REHABILITATION OF ALIEN INVADED RIPARIAN ZONES AND CATCHMENTS USING INDIGENOUS TREES: AN ASSESSMENT OF INDIGENOUS TREE WATER-USE Volume 1: Research Report Report to the WATER RESEARCH COMMISSION and DEPARTMENT OF ENVIRONMENTAL AFFAIRS by C.S. Everson (Editor)a,b,c With Contributions by: Scott-Shaw B.C, Everson C.S, Geldenhuys C.J, Starke, A, Atsame-Edda A, Schutte S, R, Mupemba Mwamba aDepartment of Plant Production and Soil Science, University of Pretoria bSchool of Agriculture, Earth & Environmental Sciences, CWRR, University of KwaZulu-Natal cSAEON, Grasslands-Forests, Wetlands Node, Pietermaritzburg WRC Report No. 2081/1/16 ISBN 978-1-4312-0828-9 August 2016 Obtainable from Water Research Commission Private Bag X03 Gezina, 0031 [email protected] or download from www.wrc.org.za DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC nor does mention of trade names or commercial products constitute endorsement or recommendation for use. © Water Research Commission Rehabilitation of Alien Invaded Riparian Zones and Catchments Using VOLUME 1 Indigenous Trees: An Assessment of Indigenous Tree Water-use EXECUTIVE SUMMARY water-use research is based on forest hydrology REPORT STRUCTURE and has focused on exotic tree species and their impacts on streamflow. In order to support the Government’s rural tree programmes, there is a This report is organized in two volumes: need to expand current research to include the water-use of indigenous trees used in forest Volume 1 (Research Report) outlines innovative expansion, the rehabilitation of degraded lands approaches to the rehabilitation of invaded and the restoration of riparian zones. One of the riparian forests and the impacts on their water- biggest problems with current rehabilitation use for three case studies (Western Cape Afro- programmes is that exotic species are used to temperate forest, Eastern Mistbelt Forest and restore the ecosystem services (e.g. water Maputaland Coastal Belt). production, plant production and soil fertility). However, this ignores the importance of Volume 2 (Guidelines Report) provides ecosystem structure and functioning (e.g. recommendations and instructions on how to biodiversity). Research and policy support in rehabilitate invaded riparian forests. South Africa is required to promote and scale-up indigenous tree planting and growing initiatives BACKGROUND in degraded areas and riparian zones. The impact of expanding the use of indigenous trees Agroforestry is receiving worldwide recognition to catchment hydrology is of critical importance as it has the potential to make an impact in in a water scarce country. It is therefore many diverse areas such as poverty alleviation, important to understand the plant water-use global climate change, carbon sequestration, (transpirational changes) brought about by tree domestication and payments for introducing indigenous trees into degraded environmental services (Hicks et al. 2014). While landscapes and alien cleared riparian zones. A this is the case in most regions of the world, it is common practice to facilitate indigenous forest clear that South Africa has lagged behind in recovery after removal of alien tree is through implementing agroforestry research. There is selective thinning of the invader plant stand to clearly a need to broaden agroforestry research stimulate the growth of natural forest seedlings beyond the traditional alley cropping approach. (Cawe & Geldenhuys 2007; Geldenhuys & Innovative approaches recently used in South Bezuidenhout 2008; Geldenhuys & Delvaux Africa include: (i) the management of regrowth 2007). An alternative is to plant relatively fast- after forest degradation to enable rural resource growing indigenous forest pioneer and early users to utilize resources while restoring the regrowth species to promote establishment of forest; and (ii) manipulation of plantation or late successional species. The question then is invasive alien tree stands towards natural forest whether these indigenous species use less water recovery with species that are also used by rural than the fast-growing invader plant stands? communities, i.e. combining forest rehabilitation There is a widespread belief in South Africa that actions with resource use needs satisfaction indigenous tree species, in contrast to the exotic (Geldenhuys 1997, 2003, 2007; Geldenhuys & trees, are water efficient and should be planted Delvaux 2007). Trees can be used for catchment more widely in land restoration programmes. rehabilitation and riparian zone management as This is based on observations that indigenous well as to combat global climate change. In trees are generally slow growing, and that order to use trees for these purposes it is growth and water-use are broadly linked. necessary to find alternatives to the use of fast However, tree water-use is technically difficult growing exotic tree species which are high and expensive to measure, and so there is scant water-users. Much of the South African tree evidence of low water-use by indigenous trees. iii Rehabilitation of Alien Invaded Riparian Zones and Catchments Using VOLUME 1 Indigenous Trees: An Assessment of Indigenous Tree Water-use This is even more so for pioneer tree species SPECIFIC AIMS more suited to the rehabilitation of degraded lands and those found re-colonising riparian 1. To investigate the water-use (transpiration zones previously invaded with exotic trees (e.g. rates) of a selection of pioneer indigenous wattle). According to Clausnitzer et al. (2011), tree species suitable for forest expansion, future riparian area conditions may not only rehabilitation programmes and riparian include increasing temperatures and zone restoration following invasive alien evaporative demands but also a higher clearing or manipulation. frequency of extreme events. Stand water 2. To review available knowledge on the relations are crucial for valuing site conditions, potential of indigenous trees for and long term observations are necessary for a rehabilitation better understanding of inter- and intra-annual 3. Determine the potential impacts of alien variability including responses to extremes or tree removal on the re-establishment of management effects (Clausnitzer et al., 2011). indigenous tree species. For many field and modelling applications, 4. Incorporate the data into a modelling accurate estimates of evapotranspiration (ET) framework for temporal and spatial are required, but are often lacking. Modelled extrapolation of water-use predictions. estimates are often used without proper 5. Provide guidelines and recommendations validation and the verification of the results is for the management of water-efficient questionable, especially in dynamic and highly- indigenous tree species for use in land sensitive riparian areas. Innovative approaches rehabilitation programmes. to forest restoration and rehabilitation of degraded land in South Africa include: • the management of regrowth after forest METHODS degradation, to enable rural resource users to utilize resources, while restoring the Study sites and species selection. forest; and • the manipulation of plantation or invasive Various climatic regions were considered in alien tree stands towards natural forest order to investigate the water-use of species recovery with species that are also used by suitable for forest expansion and rehabilitation rural communities. programmes throughout South Africa (Objective 1) and later, to incorporate the data into a In order to use trees for these purposes it is modelling framework for water-use predictions necessary to find alternatives for the use of fast- (Objective 4). The presence of a Working for growing exotic tree species, which are known to Water clearing programme in a location was be high water-users. Although the ET of the essential to the selection of a site to meet riparian zone vegetation is a critical component Objective 3. With these requirements in mind, of the overall catchment water balance, it is still the following sites were selected: Western Cape poorly understood. Due to the availability of Afro-temperate forest (Buffeljagsrivier), Eastern accurate techniques to quantify this large, and Mistbelt Forest (New Forest) and Maputaland often poorly understood, component of the Coastal Belt (Vasi pan). hydrological cycle, modelled estimates can be improved and therefore better understood, as The selection of priority species for this project can direct measurements. There is a need to was motivated by a combination of expert expand current research and to include the advice (workshops), gaps in past research, and a water-use of indigenous trees used in forest short-list of species selected by Gush et al. expansion, the rehabilitation of degraded lands (2010) on the water-use and economic value of and the restoration of riparian zones. the biomass of indigenous trees under natural and plantation conditions (WRC Project K5/1876). The primary aim was to select species that occurred in the forest biome and over various climatic regions. iv Rehabilitation of Alien Invaded Riparian Zones and Catchments Using VOLUME 1 Indigenous Trees: An Assessment of Indigenous Tree Water-use Based on this approach the alien species (e.g. rehabilitation of invaded riparian zones using Acacia mearnsii De Wild.) and the most the SWAT and ACRU hydrological catchment important indigenous tree species (e.g. Celtis models.