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
A total of 10 million ha of South Africa has been invaded by 180 alien species, which is over 8 % of the country's total area (van Wilgen et al., 2001). The majority of this invasion extent is within riparian areas that have readily available water and are difficult to manage (Kotzé et al., 2010). In South Africa there is a limited understanding of the extent to which tree species (particularly those in the riparian area) contribute to total evaporation (ET). As such, it is difficult for government organizations and scientists to justify alien tree removal and rehabilitation, unless a known hydrological benefit can be demonstrated. The deep fertile soils, with high soil moisture contents associated with riparian areas, make them ideal for plant establishment and growth (Everson et al., 2007). In South Africa, these areas are extremely vulnerable to invasion by pioneer plant species, particularly species that have historically been introduced for commercial forestry. In South Africa, there is a widespread belief (which has been supported by numerous studies: Olbrich et al., 1996; Dye et al., 2001, 2008; Everson et al., 2007; Gush and Dye, 2008, 2009; Gush et al., 2015) that indigenous tree species, in contrast to the introduced tree species, use less water and should be planted more widely in land rehabilitation programmes. Little research has been undertaken on the riparian area where the availability of water to trees is often not limited (except in severe drought conditions).
The benefits of healthy riparian zones in providing basic ecosystem services are well known (Askey-Dorin et al., 1999; Richardson et al., 2005). These benefits and the impacts of degradation through alien plant invasions were fully described in a study by Scott-Shaw et al. (2017) on the water use of plants in the Mediterranean climate of the Western Cape region of South Africa. Here we summarize the most important aspects relevant to this study.
Commercial forestry has been blamed for increasing the green water (water lost by total evaporation) and decreasing the blue water (water in rivers and dams) in areas across South Africa (Jewitt, 2006). For these reasons, invasive alien plants (IAPs), particularly introduced commercial trees, are considered to be a major threat to water resources and biodiversity.
There is a widespread belief in South Africa and globally that indigenous tree species, in contrast to the introduced trees, are water efficient and should be planted more widely in land restoration programmes. This is based on observations that indigenous trees are generally slow growing and that growth and water use are broadly linked (Everson et al., 2008; Gush, 2011).
At the ecosystem scale, a comprehensive review of numerous internationally
published studies indicates that invasive species use up to 189 % more
water than indigenous-dominated stands, particularly in tropical moist
forests (Nosetto et al., 2005; Yepez et al., 2005; Fritzsche et al., 2006). These findings,
typically outside of South Africa, are limited to mostly herbaceous species
with very few recent studies focusing on measurement of introduced trees. In
the high-rainfall areas of South Africa, invasive alien plants growing in
riparian areas (5726 km
Management of invaded riparian zones can result in hydrological gains disproportionately greater than the catchment area affected, with up to 3 times more streamflow yield than upslope areas (Scott and Lesch, 1996; Scott, 1999).
For many field and modelling applications, accurate estimates of total evaporation are required, but are often lacking. Sap flux density measurements give precise information on flow directions as well as spatial and temporal flow distribution (Vandegehuchte and Steppe, 2013). The heat pulse velocity (HPV) method is the most accurate of the available methods when compared against gravimetric methods (Steppe et al., 2010; Vandegehuchte and Steppe, 2013).
While extensive research has been undertaken on the water use of terrestrial ecosystems, little is known about riparian tree water use and growth for both indigenous and introduced tree species. This gap in knowledge has led to uncertainty and contention over riparian restoration and rehabilitation techniques. In this research, the water use was measured for various indigenous and introduced species that have been identified as economically or ecologically viable. Results from this study provide substance that may facilitate riparian habitat rehabilitation. The the New Forest site in KwaZulu-Natal, South Africa, is part of a Working for Water clearing programme. The government-funded WfW programme clears catchment areas of invasive alien plants with the aim of restoring hydrological functioning while also providing poverty relief to local communities through job creation (Turpie et al., 2008). The aim of this study was to quantify the potential hydrological benefit of the conversion of invaded stands to more pristine stands for forest management practices.
An overview of the study site, sampling design and equipment used to carry out the study has been provided in this section. Details on the heat pulse velocity technique have been documented in a previous paper (Scott-Shaw et al., 2017) and will not been repeated here.
Location of the New Forest farm research area within KwaZulu-Natal, South Africa.
The the New Forest riparian area is located at latitude
29
Approximately 80 % of the precipitation occurs in the summer months
(October to March), which mostly consists of orographically induced and
squall-line thunderstorms (Schulze, 1982). Interception from mist makes a
large contribution to the seasonal precipitation and determines the
distribution of the mistbelt forest. The long-term mean annual precipitation
(MAP) is between 941 and 1000 mm a
The New Forest farm is privately owned. The area south of the Umgeni tributary
has been planted with
Five sites, each representing frequently occurring indigenous and introduced
tree species, were instrumented for water-use monitoring. These trees
included a size range of invasive
Tree physiology and specific data required for the calculation of sap flow and upscaling.
The trees within the riparian forest were in a disturbed state. The overall
canopy height of the indigenous species was low, ranging from 4.1 to 8.3 m.
The invasive species were significantly taller, ranging from 13.1 to
16.6 m. The physical characteristics of each monitored tree are provided
in Table 1. There was variability between the stem moisture content and wood
density between species, which can be explained by the different physical
characteristics of the trees measured (variations in sap wood depth and
active xylem concentration). A forest ecology study (Everson et al.,
2016) undertaken at New Forest compiled stem density measurements for
regrowth forest, invaded riparian areas and on
A meteorological station was established on 19 September 2012 at the New Forest farm in a nearby natural grassland, 250 m from the tree monitoring sites. Rainfall using a tipping bucket rain gauge (TE525, Texas Electronics Inc., Dallas, Texas, USA) was measured at a height of 1.2 m from the ground. Air temperature and relative humidity (HMP45C, Vaisala Inc., Helsinki, Finland), solar irradiance (LI-200, LI-COR, Lincoln, Nebraska, USA), net radiation (NR-Lite, Kipp and Zonen, Delft, the Netherlands), and wind speed and direction (Model 03002, R.M. Young, Traverse City, Michigan, USA) were all measured at a height of 2 m from the ground. These were measured at a 10 s interval and the appropriate statistical outputs were recorded every hour. A flat and uniform short grassland area which was regularly mowed was selected to meet the requirement for FAO 56 reference evaporation calculation.
A heat pulse velocity system using the heat ratio algorithm (Burgess et al.,
2001) was set up to monitor long-term sap flow on all of the selected trees
over a 2-year period. The instrumentation is described further by Clulow et
al. (2013) and Scott-Shaw et al. (2017) and included hourly measurements of
sap flow heater trace using a pair of type-T thermocouple probes. Regular
maintenance was undertaken to ensure sufficient power and operation of the
equipment. An increment borer was used to take non-destructive samples for
sapwood depth, tree age, wood density and moisture content (described by
Marshall, 1958), as well as to allow for upscaling of probe measurements to
whole-tree water use (L h
The stem steady state (SSS) technique, which estimates sap flow by solving a
heat balance for a segment of stem that is supplied with a known amount of
heat (Grime and Sinclair, 1999), was implemented on the smaller trees in the
understorey that were not quantifiable using the HPV technique. Two Dynamax
Flow 32-K systems (Dynamax, Houston, TX, USA) were installed at New Forest.
Each of these systems was powered by a 12 V 100 Ah battery and consisted of a
CR1000 data logger (Campbell Scientific Inc.) and an AM16/32B multiplexer. A
voltage control unit regulated the voltage output depending on the number of
collars and the size of the collars. The gauge's insulating sheath (referred
to as a “collar”) contains a system of thermocouples that measure
temperature gradients associated with conductive heat losses vertically (up
and down the stem) and radially through the sheath (Allen and Grime, 1994).
A foam insulation and weather shield were installed around the stem in order
to sufficiently minimize extraneous thermal gradients that could influence
the heated section of the stem (Smith and Allen, 1996). The conduction of heat
vertically upwards and downwards was calculated by measuring voltages which
corresponded to the temperature difference between two points above and
below the heater (Savage et al., 2000). The radial heat was calculated by
measuring the temperature difference of the insulated layer surrounding the
heater (Savage et al., 2000). Finally, the voltage applied to the heater was
measured. These measurements allowed the energy flux (J s
The daily rainfall, solar radiation, average air temperatures and reference total evaporation at New Forest.
Hourly volumetric soil water contents (VWCs) were recorded at sites 1 and 2 within the riparian forest with three time domain reflectometry (TDR) probes (CS 615, Campbell Scientific Inc.) installed horizontally at each site. The probes were installed at depths of 0.1, 0.3 and 0.5 m below the litter layer, due to shallow soils at the site. A thick litter layer was observed throughout the site consisting of mostly indigenous leaves and large broken branches from cattle and climatic disturbances. The hourly volumetric water content measurements provided an understanding of the responses of trees to rainfall events or stressed conditions. Additional soil samples were taken to determine the distribution of roots, soil bulk density and soil water content.
Although the riparian stand had a heterogeneous composition, the availability
of detailed stem density measurements (Everson et al., 2016) allowed for a
methodology to be followed based on relevant upscaling studies (Ford et al.,
2004; Miller et al., 2007):
Medoid (representative of the population) trees were selected for sap
flow measurement:
most commonly occurring indigenous species (canopy and understorey), most commonly occurring introduced species (canopy and understorey)
and a range of size classes for each species. A species density analysis was undertaken (species and diameter
(> 50 mm) in multiple 400 m A relationship was derived between the measured whole-tree water use (using
HPV and SSS) and each representative size class and species class identified
in the density measurements. This allowed for the estimation of the stand
water flux ( The
Techniques that measure total evaporation over a spatial area are usually
ideal for upscaling sap flow measurements. However, at sites with limited
aerodynamic fetch common in riparian areas, techniques such as eddy
covariance may not always be used in the upscaling process. With the
availability of detailed stem density data, upscaling whole-tree
measurements to stand-level measurements can be done. However, limitations
exist regarding the equipment available to maximize replications.
The historical mean annual precipitation for the New Forest area is 941 mm.
During the 2-year monitoring period the area received 1164 and 1110 mm a
During periods of high solar radiation, the water vapour pressure deficit was
high and correlated to peaks in transpiration rates. An average daily air
temperature of 18.4
The monthly rainfall, monthly solar radiant density and average monthly air temperatures at New Forest averaged over 2 years.
The radial heat pulse velocity of a
Hourly heat pulse velocity of a
Hourly heat pulse velocity of an
Individual whole-tree water use showed a clear seasonal water-use trend for
the semi-deciduous and deciduous indigenous species (Fig. 6). This was
attributed to fewer daylight hours and less heat units during the winter
months than in the summer months, resulting in reduced available energy and
therefore limiting the transpiration process. The daily water use of
Daily sap flow (dotted line) and accumulated sap flow (dashed
line) averaged over 2 years (2013 and 2014) from an indigenous
The introduced
Daily water use for three
With regards to the understorey, the daily water use of the multi-stemmed
Sap flow (daily and accumulated) for each species measured at New Forest.
The daily summer water use of indigenous trees at site 1 (Table 2) showed low
water use with an average of between 9 and 15 L day
The daily summer water use of the
Hourly volumetric soil water content and the hourly rainfall at site 1 at New Forest.
Hourly soil volumetric water content and the hourly rainfall at site 2 at New Forest.
The volumetric soil water content (VWC) measured at New Forest was highly responsive to rainfall events (Figs. 8 and 9). During the wet summer season, the VWC at the indigenous site 1 (Fig. 6) ranged from 27 % in the upper horizon to 35 % in the lower horizon. This indicated a higher clay content in the lower horizon. Towards the dry season, as the vegetation continues to use water, the VWC was depleted to 10 % in the upper horizons. At the introduced site 2, the soils were uniform throughout the horizons. During the summer periods, the profile soil water averaged 27 %, whereas it depleted to 9 % or 11 mm of water per 100 mm depth of soil during the dry periods.
The soils had a dry bulk density (
The VWC at both sites did not respond significantly to rainfall events under
6 mm h
The results obtained from both the HPV and SSS techniques were used to
determine an actual annual water use per unit area of the invaded mistbelt
forest. Two hypothetical scenarios, a pristine forest and a heavily invaded
forest, were also tested. Using the stem density per size class taken from
ecological research completed in the area (Everson et al., 2016),
stands of forest were compared. As the forest did not have a closed canopy,
understorey trees were numerous as more photosynthetically active radiation
(PAR) was available throughout the stand. The water use for a 2-year
average of the riparian forest in its current state (21 % invaded) was
upscaled for all species and size classes. The total stand water use was
approximately 3.3 ML ha
Two hypothetical scenarios were applied using the existing size class
distribution for each species class and extrapolated based on an assumed
invasion level. Assuming that the site was rehabilitated to a more pristine
state, using stem density for non-invaded areas, the upscaled indigenous
stand would use 2.39 ML ha
Due to the prevalence of severe droughts in this area, these results are more likely to provide substance to land managers and decision makers, indicating the hydrological benefit of restoration and rehabilitation activities.
In South Africa, it has been well documented that introduced commercial tree
species, in contrast to indigenous tree species, use more water and if
removed would result in a net hydrological gain (Olbrich et al., 1996; Dye et
al., 2001, 2008; Everson et al., 2007; Gush and Dye, 2008, 2009, 2015). The
greater sapwood area in introduced species, as well as their fast
establishment, tree density and rapid growth, results in a greater
transpiration rate than indigenous species per unit area. The HPV and SSS
techniques have been used, both locally and internationally, on numerous
vegetation types. The accuracy of these measurements has been validated using
gravimetric methods (Burgess et al., 2001; Granier et al., 2001; O'Grady et
al., 2006; Steppe et al., 2010; Vandegehuchte and Steppe, 2013; Uddin et al.,
2014; Forster, 2017). In South Africa, the HPV technique has been shown to
provide accurate estimates of sap flow in both introduced tree species such
as
A recent study showed that introduced stands could use up to 6 times more
water than indigenous species in the riparian area (Scott-Shaw et
al., 2017). However, this difference was largely related to stem density at
a site where high winter rainfall and deep sandy soils were conducive to a
high-density mature introduced stand. The stand at New Forest, which was
highly disturbed and was in a constant state of recovery, did not have a high
stem density of mature trees in its current state. The measurements
undertaken at this site have allowed for an accurate direct comparison of
indigenous and introduced tree water use. Additionally, the measurements of
trees growing in the understorey have provided interesting findings,
indicating significant water use in the subcanopy layer. The results showed
that individual tree water use is largely inter-species specific. As the
introduced species remain active during the dry winter periods, their
cumulative water use is significantly greater than that of the indigenous
species. Small tress (< 30 mm) in the understorey can use up to
2000 L a
Spatial estimates of evapotranspiration are required but are difficult to obtain in remote areas with limited aerodynamic reach. Remote sensing could be one area where this could be useful given appropriate validation. However the nature of the “thin” riparian strip will require finer scales than provided by most remote sensing products used for evaporation modelling (e.g. Landsat 8). The use of drones could provide the best option for these narrow riparian strips in the subsequent studies. Management dynamics are important in these environments. There is potential for these data to be used in a modelling framework with specific inputs for invaded mixed riparian forests. This would provide a suitable land management tool.
The data used in this study can be requested by contacting the corresponding author.
BCSS performed the field work, data processing, and report compilation. CSE performed the field work, report compilation, and report review.
The authors declare that they have no conflict of interest.
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 project was managed and funded by the WRC, with co-funding and support provided by the Department of Economic Development, Tourism and Environmental Affairs (EDTEA). The land owner, Alfie Messenger, of the New Forest farm is acknowledged for allowing field work to be conducted on their property. Assistance in the field by Alistair Clulow, Allister Starke and Siphiwe Mfeka is much appreciated. Edited by: Dominic Mazvimavi Reviewed by: Timothy Dube and two anonymous referees