Stable states in relation to grazing in Patagonia: a 10-year experimental trial

Oliva, Gabriel; Cibils, Andrés F.; Borrelli, Pablo; Humano, Gervasio

doi:10.1006/jare.1998.0425

Cited by 52 publications

(41 citation statements)

References 13 publications

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“…Effects of grazing are often positive where the rangeland vegetation has coevolved with grazing and is adapted to it (Milchunas and Lauenroth 1993). Local adaptations or changes in plant community composition may prevent degradation; even when macroclimatic conditions are relatively stable (Mwalyosi 1992, Oliva et al 1998, Hostert et al 2003, Geerken and Ilaiwi 2004. Regardless of the indicator used, a transparent definition of degradation is vital to conduct a meaningful assessment of the non-equilibrium concept.…”

Section: Discussionmentioning

confidence: 99%

Global assessment of the non‐equilibrium concept in rangelands

Wehrden

Hanspach

Kaczensky

et al. 2012

Ecological Applications

134

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Abstract. The non-equilibrium concept of rangeland dynamics predicts that the potential for grazing-induced degradation is low in rangelands with relatively variable precipitation. To date, evidence in support of the non-equilibrium concept has been inconsistent. Using a standardized protocol, including a newly developed global map of rainfall variability, we reviewed the incidence of degradation in relation to rainfall variability across 58 published studies. We distinguished between (1) zonal degradation (i.e., degradation independent of water and key resources), (2) degradation in the presence of key resources, and (3) degradation in the presence of water. For studies not affected by proximity to permanent water or key resources, we found strong support for the non-equilibrium concept for rangelands. Zonal degradation was absent at CV (coefficient of variation) values above 33%, which has been proposed as a critical threshold. Grazing degradation was almost entirely restricted to areas with relatively stable annual precipitation as expressed by a low CV, or to rangelands with key resources or water points nearby. To better understand rangeland dynamics, we recommend that future studies use globally comparable measures of degradation and rainfall variability. Our work underlines that rangelands with relatively stable rainfall patterns, and those with access to water or key resources, are potentially vulnerable to degradation. Grazing management in such areas should incorporate strategic rest periods. Such rest periods effectively mimic natural fluctuations in herbivore populations, which are a defining characteristic of non-degraded rangelands occurring under highly variable precipitation regimes.

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Section: Discussionmentioning

confidence: 99%

Global assessment of the non‐equilibrium concept in rangelands

Wehrden

Hanspach

Kaczensky

et al. 2012

Ecological Applications

134

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“…Nevertheless, experimental evaluation of the effect of improved management on tussock loss is limited, because the process is slow. Ten years of controlled grazing at 3 stocking rates failed to show a decrease in tussock grass cover at the community scale when grazed at high intensity (Oliva et al 1998). Direct observation of the process may require longer and clearly nonviable experiments or unrealistically high stocking rates.…”

Section: Introductionmentioning

confidence: 99%

Demography of Grazed Tussock Grass Populations in Patagonia

Oliva¹,

Collantes²,

Humano³

2005

REM

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The cover of Festuca gracillima (coiró n fueguino), a native tussock grass that dominates grass steppes of Southern Patagonia and Tierra del Fuego, has diminished under continuous sheep grazing. This loss is a concern, because it also reduces forage availability in winter, biodiversity, and soil stability. In the present study, the hypothesis that tussock grass birth and mortality rates are balanced only under moderate-grazing or exclusion regimes was tested with two 5-year records of demographic data obtained from 3 sheep grazing regimes: exclosure (no grazing), low (0.0348 AU ha À1 y À1 , where AU represents animal units equivalent to the consumption of a 450-kg cow), and high (0.1043 AU ha À1 y À1 ). Tussocks were outlined in photographs and marked in the field at two 5-year intervals. The initial total number of plants for the 3 grazing regimes (n ¼ 358) increased to 384 plants at the end of the 10-year period. No recruitment from seed was observed; plant number changed as a balance of yearly rates of tussock mortality (1.48%), amalgamation (0.75%), and subdivision (2.04%). Intensely grazed populations showed greater (2.13%) mortality rates than ungrazed (1.20%) or moderately grazed (0.78%) populations. Tussocks in intensely grazed populations were smaller (167 cm 2 plant À1 ) than those in moderately grazed (197 cm 2 plant À1 ) or ungrazed (300 cm 2 plant À1 ) populations. Transition matrices showed eigenvalues of 0.701 (high grazing), 0.794 (exclosure), and 0.876 (low grazing). All growth rates of demographic models were negative; the largest rate of population decrease was found under high-intensity grazing, for which projections show that half the tussocks would be lost in 37 years. Under low-intensity grazing and exclosure, a similar tussock loss would take place in 87 and 74 years, respectively. Results show 1) the importance of vegetative processes for tussock demography, 2) the extremely slow dynamics of population changes, and 3) that tussocks may suffer increased mortality as a consequence of subdivision or fragmentation, a process that can be viewed as a small-scale example of the generalized effect of patch subdivision under grazing. ResumenFestuca gracillima (coiró n fueguino) es una gramínea mediana dominante en los pastizales del sur de la Patagonia y norte de Tierra del Fuego que ha ido perdiendo cobertura bajo pastoreo continuo. El proceso implica también una reducció n en el forraje disponible en invierno, en la estabilidad del suelo y en la biodiversidad. El presente trabajo analiza 10 añ os de demografía de poblaciones de coirones sujetos a pastoreo controlado con tres intensidades: clausura (sin pastoreo), alta (0.1043 AU ha À1 añ o À1 , AU son unidades animales equivalentes al consumo de una vaca de 450 kg), y baja (0.0348 AU ha À1 añ o À1 ). Se puso a prueba la hipó tesis de que las tasas de natalidad y mortalidad llegan al equilibrio solamente bajo pastoreo moderado o en exclusió n. Los coirones fueron identificados en mapas fotográficos y marcados en el campo en dos períodos ...

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“…Stocking rate studies from different locations in the south-western United States have indicated that a conservative stocking involving 30-35% use of average forage production is a reliable way to increase forage production and improve vegetation composition on degraded rangelands (Holechek et al, 1994(Holechek et al, , 1999Winder et al, 2000). Similarly, Oliva et al (1998) have demonstrated for the Patagonia Argentina that plant communities under moderate stocking did not lose diversity or highly palatable species after a 10-year grazing trial.…”

Section: Introductionmentioning

confidence: 93%