Patch Indices and other Variables at Patch Scale

The overall trend in ecological measurements for patch types tended to follow the degree of 1378

biomass found in a patch (Tables 2.5 and 2.7), although at the high biomass end of the 1379

continuum, differences between grass, tall grass, Schoenoplectus or S. plumosum patches 1380

tended to be small or reflect unique conditions. Therefore, Schoenoplectus patches at Vaal 1381

River, which had significantly higher above-ground biomass together with S. plumosum and 1382

tall grass patches, when compared with bare grass patches, had the highest mean EC value 1383

(Table 2.5). This patch type is defined by the sedge Schoenoplectus corymbosus (Roth ex 1384

Roem. & Schult.) J. Raynal, which is always associated with wetlands (Marnewecke and 1385

64 Kotze, 1999), was found on a single transect (vr8, Table 2.4) located close to a tailings 1386

storage facility. This transect recorded a significantly higher mean EC than any other 1387

transect at Vaal River (Table 2.4) with the exception of the transect next to it (vr9). Electrical 1388

conductivity is a measure of the presence of soluble salts in a soil solution (Benton Jones Jr., 1389

2001) and therefore may be an indicator of contaminated soil. Transect vr8, with the 1390

highest mean EC of 881.5 ± 178.5 μS/cm, could be regarded as very slightly to moderately 1391

saline (Benton Jones Jr., 2001) which is considerably lower than values quoted in the Vaal 1392

River Environmental Management Plan (VR EMP) of 1520 to 28782 μS/cm for contaminated 1393

areas (Ellis et al., 2009), some of which were located fairly close (a few 100’s of metres) to 1394

transect 8 in the vicinity of the same tailings storage facility (TSF). 1395

Seriphium plumosum patches, which also had a high mean EC (Table 2.5), were found on 1396

four contiguous transects (vr 8, 9, 10 and 11, Table 2.4) at Vaal River, two of which had the 1397

highest EC values overall. In total, three transects (vr 3, 8 and 9) had high EC values and 1398

these were also transects that had visible white evaporite crusts on the soil surface. All 1399

three transects were situated on lower slopes and flats and presumably are in close contact 1400

with the water table. Evaluation of soil profiles near vr8 and 9, as documented in the Vaal 1401

River EMP, recorded seepage at 0.6 mbgl (Ellis et al., 2009). Seepage from groundwater and 1402

capillarity together with high evaporation rates at the soil surface and low rainfall resulting 1403

in minimal leaching are believed to cause evaporite crusts or efflorescences (Dultz and 1404

Kühn, 2005). As no chemical analysis was carried out on these evaporite crusts, it is unclear 1405

whether they are the weathered soluble products of dolomite (Ca/MgCO3) which outcrops

1406

throughout the VR study site, gypsum (CaSO4) or other sulphate salts, or a product of

1407

contamination from tailings storage facilities and mining activities or a combination of all 1408

three (Meza-Figueroa et al., 2009, Carmona et al., 2008, Dultz and Kühn, 2005, Joeckel et al., 1409

2005, Singer et al., 1999, Keller et al., 1986). Values of soil EC for all sites at West Wits were 1410

very low (Table 2.6), lower than any values for Vaal River recorded in this study, suggesting 1411

that surface soils contaminated through groundwater or atmospheric pathways (Singer et 1412

al., 1999) was not an issue at the sites studied at West Wits. 1413

Seriphium plumosum patches had the highest values for the LFA indices of soil stability, 1414

infiltration and nutrient cycling at both Vaal River and West Wits (Table 2.5 and 2.7, 1415

respectively), and the highest above-ground biomass, SOM and soil pH at West Wits. This 1416

65 suggests that S. plumosum plays a highly functional role in these grasslands, which according 1417

to literature, is contradictory. S. plumosum has been considered an indicator of severe 1418

overgrazing and rangeland degradation (Snyman, 2009b, Roux, 1969a) or poor management 1419

(Jordaan, 2009) as illustrated in one of its common names, bankrupt bush, and its 1420

encroachment into Highveld grasslands has been well documented although the 1421

mechanism/s behind this encroachment remain unknown (Snyman, 2009a). The shrub is a 1422

prolific producer of seeds, some of which seem to have dormancy mechanisms, and even 1423

after three years may have high viability (Snyman, 2009a). In contradiction to the 1424

conclusions that it invades overgrazed areas and may be a pioneer species, its seedlings only 1425

germinate in low-light conditions such as those formed close to unpalatable grasses such as 1426

Cymbopogon or Elionurus species (Snyman, 2010, Lecatsas, 1962, Cohen, 1937). Snyman 1427

(2010) provided evidence that S. plumosum has allelopathic effects on its own seedlings as 1428

well as interspecific competitor seedlings through inhibition of germination and seedling 1429

development. This may explain the bare ground that tended to surround S. plumosum 1430

shrubs on transect ww3 at West Wits (Figure 2.14b). However this was the only site at both 1431

Vaal River and West Wits where there was evidence that S. plumosum may be exerting a 1432

negative effect on surrounding vegetation and no investigation has yet been conducted on 1433

allelopathic interactions between S. plumosum and adult plants of the same or different 1434

species. Furthermore, signs of remnant burnt stubble in these bare patches surrounding S. 1435

plumosum patches in transect ww3 support Snyman’s (2011) conclusion that increased 1436

temperatures due to the burning of the S. plumosum canopy cause increased local mortality 1437

of perennial grass species (Figure 2.14a and b) in the vicinity of these shrubs. 1438

According to Snyman (2012, 2011, 2010), S. plumosum is not found on soils that get water- 1439

logged, preferring well-drained soils. Yet forty-three S. plumosum patches were recorded on 1440

the 180 metres of transect vr8 (Table 2.3). This transect also had the only recorded patches 1441

at any site in this study of Schoenoplectus species, a sedge that definitely prefers at least 1442

seasonally waterlogged or wetland conditions (Marnewecke and Kotze, 1999). Possibly 1443

these S. plumosum patches are a remnant of drier conditions prior to the establishment of 1444

the TSF near the foot of transect vr8. Aerial surveys from 1980 show no TSF but it was 1445

present in 1992. Hattingh (1953) estimated that S. plumosum had an average lifespan of 1446

66 a.)

1447

b.) 1448

Figure 2.14 (a) Seriphium plumosum burning as a fire swept through transect vr15

1449

showing the increased intensity of the fire as it burns the S. plumosum 1450

canopy compared to the grass canopy. Notice the grass right up to the S. 1451

Plumosum suggesting no allelopathic response. (b) Remnant burnt grass 1452

stubble and bare patch in the vicinity of an S. Plumosum shrub on ww3 1453

transect. The bare patch and dead grass stubble possibly indicative of the 1454

increased mortality of grasses close to these shrubs through increased fire 1455

intensity when the shrub burns (Snyman, 2011). 1456

67 about 15 years which is significantly shorter than the time that the TSF has been present 1457

(> 20 but < 34 years). However, it is unknown whether or when surface/groundwater 1458

conditions might have switched from being possibly well-drained to hydromorphic with the 1459

establishment of the TSF. Seriphium plumosum shrubs have been associated with low soil 1460

nutrient conditions (SOM and % P) and Snyman (2009b) observed that they preferentially 1461

invaded abandoned marginal cultivated lands over more fertile sites. Nevertheless, in this 1462

study there was no significant difference in SOM between patch types at Vaal River, while at 1463

West Wits, SOM values were highest for S. plumosum patches and these differences were 1464

significant for bare patches but not for other vegetated patch types. Observations while 1465

sampling these patches found there was often a well developed fine litter layer of 1466

predominantly seeds shed from the shrub. Phosphorus was not examined in this study but 1467

no significant differences in soil nitrogen, either organic or inorganic, were found between 1468

patch types at West Wits. 1469

Of the seven patch types described from Vaal River, four were some form of graminoid- 1470

dominated patch, namely bare grass, sparse grass, grass patch and tall grass, which 1471

essentially describes an above-ground biomass gradient (Table 2.5). At West Wits, two of 1472

the four described patch types were graminoid-dominated patch types and were similarly 1473

named as two of the Vaal River patch types, i.e. sparse grass and grass patch, again based 1474

on differences in above-ground cover reflecting a gradient in above-ground biomass (Table 1475

2.1 and 2.7). One factor influencing or influenced by this continuum is soil moisture. 1476

However, transect averages for soil moisture tended to follow the date of sampling (not 1477

shown) with transects like vr10 and vr11 with the highest average soil moisture being 1478

sampled in mid winter whereas transects vr13 and vr14, with the lowest mean soil moisture 1479

sampled last towards the end of the dry winter (Table 2.4). But even with the influence of 1480

sampling date and winter dry-season effects, above-ground biomass (Figure 2.8) and root 1481

biomass (Figure 2.10a) were weakly correlated with soil moisture. Patch type showed 1482

significant differences based on soil moisture (Table 2.5 and 2.7) and generally followed the 1483

trend of increasing vegetation biomass although there were exceptions. For instance at Vaal 1484

River, grass patches had significantly higher above-ground biomass when compared with 1485

bare grass and biological soil crust patches, the latter having no measureable above-ground 1486

biomass. Yet soil moisture was not significantly different between the three patch types 1487

68 mentioned, although it was significantly lower than that for Schoenoplectus and S. 1488

plumosum patch types which had the highest mean above-ground biomasses (Table 2.5). As 1489

these grass patches belong to transect vr9 and this was sampled towards the end of winter, 1490

it is likely that this anomaly in low soil moisture is a result of the length of time the soil has 1491

had to dry out over the winter dry period prior to sampling. However, the dominant trend of 1492

there being a direct correlation between soil moisture and biomass, and therefore patch 1493

type, is one that is well described in the literature (Li et al., 2013, D'odorico et al., 2007, 1494

Eldridge et al., 2002, Breshears and Barnes, 1999, Greene, 1992). Bhark and Small (2003) 1495

showed that shrub and grassland canopies had higher infiltration and soil moisture than did 1496

the inter-canopy spaces and that this effect was stronger for shrubs in their study area than 1497

for grass canopies. Pockman and Small (2010) however, found grass patches in the 1498

Chihuahuan Desert had higher soil moisture after a rain event than did the shrub Larrea 1499

tridentate with bare patches the lowest. Thus the relationship between vegetation cover or 1500

above-ground biomass and soil moisture is complex. 1501

The primary determinant of soil moisture is climate (Legates et al., 2011, Noy-Meir, 1981). 1502

In the United States, Anderson (2006) describes how the central grasslands are separated 1503

into three categories which form a continuum from west to east, namely short-grass (0.3 – 1504

0.5 m tall), mixed grass (0.8 – 1.2 m) and tall grass (1.8 – 2.4 m). This continuum essentially 1505

reflects an annual precipitation gradient with the arid central continental short-grass prairie 1506

in the west receiving 260 – 375 mm annual precipitation, which increases as one moves 1507

eastward with the wetter east receiving 625 – 1200 mm (Anderson, 2006), and mixed prairie 1508

situated between the two and in the middle of the moisture gradient. It is likely that 1509

differences in precipitation underlie differences in biomass and canopy density between the 1510

two research sites in this study. At Vaal River 50% of all patches described were sparse grass 1511

and this patch type constituted 67% of the graminoid-based patches. At West Wits, 52% of 1512

patches were graminoid in nature, split slightly in favour of grass patches (56%) over sparse 1513

grass (44%) with no tall grass patches on the transect, but such tall grass patches were 1514

observed in off-transect areas of West Wits. Although the composition of the sparse and 1515

grass patches was not sampled the only obvious difference was in density of cover as 1516

reflected in the above-ground biomass. The fact that this was only significantly higher at 1517

West Wits is probably at least partially a function of the higher annual precipitation at West 1518

69 Wits (Anderson, 2006). Height in these two patch types was not measured but never got 1519

higher than knee height (0.6 m). However, tall grass patches consisting of Hyparrhenia hirta 1520

tussocks that can reach a height of 1.5 m (Van Wyk and Van Oudtshoorn, 1999), made up 1521

5% of sampled patches and was limited to particular transects at Vaal River. Although it was 1522

present at West Wits, tall grass patches were not recorded from any transects sampled 1523

there. However its presence is not related to a moisture gradient as H. hirta is regarded as a 1524

drought resistant species generally associated with climax or sub-climax grassland 1525

communities and secondary succession (Van Wyk and Van Oudtshoorn, 1999, Roux, 1969b). 1526

Soil organic matter (SOM) was significantly different between patch types at West Wits 1527

where bare patches had the lowest mean values and S. plumosum patches had the highest 1528

values (Table 2.7). However at Vaal River there were no significant differences in SOM 1529

between patch types (Table 2.5). Furthermore, all patches at Vaal River had lower mean 1530

values for SOM than did any patch type at West Wits although these differences were not 1531

tested statistically. The reasons for these differences are unknown but the formation and 1532

breakdown of SOM is a complex process (Jenkinson et al., 1990, Jenkinson and Rayner, 1533

1977) and a number of factors are known to affect SOM. Temperature (Kätterer et al., 1998, 1534

Lloyd and Taylor, 1994) and soil moisture (Yang et al., 2007, Lee et al., 2004) have been 1535

shown to influence SOM where increases in either variable lead to increased soil microbial 1536

activity and rates of SOM turnover which may result in decreased levels of SOM in soils. 1537

However, increases in both these factors can also result in increased inputs of carbon to 1538

soils through increased net primary productivity. Jenkinson and Rayner (1977) showed that 1539

reduced plant inputs reduced the long-term SOM values of a soil, and sparse grass, grass 1540

patch and S. plumosum patch above-ground mean biomasses were considerably lower at 1541

Vaal River compared to those for the same patch types respectively at West Wits (Table 2.5 1542

and 2.7). Seriphium plumosum patches at West Wits also had the highest mean above- 1543

ground biomass for any patch type at West Wits which supports the concept of higher 1544

carbon in the form of biomass inputs contributing to higher SOM values. Root biomass was 1545

not quantified at West Wits. At Vaal River, root biomass was found to be only weakly 1546

correlated with SOM and more strongly correlated with above-ground biomass (Figure 1547

2.10a). Furthermore, the relationship between root biomass and patch type was not a direct 1548

relationship between above- and below-ground biomass. This can be seen when comparing 1549

70 above and below-ground biomasses for sparse grass which had higher mean root biomass 1550

and lower above-ground shoot biomass when compared to grass patches at Vaal River. 1551

However, the general trend was that increases in above-ground biomass were mirrored in 1552

increased root biomass (Table 2.5). The effect of roots on SOM is complex as, on the one 1553

hand, they are a source of SOM (Aerts et al., 1992), while Cheng (1990) showed that living 1554

roots enhanced the rate of decomposition of SOM. However, amongst other factors, both 1555

SOM and the presence of above- and below-ground biomass are important influences on 1556

the stability of a soil and its resistance to fluvial based soil erosion (Oades, 1993, Oades, 1557

1984). 1558

The LFA index for soil stability is designed to indicate a site’s susceptibility to erosive forces 1559

and the ability of a soil to reform after disturbance (Tongway and Hindley, 2004). The index 1560

is calculated as a composite of a number of visual or tactile properties of the soil, or features 1561

at the soil surface, and uses a relative scale in which high values indicate stable, resistant 1562

soils. As the index is a visual system it does not include direct measures of SOM and root 1563

presence. Soil organic matter is indirectly estimated through evaluating the degree of 1564

incorporation of a leaf litter into soil and the visual presence of fungi in the litter. Root 1565

biomass is indirectly estimated through the basal area in grasses, and the canopy cover and 1566

density in shrubs and trees (Tongway and Hindley, 2004), on the assumption that the larger 1567

the base of a grass tuft is, the greater the root mass that will extend from it into the soil 1568

volume. The general trend at both West Wits and Vaal River was that values for LFA soil 1569

stability increased from lower values measured for bare soil to highest values for S. 1570

plumosum patches (Table 2.5 and 2.7). This trend in LFA soil stability was mirrored in most 1571

of the environmental factors that promote soil stability and resistance to erosion such as 1572

increased above-ground biomass (Chartier et al., 2013, Dlamini et al., 2011, Okin et al., 1573

2006, Abrahams et al., 1995) which was strongly correlated with LFA soil stability at West 1574

Wits and less so at Vaal River. Similarly, a weaker but still positive correlation between 1575

below-ground biomass and the LFA soil stability index supported the conclusion that this 1576

index was providing a viable relative indicator of soil stability and susceptibility to erosion 1577

(Burylo et al., 2012, De Baets et al., 2007, Cammeraat et al., 2005). 1578

Other environmental factors measured in this study also provide support for the conclusion 1579

that the LFA soil stability index is giving a valid relative indicator of soil stability. Core depth 1580

71 was taken to 10 cm but in a number of occasions stone prevented the attainment of this 1581

depth and there is a statistical correlation between depth of core and patch type. Core 1582

depth was significantly shallower in biological soil crust patches and bare grass patches 1583

(Table 2.5) compared to all other vegetated patches with the exception of tall grass patches. 1584

Also, the amount of stone greater than 2 mm present in the soil in both biological soil crust 1585

patches and bare grass patches was significantly higher than in other patch types with the 1586

exception of grass, and Schoenoplectus patches which were very few and restricted to one 1587

transect. There could be a number of explanations for the differences in soil core depths 1588

and soil stone content which are unrelated to soil stability and erosion. One such 1589

explanation could be that the sub-surface rock is closer to the surface in these patches than 1590

it is in neighbouring or nearby patch types and this leads to conditions that are not 1591

conducive to the establishment of vegetation and thus a bare patch, biological soil crust

In document Mapping landscape function with hyperspectral remote sensing of natural grasslands on gold mines (Page 84-103)