3. METHODS 1 Monitoring technique
3.2 Description of sites
For purposes of this investigation sites were selected with the following criteria:
(a) grasslands which were open communities in 1937 despite recent habitat degradations;
(b) occurring within three kilometres of the littoral dunes;
(c) at an altitude not more than 100 m;
(d) confined to Veld Type One (Acocks 1953); and (e) infrequently or not grazed.
Chromolaena was first reported as a weed-problem on the south coast in 1955 (Egberink&
Pickworth 1969). Old aerial photographs (1937 and 1966) were used to validate sites
described in point 3.2a above (Plate 1). Degradation taking place after 1966 is associated with sugarcane expansion, i.e. conversion of virgin soil to arable lands, causing grasslands unsuitable for cultivation becoming fragmented. Excluded are communal rangelands, typified by a cropped sward, or more poignantly, free of chromolaena and other woody species, in accordance with point 3.2e.
The criteria for site selection, and the rarity of coastal grassland communities (le Roux 1995), meant sites chosen were highly variable. The major variations include differences in aspect,
soil form and consequently species composition. Differences in condition in relation to chromolaena density are in keeping the objectives of the study. I did not see the large degree of variation as a discredit to the study because the primary focus was the descriptive analysis of temporal changes within communities. None of the sites chosen had been grazed in recent years. For logistical reasons, I restricted sites to an area between the Mkomazi and
Mtwalume rivers (ca. 40km apart) on the KZN south coast. Sites are briefly described below.
Coastal grasslands are low elevation fire-climax communities with a potential forest domain in the absence of fire (Tainton 1981). Inthe Transkei region of the Eastern Cape coastal grasslands characterised by short grassland are composed almost exclusively of Aristida junciformis with associations of Themeda triandra and Tristachya leucothrix (Acocks 1953).
In tall grassveld Cymbopogon species predominate, but this type of veld has a limited patchy distribution. Both community types were represented in this study.
3.2.1 Site 1
Site 1 is a three hectare open grassland at Elysium (3"O°28'52/1S, 30038'07/1E) on a lowland hillock (alt. 30 m) 500 m to the west of the littoral dunes. The hillock slopes in the south to form the mouth of the Mtwalume River. Three locales typify the terrain, namely east (Site la), crest and west (Site 1b) facing slopes (Plate 1, a and b). Soils at Site 1 are regic sands (MacVicar et at. 1991). Soil texture varies from sand (86:8% sand:clay) on the eastern aspect to sandy loam (74:15% sand:clay).
The site experiences irregular wildfires with magnitudes of Brachiaria serrata, Digitaria eriantha, Diheteropogon amplectens and Themeda triandra. The scarcity of indigenous woody species kept as scrubby shrublets (0.5 m) by fire and salt-laden sea breezes, and the absence of exotic species, fulfilled the requirements of using this site for benchmark
purposes. Monitoring was carried out during the 1992/93 biological year. Fuel loads at the start of the investigation were estimated at 2.5 and 5.0 t DM ha-1at Site la and Site lb respectively.
3.2.2 Site 2
Site 2 is a one hectare pocket of gently undulating grassland (Site 2a) bisected by a natural drainage depression (Site 2b), in a coastal forest clearing (30014'26"S, 30046'26"E) at Clansthall. Soils are regic sands with sand:clay percentages of 85:9 at 2a and 75: 19 at 2b. It is situated one kilometre from the littoral dunes and has a maximum altitude of 60 m. The grassland has been reduced in areal extent since 1937 by secondary forest expansion (Plate 1, c to e) in areas not converted to sugar cane. Site 2 was moribund in 1991, the last fire, according to the farmer, occurred in 1960, but the area was mowed by tractor in 1988.
Site 2 is characterised by open mesic grassland on well drained loamy sand (Site 2a). One metre tall chromolaena was the dominant woody species in 1991 (1.2 plants m-2, 60%
frequency), with occasional Albizia adianthifolia, Brachylaena discolor and Syzygium cordatum occurring as dwarf shrubs. Site 2b is on sandy loam soils and occurs in the drainage line. Itwas historically sedgy-grassland, but is never inundated by water.
Subsequent to 1966 the site was modified and visibly desiccated by a 1.5 m thicket-forming infestation of chromolaena (17.7 plants m-2, 100% frequency). The grass fuel loads at the start of the investigation were estimated at 13.2 and 0.3 t DM ha-1at Site 2a and Site 2b respectively.
3.2.3 Site 3
Site 3 (alt. 80 m) near Clansthall, is situated on north facing slope (30014'43"S, 30045'07"E) that drops down steeply to Mahlongwana River, 2.5 km inland from the sea. Soil form is Glenrosa (MacVicar et at. 1991) and soil texture was classed as sandy clay loam (56:23%
sand: clay). In 1937 the grasslands in this locality were quite extensive. Cultivation of
surrounding grasslands during the late 1960's resulted in the establishment of scrub forest (Plate 1, fto h). Grass cover had been reduced toca. 30% (2.7 t DM ha-I) by 2 m
Chromolaena bushes (5.2 plants m-2, 76% frequency). Combretum molle, Dichrostachys cinerea andHeteropyxis natalensis were the dominant tree species with heights ranging from
1 to 5m.
3.2.4 Site 4
Site 4 (30° 14'43"S, 30° 45 '05"E) is located on the same slope as Site 3 at an altitude of 100 m and has the same soil form and textural properties. Open grassland has been transformed to scrub forest, without a herbaceous layer, dominated by 3 m chromolaena thicket (11.7 plants m·2, 100% frequency). Canthium inerme, C. molle, D. cinerea, H natalensisandProtorhus longifolia trees up to a height of seven metres were interspersed above the chromolaena under storey.
Table 3.1 Location and specifications of belt transects used in monitoring coastal grasslands
Site Transect No. Quadrats Aspect Slope
m 1 m2 25 m2(total area m2) N=360/0° n°
Site la 60 60 0 90 15
Site Ib 80 80 0 270 16
Site 2a 25 25 10 (250) 220 14
Site 2b 25 25 10 (250) 150 2
Site 3a 30 30 0 360 18
Site 3b 25 25 10 (250) 360 17
Site 3c 20 20 0 340 18
Site 3d 15 15 0 320 15
Site 4a 25 25 10 (250) 360 21
Site 4b 25 25 10 (250) 360 21
3.3 Soils
The coastal lowland of the southern KZN monocline, with the offshore continental shelf at 110 m below sea-level, consists of buried soils of sedimentary origin underlying an
overburden of aeolian sand about 3kminland (Tinley 1985). Topsoils in the study areas have been formed by either sand accretion at sites less than 60 m above sea level (Sites 1 and 2 = regie sand) or by glacial sedimentation, Glenrosa form, (Sites 3 and 4) at altitudes above 60 m.