Bush canopies, grass species composition and biological soil crust
relations in the Kalahari, Botswana
Draft outline with aim for submission to African Journal of Ecology (September
2004).
Keywords – Bush encroachment, Kalahari, biological soil crusts, Acacia mellifera,
Grewia flava, rangeland degradation
Running title – Bush, Grass and Crust Relations in Kalahari
Abstract (200 words)
Introduction
Recent ecological research in grazed savanna rangelands has suggested that ecological
changes, notably bush encroachment, are regulated by relations between the spatial
heterogeneity of soil and vegetation communities (Scholes and Archer, 1997). Spatial
relations between bush canopies and soil chemical properties have been studied for a
number of rangelands globally. For example, extensive studies have been conducted
in the South West USA (Schlesinger et al., 1990 and others), Australia (Bennett and
Adams, 1999) and East Africa (Belsky et al., 1989, 1993; Belsky 1994 and more
recent?). However, there is a lack of analytical studies investigating the links between
bush canopy cover, grass species composition and soil chemical properties for the
extensive Kalahari rangelands of Southern Africa. The few studies that are available
tend to be theoretically based around spatial bush patterns (e.g. Skarpe, 1991; Jeltsch
et al., 1996, 1998, 1999; Weber et al., 1998) without assessing the relations to soil
properties.
The Kalahari is an extensive basin of wind-blown sand deposits that covers over 2
million km2, including 80 % of Botswana (Thomas and Shaw, 1991), and is typified
by savanna ecosystems that traditionally were maintained as grass-dominant due to
regular fires that caused bush die-back and provided grasses with the competitive
advantage.
However, with the introduction of intensive cattle grazing, enabled
through groundwater extraction and supported by a series of Government of
Botswana Agricultural Policies, there have been widespread increases in woody plant
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species cover (bush encroachment) documented across the region (e.g. Skarpe, 1990;
Perkins and Thomas, 1993; Moleele and Perkins, 1998; Moleele et al., 2002; Ringrose
et al., 2002). The main encroaching species have been identified by Moleele et al.
(2002) as Acacia tortilis, Acacia mellifera, Dichrostachys cinera, Grewia flava and
Ziziphus mucronata. In Kgalagadi District that covers the arid and semi-arid south
west of Botswana, encroachment is largely associated with A. mellifera and G. flava
(Reed and Dougill, 2002) a finding similar to the Kalahari rangelands of Namibia
(Bester, 1996).
Initial integrated soils and ecological research elsewhere in the
Kalahari has investigated the links between bush encroachment and soil properties by
testing theories of increased leaching of water and nutrients to depth, but found no
relation between bush cover and changes in profile distributions of soil water and
nutrient availability (Dougill et al., 1998, 1999). These studies, however, did not
address spatial variations in soil properties in relation to the encroaching bush species,
as suggested as important in research from other semi-arid rangelands.
This paper aims to provide a preliminary analysis of the relations between bush
canopy cover of the two main encroaching bush species in Kgalagadi District, grass
species composition and sub-canopy biological soil crust development. The focus on
biological soil crusts results from the recognition of the vital role crusts play in
affecting nutrient retention and availability in dryland soils globally (Belnap et al.,
2003) and preliminary analyses that have displayed their widespread occurrence
across Kalahari rangelands (Dougill and Thomas, 2004). Specifically, the objectives
of the paper are to: (1) assess the spatial relations of biological crust cover and
morphological crust type in association to bush canopy dimensions of A. mellifera and
G. flava; (2) determine the impact that the intensity of livestock disturbance has on
the spatial relations of crust cover around A mellifera and G. flava; (3) investigate the
relations between spatial patterns of crust cover and soil nutrient availability in subcanopy sites; (4) assess the grass species composition of sub-canopy habitats; and to
(5) provide a conceptual model of the spatial relations between bush canopy and crust
characteristics for different species and disturbance settings, that can better inform our
understanding of the processes determining bush encroachment in Kalahari
rangelands.
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Materials and Methods
Justify study site selection – all on Kalahari sands, range of different disturbance
settings (4 sites in communal rangelands near Tshabong, 1 in communal rangelands
near Tshane (SAFARI study site) and 1 in Mabuasehube National Park.
Background species make up of all sites from 30 m x 30 m analyses (assuming we can
use Candy’s for broad overview at BB sites). Two transects away from bush stem of
selected A. mellifera and G. flava within sites – chosen N and S to assess potential
impact of greater shading to S and spatial relations of litter and sediment deposition in
lee of prevailing NE dry season winds. Series of 0.5 x 0.5 m quadrats taken out from
bush stem assessing the following variables – crust cover; % crust type cover (by
morphological classification of Dougill and Thomas, 2004), unconsolidated soil
cover; litter cover; buried crust cover; grass species composition at selected sites;
canopy dimensions. In addition, soil samples collected of each crust type for soil
nutrient analysis.
Results
Journal restricts to 6 displayed items (ie. Tables and Figures). My first thoughts are
that we need the following to answer objectives set (and the first as general results
overview) –
1. Site characteristics – amended and updated version of Andy’s Table 5.1 – add
Tshane and Mabua data sets, check grass %’s for bush sub-canopies – one
anomalous result appears to be the 6 % crust cover in interspaces at grassdominant site. This is significantly lower than anything we’ve recorded before
even in disturbed Molopo and Namibian sites – does Candy’s data corroborate
this?
2. Spatial relations of crusts under A. mellifera and G. flava – Distance relations
from stem for both species (total crust or by type??). Will have to do these
from raw data sheets.
3. Disturbance and crusts in sub-canopy and interspaces – amend and add new
sites into Andy’s Figure 5.1. As in 1 need to check outlier of 6 % crust.
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4. Table of soil chemical results for different crust types (even if not directly
from different sub-canopies as we can link from proportions of crust types to
what this would mean in terms of total sub-canopy enrichment – ie. Could
possibly develop from Table 6 of T+D, submitted if nowt else?).
5. Table of grass species compositions of sub-canopy habitats at different sites
(we have Tshane and Mabua, will need to check what’s available for
Tshabong – again if necessary do have Molopo data we could fall back on if
struggling!).
6. Conceptual model – amend from Andy’s Figure 5.8 (perhaps even consider
adding buried component) and discuss how a similar one for G. flava would be
significantly different. This is crux to enable to suggest that it is preferential
crust development under A. mellifera that is a potential mechanism favouring
expansion of A. mellifera cover and bush encroachment problems.
Discussion
Implications
for
positive
feedback
mechanism
exacerbating
A.
mellifera
encroachment.
Link to whether bush encroachment equates to degradation depends on maintenance
of nutritious grass species in sub-canopy habitats that appears to be determined by the
overall levels of grazing, such that a threshold can be crossed to a degraded system
with largely only A mellifera and annual Schmidtia kalaharensis grass cover.
Need for further crust microbiological and nutrient cycling process study research.
References
Tables
Figure Legends
Figures
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