Invasive alien woody plants of the southern and southwestern Cape region , South Africa

The frequency and abundance of invasive alien plants were recorded along roadsides and at watercourse crossings in 82.9% (145/175) of the quarter degree squares in the study area (3I-35°S, 17-25°E and covering ± 90 000 km:). The sur­ vey yielded 102 species of which the most prominent (in order of prominence) in roadside and veld (natural and modified) habitats were: Acacia cyclops, A. saligna and A. mearnsii The most prominent species (in order of prominence) in streambank habitats were: A. mearnsii, A. saligna and Populus x canescens. The greatest intensity of invasion was recorded in forest and fynbos vegetation types in the relatively narrow belt stretching from the coastline to the tops of the coastal mountain ranges In the coastal lowlands Acacia cyclops and A. saligna form the most extensive and continuous stands of alien vegetation recorded anywhere in South Africa. In the and intenor invasion was largely confined to watercourses.


Survey history and objectives
This study of the southern and southwestern Cape is the sixth of eight regional surveys which together are designed to reflect invasion by woody alien plants in South Africa as a whole.Surveys have been completed for the former Transvaal (Henderson & Musil 1984), Natal and northeatern Orange Free State (Henderson 1989), Orange Free State (Henderson 1991a), northern Cape (Henderson 1991b) and eastern Cape (Henderson 1992).The survey of this area was undertaken from east to west and during the months of March and October 1990, January and December 1991, February and November 1992, and May 1993.The objectives of the survey were: to produce a check list of the major invasive alien woody plants of stream bank.roadside and veld habitats in the study area: to determine the pattern of alien woody invasion as a whole and for individual species; to attempt to relate distribution to environmental factors and to determine which are the most prominent and potentially important invaders.

The study area
The study area lies between latitudes 31° and 35°S and longitudes 17° and 25CE (Figure 1).and occupies an area of approximately 90 (MX) km;.It is bounded in the south by the Indian Ocean and in the southwest by the Atlantic Ocean.The topography is dominated bv the mountains of the Cape Fold Belt.These mountains occur for the most part in subparallel ranges with an average height of 1 (KM) to 1 500 m and with individual peaks reaching over 2 (KM) m.In the south these ranges strike from east to west whereas in the west the strike is more Changes in the rainfall can be very abrupt, and are usu ally associated with the topography.During winter, snow falls regularly on the higher mountains, but the lowlands enjoy an equable climate and frost is rare except in some of the deep valleys of the interior (Taylor 1978: Linder 1991;Jury 1993).
For the purposes of this survey and in keeping with previous surveys, the vegetation of the study area has been subdivided according to the biomes of southern Africa defined by Rutherford & Westfall (1986) and Acocks's Veld types o f South Africa (1988).There are four biomes in the study area, namely the Succulent Karoo, Savanna, Forest and Fynbos Biomes (Figure 2).Eleven Acocks Veld Types occur in the study area and have been grouped into nine broad veld type categories for the purposes of this survey (Table 1; Figure 2).Succulent karoo occupies the region commonly known as the Little Karoo.The vegetation has a predom inance of succulents, and dwarf trees and shrubs are numerous.It occupies rocky, hilly country, at elevations ranging from 300-600 m. receiving 150-300 mm of rain per annum (Acocks 1988).
Savanna is represented by a vegetation type comm on ly known as succulent mountain scrub which occurs on steep mountain slopes.It is typically a dense scrub dom inated by Portulacaria afra.The altitude ranges from 400-1 060 m, and rainfall ranges from 250-300 mm per annum (Acocks 1988;Low <N : Rebelo 1996).
The only forest area of biome dimension in South Africa occurs in the southern Cape at Knysna (Rutherford <k Westfall 1986).Before the southern Cape forests were heavily exploited and reduced to their pre sent remnant areas, they covered most of the southern Cape coastal region between Mossel Bay and Humansdorp (Von Breitenbach 1972).Today the Knysna Forest and many smaller forest patches occur in a nar row.irregular belt along the southern coastal shelf and foothills of the Outemqua and Tsitsikamma Mountains (Von Breitenbach 1972).Forest growth is favoured by mild temperatures and a high, well-distributed ram tall (Von Breitenbach 1972).The mean annual rainfall is 800-1 000 mm (Schulze & McGee 1978), but ranges from 500-1 200 mm (Geldenhuys 1993).Altitude ranges from sea level to 1 220 m at the tops of the mountains only 10-37 km inland (Geldenhuys 1993).Small forest outliers occur in fire-free habitats westwards towards the Cape Peninsula (Taylor 1978).
Strandveld is a low-growing, sem isucculent and shrubby vegetation type confined to the sandy coastal plains of the west coast.It is transitional between coastal fynbos and succulent karoo.It receives 50-300 mm of rain per annum, mainly in winter (Acocks 1988).
Coastal renosterveld occurs in a western and southern belt and is situated between the fynbos of the mountains and of the coastal plain.The terrain is undulating and the soils are clayey.Rainfall ranges from 300-600 mm per annum.Most of the original vegetation has been ploughed up for the grow ing of wheat.Scrub relics that remain are dense, thorny and semisucculent.Elsewhere the scrub has been replaced mainly by renosterbos (Acocks 1988).
Coastal fynbos occurs on sand and limestone in the west and south coastal belts.The vegetation is a more or less open scrub lacking the dense thorniness and semi succulence of the previous veld type.The altitude ranges from 0-300 m and rainfall from 300-500 mm per annum (Acocks 1988).
Mountain fynbos is the most widespread vegetation type in the Fynbos Biome.occurring mainly along the Cape Fold Belt from north of Nieuwoudtville to Cape Town and Cape Agulhas and to Humansdorp in the east.Altitude ranges from 0-2 200 m.and rainfall from 200 to more than 2 000 mm per annum, occurring mainly in the winter months (Low & Rebelo 1996).Summers are hot and dr\ and conducive to veld fires.The vegetation is an open to closed shruhland.
Mountain fynbos & forest occurs outside the Forest Biome defined by Rutherford and Westfall (1986) but within the original limits of the Knysna Forest.Acocks Veld Type 4, mapped by Acocks (1988).This region con tains wet mountain fynbos (National Committee for Remote Sensing, CSIR 1983), numerous small patches of forest, including the Tsitsikamma Forest, and exten sive timber plantations of mainly pine, but also gums and Acacia melanoxylon (Van der Zel 1988).The mean annu al rainfall is 800-1 000 mm (Schulze & McGee 1978) and uniformly distributed throughout the year.

Sampling method
The method used in this survey was the same as in the previous survey of the eastern Cape (Henderson 1992).The presence and abundance of all alien trees, large shrubs and conspicuous climbers which appeared to be spreading spontaneously (naturalised) were recorded for each veld type category, habitat type (roadsides and adjoining veld, and streambanks) and quarter degree/fif teen minute square traversed by road.Thirty three quar ter degree squares were selected for more intensive sur veying (Figure 1).They may be used at a later date for a quick resurvey of the study area to assess any changes that may have taken place.
Recordings of roadside and veld invaders were made from a moving vehicle along road transects of between five and 10 km in length.The average transect length was 6.4 km for the general survey area and 5.0 km for the intensive sites.Recordings of streambank invaders were made at virtually all watercourse crossings on the survey route.Details of the roads traversed are lodged in the Plant Protection Research Institute (PPRI).Pretoria.As on the previous occasions the survey was undertaken in a minibus, with one driver and one recorder (the author).The average speed was 60 km/h but ranged from about 20 km/h in densely vegetated areas to 100 km/h in sparsely vegetated areas.
All the raw data for this survey as well as the previ ously completed regional surveys have been computer ized using the data management system DataEase.This database is housed in Pretoria at the PPRI.

Abundance ratings
The abundance ratings for roadside and veld habitats and streambank habitats are given in Table 2.

Sampling level achieved
The sampling level achieved was 82.9% (145 out of the total 175 quarter degree squares) at an average of 44.5 km travelled per square.An average of 29.3 km of road transects were sampled per quarter degree square for abundance estimates of roadside and veld invaders.
The veld type coverage in terms of quarter degree squares and road transects sampled, kilometres travelled and watercourse recordings made, is given in Table 3. Statistics for streambank, roadside and veld habitats are given in Tables 4 and 5.

Frequency
The percentage frequency of occurrence of a species x in a given category (veld type, biome or study area) y was calculated as follows: frequency = no. of watercourse recordings/road transects in category y having species x total no. of watercourse recordings/road transects in category y x 100

Prominence value
The prominence value is a combined measure of a species' frequency and abundance relative to that of all other species, within a given vegetation category (veld type, biome or study area).
In streambank habitats the prominence value for a species x in category y was calculated as follows: total weighted abundance of species x in category y prominence value = sum of the weighted abundances of all species in category y + frequency of species x in category y sum frequency of all species in category y x 100 The abundance ratings were weighted according to the minimum percentage cover in each scale rating (see Table 2).Thus ratings 7, 6. 5 and 4 had weighted values of 75, 50, 25 and 5 respectively.Ratings 1, 2 and 3 each had weighted values of 1.
In roadside and veld habitats the prominence value for a species x in category y was calculated as follows: prominence value = total abundance* of species x in category y __________________ sum of the abundances* of all species in category y + frequency of species x in category y sum frequencies of all species in category y -x 100 x 100 The highest prominence values in a given category which add up to approximately 160 points out of a total of 200 are printed in bold in Tables 6, 7, 8 and 9.The cut off point of 160 points is arbitrary but represents 80% of the summed prominence values.
Mean species abundance rating in roadside and veld habitats (Tables 8 & 9) The mean species abundance rating** of a species x in a given category (veld type, biome or study area) y was calculated as follows: total no. of individuals or groups of species x in category y ___________ ______________ mean no. of individuals or groups " total distance along which species x was per 10 km rated in category y x 10 Mean abundance o f invaders per km in roadside and veld habitats (Table 5) The mean abundance of invaders per km in a given category (veld type, biome or study area) y/quartcr degree square / was calculated as follows: mean abundance =

RESULTS
The survey yielded 102 naturalised alien species.These species are listed in the Appendix together with a further 34 species which were obtained from various lit erature and other sources.The distributions of 20 of the most prominent species are given in Figures 6 and 7 and a further 10 potentially important species are given in Figure 8.
x loo The streambank habitat total abundance* of all species in category y/quarter degree square z total kilometres rated for abundance estimates in category y/quarter degree square z each abundance rating was expressed in numbers of individuals or groups recorded per transect (Table 2) To be both conservative and consistent the minimum number was used in each instance, e g an abundance rating of 5 over 10 km = 50 and an abundance rating of 5 over 5 km = 25 mean no. of individuals or groups per 10 km converted to rating (Table 2) The whole study area One thousand and thirty-six watercourse crossings were sampled in which 75 species were recorded, with up to eight species in one sample.Invaders were present at 73.5% of all crossings and 30.9% of all crossings were heavily invaded (Table 4).

Analysis according to veld type
Overall the Fynbos Biome was the most heavily invaded in terms of percentage crossings invaded and percentage crossings heavily invaded.Invasion was most intense in mountain and coastal fynbos where the highest percentages of heavily invaded crossings were recorded.Very few watercourses were sampled in the Forest Biome.Most recordings were of small streams and it was difficult to see beyond 10 m.

Frequency
Acacia mearnsii was the most frequently recorded invader in the study area (29.4%).Only this species.A. saligna (20.7%) and Populus x canescens (13.6%) were recorded at 107 or more crossings in the whole study area (Table 7).

Prominence
The most prominent invader in the whole study area was Acacia mearnsii with a prominence value of 61.6 out of a combined total for all species of 200 (Table 7).The next most prominent invaders were A. saligna (36.9) and Populus x canescens (19.5).
In the Fynbos Biome Acacia mearnsii was the most prominent invader in four of the six veld type categories, namclv mountain fynbos, mountain tvnbos & torest.Acacia melanoxxlon was the most prominent invader in the Forest Biome, followed by Acacia mearnsii.Populus x canescens was the most prominent invader in the Savanna Biome and .4. saligna was the most promi nent invader in the Succulent Karoo Biome.

Roadside and veld habitats
The whole studx area were recorded.Up to 45 species were recorded per quar ter degree square.Naturalised species were recorded in 93.8% of all transects sampled and 45.8% of all transects were heavily invaded (Table 5).

Analxsis according to veld type
The most intense invasion was recorded in mountain fynbos <fc forest in the Fynbos Biome.closely followed by the Forest Biome where the highest percentages of transects (96.1% and 88.9% respectively) were heavily invaded.The mean abundance of invaders per km reached a maximum in coastal fynbos (29.8).The least invasion was recorded in the Savanna Biome.where no transects were heavilv invaded.
One hundred and forty-five quarter degree squares It must be noted here that although invasion w as intense and 660 road transects were sampled in which 96 species m the Forest Biome.most invasion was recorded in the dis-  Most of the higher parts of the mountain fynbos were inaccessible by road and therefore undersampled in this survey.As a consequence the mountain species such as Hake a spp., particularly H. sericea.and Pinus spp.were no doubt under-recorded in this survey.The accessible parts of mountain lvnbos were the valleys and this is where most recordings were done.The best data on the extent of woody plant invasions in the higher altitude areas can be obtained from studies of the Cape Peninsula mountains (Moll & Trinder-Smith 1992;Richardson etal. 1996).
The most frequently recorded species in the Fynbos Biome were Acacia cyclops, A. saligna and A. mearnsii.In the Forest Biome A. melanoxylon, A. mearnsii, Pinus pinaster and P. radiata were the most frequent species.In the Savanna Biome Nicotiana glauca, Opuntia ficusindica and Agave americana were the most Irequent species.In the Succulent Karoo Biome Nicotiana glau ca, Opuntia ficus-indica and A. saligna were the most frequent species.

Prominence
Acacia cyclops scored the highest prominence value of 45.2 in the study area.The next most prominent species were Acacia saligna (24.8) and A. mearnsii (24.0) (Table 9).
In the Fynbos Biome. the three aforementioned species were the most prominent invaders, followed by Pinus pinaster.In the Forest Biome Acacia melanoxylon was the most prominent invader followed by A. m earn sii, Pinus pinaster and P. radiata.In the Savanna Biome Nicotiana glauca and Opuntia ficus-indica were the most prominent species.In the Succulent Karoo Biome A. saligna was the most prominent species.

Patterns of invasion
Alien plant invasion was recorded in streambank.roadside and veld habitats throughout the southern and southwestern Cape (Figures 3, 4 & 5).Most invasion, however, was encountered within the relatively narrow belt stretching from the coastline to the tops of the coastal mountain ranges.
A comparison of Figures 3 and 4 shows that similar patterns of invasion were recorded in streambank.road side and veld habitats except that in the dry inland areas of the Little Karoo, centred around Ladismith and Oudtshoorn.there was more severe invasion of the streambank habitat than of roadside and veld habitats.

Prominent and potentially important species
Acacia species were overall the most prominent invaders in the studs area with one or more species being the most prominent in every vegetation categor\ with the   exception of savanna.Acacia mearnsii, A. saligna and A. cyclops were the top three most prominent invaders with their combined prominence values amounting to 201 out of a total of 400 points for all species.In streambank habitats the Acacia species were dominant with five species (A. mearnsii, A. saligna, A. cyclops, A. longifolia, A. melanoxylon) amongst the top six most prominent species.In roadside and veld habitats four of the top six most prominent species were Acacia species (A. cyclops, A. satigna, A. mearnsii and A. melanoxylon).
Acacia m earnsii (Figure 6D) was a prominent invad er in all vegetation categories with the exception of savanna, strandveld and coastal fynbos.It was recorded predom inantly along watercourses and was the most prom inent riverine invader in the Fynbos Biome and the whole study area.Unlike in M pumalanga (of the former Transvaal) and KwaZulu-Natal, where it has been cultivated on a grand scale for tannin, it has been cultivated only on a small scale in the Cape, yet has spread widely.This is a reflection of its aggressiveness as an invader.Acacia saligna (Figure 6G) was a prominent invader in all vegetation categories with the exception of savanna and forest.It was the next most prominent riverine invad er in the study area after A. mearnsii and unlike A. m eamsii was prominent in strandveld and coastal fynbos.It was the second most prominent invader of roadsides and veld in the Fynbos Biome, after A. cyclops.Together with A. cyclops it formed extensive stands on the Cape Flats, where it was used for the stabilization of driftsands and was grown for tannin from 1850 until 1910 (Stirton 1978).It was most abundant in a broad band stretching from Bredasdorp northwestwards to Clan william.It scored a highest abundance rating of 8 in the vicinity of Robberg (Plettenberg Bay) and Bredasdorp.A gall-forming rust fungus Uromycladium tepperianum, introduced for the biocontrol of A. saligna (Morris 1991), was only noted as abundant north of Cape Town between Atlantis and Kalkbaskraal in May 1993.Four years later, in 1997, it is abundant throughout the range of A. saligna.Acacia cyclops (Figure 6A) was the most prominent invader in roadside and veld habitats in the Fynbos  Biome and in the whole study area.It was prominent along watercourses only in coastal fynbos.It formed very extensive stands on the Cape Flats, where it was established as a sand-binder from 1876-1886 (Stirton 1978).In the vicinity of M itchell's Plain, Strandfontein and Muizenberg it formed almost pure stands for dis tances up to 10 km or more, thereby scoring the maxi mum abundance rating.It also scored the maximum rat ing in the vicinity of Cape Agulhas, Struisbaai and Quoin Point.It was exceedingly abundant (scoring an abun dance rating of 7 or more) in coastal areas stretching all the way from Saldanha Bay in the west to Humansdorp in the east, a distance of approximately 1 000 km.
Acacia melanoxylon (Figure 6E) was concentrated in the forested area defined by the limits of Acocks's Veld Type 4, Knysna Forest.It was exceedingly abundant and the most prominent invader in the Forest Biome.The only other areas where it was recorded as abundant were in the vicinity of Wolseley and in the Riviersonderend Valley near Greyton.The abundance of this species around Knysna can be attributed to its use as a commer cial timber tree and its consequent spread.It was used extensively from 1856 as a forest replacement species in the Knysna Forest, where it was planted in gaps created by the felling of indigenous trees (Stirton 1978).The indigenous forest pioneer tree.Virgilia oroboides was a widespread weed of roadsides and probably provides strong competition for A. melanoxylon and A. mearnsii.Acacia longifolia (Figure 6C) was concentrated in a broad belt from Bredasdorp westwards to Cape Town and northwards as far as Ceres.Two other foci occurred eastwards around Knysna and Klipdrif.east of Storms River.It was absent from succulent karoo, savanna, strandveld and mountain renosterveld.In the remaining wetter vegetation categories, particularly coastal and mountain fynbos, it was more prominent in streambank habitats than roadside and veld habitats.Overall for the Fynbos Biome. it rated fifth and sixth most prominent in streambanks and roadsides/veld respectively.Galled plants, caused by the introduced wasp Trichilogaster acaciaelongifoliae for biocontrol (Dennill & Donnelly 1991), were very much in evidence throughout its distri bution.
The biocontrol programme against A. longifolia has reduced seed production and the overall biomass of pop ulations; in some instances h has caused mortality of adult trees (Dennill & Donnelly 1991).Now there also appears to be a reduction in the extent and abundance of this invader.Comparison of the distribution of A. longi folia in Richardson et al. (1992).which was based on reports from the mid 1980's, and with the present survey, shows a marked reduction in the extent and abundance of A. longifolia in the southern Cape.
Acacia dealbata (Figure 6B) was virtually restricted to watercourses in mountain fynbos.Although recorded at very few localities it was almost always abundant, forming dense stands.These localities were: Bergkloof River, near H erbertsdale; Uniondale Poort, Holdrif River; Riviersonderend; Dwars River tributary, near Stellenbosch.Acacia pycnantha (Figure 6F) was restrict ed to the Fynbos Biome and was most prominent in road sides and veld habitats in mountain fynbos.It was exceedingly abundant i.e. scoring a 7 in the vicinity of Wolseley.
Populus x canescens (Figure 7G) was a widespread invader of watercourses and was recorded in all vegeta tion categories with the exception of the Forest Biome.It was the third most prominent riverine invader in the whole study area and in the Fynbos Biome.
Pinus pinaster (Figure 7E) was a prominent invader of mountain slopes in both the Forest and Fynbos Biomes.It has been used extensively for afforestation and its distribution is a reflection of planting patterns (Stirton 1978).Pinus radiata (Figure 7F) has been plant ed on a smaller scale and was less prominent than P. pinaster.These two species were the third and fourth most prominent invaders of roadside and veld habitats in the Forest Biome.They are the most prevalent species on the Cape Peninsula mountains (Moll & Trinder-Smith 1992).
Rubus fruticosus (Figure 71) occurred mainly in the wetter vegetation types.It was most prominent in m oun tain fynbos & forest where it formed thickets along road sides and on the margins of plantations or forest.In pre vious publications (Henderson 1989;Henderson 1992) reference was made to R. affinis.This is one of about 2 000 species belonging to the Rubus fruticosus complex (Tutin et al. 1968) and was regarded by Spies & Du Plessis (1985) to occur in South Africa.In this publica tion reference is made only to R. fruticosus L. aggregate which is in keeping with the Pretoria National Herbarium.
Hakea sericea (Figure 7A) was only rated tenth most prominent invader in mountain fynbos.This is an under estimate and resulted from the undersampling of moun tain slopes.From a distance Hakea spp.are also more difficult to distinguish from the fynbos than for example the Pinus spp., which invade the same habitats.Richardson et al. (1992) gives a very different picture of the distribution and abundance of H. sericea, which they said occurred in 30°Ic of the quarter degree squares in the Fynbos Biome and formed dense stands in 19% of squares.These records are based on Macdonald et al. (1985) but updated from various sources (D.M. Richardson pers.comm.).
Eucalyptus spp.were prominent invaders of the Forest Biome and mountain fynbos & forest.E. diversicolor (Figure 61) has been cultivated commercially around Knysna and was the most prominent species in this region.It was often difficult to distinguish the Eucalyptus spp.and then they were recorded collective ly.In the Knysna area it was not clear how abundant E. cladocalyx was.It was.however, regarded as one of the three problem species in the area, together with E. diversicolor and E. microcorys (forester at Woodville State Forest pers.comm ).
Eucalyptus lehmannii (Figure 6 J) was the third most prominent invader in roadsides and veld habitats in coastal fynbos.It was most abundant in the coastal belt stretching from Bredasdorp westwards to the Cape Peninsula.E. cf.camaldulensis (Figure 8B) was mainly an invader of watercourses and was most prominent in the Succulent Karoo Biome along the Bree River.It was also abundant along some watercourses in the Fynbos Biome, such as the Olifants River near Clanwilliam, Riviersonderend and Berg Rivers.E. exserta, which could be mistaken for E. camaldulensis was abundant on the Berg River near Langebaan and locally prominent on the Bree River near Robertson.
Leptospermum laevigatum (Figure 7B) was the sixth most prominent invader in roadside and veld habitats in coastal fynbos.It has been used mainly as a hedge or windbreak in coastal towns from where it has spread into the adjacent fynbos.It was most abundant on sandy soils from Bredasdorp westwards to Fish Hoek on the Cape Peninsula.It was exceedingly abundant around Hermanus and Kleinmond.
Sesbania punicea (Figure 7J) was recorded in all veg etation categories except savanna, strandveld and moun tain renosterveld.It was most abundant along water courses and in particular the Bree River in the Succulent Karoo Biome near Worcester.This species could have been underestimated in this survey as it is easily over looked when not in flower.
Opuntia ficus-indica (Figure 7D) was by far the most widespread invader in the study area, being recorded in 71.5% of all road transects.It rated as the fifth most prominent invader in roadside and veld habitats in the study area but it was rarely abundant.It was only in the dry coastal bush around Mossel Bay that it scored an abundance rating of 5; elsewhere it occurred only as scat tered individuals or small clumps.
Nicotiana glauca (Figure 1C) was the second most widespread invader, being recorded in 63.6% of all road transects.It was only recorded in disturbed sites such as along roads, railway lines, river banks, dry river beds, and in quarries and rubble heaps.It was a prominent invader of watercourses in the Succulent Karoo and Savanna Biomes but was never abundant.The only places where it was recorded as very abundant was on the roadside and floodplain adjacent to the Verlorevlei, north of Piketberg and near Elandsbaai.
Arundo donax (Figure 6H) was a widespread invader and was absent only from strandveld and forest.It was a prominent invader of watercourses in coastal renoster veld, succulent karoo and savanna.Flowering plants were only seen in the coastal belt near Humansdorp, Knysna and Wellington.
Ricinus communis (Figure 7H) was a widespread invader of roadside, veld and streambank habitats and was prominent in coastal renosterveld, savanna and suc culent karoo.It was recorded in all vegetation categories but was only abundant in the Kogm anskloof near Montagu and in the Kafferkuilsrivier valley near Stilbaai.
Agave americana was prominent in succulent karoo and savanna but was never abundant.Its distribution is a reflection of where it has been planted.It appeared to have spread from seed in two localities within the succu lent karoo.These were north of Barrydale between the Anysberg and Warmwaterberg and in the Touws River valley south of Ladismith.
Atriplex nummularia was most frequently recorded in roadside and veld habitats in the succulent karoo, but it was never abundant.Judging from observations in the Great Karoo in the central Cape (Henderson in prep.) it is likely to become a prominent invader of seasonal and episodic watercourses in the succulent karoo of the Western Cape.
Salix babylonica was recorded along watercourses in all vegetation categories with the exception of strand veld.It was seldom abundant and its distribution was largely a reflection of where it has been planted.It was most prominent in mountain renosterveld where it was ranked third after Acacia mearnsii and Populus x canescens.
Species which scored abundance ratings of 5 or more but were not rated as prominent were: Hakea gibbosa and H. drupacea (= 11. suaveolens), Paraserianthes lophantha, Pinus cf.canariensis, Prosopis spp.and Lavatera arborea.Hakea gibbosa (Figure 8D) was only recorded as abundant near Stanford on the Akkedisberg Pass.H. drupacea (Figure 8C) was only recorded as abundant on the Cape Peninsula between Simonstown and Smitswinkelbaai.If one compares the distributions of H. gibbosa and H. drupacea with those in Stirton (1978) they appear to have changed little in the past twenty years.
Paraserianthes lophantha (Figure 8G) was most fre quently recorded in mountain fynbos & forest and moun tain fynbos.It has a preference for moist, low-lying sites.Pinus cf.canariensis was only recorded as abundant on dry mountain slopes near Paarl on the road to Franschhoek.It was locally common around Ceres and Tulbagh.Prosopis spp.were only locally abundant on the plains to the east of Piketberg.They have been plant ed in this area and were just starting to spread as mostly small plants were seen along roadsides and along water courses.Lavatera arborea was recorded along roadsides in many coastal towns; it was common to abundant near Saldanha Bay on the west coast.
Species that were only locally common (i.e.scoring abundance ratings of 4) were: Tamarix spp.along water courses in succulent karoo and savanna near Oudtshoorn and in coastal renosterveld on the Gouritz.River south of Mossel Bay; Quercus robur (Figure 81) and Quercus spp.Several other species were locally common but were not included in formal recordings.These were: Hedychium sp., Ipomoea cf.purpurea, Solarium mauritianum and Spartium junceum on the Constantia Nek Road near Hout Bay on the Cape Peninsula.Spartium junceum (Figure 8J) was recorded in several localities, but mainly close to habitation and plantings.This species should be closely watched as it is showing signs of becoming invasive.Phytolacca dioica was locally com mon around Knysna.
Metrosideros excelsa is a potentially important invad er of fynbos on moist, peaty soils.It was locally common at Betty's Bay where it has spread from plantings and threatens about three km of fynbos on the seafront, but has been controlled at least since the early 1980's (R. Attwell pers.comm.).It has also spread from plantings around Hermanus (M.J.Wells pers.comm.).Small groups of naturalised plants were recorded on roadsides in the Cape Peninsula.These were on the Ou Kaapseweg, on the Steenberg; and on Boyes Drive between Muizenberg and Kalkbaai.In both cases they were in close proximity to suburban gardens and culti vated plants.
Pittosporum undulatum is a potentially important invader of fire-free, wooded or forested areas.It is a common hedge and ornamental garden plant in the Western Cape.In this survey it was seldom recorded as naturalised, but this could partly be due to its inconspic uousness in the habitats it invades.It was recorded in the river valley between Stellenbosch and Jonkershoek.and also on the Cape Peninsula on Boyes Drive at Kalk Bay.According to D.M. Richardson, pers.comm., it forms dense stands in the Jonkershoek State Forest and also at Newlands and Tokai State Forests on the Cape Peninsula.This species has become an important invader of forests in many parts of the world including Jamaica, New Zealand and even in Australia, where it is indigenous, but has invaded vegetation outside of its natural range (Gleadow & Ashton 1981).Recent reports of diseased plants caused by a pathogen in the Western Cape may help to curb the invasiveness of P. undulatum in South Africa (M.J. Morris pers.comm.).8E), a tree with some what succulent leaves and adapted to windy, salt-laden air.has been planted as a windbreak and for shade in all coastal towns in the study area.Occasional seedling spread was recorded throughout its range but was most noticeable in strandveld from the Cape Peninsula north wards to the west coast.It appeared to be better adapted to the more arid and semisucculence of the strandveld rather than coastal fynbos.

M xoporum tenuifolium (Figure
Nerium oleander (Figure 8F) has invaded watercours es in dry mountain valleys in several parts of the study area.It was most evident along the Gamka, Huis and Gouritz Rivers in the Ladismilh and Calitzdorp Districts.It was also recorded near Robertson along the Bree River, at Citrusdal through the Piekenierskloof (Grey's Pass), and al Wuppertal on the Tra-Tra River.It is known to occur in the Cederbcrg (Stirton 1978) but this region was not surveyed due to bad weather conditions at the time.
Bothalia 28,1 (1998) Acacia elata (Figure 8A) was most frequently record ed in the mountain fynbos & forest and forest vegetation categories.Virtually all recordings were of seedling spread from plantings in urban areas or close to habita tion.On a few occasions, however, seedlings were noted on roadsides far from any plantings e.g. on the Franschhoek and Du Toits Kloof Passes.

Relation of invasion to historical and environmental factors
Alien plant invasion in the southern and southwestern Cape has been greatly influenced by the deliberate intro duction and large scale planting of alien plant species which were adapted to the prevailing environmental con ditions.For the first 150 years after the colonisation of the Cape in 1652.plants of mainly European origin were introduced.Only a few of these plants have become invasive such as Pinus pinaster a native of the Mediterranean and adapted to the climate and soils of the southern Cape.It was only after 1830, when tree and shrub species were intentionally imported from areas of sim ilar clim ate, especially southern and western Australia, and were extensively propagated, that most of the important invaders of natural vegetation became established (Richardson et al. 1992).
By 1865 all of the most prominent Acacia, Hakea and Pinus species mentioned in this survey, had been intro duced to the Cape Town region (McGibbon 1858;Shaugnessy 1986).The government forestry authority played a major role in attempting to establish many of these plants both on the Cape Flats and on Table Mountain and adjoining mountains (Shaugnessy 1986).They also encouraged private growers to establish alien plantations.The reasons for the plantings were various: timber, fuel, shelter, driftsand stabilisation, tannin pro duction and simply beautification of a landscape that was perceived at the time to be 'bleak and naked' (Shaugnessy 1986).Forestry officials even justified the afforestation of Table Mountain in terms of improved water supply and fire control (Shaugnessy 1986) which is exactly opposite to today's thinking.
In the southern Cape forests, fast-growing alien trees, particularly Acacia, Eucalyptus.Pinus and Quercus species, were planted in forest gaps and in plantations along the margins of forests.Acacia melanoxylon.the most prominent invader in the forested areas of the southern Cape today, occurred as large trees around George and Knysna bv 1876 (Geldenhuys et al. 1986).
The pattern of invasion of several of the most wide spread species today, such as Pinus pinaster.Acacia cyclops and A. saligna is a reflection of where they were planted.The disturbance of the soil and destruction of indigenous vegetation when establishing plantations of these alien species would have creatcd conditions favourable for their further spread and also for the spread of other alien species.Later the abandonment of many of the plantations left stands of aliens which could act as a seed source for future generations of these species (Shaugnessy 1986).
Successful invaders in the Fynbos Biome have to be either tolerant of, or adapted to, nutrient-poor sandy soils and periodic high-intensity fires, particularly in moun tain fynbos.Fire may encourage invasion by activating mass seed release from serotinous cones (as in Hakea sericea and Pinus pinaster), and by stimulating germina tion of soil-stored seeds as in Acacia saligna and A. longifolia (Richardson et al. 1992).The aforementioned Acacia species show a further adaptation which enables their seeds to avoid destruction by fire on the soil sur face.Their seeds, with fleshy attachments that are eaten by ants, are buried in caches below the soil surface (Dean et al. 1986).A. saligna also has the ability to coppice after fire.
The Fynbos Biome is subjected to strong winds throughout the year and this has facilitated the rapid spread of species with winged seeds, mainly those of the Hakea and Pinus species.Whereas seed dispersal in mountain fynbos is mainly by wind and water; in the lowlands birds, mammals (including humans and their implements), ants and water are important.For example, Acacia cyclops seed is dispersed by mammals, such as the striped field mouse and the chacma baboon (Stirton 1978), and many indigenous birds as well as the intro duced European starling (Glyphis et al. 1981).The large seeds of Pinus pinea are dispersed by the grey squirrel (Richardson et al. 1994).
Watercourses have played an important role in the long-range dispersal of alien plants throughout the study area.Species that have depended mainly on water disper sal include the hard-seeded legumes which are not birdor ant-dispersed and are otherwise relatively immobile such as Acacia dealbata, A. mearnsii, Paraserianthes lophantha and Sesbania punicea.Watercourses have also enabled some species, for example, A. saligna and A. mearnsii, to penetrate the dry interior of the Succulent Karoo and Savanna Biomes.Nerium oleander is confined to riverbeds and is dependant on moisture for the germi nation and survival of its seedlings.

SOME IDEAS FOR THE FUTURE
Since this survey was completed in 1993, a national programme for the removal of alien plant invaders in the water catchments of South Africa has been initiated.The project is government-aided and is called the Working fo r Water Programme of the Department of Water Affairs and Forestry.The 1997/98 budget stands at R 115 million and the prospects for further funding are promising (Preston 1997).It is envisaged that a large proportion of the funds will be derived from water levies which are likely to become a long-term source of funding (H.G. Zimmermann pers. comm.).The proposed 'interception of water levy' is aimed primarily at the forestry industry which is one of the major water users in the country (Yeld 1997).Clearing operations were started in October 1995 and by the end of March 1997, 71 289 hectares had been cleared (Willems 1997).However the ultimate suc cess of the project will depend on very strict follow-up operations over an extended period of time.If these con ditions are not met then the problem could be exacerbat ed rather than improved.
Many of the problem species are also useful plants and several are major commercial forestry crops.There is a growing call for instituting the 'polluter pays' princi ple, whereby the parties that benefit through the propa gation of these species must contribute to the costs of controlling their spread from sites of propagation (Ivey & Heydenrych 1995;Richardson et al. 1997;Yeld 1997).
Prevention is better than cure!Every attempt should be made to prevent other, new species from becoming invasive.The possibility of developing sterile cultivars of commercially important species needs investigating.The most important step should be the screening of alien plant species for potential invasiveness before they are introduced and widely planted.To this end, an expert system has been developed to assist authorities in screen ing plants for their invasive potential (Tucker & Richardson 1995).
Biological control using mainly introduced insects and pathogens could play an increasingly important role in the long-term and sustainable control of invasive plant species in the study area.Some programmes have already been so successful, especially those against Acacia saligna and A. longifolia, that they could ulti mately lead to the destruction of all the dense infesta tions of these species and severely restrict any further spread.
This paper has focused on woody plants which are the most prominent invaders of fynbos.However herbaceous alien plants are also important in some systems (Vlok 1988).There is an urgent need to assess the extent of invasions by alien herbs, especially in the remaining veg etation on the lowlands, and to determine what impacts they are having.
FIGURE 3.-Invasion in stream bank habitats in terms of the intensity o f invasion of watercourse crossings and species diversity per quarter degree square.

FIGURE
FIGURE 4.-Invasion in roadside and veld habitats in terms of the intensity of invasion of road transects and species diversity per quarter degree square.

FIGURE 5 .
FIGURE 5.-Invasion in roadside and veld habitats in terms of the mean abundance of in vaders per kilometre in each quarter degree square along watercourses near Stellenbosch and Swellendam; Eucalyptus cladocalyx and E. gomphocephala in several localities in mountain fynbos and coastal renosterveld; Opuntia vulgaris in coastal renosterveld near Mossel Bay; Passiflora caerulea along watercourses in coastal renosterveld near Riversdale; Pinus pinea in coastal renosterveld near Stellenbosch; and P. halepensis (Figure 8H) in mountain fynbos near Joubertina in the Lang Kloof.According to D M. Richardson, pers.comm., P halepensis has been underestimated in this study and it forms dense stands in the Jonkershoek Valley and on the Cape Peninsula near M iller's Point.

TABLE 1 .
-Veld type categories in study area and equivalent Acocks Veld Type groupings and Veld Type numbers Biome' and veld type category* Acocks Veld Type grouping Acocks Veld Type No.

TABLE 3 .
Rutherford & Westfall 1986.biome, veld type category and study area this represents the distance along which abundance recordings were made.Total distance along which observations were made is approximately one and a half times that given; * according to Henderson; * according toRutherford & Westfall 1986.
* TABLE 4.-Statistics for streambanks in each biome, veld type category and study area * one or more species scored an abundance rating of 5 or more; ** invaders present; T according to Henderson; * according to Rutherford & Westfall 1986.

TABLE 5 .
-Statistics for roadside and veld habitats in each biome, veld type category and study area

TABLE 6 .
__Alien species occurring in streambank habitats of Succulent Karoo.Savanna and Forest Biomes and in strandveld and mountain renosterveld of Fynbos Biome included in a formal recording at a watercourse crossing.Bold numbers: the highest prominence values in a given category which add up to ± 80^ of the summed values (see text).
Ffrequency of occurrence: I. % crossings heavily invaded; P. prominence value; * species occurring in the given category but not coastal renosterveld and mountain renosterveld.,4.saligna was the most prominent invader in the remaining two categories, namely strandveld and coastal fynbos..4. cyclops was a close second in coastal fynbos.

TABLE 7 .
-Alien species occurring in streambank habitats in coastal renosterveld, coastal fynbos, mountain fynbos, mountain fynbos & forest of Fynbos Biome and study area 1 5 i ^ ^1 3

TABLE 7 (
cont.).-Alien species occurring in stream bank habitats in coastal renosterveld.coastalfynbos.mountainfynbos.mountainfynbos & forest of Fynbos Biome and study area F, % frequency of occurrence; I. % crossings heavily invaded; P. prominence value; * species occurring in the given category but not included in a formal recording at a watercourse crossing.Bold numbers: the highest prominence values in a given category which add up to ± 80% of the summed values (see text).turbed areas between the remaining indigenous forest patches.Only small sections of forest are accessible by road.The following species were recorded along roadsides, margins or other gaps in indigenous forest: Acacia meamsii, A. melanoxylon, Cinnamomum camphora, Cortaderia selloana, Eucalyptus diversicolor, Pinus pinaster, P. radiata, and Rubus fruticosus.They were never abundant but occurred as single plants or small groups.

TABLE 8 .
-Alien species occurring in roadside and veld habitats of Succulent Karoo.Savanna and Forest Biomes and in strandveld and moun tain renosterveld of Fynbos Biome F, % frequency of occurrence; A, mean abundance rating; P, prominence value; * species occurring in the given category but not included in a formal recording in a road transect Bold numbers: the highest prominence values in a given category w hich add up to ± 80% of the summed values (see text).

TABLE 9 .
-Alien species occurring in roadside and veld habitats in coastal renosterveld, coastal fynbos, mountain fynbos, m ountain fynbos & forest of Fynbos Biome and study area

TABLE 9 (
cont.).-Alien species occurring in roadside and veld habitats in coastal renosterveld, coastal fynbos, mountain fynbos, mountain fynbos & forest of Fynbos piome and study area