A compensatory forestry approach to clearing alien invasive vegetation from riparian zones in a South African basin

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1 FRIEND 2002 Regional Hydrolog}': Bridging the Gap between Research and Practice (Proceedings of tile Fourth International FRIIiND Conference held at Cape Town. South Africa. March 2002). IAHS Publ. no A compensatory forestry approach to clearing alien invasive vegetation from riparian zones in a South African basin G. P. W. JEWITT, M. C. HORAN School of Bioresources Engineering und Environmental Hydrology, University of Natal, Private Bag XOI, Scottsvi/le 3209, KwaZulu-Natal, South Africa iewittg(j7).nti.ac.za K. B. MEIER Land Resources International, PO Box 252. Pinetown 3610, KwaZiilu-Nala! South Africa Abstract In South Africa, it is accepted that widespread invasive alien vegetation is impacting negatively on biodiversity and water resources. Water resource management now includes consideration of invasive alien plant management. It has been suggested that in some areas of the countiy, the clearing of alien vegetation in riparian zones may be compensated for by the establishment of commercial plantations elsewhere in a basin, and still result in an improvement of the water supply and an economic benefit to the grower. A case study in KwaZulu-Natal province in which hydrological impacts of possible land-use changes associated with this so-called "compensatory forestry" are simulated with the use of the ACRU agrohydrological model, and the results then linked to an economic analysis is presented. Results indicate that clearing of alien invasive vegetation in the riparian zones and the establishment of commercial timber plantations in optimal areas of the basins will have both hydrological and economic benefits. Key words compensatory forestry; alien invasive vegetation; riparian zone; ACRU; South Africa; working for water INTRODUCTION Management of water resources in South Africa is complicated by high variability of runoff characterized by unpredictable extremes in the form of droughts and floods. In addition to these challenges, planners and managers are faced with increasingly complex issues arising from growing competition for water resources, a National Water Act (NWA) which strives for an equitable distribution of water for all users, including a so-called "Reserve" for basic human and environmental requirements. Concerns for the protection of water resources have led to the control of commercial afforestation and forestry practices since Furthermore, it is estimated that some ha of land are infested to some degree with alien invasive vegetation (AIV). The removal of such vegetation, in particular that in riparian zones which is assumed to use more water than the indigenous vegetation it replaces, has been the focus of the so-called "Working for Water" programme. This initiative has amongst its aims, the enhancement of water supplies in basins heavily infested by alien vegetation species and the social and economic upliftment of the communities from which the labour for the programme is drawn.

2 398 G. P. W. Jewitt et al. It has been suggested that in areas of the country where a moratorium on additional commercial afforestation has been enforced because of downstream water supply concerns, that the clearing of alien vegetation in the riparian zones may be compensated for by the establishment of commercial plantations elsewhere in a basin and still result in an improvement of the water supply and an economic benefit to the grower. However, this is an assumption that requires testing before being implemented. In this paper, we present a case study in which impacts of land-use changes associated with a so-called "compensatory forestry" approach in the Pongola- Bivane basin in northern KwaZulu-Natal province, South Africa, are simulated with the use of the ACRU agrohydrological model, and the results then linked to an economic analysis. The analysis provides a useful example of the role of research and the incorporation of research findings in "on the ground" water resources management" by illustrating how practitioners' intuition, research, field assessment and simulation modelling are integrated through a practical application concerning a real hydrological and water resources planning issue. ALIEN INVASIVE VEGETATION IN SOUTH AFRICA It has been suggested that invading alien plants reduce the annual flow in South Africa's rivers by 7% each year (Versfeld et al., 1998). In KwaZulu-Natal province, over ha, or more than 10% of the surface area, is infested to some degree. Acacia species (wattle) are the most widespread invasive species (Versfeld et al., 1998). Riparian areas are usually found to be the most heavily infested areas of a basin as seed dispersion and tree establishment conditions are optimal. It is often suggested that, because of the continuous availability of water, trees growing in the riparian zone use substantially more water than those growing elsewhere in the basin, though there has been little conclusive research to support this assumption. THE ACRU AGROHYDROLOGICAL MODELLING SYSTEM The ACRU model is a multipurpose and multi-level integrated physical conceptual model that can simulate streamflow, total evaporation, and land cover/management and abstraction impacts on water resources at a daily time step (Schulze, 1995). The model revolves around multi-layer soil water budgeting (Fig. 1): Streamflow is generated as stormflow and baseflow dependent upon the magnitude of daily rainfall in relation to dynamic soil water budgeting. Components of the soil water budget are integrated with modules in the ACRU system to simulate many other basin components including irrigation requirements and sediment yield. Spatial variation of rainfall, soils and land cover is facilitated by operating the model in "distributed" mode, in which case the basin to be modelled is subdivided into sub-basins. Within these sub-basins, units of similar hydrological response, based largely on land-use zones, are designated to facilitate simulation of land-use changes, or isolation of, for example, riparian zones. Land cover and land use affect hydrological responses through canopy and litter interception, infiltration of rainfall into the soil and the rates of evaporation and transpiration of soil water from the soil.

3 Clearing alien invasive vegetation from riparian zones in a South African basin 399 Fig. 1 The ACRU agrohydrological modelling system (Schulze, 1995). The principal applications of the model are in the assessment of environmental and land-use related impacts for water resources planning. Many verification studies have been performed, both on internal state variables and final model output (Schulze, 1995). In particular, forest water use and the impacts on streamflows have been verified at several locations in South Africa (Jewitt & Schulze, 1999). THE PONGOLA-BIVANE STUDY The Pongola-Bivane basin encompasses an area of km 2 covering the northern border between KwaZulu-Natal and Mpumalanga provinces of South Africa with a small area extending into Swaziland. Altitudes range from 714 m in the east to 1750 m in the west. Mean annual precipitation (MAP) ranges from 1071 mm in the west to 747 mm in the east (Fig. 2). For the purposes of simulation with the ACRU model, the basin has been delineated into 20 sub-basins, generally corresponding with the Department of Water Affairs and Forestry (DWAF) designated quaternary basins (QC) for the area (Fig. 2).

4 Fig. 2 Location and sub-basin delineation ofthe Pongola-Bivane basin.

5 Clearing alien invasive vegetation from riparian zones in a South African basin 401 Within this basin DWAF has imposed a moratorium on permits being issued for additional commercial afforestation because of water resource concerns. However, a detailed land-use study based on : aerial photographs, showed that approximately 130 km" of the basin's riparian area is infested by acacia species (Schulze et al., 1998). It has been suggested that a commitment by stakeholders to the removal of AIV, may be compensated by a partial lifting of the afforestation moratorium under specific circumstances. The aims of this study were therefore, to assess the impact of AIV on the mean annual runoff, seasonal runoff and low flows in the Pongola-Bivane basin, and to estimate the area of compensatory forestry that could be established in each quaternary basin. Basin information Various physiographic data including rainfall, soils and land-use information are required as input to ACRU (Fig. 1). Soils information was obtained from a national soils database and rainfall data from the South African Weather Bureau for the period Land use was derived from 1: aerial photographs flown in 1996 and updated by intensive field work in mid 1997 and In the absence of detailed soils information, a South African "standard" is that a strip 30 m from the river channel can be designated as the riparian zone. With the use of geographical information system (GIS) tools, a 60-m wide network of polygons following the river patterns could be demarcated, the area of which could then be calculated and the percentage of alien vegetation of the riparian zone occurring within each sub-basin could then be calculated (Table 1). The ACRU soils and vegetation parameters could then be adjusted for simulation of the hydrological processes occurring within each riparian zone. These assumptions of riparian zone area and percentage infestation concur with findings of field evaluations undertaken in Table 1 Estimated riparian zone area, densely infested area and percentage dense infestation of riparian zones in the Pongola-Bivane study area. QC number Estimated riparian area (km 2 ) Riparian infestation (km 2 ) % Riparian infestation QC number Estimated riparian area (km 2 ) Riparian infestation (km 2 ) % Riparian infestation Assumptions made in this study In order to perform the compensation analysis for afforestation in lieu of riparian clearance certain assumptions were necessary:

6 402 G. P. W. Jewitt et al. (a) The AIV in the riparian zone was assumed to be dense 5-year-old wattle with a grass and litter understorey. (b) Any basin commercially afforested to 20% or more was not permitted any extra (compensatory) afforestation. This is a general rule applied to QCs in recent South African afforestation license applications. (c) Compensatory afforestation is estimated for each basin individually, before the influences of upstream contributions are considered. (d) The cleared riparian zone was deemed to consist of the indigenous vegetation of the area, most commonly a grass known as North East Mountain Sourveld. (e) Any compensatory afforestation would consist of the genera already planted in that basin in the same proportion as current conditions. RESULTS Streamflow in the basin was simulated with model configurations which represent riparian zones in both cleared and infested states. Analyses are focussed on median annual and 4-month low flow conditions. The simulated impacts of clearance at the outlet of the basin are depicted in Fig. 3. It should be noted that the results suggest the most significant improvements in streamflow will be obtained in the drier winter months of the year. The differences in median annual streamflow between existing and cleared conditions provide a basis for the estimation of an area of compensatory commercial afforestation for each sub-basin through an optimization procedure, in which the area of commercial afforestation in each basin was increased until the simulated median annual streamflow matched that estimated under existing conditions. As noted previ- Fig. 3 zone. Comparison of accumulated streamflows before and after clearing the riparian

7 Table 2 Differences in streamflows and areas between present land cover conditions, and estimated compensatory afforestation after alien clearance from the riparian zone. QC Basin area Present (infested) Compensatory Difference Difference Present Total Additional % Basin % New number conditions: afforestation conditions: in median in 4-month afforesta afforestation comp. afforested (compen Streamflow 4-month Streamflow 4-month conditions low flow tion incl. Afforesta after satory) (mm) low flow (mm) low flow Compensa tion compensa forest area median mean median mean tory tion conditions conditions conditions conditions (km 2 ) (annual) (mm) (annual) (mm) (mm)* (mm) (km 2 ) (km 2 ) (km 2 ) t * T lt f f t f f t t t * A negative value in the difference column indicates that increase in streamflow caused by riparian clearance is greater than the reduction caused by afforestation due to the threshold 20% being reached in that basin, or upstream, t Basins which are already 20% afforested.

8 404 G. P. W. Jewitt et al. ously, sub-basins where afforestation already exceeds 20% were not considered for compensatory forestry and the water "available" from clearing riparian zones in these basins is transferred to the next downstream sub-basin. Thus, the estimated areas of compensatory forestry may reflect more than would be the case if only the water produced from clearing in that particular sub-basin was available. The results of this study are presented in Table 2. In order not to prejudge applications for new afforestation in the basin, the climatic suitability of these areas for commercial forestry was not considered. ECONOMIC FEASIBILITY OF COMPENSATORY FORESTRY The potential economic benefits of this approach were assessed by comparison of the costs of clearing AIV and the economic benefits that could be obtained from the establishment and ultimate harvesting of an area of commercial forestry. This comparison is made for different ratios of the area of compensatory forestry established vs the area of riparian zone cleared, ranging from 1 ha commercial forestry for clearing 1 ha of riparian zone (1:1) to 5 ha commercial forestry for clearing 1 ha of riparian zone (5:1). The cost of clearing AIV in the study area, was based on information from existing Working for Water projects in KwaZulu-Natal. This cost equates to ZAR 6000 per ha (a) 1 ha 2 ha "^3ha ha 5 ha Planning Period (years) Fig. 4 Economic benefits for different ratios of compensatory forestry at a 6% discount rate: (a) costs and benefits are divided equally through the 12-year rotation, and (b) costs and benefits are assumed to occur within the actual year of rotation.

9 Clearing alien invasive vegetation from riparian zones in a South African basin 405 (ZAR = South African rand) to treat densely infested acacia species through an initial clearing operation as well as three follow-up operations. The costs per hectare of implementing and running forestry operations in this area for a 12-year forestry rotation were estimated as: (a) ZAR 3000 per annum for the first 2 years of rotation for crop establishment, (b) ZAR 750 per annum for the next 8 years of rotation for silviculture, and (c) ZAR 6000 per annum for the last 2 years of rotation for harvest and transport costs. Presently, pulp timber can be sold in the Pongola area for ZAR 205 per ton. It is reasonable to assume a yield of approximately 200 tonnes of timber per ha of forestry harvested. Hence the gross profit on 1 ha of timber would equate to ZAR over a 12-year rotation. The ratios of the cost of clearing 1 ha of AIV vs the benefit obtained for compensated forest area have been summarized in Fig. 4(a). The costs vs benefits in this graph are indicated as a Nett Present Value (NPV) at 6% discount rate over 20 years. An assumption made in this example is that the costs and benefits are divided equally through the 12-year rotation. However, if the costs and benefits are assumed to occur within their actual year of rotation, as would be expected if compensatory forest permits are issued, the resulting NPV is less attractive as illustrated in Fig. 4(b). CONCLUSIONS Alien invasive vegetation in the riparian zones of the Pongola-Bivane basin uses a substantial amount of water and there are definite economic benefits to the implementation of alien vegetation clearing in return for compensatoiy forest permits even when a compensation ratio of 1:1 is considered. However, since timber is a long rotation crop, cash flow constraints during the first rotation may hinder the implementation of this concept. Results have shown that the establishment of new commercial afforestation in these basins can compensate for clearing AIV from the riparian zones and that this is both hydrologically and economically feasible. Acknowledgements The support by SAPPI of the senior author is gratefully acknowledged. The DWAF and the Water Research Commission of South Africa are acknowledged for the support of this research. GIS analyses were performed and ACRU was run on the computing equipment of the Computing Centre for Water Research. REFERENCES Jewitt, G. P. W. & Schulze, R. E. (1999) Verification ofthe ACRU model for forest hydrology applications. Water SA 25(4), 483^189. Schulze, R. E. (1995) Hydrology and agrohydrology: a text to accompany the ACRU 3.00 Agrohydrological Modelling System. WRC Report no. TT69/95, Water Research Commission, Pretoria, South Africa. Schulze, R. E., Horan, M. J. C, Shange, S. N., Ndlela, R. M. & Perks,!.. A. (1998) Hydrological impacts of land use practices in the Pongola-Bivane catchment. ACRUcons Report 26, University of Natal, Pietermaritzhttrg. South Africa. Versfeld, D. B., Le Maitre, D. C. & Chapman, R. A. (1998) Alien invading plants and water resources in South Africa: a preliminary assessment. WRC Report no. TT99/98, Water Research Commission, Pretoria, South Africa.