivalent by multiplying by the area under apple orchards in each watershed. The volumetric equivalents were then summed for all QCs in the Water Management Area to calculate the overall water footprint for apple production in the basin. Orchard-scale WF, taking into account all water uses and a fruit yield of 61.5 t.ha−1, was 212.1 m3. t−1, comprising 62.7% WFblue, 14.9% WFgreen and 22.5% WFgrey. Irrigation thus contributed the bulk of the WF in the apple production chain. Resultant water productivity (WP) figures for the orchard averaged 4.72 kg.m¯³. Scaling up the WF estimates to QC level gave an average value of 228.4 m3. t−1 (WP = 4.41 kg.m¯³). Accurate crop coefficients, representative weather / ETo data and reliable crop areas within each QC are critical requirements in terms of upscaling WF estimates, where the information has potential application in water allocation decisions, Water-Energy-Food (WEF) Nexus cost-benefit analyses and other water resource management decisions.
A number of studies undertaken in South Africa to quantify the green water-use (total evaporation) of introduced commercial forestry species have shown conclusively that green water-use from ...commercial forest plantations is substantially higher than from the original grasslands or fynbos that were replaced by afforestation. Green water can be categorised into productive (transpiration) and non-productive (canopy and litter interception and soil evaporation) fluxes. There is a widespread perception within South Africa that indigenous tree species, in contrast to commercial forestry genera/species, are water-wise and should thus be planted more extensively in view of their more efficient use of water. However, information on the water-use of indigenous trees and forests is scarce and indirect, and the relative contributions of transpiration, canopy interception and litter interception to total evaporation have until now not been investigated in South Africa. To quantify these fluxes, both field measurements and modelling were undertaken. In this study, green water-use by indigenous Podocarpus henkelii and an exotic species, Pinus patula , were compared. The results from this study showed that the productive green water-use by P. henkelii and P. patula was 41.0% and 95.9% of gross precipitation, respectively, over the 18-month period of this study. The non-productive canopy and litter interception by P. henkelii accounted for 29.8% and 6.2%, respectively, while canopy and litter interception accounted for 22.1% and 10.7%, respectively, for P. patula . The productive green water-use efficiency (WUE) of P. henkelli and P. patula is 7.14 g mm ⁻¹ and 25.21 g mm ⁻¹, respectively, in comparison with the total green WUE of 3.8 g mm ⁻¹ and 18.8 g mm ⁻¹. From a water resources management and planning perspective it is important to consider the total green WUE, but also to have a good understanding of the relative contributions of each component of the green water fluxes so that water abstracted from the soil can be differentiated from the water that does not reach the soil due to losses of canopy and litter interception and does not get lumped as one evaporative loss.
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