THE WAR AGAINST ALIEN IN­VA­SIVE TREES IN SOUTH AFRICA

Tourism Tattler - - ENVIRONMENT -

Black wat­tle (Aca­cia mearn­sii) seed pods. A ma­ture wat­tle for­est can have up to 70,000 seeds per square me­ter of for­est lit­ter. Seeds may re­main vi­able for up to 50 years. The ‘big guns' in the ar­se­nal of in­vaders are the Aus­tralian wat­tles (Aca­cia spp.), gum trees (Eu­ca­lyp­tus spp.), pines, poplars, weep­ing wil­lows and mesquite. By 2010, these plants had spread over 16% of South Africa's land area – some 20 mil­lion hectares – which is dra­mat­i­cally greater than their es­ti­mated foot­print from fif­teen years ear­lier.

There are 981 species of wat­tles or aca­cias (racosperma), and all but 10 of these are na­tive to Aus­tralia. Thir­teen types of aca­cia are clas­si­fied as in­va­sive plants in South Africa (read more on page 18).

Many alien in­va­sive plants change the chem­i­cal com­po­si­tion of the soil in which they grow. Black wat­tles, for in­stance, load the soil with ni­tro­gen, im­pact­ing threat­ened grass­land species that thrive on low ni­tro­gen soils and mod­i­fy­ing the en­vi­ron­ment for other in­vaders. Gum trees poi­son the soil for other plants. The re­sult is that in­dige­nous plants die out around these trees.

Other in­vaders might be toxic or un­palat­able to graz­ing an­i­mals and of­ten out-com­pete fod­der plants – thus re­duc­ing the food source avail­able for live­stock. Re­search es­ti­mates that if in­vaders are al­lowed to spread, they could re­duce the coun­try's car­ry­ing ca­pac­ity for large graz­ing an­i­mals by more than two-thirds. Sim­i­larly these in­vaders change the fire pat­tern in the area, which can be cat­a­strophic, even for fire-adapted vege­ta­tion types like fyn­bos and grass­lands. The trees grow big, with heavy wooden stems and branches, and there­fore carry a higher fuel load. When fire burns through a for­est of black wat­tles, in­stead of mov­ing through hot and fast, as it would in in­dige­nous vege­ta­tion, the fire will burn much longer. This bakes the soil and kills in­dige­nous seeds.

Sci­en­tists have long sus­pected that dense thick­ets of alien in­va­sive plants are much thirstier than their in­dige­nous coun­ter­parts. To un­der­stand pre­cisely how much wa­ter they use, re­searchers from the Univer­sity of KwaZulu-Na­tal and the Coun­cil for Sci­en­tific and In­dus­trial Re­search (CSIR) set up a mon­i­tor­ing process over a pe­riod of thir­teen years, which proved that black wat­tle can con­sume up to 44 per­cent of the wa­ter out of a catch­ment area.

Other ex­per­i­ments from around the coun­try show that heav­ily in­vaded patches of black wat­tle, gum or pine trees use even more wa­ter than this, fur­ther im­pact­ing stream flow and wa­ter qual­ity. In ar­eas where these stands of in­va­sive species have been cleared from the wa­ter catch­ments, streams and rivers have be­gun to flow again af­ter be­ing dry for years, thus rein­vig­o­rat­ing the de­funct ecosys­tem. The en­vi­ron­men­tal de­cay that comes with this sort of in­va­sion is like a form of pol­lu­tion. But un­like an oil spill or lit­ter dumped in the veld, this form of pol­lu­tion self-repli­cates. Once these in­va­sive species have gained enough mo­men­tum, they keep on spreading. Black wat­tle seeds, for in­stance, can re­main vi­able in the soil for up to half a century. Ac­cord­ing to the Na­tional Bio­di­ver­sity As­sess­ment 2011, sci­en­tists cal­cu­late that the pro­fuse mass of in­va­sive plants uses about 7% of the coun­try's to­tal an­nual runoff (equiv­a­lent to 18 large dams), wa­ter which should end up in our rivers and dams. If left unchecked, they will con­tinue to spread, even­tu­ally us­ing up nearly 60% of our yearly runoff.

But their im­pact isn't just on the vol­ume of wa­ter they ab­sorb. When it rains, in­stead of the wa­ter be­ing slowed down by grass and shrubs, and per­co­lat­ing down into the soil to recharge the ground­wa­ter and trickle through to nearby rivers, the wa­ter rushes across the sur­face of the ground, shear­ing off layer upon layer of top soil. Rivers be­come clogged with this sed­i­ment.

Re­searchers es­ti­mate that South Africa now loses the equiv­a­lent of R6.5 bil­lion worth of ecosys­tem ser­vices an­nu­ally due to the dra­matic ex­pan­sion of these in­vaders.

The mes­sage is clear: if we want to re­ha­bil­i­tate South Africa's sa­vanna biome to its for­mer glory, and con­serve al­ready scarce wa­ter re­serves, in­va­sive alien trees need to be erad­i­cated. This is not a once-off ef­fort, but will need to be main­tained for decades.

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