Spatial dynamics of biological soil crusts: bush canopies, litter and
burial in Kalahari rangelands
1Berkeley,
A., 1*Thomas, A.D. and 2Dougill, A.J.
– Department of Environmental and Geographical Sciences, Manchester
Metropolitan University, John Dalton Building, Chester Street, Manchester, M1 5GD,
U.K; A.Berkeley@mmu.ac.uk ; A.D.Thomas@mmu.ac.uk
1
– School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK;
adougill@env.leeds.ac.uk
2
* - Corresponding author – Dr Andrew Thomas, A.D.Thomas@mmu.ac.uk
Keywords: Biological soil crusts; Kalahari; Bush encroachment; Acacia mellifera;
Grewia flava; Crust burial
Running Title – Bush, crust and litter relations in Kalahari
Abstract
Intensive grazing of Kalahari rangelands has led to bush encroachment, notably of
Acacia mellifera and Grewia flava. The mechanisms causing this process, and the
ecological stability of bush encroached ecosystems, remains uncertain. Past studies
suggest that bush-soil relations may enhance bush competitive dominance. This study
aims to investigate one element of the bush-soil relation, by examining the spatial
distribution of biological soil crusts in three vegetation sub-habitats at sites of
different disturbance. Crust burial, by litter and sediment, was also assessed and
modelled to analyse the dynamics of bush-crust relations. Results display enhanced
cyanobacterial crust cover under A. mellifera canopies and that unlike G. flava
canopies this enhanced crust cover remains under A. mellifera even at disturbed sites.
This canopy-crust association suggests A. mellifera encroachment will exhibit
intrinsic resilience due to the crusts ability to stabilise the soil surface and increase
water and nutrient retention. Crust burial by litter and sediment that accumulates
under larger bushes restricts crust development under canopies. Disturbance restricts
crust development in bush interspaces and under G. flava. These two mechanisms
combine to restrict crust development to an observed 40 % threshold, with non-linear
spatial models required to explain spatial patterns of crust dynamics.
Introduction
Livestock farming in the Kalahari is typified by the use of boreholes that provide
groundwater reserves to cattle. Intensive grazing pressure around these waterpoints,
has led to concerns over rangeland degradation (e.g. Moleele & Perkins, 1998;
Dougill et al., 1999; Moleele et al., 2002), notably over the increased dominance of
woody bush species over grasses. This process, referred to as bush encroachment, has
been linked to spatial heterogeneity of soil resources, and the reorganisation of
nutrients into ‘islands of fertility’ (Titus et al., 2002) that can contribute to the
competitive advantage of encroaching bush species (Schlesinger et al., 1990; Dougill
& Thomas, 2004). This paper aims to improve understanding of the mechanisms
controlling relations between the encroaching bush cover and sub canopy soil
biochemical characteristics that will control future ecological changes in Kalahari
rangelands.
One component of the Kalahari system that has been largely overlooked in past
research are biological soil crusts, comprising cyanobacteria, green algae, lichens,
mosses, microfungi and other bacteria (Belnap et al., 2003). Biological soil crusts are
present in all arid and semi-arid regions (Belnap & Lange, 2003). The ecological roles
of these crusts include; increasing soil surface stability by binding erodible soil
particles into aggregates thus decreasing erosion by wind and water (Eldridge & Leys,
2003); fixing atmospheric nitrogen (Aranibar et al., 2003), and sequestering CO2 into
organic carbon (Zaady et al., 2000).
Although crusts are usually associated with finer grain soils, Dougill & Thomas
(2004) have documented a cyanobacterial soil crust cover of between 19 - 40 % at a
range of disturbed sites on Kalahari sand soils. Fundamental to understanding the
ecological significance of biological soil crusts in the Kalahari is a comprehension of
their spatial distribution. Several factors are recognised as influencing crust
distribution and development, especially substrate, vegetation type and cover, and
disturbance levels (Belnap et al., 2003). Thomas and Dougill (submitted) have
documented the differences in biological crust cover for several substrate types in the
Southern Kalahari. Although differences existed in crust cover under different bush
species, and in the interspaces (Thomas et al., 2002; Thomas and Dougill, submitted),
the exact relationship between crust cover, vegetation cover and disturbance regime
remains uncertain.
It has been demonstrated that plants growing in crusted soils may exhibit enhanced
nutrient levels, compared to those growing on non-crusted surfaces (Belnap, 2002).
Conversely, it is also reported that vegetation and biotic crust cover are negatively
related due to the effects of competition for light (Malam Issa et al., 1999) and
nutrients (Harper & Belnap, 2001). It is generally accepted that trampling, as a result
of grazing, damages biologically crusted surfaces (e.g. Eldridge, 1998). It follows
that, in areas of intense grazing such as around boreholes, the spatial distribution of
biological soil crusts will be limited. The hypothesis that crust cover increases with
distance from borehole (i.e. with decreasing disturbance) is yet to be examined and
may be complicated by increases in bush cover away from waterpoints (Ward et al.,
2000).
Zaady & Bouskila (2002) describe disturbance as the key factor in determining 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 determining the distribution of biological soil crusts. In this context
bush canopies may represent quasi-discrete environments, in which the response of
crusts to local disturbance regimes is altered. This phenomenon is yet to be tested with
reference to disturbance intensity (i.e., with disturbance as the independent variable),
but could be vital in controlling the response of the Kalahari ecosystem to grazing
related disturbance, and thus the relative abundance of grasses and shrubs. The
concentration of leaf litter below bush canopies complicates the situation. Litter may
smother crusts and prevent photosynthesis, or, alternatively, may only shade crust and
provide a moister habitat conducive to crust development. That biological soil crusts
may develop differentially within these sub-canopy habitats has important
implications in terms of the spatial heterogeneity of resources, ecosystem resilience
and long-term ecological stability of rangelands.
It is probable that the roles of vegetation and disturbance on biological crust
distribution are not mutually independent of one another. The aim of this study is to
describe the distribution of biological soil crusts at grazed Kalahari study sites in
terms of the overlapping domains of vegetation and disturbance. In order to address
this aim the following objectives were chosen: i) to test models that suggest that there
are species-specific, sub-canopy protection impacts on the form and characteristics of
biological soil crusts; ii) to determine the impact of plant litter and sediment burial on
the distribution of biological soil crusts.
Materials and Methods
Site Selection
Research was undertaken during July 2003 on communal grazing lands adjacent to
Berrybush Farm, near Tsabong, Southern Kgalagadi District, Botswana (Figure 1).
Four sites, at different settings around a borehole, were selected for data collection.
Disturbance was quantified at each site using a disturbance index rather than the
proxy of distance from borehole. The closest and furthest sites, with respect to the
borehole, correspond to the ‘sacrificial zone’ (Site 1) and ‘un-encroached zone’ (Site
4) of the piosphere model described by Moleele et al. (2002), with the intermediate
sites representing the ‘bush encroached’ (Site 2) and ‘mixed bush and grass’ (Site 3)
zones respectively (Figure 1).
Quantification of disturbance
At each site, disturbance levels were quantified using cattle track and dung frequency
(as per Dougill & Thomas, 2004). At each site, a 50 m x 50 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) laying within 0.5 m either side of the gridline. The 0.5 m
value is arbitrary and for the sake of consistency only.
Assessment of biological crust cover in interspaces
Crust cover data were estimated within a 0.5 m x 0.5 m quadrat at intervals of 10 m
inside the 50 m x 50 m grid. Percentage cover was estimated for each successionary
stage of biological soil crust (according to the morphological classification system of
Dougill & Thomas, 2004), buried crust, unconsolidated soil, litter and grass within
five 0.5 m x 0.5 m quadrats at each site.
Assessment of crust cover beneath bush canopies
The two most common bush encroaching species in the Southern Kalahari were
selected for sampling, the thorny Acacia mellifera and the non-thorny Grewia flava
(Reed & Dougill, 2002). The canopy dimensions of every bush within the 50 m x 50
m quadrat were measured. Crust cover estimates were taken in several 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. Within each quadrat, crust cover was quantified, as well as
buried crust, unconsolidated substrate and litter.
Measuring and predicting buried crust
Buried crust was estimated within the same 0.5 m x 0.5 m quadrats, by carefully
prodding the unconsolidated surface at numerous points to determine buried
consolidated layers.
Given that biological crust cover has been shown to reduce sediment entrainment
(Belnap & Gillette, 1998; Eldridge & Leys, 2003), it is reasonable to assume that the
magnitude of sediment redistribution at a given site is inversely proportional to the
area of crusted surface. The occurrence of buried crust will be proportional to both
crust cover and the amount of ground which is unconsolidated and can be written as:
Cburied = kC(100 – C)
where Cburied is the amount of crust buried, C is the percentage of ground crusted (the
sum 100 – c representing the percentage area uncrusted), and k is the constant of
proportionality which, in this case, describes the combined influences of climate,
grain size, and vegetation. This model predicts maximum values for crust burial at
those sites where crust cover and unconsolidated substrate share a mutual maximum
(i.e. ~ 50% each) and minimum values of buried crust where the crust cover is either
too high (too little unconsolidated substrate for reworking), or too low (probability of
burial too low). So it seems, theoretically at least, that the process of crust burial is a
trade off between sufficient crust cover to be buried and sufficient unconsolidated
substrate to supply the material for burial.
Results
Bush canopies and biological crust cover
Table 1 summarises the results from all sites and sub-habitats. In order to test the
hypothesis that A. mellifera sub-canopies exhibit enhanced crust cover, analyses were
required between sites and between sub-habitats (Figure 2). One-way ANOVA
showed that there is a significant difference in interspace crust cover between sites
characterised by different levels of disturbance (F3, 140 = 42.683, p < 0.01). A
Bonferroni adjustment demonstrated that at the bush encroached and least disturbed
site 2, crust cover is significantly greater than at the mixed grass and bush site 3 (p <
0.01). Crust cover at site 3 is also significantly greater than at both the sacrifice zone
(site 1) and the un-encroached site 4 (p < 0.01). Similarly, crust cover beneath the
canopy of G. flava differed significantly between sites (F3, 252 = 27.837, p < 0.01).
Beneath A. mellifera, however, there was no statistically significant difference in crust
cover between sites (F3, 504 = 1.862, p = 0.135). A. mellifera equalizes the effects of
local disturbance by protecting the sub-canopy soil from disturbance, whereas crust
cover under G. flava varies significantly across the disturbance gradient as the bush
offers little protection from grazing.
This pattern is also apparent when analysing crust cover under bush canopies and in
neighbouring interspaces at each site. At the most disturbed sacrifice zone (site 1),
interspace and G. flava sub-canopy crust cover were not statistically significantly
different, although crust under A. mellifera bushes is significantly higher than under
G. flava or in the interspaces (p < 0.01; Figure 2). At the least disturbed bush
encroached site, there were no differences in sub-habitat crust cover (F2, 225 = 0.449, p
= 0.639). At the mixed bush and grass site, G. flava sub-canopies had significantly
higher crust cover (F2, 204 = 3.939, p < 0.05). Finally, at the un-encroached site A.
mellifera sub-canopies had significantly greater crust cover than under G. flava (p <
0.05) and the interspaces (p < 0.01).
Litter and biological crust cover
By comparing bush-averaged values for crust cover and litter cover, a statistically
significant, negative relationship is present for the sub-canopy environment of A.
mellifera (F1, 63 = 16.21, p < 0.01, R2 = 20.46%; Figure 3a). Specifically, those bushes
with higher sub-canopy litter covers have significantly lower biological crust cover.
Furthermore, the variability in litter density beneath A. mellifera is related to bush
size. As A. mellifera grow larger, the proportion of ground covered by litter increases
(F1, 63 = 7.42, p < 0.01, R2 = 10.53%; Figure 3b). In contrast, no significant statistical
relationship between litter and biotic crust, or between litter and bush size, was
detected beneath G. flava canopies. If litter has a detrimental effect on crust
development, and the amount of litter is a function of bush size, it follows that larger
bushes should have less crust cover. This is demonstrated for A. mellifera (Figure 3c)
where sub-canopy crust cover is a function of bush size, with sub-canopy crust area
decreasing with increasing bush size (F1, 63 = 61.46, p < 0.001, R2 = 49.38%). No
such relationship exists for G. flava.
Additional support for the deterministic role of litter on crust development beneath the
canopy of A. mellifera is revealed when comparing the north and south axes of the
bush. North facing sides of A. mellifera have significantly less litter than the south
facing sides (paired t test; t = 6.996, df = 64, p < 0.01), but significantly more
biological crust cover (t = 3.546, df = 64, p < 0.01). Whilst G. flava also exhibited a
statistically greater litter load beneath its southern facing portion (t = 3.278, df = 62, p
< 0.01), crust characteristics in the two directions were statistically indistinguishable
(t = 0.210, df = 62, p = 0.417). Figure 3d shows the nature of the relationship between
crust and litter. Litter cover increases from the canopy edge towards the base,
eventually gaining a density great enough to produce a decline in crust cover.
Maximum biological crust development occurs between the disturbance-affected
canopy edge and the litter-dense bush interior (Figure 3d).
Biological crust burial
Buried crust was universally present across all sites and within all sub-habitats (Table
1). Figure 4 compares the prevalence of interspace crust burial with that predicted by
the mathematical model introduced earlier. As predicted by the model, low biotic
crust cover (sites 1 & 4, 6 – 12 %) appears to produce a low incidence of crust burial
(1 - 5 %), whilst those sites with the highest values for crust burial (sites 2 & 3, 10 –
27 %) host intermediate biotic crust cover (33 – 47 %).
Discussion
The limited range of livestock and smallstock, whose physiology constrains them to
graze within several kilometres of drinking water, has the effect of concentrating them
into stocking densities greater than those associated with nomadic pastoralism or
wildlife (Leggett et al., 2003). Because of the intense, localized grazing pressure, a
zone of decreasing intensity of disturbance (or piosphere) radiates from waterpoints
(Moleele & Perkins, 1998). This adds a new environmental gradient to the ecology of
a region subject to otherwise relatively homogenous environmental conditions. This
has led to the encroachment of bush species, notably A. mellifera and G. flava
(Moleele & Perkins, 1998; Reed & Dougill, 2002). The mechanism appears speciesspecific, owing much to the selectivity of browsing livestock, but also to the
relationship between bush canopies and the underlying soil properties. It has been
suggested that once established the bush encroachers may monopolize soil moisture
and nutrients (Moleele et al., 2002), preventing the original vegetation from reestablishing, as nutrient and water retention is increased in sub canopy habitats
resulting from increased crust cover (Dougill & Thomas, 2004).
Results presented in this paper describe the spatial distribution of enhanced
cyanobacterial crust cover found beneath the canopies of shrubs. Furthermore, this
study has shown that, whilst crust cover in the shrub interspaces and beneath the
canopy of G. flava varies significantly across a disturbance gradient, biological crust
cover beneath A. mellifera remains at the same elevated level. This displays the
species-specific association between canopy and crust development that is facilitated
best by the dense, thorny nature of the A. mellifera canopy. In contrast, at the least
disturbed site the sub-canopies of A. mellifera and G. flava and the interspace shared
similar levels of crust cover. This is an important result as it shows that when
disturbance is limited, each environment provides an equally suitable habitat for crust
development and that without disturbance, localised differences in crust cover
disappear.
Aranibar et al. (2004) found that, although no Acacia species showed evidence of
direct nitrogen fixation they nevertheless maintain a high N content, suggesting
another mechanism of N acquisition. If it can be demonstrated that A. mellifera are the
recipients of crust associated nutrients (as demonstrated elsewhere for other species,
e.g. Evans & Belnap, 1999; Harper & Belnap, 2001) then an important symbiosis may
be revealed. Such a relationship would suggest that the alternative stability domain
established with bush encroachment exhibiting intrinsic resilience due to the
association between bush canopies and sub-canopy biological soil crust development.
Sub-canopy litter cover per unit area increases with A. mellifera size (Figure 3b) and
has a detrimental effect upon biological crust development (Figure 3a) with biological
crust area reducing with bush dimensions (Figure 3c). Figure 3d shows that the
distribution of crust and litter beneath the canopy of A. mellifera is not uniform or
random, but sorted into an interior dominated by litter and an outer concentric zone of
increased biological crust development. It follows that the increase in litter cover
with bush size, and corresponding decrease in area-relative crust cover, is mediated
through a migrating outward of the litter-dominated bush interior as total bush volume
becomes gradually larger. Figure 5 demonstrates this schematically but is based on the
logarithmic model used in Figure 3c and the data in Figures 3a and b. At relatively
small bush sizes most of the sub-canopy floor is crust dominated, with only a small
area dominated by litter. As the area underneath the bush increases the zone of litter
dominance increases in proportion with bush volume and thus spreads outwards,
pushing the zone conducive to crust growth further out. At this stage, the absolute
area covered by crust may still be increasing with bush canopy growth. However,
eventually the litter load increases more rapidly than canopy edge is advancing,
resulting in the zone of litter dominance expanding at the expense of the crust
dominant zone. According to the model presented here, the biological crust is
progressively pushed towards the bush exterior until at a radius of 7.4 m the bush
produces enough litter to cover the entire sub-canopy zone with sufficient a density to
prevent photosynthesis and biological crust development. Field observations suggest
that A. mellifera rarely reach such sizes and thus the symbiotic relationship between
A. mellifera and biological crust communities may be sustained throughout the life
cycle of the bush.
Sediment burial will have a similar smothering effect on crusts and prevent
photosynthesis as light is excluded from the cyanobacteria. The process has received
only passing references in the literature. For example, Harper & Belnap (2001)
sampled from a station “where wind-borne sediment deposition precluded the growth
of crustal organisms” (p. ??), and Belnap (2002) ascribes long term declines in
nitrogenase activity in disturbed crusts to the death of buried material. This study has
presented the first survey of the occurrence of biological crust burial. Buried crusts
were found at all sites, and within all sub-habitats, but to differing degrees. The
mathematical model presented to describe crust burial in terms of biological crust
cover appears to provide an approximation of the burial observed (Figure 4).
The data show crust burial to be widespread across the study area. It is unclear what
happens to biological crusts after burial. According to Belnap & Gillette (1998), 75%
of the photosynthetic biomass of biological crusts is from organisms in the top 3 mm,
and sediment burial results in the death of these organisms. Should crust burial be
shown to cause microbiological communities to die, or make them metabolically
dormant, then the model proposed may have important consequences. If crust burial is
at a maximum where crust cover and unconsolidated soil are approximately equal (i.e.
around 50% each) then it may provide a negative feedback on spatial crust growth; as
the crust cover at a given site increases from low values crust burial will also become
more evident, dampening any further growth. This may explain the apparent 40 %
limit on crust cover at disturbed sites in the Kalahari seen in several studies including
this one (Thomas et al., 2002; Aranibar et al., 2003; Dougill & Thomas, 2004).
Conclusion
This study has presented data that support the view that A. mellifera mitigates the
effects of cattle-related disturbance beneath its canopy. The protection offered by this
bush to the sub-canopy soil permits the enhanced development of a biological crust
community, when compared to the more disturbance-intense bush interspaces and the
sub-canopy of G. flava. It has been reported in the literature that biological soil crusts
provide additional nutrients to those plants growing in crusted soils. Consequently, it
seems reasonable to suggest that the ability of A. mellifera to withstand drought and
grazing and its association with a significant sub-canopy biological crust cover, even
in disturbed areas, leads to the stability of the bush encroached ecosystem state that is
now prevalent across much of the Kalahari. Litter accumulation under A. mellifera
imparts a net negative effect on biological crust development, the important variable
being litter density as opposed to any in situ litter characteristics. The incidence of
buried crusts highlights the ubiquity of crust burial, and suggests this as an important
(but under-researched) process regulating the spatial occurrence of biological soil
crusts. A model is proposed relating the extent of crust burial to biological crust
cover, portraying crust burial as a non-linear system with potential feedbacks of direct
consequence for the spatial development of biological crust communities.
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Figure 1: Study site location
Figure 2: Between site and sub-habitat differences in crust cover with respect to
disturbance
Figure 4: Relationship between biotic crust cover and crust burial. The model
predicts a low occurrence of buried crusts at the two extremes of crust cover, i.e. very
low and very high values for crust cover. Note that the absolute values predicted here
are not important as sediment redistribution might realistically be site and seasonal
specific. What is being proposed here is that crust burial will be relative to crust cover
in the mathematical form described in the text, and hence, the graphical form shown
here, across the potential values for crust cover (i.e. 0-100%). The steepness and
vertical position of the curve may alter according to site characteristics but the basic
shape, and consequently values relative to each other, will remain similar. The data
approximates this shape, supporting the model, however too few data points are
present and the spread of values leaves much uncertainty.
Figure 5: Proposed model of crust/litter dynamics beneath the canopy of Acacia
mellifera (see text)