Acacia Mearnsii) in the Tsomo Valley in Eastern Cape: Consequences on the Water Recharge and Soil Dynamics (An Ongoing Study)

Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2009 ▪ Vol. 9 ▪ No. 2

Impact of the removal of black wattle (Acacia mearnsii) in the Tsomo Valley in Eastern Cape: Consequences on the water recharge and soil dynamics (an ongoing study)

HPM Moyo, S Dube and AO Fatunbi Department of Livestock and Pasture Sciences, University of Fort Hare Email: [email protected]

lack wattle (Acacia mearnsii) (Galatowitsch and Richardson is a fast growing leguminous 2004). Acacia mearnsii ranks first in B (nitrogen fixing) tree and it is water use among invasive species, often used as a commercial source using 25% of the total amount, and of tannins and a source of fire wood is estimated to reduce mean annual for local communities (DWAF 1997). runoff by 7% in South Africa (Dye Riparian ecosystems are widely re- and Jarmain 2004). The invasive garded as being highly prone to in- ability of this species is partly due to vasion by alien plants, especially A. its ability to produce large numbers mearnsii, largely because of their of long-lived seeds (which may be dynamic hydrology, nutrient levels, triggered to germinate by fires) and and ability to disperse propagules the development of a large crown (Galatowitsch and Richardson that shades other vegetation (Nyoka 2004). Disturbing native vegetation 2003). also causes invasion as this often Acacia mearnsii threatens na- prepares a seed bed for invader spe- tive habitats by competing with in- cies (DWAF 1997). Native fynbos digenous vegetation, replacing grass tree species lack mycorrhyzal asso- communities; reducing native biodi- ciates and therefore are less efficient versity and increasing water loss at nutrient uptake (Smita 1998), from the riparian zones (Nyoka leading to them being out-competed 2003). Non-indigenous species, like by the invasive tree species. Acacia mearnsii, with evapotranspi- Acacia mearnsii is now consid- ration rates higher than those of the ered to be a major invasive tree spe- native flora significantly alter hydro- cies in South Africa covering an esti- logical regimes and lower water ta- mated 2.5 million hectares of land bles (Dye and Jarmain 2004). Soil and it is among the top ten invading under A. mearnsii becomes desic- species in South Africa cated more quickly than it does un-

38 Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2009 ▪ Vol. 9 ▪ No. 2

A natural well occurring near Acacia stands der grass (Dye and Jarmain 2004). pollinators (Chornesky and Randall The presence of other Acacia 2003). species of Australian origin in stream The invasion of grassland by bank habitats throughout the sub- tree species is known to have some continent suggests that their ability negative effects on the sustainability to take advantage of more than ade- of the grassland globally but the ef- quate soil water at some season fects of this encroachment on (and to survive the intervening ground water and salt fluxes remains drought) favors their proliferation poorly understood (Jobbagy and (Bromilow 1995). These species of- Jackson 2004). Acacia mearnsii sta- ten form dense stands, maintain a bilises sediments by colonising de- high green leaf area throughout the posits but its shallow root system is year, and replace seasonally dor- easily washed out during floods, re- mant grasslands, permitting continu- leasing sediments and blocking ously high rates of total evaporation bridge arches and storm drainage (Hess et al. 2006). Acacia mearnsii systems (Surridge 2006). Once A. stands can displace native species mearnsii is established, it alters the by hybridisation and altering the be- ecosystem function through its high havior and abundance of native plant evapotranspiration rate and as such

39 Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2009 ▪ Vol. 9 ▪ No. 2

Woody detritus near water path affects soil microbial functioning References (Galatowitsch and Richardson 2004). Bromilow C 1995. Problem plants of South Our ongoing research work Africa. Briza Publications, Pretoria aims at determining the effect of the Campbell P 1993. Wattle control. Plant Protection Research Institute, hand- removal of A. mearnsii on the re- book no. 3, Pretoria. charge of natural wells. This is in Chornesky EA and Randall JM. 2003. The order to quantify the potential loss threats of invasive alien species to and or gain of the scarce water re- bio-diversity: setting a future course. sources that could accrue with the Annals of the Missouri Botanical Gar- den 90: 67-76. removal of the trees. Furthermore, Department of Water Affairs and Forestry we aimed at determining the effect of 1997. Working for Water Programme: the A. mearnsii on the soil chemical Annual Report 1996/1997. Depart- and biological properties. This will ment of water Affairs and Forestry, help us to provide information on the Pretoria. effects of A. mearnsii control on the Dye P and Jarmain C 2004. Water use by Black Wattle (Acacia mearnsii): Impli- regrowth, regeneration and species cations for the link between removal structure of native grass species. of invading trees and catchment streamflow response. South African

40 Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2009 ▪ Vol. 9 ▪ No. 2

Dense population of Acacia mearnsii trees in a riparian agroecology at Elliot in Eastern Cape Province of South Africa

Journal of Science 100, January/ Smita M 1998. The invasion of South Afri- February 2004. can Fynbos by an Australian Immi- Galatowitsch S and Richardson DM 2004. grant. Conservation Biology 13: 735- Riparian scrub recovery after clearing 743 . of invasive alien trees in headwater Surridge M H 2006. The threat of invasive streams of the Western Cape, South species to WWF’s Global 200 Eco- Africa. Biological Conservation 122 regions. Journal of Applied Ecology (4): 509-521. 43: 442-457. Hess HD, Tiemann TT, Noto F, Carulla JE and Kreuzer M 2006. Strategic use of tannins as means to limit methane emission from ruminant livestock. Science Direct 1293: 164-167. Jobbagy EG and Jackson RB 2004. Groundwater use and salinization with grassland afforestation. Global Change Biology 10: 1299-1312 Nyoka BI 2003. Biosecurity in Forestry: A Case Study on the Status of Invasive Forest Trees Species in Southern Africa. Forest Biosecurity Working Paper FBS/1E. Forestry Department. FAO, Rome.

41 Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2009 ▪ Vol. 9 ▪ No. 2

ANALYTICAL LABORATORY

EQUIPMENT AVAILABLE ANALYSIS

Drying ovens We offer a wide range of instruments for Spectrophotometer sample preparation and analysis. Mill grinder Balances The Smartchem Discrete Chemistry Analyser Orbital shaker can perform the following analysis: Smartchem discrete chemistry analyser Acidity, alkalinity, aluminium, ammonia, iron, Microwave digest system magnesium, nitrate, calcium, chloride, chlorine, Water system (de-ionised water) chromium, fluoride, hardness, nitrate, Hotplate/magnetic stirrer phosphate, phenol, silicate, sulfate, total Fume hood phosphorus, urea, etc. Glassware General lab consumables Costs: pH & conductivity meter A daily fee of R50 will be charged for the use of the laboratory. Tests will be charged separately

For bookings or more information, contact Louise Goosen at [email protected] or (013) 735 4183