United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical 

Report

RMRS-GTR-316

February 2014

Review of Climate Change Impacts on 

Future Carbon Stores and Management 

of Warm Deserts of the United States

Michell L. Thomey, Paulette L. Ford, Matt C. Reeves, 

Deborah M. Finch, Marcy E. Litvak, and Scott L. Collins
Thomey, Michell L.; Ford, Paulette L.; Reeves, Matt C.; Finch, Deborah M.; Litvak, Marcy E.; Collins, Scott L. 2014. Review of 

climate change impacts on future carbon stores and management of warm deserts of the United States. Gen. Tech. Rep. 

RMRS-GTR-316. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 26 p.

Abstract

Reducing atmospheric carbon dioxide (CO2) concentration through enhanced terrestrial carbon storage may help slow or reverse 

the rate of global climate change. As a result, Federal land management agencies, such as the U.S. Department of Agriculture Forest Service and U.S. Department of the Interior Bureau of Land Management, are implementing management policies to increase 

carbon storage. However, information on how projected southwestern climate changes might affect the balance between CO2

 uptake 

and loss on semiarid rangelands is not easily accessible to land managers. We summarize studies that focus on key components 

of carbon exchange, including photosynthesis, soil respiration, and plant productivity, across the warm deserts of North America to 

determine if common trends exist that can be utilized in management. We also provide an overview of how management practices 

can influence carbon sequestration in this region and discuss the U.S. Department of Agriculture Forest Service Climate Change 

Scorecard. Since desertification is projected to increase in the future, management strategies that increase carbon sequestration 

or decrease carbon loss are especially important. This requires managers to thoughtfully consider management practices that do 

not impede sequestration during critical times. For a popular version of the GTR see Rangelands February 2014.

Keywords: Global climate change; carbon dioxide (CO2

); carbon sequestration in arid-semiarid Ecosystems; Mojave, Sonoran 

and Chihuahuan Deserts; land management to increase carbon storage

Authors

Michell L. Thomey received her Ph.D. from the University of 

New Mexico in 2012. Currently, she is a Postdoctoral Associate 

in the Department of Biology at the University of New Mexico 

where she oversees the Extreme Drought in Grasslands Experiment (EDGE). Her research is focused on understanding 

the linkages between environmental change and plant-plant 

interactions and feedbacks to plant community structure and 

energy and carbon fluxes typically at transition zones.

Paulette L. Ford is a Research Ecologist with the USDA Forest 

Service, Rocky Mountain Research Station in Albuquerque, New 

Mexico. She received her M.S. in Biology from the University of 

New Mexico, and her Ph.D. in Renewable Natural Resources 

Studies and Ecology and Evolutionary Biology from the University of Arizona. Her interests include the role of disturbance 

(fire and drought) in structuring grassland, shrubland and desert 

communities. Paulette’s long-term (18-year) research uses an 

experimental framework to analyze the effects of season and 

frequency of fire on shortgrass steppe under a changing climate.

Matt C. Reeves is a Post-Doc Research Ecologist with the 

USDA, Forest Service, Rocky Mountain Research Station, 

Missoula Montana. He received his BS in Range Management, MS in Environmental Resources and his PhD in Remote 

Sensing and Ecological modeling of non-forest and agricultural 

environments. As a rangeland professional, he studies effects 

of climate change, develops decision support systems and 

does large area inventory and monitoring. At a young age, 

Dr. Reeves was keenly interested in plants, animals and their 

interactions and spent countless hours exploring the outdoors. 

He is an avid conservationist and a consummate backpacker, 

hunter, and fisherman. 

Deborah M. Finch received her Bachelor’s Degree in Wildlife 

Management from Humboldt State University, Arcata, CA, her 

Master’s in Zoology and Physiology from Arizona State University, Phoenix, and her Ph.D. in Zoology and Range Science 

from University of Wyoming, Laramie. Deborah was a research 

wildlife biologist employed by the USDA Forest Service, Rocky 

Mountain Research Station from 1978 to 2010, and is now 

a science program manager. Her research interests include 

ecosystem restoration using prescribed fire and fuel removal, 

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ecosystem ecology, climate change, avian reproductive ecology 

and habitat relationships, invasive plants, at-risk species, and 

technology transfer. She supervises and coordinates a team 

of scientists who conduct research on restoration methods, fire 

ecology, invasive species, wildlife ecology and climate change. 

Marcy E. Litvak received her PhD from the University of 

Colorado in 1999. She was an Assistant Professor in the Section of Integrative Biology at the University of Texas in Austin 

from 2001-2004, then moved to the Biology Department at the 

University of New Mexico in 2005 where she was an Assistant 

and now Associate Professor. She is currently Associate Chair 

of the Biology Department at University of New Mexico. Her 

research focuses on understanding how large scale ecosystem 

processes such as carbon sequestration, evapotranspiration, 

and energy balance vary across ecological gradients in the 

Southwestern US.

Scott L. Collins received his PhD from the University of 

Oklahoma in 1981. From 1985-1994 he was an Assistant and 

Associate Professor of Botany at the University of Oklahoma. 

In 1992 he moved to the National Science Foundation where he 

served as a Program Director in the Division of Environmental 

Biology for multiple research programs. In 2003 he moved to the 

University of New Mexico where he is now a Regent’s Professor 

of Biology and the Loren Potter Chair of Plant Ecology. Collins 

is also Chair of the US Long-term Ecological Research Network 

and Past President of the Ecological Society of America. His 

research focuses on the interactive effects of fire, grazing and 

climate change on grassland ecosystems.

Acknowledgments

Funding in support of this research was provided by a grant 

from the USDA Forest Service, Western Wildland Environmental 

Threat Assessment Center. This research was completed in 

partial fulfillment of the requirements for the degree of Doctor 

of Philosophy from the University of New Mexico. We thank 

Jesse Nippert for providing comments to the dissertation 

chapter; Sandra Brantley, Rosemary Pendleton, Linda Joyce, 

and Carly Woodlief for comments or reviews; and Yancey 

Ranspot for technical help. We thank Dr. Zhiliang Zhu and his 

LANDCARBON team for enabling us to use data for Figure 2. 

Contents

Introduction................................................................................................... 1

The Deserts of North America........................................................................ 3

Carbon Cycling in North American Warm Deserts ........................................ 7

Precipitation.................................................................................................. 7

CO2 .................................................................................................... 8

Warming ....................................................................................................... 9

Environmental Hurdles................................................................................ 10

Disturbance.................................................................................... 11

Invasive Species.............................................................................. 12

Management and Policy Implications........................................................... 13

Carbon Sequestration and Land Management.............................................. 13

The Climate Change Scorecard.................................................................... 13

Carbon Sequestration and Storage Defined.................................................. 14

Measuring Stored Carbon ............................................................................ 15

Monitoring Considerations .......................................................................... 16

Effects of Management Actions and Disturbances on Carbon Storage .......... 16

Balancing Carbon Sequestration with Other Services................................... 18

Literature Cited............................................................................................ 19

Appendix: Table of Abbreviations................................................................. 26

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 1

Review of Climate Change Impacts on Future Carbon Stores 

and Management of Warm Deserts of the United States

Michell L. Thomey, Paulette L. Ford, Matt C. Reeves, Deborah M. Finch, 

Marcy E. Litvak, and Scott L. Collins

Introduction _____________________________________

In the terrestrial carbon cycle, CO2 acquired from the atmosphere through photosynthesis is assimilated in plants (GPP, Appendix 1) while CO2 is also emitted back to the 

atmosphere as the byproduct of autotrophic and heterotrophic respiration (hereafter, 

Reco) (Kondratyev and others 2003; Adams 2009). Together, these metabolic processes 

serve as the engine that drives the terrestrial carbon cycle (Chapin and others 2002).

Because of fossil CO2 emissions, ecosystems are subject to shifts in climate patterns 

(Karl and others 2009; Allison and others 2011). While the degree of impact may vary 

by latitude, increasing CO2 concentrations, warmer temperatures, and changes in the 

frequency of extreme weather and climate events will alter the balance between GPP 

(net CO2 uptake) and Reco (CO2 loss) or net ecosystem exchange (NEE) and will influence the amount of CO2 remaining in the atmosphere (Adams 2009).

Although arid-semiarid regions are characterized by low primary productivity 

(Figure 1), they cover ~45% of the land surface (Schimel 2010) and, therefore, collectively contribute significantly to the global carbon cycle. This biome stores ~199 Pg C 

in vegetation and in soil organic carbon (Janzen 2004). Moreover, the soils also contain 

inorganic carbon as carbonate (CaCO3), which sequesters an additional 800-1700 Pg C 

(Schlesinger and others 2000). For this reason, arid-semiarid soils are considered the 

third largest global pool of carbon (Emmerich 2003). In these water-limited ecosystems, 

CO2 loss through Reco frequently exceeds GPP as net CO2 uptake is limited to periods of 

favorable rainfall (Anderson-Teixeira and others 2011). Arid-semiarid ecosystems are 

highly responsive to precipitation variability and, therefore, will be affected by predicted 

increases in extreme precipitation events, increased CO2 concentrations, and increased 

temperatures (Smith and others 1997; Heisler-White and others 2008; Diffenbaugh and 

others 2008). Therefore, it is important to understand how these elements of climate 

change might affect NEE (Figure 2).

In this review, we summarize the literature from the warm deserts of North America 

and consider leaf (Anet), soil (Rs), and plant community (aboveground net primary 

productivity; ANPP) studies from the Mojave, Sonoran, and Chihuahuan Deserts to 

determine if common trends exist across measurement scales that can be utilized to 

effectively manage the warm deserts. Furthermore, we investigate the potential management strategies that may increase carbon sequestration or minimize carbon losses in 

this region. We begin by describing the warm deserts in the southwestern United States 

and consider climate change projections for this region. We then examine how climate 

change may affect NEE and carbon sequestration.
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 Figure 1—Rangeland net primary production estimated from the Moderate Resolution Imaging Spectroradiometer (MODIS) NPP Collection 4.5 product (Running and others 2004). Note the predominantly low net primary production that characterizes deserts. 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 3

Figure 2—Soil organic carbon adapted from the LANDCARBON Project (Zhu and Reed [2012]). Note the relationship between mean annual precipitation and soil organic carbon. Generally speaking, greater precipitation enables greater rates of 

carbon accumulation. Mean annual precipitation was generated from the Parameter-elevation Regressions on Independent 

Slopes Model (PRISM) data (Daly and others [2001]) (http://www.prism.oregonstate.edu/). Deserts exhibit both low annual 

precipitation and low carbon storage ability relative to other biomes. 

The Deserts of North America ______________________

The North American Desert biome is divided into cold and warm deserts. The cold 

desert, including the Great Basin and Colorado Plateau, is primarily located in Nevada 

but also extends into portions of Utah, Colorado, Arizona, New Mexico, Oregon, Washington, Idaho, and Wyoming (Barbour and Billings 2000). Compared to warm deserts, 

cold deserts: occur at higher elevations, have more days with temperatures that reach 

freezing, receive precipitation that mostly falls as snow during the winter, and maintain 

deeper soil water recharge due to lower evapotranspiration and the slow release of water 

through snowmelt (Smith and others 1997). The warm deserts are divided into three 

types: (1) Mojave (Figure 3), (2) Sonoran (Figure 4), and (3) Chihuahuan (Figure 5). 

While the largest area of the Sonoran and Chihuahuan Deserts is located in Mexico 

(Figure 1), in the United States these desert types are distributed from Nevada into 

southern California (Mojave) and east through southern Arizona (Sonoran), southern 

New Mexico (Chihuahuan), and western Texas (Chihuahuan). We limit the remainder 

of our discussion to the warm desert types.
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

Figure 3—Mojave Desert. Photo credit: Ken Cole, U.S. Geological Survey.

Figure 4—Sonoran Desert. Photo credit: U.S. Fish and Wildlife Service.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 5

A common trait of the Mojave, Sonoran, and Chihuahuan Deserts is that they are 

located on the leeward (e.g., rain shadow) side of the Sierra Nevada, Peninsular and 

Sierra Madre Occidental, and Sierra Madre Oriental mountain ranges, respectively. 

Another shared trait is that precipitation is spatially and temporally variable and occurs 

in discrete packages (i.e., “pulses”) that initiate biotic activity followed by a dry interpulse period when the ecosystem returns to a less biologically active state (Noy-Meir 

1973). Finally, in the southwestern United States, short-term variability in winter rainfall 

derived from the Pacific is largely influenced by the El Niño and La Niña phases of the 

Southern Oscillation (Merideth 2001). The El Niño phase is largely associated with the 

high pressure system over Darwin, Australia, that temporarily reverses position with 

the low pressure system located over Tahiti (Barry and Chorley 2003). This exchange 

of pressure systems results in the breakdown of the trade winds, warmer (SST) along 

the coast of South America, and a low pressure zone of rising air and increased precipitation, leading to above-average rainfall across the warm deserts (Merideth 2001). In 

contrast, the La Niña phase is characterized by a strengthening of the trade winds that 

bring below-normal SST along the coast of South America and above-normal SST to 

the western Pacific Ocean (Duxbury and Duxbury 1993). This shift in SST results in the 

northward displacement of the subtropical and mid-latitude jet streams, leading to dry 

winters (Duxbury and Duxbury 1993; Barry and Chorley 2003) across the warm deserts. 

Beyond these similarities, the warm deserts are distinguished by climatic differences. 

Two main differences are: (1) the precipitation gradient with rainfall increasing from the 

Figure 5—Chihuahuan Desert. Photo credit: Sevilleta LTER Program.
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

Mojave Desert east to the Chihuahaun Desert, and (2) the seasonality of precipitation 

with a winter dominant rainfall in the Mojave, summer and winter dominant rainfall 

(bimodal) in the Sonoran, and summer dominant rainfall in the Chihuahuan Desert with 

some winter precipitation inputs.

The Mojave Desert is the driest of the warm deserts. The mean annual temperature 

range is from 1.5 to 28.7 °C and winter snowfall can occur (Beatley 1974; Rundel and 

Gibson 1996). Average rainfall is 137-168 mm/year and is characterized by a winter 

dominant rainfall pattern in which most rainfall occurs from October through April as 

a result of west-to-east frontal systems originating over the Pacific Ocean (Smith and 

others 1997; Hereford and others 2006). This rainfall pattern is particularly important 

for the C3 perennial shrubs (Larrea tridentata, Ambrosia dumosa, Atriplex hymenelytra, 

A. polycarpa, and Coleogyne ramosissima) that dominate the landscape (Beatley 1974; 

Rundel and Gibson 1996; MacMahon 2000). The eastern margins of the Mojave can 

experience a winter/summer (e.g., bimodal) rainfall pattern when the NAMS (North 

American Monsoon System) expands into the region (Rundel and Gibson 1996). For 

example, Hereford and others (2006) found a winter dominant rainfall pattern at 90% of 

weather stations located west of 117°W longitude (Barstow, California) while a bimodal 

rainfall pattern occurred at 70% of the weather stations located east of this longitude. 

The eastern region is also referred to as the Colorado Desert and is considered to be an 

extension of the Sonoran Desert (Schoenherr 1992).

The Sonoran Desert is the warmest of the desert types and is defined as subtropical 

in nature (Rundel and Gibson 1996). Because this desert type rarely has freezing temperatures or winter snowfall, it is distinguished, in part, by cold intolerant succulents 

(Carnegiea gigantea). In addition, the Sonoran Desert supports a variety of tree species (Cericdium macrophyllum, Acacia greggii, Olneya tesota, Prosopis glandulosa, 

and Fouquieria splendens), perennial grasses (Pleuraphis rigida), and shrubs (Hyptis 

emoryi, Atriplex ploycarpa, and Larrea tridentata) (Nielson 1987; MacMahon 2000). 

The annual temperatures range from 12 °C to 30 °C (Smith and others 1997). While 

rainfall varies across the region, mean annual rainfall is 258-311 mm/year and occurs 

in a bimodal pattern (Bahre and Shelton 1993; MacMahon 2000). In contrast to the 

Mojave, summer rainfall in the Sonoran Desert is produced by the NAMS (Smith and 

others 1997). During the summer monsoon season (July-September), the Bermuda high 

pressure cell shifts from the Atlantic coast to the northwest. Meanwhile, land and ocean 

temperatures diverge, resulting in a shift in wind patterns. This brings in moisture mostly 

from the Gulf of Mexico and also from the Gulf of California giving way to convective 

storm patterns (Barry and Chorley 2003). Overall, this bimodal precipitation pattern 

leads to high plant diversity, including winter active as well as summer active species 

(Barbour and Billings 2000).

The Chihuahuan Desert is located at the wettest end of the precipitation gradient. This 

desert community exhibits higher elevations and cooler temperatures than the Mojave 

and Sonoran Deserts (Smith and others 1997). The majority of precipitation in this region 

falls during the summer monsoon season (July-September) as a result of the NAMS, 

but winter rainfall and snowfall results in a weak bimodal signal. Annual precipitation 

averages 250 mm/year (77-507 mm/year) and the mean annual temperature is 14.7 °C 

(13.5 °C-16.3 °C) (Wainwright 2006). These climate factors contribute to a marked 

increase in perennial grass dominance (Bouteloua eriopoda, B. gracilis, Sporobolus

spp., Pleuraphis jamesii, Muhlenbergia spp., and Aristida purpurea) (MacMahon 2000; 

Whitford 2002; Peters and Gibbens 2006). Other dominant species are Larrea tridentata, 

Gutierrezia sarothrae, and Ephedra trifurca.

Throughout the southwestern United States, climate models consistently project increased aridity, seasonal shifts in precipitation, and more extreme precipitation events 

(Diffenbaugh and others 2005; Seager and others 2007; Schoof and others 2010). Already, 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 7

temperatures are increasing faster in the southwestern United States than anywhere in 

the nation (USGCRP 2009). While droughts and megadroughts are a trait of the regional 

climate, a poleward shift in the subtropical dry zones, for example, would give way to 

a drying pattern that the Southwest has never experienced (Fawcett and others 2011; 

Seager and others 2007). Moreover, with a novel base state of aridity, these ecosystems 

may not be able to recover from periodic drought events (Gutzler and Robbins 2010).

Carbon Cycling in North American Warm Deserts______

Precipitation

For each warm desert type, the majority of precipitation events are ≤5 mm and primarily increase microbial activity and CO2 efflux to the atmosphere, whereas precipitation 

events ≥10 mm are required for GPP to exceed Reco (Huxman and others 2004; Loik and 

others 2004; Kurc and Small 2007; Svejcar and others 2008). Between rainfall events, 

GPP and microbial activity decrease although a substantial portion of deep autotrophic 

root respiration from >20-50 cm depth can occur in desert shrublands (Breecker and 

others 2012). At the soil surface, inorganic nitrogen, microbial biomass, and soil organic 

matter accumulate (Austin and others 2004). When dry soil is re-wet with a large (≥10 

mm) precipitation event, microbial activity quickly increases but net CO2 uptake is 

not immediate due to plant acclimation and growth of new roots and leaves (Ogle and 

Reynolds 2004). In the Sonoran and Chihuahuan Deserts, there is consistently a two- to 

three-day lag following a precipitation event before GPP surpasses Reco losses (Potts 

and others 2006; Kurc and Small 2007; Scott and others 2010; Hamerlynck and others 

2011). The delayed increase in NEE may also vary due to differences in soil moisture 

and the growing season. For example, Wohlfhart and others (2008) observed a three- to 

four-day lag prior to an increase in NEE following a summer rainfall event in the Mojave 

Desert, while NEE increased within the same day following a spring rainfall event that 

occurred at peak soil moisture and primary productivity.

Regardless of plant community structure, similar to ANPP, the variability in NEE is 

mostly driven by large rainfall events (Kurc and Benton 2010). This trend is intuitive 

in the Mojave Desert where deeper rooted C3 shrubs are dominant, but it is less obvious in desert grasslands where the majority of C4 roots are located in the top 30 cm of 

the soil (Gibbens and Lenz 2001; McCulley and others 2004). Nevertheless, research 

indicates that a significant increase in Anet of the dominant Chihuahuan Desert grass, 

Bouteloua eriopoda, in response to small (5 mm) rainfall events does not always translate 

into a significant increase in ANPP or NEE (Kurc and Small 2007; Anderson-Teixeira 

and others 2011; Thomey and others 2011). Another factor that contributes to the variability in NEE is leaf area (Flanagan and others 2002; Polley and others 2010a). At the 

leaf level, research shows that native grass Anet development tracks leaf area in desert 

grasslands (Ignace and others 2007; Hamerlynck and others 2011). In a semiarid riparian ecosystem, the variability in GPP under drought conditions was attributed to leaf 

area in grassland as opposed to stomatal adjustments in woodlands (Jenerette and others 

2009). Moreover, Comstock and Ehleringer (1986) deemed total plant canopy leaf area 

in Encelia frutescens as the main factor limiting whole-plant carbon gain. Likewise, at 

the community scale, Knapp and Smith (2001) also noted that ANPP is constrained by 

leaf area and plant density in Chihuahuan Desert grassland. Thus, the variability in NEE 

across the warm deserts is attributed to large precipitation events that extend periods of 

increased soil water availability leading to improved leaf area and net CO2 uptake. In 

contrast, small precipitation events can trigger a physiological response (e.g., increase 

Anet) but are quickly lost by evapotranspiration. Although water is the most limiting 

resource in desert ecosystems, a threshold exists such that precipitation pulses may 
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

initiate Anet and limited plant growth (ANPP) but net CO2 uptake does not surpass Reco

(Fernández 2007; Muldavin and others 2008; Kurc and Small 2007).

In addition to pulse size, antecedent soil moisture along with the seasonal timing of 

rainfall affects CO2 uptake. In the Sonoran Desert, the magnitude of the response of Anet

was greater when water was added to dry soil when compared to wet soils where desert 

grasses had already reached their physiological capacity (Ignace and others 2007). Similarly, Patrick and others (2009) observed that the Chihuahuan Desert shrub, Dasylirion 

leiophyllum, did not increase Anet when additional water was supplemented to wet soils. 

This trend is also reflected in NEE but for different reasons. For example, CO2 uptake 

was enhanced when a large rainfall event occurred following a pre-monsoon season 

drought because Reco was limited likely by substrate supply or nitrogen availability (Austin and others 2004; Potts and others 2006). The response of CO2 uptake to the seasonal 

timing of rainfall is often confounded by the dominant plant type. For example, desert 

grassland ANPP is strongly correlated with summer (August) precipitation, and ANPP 

in desert shrublands responds mostly to winter precipitation (Cable 1975; Huenneke and 

others 2002; Muldavin and others 2008). This affects NEE in several ways. In desert 

grasslands, NEE will likely not increase if above-average rainfall is evenly distributed 

throughout the year when grasses are dormant rather than falling during the peak summer growing season (Mielnick and others 2005). On the other hand, desert shrublands 

act as a strong source of CO2 if below-average rainfall occurs during the winter season 

(Scott and others 2009). In general, soil moisture and seasonal patterns of precipitation 

affect the biotic response and drive the variability in NEE (Polley and others 2010b).

CO2

Arid-semiarid ecosystems are projected to be the most responsive to elevated CO2

concentrations largely because decreased gs will lead to higher plant WUE (Mooney 

and others 1991; Melillo and others 1993; Smith and others 1997). In arid environments, these physiological adjustments have the potential to decrease ET and increase 

θ (Morgan and others 2004). However, the effect of CO2 extends beyond WUE (Field 

and others 1995). For example, elevated CO2 concentrations are predicted to increase 

primary productivity, particularly in C3 species due to the suppression of oxygenase 

activity of Rubisco (Lambers and others 2008). Litter quality is another factor that may 

be impacted as higher CO2 concentrations increase C:N ratios and decrease decomposition as well as soil N availability (Strain and Bazzaz 1983; Mooney and others 1991). 

The impact of increased atmospheric CO2 concentrations will vary by ecosystem (Field 

and others 1995).

The most intensive experiment to study the response of arid ecosystems to elevated 

CO2 occurred in the Mojave Desert at the Nevada Desert Free-Air CO2 Enrichment 

Facility (NDFF). Experimental plots were continuously exposed to CO2 concentrations 

of ~550 μmol mol-1. Following seven years of treatment, the highest mean daily NEE 

in elevated CO2 plots occurred during periods of peak plant cover or high soil moisture 

(Jasoni and others 2005). Soil moisture remained an important driver of the effects of 

CO2 throughout all studies at the NDFF. For instance, belowground root respiration of 

the dominant shrubs Larrea tridentata and Ambrosia dumosa was unaffected by elevated 

CO2 but, instead, reached peak rates when soils were near their maximum water content 

(Clark and others 2010). Although elevated CO2 slightly increased the C:N ratio of plant 

litter at the soil surface, decomposition was affected more by the proportion of nondecomposable (high lignin and secondary compounds) to decomposable species present 

in the litter, which varied with inter-annual differences in precipitation (Weatherly and 

others 2003). Additional data from the NDFF also indicate that CO2 exposure does 

not affect litter quality through increased C:N ratios (Billings and others 2003). On 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 9

the other hand, leaf-level photosynthesis was enhanced by elevated CO2 (Hamerlynck 

and others 2000; Naumburg and others 2003; Houseman and others 2003). Moreover, 

gs in Larrea tridentata decreased by 25-50% in response to elevated CO2, but these 

patterns were most pronounced (Anet) in or limited (decreased gs) to years with aboveaverage precipitation (Naumburg and others 2003). Similarly, elevated CO2 increased 

aboveground production of L. tridentata, Ambrosia dumosa, and Krameria erecta, 

but this response was also limited to years with high rainfall (Smith and others 2000; 

Housemen and others 2006). Based on these results, an increase in atmospheric CO2

concentrations in arid ecosystems may enhance carbon sequestration but only in years 

with above-average rainfall.

The degree to which CO2 affects ecosystem processes depends on lower gs and ET 

that combine to increase soil θ (Field and others 1995). Overall at the NDFF, higher 

atmospheric CO2 concentrations did not conserve soil θ (Nowak and others 2004). 

However, soil θ was higher under elevated CO2 during wet years (Nowak and others 2004) and this corresponded with a decrease in gs of the dominant shrub Larrea 

tridentata (Naumburg and others 2003). In addition, at the NDFF, down-regulation 

of photosynthesis occurred in L. tridentata and Lycium andersonii shrubs grown 

under elevated CO2 concentrations (Huxman and others 1998; Pataki and others 

2000; Hamerlynck and others 2002). This can occur in C3 species when carbohydrates 

accumulate and directly inhibit photosynthesis or when less photosynthetic enzymes 

are produced (e.g., Rubisco) in response to elevated CO2 concentrations (Stitt 1991). 

This response to long-term elevated CO2 will decrease carbon sequestration capacity. 

Another factor to consider in a CO2 enriched environment is that while lower gs can 

lead to favorable WUE, the reduction in transpiration could increase leaf temperatures, 

offset reductions in gs, and affect photosynthesis (Bazzaz 1990). Higher leaf temperatures did not occur for the species that exhibited decreased gs at the NDFF (Nowak and 

others 2001). Generally, desert ecosystems may be exempt from this for the following 

reasons: (1) small leaves track air temperature due to high convective heat transfer, and 

(2) heatloss by transpiration in desert species is a minimal component in leaf energy 

balance due to water limitations (Lambers and others 2008). One last issue to consider 

is that increased primary productivity due to higher CO2 can only be sustained with 

continued availability of soil N (Schaeffer and others 2007).

Warming

In addition to precipitation and CO2, warmer temperatures can directly affect primary 

productivity and respiration or indirectly affect carbon balance by altering soil moisture, 

N availability, and species composition (Shaver and others 2000). In a meta-analysis 

from 85 warming studies established around the globe, Wu and others (2011) found 

that warming generally stimulated plant growth (above- and belowground net primary 

productivity), respiration, and NEE. Moreover, a 0.3-6.0 °C increase in temperature 

increased mean N mineralization by 46% across ecosystems ranging from tundra to 

grasslands (Rustad and others 2001). Unfortunately, desert ecosystems are rarely included in warming studies. However, we would expect that warmer temperatures alone 

would intensify soil water limitation and, therefore, decrease respiration and primary 

productivity, but these responses may be varied (Shaver and others 2000; Heimann and 

Reichstein 2008; Shen and others 2009). For example, in Chihuahuan Desert shrublands, 

van Gestel and others (2011) found that by reducing the daily range of soil temperature, 

soil moisture and soil respiration increased and N availability decreased while Anet

of Larrea tridentata was not affected. Warmer minimum nighttime air temperatures 

(Figure 6) had an opposing effect on vegetation cover in Chihuahuan Desert grassland. 

A 2.5 °C increase in minimum nighttime air temperatures significantly increased cover 
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

of the Chihuahuan Desert grass Bouteloua eriopoda and C3 shrub Gutierrezia sarothrae

while warmer temperatures did not affect Bouteloua gracilis cover (Collins and others 

2010). Also in the Chihuahuan Desert grassland, Bell and others (2008) found that while 

bacterial response tracked precipitation, soil temperature was the major determinant 

of fungal carbon use. Since community composition and microbial activity are sensitive to warmer temperatures in arid ecosystems, additional research is needed to fully 

understand how this element of climate change may re-shape these communities.

Environmental Hurdles

Environmental hurdles limit our ability to increase carbon sequestration in aridsemiarid ecosystems. These barriers alter the carbon cycle, are often exacerbated by 

climate change, and will affect the options available to land managers to enhance carbon sequestration. In this section, we focus on invasive species and disturbance regime 

(periodic destruction or removal of ecosystem components [Hobbs 2009]) as it relates 

to grazing, fire, and the carbon cycle.

Figure 6—Wet and dry deposition collector (foreground) and a long-term climate change experiment 

(background) in mixed species composition desert grassland in the Sevilleta National Wildlife Refuge. 

The climate change experiment is designed to determine the individual and interactive effects of increased 

winter precipitation, nighttime warming, and nitrogen deposition on this desert grassland ecosystem. 

Variables measured in each replicate (N = 5) are: air and soil temperature, soil moisture, plant available 

soil nitrogen, aboveground net primary production (ANPP), and plant species composition. In addition, 

a subset of the treatment also continuously measures soil respiration. See http://sev.lternet.edu/projects 

for more information on this and other climate change related research being conducted as part of the 

National Science Foundation-funded Sevilleta Long-Term Ecological Research Program. Photo credit: 

Sevilleta LTER Program.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 11

Disturbance—In many ecosystems, periodic disturbance is typically followed by a 

period of succession where an ecosystem is gradually restored to a similar pre-disturbance 

state (Hobbs 2009). Succession in arid-semiarid ecosystems, however, may not occur 

along this trajectory. Across the warm deserts, the capacity to regenerate from disturbance 

(e.g., resilience) is directly linked to resource availability (Brooks and Chambers 2011). 

As such, low primary productivity and annual rainfall in the Mojave Desert results in 

low resilience to disturbance, whereas greater primary productivity and annual rainfall 

leads to increased resilience to disturbance in the Chihuahuan Desert. Moreover, novel 

disturbances, such as increased fire frequency in the Mojave Desert, result in a succession 

process that is varied or unknown. For example, burned Larrea tridentata communities 

converged with unburned L. tridentata communities over 19 years, while burned and 

unburned Coleogyne ramosissima communities lacked convergence irrespective of time 

since fire (Engel and Abella 2011). In contrast, the debate continues over the role of 

grazing intensity and fire frequency in the Sonoran and Chihuahuan Deserts.

In general, fire and grazing can affect ecosystems through a variety of mechanisms 

that act on components of the carbon cycle. For example, both grazing and grazing exclusion have been found to promote shrub encroachment in Sonoran and Chihuahuan 

Desert grasslands (Asner and others 2004; Yanoff and Muldavin 2008; Browning and 

Archer 2011). The change in species composition from grassland to shrubland would 

be expected to affect spatial variability of soil organic carbon through changes in 

above- and belowground plant growth as well as N availability, especially if the shrub 

was a legume such as mesquite (Piñeiro and others 2010). Emerging evidence in the 

Chihuahuan Desert suggests that creosote bush shrublands provide a larger carbon sink 

than the black grama grasslands they are invading (Figure 7).

Figure 7—Shrub encroachment—invasion of native C4 grassland by native C3 shrub species—is occurring globally. In the Southwest, many grasslands are being invaded by creosote bush (Larrea tridentata, 

pictured) and mesquite (Prosopis spp.). Causes of shrub encroachment vary regionally but generally 

include interactions among several factors, such as overgrazing, drought, elevated atmospheric CO2, 

fire suppression, and global warming. At the Sevilleta National Wildlife Refuge, shrub encroachment into 

grasslands results in higher night time temperatures during winter, lower plant species diversity, increased 

soil resource heterogeneity, and increased organic matter and nutrient losses via runoff. However, evidence 

suggests that creosote dominated shrublands are a stronger carbon sink than the grasslands they replace. 

Photo credit: Sevilleta LTER Program.
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

Climate change is likely to increase fire frequency in the Southwest, which may 

facilitate a change in species composition by invasive species or be perpetuated by the 

presence of invasive grasses across the warm deserts (Abatzoglou and Kolden 2011). 

Although there exists positive feedback among climate, disturbance, invasive species, 

and the carbon cycle, changes in carbon cycling associated with disturbance are also 

significant in the absence of invasive species. A study by Vargas and others (2012) in 

Chihuahuan Desert grassland consisting largely of the native perennial grass Bouteloua 

eriopoda and others showed that fire decreased Rs and NPP by an average of ~70%. 

Another study found that the native Chihuahuan Desert grasses B. eriopoda and Aristida 

purpurea increased Anet and Gs in response to fire likely due to an increase in soil N 

availability (Allred and Snyder 2008). Additional studies are needed to fully understand 

how arid-semiarid ecosystems respond and recover from disturbance as climate change 

will likely increase the susceptibility of these ecosystems to disturbance and biological 

invasions.

Invasive Species—In this section, we define “invasive” as any native or non-native 

species that causes or is likely to cause social, economic, or ecological harm (Runyon 

and others 2012). Across the Mojave, Sonoran, and Chihuahuan Deserts, high temperatures, limited water availability, and nutrient deficient soils create an environment where 

a specific suite of native plants persist (Rao and others 2011). However, increasing 

CO2 concentrations, precipitation variability, and increasing N deposition modify the 

availability of limiting resources and may increase invasive species success in aridsemiarid ecosystems (Davis and others 2000). For example, when grown at elevated 

CO2 concentrations, plant density, biomass, and seed rain were significantly higher in 

the non-native annual Bromusmadritensis ssp. rubens compared to native annuals during 

a wet year in the Mojave Desert (Smith and others 2000). The favorable response of B. 

madritensis spp. rubens is attributed to a decrease in the energetic cost of aboveground 

biomass construction under elevated atmospheric CO2 concentrations (Nagel and others 

2004). In another study, Suazo and others (2012) found that non-native annual species 

increased in density (Schismus spp.) and allocated more resources to seed production 

(Brassica tournefortii) in response to disturbance as well as supplemental watering. 

This indicates that invasive species establishment in desert ecosystems is also favored 

by multiple aspects of climate change.

Once established, invasive species can alter ecosystem processes that feed back to 

the climate system (Dukes and Mooney 1999). This has been observed with shrub 

encroachment in the Chihuahuan and Sonoran Deserts. When experimental rainfall is 

-80%, -50%, +50%, or +80% of ambient rainfall, the C3 shrub Prosopis glandulosa

consistently maintains higher Anet and Ψpd compared with the C4 grass Bouteloua eriopoda in the Chihuahuan Desert (Throop and others 2012). Similarly, in the Sonoran 

Desert, Prosopis velutina sustained higher Anet and photosynthetic function across a 

greater temperature range and under dry pre-monsoon conditions compared with the 

native C4 grass Sporobulus wrightii (Barron-Gafford and others 2012). These results 

are especially interesting since C4 species usually maintain higher rates of Anet and 

WUE under warmer and drier environmental conditions (Lambers and others 2008). 

However, in the Barron-Gafford and others (2012) study, the deep-rooted C3 shrub 

outperformed the C4 grass because it had access to groundwater while significantly 

higher Anet in S. wrightii was limited to brief periods of higher water availability during the monsoon season. Experimental results also suggest that shrub encroachment 

can influence Rs because of shrub-induced changes in microsite conditions. Research 

by Cable and others (2012) showed that Rs was higher near the trunk and under the 

canopies of large Prosopis velutina but temperature sensitivity of Rs was greatest at 

the canopy edge. Furthermore, Rs under large P. velutina canopies was more similar 

to Rs under grass canopies as opposed to that measured under the canopy of medium 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 13

sized P. velutina (Cable and others 2012). The differences in Rs with varied microsite 

conditions are likely due to higher root biomass in large P. velutina or to differences 

in heterotrophic and autotrophic response patterns from the trunk to canopy edge. At 

the continental scale, shrub encroachment into Chihuahuan Desert grassland results in 

lower ANPP compared to mesic sites where shrubs have invaded (Huenneke and others 

2002; Knapp and others 2008; Barger and others 2011). These differences in primary 

productivity and Rs in response to shrub encroachment stand to alter the source-sink 

strength of warm desert ecosystems.

Management and Policy Implications ________________

Carbon Sequestration and Land Management

According to the studies, the elements of climate change will probably have contrasting 

effects on NEE and carbon uptake in the warm deserts. Nevertheless, increased precipitation variability will substantially affect the capacity of these water-limited ecosystems 

to exploit increased atmospheric CO2 concentrations and persist during prolonged 

droughts. Certainly, climate change mitigation in arid-semiarid ecosystems has a low 

potential when compared with more productive ecosystems (Brown and others 2010). 

However, arid-semiarid systems cover ~45% of the terrestrial surface (Schimel 2010) 

and desertification is projected to increase (IPCC 2007), making management strategies 

that increase C sequestration or decrease carbon loss worth pursuing.

Among the elements of climate change, precipitation variability is the least predictable 

but remains the most influential component in arid-semiarid ecosystems. Consequently, 

the most beneficial and cost-effective land management strategies should work to restore degraded lands and improve their potential for storing atmospheric CO2 (Follet 

and Reed 2010; Laca and others 2010; Piñeiro and others 2010), maximize carbon gain 

during peak productivity (Svejcar and others 2008), and combine eddy co-variance 

techniques with field based measurements and predictive modeling to understand how 

arid-semiarid ecosystems function and to predict effective management strategies 

(Brown and others 2010).

Presently, CO2 concentrations are increasing by roughly 2 ppm annually while mean 

global temperatures are projected to increase 1.1-6.4 °C by the end of this century 

(IPCC 2007). Since CO2 is the most prevalent greenhouse gas, prevailing scientific 

theory suggests reducing atmospheric CO2 concentration via enhanced carbon storage 

may slow or reverse the rate of global climate change. As a result, some Federal land 

management agencies are now implementing management policies to increase carbon 

sequestration.

The Climate Change Scorecard

The USDA Forest Service developed the National Roadmap for Responding to Climate 

Change (http://www.fs.fed.us/climatechange/pdf/Roadmapfinal.pdf), which includes 

three interconnected responses:

1. Assess current risks, vulnerabilities, policies, and gaps in knowledge,

2. Engage employees and stakeholders to seek solutions, and

3. Manage for resilience, in ecosystems as well as in human communities, through 

adaptation, mitigation, and sustainable consumption strategies.

The Forest Service is using the Climate Change Scorecard to track progress on this 

Roadmap. Element Nine of the Climate Change Scorecard addresses carbon sequestration and provides fundamental questions for managers to address when reporting on 
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

their accomplishments towards developing land management strategies in the context of 

climate change. The Scorecard defines the required baseline assessment of carbon stocks, 

including the consideration of management practices to increase carbon sequestration 

or reduce emissions. The scorecard evaluation process contains provisions for: (1) developing a baseline assessment of carbon stocks; (2) understanding how management 

practices and disturbances influence carbon stocks, sequestration, and emissions; and 

(3) balancing carbon stewardship with the management of other benefits. These provisions are linked to complicated biological processes, which are often poorly understood 

and represented by confounding research. However, helpful guidelines and generalizations can be made. The following section provides an overview of how management 

practices influence carbon sequestration. We focus on fire and grazing as management 

tools (Figures 7 and 8).

Carbon Sequestration and Storage Defined

Carbon sequestration includes any process for capturing and storing atmospheric CO2 

long term. Since a large portion of carbon resides in soils of arid-semiarid ecosystems, 

a clear definition and understanding of carbon sequestration in non-forest ecosystems is 

necessary to estimate the management effects on carbon stores (Table 1). Carbon pools 

contained in the soil emanate from both organic and inorganic sources. Soil carbon is 

most often characterized as SOC or SIC. SOC is a component of soil organic matter 

(SOM), which includes all materials of biological origin regardless of origin or state 

of decomposition (Baldock and Skjemstad 1999). Organic matter contains roughly 

60% carbon (Bell and Lawrence 2009), meaning that soil with 1.7% SOM will yield 

roughly 1% SOC. Whereas SOC originates from carbon fixation during photosynthesis 

and microbial decomposition, SIC (e.g., CaCO3) is the result of mineral weathering. 

Figure 8—Controlled burn in Chihuahuan Desert grassland at the Sevilleta National Wildlife Refuge, New 

Mexico. Photo credit: U.S. Fish and Wildlife Service.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 15

SIC concentrations tend to be much more stable through time because inorganic carbon compounds are not available as microbial substrate. Given that most management 

techniques are administered to influence vegetation, it follows that the rate of organic 

carbon accumulation is directly influenced by management actions. As a result, this section focuses primarily on SOC, which is directly influenced by management activities.

Prior to understanding how management actions can alter carbon stores or developing 

a monitoring protocol, a baseline assessment is needed to estimate the current amount 

of carbon stored in an area. Since far more carbon is stored below- than aboveground 

(Allen-Diaz and others 1995), it follows that any baseline assessment should include 

a substantial focus on belowground carbon pools. Creating a baseline assessment of 

carbon stocks on rangelands poses unique challenges. Unlike forested landscapes, few 

agency standards or resources exist for collecting information on non-forest environments. For example, there exist no codified provisions for the FIA Program (FIA 2010) 

for evaluating non-forested stands. In addition, a greater proportion of carbon is stored 

belowground in rangelands compared with forests or woodlands, thereby requiring 

expensive soil analyses for quantifying baseline carbon stores. Methods for estimating 

carbon stores can be categorized as direct measures, indirect measures, and modeling. 

Each category includes advantages and disadvantages and varying levels of cost, complexity, and accuracy.

Measuring Stored Carbon

Direct measures generally include field and laboratory measurements and eddy covariance techniques (Baldochi and others 1988; Verma 1990; Lee and others 2004; Burba 

and Anderson 2010). Laboratory measurements for soil carbon include techniques such 

as dry combustion or bulk density testing and are nicely described in Donovan (2012; 

measuring soil carbon change). Four promising field-based measurement techniques 

include Laser Induced Breakdown Spectroscopy (Cremers and others 2001), Inelastic 

Neutron Scattering (Wielopolski and others 2000), Mid-Infrared Reflectance Spectroscopy (McCarty and others 2002), and Visible-Near Infrared diffuse reflectance 

Table	
  1.	
  “Pools” (or	
  types)	
  of	
  soil	
  organic	
  carbon	
  (adapted	
  from	
  Liddicoat	
  and	
  Others	
  2010).	
  

Organic	
  carbon	
  pool Size Stability Turnover	
  

time

Key	
  functions

(1)	
  Crop	
  residues	
  

Shoot	
  and	
  root	
  residues	
  on	
  and	
  in	
  

the	
  soil

>	
  2mm Labile	
  (readily	
  

available)

Days Provide	
  energy	
  and	
  nutrients	
  to	
  biological	
  

processes;	
  readily	
  broken	
  down	
  providing	
  soil	
  

conditions	
  that	
  favor	
  soil	
  biology.

(2)	
  Particulate	
  organic	
  matter	
  	
  

(POM)

Smaller	
  plant	
  debris

0.05-­‐2mm These	
  are	
  broken	
  down	
  relatively	
  quickly	
  in	
  

suitable	
  conditions	
  but	
  more	
  slowly	
  than	
  crop	
  

residues.	
  	
  Important	
  for	
  soil	
  structure,	
  

provision	
  of	
  energy	
  for	
  biological	
  processes	
  

and	
  nutrients.

(3)	
  Humus

Decomposed	
  material	
  dominated	
  

by	
  molecules	
  stuck	
  to	
  soil	
  minerals

<	
  0.05mm This	
  plays	
  a	
  role	
  in	
  all	
  key	
  soil	
  functions,	
  but	
  is	
  

particularly	
  important	
  in	
  the	
  retention	
  and	
  

provision	
  of	
  nutrients	
  (e.g.	
  the	
  majority	
  of	
  

available	
  N	
  is	
  found	
  in	
  the	
  humus	
  fraction).

(4)	
  Recalcitrant	
  organic	
  matter

Biologically	
  stable,	
  dominated	
  by	
  

pieces	
  of	
  charcoal

Variable Very	
  stable	
  /	
  

relatively	
  inert

Hundreds	
  

of	
  years

Decomposes	
  very	
  slowly	
  and	
  if	
  present	
  in	
  large	
  

enough	
  quantities	
  can	
  contribute	
  to	
  cation	
  

exchange	
  capacity	
  as	
  well	
  as	
  controlling	
  soil	
  

temperature.
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

spectroscopy. The USDA Natural Resources Conservation Service is systematically 

measuring soil carbon using the visible-near infrared method and it is conceivable that 

a similar approach could be used by Forest Service field units to fulfill the obligations 

of the Climate Change Scorecard.

Although most carbon resides belowground as SOC, some rangeland ecosystems 

exhibit a substantial amount of carbon stored aboveground. Aboveground carbon stores 

are not always considered in sequestration studies due to the relative ease with which 

shrubs can disappear from a stand. Nevertheless, many stands support significant 

aboveground carbon stores, which are important to evaluate when a full accounting 

of carbon stocks is required. Inventorying and monitoring aboveground carbon stocks 

can easily be achieved using destructive sampling or allometric equations (e.g., Means 

and others 1994; BIOPAK Software, http://andrewsforest.oregonstate.edu/data/tools/

software/biopak.cfm?topnav=149) that relate stand structure to biomass components. 

Once a baseline assessment of carbon stores has been conducted, the more challenging 

task of monitoring could be considered.

Monitoring Considerations

Monitoring changes in carbon stores is critical to determining the efficacy of various 

types and levels of landscape treatments and disturbance regimes. Monitoring carbon 

stocks in rangeland ecosystems is inherently difficult because the change in mass of 

carbon is small relative to the current carbon store and usually takes years or decades 

before significant trends can be revealed.

SOC is often monitored using field or laboratory measures, indirect observations such 

as remote sensing, and ecosystem modeling (Brown and others 2010) or a combination 

of these techniques (Hunt and others 2004). Properly calibrated ecosystem simulation 

models, such as Comet-VR (http://www.cometvr.colostate.edu/), provide a suitable 

framework for estimating the effects of disturbances and land management techniques 

on carbon stores of rangelands. The most common rangeland management practices that 

have a meaningful impact on SOC are fire and herbivory. The effects of these practices 

on carbon stores and rates of sequestration are often inter-linked, confounded, and poorly 

understood across the wide range of non-forest ecosystems.

Effects of Management Actions and Disturbances on Carbon 

Storage

Fire in most non-forest systems has the obvious effect of reducing aboveground carbon stores (Figure 8). However, beyond the initial and obvious response, the effects of 

fire on carbon sequestration on non-forest landscapes are complicated to quantify. Fires 

can change plant diversity and structure by changing the availability of light, water, 

and nutrients. Such alterations in ecosystem structure and physical environment often 

modify primary productivity and nutrient fluxes that subsequently alter carbon storage in 

plants and soils (Ojima and others 1990; 1994; Rice and others 1998; Reich and others 

2001). The direction and magnitude of these changes, however, depend on vegetation 

type, fire frequency, fire duration, and season of fire occurrence. Changes in SOC in 

response to fire are difficult to determine over short time periods due to large quantities 

of carbon stored in soil and because changes in SOC storage occur slowly relative to 

the quantity of SOC. In general, fires tend to enhance ANPP in grasslands and savannas 

during the first several growing seasons after the fire event (Rice and Owensby 2000; 

Ansley and others 2002; Santos and others 2003; Bremer and Ham 2010). Fires also 

appear to accelerate root production, increase root biomass, and increase C:N ratios of 

root tissue (Blair 1997; Rice and others 1998; Johnson and Matchett 2001; Reich and 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 17

others 2001) in some grasslands. While increased production may be observed over short 

time periods, it does not seem to cause commensurate increases in SOC over longer time 

periods (up to 50 years) (Sharrow and Wright 1977; Ansley and others 2010). Indeed, 

most studies indicate that C stocks only slightly decrease or demonstrate no change for 

up to 50 years of annual-biennial fire treatment (Rice and Owensby 2000). Despite this 

generality, characterizing these effects in a specific manner is complicated by a lack of 

controlled studies of sufficient duration for observing changes in SOC in response to 

fire characteristics. What’s more, most studies aimed at fire effects on carbon dynamics 

have been conducted in relatively productive systems such as tallgrass prairie, which 

shows little decline in SOC with repeated fires (Rice and Owensby 2000).

These generalities depend on many other factors, resulting in complicated feedbacks 

and interactions between fire and other disturbance types. A lack of grazing can alter species composition and has been postulated to enhance shrub encroachment on 

semi-desert and desert grasslands (Asner and others 2004; Yanoff and Muldavin 2008; 

Browning and Archer 2011). Altered species composition modifies fire frequencies 

and intensities, which affect SOC through changes in above- and belowground primary 

production. Similarly, the ultimate effects of herbivory are not well understood and vary 

between and even within various rangeland ecosystems, making generalities or widely 

applicable guidelines difficult to develop.

One overarching factor, however, is that rangelands function as carbon sinks for 

relatively short periods (Svejcar and others 2008). This fact requires managers to 

thoughtfully consider management practices that do not impede sequestration during 

critical times. Sustaining carbon uptake relies on growth and development of vegetation, which requires careful balance with grazing, which affects C sequestration (Derner 

and Schuman 2007). In both short- and mixed–grass, grazing at light, moderate, and 

heavy stocking rates tends to increase SOC (Derner and others 1997, 2006; Reeder 

and Schuman 2002; Reeder and others 2004) in the surface 30 cm, while moderate and 

heavy grazing over many decades appears to increase SOC down to the 60-cm level in 

northern mixed-prairie (Liebig and others 2010). Although grazing can increase SOC, 

annual production can decrease as a result of altered species composition (Figure 9). 

Figure 9—A lone steer grazing in Chihuahuan Desert on the Jornada Experimental Range in southern New 

Mexico. Photo credit: Scott Collins.
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

For example, heavy stocking rates can increase the relative proportion of warm season 

species that translocate greater amounts of C to belowground structures, often resulting 

in increased SOC (Coupland and Van Dyne 1979; Schuman and others 1999; Derner and 

Hart 2007). The general increase in SOC observed in the previously mentioned studies, however, must be interpreted with caution and will not always be applicable. For 

example, when heavy grazing is coupled with sustained or severe drought, significant 

losses of SOC can occur (Ingram and others 2008). This supposition is strengthened 

by Zhang and others (2010) who reported that more xeric rangelands tend to be in C 

balance or are C sources whereas more mesic rangelands function primarily as C sinks.

Balancing Carbon Sequestration with Other Services

Carbon sequestration and storage, as a greenhouse gas mitigation strategy, can be 

viewed as an ecosystem service. Management actions influencing carbon storage and 

emissions (i.e., encouraging shrub encroachment into grasslands) must ultimately be 

balanced with the need to sustainably produce other goods and services such as wildlife 

habitat, livestock, clean air and water, aesthetic properties, and other natural amenities 

(Figure 9). Addressing this need requires both the art and science of range management 

in addition to more comprehensive research aimed at understanding tradeoffs between 

carbon storage and other products or services. In many cases, production of multiple 

goods and services can be simultaneously enhanced through proper management 

techniques. For example, nutrient cycling (Rau and others 2010), forage (Sandell and 

Knezevic 2010), biotic integrity, biodiversity (Catovsky and others 2002), and carbon 

sequestration (Rau and others 2010) can be enhanced or maintained through removal of 

some invasive species. In contrast to this type of win-win scenario, tradeoffs between 

various management outcomes will depend on resource values and sometimes produce 

one-sided outcomes. 

As an example, in the Chihuahuan Desert, consider the tradeoff between increased 

carbon capture and storage versus water abundance and availability. In some regions, 

simply aiding the spread and rate of encroachment by shrubs such as Juniperus or 

Prosopis species can capture more carbon. However, shrub encroachment leads to lower 

biodiversity, higher regional temperatures, increased surface runoff and erosion, and 

lowered water tables that could ultimately lower the amount of available surface water.

The relative value of various goods and services in the future will likely change with 

our changing climate, but the magnitude of these changes is subject to public perceptions 

as well as environmental, political, and socio-economic factors. Thus, it is conceivable 

that future carbon capture and storage may be less of a priority compared to a dependable supply of clean water or open space and winter range for ungulates (Figure 10).

Balancing societal needs and desires with ecological constraints is not easy, and the 

added consideration of carbon storage creates a multi-dimensional problem for which 

decision support systems may be invoked to understand tradeoffs between potential 

management actions. Toward this goal, the Sustainable Rangeland Roundtable, a group 

of private and public organizations and agencies, developed a forum to discuss rangeland 

sustainability and assessment to integrate social, economic, and ecological disciplines 

related to rangelands. In this regard, the Integrated Social, Economic, and Ecologic 

Conceptual Framework created by the Sustainable Rangeland Roundtable enables assessment of tradeoffs and implications of management decisions in a multi-dimensional 

manner. The likelihood of the Forest Service or Bureau of Land Management achieving carbon storage planning goals will be increased by use of decision support systems 

capable of integrating simultaneous multi-dimensional analyses.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014 19

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26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-316. 2014

Appendix _______________________________________

Table of Abbreviations

θ Soil water content

Anet Photosynthesis

ANPP Aboveground net primary productivity

C Carbon

CaCO3 Calcium carbonate

CO2 Carbon dioxide

ET Evapotranspiration

FIA Forest Inventory and Analysis

GPP Gross primary productivity

Gs Leaf conductance

mm Millimeters

N Nitrogen

NAMS North American Monsoon System

NDFF Nevada Desert Free-Air CO2 Enrichment Facility

NEE Net ecosystem exchange

Pg Petagram

Reco Ecosystem respiration

Rs Soil respiration

SIC Soil inorganic carbon

SOC Soil organic carbon

SST Sea surface temperature

WUE Water use efficiency

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