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The Savory Method Can Not Green Deserts or Reverse Climate Change

DOI:10.2111/rangelands-d-13-00044.1
Authors:
David Briske at Texas A&M University
David Briske
Brandon Bestelmeyer at United States Department of Agriculture
Brandon Bestelmeyer
Joel R Brown at Jornada Experimental Range
Joel R Brown
  • Jornada Experimental Range
Samuel D. Fuhlendorf at Oklahoma State University - Stillwater
Samuel D. Fuhlendorf
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Abstract

The article presents a counter response to Allan Savory's statement that his planned grazing method was necessary to reverse desertification and climate change in the video 'How to green the world's deserts and reverse climate change' that was presented in session 7 at the 2013 TED (Technology, Entertainment, Design) Conference on February 27, 2013 in Long Beach, California. The pressing challenge is to develop broad approaches that can be implemented at multiple levels of social organization to minimize these pervasive and complex issues confronting rangeland sustainability. The tactic of discrediting science detracts from progress toward this goal, because it continues to oversimplify the complexity of rangeland systems and to promote narrowly focused technological solutions. Given that there are about five billion hectares of rangeland globally, it is relatively simple to calculate that each hectare of rangeland would have to sequester an additional two tons of C each year.
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73
October 2013
72 Rangelands
Allan Savory stated that his “planned grazing”
method was necessary to reverse two of the
world’s most challenging and interlinked global
change processes—desertification and climate
change—in the video “How to green the world’s deserts and
reverse climate change” that was presented in session 7 at the
2013 TED (Technology, Entertainment, Design) Confer-
ence on 27 February 2013 in Long Beach, California.i As
members of the scientific community, it is our obligation to
evaluate the claims made in this video relative to the existing
scientific information on this topic.
We find all of Mr Savory’s major claims to be unfounded and
we express deep concern that they have the potential to under-
mine proven, practical approaches to rangeland management and
restoration that are supported by a global community of prac-
titioners and scientists. The two major assumptions underlying
Mr Savory’s presentation are alone sufficient grounds for extreme
skepticism. First is that humans have misunderstood the basic
grassland–grazer relationships for centuries and that only he
knows the true nature of this relationship. Second is that use of
intensive, concentrated livestock grazing, specific to the method
that Mr Savory developed, is the only viable solution to reverse
desertification and climate change. In addition to challenging
these two highly problematic assumptions, we present a spe-
cific critique of three invalid arguments that Mr Savory used to
claim that his grazing method can reverse desertification, climate
change, and alleviate human suffering and death (19:00 in video).
Invalid Argument 1: All Nonforested Lands Are
Degraded
Mr Savory claims that all nonforested land on the planet
is degrading and that the reasons for degradation are not
understood (2:40 and 7:16 in video). This is absolutely false
i
A video of the talk is available online at: http://www.ted.com/talks/
allan_savory_how_to_green_the_world_s_deserts_and_reverse_
climate_change.html
in both respects because many rangelands are well managed
and deserts are a consequence of climate and soil factors,
in addition to inappropriate management. He justified this
claim by discrediting rangeland science as indicated in his
statement that “a century of rangeland management has
increased land degradation.” Global rangeland degradation
represents a serious concern, but it is often a consequence
of increasing human and livestock populations, land frag-
mentation, changes to land tenure, and poverty, rather than
invalid or insufficient scientific information.1 The press-
ing challenge is to develop broad approaches that can be
implemented at multiple levels of social organization to
minimize these pervasive and complex issues confronting
rangeland sustainability. The tactic of discrediting science
detracts from progress toward this goal, because it continues
to oversimplify the complexity of rangeland systems and to
promote narrowly focused technological solutions.2 How-
ever, this ploy is the only alternative available to Mr Savory
because his claims are not only unsupported by scientific
information, but they are often in direct conflict with it.3
Invalid Argument 2: Rangelands Can Store All
Fossil Fuel Carbon in the Atmosphere
Mr Savory’s claim that his grazing method can reduce atmo-
spheric carbon (C) concentrations to preindustrial levels (a 30%
reduction; 400 vs. 280 ppm CO2) (19:30 in video) is an enor-
mous misrepresentation of the global carbon cycle and climate
change science. Fossil fuel combustion, followed distantly by
deforestation, land conversion, and degradation are the major
contributors to increasing atmospheric C and global warming.
Consequently, strategies to offset climate change by increasing
C storage in soils and vegetation, described as C sequestration,
are extremely limited relative to the current rate of global C
emissions. Rangelands are known to be very weak sinks for at-
mospheric C because plant production is water limited and more
C is often released into the atmosphere from soil respiration
than is take up by vegetation, especially during drought periods.4
View Point
The Savory Method Can Not Green
Deserts or Reverse Climate Change
A response to the Allan Savory TED video
By David D. Briske, Brandon T. Bestelmeyer, Joel R. Brown, Samuel D. Fuhlendorf,
and H. Wayne Polley
73
October 2013
72 Rangelands
We present a few key values from the global C cycle to
identify the inaccuracy of Mr Savory’s claims regarding the
potential for his grazing method, or any grazing method, to
sequester C. In 2012, global greenhouse gas emissions were
estimated at about 50 billion metric tons (CO2 equivalents;
CO2e).1 In order to offset these current emissions, rangelands
would have to sequester approximately 13.6 billion tons of C
annually.ii Given that there are about five billion hectares of
rangeland globally, it is relatively simple to calculate that each
hectare of rangeland would have to sequester an additional
two tons of C each year. Credible estimates of the poten-
tial for rangeland C sequestration are generally less than 0.25
tons C per hectare per year, which is eight-fold less than Mr
Savory’s claims would require.
Even this estimate of the large discrepancy in rangeland
C sequestration is extremely conservative because of the in-
herent ecological limitations that control plant production
and C sequestration. Grass biomass is about 40% C, so that
sequestering 2.5 tons of C per hectare would require that ap-
proximately 6.25 tons per hectare (6,250 kg or 12,500 lbs) of
dry matter be produced each year. Given that the vast major-
ity of the world’s rangelands are arid and semiarid, the op-
portunities for achieving these levels of plant production are
all but impossible.5 In addition, only a portion of the C in
plant biomass production is stored in the soil as organic C,
while the majority is released back into the atmosphere as
CO2 from plant and soil microbial respiration (a cellular bio-
chemical process that releases carbon dioxide during energy
production, it is driven largely by temperature and soil water
availability6).
Finally, the capacities of soils to sequester C do not in-
crease indefinitely, but they encounter upper limits set by cli-
mate, vegetation, and soil characteristics. Most estimates set
the C sequestration potential of global rangelands between
one and two billion tons per year, a significant amount to
be sure, but hardly sufficient to offset current C emissions
(50 billion tons in 2012). In addition, rain-fed rangelands are
estimated to attain new upper limits on C sequestration in
about two decades following major improvements in man-
agement strategies. Consequently, currently recommended
rangeland management strategies place greater emphasis on
the conservation of existing soil C, rather than the sequestra-
tion of additional C.7
Invalid Argument 3: Intensive Grazing is
Necessary to Prevent Rangeland Degradation
The ecological benefits of concentrated livestock grazing or
“hoof action” to rangeland restoration and C sequestration
are grossly overstated and without supporting evidence, other
than for a few select photos.3 Two of the photos presented in
ii
The atomic weight of CO2 is 44: carbon = 12, oxygen = 16. Therefore
CO2 = 12 + 16 + 16 = 44. For our purposes, 13.6 billion tons of C must
be stored in the soil to offset 50 billion tons of atmospheric CO2 (50
× 0.27).
the video were misrepresented. One, occurring within Chaco
Culture National Historical Park, was identified in a web
comment on the TED talk by Bernard Foy (8 March 2013)
as an area that is slowly recovering from a historical period of
mismanaged grazing, rather than as a consequence of grazing
exclusion as indicated by Mr Savory. Another set of repeat
photographs, assembled by one of the authors (Bestelmeyer),
were inappropriately associated with Jornada Experimental
Range, but were actually of a small patch of desert grassland
within the Las Cruces International Airport in southwestern
New Mexico (toward the top of the “X” runways, which can
be found in Google Earth).
We briefly describe this misrepresentation of the desert
grassland example in southern New Mexico to disprove Mr
Savory’s claim that nonuse by livestock contributes to range-
land degradation. This area has been, as Mr Savory indicates,
ungrazed by domestic livestock since the 1950s and there
had been an obvious decline in grass cover in spite of grazing
exclusion (as of 2003). Wind erosion of sandy soils in the
surrounding desertified landscape was proposed as a cause
for this decline, rather than grazing exclusion.8 Wind moves
large amounts of sand that are deposited on and trigger ero-
sion around remnant plants, causing mortality and limiting
reproduction, even in the absence of grazing. This grass patch
is a relict within a desertifying landscape that only exists at all
because it was fenced.
The pertinent question in this case, however, is: could Mr
Savory’s grazing method have prevented or now restore this
remnant grassland patch? We have tested the effects of in-
tensive grazing and rest from grazing on the dominant grass,
black grama (Bouteloua eriopoda), in this desert grassland.9
Grass cover increases dramatically with rest and intensive
grazing delays this recovery. Most relevant to Mr Savory’s
claims, we found no evidence that grass cover had declined
with 13 years of rest in a noneroding landscape. Erosion and
drought can act suddenly in desert grasslands of the south-
western United States to produce widespread mortality of the
dominant perennial grass black grama.10 Fortunately, live-
stock producers of the region recognize that flexible grazing
management is needed to sustain these arid grasslands.
It is also useful to consider the proposed mechanisms by
which grazing should benefit perennial grasses like black
grama. Many of the soils in this desert grassland are sandy
and crusting does not limit infiltration, so the mechanical ac-
tion of hooves does not increase infiltration. In contrast, the
biological crusts implicated as a contributor to desertification
by Mr Savory are known to stabilize the sandy soil surface and
protect it from wind erosion and carbon loss.11,12 Similarly,
experimental data from Botswana confirm the importance of
soil biological crusts for cycling rangeland C and specifically
indicate that intensive grazing, which destroys these crusts
through trampling and burial, will adversely affect C seques-
tration and storage.13 This research concluded that managed
grazing, where soil surfaces are only lightly disturbed, would
help maintain a positive C balance in African rangelands.
PB
October 2013
74 Rangelands
Our Final Take
Progress regarding rangeland management is being made in
many portions of the globe, but there are significant environ-
mental, social, and political challenges to overcome, includ-
ing human population growth, climate change, poverty, war,
and inadequate education. Mr Savory’s attempts to divide
science and management perspectives and his aggressive pro-
motion of a narrowly focused and widely challenged graz-
ing method only serve to weaken global efforts to promote
rangeland restoration and C sequestration. The false sense of
hope created by his promises, expressly regarding some of the
most desperate communities, are especially troubling. Scien-
tific evidence unmistakably demonstrates the inability of Mr
Savory’s grazing method to reverse rangeland degradation or
climate change, and it strongly suggests that it might actually
accelerate these processes.
References
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gas emissions gap widening as nations head to crucial climate
talks in Doha. Available at: http://www.unep.org/newscentre/
default.aspx?DocumentID=2698&ArticleID=9335. Accessed
24 June 2013.
2. Briske, D. D., N. F. Sayre, L. Huntsinger, M. Fernandez-
Gimenez, B. Budd, and J. D. Derner. 2011. Origin, persis-
tence, and resolution of the rotational grazing debate: inte-
grating human dimensions into rangeland research. Rangeland
Ecology & Management 64:325–334.
3. Briske, D. D., J. D. Derner, J. R. Brown, S. D. Fuhlendorf,
W. R. Teague, K. M. Havstad, R. L. Gillen, A. J. Ash, and
W. D. Wil lms. 2008. Rotational grazing on rangelands: rec-
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dra, G. E. Schuman, P. L. Sims, and K. Snyder. 2008. Car-
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5. Yang, Y., J. Fang, W. Ma, and W. Wang. 2008. Relationship
between variability in aboveground net primary production and
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35:L23710.
6. Huxman, T. E., K. A. Knyder, D. Tissue, A. J. Leffler, K.
Ogle, W. T. Pockman, D. R. Sandquist, D. L. Potts, and S.
Schwinning. 2004. Precipitation pulses and carbon fluxes in
semiarid and arid ecosystems. Oecologia 141:254–268.
7. Booker, K., L. Huntsinger, J. W. Bartolome, N. F. Sayre,
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in the United State? Global Environmental Change 23:240–251.
8. Bestelmeyer, B. T., D. A. Trujillo, A. J. Tugel, and K. M.
Havstad. 2006. A multi-scale classification of vegetation dy-
namics in arid lands: What is the right scale for models, moni-
toring, and restoration? Journal of Arid Environments 65:296–
318.
9. Bestelmeyer, B. T., M. C. Duniway, D. K. James, L. M. Bur-
kett, and K. M. Havstad. 2013. A test of critical thresholds
and their indicators in a desertification-prone ecosystem: more
resilience than we thought. Ecology Letters 16:339–345.
10. Bestelmeyer, B. T., A. M. Ellison, W. R. Fraser, K. B. Gor-
man, S. J. Holbrook, C. M. Laney, M. D. Ohman, D. P. C.
Peters, F. C. Pillsbury, A. Rassweiler, R. J. Schmitt, and
S. Sharma. 2011. Analysis of abrupt transitions in ecological
systems. Ecosphere 2:art129.
11. Marticorena, B., G. Bergametti, D. Gillette, and J.
Belnap. 1997. Factors controlling threshold friction velocity in
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Authors are Professor, Dept of Ecosystem Science and Manage-
ment, Texas A&M University, College Station, TX 77843,
USA, dbriske@tamu.edu (Briske); Research Ecologist, USDA-
ARS, Jornada Experimental Range and Jornada Basin LTER,
New Mexico State University, Las Cruces, NM 88003, USA
(Bestelmeyer); Research Scientist, USDA-NRCS, Jornada Ex-
perimental Range, Las Cruces, NM 88003 (Brown); Starkey’s
Distinguished Professor, Dept of Natural Resources Ecology
& Management, Oklahoma State University, Stillwater, OK
74077, USA (Fuhlendorf); Research Ecologist, USDA-ARS,
Grassland, Soil & Water Research Laboratory, Temple, TX
76502, USA (Polley).
Rangelands 35(5):72–74
doi: 10.2111/RANGELANDS-D-13-00044.1
© 2013 The Society for Range Management
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Full-text available
  • Sep 2020
Herbivore-carnivore interactions are fundamental to grassland ecosystem functionality and to the human cultures that have long depended on these ecosystems for their nutrition. However, a large literature has developed during the past century indicating that animal agriculture is responsible for numerous negative environmental impacts. In this paper, I review literature on some of the environmental impacts of two different livestock management approaches, industrial-conventional (IC) management and regenerative-multi-paddock (RM) management. I consider the null hypothesis that the environmental impacts of ruminant livestock production are independent of the approach used to manage animals and grazing lands. It evident in the literature that managed grazing ecosystems are complex, and for certain system attributes, such as forage quality and plant community structure, the better management system is difficult to discern. In other areas definitive differences in impacts appear clearly management dependent. For instance, the soils of RM grasslands exhibit higher microbial biomass and diversity, and higher fungal: bacterial ratios than IC soils. Several impacts associated with livestock production appear to have less to do with grazing, per se, and more to do with support factors, such as feed production and manure management. The compilation of data from numerous sources suggests that RM management may reduce blue withdrawals and GHG emissions by >50%, relative to IC management. Accumulating data also suggest that a significant portion of anthropogenic CO2-eq emissions can be removed from the atmosphere and stored in the soil by applying RM management practices. Finally, it is suggested that while research design may affect the outcomes of some studies, the quality and quantity of the science may not resolve many discrepancies in the data. It is suggested that the viability and sustainability of animal agriculture may depend upon broadening the goals of practitioners to include both food production and the restoration and protection of agricultural ecosystem services.
The Society for Range Management: Bridging gaps and setting directions
Article
On the Ground •The Society for Range Management and the profession of rangeland ecology were founded about 80 years ago to bring scientific information to the management of rangelands. Sustaining a strong connection between science and management set the foundation for the rangeland profession, though this connection has been challenging to sustain. •An era of collaborative research and conservation has fueled projects that include a diversity of individuals and organizations and confirm the importance of information from both experimentation and experience. •Successfully addressing contemporary challenges to rangelands will depend on old-fashioned actions like conversation, a commitment to rangelands communities and landscapes, and engaging a wide variety of backgrounds and experiences to find solutions.
A Fair Share: Bibliography
Chapter
Full-text available
  • Jun 2018
Origin, Persistence, and Resolution of the Rotational Grazing Debate: Integrating Human Dimensions Into Rangeland Research
Article
Full-text available
  • Jul 2011
The debate regarding the benefits of rotational grazing has eluded resolution within the US rangeland profession for more than 60 yr. This forum examines the origin of the debate and the major reasons for its persistence in an attempt to identify common ground for resolution, and to search for meaningful lessons from this central chapter in the history of the US rangeland profession. Rotational grazing was a component of the institutional and scientific response to severe rangeland degradation at the turn of the 20th century, and it has since become the professional norm for grazing management. Managers have found that rotational grazing systems can work for diverse management purposes, but scientific experiments have demonstrated that they do not necessarily work for specific ecological purposes. These interpretations appear contradictory, but we contend that they can be reconciled by evaluation within the context of complex adaptive systems in which human variables such as goal setting, experiential knowledge, and decision making are given equal importance to biophysical variables. The scientific evidence refuting the ecological benefits of rotational grazing is robust, but also narrowly focused, because it derives from experiments that intentionally excluded these human variables. Consequently, the profession has attempted to answer a broad, complex question—whether or not managers should adopt rotational grazing—with necessarily narrow experimental research focused exclusively on ecological processes. The rotational grazing debate persists because the rangeland profession has not yet developed a management and research framework capable of incorporating both the social and biophysical components of complex adaptive systems. We recommend moving beyond the debate over whether or not rotational grazing works by focusing on adaptive management and the integration of experiential and experimental, as well as social and biophysical, knowledge to provide a more comprehensive framework for the management of rangeland systems. El debate sobre los beneficios del pastoreo rotativo ha eludido una resolución en el ámbito de la profesión del manejo de los pastizales naturales en los EE.UU. por más de 60 años. Este foro examina el origen del debate y las principales razones de su persistencia en un intento por identificar un terreno común para su resolución y para buscar lecciones sustantivas de este capítulo central de la historia de la profesión del manejo de los pastizales naturales de EE.UU. El pastoreo rotativo fue un componente de la respuesta institucional y científica a la severa degradación de pastizales que ocurrió a comienzos del siglo 20 y desde entonces se ha transformado en la norma profesional para el manejo del pastoreo. Quienes manejan el pastoreo han encontrado que los sistemas rotativos pueden facilitar el logro de objetivos de manejo diversos, pero los experimentos científicos han demostrado que los sistemas de pastoreo no necesariamente contribuyen al logro de objetivos ecológicos específicos. Estas interpretaciones parecen contradictorias, pero sostenemos que pueden ser reconciliadas si son evaluadas dentro del contexto de sistemas adaptativos complejos en los que las variables humanas tales como el fijar metas, el conocimiento empírico, y la toma de decisiones reciben la misma importancia que las variables biofísicas. La evidencia científica refutando los beneficios ecológicos del pastoreo rotativo es robusta, pero con un foco estrecho, porque proviene de experimentos en los que se excluyeron intencionalmente las variables humanas. Por lo tanto, la profesión ha intentado dar respuesta a una pregunta amplia y compleja - si se debiera o no adoptar el pastoreo rotativo — con un enfoque necesariamente estrecho de investigación de procesos ecológicos. El debate sobre el pastoreo rotativo persiste porque la profesión de manejo de los pastizales aún no ha generado un marco de manejo e investigación que pueda incorporar tanto los componentes sociales como biofísicos de sistemas adaptativos complejos. Recomendamos ir más allá del debate sobre si el pastoreo rotativo "funciona" o no, y enfocar el debate en el manejo adaptativo y la integración de conocimiento experimental y empírico, así como el conocimiento social y biofísico, para proveer un marco más amplio para el manejo de sistemas de pastizales naturales.
What can ecological science tell us about opportunities for carbon sequestration on arid rangelands in the United States?
Article
Full-text available
Scientific interest in carbon sequestration on rangelands is largely driven by their extent, while the interest of ranchers in the United States centers on opportunities to enhance revenue streams. Rangelands cover approximately 30% of the earth's ice-free land surface and hold an equivalent amount of the world's terrestrial carbon. Rangelands are grasslands, shrublands, and savannas and cover 312 million hectares in the United States. On the arid and semi-arid sites typical of rangelands annual fluxes are small and unpredictable over time and space, varying primarily with precipitation, but also with soils and vegetation. There is broad scientific consensus that non-equilibrium ecological models better explain the dynamics of such rangelands than equilibrium models, yet current and proposed carbon sequestration policies and associated grazing management recommendations in the United States often do not incorporate this developing scientific understanding of rangeland dynamics. Carbon uptake on arid and semi-arid rangelands is most often controlled by abiotic factors not easily changed by management of grazing or vegetation. Additionality may be impossible to achieve consistently through management on rangelands near the more xeric end of a rangeland climatic gradient. This point is illustrated by a preliminary examination of efforts to develop voluntary cap and trade markets for carbon credits in the United States, and options including payment for ecosystem services or avoided conversion, and carbon taxation. A preliminary analysis focusing on cap and trade and payment for avoided conversion or ecosystem services illustrates the misalignment between policies targeting vegetation management for enhanced carbon uptake and non-equilibrium carbon dynamics on arid United States rangelands. It is possible that current proposed carbon policy as exemplified by carbon credit exchange or offsets will result in a net increase in emissions, as well as investment in failed management. Rather than focusing on annual fluxes, policy and management initiatives should seek long-term protection of rangelands and rangeland soils to conserve carbon, and a broader range of environmental and social benefits.
Rotational Grazing on Rangelands: Reconciliation of Perception and Experimental Evidence
Article
Full-text available
  • Jan 2008
In spite of overwhelming experimental evidence to the contrary, rotational grazing continues to be promoted and implemented as the only viable grazing strategy. The goals of this synthesis are to 1) reevaluate the complexity, underlying assumptions, and ecological processes of grazed ecosystems, 2) summarize plant and animal production responses to rotational and continuous grazing, 3) characterize the prevailing perceptions influencing the assessment of rotational and continuous grazing, and 4) attempt to direct the profession toward a reconciliation of perceptions advocating support for rotational grazing systems with that of the experimental evidence. The ecological relationships of grazing systems have been reasonably well resolved, at the scales investigated, and a continuation of costly grazing experiments adhering to conventional research protocols will yield little additional information. Plant production was equal or greater in continuous compared to rotational grazing in 87% (20 of 23) of the experiments. Similarly, animal production per head and per area were equal or greater in continuous compared to rotational grazing in 92% (35 of 38) and 84% (27 of 32) of the experiments, respectively. These experimental data demonstrate that a set of potentially effective grazing strategies exist, none of which have unique properties that set one apart from the other in terms of ecological effectiveness. The performance of rangeland grazing strategies are similarly constrained by several ecological variables establishing that differences among them are dependent on the effectiveness of management models, rather than the occurrence of unique ecological phenomena. Continued advocacy for rotational grazing as a superior strategy of grazing on rangelands is founded on perception and anecdotal interpretations, rather than an objective assessment of the vast experimental evidence. We recommend that these evidence-based conclusions be explicitly incorporated into management and policy decisions addressing this predominant land use on rangelands.
Relationship between variability in aboveground net primary production and precipitation in global grasslands. Geophysical Research Letters
Article
Full-text available
Aboveground net primary production (ANPP) is strongly correlated with annual precipitation (AP) in grassland ecosystems. However, the relationship between the interannual variation in ANPP and the variability in precipitation remains controversial. In this study, we used long-term data of biomass and precipitation from 118 sites across global grasslands to examine the relationship between variability in ANPP and AP. Our results showed that ANPP increased with precipitation, but leveled off in humid regions, and that increased variation in precipitation led to an increase in the variability in ANPP. The relative ANPP maxima significantly increased with relative AP maxima and the relative ANPP minima also positively correlated with relative AP minima. These suggest that the fluctuations in precipitation can alter the growth of grasslands, which should be incorporated into the prediction and modeling of climate changes.
Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana
Article
Full-text available
Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO(2) efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO(2) efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO(2) efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO(2) efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO(2) efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.
Carbon Fluxes on North American Rangelands
Article
Full-text available
  • Sep 2008
Rangelands account for almost half of the earth's land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.
Rotational Grazing on Rangelands: Reconciliation of Perception and Experimental Evidence
Article
Full-text available
  • Jan 2008
In spite of overwhelming experimental evidence to the contrary, rotational grazing continues to be promoted and implemented as the only viable grazing strategy. The goals of this synthesis are to 1) reevaluate the complexity, underlying assumptions, and ecological processes of grazed ecosystems, 2) summarize plant and animal production responses to rotational and continuous grazing, 3) characterize the prevailing perceptions influencing the assessment of rotational and continuous grazing, and 4) attempt to direct the profession toward a reconciliation of perceptions advocating support for rotational grazing systems with that of the experimental evidence. The ecological relationships of grazing systems have been reasonably well resolved, at the scales investigated, and a continuation of costly grazing experiments adhering to conventional research protocols will yield little additional information. Plant production was equal or greater in continuous compared to rotational grazing in 87% (20 of 23) of the experiments. Similarly, animal production per head and per area were equal or greater in continuous compared to rotational grazing in 92% (35 of 38) and 84% (27 of 32) of the experiments, respectively. These experimental data demonstrate that a set of potentially effective grazing strategies exist, none of which have unique properties that set one apart from the other in terms of ecological effectiveness. The performance of rangeland grazing strategies are similarly constrained by several ecological variables establishing that differences among them are dependent on the effectiveness of management models, rather than the occurrence of unique ecological phenomena. Continued advocacy for rotational grazing as a superior strategy of grazing on rangelands is founded on perception and anecdotal interpretations, rather than an objective assessment of the vast experimental evidence. We recommend that these evidence-based conclusions be explicitly incorporated into management and policy decisions addressing this predominant land use on rangelands.
A test of critical thresholds and their indicators in a desertification-prone ecosystem: More resilience than we thought
Article
Theoretical models predict that drylands can cross critical thresholds, but experimental manipulations to evaluate them are non-existent. We used a long-term (13-year) pulse-perturbation experiment featuring heavy grazing and shrub removal to determine if critical thresholds and their determinants can be demonstrated in Chihuahuan Desert grasslands. We asked if cover values or patch-size metrics could predict vegetation recovery, supporting their use as early-warning indicators. We found that season of grazing, but not the presence of competing shrubs, mediated the severity of grazing impacts on dominant grasses. Recovery occurred at the same rate irrespective of grazing history, suggesting that critical thresholds were not crossed, even at low cover levels. Grass cover, but not patch size metrics, predicted variation in recovery rates. Some transition-prone ecosystems are surprisingly resilient; management of grazing impacts and simple cover measurements can be used to avert undesired transitions and initiate restoration.
A multi-scale classification of vegetation dynamics in arid lands: What is the right scale for models, monitoring, and restoration?
Article
  • Apr 2006
Measurements of vegetation and soil dynamics used to anticipate (or reverse) catastrophic transitions in arid and semi-arid rangelands are often difficult to interpret. This situation is due, in part, to a lack of empirically based conceptual models that incorporate the effects of multiple processes, scale, spatio-temporal pattern, and soils. Using observations of multi-temporal data from the Chihuahuan Desert, we describe a new approach to classifying vegetation dynamics based on multiple scales of vegetation and soil pattern as well as cross-scale interactions. We propose the existence of six types of mechanisms driving vegetation change including (1) stability, (2) size oscillation of plants, (3) loss and reestablishment of plants within functional groups, (4) loss of one plant functional group and replacement by another, (5) spatial reorganization of vegetation patches, and (6) cascading transitions that spread from small to broad scales. We provide evidence for the existence of these mechanisms, the species involved, and the geomorphic components on which they are observed in the Chihuahuan Desert. These mechanisms highlight the kinds of multi-scale observations that are needed to detect or interpret change and emphasize the importance of soil surface properties for interpreting vegetation change. The classification is potentially general across arid and semi-arid ecosystems and links spatial and temporal patterns in vegetation with ecological and geomorphic processes, monitoring, and restoration strategies.
Analysis of abrupt transitions in ecological systems
Article
  • Dec 2011
The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them.We apply such an approach to 30–60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.