Human Influence on Grazer Impacts: the role of socio-economic and political factors in herd movement

Megan McSherryPrinceton University

Grazing and soil carbon: A comparison of grassland types and management strategies

Grazing & Soil Carbon WorldwideGrasslands cover nearly 40% of earth and are found in every region of the world (Derner and Schuman 2007)

Grazing most prevalent land use on grasslands (Sanderson et al. 2004, Oenema et al. 1997)

National Geographic

Much of the recent work on efforts to mitigate the effects of global climate change have focused on preserving or restoring carbon stored in forests. What has been given relatively less focus is the potential for sequestering carbon in grassland soils, despite the fact that grasslands represent a huge potential reservoir for carbon, as shown in yellow on this map, by their extent over approximately 40% of the Earths land mass. This potential however, is dependent upon the effects of grazing by large herbivores, the #1 land use on grasslands. Much uncertainty exists regarding the conditions under which grazing promotes soil carbon storage. Therefore the goal of this study is to assess the potential for increasing soil carbon storage under grazing across two different types of grasslands and under two different herding strategies. These comparisons will be made across grasslands in Kenya and in Mongolia. These differences, as well as effects of grazing by different sized herbivores, will be assessed in order to determine the future potential for carbon credit programs in these communities.2

Grazing & Soil CarbonGrasslands have the ability to store 10-30% of global soil organic carbon (SOC) (Derner and Schuman 2007)

Increase allocation of C inputs belowground (through species shifts and/or increase in root density)

By reducing biomass available to fires, depositing as dung

Huge potential for C sequestration

Help to mitigate global climate change (rise in atmospheric CO2)

Potential to provide additional income to herding communities through selling of carbon credits

There are many ways that grazing can positively impact carbon storage on grasslands, including increasing allocation of carbon inputs belowground (for example, through compositional shifts to species with higher root to shoot ratios or increased productivity), as well as by reducing, through consumption, available biomass for fires, and instead returning carbon back to the land as dung. Given this potential in sequestering carbon, grazed lands could potentially aid in the mitigation of global climate change as well as potentially provide added income to communities through the selling of carbon credits. This would not only provide added revenue to often-impoverished communities but would help in preserving the pastoralist livelihood which if managed appropriately, is largely coexistent with wildlife conservation goals. 3

Impacts of Grazing on SOCWidely studied but results mixed (Milchunas & Lauenroth 1993; Derner et al. 2006)

Associated with losses in SOC density of over 300 g/m2/yr. and gains of up to 200 g/m2/yr.

Sign of grazer effect (+ or -) highly context-dependent (McSherry & Ritchie 2013)

Interactions between variables extremely important (McSherry & Ritchie 2013)

While the effects of large grazers on soil organic carbon have been widely studied, the results have been largely mixed, with some studies finding grazers to positively affect SOC while others have found grazers to have negative effects. Effects are inconsistent owing to the great variation across grasslands in climate, soils, and vegetation as well as in types of grazing practices. In my 2013 global review of studies that analyzed effects of grazers on SOC density, I found that while grazer effects on SOC are potentially large, with annual changes as much as 1.5 tons/ha/year, the sign of grazer effect (whether it is positive or negative), is highly dependent on the environmental context of the grazed system, with interactions in environmental and biotic conditions producing variable effects.4

McSherry & Ritchie 2013

One such interaction that influences the effect of grazers on soil carbon is between grazing intensity and the dominant grass vegetation type of the grassland, specifically whether grasses utilize the C3 pathway of photosynthesis or C4. Higher grazing intensities are associated with increased SOC in C4grasslands but with lower SOC in grasslands dominated by C3 grasses. In these systems, only light levels of grazing produce positive effects of grazers on carbon storage. On the other hand, in C4, grazing-adapted systems, light grazing or a lack of grazing shifts the system from a carbon sink to a carbon source.5

Proposed Study Compare grazer effects on SOC density

Across vegetation type:

C3 grassland (Patagonia, Argentina) vs. C4 grassland (Kenya)

Across pastoralist herding strategy:

Horseback vs. herding by foot

Across variation in herd composition:Sheep, cattle, native herbivores

In order to learn more about the conditions under which grazing has positive effects on SOC and can contribute to increased C sequestration, I am proposing a study that will compare grazer effects on SOC across pastoralist rangelands, particularly across dominant vegetation types but also across differing pastoralist herding strategies and across variation in herd composition.6

Proposed Study DesignSmall Stock OnlyReference- All GrazersTotal Exclusion - UngrazedLarge & Small Stock- Herded

Sets of exclosures to measure effects of different aspects of herd composition

Measure % OC, soil bulk density, soil texture (% clay, silt, sand), and growth rates of plant roots and arbuscular mycorrhizal fungi

One aspect of the study design will be to create plots with varying access to different types of grazers in order to isolate the effects of each on SOC density, as compared to SOC density of an ungrazed plot. These plots will then be replicated across vegetation types to compare effects on C3 dominated grasslands and C4 dominated grasslands. I will measure SOC density in each of these plots as well as associations with above and belowground biomass.7

Immediate GoalsFind comparable communities

Further develop sampling protocol and study design

Assess potential for SOC storage in each community

Assess individual herder willingness to adapt grazing strategies aimed at increasing SOC storageFuture Goals

The next steps in this study will be finding appropriate rangelands for the aforementioned comparisons as well as developing a detailed sampling protocol and study design. In the future, with results from this study, we hope to be able to assess the potential for SOC storage in each community as well as assess individual pastoralists willingness to adapt recommended grazing practices aimed at increasing soil carbon storage.8

Questions?

McSherry & Ritchie 2013F = 5.929, p = 0.002

Economic Trade-Offs Grazing IntensityOptimalDifficult to Implement

Ultimately, the success of a carbon credits program will depend on the economic trade-offs facing the candidate communities. If carbon storage and cattle production can be maximized, or at least produce positive balances at similar grazing intensities, then human likelihood to adapt becomes greater. However, if SOC storage and cattle production optimums fall at different levels of grazing intensity, it will become a question of how much behavior needs to change in order to achieve carbon storage and at what cost to livestock production. And then, is the loss in animal production made up for by the potential income from carbon credits?11