Spatial and temporal distribution of cyanobacterial soil crusts in the Kalahari: implications for soil surface properties
Thomas, A.D.1 and Dougill, A.J.2
1 – Department of Environmental and Geographical Sciences, Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester, M1 5GD, U.K. Tel. +44 (0) 161 247 1568. Fax. +44 (0) 161 247 xxxx. Email:
Corresponding Author.
2 – School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK. Tel. +44 (0) 113 343 6782. Fax. +44 (0) 113 343 6716. Email:
Abstract
Keywords
Introduction
Extensive research reviews exist assessing the role of surface vegetation on dryland geomorphological processes (e.g. Thornes, 1990; Bullard, 1997), in particular on the role that surface vegetation has in controlling aeolian wind erosion (Tsoar & Møller, 1986; Lancaster & Baas, 1998). Specifically for the Kalahari region of Southern Africa, research has focused on the link between surface vegetation cover and dune mobility in the arid southwest of Botswana (Wiggs et al., 1995) and on the controlling influence of bush cover on nebkha formation in the mixed farming areas in the dry sub-humid Molopo Basin (Dougill & Thomas, 2002). Much less attention has been paid to aeolian erosion processes in the more extensive semi-arid savanna rangelands that typify much of Botswana, Eastern Namibia and Northern South Africa (Thomas, 2002), or to the possible controlling influence of cyanobacteria assemblages that colonise biological soil crusts on the soil surface (Dougill & Thomas, 2004). Biological soil crusts have been shown in other dryland regions to decrease surface erodibility significantly due to a number of contributory factors. Biological crusts bind sand particles together (Belnap et al., 2003). Crusts also provide notable elasticity to the soil surface that reduces the force of grain impact (McKenna Neuman & Maxwell, 2002). In addition, crusts also provide an enhanced nitrogen source that will increase the surface vegetation cover (and thus landscape roughness) in dryland ecosystems that are typically N-limited (Scholes & Walker, 1993). This paper aims to report on recent research findings from a range of locations in the semi-arid Kalahari that focus on the impact of biological soil crusts on spatial and temporal patterns of soil surface properties that regulate aeolian erosion processes.
Biological Soil Crusts and Erodibility
Biological soil crusts are present in all arid and semi-arid regions (Belnap & Lange, 2003) and comprise cyanobacteria, algae, lichens, mosses, microfungi and other bacteria (Belnap, Büdel and Lange, 2003). Fundamental to understanding the geomorphological significance of cyanobacterial soil crusts is a comprehension of their spatial distribution and their dynamic influence on surface erodibility.
Several factors are recognised as influencing crust distribution and development, especially substrate, vegetation type and cover, and disturbance levels (Belnap et al., 2003). It has been shown that vegetation and biological crust cover are inversely proportional due to the effects of competition for light (Malam Issa et al., 1999) and nutrients (Harper & Belnap, 2001). It is also generally accepted that trampling, as a result of grazing, damages biologically-crusted surfaces (e.g. Eldridge, 1998), thus in areas of intense grazing, the spatial distribution of crusts will be limited. Zaady & Bouskila (2002) describe disturbance as the key factor in determining biological crust development in areas where physical conditions are relatively constant. Given the spatial homogeneity of the Kalahari, in terms of altitude, relief and surface water (Thomas & Shaw, 1993), it is reasonable to impart a significant role to grazing disturbances in affecting the distribution of cyanobacterial soil crusts. In this context, Berkeley et al. (2005) have shown that on a localised scale the canopies of woody plants represent quasi-discrete environments, in which the response of crusts to disturbance regimes is altered. This displays the need to base studies of spatial patterns of crust distribution and dynamics on three scales of analysis: a landscape scale to investigate the role of different land uses; a farm scale to assess differences in grazing intensity; and a localised ‘plot’ scale to analyse inter-relationships between crust and surface vegetation communities.
Assessments of the spatial patterns and temporal dynamics of biological crust cover are vital to improving geomorphological assessments of drylands due to the direct and indirect impacts that crusts have on reducing the erodibility of the soil surface (e.g. Van der Ancker, 1985; Danin et al., 1989; Yair, 1990; Zobeck, 1991; Belnap & Gillette, 1998; Rice & McEwan, 2001; McKenna Neuman & Maxwell, 2002; Belnap et al., 2003; Eldridge & Leys, 2003; Goosens, 2004). Erodibility (the degree to which a surface is susceptible to erosion) assessment has received less attention than erosivity (the potential to erode the surface), partly due to the difficulty of measuring surface erodibility (Geeves et al., 2000). This difficulty in assessment is exacerbated by the control of inter-related textural, mineralogical, chemical, hydrological and biological surface characteristics on erodibility, each of which varies in space and time. There is a recognised need to improve information available on the soil surface conditions, such as cohesive strength and roughness, that control erodibility if wind erosion models are to be improved to incorporate the evolution of soil surface conditions (Sokolik & Toon, 1996; Shao & Leslie, 1997; Chappell et al., 2005). Such advances are essential if dust and erosion models are to be linked to global climate change and regional biogeochemical cycle models.
Dougill & Thomas (2004) have previously documented a cyanobacterial soil crust cover of between 19 - 40 % at a range of disturbed sites on Kalahari Sands. They identified three morphologically distinct crusts: a weakly consolidated crust with no surface discolouration (type 1); a more consolidated crust with a black or brown speckled surface (type 2); and a crust with a bumpy surface with an intensely coloured black/brown surface (type 3). This paper builds on this preliminary analysis by addressing the following research objectives:
- To determine the controls on the distribution of soil crusts on Kalahari Sands at regional, farm and plot scales.
- To quantify recovery of cyanobacterial crust cover after disturbance.
- To examine the implications of cyanobacterial crusts for soil surface roughness, strength, and total nitrogen content.
Study Area
Kalahari Sands are ancient wind-blown deposits that cover an area of over 2.5 million km2 of Southern Africa, including over 80 % of Botswana (Thomas & Shaw, 1991). Kalahari Sand soils typically consist of over 95 % fine sand sized sediments and are predominantly deep, structureless and nutrient deficient (Dougill et al., 1998). The Kalahari includes areas of both active sediment movement and stable surfaces, with much research focusing on the importance of surface vegetation cover (Bullard, 1994; Wiggs et al., 1994, 1995) and climate change (Thomas et al., 1997; Knight et al., 2004) in affecting sediment mobility. The importance of soil surface characteristics, especially the development of biological soil crusts, has been largely overlooked. The presence of crusts has been largely focused on their role in affecting nitrogen cycling (Skarpe & Hendrikson, 1987; Aranibar et al., 2003, 2004; Dougill & Thomas, 2004; Berkeley et al., 2005). All these studies focused on highly disturbed communal rangeland sites and have estimated crust covers of between 19 and 40 %. No empirical estimates exist of crust cover in lightly disturbed sites where crust cover can be expected to be greater (Belnap et al., 2003).
Land use across the Kalahari is predominantly pastoral and is made possible by boreholes that allow access to groundwater, although large tracts remain zoned for wildlife conservation or management as either National Parks or Wildlife Management Areas. Concentrated grazing pressure close to boreholes adds a new environmental gradient to an otherwise relatively homogenous environment (Thomas & Shaw, 1993). This leads to localised sediment mobility in a spatially confined ‘sacrifice zone’ close to boreholes (Perkins & Thomas, 1993) and a more extensive ‘bush encroached zone’ where woody shrubs, typically A. mellifera, dominate. The region has a highly seasonal summer rainfall regime with very high inter-annual variability that increases with declining mean annual rainfall, with inter-annual variability of over 50 % found in sites in the arid south-west of Botswana where mean annual rainfall is less than 200 mm (Bhalotra, 1987). The study sites investigated in this study lie (Figure 1) on a transect across the semi-arid Kalahari between Tsabong (c. 300 mm mean annual rainfall; 45 % inter-annual variability) and Okwa (c. 400 mm mean annual rainfall; 40 % inter-annual variability). The rainfall regime over the past seven years for one of the study sites is shown in Figure 2 notably displaying the below average rainfall experienced since 2002 when sampling and crust recovery studies have been undertaken.
Figure 2. Mean annual rainfall at Berry Bush Farm, Tsabong September 1997 to August 2004
Research Design and Methods
Research was undertaken during July 2003 at five locations of differing land use, and a total of 10 sites in total (as sites of different grazing intensity measured at Tshane and Tsabong, and of different vegetation communities in Mabuasehube National Park) on Kalahari Sands in Kgalagadi District, Botswana (Figure 1). Disturbance was quantified at each site using a disturbance index using cattle track frequency and dung density (as per Dougill & Thomas, 2004 and Berkeley et al., 2005). At each site, a 30 m x 30 m grid was established. The grid was crossed at 10 m intervals in two perpendicular directions. Cattle tracks and dung were counted along each of these gridlines, cattle tracks being defined as well established ‘routes’, and a dung ‘count’ being a single or collection of pats (as opposed to total fragments) within an arbitrary 0.5 m either side of the gridline.
Crust cover data were estimated at three scales. To assess the impact of regional variations in land use, 5 study locations were sampled (Figure 1). These encompassed a range of grazing intensities from a lightly-grazed National Park, wildlife management zone to more intensely grazed communal rangelands and commercial ranches (encompassing 10 sites as listed in Table 1). Within each site, localised measurements of crust-vegetation relationships were taken using a 1 m x 1 m quadrat sampled at regular intervals of 2 m inside 30 m x 30 m grid such a total of 240 quadrats were examined (show as a Figure to explain this diagrammatically?). Percentage cover was estimated for each cyanobacterial soil crust type (according to the morphological classification system of Dougill & Thomas, 2004), unconsolidated soil, litter and cover of each vegetation species within the quadrats. Detailed crust associations with the two most common shrub species in the Southern Kalahari (Acacia mellifera and Grewia flava) were also examined. The canopy dimensions of three shrubs of each species were measured and crust cover estimated in a series of 0.5 m x 0.5 m quadrats, adjacent to one another along a line extending from the bowl to the canopy edge in a northerly and southerly direction to account for any orientation-controlled differences in cover.
To assess recovery of crust cover, two 5 m x 5 m plots were established in July 2003 at a commercial ranch site (Berry Bush farm) neighbouring the Tsabong sample sites. These plots were located at a severely disturbed site close to the water point and farm where no crust cover was found. One plot was fenced to exclude all grazing animals and to compare crust cover characteristics of the ungrazed plot to the neighbouring unfenced plot where grazing was still possible, though cattle were removed from the farm shortly after plot establishment in favour of camel and game. Sixteen 1 m x 1 m quadrats were surveyed for crust type and vegetation cover in each plot in both July 2003 and November 2004 to assess the changes in crust cover and morphology following removal of disturbance.
Properties – OM/TN;
Chlorophyll
Strength and Roughness – Tom
Results and Analysis
Controls on the distribution of soil crusts at a variety of spatial scales
Cyanobacterial Crust Cover (%) / Vegetation Cover (%) / DisturbanceIndex
Site (n = 240 at each site) / BC1 / BC2 / BC3 / Total / Shrub / Grass
Tshane 1
Tshane 2
Mabua 1
Mabua 2
Okwa WMZ
Tsabong Mixed
Tsabong Bush Encroached
Tsabong Pan
Tsabong Borehole
Draihoek Bush Encroached / 40
42
75
64
80
26
24
21
11
55 / 4
2
10
24
5
39
36
48
0
8 / 0
0
1
7
1
2
4
15
0
1 / 44 (36)
44 (38)
86 (17)
95 (8)
86 (22)
68 (44)
64 (45)
84 (29)
11 (18)
64 (35) / 27 (40)
26 (40)
7 (23)
1 (9)
4 (19)
22 (30)
26 (28)
2 (12)
32 (44) / 2 (0.2)
5 (6)
94 (20)
17 (11)
22 (22)
-
-
-
-
0.2 (0.9) / 13.2
7.5
0.0
0.1
0.0
1.6
2.1
2.4
15.4
8.2
I can happily run through SPSS on this – ANOVA using a Post-Scheffe’s test should account for the fact that all data may not be normally distributed.
Surfer Plots and vegetation Patterns (for veg pattern stuff refer to Berkeley et al)
Figure . Total Cyanobacterial Crust Cover within 30m x 28 m plots at a) Okwa Wildlife Management Zone and b) Tshane communal grazing area
Figure . The relationship between different vegetation types and cyanobacterial crust cover within 30m x 28 m plots at a) Okwa Wildlife Management Zone and b) Tshane communal grazing area
Recovery of cyanobacterial crust cover after disturbance
Cyanobacterial Crust Cover (%) / VegetationSite (n = 240 at each site) / BC1 / BC2 / BC3 / Total / Shrub / Grass
Open Plot
August 2003
November 2004
Closed Plot
August 2003
November 2004 / 0
77
0
93 / 0
13
0
6 / 0
0.3
0
0 / 0
90.3
0
99 / 8
11
4
5 / 0
2
0
1.4
Chlorophyll data (response to wetting) – refer to last paper (JAE)
To examine the implications of cyanobacterial crusts for soil surface roughness, strength, and total nitrogen content.
Organic Matter % / Ammonium µg/g-1 / Total N µg/g-11 / Phosphate µg/g-1 / pH / Strength (kg cm-2)All Sites
Unconsolidated
BC1
BC2
BC3 / n= 278
1.14 (0.93)
0.92 (0.49)
1.04 (0.4)
1.68 (0.53) / n = 145
76.4 (48.7)
101.3 (52.3)
80.6 (44.1)
89.4 (51.8) / n = 98
183.1 (191.1)
172.9 (89.2)
341.3 (221.3)
652.2 (856.5) / n = 92
2.7 (1.2)
2.8 (1.1)
3.1 (1.5)
2.5 (0.9) / n = 192
5.9 (0.4)
5.8 (0.5)
5.8 (0.6)
5.9 (0.3) / n = 66
0
0.56 (0.8)
0.73 (0.8)
1.50 (0.9)
Discussion
The research findings presented here provide a first assessment of the spatial variability of biological soil crusts on both local and regional scales for the Southern Kalahari. These findings display that the spatial extent of crust cover is far more extensive than suggested in previous analyses that focused solely on regularly disturbed communal rangeland sites (e.g. Skarpe & Henriksson, 1987; Aranibar et al., 2003; Dougill & Thomas, 2004). Previous taxonomic analyses of these crusts (Thomas & Dougill, in press) have shown that they are dominantly comprised of only two main cyanobacteria species, Microcoleus and Scytonema. Both these species are capable of binding fine sand particles together, thus decreasing surface sediment mobility and explaining the lack of major aeolian erosion events, or dust losses, observed from the Kalahari sandveld (TOMS style RS references??). This is exacerbated by the large portions of Botswana (approximately 46 %) that has protected status as either National Park or Wildlife Management Area. Our findings show that these areas typically have a cyanobacterial mean crust cover of over 90 % that can explain the low levels of surface erosion observed. The Kalahari, appears notably different to many other drylands globally where with low disturbance levels where crusts become dominated by lichens and mosses (e.g. refs??). In particular, surface cohesivity and strength is provided to Kalahari soils by Microcoleus species that typically occur in bundles surrounded by a polysaccharide sheath. This filamentous sheath material forms a web of cohesive material in the upper soil layer following cycles of wetting and drying (Belnap et al., 2003).
On a localised (site) scale, crust distribution is related directly on a species-specific basis to the surface vegetation cover, notably bush and tree canopies that determine the spatial patterns of surface disturbance by grazing animals. This has important implications on the patterns of potentially erodible sediment source areas (in open areas between bush canopies) and stable zones (notably under A. mellifera canopies) where crusts can continue to develop and thus significantly increase their surface strength, roughness and total-N content further. This localised spatial heterogeneity of crust cover offers a potential mechanism that can contribute to the rapid expansion of bush encroachment by increasing the nitrogen content and water retention capacity of soils under A. mellifera canopies (Hagos & Smit, 2005). There are similarities in such a spatial heterogeneity explanation of bush encroachment to hypotheses suggested for the SW USA (e.g. Schlesinger et al., 1990; Titus et al., 2002). However, unlike these Mojave Desert examples where sediment redistribution by water erosion leads to ‘islands of fertility’ under bush canopies where sediment is deposited, the increase in soil resource heterogeneity in the Kalahari appears to occur without significant erosion, even on such a localised scale. Indeed, when significant localised wind erosion does occur it leads to the formation of small nebkha dunes as seen in the Molopo Basin (Dougill & Thomas, 2002) and around major settlements such as Tshabong and Tshane. Crust formation in such cases is likely to be restricted by both the burial of the crust and the surface disruption by saltating sand grains (McKenna Neuman & Maxwell, 2002). Our findings suggest that erosion rates in the Kalahari rarely reach levels such that the redevelopment of the cyanobacterial soil cover is threatened. Further research is required to assess the role of crusts in suppressing erosion rates in more arid sites in south-west Botswana, where the crests of linear dunes appear active only when surface vegetation cover is below an approximately 14 % threshold (Wiggs et al., 1995).
The ability of crusts to redevelop rapidly following disturbances is clearly portrayed by the recovery of crust cover on the experimental plots established in this study. This recovery has occurred despite the proximity of plots to sparsely vegetated surfaces and the low rainfall at the site since their establishment and offers a plausible explanation for the resilience of Kalahari soils that has often been inferred in past studies (e.g. Perkins & Thomas, 1993; Dougill et al., 1998).
Erosivity – roughness, strength, total-N acting together to reduce surface erosivity.
Need for further field process-based research.
Conclusions
Acknowledgements
Usual Research permit reference and help in field from Andy B, Angela C and Jill and Keith Thomas, Berry Bush Farm, Tshabong.
References
I’m happy to do these next Thursday / Friday if aiming for April 1st deadline
Tables
Figures