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Question: Biological soil crust (BSC) communities can be used in the identification and monitoring of degradation. A key question is how landscape-scale livestock disturbance and other local-scale factors influence BSC communities. We hypothesize that at the landscape scale, increased grazing pressure would lead to decreasing cover of BSC, but at the local scale biotic interactions between BSC and vascular plants would modulate the influence of grazing on BSC. Methods: Spatially explicit sampling of vegetation composition and cover was conducted using point-sampling methods in digital images along two disturbance gradients in the central Monte Desert in Argentina. Results: The grazing gradient is the major determinant of changes in the structure of plant communities at the landscape scale. Approximately 1500 to 2000 m from a watering point, there is a threshold in vegetation structure associated with a nonlinear trend of the BSC, herb, grass and shrub cover. Bivariate spatial patterns show attraction between BSC and shrubs in the vicinity of settlements, and repulsion between BSC and both grasses and litter in less disturbed sites. Conclusion: Grazing affects BSC directly through trampling and indirectly by altering vascular plant communities that interact with the BSC communities. Both these effects vary according to the spatial scale being considered. The results evidence that understanding of livestock impact in structuring arid ecosystems requires an integrated analysis of BSC and vascular plant communities at different spatial scales.
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Journal of Vegetation Science
Grazing effects on biological soil crusts and their
interaction with shrubs and grasses in an arid
Solana Tabeni, Irene A. Garibotti, Clara Pissolito & Julieta N. Aranibar
Cryptogams; Degradation; Livestock; Monte
Desert; Spatial distribution; Species response
curves; Thresholds; Watering points
BSC = Biologicalsoil crust
Brummitt & Powell (1992)
Received 31 October 2013
Accepted 14 April 2014
Co-ordinating Editor: Rasmus Ejrnæs
Tabeni, S. (Corresponding author, Instituto
Argentino de Investigaciones de las Zonas
Aridas (IADIZA), Centro Cient
ıfico Tecnol
(CCT) CONICET, MENDOZA, Av. A. RuizLeal s/n,
CP 5500, C.C.507 Mendoza, Argentina
Garibotti, I.A. (ireneg@mendoza-, Pissolito, C.
( &
Aranibar, J.N. (jaranibar@mendoza- Instituto Argentino de
ıa, Glaciolog
ıa y Cienc ias Ambientales
Ruiz Leal s/n, CP 5500, C. C. 503 Mendoza,
Aranibar, J.N. : Instituto de Ciencias B
(ICB), Universidad Nacional de Cuyo, Padre
Contreras 1300, CP 5502 Mendoza, Argentina
Question: Biological soil crust (BSC) communities can be used in the identifica-
tion and monitoring of degradation. A key question is how landscape-scale live-
stock disturbance and other local-scale factors influence BSC communities. We
hypothesize that at the landscape scale, increased grazing pressure would lead to
decreasing cover of BSC, but at the local scale biotic interactions between BSC
and vascular plants would modulate the influence of grazing on BSC.
Methods: Spatially explicit sampling of vegetation composition and cover was
conducted using point-sampling methods in digital images along two distur-
bance gradients in the central Monte Desert in Argentina.
Results: The grazing gradient is the major determinant of changes in the struc-
ture of plant communities at the landscape scale. Approximately 1500 to
2000 m from a watering point, there is a threshold in vegetation structure asso-
ciated with a nonlinear trend of the BSC, herb, grass and shrub cover. Bivariate
spatial patterns show attraction between BSC and shrubs in the vicinity of settle-
ments, and repulsion between BSC and both grasses and litter in less disturbed
Conclusion: Grazing affects BSC directly through trampling and indirectly by
altering vascular plant communities that interact with the BSC communities.
Both these effects vary according to the spatial scale being considered. The
results evidence that understanding of livestock impact in structuring arid eco-
systems requires an integrated analysis of BSC and vascular plant communities
at different spatial scales.
Arid and semi-arid ecosystems are experiencing land deg-
radation at different scales. This has led to increasing
research aiming to detect early signs of degradation and
predict potential undesirable transitions (Kefi et al. 2007).
Non-irrigated arid ecosystems used for intensive or exten-
sive grazing are some of the areas most at risk of desertifi-
cation (Dawelbait & Morari 2012). Grazing disrupts
vegetation organization patterns that unbalance the flow
of materials and organisms across the landscape (Alados
et al. 2011). Much of the research has focused on the study
of vascular plant patches, and less is known about the
impact of grazing on microbiotic organisms growing on the
ground surface (Bowker et al. 2006). It seems that micro-
biotic communities are more vulnerable to degradation
than their associated vascular plant communities (Bowker
et al. 2008).
Biological soil crusts (BSC) are specialized microbiotic
communities composed of mosses, lichens, liverworts,
Journal of Vegetation Science
Doi: 10.1111/jvs.12204©2014 International Association for Vegetation Science
cyanobacteria and algae. They reach up to 70% of the
living ground cover in some arid lands (Belnap et al.
2004), helping to stabilize the soil, regulate infiltration and
run-off patterns, fix atmospheric N and C, retain moisture
and in some cases facilitate vascular plant establishment
(Bowker et al. 2011). Because of their importance in
the functioning of desert ecosystems, it is highly relevant
to understand the impact that grazing has on BSC
communities and how this affects the process of
desertification (Bowker et al. 2006; Eldridge et al. 2010).
Disturbance by livestock reduces lichen and moss cover
and affects the functional role of BSC (Bowker et al.
2006). However, the interaction between vascular plants
and microbiotic communities is complex, modulating the
overall effect of disturbance on their diversity and cover.
At low to intermediate levels of disturbance, bare patches
provide suitable habitat for microbiotic communities that
can compete with vascular plants for resources (Eldridge
et al. 2010). In these cases, BSC and vascular plant cover
are inversely related (Muscha & Hild 2006). In other cases,
vascular plants have positive effects on BSC by ameliorat-
ing extreme environmental microclimate conditions and
modifying locally available resources (Maestre et al. 2002;
Zhang et al. 2013). For example, grasses appear to provide
microsites that facilitate BSC establishment and develop-
ment along water-centred grazing gradients (Williams
et al. 2008; Jimenez Aguilar et al. 2009).
The composition and ecology of BSC has largely been
overlooked in the context of degradation and desertifica-
tion in South America (B
udel 2001; Toledo & Florentino
2009). In the southern Monte of Argentina it has been
observed that grazing reduces BSC richness and cover,
and that BSC are functionally relevant at retaining vascu-
lar plant seeds (Scutari et al. 2004; Bertiller & Ares
2011). In the central Monte of Argentina cattle have a
negative impact on BSC communities that mainly
develop under the protection of vascular plants in dis-
turbed areas. Recovery of BSC communities is successful
on sites where cattle have been removed for over 40 yrs
omez et al. 2012).
The process of desertification is increasing along the
South American Arid Diagonal for anthropogenic and cli-
mate change reasons (Abraham et al. 2009). In this study,
we examine the effect of grazing on the cover and spatial
pattern of BSC at landscape and local spatial scales in the
central Monte Desert in Argentina. Specifically, the follow-
ing hypotheses were evaluated. (i) At the landscape scale,
increased grazing pressure would lead to a decreasing
cover of BSC because livestock disrupt BSC cover
through trampling. (ii) At the local scale, grazing has an
indirect effect on BSC cover and distribution through
affecting vascular plant communities, and consequently
biotic interactions.
Study area
The Argentinean central Monte Desert covers a wide
northwestsoutheast strip that is part of the South Ameri-
can Arid Diagonal. The study sites are located inthe Telteca
Natural Reserve (32°200S, 68°00°W, 20 700 ha), central
part of the Monte Desert. The climate is arid (total annual
precipitation about 160 mm), with cold dry winters (mean
temperature <10 °C) and warm rainy summers (mean
temperature >20 °C). The dominant landscape is a gently
undulating floodplain that presents sandysilt soils mainly
of aeolian origin (Gonz
alez Loyarte et al. 2000). Vegeta-
tion is relatively homogeneous in physiognomy and floris-
tic composition (Abraham et al. 2009). Native woodlands
of Prosopis flexuosa are confined to inter-dune valleys
alez Loyarte et al. 2000).The lower stratum is a
shrub layer composed mainly by Larrea divaricata,Suaeda
divaricata,Tricomaria usillo,Atriplex lampa,Capparis atami-
squea and Lycium tenuispinosum.
In these non-irrigated areas, people live in scattered
livestock settlements supplied by hand-drilled water
wells (Abraham et al. 2009). The water table lies
between 6 and 15 m, and there are no surface water
sources (Torres 2008). The economy is subsistence, with-
out any livestock management strategy, neither in time
(continuous grazing) or space (no fencing) (Torres 2008).
The impact of livestock activity on vegetation is evident,
with changes up to 15% in cover in a 2-km area around
the settlements (Goir
an et al. 2012). Unfortunately,
there is a paucity of grazing intensity data for the area
(Meglioli et al. 2013). Other human impacts are minor
as there is no other economic activity in the area (Torres
Sampling of BSC and vascular plant cover and
Vegetation surveys were performed in early summer
along two transects extending from highly impacted set-
tlements (La Primavera and Las Delicias settlements) to
less disturbed sites. Sampling distances were ca. 10, 500,
900, 1500 and 2500 m from watering points. Landscape
homogeneity and the minimal possible influence of con-
founding factors in the area (i.e. absence of surface water
sources or other disturbance impacts), makes water-
centred grazing gradients a suitable indirect approach to
studying the effect of different grazing intensities on bio-
logical communities (Landsberg et al. 2003; Sasaki et al.
2008). We limited sampling sites to inter-dune spaces
with a slight slope in order to minimize the influence of
topography. Conditions are similar at both studied gradi-
ents since they are relatively close in the area. Our study
Journal of Vegetation Science
2Doi: 10.1111/jvs.12204 ©2014 International Association for Vegetation Science
Spatial distribution of biological soilcrust S. Tabeni et al.
focused on two individual sites that we considered as
representative of the study area since the area is rela-
tively homogeneous in its climatic, physical and biologi-
cal characteristics. This sampling strategy was chosen in
order to achieve a high spatial resolution of BSC distribu-
tion at the local scale, according to the methods
described below, although it confers a lower resolution
at the landscape scale.
At each sampling site we mapped BSC and vascular
plant cover in grids of 10 95 m divided into 200 square
quadrats of 0.25 cm
. We used high-resolution, image-
based methods (Booth et al. 2008). Vertical images of each
sampling quadrat were obtained using a digital camera
(6.1 MP, focal length =1855 mm, equivalent to 27
82.5 mm format) positioned ca. 1 m above ground level,
with the focal plane oriented parallel to the ground.
Planar scale was provided through a rigid square frame
that delimited the sampling quadrat. Digital images were
processed in order to remove linear distortion along the
measurement plane defined by the rigid frame. The
resulting dimension of each image was 1500 pixels, in
which each pixel corresponds to 0.3 mm in the field. This
method has been shown to be successful in detecting
vegetative cover changes due to grazing (Booth et al.
2008). Photographic methods have also been used for
monitoring lichens, mosses and the BSC, indicating a
close fit with other standard field monitoring methods
(e.g. Vanha-Majamaa et al. 2000; Benavides & Sastre-De
us 2009; Jespersen 2013).
The spatial distribution of BSC and vascular plant cover
was mapped through point-sampling of digital images
using SamplePoint software, based on a 225-point system-
atic grid within each photograph (Booth et al. 2006). Sam-
ple points were classified into 17 different categories,
including species of shrub (Prosopis flexuosa,Geoffroea decort-
icans,Bulnesia retama,Lycium chilense,L. tenuispinosum,Con-
dalia microphylla,Atriplex lampa,Suaeda divaricata,Ximenia
americana,Capparis atamisquea,Grahamia bracteata and an
unidentified shrub), perennial grass, herb,BSC, litter and
bare soil. This resulted in a database with 405 000 data
points for presence/absence of the different species or veg-
etation group at each point. To estimate ground cover as a
percentage of the total surface area, each sample quadrat
was divided into nine sub-quadrats, and data within each
sub-quadrat (25 data points) were averaged, resulting in a
database with 16,200 data points that were used in the
following statistical analyses.
We used hand lenses to perform floristic surveys of the
crusts present within each sampling grid. Voucher speci-
mens of the different lichen and moss species were col-
lected for later taxonomic identification in the laboratory.
The presence of cyanobacteria was checked using a com-
pound light microscope, but they were not identified to
species level. Nomenclature follows Brummitt & Powell
Statistical analyses
We applied a detrended correspondence analysis (DCA) to
identify general systematic changes in the composition and
cover of the vegetation community. This method is ade-
quate since we expected species composition to be mainly
determined by the grazing gradient, and because the ordi-
nation axes can be interpreted in terms of the average SD
of species turnover among sites and can be used to predict
species distributions (Økland 1986; Jongman et al. 1995;
Ejrnaes 2000; Rydgren et al. 2003). The matrix data con-
sisted of the entire data set of percentage ground vegeta-
tion covers. Five vegetation variables were included in the
analysis, i.e. BSC, herbs, grasses, Lycium spp. and all other
species of shrub combined into a unique variable (‘other
shrubs’). Cover of Lycium spp. was considered an indepen-
dent variable since it is the dominant shrub genus in the
area. Detrending was performed by segment, and down-
weighting of rare species was applied. The length of the
first DCA axis was >3 SD units, indicating that use of a uni-
modal model of species distributions is adequate (Jongman
et al. 1995).
We also analysed the response of single vegetation
groups to the disturbance gradient and to the ecological
gradient defined by DCA axes. Ordination axes are surro-
gates of complex gradients of factors that vary more or less
in relation to each other (Økland 1986). For this purpose,
we applied a smoothing-fitting method based on locally
weighted polynomial regressions (LOESS), using cover of
vegetation groups as dependent variable and distance from
watering point or DCA axis 1 as independent variable.
These analyses allowed us to explore the major trends of
variability in the cover of the different vegetation groups
(whether linear, uni- or multimodal, or no trend). We
selected the best smoothing parameters examining plots of
the fit residuals vs the predictor variable, and we chose the
model yielding no clearly discernible information on the fit
residuals (Jacoby 2000).
Spatial pattern of vegetation groups
To test the influence of disturbance gradient on the spatial
distribution of BSC and the changes in biotic interactions
we conducted univariate and bivariate point pattern analy-
ses (Wiegand & Moloney 2004). Second-order statistics
such as the O-ring function characterize the number of
points encountered in the neighbourhood of a ring of
radius rcentred on an arbitrary point of the pattern, allow-
ing interpretation of spatial structure in terms of interac-
tions (Wiegand & Moloney 2004).
Journal of Vegetation Science
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S. Tabeni et al. Spatial distribution of biologicalsoil crust
First, we performed univariate analyses on each of the
nine grids to examine whether spatial distributions of BSC
along grazing gradients were random, clumped or regular.
We chose a heterogeneous Poisson null model because we
observed a heterogeneous density distribution of BSC
points, corresponding to non-constant, first-order effects.
We used a circular moving window estimator and selected
a radius r=5 and 10 cells, corresponding to 17 and 35 cm,
Bivariate O-ring statistics were performed to test the
hypothesis that the distribution of BSC is independent of
the distribution of vascular plants. We tested the relation-
ship of BSC cover against the cover of grasses, shrubs and
vascular plant litter cover. In addition, we tested the exis-
tence of the interaction between shrubs and grasses. To
detect departure from independence, we applied a toroidal
shift null model that indicates the existence of attraction or
repulsion between the two patterns. The significant depar-
ture from univariate and bivariate null models was tested
by constructing confidence envelopes with 999 Monte
Carlo simulations (Wiegand & Moloney 2004).
Biological soil crust composition
Biological soil crusts are composed of free cyanobacteria,
coccoid green algae, lichens and mosses. All soil crusts
analysed contain cyanobacteria, and the proportion of
mosses in the crusts is minor. Dominant lichen species are
Collema coccophorum,C. tenax,Fuscopannaria sp.,Heppialuto-
sa,Leptogium sp., Placidium squamulosum and Placynthium
nigrum. Mosses corresponded mainly to the species Crossidi-
um sp. and Tortula inermis. A close inspection of the crusts
made in the field with hand lenses did not indicate evident
taxonomic differences along the studied gradients.
Vegetation variations along grazing gradients
The DCA ordination of sampling plots according to their
vegetation composition indicates that the first and sec-
ond axes represent the main compositional gradients
in the data set, accounting for 27.2% and 19.7% of
species variability, respectively (Fig. 1). Higher order
axes explain less than 3% of species variability each, so
they are considered unimportant. The first ordination
axis is closely associated with the disturbance gradient,
consecutively separating the centroids of sampling sites
from sites close to watering points to less disturbed sites.
The second ordination axis is mostly related to variability
within the studied sites. High local-scale variability was
expected, given the very small quadrat size used for
vegetation sampling and the typical patch spatial pattern
of vegetation in arid environments.
The nonparametric smoothing of the geographic dis-
tance from watering points against the ecological distance
estimated by DCA1 shows that the relationship approaches
a piece-wise model comprising two lines (Fig. 2), indicat-
ing a change in the rate of species turnover between 1500
to 2000 m from watering points. The relationship depicts
that vegetation changes are faster at sites closer to watering
points than at more distant and less disturbed sites.
Figure 3 shows the response pattern of vegetation
groups along grazing and ecological gradients. BSC, herbs
and ‘other shrubs’ (those not pertaining to the genus
Lycium) show a similar pattern along the geographic
gradient, with a peak in cover at ca. 500 m from watering
points and lower cover further away. The cover of Lycium
shrubs shows an indefinite response along the disturbance
gradient. Grasses show relatively low cover near the
watering points (approximately the closest 500 m), and a
rapid cover increase further along the gradient, resembling
a truncated unimodal curve (Fig. 3a).
Fig. 1. Ordination diagram of the DCA of vegetation composition and
cover for sampling plots. Vegetation trends are displayed as lines
connecting the centroids of the successive sites, with increasing size of
symbols corresponding to increasing distance from watering points. White
and grey symbols correspond to the two grazing gradients analysed from
the human settlements (La Primavera, LP and Las Delicias, LD) in the
Telteca Reserve (Mendoza, Argentina).
Fig. 2. Nonparametric smoothing showing the relationship between
distance from watering points and sample plot scores of DCA1. The
geographic gradient represents the changes in disturbance intensity as
distance from settlements increases, and the scores of DCA axis represent
an ecological gradient of floristic composition changes.
Journal of Vegetation Science
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Spatial distribution of biological soilcrust S. Tabeni et al.
The patterns of response of vegetation groups to the eco-
logical gradient depict their relative occurrence along the
community gradient (Fig. 3b). Lycium and all other shrubs
have the same location on the gradient, with a response
pattern approaching a truncated distribution model, with
their optima at the initial part of the community composi-
tion gradient. The behaviour of BSC suggests a unimodal
shape of response change along the community composi-
tion gradient, with an optimum associated with a relatively
high cover of Lycium spp. and an intermediate cover of
grasses. Grasses exhibit a bimodal structure in the response
curve, with one maximum at sites with high shrub cover
and the other at sites with relatively low shrub cover.
Herbs show increased cover at the end of the community
gradient, coinciding with a high cover of grasses.
Spatial patterns
Univariate tests indicate an aggregated spatial arrangement
of BSC at all six sampling sites where it was found with
relatively high cover (Fig. 4). The BSC occurred in clumps
ranging from 7 cm (two cells) to 21 cm (six cells) along
the gradient. The differences in aggregation scales seem
not to be related to the grazing intensity gradient.
The bivariate O-ring statistic suggests interaction
between shrubs and grasses at most of the studied sites
(Table 1). The sites located at distances <1500 m from the
watering point show, on average, more points of grasses
closer to shrubs than would be expected under indepen-
dence, thus indicating attraction. However, the distribu-
tion patterns of shrubs and grasses show repulsion with
increasing distance from the watering point.
The relationships between distribution patterns of BSC
and vascular components show a variety of responses
(Table 1). At ca. 500 m from settlements, BSC shows a sig-
nificant attraction to shrub cover, with an aggregation
scale 70 cm. This is the site with highest cover of BSC
(Fig. 3a). At distances >1500 m from the watering point,
the distribution of BSC was independent from shrubs. In
contrast, the interaction between BSC and grasses and
between BSC and vascular plant litter was determined
through a random pattern near the settlements and a
repulsion response on most of the sites further away from
the watering point (distance >900 m).
In arid areas, livestock rely on water from waterholes, gen-
erating gradients of decreasing grazing activity with
increasing distances from animal concentration areas, such
as corrals and watering points. Earlier studies describe
decreasing linear responses of vascular plant diversity and
cover along the grazing gradients, but more recent studies
show thresholds in the patterns of vegetation changes and
non-linear responses of species (Landsberg et al. 2003;
Briske et al. 2005; Sasaki et al. 2008). However, few stud-
ies have included the responses of BSC communities, in
spite of their importance as an integral component of
desert environments (Belnap & Weber 2013).
As expected, our results show that the gradient from
watering points is the major determinant of community
structure at the landscape scale, determining changes
in the relative cover of both BSC and vascular plants
(Figs 1, 3a). Our survey design minimized the influence of
confounding factors other than grazing impact along the
water-centred gradients, thus the grazing gradient appears
as the prevailing disturbance regime. In addition, our
results show that the main ecological gradient is not line-
arly related to the geographic gradient, showing a break-
point between 1500 and 2000 m from settlements (Fig. 2).
According to Briske et al. (2005), an abrupt change in
floristic composition along disturbance or natural
gradients suggests the existence of an ecological threshold.
Fig. 3. Nonparametric smoothing showing the response pattern of cover
of the different vegetation groups to: (a) disturbance gradient (distance
from watering point) and (b) ecological gradient (sample plot scores of
Journal of Vegetation Science
Doi: 10.1111/jvs.12204©2014 International Association for Vegetation Science
S. Tabeni et al. Spatial distribution of biologicalsoil crust
Significant changes in vegetation cover within 2000 m of
the watering points have been found from satellite imag-
ery within the same area of our study (Goir
an et al. 2012).
Our study reinforces these results, indicating that changes
in vegetation cover are associated with major community
structural changes. Probably the threshold indicates differ-
ences between the vegetation communities developing
within and outside the area of major influence of the graz-
ers. A concentrated effect of grazing on the vegetation in
small areas in the close proximity to watering pointsis typi-
cal of rangelands not subjected to animal management
strategies (Andrew & Lange 1986; Meglioli et al. 2013).
The structural threshold we found between 1500 and
2000 m from watering points is mainly associated with an
increasing dominance of grasses in the community
(Fig. 3a,b). It is well known that selective grazing of live-
stock on grasses determines positive response curves for
reduced grazing intensity (Williams et al. 2008; Wesuls
et al. 2013). However, we found a bimodal pattern of grass
response to the ecological gradient (Fig. 3b), suggesting
that other factors might also be important for explaining
the observed vegetation changes. Bimodal curves along
500 m
900 m
1500 m
Minimum distance from settlement
2500 m
O11(r) O11(r)
Spatial scale r [cells]
‘La Primavera’ settlement ‘Las Delicias’ settlement
Univariate O-ring statistic (W-M)
Univariate O-ring statistic (W-M) Univariate O-ring statistic (W-M)
Univariate O-ring statistic (W-M)
Univariate O-ring statistic (W-M)
Insufficient data
Univariate O-ring statistic (W-M)
10 11 12 13 14 15 16 17 18 19 20
Spatial scale r [cells]
10 11 12 13 14 15 16 17 18 19 20 0 1 2 3 4 5 6 7 8 9
Spatial scale r [cells]
10 11 12 13 14 15 16 17 18 19 20
Spatial scale r [cells]
10 11 12 13 14 15 16 17 18 19 20 01234567 89
Spatial scale r [cells]
10 11 12 13 14 15 16 17 18 19 20
0 123456789
Spatial scale r [cells]
10 11 12 13 14 15 16 17 18 19 20
Fig. 4. Univariate point pattern analysis using the O-ring statistic (bold lines) in the sampling grids. Dashed lines indicate 95% confidence envelopes for the
null model. Insufficient data indicate that the analysis could not be performed due to low cover of BSC in the study site.
Journal of Vegetation Science
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Spatial distribution of biological soilcrust S. Tabeni et al.
ecological gradients are usually related to mechanisms of
species competitive interaction (Austin & Smith 1989).
Our results of the bivariate analysis of the distribution of
grass and shrub covers within grids (local scale) concur
with this interpretation, showing a pattern of attraction on
heavily grazed sites and of repulsion on moderate to less
grazed sites (Table 1). Attraction is interpreted as a positive
relationship between shrubs and grasses, probably related
to the protection of grasses from being grazed beneath
shrub canopies, and repulsion might be associated with the
colonization of shrub interspaces by grasses as disturbance
through grazing is relaxed.
The BSC are sensitive to trampling by livestock, thus
reducing their cover as intensity of disturbance increases
(Berkeley et al. 2005; Thomas & Dougill 2006, 2007). Wil-
liams et al. (2008) found exponential increases in the
cover of cyanobacteria-dominated BSC as density of cattle
decreased along a 500-m long, water-centred grazing gra-
dient. To the best of our knowledge, our results are the first
to provide insight into the responses of BSC cover along
larger grazing gradients (2500 m), showing a non-linear
response pattern with a peak at an intermediate level of
grazing (Fig. 3a). This pattern has been commonly
reported for several groups of vascular plants, and is inter-
preted to result from the ability of the species to tolerate
intermediate grazing pressures, while plant decline at less
disturbed sites is due to competitive interactions (Sasaki
et al. 2008; Wesuls et al. 2013).
The distribution of increased BSC cover found at inter-
mediate levels of disturbance (500 m from watering
points) is closely related to the distribution of shrub cover,
while at less disturbed sites further away from watering
points, the distributions of BSC and shrubs are indepen-
dent (Table 1). This suggests facilitation of BSC develop-
ment beneath shrub canopies at relatively disturbed
sites, while this interaction is not apparent at less disturbed
sites. These results agree with those found in Kalahari
rangelands in showing that BSC development is restricted
to sites beneath shrub canopies at high levels of distur-
bance, while interspaces and shrub subcanopies provide
equally suitable habitat for BSC development under lim-
ited disturbance (Berkeley et al. 2005). They are also in
agreement with the results of a previous study in the
Argentinean central Monte Desert, which found that BSC
communities are restricted to Larrea subcanopies at dis-
turbed sites, but successfully recover in open spaces at sites
where cattle have been removed for more than 40 yrs
omez et al. 2012).
The distribution of BSC cover shows patterns of repul-
sion towards grass and litter cover at sites with relatively
low disturbance further than 900 m from watering points
(Table 1). This result is in contrast to those studies that
found a significant association between grasses and BSC
due to modification of soil properties and buffering of abi-
otic stressors beneath tussocks (Bowker 2007; Read et al.
2008), but agrees with others that report the spatial segre-
gation of BSC and grasses due to competition for natural
resources or living space (Bowker et al. 2010; Peterson
2013). According to Bowker et al. (2010), occupation of
more living space is the most important competitive factor
structuring BSC communities, given that more space
secures access to more resources. In addition, the observed
negative spatial relationship between BSC and plant leaf
litter can be related to BSC burial reducing the availability
of light for photosynthesis (Berkeley et al. 2005; Thomas &
Dougill 2007). Therefore, the diminished availability of
bare soil due to both the increased cover of grasses and lit-
ter fall has a detrimental effect on the development of BSC
at relatively undisturbed sites in our study area.
Our study analysed the changes in vascular plant and BSC
in an arid ecosystem of central Argentina as a function of
Table 1. Results of the bivariate point pattern analysis along a grazing disturbance gradient showing the scales of attraction and repulsion between the
cover of different vegetation groups and the biological soil crust (BSC). Not available (na), indicates that the analysis could not be performed due tolow
cover or absence of BSC in the sites. Names of the sites as in Fig. 1.
Distance from watering point (m)
10 500 900 1500 2500
Shrubs 9Grasses Attraction
(70 cm)
(70 cm)
(1431 cm)
Random Attraction
(63 cm)
(17 cm)
Random Repulsion
(5670 cm)
(63 cm)
Shrubs 9BSC na na Attraction
(1070 cm)
Random Random Random na na Random
Grasses 9BSC na na Random Repulsion
(6370 cm)
Random Repulsion
(1756 cm)
na na Random
Litter 9BSC na na Random Repulsion
(10 cm)
Random Repulsion
(4270 cm)
na na Repulsion
(17 cm)
Journal of Vegetation Science
Doi: 10.1111/jvs.12204©2014 International Association for Vegetation Science
S. Tabeni et al. Spatial distribution of biologicalsoil crust
major disturbance by livestock. The results suggest
nonlinear responses of the different vegetation groups
and the BSC to the disturbance intensity gradient.
Development of grasses is possible in relatively highly
impacted sites due to their being protected under shrub
canopies. At these sites, development of BSC is also
facilitated beneath shrubs. However, as livestock distur-
bance relaxes, it seems that increase in grass cover and
plant leaf litter in interspaces between shrubs limits
development of the BSC. Further studies should analyse
the generality of our results for multiple ecological sites,
and how the changes in vegetation patterns relate to
processes that regulate the functioning of this desert
This study was partially funded by ANPCyT (grant PICT
1417). We also are most grateful to Nelly Horak forrevising
the English version of the manuscript. We thank Benjamin
Bender, Agustina Barros and Mauro Britos Navarro for
assistance with analysis of the images.
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S. Tabeni et al. Spatial distribution of biologicalsoil crust
Graphical Abstract
The contents of this page will be used as part of the graphical abstract of html only. It will not be published as part
of main.
Evidence suggests that the main biotic factors structuring biological soil crust communities in areas disturbed by grazing
are the presence of shrubs and grasses, with opposite patterns across the gradient. There is a relationship of attraction
between BSC and shrubs in the vicinity of the settlements, and of repulsion between BSC and both grasses and litter in less
disturbed sites.
... In the Monte desert, a large ecoregion of South America, BSC are widely distributed in areas with livestock exclusion and contribute to different soil properties, such as organic matter accumulation, roughness, infiltration, and stability (Garcia et al., 2015;Garibotti et al., 2018;Gómez et al., 2012;Navas Romero et al., 2019;Tabeni et al., 2014). BSC generally increase soil organic matter and nutrients, but they may also decrease available nitrogen and phosphorus during periods of higher demand. ...
... Furthermore, development of cyanobacteria or moss BSC may determine shrub-grass dynamics in restoration projects, by regulating water availability to vascular plants, and suppress the expansion of invasive exotic plants (Chen et al., 2019;Song et al., 2017). Vascular plants can facilitate or inhibit BSC development (Garcia et al., 2015;Tabeni et al., 2014). Shrubs may favor BSC growth by providing shaded and more humid micro-habitats, and protecting them from trampling. ...
... Shrubs may favor BSC growth by providing shaded and more humid micro-habitats, and protecting them from trampling. However, grasses, trees and shrubs with high litter production may bury BSC and inhibit their photosynthetic capacity and development (Berkeley et al., 2005;Tabeni et al., 2014). ...
Biological soil crusts (BSC) are widespread in the Monte desert. BSC, by fixing atmospheric N and retaining soil moisture, may enhance grass emergence, growth, and nutritional value. Our aim was to evaluate the effect of different functional types of BSC from the Monte desert on soil moisture and nutrients, N fixation, respiration, grass emergence, growth, and C/N. We sowed Leptochloa crinita seeds in pots with different BSC types (dominated by cyanobacteria, squamulose phycolichens, squamulose cyanolichens, gelatinous cyanolichens, and mosses) under two irrigation treatments: well watered and drought. We determined soil, BSC and grass properties related to nitrogen, phosphorus, and water cycles. In soils under BSC, we determined moisture, nitrate, and phosphate at two depths. In BSC, we determined total N, organic matter, ammonium, respiration, and δ¹⁵N. Finally, in grasses growing under BSC, we determined seedling emergence, grass biomass, C/N, and δ¹⁵N. All BSC types except cyanobacteria increased soil total N, ammonium, and respiration rates compared to bare soils under drought conditions. Cyanobacteria BSC increased soil moisture under drought conditions, while squamulose phycolichens and mosses increased it under well watered conditions. All BSC types increased underlying soil nitrate under well watered conditions, and decreased phosphate in at least one experimental condition. All BSC types improved at least one grass variable: cyanobacteria decreased C/N; squamulose cyanolichens increased emergence and decreased C/N; squamulose phycolichens increased emergence and biomass; gelatinous cyanolichens decreased C/N; and mosses increased emergence, biomass, and decreased C/N. The differential effects of each BSC type on soil and grass variables, under drought and well watered conditions, suggest the importance of BSC functional diversity on ecosystem functions of water regulation and nutrient cycling.
... The microclimate generated under shrub canopies have been frequently identified to benefit plants and thus utilized in reforestation projects (Gómez-Aparicio 2009;Pescador et al. 2014). The facilitative effect of shrubs can also extend beyond plants and can include benefits to animals Filazzola et al. 2017), and biological soil crusts (Tabeni et al. 2014). ...
... Initiatives in California, such as the retirement of agricultural land for endemic species , intend on restoring significant portions of the San Joaquin desert and planting shrubs could be a viable strategy. Identifying the basic ecological interactions associated with E. californica can inform restoration strategies by using a "bottom-up" approach of planting shrubs to facilitate the neighbouring plants (van Zonneveld et al. 2012;Michalet et al. 2015;Filazzola et al. 2018), animals (Schmidt & Whelan 1999;Ruttan et al. 2016;Filazzola et al. 2017), and soil characteristics (Wu & Yang 2013;Tabeni et al. 2014). E. californica is a good candidate species for restoration using this approach because the seeds have high rates of viability and we observed significant emergence rates relative to other desert shrub species. ...
Full-text available
Questions The biodiversity of deserts is becoming increasingly threatened due to global change including the introduction of invasive species. Desert shrubs are a foundational species that can facilitate native plant communities but can also benefit exotic species. The influence of exotic plants on the establishment of benefactors from seeds or seedlings is a critical knowledge gap. We tested if the establishment, survival, or growth of seedlings for a benefactor shrub species in California was reduced by the invasive grasses that they facilitate in the field. Location San Joaquin Desert, California, USA Methods We conducted a field survey to determine if a native shrub, Ephedra californica , facilitated the invasive grass Bromus madritensis . Using seed collected from the field, we conducted a competition experiment on E. californica , using a densities series of B. madritensis and under manipulated conditions of light, water, and simulated herbivory. We measured seedling establishment, survival, and biomass of E. californica and B. madritensis. Results In the field, E. californica facilitated B. madritnesis within the shrub canopy. In the competition experiment, B. madritnesis had consistent negative effects on E. californica emergence and seedling survival at all resource and herbivory levels. The emergence and survival of E. californica was reduced in low light, but none of the manipulated conditions increased the competitive effect of B. madritensis. Conclusions Reciprocal costs of facilitation by shrubs were evident in emergence and seedling survival but not growth once established. Water, herbivory, and shade did not mitigate these costs, but also did not exacerbate competition from exotics. Direct competition with exotic plant species is most significant impact tested here on dryland shrub species and manipulations to resources or herbivory may not effectively promote shrub recruitment. Native shrubs are well‐adapted to variable desert conditions and could be effective foundational species if invasive grasses are reduced.
... Contrarily, in the inter-shrub spaces (flat microtopographic positions), where trampling and defoliation concentrate, the moss biocrust cover was higher in the absence of grazing. In the ungrazed condition, undisturbed grasses may moderate the physical environment (radiation, temperature, humidity) at ground level facilitating growth of moss biocrust (Maestre, 2003;Concostrina-Zubiri et al., 2014;Tabeni et al., 2014), which may explain the positive correlation observed in this study between moss biocrust cover and grass cover in the inter-shrub spaces exposed to the sun. Moreover, the fact that the highest moss biocrust cover under the experimental conditions was observed in the chained ungrazed condition suggests that the moss species in the study area are early successional species, since they developed fast after exclusion of the mechanical disturbance and livestock. ...
... Biocrusts have many roles in ecosystems, such as increasing soil fertility, providing stability to soils and preventing erosion, and interacting with vascular plants (Chamizo et al., 2017;Rodriguez-Caballero et al., 2018;Tabeni et al., 2014). They can tolerate extreme temperatures, long desiccation periods, and high radiation (Belnap, 2003a). ...
In arid areas, biological activity responds to precipitation pulses, with different activation thresholds for different organisms. Biocrusts, communities of cyanobacteria, green algae, lichens and mosses living in the soil surface, contribute to soil fertilization by fixing atmospheric nitrogen (N), carbon (C), and solubilizing phosphorus (P). In this study, we hypothesized that biocrusts respond to low simulated precipitation pulses, lower than 10 mm, increasing nutrient availability. We expected biocrust organisms to increase soil net CO2 exchange, N fixation, soil N and P availability in response to small water pulses. In order to evaluate these predictions, we artificially irrigated soils with different covers, including those dominated by bare soil, mosses, and lichens, simulating precipitation pulses of 1, 3, 5, and 10 mm. We measured variables that change at short time scales (hourly), such as net CO2 exchange, N fixation and soil nitrate and phosphate availability, and those that change at larger time scales (years), such as organic matter and total N. Organic matter, total N, and phosphate concentrations were higher in soils with moss dominated biocrusts than in bare soils, indicating their importance to soil fertility in the long term. Soil net CO2 exchange, phosphate, and nitrate availability responded to the simulated precipitation pulses in all soil covers. Net CO2 exchange increased in all soil covers, but phosphate availability decreased in response to precipitation in moss dominated biocrusts. Nitrate availability was lower in lichen dominated biocrusts, although it increased in all soil covers in response to precipitation. These results show that although all the soils analyzed respond to low simulated precipitation pulses, biocrust organisms may retain soil nutrients and decrease availability to other organisms.
... Goat-induced disturbance, which leads to a lower abundance of biocrust presence, except underneath shrubs, is therefore a plausible mechanism behind the unsuspected positive association observed between biocrusts and smallstatured shrubs, when plant presence usually would reduce biocrust distribution. Although more subject to deposition of litter and competition for light, biocrusts beneath small shrubs likely benefit from reduced trampling, particularly beneath less palatable shrubs, such as latex-bearing Jatropha and Croton species, and reduced evaporation through shading (Bowker et al., 2005;, especially at sites with high disturbance (Tabeni et al., 2014). ...
... Goat-induced disturbance, which leads to a lower abundance of biocrust presence, except underneath shrubs, is therefore a plausible mechanism behind the unsuspected positive association observed between biocrusts and smallstatured shrubs, when plant presence usually would reduce biocrust distribution. Although more subject to deposition of litter and competition for light, biocrusts beneath small shrubs likely benefit from reduced trampling, particularly beneath less palatable shrubs, such as latex-bearing Jatropha and Croton species, and reduced evaporation through shading (Bowker et al., 2005;, especially at sites with high disturbance (Tabeni et al., 2014). ...
Full-text available
Biological soil crusts (BSCs) develop when various combinations of diminutive cyanobacteria, eukaryotic algae, non-lichenized fungi, lichens, and/or bryophytes occupy the upper few millimeters of the soil and raw material. They can be present in a wide range of ecological, including successional, and climatic conditions when and where disturbance and/or aridity have resulted in opportunities for colonization. However, they are most prevalent in arid, semiarid and polar ecosystems where vascular plant cover and diversity are characteristically low, leaving large areas available for colonization by some combination of the organismal groups mentioned above. The ecological roles of BSCs are numerous and diverse, and include the collection, accumulation and cycling of essential airborne and soil nutrients, redistribution of precipitated water, and soil formation and stabilization.
... Our results, could be related to the capacity of Pottiaceae for their adaptation to dry habitats and high tolerance to the heat and radiation as it was previously postulated by Rivera-Aguilar et al. (2006). The species of mosses were coincident with those ones observed and described before in Argentina (Gómez et al., 2012;Tabeni et al., 2014;Garibotti et al., 2018). Bryum argenteum was the most frequent species (Table 2, Fig. 8). ...
Biological soil crusts (biocrusts) play a fundamental role in arid and semiarid areas of South America. Nevertheless, little attention has been paid to the distribution and coverage of them, and studies about biocrusts are still scarce especially in Argentina. The goal of this contribution was to analyze the coverage of biocrusts and their main biological components along an aridity gradient in the central-west Argentina. The coverage percentage of each component was recorded using the point-quadrat method in transects along a gradient consisting of three bioclimatic sites: semiarid, arid, and hyperarid. The diversity of components in biocrusts was recorded in each studied site. The semiarid site had the highest record of species followed by the arid and hyperarid site. The hyperarid site had the highest record of cyanobacteria species and the semiarid site of lichens. The arid site was the system with the highest coverage of biocrusts followed by the hyperarid site. The semiarid site presented the lowest value of coverage. Cyanobacteria were the dominant group in the hyperarid site. Additionally, cyanobacteria together with lichens were dominant in the arid site. The coverage of the studied organisms showed variations in the semiarid site. Results found in this contribution supported the idea that the coverage and organisms in biocrusts showed changes along an aridity gradient. Differences about the dominance of species and the coverage of biocrusts could be reflected in the functioning role of them in the different sites.
... Regardless of the platform used, or the ecosystem of interest, SampleFreq can be used to measure plant frequency. As an example, vegetation point classification from nadir images using the closely-related program SamplePoint has been completed across many ecosystems with varying plant communities, including grasslands, temperate and tropical forests, alpine tundra, salt marshes and deserts (Guo et al., 2016, Parrish et al., 2017, Skipper et al., 2013, Goonan et al., 2009, Bacopoulos et al., 2018,Tabeni et al., 2014 as well as agricultural settings (Nielsen et al., 2015). We therefore think it reasonable to anticipate successful use of SampleFreq in these ecosystems, with the caveats that image resolution must be adequate for identification of the species of interest, and that the vegetation canopy allows determination of where a plant is rooted. ...
As our understanding of ecological systems grows, natural resource management becomes ever more dependent on timely, accurate, and inexpensively-collected monitoring data to support management decisions. Vegetation cover, density, and frequency are abundance metrics used in resource management; however, frequency data can be collected more quickly than density data and with more repeatability and less sensitivity to inter- and intra-seasonal variation in plant morphology. Moreover, frequency is perhaps the best method for monitoring invasive species across extensive areas. A limitation to the use of frequency data is that plot size affects frequency. The optimal plot size is one that yields measurements suitably removed from 0 or 100% to allow detection of both upward and downward frequency trends, yet the optimum plot size cannot be known before sampling. We addressed this conundrum by developing SampleFreq software that facilitates frequency measurements from digital nadir images of any scale with up to 10 nested plot sizes within the confines of the image dimensions. We conducted accuracy and agreement tests of the software using both artificial populations and field plots. Using artificial population plots, accuracy across all users was 93.4% with a repeatability coefficient of 1.4%, indicating high precision. In a field test, SampleFreq and standard field data averaged a 3.4% difference, and were within approximately 10.5% of each other 95% of the time. From the same field test, SampleFreq repeatability coefficient was 6.7%, while the field method was 4.3%, illustrating that both methods have relatively high precision. Because SampleFreq has high potential accuracy, high agreement with field data, and high precision across a range of users, we recommend using SampleFreq with nadir digital images as a suitable alternative method for monitoring plant frequency.
This paper was accepted and the full text will be available after proofreading. Spatial patchiness in resources is a pervasive feature of drylands worldwide. This patchiness manifests itself as two distinct geomorphic zones that are characterised by the loss (runoff) or gain (runon) of resources such as water, seed and organic matter. Most studies have examined how vascular plants vary across these two distinct zones, but there are few studies of the non-vascular components of these patterned landscapes. We examined the distribution of soil crust lichens and bryophytes (moss, liverworts) in relation to three geomorphic positions within a patterned Acacia aneura woodland grazed by sheep and kangaroos. We found that the distribution of biocrust taxa was strongly related to geomorphic position, with lichens dominating the runoff zones and liverworts restricted to the runon areas (groves). The runon zones were characterised by a greater cover of litter, and greater species richness, cover, and functional richness of biocrusts, while biocrusts in the runoff zones had greater functional diversity and dispersion. Importantly, biocrusts growing in the runon areas were taller, had shorter rhizines, and a greater capacity to trap sediment and absorb water. There were no effects of grazing intensity nor herbivore type (sheep cf. kangaroo) on any diversity or functional measures of biocrusts. Overall, our study indicates that the functional attributes of biocrusts align with those reported for vascular plants (e.g., perennial grasses), by affecting hydrological processes and sediment capture, albeit at smaller spatial scales. Biocrusts may be important, therefore, for sequestering small falls of water emanating from resource-shedding zones, thereby reinforcing landscape patchiness, particularly as hotter and drier climates reduce the cover of vascular plants in drylands.
Full-text available
Reevaluating assumptions about the ecology and management of sagebrush and salt desert shrub systems in the Great Basin and Intermountain West is a proper role for science. These are complex rangeland ecosystems, and our management applications need to account for this complexity. Understanding and reckoning this complexity is vital to the future existence of these rangeland systems and their ability to provide critical goods and ecosystem services to society. The most influential ecological claim of the past 40 yr is based on ideas presented by Mack and Thompson (1982), that Great Basin and Intermountain West plant communities evolved with few or perhaps no large hooved-grazing animals. Our thesis asserts that Mack and Thompson's position is based on 1) an oversimplification of complex, heterogeneous, and diverse ecosystems; 2) a poor understanding of science, both in 1982 and now; and 3) the attribution of all recent ecological changes to a single land use. We review the archaeological and historical record of vegetation and large grazing animals in the region and then revisit Mack and Thompson's (1982) interpretations of the rangeland plants and plant communities, forage quality and nutrition, and soil biotic crusts east and west of the Rocky Mountains, adding the information necessary for a more comprehensive interpretation. We finish by proposing an alternative paradigm to guide management and conservation of sagebrush and salt desert systems of the Great Basin and Intermountain West and beyond.
Full-text available
Introduction Sagebrush ecosystems in western North America are being replaced by the invasion of annual grasses, particularly Bromus tectorum. In experimental situations and in localized landscapes, prior studies have documented that biological soil crusts (biocrusts) can reduce annual grass presence and that biocrusts are highly vulnerable to physical disturbance. Practical conservation would benefit from verification of these patterns at scales that matter to local economies. This study tests if these patterns appear at a regional scale. Methods A previously collected data set of vegetation provided sampling of biocrust cover across the Great Basin within the state of Nevada, USA. Data were analyzed with non-parametric methods including odds ratios and generalized additive models (GAM). Results From a data set of 608 vegetation plots within the Great Basin ecoregion, proportion of plots with high annual grass cover differed between sites with high versus low biocrust cover (p = 0.0015). A negative relationship between annual grass cover and biocrust cover was confirmed with GAM (p = 0.009). For a model of biocrust cover, cattle disturbance was found to be an explanatory variable (p < 0.00001). Conclusions The patterns do appear at the regional scale, with high levels of cattle activity corresponding to low cover of biocrusts, and low cover of biocrusts corresponding to high cover of annual grasses.
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The biology and ecology of biological soil crusts, a soil surface community of mosses, lichens, cyanobacteria, green algae, fungi, and bacteria, have only recently been a topic of research. Most efforts began in the western U.S. (Cameron, Harper, Rushforth, and St. Clair), Australia (Rogers), and Israel (Friedmann, Evenari, and Lange) in the late 1960s and 1970s (e.g., Friedmann et al. 1967; Evenari 1985, reviewed in Harper and Marble 1988). However, these groups worked independently of each other and, in fact, were often not aware of each other’ sw ork. In addition, biological soil crust communities were seen as more a novelty than a critical component of dryland ecosystems. Since then, researchers have investigated many different aspects of these communities and have shown that although small to microscopic, biological soil crusts are critical in many ecological processes of deserts. They often cover most of desert soil surfaces and substantially
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This article synthesizes the ecological concepts and perspectives underpinning the development and application of state- and-transition models, thresholds, and rangeland health. Introduction of the multiple stable state concept paved the way for the development of these alternative evaluation procedures by hypothesizing that multiple stable plant communities can potentially occupy individual ecological sites. Vegetation evaluation procedures must be able to assess continuous and reversible as well as discontinuous and nonreversible vegetation dynamics because both patterns occur and neither pattern alone provides a complete assessment of vegetation dynamics on all rangelands. Continuous and reversible vegetation dynamics prevail within stable vegetation states, whereas discontinuous and nonreversible dynamics occur when thresholds are surpassed and one stable state replaces another. State-and-transition models can accommodate both categories of vegetation dynamics because they represent vegetation change along several axes, including fire regimes, weather variability, and management prescriptions, in addition to the succession-grazing axis associated with the traditional range model. Ecological thresholds have become a focal point of state-and-transition models because threshold identification is necessary for recognition of the various stable plant communities than can potentially occupy an ecological site. Thresholds are difficult to define and quantify because they represent a complex series of interacting components, rather than discrete boundaries in time and space. Threshold components can be categorized broadly as structural and functional based on compositional and spatial vegetation attributes, and on modification of ecosystem processes, respectively. State-and-transition models and rangeland health procedures have developed in parallel, rather than as components of an integrated framework, because the two procedures primarily rely on structural and functional thresholds, respectively. It may be prudent for rangeland professionals to consider the introduction of these alternative evaluation procedures as the beginning of a long-term developmental process, rather than as an end point marked by the adoption of an alternative set of standardized evaluation procedures. Resumen
1. The stress-gradient hypothesis (SGH) predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients, with facilitation being more common when abiotic stress is high. The effect of competition intensity on species richness is generally thought to be negative, but tests along true stress gradients are lacking. This body of research has primarily focused on vascular plants and could be well informed by broadening to other communities.
Soil-associated lichen species characteristic of north-eastern Patagonia are classified by cluster analysis into six groups using ecological and morphological characters. The constancy of species, the total number of species, the number of species per crust and the relative frequency of species are analysed at sites with different grazing levels: three non-grazed, three regulary grazed, and three heavily grazed. Using the results, the potential use of lichen groups as bioindicators of rangeland conservation and degradation are explored. Species of three lichen groups (group A: lichens growing on calcareous gravels; group C: terricolous lichens with pale, crustose non-areolate thalli; group F: terricolous lichens with pale, areolate thalli) are identified as sensitive to grazing, and most of the species forming these groups are suggested as potential bioindicators of grazing disturbance. Thus, Rinodinabischoffii,Caloplacaholocarpa, Catillarialenticularis,Acarosporaheppii (group A); Lecanoradispersa and Rinodinamucronatula (group C); and Psoradecipiens (group F) are the species most sensitive to grazing disturbance. Conversely, species of group D (terricolous lichens with dark, foliose thallus: Collemacoccophorum), and group B (lichens growing on siliceous gravels: Aspiciliacontorta) may be indicated as the most resistant to grazing disturbance.
This study investigated the distribution pattern of biological soil crust (BSC) in Artemisia ordosica communities in Mu Us Sandy Land. Three experimental sites were selected according to grazing pressure gradient. In each experimental site, the total vegetation cover, A. ordosica cover, BSC cover, litter-fall cover, BSC degree of fragmentation, BSC thickness and soil properties were investigated in both fixed and semi-fixed sand dunes and simultaneously analyzed in the laboratory. The results showed that at the same grazing pressure, BSC cover and composition were significantly affected by the fixation degree of sand dunes. In addition, BSC cover in the fixed sand dunes was 83.74% on average, whereas it is proportionally dominated by 28% mosses, 21% lichens, and 51% algae. Meanwhile, BSC cover in the semi-fixed sand dunes was 23.54% on average, which is proportionally dominated by 6.3% mosses, 2.5% lichens, and 91.2% algae. Fine sand, organic matter, and total nitrogen (N) contents in the fixed sand dunes were all significantly higher than those in the semi-fixed sand dunes. Litter-fall cover decreased along the grazing pressure gradient, whereas BSC fragmentation degree increased. Fine sand content decreased along with the increase of grazing pressure, whereas medium sand content increased in both fixed and semi-fixed dunes. The organic matter and total N contents in the no grazing site were significantly higher than those in light and normal grazing sites. However, there were no significant differences between the light and normal grazing sites. In addition, there were also no significant differences in BSC thickness between the light and normal grazing sites in the fixed sand dunes. However, a significant decrease was observed in both BSC cover and thickness in the normal grazing site. The BSC in the semi-fixed dunes was more sensitive to disturbance.
The growing population and land-use pressures in arid areas demands a better understanding of the interactions between humans and the environment. Patterns of human occupation are linked to natural resources, such as water and forest resources. In the Monte Desert, shallow groundwater sustains phreatophyte forests and extensive livestock settlements. This study aims to detect environmental factors (i.e., water availability, roads) that affect settlement distribution, estimate the area of influence of individual settlements on surrounding vegetation, and the impact of settlement densities on vegetation cover at a regional scale, using spatial analysis, remote sensing, and vegetation surveys. We found a heterogeneous and aggregated spatial distribution of settlements, with higher densities near rivers and an old-river bed. We detected vegetation changes associated with land-use with the spectral vegetation index SATVI (soil adjusted total vegetation index), which were not detected with NDVI and SAVI (normalized difference vegetation index and soil adjusted vegetation index, respectively). Vegetation was reduced within 2 km of settlements, outside a fenced reserve, and in areas with high settlement densities. Our study suggests that future changes on water availability may affect livestock settlement spatial distribution, densities, and vegetation, pointing to the need of integrated land-use management.
Two Landsat images, acquired in 1987 and 2008, were analyzed to evaluate desertification processes in central North Kurdufan State (Sudan). Spectral Mixture Analysis (SMA) and multitemporal comparison techniques (change vector analysis) were applied to estimate the long-term desertification/re-growing of vegetation cover over time and in space.Site-specific interactions between natural processes and human activity played a pivotal role in desertification. Over the last 21 years, desertification significantly prevailed over vegetation re-growth, particularly in areas around rural villages. Changes in land use and mismanagement of natural resources were the main driving factors affecting degradation. More than 120,000 km2 were estimated as being subjected to a medium-high desertification rate. Conversely, the reforestation measures, adopted by the Government in the last decade and sustained by higher rainfall, resulted in low-medium re-growth conditions over an area of about 20,000 km2.Site-specific strategies which take into account the interactions of the driving factors at local scale are thus necessary to combat desertification, avoiding any implementation of untargeted measures. In order to identify the soundest strategies, high-resolution tools must be applied. In this study the application of spectral mixture analysis to Landsat data appeared to be a consistent, accurate and low-cost technique to identify risk areas.
Disturbance by domestic grazing is recognized as the most widespread stressor of biological soil crust (BSC) communities. To assess the recovery of the BSC after grazing exclusion, we estimated the composition, cover, and spatial distribution of biological soil crusts, and their influence on soil nitrogen in a protected area after 40 years of grazing exclusion (Reserve MaB of Ñacuñán), and in its surrounding grazed matrix in the central Monte Desert. We considered two spatial scales: at the landscape scale we estimated vegetation and BSC cover in paired grazed and ungrazed sites of Larrea shrublands; at the microsite scale we assessed the influence of the dominant vascular plant, Larrea cuneifolia, on crust cover, and the influence of crust cover on soil nitrogen concentration. Grazing has a negative impact on soil crusts, which only develop under the protection of vascular plants in grazing areas. Grazing exclusion favors crust recovery, allowing black, lichen dominated crusts to develop in exposed areas between shrub canopies. The cover of the moss-dominated crusts was not significantly different at any of the two spatial scales analyzed. Soil nitrogen was higher in areas under L. cuneifolia and without BSC cover, suggesting that litterfall inputs currently exceed those from soil crust N2 fixation, perhaps because crust function has not yet recovered.