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Biogeomorphology and biological soil crusts: A symbiotic research relationship

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Consisting of various cyanobacteria, algae, lichens, and mosses, biological soil crusts (BSC) represent microbial ecosystems essential for many arid and semi-arid regions. Their structure and function have been researched intensely with little attention to spatial characteristics. Because it studies biota-landform interactions, biogeomorphology as a discipline stands poised to significantly narrow this apparent BSC research gap. While specific in scope, this article nonetheless outlines several key points and possible research agendas centered on the discipline of biogeomorphology that could enhance BSC research agendas. It first introduces readers to basic BSC concepts and how they have been traditionally studied with an ecological focus, noting how the discipline of biogeomorphology might influence this traditional research agenda. Then, after offering an analysis of BSC research related to remote sensing, the article then turns to how biogeomorphology stands at the forefront to conduct important BSC research through incorporation of weathering science.
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Géomorphologie : relief,
processus, environnement
4/2010 (2010)
Varia
................................................................................................................................................................................................................................................................................................
Casey DuaneAllen
Biogeomorphology and biological
soil crusts: a symbiotic research
relationship
................................................................................................................................................................................................................................................................................................
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Géomorphologie : relief, processus, environnement [En ligne], 4/2010|2010, mis en ligne le 01 décembre 2012,
consulté le 02 décembre 2012. URL: http://geomorphologie.revues.org/8071; DOI: 10.4000/geomorphologie.8071
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Version abrégée en français
A partir d’une recension de la bibliographie, cet article
dresse le bilan des connexions entre la biogéomorphologie et
l’étude des croûtes cryptogamiques (CC). Constituées de
nombreux lichens, mousses, algues ou cyanobactéries, les
croûtes cryptogamiques constituent un écosystème microbien
essentiel dans les régions arides et semi-arides et, dans de
nombreux cas, y couvrent le sol de façon dominante (Belnap
et Lange 2003 ; fig. 1). Les CC jouent également un rôle clé
dans l’analyse des changements écologiques à long terme, et
plus spécifiquement dans le registre du stockage du gaz car-
bonique (fig. 2). Leur structure et leur fonction ont fait l’ob-
jet de nombreuses recherches et une classification morpho-
logique a même été proposée (fig. 3 et fig. 4) mais peu de tra-
vaux ont pris en compte leurs caractéristiques spatiales. La
biogéomorphologie, en tant que discipline étudiant les inter-
actions entre les formes de terrain et le vivant, permet de pal-
lier sensiblement cette carence apparente. En retour, cela
permettrait de réduire les insuffisances de la discipline sou-
lignées par L.A. Naylor et al. (2002), parce que les CC per-
mettent de tester le modèle holistique bioérosion / bioprotec-
tion / bioconstruction proposé par ces auteurs.
Bien que l’objectif principal de cet article soit d’examiner
le rôle important que la biogéomorphologie joue en termes
de recherches spécifiques sur les CC, cet exposé synoptique
se concentre plus spécifiquement sur les techniques biogéo-
morphologiques qui pourraient être employées pour étudier
non seulement les CCmais aussi la biogéomorphologie des
CC elle-même (un aspect totalement inédit à ce jour). L’ar-
Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Biogeomorphology and biological soil crusts:
a symbiotic research relationship
Biogéomorphologie et croûtes cryptogamiques :
relations symbiotiques de la recherche
Casey Duane Allen*
*University of Colorado Denver – PO Box 173364 – CB 172 – Denver – CO 80217-3364. E-mail: casey.allen@ucdenver.edu
Abstract
Consisting of various cyanobacteria, algae, lichens, and mosses, biological soil crusts (BSC) represent microbial ecosystems essential
for many arid and semi-arid regions. Their structure and function have been researched intensely with little attention to spatial charac-
teristics. Because it studies biota-landform interactions, biogeomorphology as a discipline stands poised to significantly narrow this
apparent BSC research gap. While specific in scope, this article nonetheless outlines several key points and possible research agendas
centered on the discipline of biogeomorphology that could enhance BSC research agendas. It first introduces readers to basic BSC con-
cepts and how they have been traditionally studied with an ecological focus, noting how the discipline of biogeomorphology might
influence this traditional research agenda. Then, after offering an analysis of BSC research related to remote sensing, the article then
turns to how biogeomorphology stands at the forefront to conduct important BSC research through incorporation of weathering science.
Key words: biological soil crusts, biogeomorphology, remote sensing, weathering.
Résumé
Constituées de nombreux lichens, mousses, algues ou cyanobactéries, les croûtes cryptogamiques (CC) constituent un écosystème
microbien essentiel dans les régions arides et semi-arides. Leur structure et leur fonction ont fait l’objet de nombreuses recherches mais
peu ont pris en compte leurs caractéristiques spatiales. La biogéomorphologie, en tant que discipline étudiant les interactions entre les
formes de terrain et le vivant, permet de pallier sensiblement ce manque. Cet article se propose de souligner les apports cruciaux de
la biogéomorphologie dans le domaine des CC et les pistes restant à explorer. Pour ce faire, sont rappelés les concepts de base des CC,
la méthodologie d’étude habituelle issue des recherches écologiques et la manière dont la biogéomorphologie pourrait faire évoluer
cette approche traditionnelle. Ensuite, après avoir rappelé l’apport de la télédétection pour la connaissance des CC, l’article montre
comment la biogéomorphologie via sa contribution à la connaissance de la météorisation peut devenir une discipline fondamentale
dans l’amélioration de la connaissance des CC.
Mots clés : croûtes cryptogamiques, biogéomorphologie, télédétection, météorisation.
ticle présente tout d’abord les concepts de base définissant
les CC, la méthodologie d’étude habituelle issue des re-
cherches écologiques et de quelles manières la biogéomor-
phologie pourrait faire évoluer cette approche traditionnel-
le. Ensuite, après une présentation rapide des concepts de la
télédétection appliquée aux études environnementales, une
analyse complète des recherches sur les CC par télédétection
est proposée, soulignant une fois encore de quelle manière la
biogéomorphologie pourrait contribuer à cet axe de re-
cherche sur les CC. Cette section inclut l’examen des tech-
niques de télédétection passées et présentes, ainsi que des
méthodes de télédétection «basiques » (e.g., la vision hu-
maine ; fig. 5) ou plus avancés (e.g., les algorithmes spéci-
fiques employés pour déterminer les types d’espèces par
imagerie satellitale). L’article détaille ensuite la manière
dont la biogéomorphologie qui se place en première ligne
sur le front de la recherche sur les CC pourrait contribuer au
comblement de lacunes par l’incorporation de son savoir sur
la météorisation (fig. 6). Faire appel à la météorisation pour
étudier les CC représente une voie novatrice qui n’a pas en-
core été entièrement exploitée par la biogéomorphologie ou
par la recherche sur les CC. Pourtant, cet axe de développe-
ment recèle un fort potentiel heuristique. L’article défend
l’idée que la météorisation superficielle peut être le chaînon
manquant reliant les recherches connectées aux CC avec la
biogéomorphologie, et représente l’un des domaines de la
biogéomorphologie qui, bien que fondamental, est souvent
négligé en tant que sujet principal de recherches (ce qui n’est
d’ailleurs pas spécifique aux recherches sur les CC). Ce rôle
fondamental dévolue à la météorisation est justifié par sa ca-
pacité à faire des sauts scalaires, un manque commun à la
recherche sur les CC (Belnap et Lange, 2003) et à la bio-
géomorphologie (Naylor et al., 2002).
De plus, parce que la biogéomorphologie est une discipli-
ne concernée par les propriétés spatiales telles que la géo-
métrie du relief, le climat et les modalités de la météorisation
(avec la capacité de les relier à travers de grands espaces),
elle représente un «champ» naturel pour les recherches sur
les CC. Au moyen d’une analyse approfondie de la littératu-
re, cet article suggère que la biogéomorphologie peut offrir à
la recherche sur les CC de nouveaux modèles, méthodes, ap-
proches, et techniques qui développeront davantage le do-
maine de recherche, tout en contribuant simultanément à l’ex-
tension de l’espace d’étude de la biogéomorphologie. Par
exemple, puisque les régions urbaines continuent à s’étendre
au détriment des terres non cultivées, les modèles dynamiques
spatiaux des CC élaborés par la biogéomorphologie peuvent
avoir une influence sur les politiques environnementales. En
fin de compte, indépendamment des résultats potentiels des
recherches consacrées aux CC, la biogéomorphologie, en in-
tégrant la spatialisation des processus et la météorisation, de-
meure un moteur puissant de la recherche environnementale,
dont les CC ne sont qu’un aspect.
Introduction
For much of Earth’s early history, the terrestrial surface
hosted little more than the primitive plant life of algae,
fungi, cyanobacteria (blue-green algae), and perhaps lichens
(Budyko and Ronov, 1979; Berner, 1994; Berner and Ko-
thavala, 2001). Found predominantly in arid and semi-arid
regions, today’s analog of biological soil crusts (BSCs)
contain various species of these same organisms. Interac-
tions between BSC organisms and calcium-containing mi-
nerals were key to the habitability of Earth, as they led to the
draw-down of carbon dioxide (Brady and Caroll, 1994; Ber-
ner, 1995). Mesozoic development of higher order land
plants led to further reduction in carbon dioxide (Drever,
1994), and Miocene development of the Himalaya Moun-
tains and the further evolution of plant life with C4photo-
synthetic pathways reduced carbon dioxide enough to let
earth cross over into the Pleistocene glacial condition (Mol-
nar et al., 1993). Given their potential importance to early
Earth and relatively unknown importance to global cycling
related to today’s Earth – and while more studies related to
cause and effect between landform and plants are being
conducted (Bornyasz et al., 2005) – it is perhaps surprising
that little research exists to answer a fundamental question of
exactly how BSCs respond to disturbance in different geo-
graphic (spatial) contexts. BSCs are known to exist in nearly
every arid and semiarid ecosystem in the world, and even in
microclimates of some temperate regions; only evergreen
rainforests climatic region lacks BSCs (Büdel, 2003). Yet
BSC research, as an interdisciplinary field of study, is unable
to articulate clear connections between spatial controls - spe-
cifically landform type and effects of BSC components on
weathering rates - and initial responses to disturbance.
This problem could be addressed at an infrastructural
level in the field of biogeomorphology. In a 2002 special
issue of Geomorphology, a review of biogeomorphology by
L.A. Naylor et al. (2002) identified key research needs si-
milar to those deficiencies facing BSC-related research des-
cribed by J. Belnap and O.L. Lange (2003). These include:
(i) expanding research to variable spatial and temporal
scales; (ii) creating new approaches for modeling and devi-
sing assessment techniques that will link bioprocesses
across the system; (iii) implementing a more holistic ap-
proach to studying biota-landform relationships; (iv) explo-
ring the effects of multiple processes in shaping landforms;
(v) bridging the disparity between short time scale biotic
processes and longer scale landform processes and develop-
ment; (vi) utilising new theoretical advances in geosciences.
To help satisfy these deficiencies, L.A. Naylor et al. (2002)
proposed an interactive and dynamic three-fold spectrum of
how biota and geomorphology interact. This spectrum
consists of ‘bioconstruction’, how biota construct or de-
construct landforms; ‘bioerosion’, how biota help or hinder
landform erosion; and ‘bioprotection’, how biota protect
and/or fail to protect landforms. One significant way to
connect this biogeomorphological research triumvirate rests
in the oft-overlooked, but extremely important science of
weathering. While weathering studies abound in geomor-
phological research, few studies apply weathering science to
BSC-related research. The overarching framework of this re-
view takes guidance from this three-fold conceptual structu-
re of identifying potential linkages of spatial aspects across
348 Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Casey Duane Allen
landforms, while also taking into account the broader role of
soil crusts in general (both biological and chemical) as noted
by H.A. Viles (2008), and addressing identified gaps in the
BSC literature related to spatial dynamics (Belnap and Lange,
2003). Indeed, as discussed in this article, perhaps weathering
science is the missing piece – the ‘glue’ – that connects bio-
geomorphology to BSC and BSC-related research agendas.
Though it seeks to examine the overall important role bio-
geomorphology can play in regards to BSC research, this re-
view paper supports a more specifically-focused agenda, ar-
guing not only for using biogeomorphological techniques to
study BSCs, but also advocating for studying the biogeo-
morphology of BSCs themselves. To that end, the first sec-
tion focuses on how BSCs are studied from the traditional
ecological point of view, and how the discipline of biogeo-
morphology stands ready to engage in and expand the usual
techniques used in BSC research (i.e., those research endea-
vors with an inherent ecological focus). Next, after a brief
overview of remote sensing (RS) as an assessment method,
insight is offered into past and current RS-BSC research ef-
forts, and how RS can be used to gain a better understanding
of BSC biogeomorphology. Then, before concluding, the ar-
ticle offers a brief overview (because there is so little related
research) of the anticipated significance BSC research can
have on narrowing the present gap in BSC-related research
agendas through the disciplines of biogeomorphology and
weathering, including it potential for carbon sequestration.
Soil crusts studied with an ecological
focus
Found in every major arid/semi-arid biome in the world,
biological soil crusts (BSCs) can account for up to 70% of
the ground cover in some of these areas (fig. 1). Despite
their inherent presence in desert biomes, BSC research tends
to focus on three research questions: (i) Composition, i.e.
what kinds of creatures live in the BSC; (ii) Nitrogen fixa-
tion, i.e. how BSCs fix atmospheric nitrogen in the soil to
promote higher plant growth; and (iii) Disturbance recove-
ry, i.e. how BSCs recover from disturbance, prima-
rily focusing on recovery from grazing. This section
demonstrates that the literature on BSCs is remar-
kably rich with ecological insights, focusing on pro-
cesses and nutrient cycling (particularly nitrogen),
creating an ecologically-important symbiotic soil-
plant-atmosphere relationship (Rychert and Sku-
jins, 1974; Harper and Pendleton, 1993; Brady and
Weil, 2008). Some of this literature also focuses on
BSCs’ roll in reducing soil erosion by binding
‘loose’ soils together and aiding in water retention and dis-
persion (MacGregor and Johnson, 1971). Meant as no criti-
cism to the researchers focusing first on an understanding of
these processes, the relationships of BSCs with respect to
landform distribution, (micro)climate, and weathering have
not been aggressively pursued (Belnap and Lange, 2003).
The discipline of biogeomorphology can fill two specific
gaps in BSC-related literature. First, BSC-related studies in-
clude a lack of comparative landform-climate studies ‘spe-
cifically’ over large areas, ‘without’ using ground cover
and/or soil texture as surrogates (e.g., expanding study areas
to larger than single landform-sizes, such as a dune field or
catchment basin). Second, studies that relate BSCs to wea-
thering rates, mechanisms, forms, and processes are few and
far between.
A fundamental ecological function of BSCs in the land-
scape rests in stabilizing desert surfaces (Belnap and Gillet-
te, 1998; Yair, 2003), yet BSC integrity is compromised by
even the smallest disturbance (e.g., a human footprint) and
devastated even more by larger events (e.g., off-road ve-
hicle). J. Belnap and D.A. Gillette (1998), for example, dis-
covered that BSCs have significantly higher threshold fric-
tion velocities than bare soil and disturbed BSCs. Indeed,
not only do BSCs hold fragile desert soil together, but they
also play a key role in preventing deflation of fine particu-
lates into the atmosphere. In valleys dominated by an urban
heat island that traps fine particulates and decreases air qua-
lity, understanding BSCs through a biogeomorphological
lens can play an integral role in the management of arid and
semi-arid region cities.
Important human impact studies on soil crusts (Cole,
1990; Belnap, 1993, 1995, 1996) note that valuable fiber
connections were easily broken during dry seasons. These
studies also found that recovery time for disturbed soil
crusts takes anywhere from six weeks for initial cyanobac-
terial growth to 20 years or more for larger ecosystem-es-
sential lichen and moss growth, putting to rest the previous-
ly-accepted view of soil crust communities taking centuries
to recover. Yet while most human disturbances do not kill
349
Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Biogeomorphology and biological soil crusts
Fig. 1 – Biological soil crusts as dominant ground
cover in an arid, badlands-topography biome, Grand
Staircase-Escalante National Monument, UT (USA).
Photo by author.
Fig. 1 – Les croûtes cryptogamiques comme couver-
ture dominante des sols au sein d’un biome aride et
à topographie ravinée, Grand Staircase-Escalante
National Monument, Utah (Etats-Unis). Photographie
de l’auteur.
BSC microorganisms directly, water must be available for
repair mechanisms to function, something lacking in the
arid and semi-arid BSC habitats (Belnap, 1993; Belnap and
Lange, 2003). In order to obtain recovery data, these studies,
and subsequent later studies (Belnap and Gillette, 1998;
Belnap et al., 2004), focused specifically on recovery rates
for soil crusts over longer temporal scales (e.g., more than
one year) and further, only focused on microorganism reco-
very, paying no attention to the type of landform where re-
covery was measured, nor to the effects of BSC microcli-
mate or seasonal precipitation regimes (key components of
biogeomorphological research).
BSCs can also assist in monitoring long-term ecological
research. Because most mature crusts contain not only cya-
nobacteria and algae but also mosses and lichens, BSCs
represent natural long-term storage systems of carbon in
arid/semi-arid regions. Yet if left undisturbed, BSCs main-
tain extremely long life spans lasting centuries (Belnap and
Lange, 2003) and have been found to, since early environ-
ments, contribute greatly to the ecosystem (Chacon-Baca et
al., 2002; Beraldi-Campesi et al., 2004; Beraldi-Campesi
and Cevallos-Ferriz, 2005). Understanding BSC resilience
and recovery among landforms near desert cities, and the
affect BSCs have on local arid and semi-arid ecosystems,
may also lead to effective models for carbon sequestration
in these settings. Soil organic carbon (SOC) plays a consi-
derable role in the global carbon cycle, containing more than
triple the amount of organic C found in living biomass and
atmospheric CO2(Lal, 2004b). Increasing SOC storage by a
mere 5% could decrease atmospheric CO2as much as 16%
(Baldock, 2007). While C sequestration in soil remains a
slow process, it may represent an efficient natural strategy to
offset increased atmospheric CO2brought on by fossil fuel
emissions (Baldock, 2007). Some estimates suggest seques-
tration rates of up to 150 Pg CO2-C over the next century are
possible if predictive accuracy can be increased (Houghton,
1995; Lal et al., 1998; Lal, 2004a). Representing ‘natural’
long-term storage facilities for carbon in arid regions – espe-
cially relative to their mass – most mature BSCs contain not
only basic carbon producers such as cyanobacteria and algae,
but also carbon-rich producers such as mosses and lichens
(fig. 2). The avenue of BSCs as carbon sequestration sites
has been only explored slightly (Beymer and Klopatek,
1991; Evans and Belnap, 1999; Evans and Lange, 2001), and
research into carbon sequestration ‘specifically’ could prove
beneficial, as BSCs are suspected to have a significant C
draw-down potential, especially relative to their mass and
composition, and specifically in conjunction with potential
soil organic carbon drawdown from soil in general (Jeffries
et al., 1989; Beymer and Klopatek, 1991; Jeffries et al., 1993
a and b; Palmqvist et al., 1994; Palmqvist, 1995; Ziegler and
Lüttge, 1998; Lange, 2000; Palmqvist, 2000; Belnap et al.,
2003a; Evans and Lange, 2003; Baldock, 2007).
Besides the enormous amounts of BSC studies relating to
human disturbances and nitrogen fixation capabilities of soil
crusts and the limited studies relating specifically to BSC-
related carbon cycling and sequestration, a great deal of re-
search also focuses on soil crust composition and taxonomy
(Cameron et al., 1965; Follmann, 1965; Cameron et al.,
1966; Forest and Weston, 1966; Soriano, 1983; Belnap and
Gardner, 1993; Bouza and Del Valle, 1993; Flechtner et al.,
1998; Maya et al., 2002; Redfield et al., 2002). Researchers
recognize four types of crusts based on visual morphology
and potential evapotranspiration (PET): smooth, rugose
(fig. 3), pinnacled (fig. 4), and rolling. Per J. Belnap and
O.L. Lange (2003, p. 180) ‘smooth crusts occur in hyper-
arid and arid hot deserts with the highest PET; rugose
crusts… in hot, arid deserts with slightly lower PET… oc-
curring where soil freezing does not occur’. Pinnacled
crusts, however, occur in arid and semi arid cold deserts
where soil freezing does occur, and ‘lower PET supports a
higher biomass of mosses and lichens than hot deserts’.
Found in ‘even colder semiarid and cool and cold deserts,
where soils freeze… rolling crusts have an even lower PET
that ‘…supports a larger biomass of lichens, mosses, and
vascular plants than found in less moist deserts’ (ibid.).
While taxonomic endeavors give researchers a common
vernacular, the specific morphological names were genera-
ted from, mainly, climatic characteristics, regimes,
and/or biomes. This climate-based morphological
classification lends itself well to on-the-ground
classification over small areas, but may not be ap-
350 Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Casey Duane Allen
Fig. 2 – Lichen- and moss-dominated BSC, near Zion
National Park, UT (USA). It is thought that these types of
BSCs may play an important role in the global carbon cycle
– especially relative to their mass and relationship with po-
tential SOC drawdown in conjunction with soils – including
perhaps unidentified carbon sequestration sites. Photo by
author.
Fig. 2 – Croûtes cryptogamiques à lichens et mousses,
à proximité du Parc national de Zion, Utah (Etats-Unis).
Il est pensable que ce type de CC puisse jouer un rôle
important dans le cycle global du carbone – en regard
de leur masse et de leurs liens avec la chute du carbo-
ne organique des sols – incluant peut-être des sites de
séquestration du carbone encore inconnus. Photogra-
phie de l’auteur.
plicable to larger-area studies, as most BSC climate-related
studies focus ‘specifically’ on microclimate and influences
on smaller, local ecosystems, rather than generating an ex-
panded, more holistic view or model (cf. the many local-
based climate studies such as: L.C. Pearson and D.B. La-
wrence, 1965; A. Goudie, 1972; G.J. Kidron, 1992; G.J. Ki-
dron et al., 1995 a and b; G.J. Kidron and A. Yair, 1997;
B. Sundberg et al., 1999; G.J. Kidron et al., 2000; M. Veste
et al., 2001 a and b; C.D. Allen, 2005; S.L. Ustin et al.,
2009). Large spatial area BSC assessments would be a va-
luable addition to potential BSC-related research agendas
such as assessing climate change. Lichens, as a species, re-
main very susceptible to atmospheric disturbances, and may
be able to aid researchers studying anthropogenic climate
change factors, though using lichens in this manner is only in
its infant stage and has not yet been fully investigated (Theo-
dore Crusberg, personal communication, January 2009). Ne-
vertheless, these areas of potential research represent short-
comings in BSC-related research that the discipline of bio-
geomorphology might be able to address.
Regardless of type or classification however, BSC com-
munities remain an important part of arid region sustainabi-
lity today. They are an essential first step to producing and
protecting arable soils in the fragile desert ecosystem (Bel-
nap, 1995; Belnap and Lange, 2003; Belnap et al., 2004).
The delicate microorganisms that comprise BSCs live a te-
nuous existence subject to the uncertainties of climate, dis-
persal, animals, and humans (Cole, 1990; Belnap and Lange,
2003). Disturbances of any kind increase the damage to and
destruction of BSCs (Cole, 1990; Belnap et al., 1994), espe-
cially in the unprotected and increasingly-touristed arid re-
gions of North and Latin America (Maya et al., 2002; Ro-
sentreter and Belnap, 2003). Thus, future BSC research
needs that can be addressed by biogeomorphic research tech-
niques and principles rest in the dual problems of: (i) deve-
loping better understanding of large-area spatial dynamics
of BSCs (e.g., across landforms, across biomes); and (ii) de-
veloping better understanding of biogeomorphic circum-
stances BSCs need to initiate their disturbance recovery.
Furthermore, research challenges faced in the field of bio-
geomorphology are similar to those faced in BSC-related re-
search (i.e., small area research focused on a limited set of
questions). In short, because they can be monitored, studied,
and modeled in fine detail ‘and’ across larger areas, BSCs
are uniquely positioned to address deficiencies in both BSC-
related and biogeomorphology arenas. One important BSC
assessment and monitoring technique that may make a solid
contribution to this arena – especially when studied using a
biogeomorphologic focus – is remote sensing.
Remote sensing and BSCs
At its most basic, remote sensing (RS) is used to detect or
infer the properties of a substance without being in direct
physical contact. RS tools measure the electromagnetic (EM)
energy flow from surface phenomena in and across different
regions of the wavelength spectrum. Energy can be emitted
from an object due to its temperature or reflectance from a
natural (e.g., sun) or artificial (e.g., radar) source. Energy re-
flected from surfaces varies as a function of wavelength.
When it comes to soil, the most important factor affecting re-
flectance is the soil mineralogy (e.g., iron oxides, clay mine-
rals, carbonates), though soil reflectance, soil-water content,
organic matter content, soil texture, and soil roughness are
also factors (Karnieli et al., 2003). Remotely sensed images
combine different spectra channels to create specific indices
to assess percent of ‘cover’ and accompanying biophysical
condition. The most widely used index for these purposes is
the Normalized Difference Vegetation Index (NDVI). The
NDVI values range from -1 to +1, with denser and/or heal-
thier vegetation having higher positive values. A.R. Huete et
al. (1984), however, note that in arid regions, exposed soils
have a significant effect on NDVI values. Still, vegetation in-
dices generated via satellite-borne sensors usually consist of
351
Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Biogeomorphology and biological soil crusts
Fig. 3 – Rugose biological soil crust, McDowell Mountain Re-
gional Park, Sonora Desert, AZ (USA). Photo by author.
Fig. 3 – Croûte cryptogamique rugueuse, Parc régional de la
McDowell Mountain, Désert du Sonora, Arizona (Etats-Unis).
Photographie de l’auteur.
Fig. 4 – Pinnacled BSC, near Zion National Park, UT (USA).
Photo by author.
Fig. 4 – Croûte cryptogamique à pinacles, à proximité du Parc
national de Zion, Utah (Etats-Unis). Photographie de l’auteur.
measurements in a few channels, resulting in a coarse spec-
tral resolution (referred to as broad-band, multispectral sen-
sors, e.g., Landsat TM). Yet E. Zaady et al. (2007) success-
fully used the NDVI in conjunction with the Brightness
Index (BI) to monitor BSC succession and regeneration rates
over long periods (years) and correlated findings with slope
aspect (i.e., difference of BSC spatial distribution between
north- and south-facing slopes). This technique could prove
useful in the biogeomorphological arena.
In contrast to broad-band multispectral sensors, hyper-
spectral RS measures EM reflectance in hundreds of conti-
nuous narrow-bands (e.g., the Airborne Visible and Infra-
Red Imaging Spectrometer (AVRIS) operated by NASA /
JPL; Green et al., 1998). The ability to determine and iden-
tify absorption features in a reflectance spectrum that arise
from chemical bonds present in the surface materials gives
hyperspectral RS a sizeable advantage over multispectral
RS. This has specific implications for BSC-related spatial
studies, because hyperspectral imaging can more easily dis-
criminate between shape and wavelength position of ab-
sorption features instead of merely sensing the differential
reflectance level in two channels. Yet multispectral and hy-
perspectral imaging are generally used in non-BSC research
arenas such as geologic mapping (Lillesand and Keifer,
2002), environmental studies (Swayze et al., 2000), vegeta-
tion cover identification (Martin et al., 1998; Roberts et al.,
1998), and vegetation biochemical composition (Kokaly
and Clark, 1999). For small area assessments, such as small
plots of BSCs on differing landforms however, hyperspec-
tral imaging remains relatively expensive to conduct, while
multispectral imaging offers limited coverage, although
Landsat TM has been used to successfully distinguish BSCs
from bare ground (Wessels and van Vuuren, 1986). Less ex-
pensive and more conducive to on-the-ground fieldwork, a
hand-held spectroradiometer offers coverage over most of
the EM spectrum necessary to assess BSC biogeomorpholo-
gy, but would only be feasible over smaller areas as well.
Even capturing BSC communities on differing landforms
and/or across biomes and climatic regimes using repeat di-
gital photography from the ground can prove useful (Bow-
ker et al., 2008), as BSCs are more active over short-time
scales than formerly thought (Belnap et al., 2006), but this
requires a longer timeframe. At a more general remote sen-
sing level, A. Karnieli et al. (2003) outline how differing
forms of RS, such as spacecraft versus aircraft, might be
used in conjunction with more traditional in situ and labora-
tory techniques to quantify not only BSCs, but also higher
plants and bare soils, using examples from Israel, the Colo-
rado Plateau, and the rangelands of Southeastern Idaho.
Globally speaking, BSC spectra have similar signatures
even when species composition vary, and thus can be distin-
guished from other ground components, allowing for map-
ping based solely on RS data (Karnieli et al., 2003).
Overall, RS provides an opportunity to expand in situ
BSC and BSC-related studies, cutting-down on the costs,
time, and energy used in ground truthing. Large areal as-
sessments are important to understanding spatial dynamics
of BSCs across the larger ecosystem (Belnap and Lange,
2003). Yet even though BSCs are found in nearly every en-
vironment in the world (Büdel, 2003) - and while scientific
and research endeavors related to BSCs are growing - very
few studies have been conducted on how RS can enhance
the study of BSC spatiality. Of those studies published, ac-
cording to A. Karnieli et al. (2003), D.C.J. Wessels and
D.R.J. van Vuuren (1986) were the first researchers to detect
and map BSCs using solely satellite imagery. Their work in
the Namib Desert using Landsat TM imagery led to discri-
mination between lichen covered, bare, and vegetation-co-
vered surfaces. Later, however, Y.M. Zhang et al. (2007)
discovered that using Landsat imagery to assess BSC cove-
rage is only workable when BSCs represent more than 33%
of the field of view. Aside from these two studies focused on
using Landsat TM data, relatively few publications have
specifically studied BSCs using some kind of spectral ima-
gery and/or RS - even to map them - although studies such
as J. Chen et al. (2005) developed an algorithm-based BSC-
specific index for mapping. Other notable BSC-specific RS
algorithms include E. Ben-Dor et al. (2003) who measured
BSC structure in specific wavelengths and A. Karnieli et al.
(1995, 1996, 1997) who used spectral reflectance as a dia-
gnostic tool have proved valuable, and the more advanced
BSC-specific Continuum Removal Crust Identification Al-
gorithm (CRCIA) developed by B. Weber et al. (2008) using
hyperspectral datasets.
One characteristic of BSCs that has been noticed – at a
basic ground level by C.D. Allen (2005) and on a spectral
scale by A. Karnieli and H. Tsoar (1995), A. Karnieli et al.
(1996, 1997, 2003), S.B. Fang et al. (2008), and S.L. Ustin et
al. (2009) – is that BSC spectral reflectance changes due to
precipitation (fig. 5). This change in spectral reflectance ap-
parently alters both the NDVI and the pigment absorption,
which can sometimes be misinterpreted in RS analysis (Kar-
nieli et al., 2003; Fang et al., 2008). Further, while distingui-
shing between individual BSC stands might not be possible
using satellite imagery, this apparently does not affect the abi-
lity of using high-altitude RS to monitor local or regional
changes in BSCs (Ustin et al., 2009). Thus, while high-altitu-
de RS can be used for BSC assessment, there still remains a
need for on-the-ground, small-area assessment. For example,
after using a BSC-specific algorithm to identify and poten-
tially map BSC stands over large areas, a high-resolution
image of a specific stand could be taken closer to the ground,
and then post-processed using remote sensing software (e.g.,
ERDAS Imagine) to determine which specific wave-lengths
represent BSCs versus bare ground, rock type, and higher
plant life. This type of remotely sensed data could be obtained
through the use of ‘pole cameras’, balloon (or kite) photogra-
phy, or even low-altitude (aerial) remote controlled vehicles
with a still and/or video camera mount, yielding truly rich da-
tasets from which to analyze BSC biogeomorphology.
BSC biogeomorphology and
weathering science
When it comes to BSCs, big picture linkages remain a re-
search deficit, as J. Belnap and O.L. Lange (2003) clearly
352 Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Casey Duane Allen
articulate, and contribute to the need for a better linkage bet-
ween BSC-related research and the larger conceptual need
for a systemic conceptual biogeomorphic framework, as
L.A. Naylor et al. (2002) argue. With specific regards to bio-
geomorphology, BSC research should help fill the gap bet-
ween the biogeomorphology triumvirate (bioconstruction,
bioerosion, and bioprotection; Naylor et al., 2002) by buil-
ding strong connections through innovative environmental
models by assessing spatial effects of human disturbances of
BSCs. This in turn would lead to research regarding BSC re-
siliency, opening new possibilities for ecosystem sustainabili-
ty research, as suggested in the work of M.A. Bornyasz et al.
(2005), and important emerging fields of study related to
BSCs, such as geobiology, a field of study that combines earth
science and biology and influences environmental decision
making and the larger arena of biocomplexity (Naylor et al.,
2002; Noffke, 2005), much as BSCs could do if specific land-
form type, differing climatic regimes (even meta-analyses),
and weathering parameters were studied through a biogeo-
morphological lens. Indeed, when it comes to BSCs and wea-
thering science, few disciplines are better prepared to assess
linkages than those trained in biogeomorphology, where the
interaction between biota and landform - usually through wea-
thering - comes to the forefront.
Researchers such as C. Ollier (1974) discuss weathering
and landforms in detail and C. Ollier and C. Pain (1996) dis-
cuss weathering in the context of soils in general, yet few
studies focus specifically on weathering as related to BSCs,
though recently C. Ollier and H. Sheth (2008) discuss duri-
crusts in relation to soil formation (in India) and H. Mura-
kami and S. Ishihara (2008) studied rare earth elements in
weathered crusts of China and Japan. Because it focuses on
spatial relationships (e.g., the weathering-climate-landform-
organisms continuum) biogeomorphology, as a discipline,
should stand at the forefront of future BSC research endea-
vors, especially when it comes to weathering.
While BSCs have been studied in nearly every environ-
ment around the globe, most studies remain ecologically-
specific, only taking into account ‘specific’ landform and
climate types randomly (for locational overviews, see chap-
ters 2-11 in J. Belnap and O.L. Lange, 2003). Nevertheless,
important studies related to local scale BSC landform type
and local climate can be found, though they seem to focus
on ‘specific’ dune fields and ‘seasonal’ rainfall in the Negev
Desert (Kidron and Yair, 1997; Kidron et al., 2002, 2003,
2009), “specific” alluvial fans (Barker et al., 2005) or dunes
(Brostoff et al., 2005) in the Mojave Desert, ‘specific’ pedi-
ments in the Sonora Desert (Allen, 2005; Beraldi-Campesi
et al., 2009), and mostly basin or other ‘fill’ in the Colorado
Plateau (Belnap, 1990; Lange et al., 1997; Bowker et al.,
2002; Belnap et al., 2003b; Belnap, 2006). Other locations
that have received limited study in relation to “local” land-
form type and climate include: Namib Desert (Goudie,
1972; Lange et al., 1994; Lalley and Viles, 2008), Antarcti-
ca (Boyd et al., 1966; Wynn-Williams, 1993; Green and
Broady, 2003), and the Atacama (Rundel et al., 1991; War-
ren-Rhodes et al., 2007). While it may be possible to
conduct a meta-analysis of broader landform-climate rela-
tions from these studies, no specific model has yet been de-
veloped, though such models have been suggested in re-
gards to general biogeomorphic and ecological research
(Viles et al., 2008) and more specifically to a biogeomor-
phic approach to rock weathering (Viles, 1995). Neverthe-
less, such overarching studies would probably be constrai-
ned by comparative time-scales. That is, while studies do
exist, they have occurred at disparate timeframes, some in
the 1970s, some in the 1980s, some in the 1990s, etc., and
correlating ‘old’ data with newer data might pose a problem.
As epilithic organisms, however, BSCs play an intimate
role in weathering processes through bedrock colonization
and trapping dust that eventually filters into rock through
fissures, where their small size allows for penetration into
interstices of sands and silts, especially compared to higher
plants (Hunt, 1979). With increased weathering, more Ca-
silicates are exposed to carbonic acid, improving porosity
and permeability because rock fragments (from weathering)
eventually become sedimentary rocks (e.g., limestone and
subsequent CO2storage, sandstone with high silica content;
fig. 6). And when it comes to weathering-BSC relationships
specifically, studies are fewer, and seem to only account for
BSC-weathering related research when consequential to
other topical investigations.
One of the first such studies was performed by N.A. Kra-
sil’nikov (1949), and focused specifically on mountain
rocks, though his later study in the same area focused on
high-altitude nitrogen fixing potential of microflora (Kra-
sil’nikov, 1956), a notable and continuing research thread
still prevalent in BSC-ecological research fields today [see
F.L. Pérez (1997) for work in the Andes, and R. Türk and G.
Gärtner (2003) for research overview in the Alps]. While
353
Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Biogeomorphology and biological soil crusts
Fig. 5 – Spectral difference between wet (left) and dry (right) ru-
gose BSC, Snow Canyon State Park, UT (USA). Real-time video of
change in spectral reflectance due to precipitation available here:
http://www.youtube.com/watch?v= 1ybZM9MRjxM. Photo by author.
Fig. 5 – Différence spectrale entre des croûtes cryptogamiques
rugueuses humide (gauche) et sèche (droite), Snow Canyon
State Park, Utah (Etats-Unis). Vidéo en temps réel du changement
de la réflectance spectrale dû aux précipitations disponible ici :
http://www.youtube.com/watch?v= 1ybZM9MRjxM. Photographie de
l’auteur.
many studies continue Krasil’nikov’s nitrogen-fixing stu-
dies, BSC-weathering related research in alpine environ-
ments remains scant, usually leaving the relationship – if it
is noted at all – as a side note (Gold, 1998; Dickson, 2000;
Zielke et al., 2002). Studies relating to CO2and nitrogen
fixation in alpine environments abound, but again, these stu-
dies neglect to mention the important BSC-weathering
connection (Alexander and Schell, 1973; Forman and Dow-
den, 1977; Wojciechowski and Heimbrook, 1984; Henry and
Svoboda, 1986; Chapin, 1996; Liengen and Olsen, 1997;
Dickson, 2000; Zielke et al., 2002).
Contemporary climate-weathering studies relatable to bio-
geomorphology-BSC research agendas include P.V. Brady et
al. (1999) who centered on lichens in relation to silicate wea-
thering, J.D. Brotherson et al. (1985) focusing specifically on
plant communities, B. Büdel’s (1999) work in topical envi-
ronments, R. Chen et al. (2009) who studied mineral compo-
nents of BSCs, A. Danin’s (1983) work on cyanobacteria
weathering limestone, H. Murakami and S. Ishihara (2008)’s
study using rare earth elements, and Viles’ broad ecological
scope of rock decay (Viles, 1995). Notwithstanding the
contribution of these landform-climate and weathering-BSC
studies to biogeomorphological-BSC research, these two ove-
rarching research foci still represent areas where biogeomor-
phology can make significant contributions. In fact, H.A.
Viles (2008) strongly suggests BSCs may be the key to un-
derstanding weathering in arid and semi-arid regions. The
path seems clear then for biogeomorphology to take the lead
in – or at least pave the way for – future, and very signifi-
cant, spatially-based BSC research (i.e., broader climate-
landform-weathering-BSC relationships).
Conclusion
A first step in understanding key issues of how biogeo-
morphology can influence BSC research rests in evaluation
of BSC spatial characteristics, specifically landform, clima-
te, and weathering parameters (and, if possible link these
across large areas). This article seeks to discuss these resear-
ch topics in relation to each other generally, and how
biogeomorphology might inform them specifically. Because
a fundamental ecological function of BSCs in the landscape
rests in stabilizing desert surfaces (Belnap and Lange,
2003), it then follows that if particular landforms (at a regio-
nal scale instead of the more-studied local scale) are more
frequently ‘used’ (e.g., tourism and/or ecotourism) or ‘inva-
ded’ (e.g., urban sprawl) by humans, understanding BSC
recovery across landforms, biomes, and seasonality (clima-
te), could enhance their preservation, or at least augment
associated land management practices. As cities expand, and
begin to sprawl across the landscape, there exists an inherent
need for careful preservation and management of the sur-
rounding regions, especially in ecologically fragile biomes.
Creating new, widely-applicable spatial dynamic models of
BSCs, using the discipline of biogeomorphology as a guide,
could alter environmental decision making by, for example,
restricting recreational and/or developmental activities on
certain types of landforms and/or certain climatic regimes.
When conducted through a biogeomorphological lens, BSC
research will also open new possibilities for resiliency and
ecosystem sustainability research (e.g., geobiology). Yet
even regardless of potential research outcomes it remains
clear that, as a discipline, biogeomorphology stands poised
to offer strong contributions to BSC research agendas and
could have long-lasting impacts on the field. There exists a
fundamental need for researchers who study spatial interac-
tions between landform, biota, and climate - biogeomorpho-
logists - to become involved in and work alongside our more
ecologically-trained colleagues (Viles et al., 2008). By be-
coming more engaged in BSC research, biogeomorpholo-
gists can also help narrow already-identified research gaps
in this fundamental field of inquiry through their inherent
focus on spatiality. Indeed, it is precisely the focus on spa-
tiality that current BSC research lacks, and where biogeo-
morphology as a discipline remains strongest.
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Article soumis le 23 novembre 2009, accepté le 24 août 2010.
358 Géomorphologie : relief, processus, environnement, 2010, n° 4, p. 347-358
Casey Duane Allen
... These highly specialised communities form a biological crust immediately on top of or within the first millimetres of the soil surface ( Büdel, 2005). Biocrusts preferably occur under harsh conditions of temperature or light, where vascular vegetation tends to be rare ( Allen, 2010). Therefore, biocrusts are generally widespread under dryland conditions ( Berkeley et al., 2005;Belnap, 2006;Büdel et al., 2009), whereas under mesic conditions they mostly occur as a successional stage after disturbance or in environments under regularly disturbed regimes ( Büdel et al., 2014). ...
... Furthermore, descriptions of different biocrust types in mesic vegetation zones and investigations in southeast Asia are rare ( Bowker et al., 2016). Functional roles of biocrusts have been investigated for decades, but less attention has been paid to their spatial distribution and characteristics ( Allen, 2010). Biocrust cover varies across spatial scales (from centimetres to kilometres), and it could be shown that it depends not only on the surrounding vascular vegetation cover but also on soils, geomorphology, and (micro-)topography or terrain ( Evans and Johansen, 1999;Ullmann and Büdel, 2003;Kidron et al., 2009;Bowker et al., 2016) in arid, semi-arid, temperate and boreal environments. ...
... These effects differ with regard to soil texture, surface roughness, water repellency and finally different crust species and developmental stages ( Warren, 2003;Belnap and Büdel, 2016). However, studies that directly relate different types of biocrust cover to rates of soil erosion are few ( Allen, 2010). Furthermore, the influence of biocrusts on sediment delivery and runoff has mostly been investigated in arid and semi-arid climates and humid climates have been largely disregarded ( Belnap and Lange, 2003;Weber et al., 2016). ...
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This study investigated the development of biological soil crusts (biocrusts) in an early successional subtropical forest plantation and their impact on soil erosion. Within a biodiversity and ecosystem functioning experiment in southeast China (biodiversity and ecosystem functioning (BEF) China), the effect of these biocrusts on sediment delivery and runoff was assessed within micro-scale runoff plots under natural rainfall, and biocrust cover was surveyed over a 5-year period. Results showed that biocrusts occurred widely in the experimental forest ecosystem and developed from initial light cyanobacteria- and algae-dominated crusts to later-stage bryophyte-dominated crusts within only 3 years. Biocrust cover was still increasing after 6 years of tree growth. Within later-stage crusts, 25 bryophyte species were determined. Surrounding vegetation cover and terrain attributes significantly influenced the development of biocrusts. Besides high crown cover and leaf area index, the development of biocrusts was favoured by low slope gradients, slope orientations towards the incident sunlight and the altitude of the research plots. Measurements showed that bryophyte-dominated biocrusts strongly decreased soil erosion, being more effective than abiotic soil surface cover. Hence, their significant role in mitigating sediment delivery and runoff generation in mesic forest environments and their ability to quickly colonise soil surfaces after disturbance are of particular interest for soil erosion control in early-stage forest plantations.
... Biological soil crusts (BSCs) consist of assemblages of living organisms on soil or rock surfaces in arid and semiarid areas. Typically composed of cyanobacteria, fungi, lichens, and algae, they cover a wide variety of undisturbed Sonoran Desert soils ( Fig. 10.6) and protect desert surfaces from erosional shear stresses imposed by overland flow and strong winds (Allen, 2005(Allen, , 2010. ...
... An individual walking on desert landforms, before massive land-use change associated with cattle grazing and urban expansion, likely would have experienced very different surface conditions than found by the average hiker today. Extensive areas once hosted desert pavements, BSCs (Allen, 2005(Allen, , 2010, and interlocking colluvium on steeper slopes that provided a net-armoring effect (Bowker et al., 2008;Granger et al., 2001;Seong et al., 2016a). Today, only patches of such armored surfaces remain, providing glimpses into the original land surfaces. ...
... Grazing impacts erosion processes in our watersheds by reducing vegetation cover and by the removal of biological soil crusts (BSCs) that once protected desert surfaces from wind and water erosion [42,43]. Naturally, BSCs were much more extensive in the region [44], but currently remain only as isolated patches-all due to human-induced disturbances such as cattle grazing [32]. ...
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Land use changes often lead to soil erosion, land degradation, and environmental deterioration. However, little is known about just how much humans accelerate erosion compared to natural background rates in non-agricultural settings, despite its importance to knowing the magnitude of soil degradation. The lack of understanding of anthropogenic acceleration is especially true for arid regions. Thus, we used 10Be catchment averaged denudation rates (CADRs) to obtain natural rates of soil erosion in and around the Phoenix metropolitan region, Arizona, United States. We then measured the acceleration of soil erosion by grazing, wildfire, and urban construction by comparing CADRs to erosion rates for the same watersheds, finding that: (i) grazing sometimes can increase sediment yields by up to 2.3–2.6x, (ii) human-set wildfires increased sediment yields by up to 9.7–10.4x, (iii) after some post-fire vegetation recovered, sediment yield was then up to 4.2–4.5x the background yield, (iv) construction increased sediment yields by up to 5.0–5.6x, and (v) the sealing of urban surfaces led to one-tenth to one-half of the background sediment yields. The acceleration of erosion at the urban–rural interface in arid lands highlights the need for sustainable management of arid-region soils.
... Furthermore, different biocrust community types can exhibit different spectral characteristics depending on their relative cover and soil surface reflectance (Rodriguez-Caballero et al. 2014). While satellite remote sensing is appropriate for large-scale biocrust assessment, there remains a need for small-area, high-resolution assessments to better characterize spatial heterogeneity in biocrust condition and extent that will facilitate better understanding of biocrust-ecosystem interactions at local scales (Duane Allen, 2010;Rozenstein and Adamowski, 2017), as well as provide data at an intermediary scale to link field studies to satellite data. ...
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Biological soil crusts (biocrusts) occur in drylands globally where they support ecosystem functioning by increasing soil stability, reducing dust emissions and modifying soil resource availability (e.g. water, nutrients). Determining biocrust condition and extent across landscapes continues to present considerable challenges to scientists and land managers. Biocrusts grow in patches, cover vast expanses of rugged terrain and are vulnerable to physical disturbance associated with ground-based mapping techniques. As such, remote sensing offers promising opportunities to map and monitor biocrusts. While satellite-based remote sensing has been used to detect biocrusts at relatively large spatial scales, few studies have used high-resolution imagery from Unmanned Aerial Systems (UAS) to map fine-scale patterns of biocrusts. We collected sub-centimeter, true color 3-band imagery at 10 plots in sagebrush and pinyon-juniper woodland communities in a semiarid ecosystem in the southwestern US and used object-based image analysis (OBIA) to segment and classify the imagery into maps of light and dark biocrusts, bare soil, rock and various vegetation covers. We used field data to validate the classifications and assessed the spatial distribution and configuration of different classes using fragmentation metrics. Map accuracies ranged from 46 to 77% (average 65%) and were higher in pinyon-juniper (av-erage 70%) versus sagebrush (average 60%) plots. Biocrust classes showed generally high accuracies at both pinyon-juniper plots (average dark crust = 70%; light crust = 80%) and sagebrush plots (average dark crust = 69%; light crust = 77%). Point cloud density, sun elevation and spectral confusion between vegetation cover explained some differences in accuracy across plots. Spatial analyses of classified maps showed that biocrust patches in pinyon-juniper plots were generally larger, more aggregated and contiguous than in sage-brush plots. Pinyon-juniper plots also had greater patch richness and a lower Shannon evenness index than sagebrush plots, suggesting greater soil cover heterogeneity in this plant community type.
... Research about the presence of biocrusts, their taxonomic composition and functional role in different habitats as well as their susceptibility to disturbance have initially been concentrated in arid and semiarid regions ( Belnap et al., 2001). In most situations, there is a clear climatic limitation to vascular plants, implying that the relationships between biocrust and vascular plants can't be properly assigned as well as the role played by biocrusts on successional trajectories or vegetation dynamics beyond biocrust succession itself (Duane Allen, 2010). In fact, little attention has been devoted to tropical ecosystem dominated by vascular plants, such as dry forests, where according to theory and the state of research, biocrusts are not expected to be either abundant or ecologically relevant ( Belnap et al., 2001;Maestre and Cortina, 2002;Seitz et al., 2017). ...
Article
Full-text available
Biological soil crusts (biocrusts) have been recognized as key ecological players in arid and semiarid regions at both local and global scales. They are important biodiversity components, provide critical ecosystem services, and strongly influence soil-plant relationships, and successional trajectories via facilitative, competitive, and edaphic engineering effects. Despite these important ecological roles, very little is known about biocrusts in seasonally dry tropical forests. Here we present a first baseline study on biocrust cover and ecosystem service provision in a human-modified landscape of the Brazilian Caatinga, South America's largest tropical dry forest. More specifically, we explored (1) across a network of 34 0.1 ha permanent plots the impact of disturbance, soil, precipitation, and vegetation-related parameters on biocrust cover in different stages of forest regeneration, and (2) the effect of disturbance on species composition, growth and soil organic carbon sequestration comparing early and late successional communities in two case study sites at opposite ends of the disturbance gradient. Our findings revealed that biocrusts are a conspicuous component of the Caatinga ecosystem with at least 50 different taxa of cyanobacteria, algae, lichens and bryophytes (cyanobacteria and bryophytes dominating) covering nearly 10% of the total land surface and doubling soil organic carbon content relative to bare topsoil. High litter cover, high disturbance by goats, and low soil compaction were the leading drivers for reduced biocrust cover, while precipitation was not associated Second-growth forests supported anequally spaced biocrust cover, while in old-growth-forests biocrust cover was patchy. Disturbance reduced biocrust growth by two thirds and carbon sequestration by half. In synthesis, biocrusts increase soil organic carbon (SOC) in dry forests and as they double the SOC content in disturbed areas, may be capable of counterbalancing disturbance-induced soil degradation in this ecosystem. As they fix and fertilize depauperated soils, they may play a substantial role in vegetation regeneration in the human-modified Caatinga, and may have an extended ecological role due to the ever-increasing human encroachment on natural landscapes. Even though biocrusts benefit from human presence in dry forests, high levels of anthropogenic disturbance could threaten biocrust-provided ecosystem services, and call for further, in-depth studies to elucidate the underlying mechanisms.
... While progress has been made since then, this notion still holds true, and is re-iterated from time to time (e.g. Duane Allen, 2010). The journey to discover the links between microbial community structure and terrestrial surface biosphere observations has only just began (Hamada et al., 2014). ...
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The paradigm and models of traditional soil science lack the ability to adequately address issues of soil dynamics, environmental integration, and change. Unexplainable research results obtained from traditional soil studies applied to non-traditional soil phenomena in physical geography, archaeology and ecology speak to the current need for soil science to move beyond description and classification and into a dynamic process-oriented soil science capable of providing explanations. Soils do not behave as static inert geologic detritus affected by climate, organisms, relief, and parent material through time, but instead soils behave as self-organizing systems dynamically interrelating with their environment. Recognition of this dynamic behaviour required a re-examination of how scientists in general think and in how modern soil science specifically evolved its basic paradigms and models. This book examines the dynamics of soil organic carbon and demonstrates the self-organizing nature of soil through time as soil responds to a wide range of environmental and human perturbations. Makes soil science accessible to a wider audience by integrating soil science with biology, geography and archaeology Demonstrates universal application by including case studies from around the world Avoids pitfalls of determinism and vitalism by being well founded in the philosophy of science.
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Where coastal topography is low and flat, the clouds dissipate inward over broad areas with little biological impact, but where isolated mountains or steep coastal slopes intercept the clouds, a fog-zone develops. This moisture allows the development of plant communities termed lomas formations. These floristic assemblages function as islands separated by hyperarid habitat devoid of plant life. Since growth is dependent upon available moisture, an understanding of climatic patterns is essential in efforts to interpret present-day plant distributions. Topography and substrate combine to influence patterns of moisture availability. The ecological requirements and tolerances of individual species ultimately determines community composition. Species endemism exceeds 40% and suggests that the lomas formations have evolved in isolation from their nearest geographic neighbors in the Andes. While the arid environment is continuous, there appears to be a significant barrier to dispersal between 18° and 22°S latitude in extreme N Chile, Less than 7% of a total flora, estimated at nearly 1000 species, occur on both side of this region. -from Authors
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The influence of environmental factors on the nitrogen fixation activity in soil and vegetation samples from different types of plant communities from the Sassen Valley (78°N, 16°E), Svalbard, Norway, was measured under controlled laboratory conditions using the acetylene reduction assay throughout the summers of 1997 and 2000. Samples for study were chosen from six sites along a 2-km-long transect representing different types of arctic vegetation. The influence of temperature, soil water content, and light intensity on acetylene reduction rates was studied. Samples from all sites showed low and almost constant acetylene reduction rates between 0 and 10°C. Above 10°C the activity of all samples increased rapidly and reached its maximum at about 25 and 32°C for the samples with free-living cyanobacteria and moss-associated cyanobacteria, respectively. There was a significant water-dependent increase of acetylene reduction activity for all types of vegetation. The samples showed a clear response to varying light conditions, i.e. a rapid decrease in acetylene reduction rates when light intensity decreased from 140 to 80 μmol m–2 s–l depending on the type of vegetation.
Chapter
Continental Antarctica possesses a characteristic flora and fauna, which varies both qualitatively and quantitatively among the different habitats. Bacteria and other micro-organisms are usually present in numbers far lower than those encountered in temperature regions. In a few areas of the Taylor and Wright dry valleys, no microbes could be detected, either microscopically or culturally. However, in the rookeries of Adélie penguins where organic matter is high and in areas either directly or indirectly contaminated by man, the numbers of bacteria found were within the range of temperate soils. There are a number of aspects of the physical and chemical environment which have a profound effect on growth and metabolism of the soil microflora. These same factors play an important role in limiting the flora to lichens and mosses as the highest type of plants and the growth of animals to no forms higher than insects and related arthropods. Metabolic activity can be demonstrated during the short growing season, although the rate is insignificant when equated to soil fertility and potential plant nitrogen. This activity cannot be ignored, however, for its products are possible food for other members of the food chain of this region.