Argentina: Semiarid Rangeland Restoration

Project Stage:
Completed

Start Date:
2001-01-01

End Date:
2006-12-31

Primary Causes of Degradation

Degradation Description

The temperate semiarid grasslands in this part of Argentina have a history of over 100 years of grazing by sheep and cattle. In the uplands, compositional changes associated with grazing include the replacement of palatable midgrasses by palatable shortgrasses. Under the conditions of long-term, heavy grazing, the seral grasses commonly give way to increasing cover or even invastion by unpalatable grasses. Grazing caused a negative pressure on the palatable grasses, allowing for an increase in the unpalatable grasses, who also had poor litter quality, which is thought to translate into decreased nutrient turnover and reduced soil nutrient availability. In some areas nitrogen and phosophorus availability were 50% to 600% higher in patches dominated by palatable than in patches dominated by unpalatable grasses.

Reference Ecosystem Description

The range was dominated by perennial cool-season bunchgrasses, with late seral dominants represented by palatable midgrasses such as Poa ligularis and Stipa clarazii. The subordinate species are mostly palatable shortgrasses like Stipa tenuis, Piptochaetium napostaense along with a much smaller frequency of unpalatable grasses such as Stipa ichu, Stipa tenuissima, Stipa trichotoma, Stipa speciosa, Stipa ambigua, and Stipa brachychaeta. The Caldenal grassland had frequent fires, however, it is thought that the grassland had more frequent occurrence prior to the introduction of livestock ranching and the reduction of fine fuels along with the building of firebreaks. Presently, fires occur naturally once every 10 years.

Project Goals

The project intended to conduct a study to assess the potential of mechanical disturbance in combination with seeding to overcome those limiting factors in a degraded grassland excluded from livestock grazing. It was though that (1) disking stands dominated by unpalatable grasses would open safe sites for seedling establishment, (2) Poa ligularis, a late seral dominant palatable grass species, would then readily establish after seeding, and (3) P. ligularis would persist at the site by competitively excluding both remnants tillers of unpalatable grasses surviving disking, and seedlings of unpalatable grasses established after the mechanical treatment.

Monitoring

The project does not have a monitoring plan.

Stakeholders

Grazing has a significant history in this part of Argentina and continues to be essential to the local agricultural economy. The increase of unpalatable species in the rangelands has a negative effect on the livestock economy, so this type of research has been deemed critical to local landholders and ranchers.

Description of Project Activities:
In March 2001, the site was burned to eliminate standing dead biomass, and a month later 10 blocks were uniformly distributed on the site. Within each block, two experimental units (8 m x 8 m) were randomly assigned to one of the two treatments: "˜disking and seeding' and untreated control. Disking consisted of one pass of a 2.65-m-wide disk ripper. Seeds of P. ligularis (60% germination power) were manually spread at an approximate density of 150 seed/sq m. Seeds had been hand harvested from a 30-year-old grazing exclosure located 30 km away from the study site. To determine the effectiveness of the mechanical disturbance, the density of unpalatable grasses was assessed in December, 2002. Measurements were made on four 0.25 m x 6 m randomly located plots within each experimental unit. Density of unpalatable grasses was assessed by counting the number of plants per plot. Seedlings established after disking were differentiated from intact tussocks and tillers that survived mechanical disturbance.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
Mechanical disturbance combined with seeding of P. ligularis was markedly detrimental to density, cover and biomass of unpalatable grasses. The most probable reason for the success of the "˜disked and seeded' treatment was reduced competition from large tussock of unpalatable grasses, combined with adequate soil moisture for seedling establishment after seeding. Both surviving tillers and seedlings of unpalatable grasses were out-competed by P. ligularis. This species posses traits such as the growth of large tussocks, high leaf nitrogen content, high specific leaf area, high relative growth rate, and low tissue density, which suggest a high potential to compete for resources and become a dominant species. Although the competitive ability of P. ligularis in particular has not been studied, results from a previous study showed a higher ability to compete for resources in palatable than unpalatable grasses in the system under study. Annual rainfall in the first 4 years of the study was close to or above the long-term average (400 mm), which may have contributed to the expression of the competitive ability of P. ligularis. Regardless of wet (2004) or dry (2006) condition, however, P. ligularis contributed the most (88 - 100%) to the end-of-season standing crop in the "˜disked and seeded' treatment. The replacement of palatable by unpalatable grasses, induced by livestock overgrazing, results in the ecological degradation of rangelands. Unpalatable grasses possess traits such as leaf silicification, leaf sclerophylly, large size, and longevity that lead to reduced primary productivity, decelerated nutrient cycling, and decreased plant species diversity. Such floristic changes also lead to economic and social problems because of their negative impact on livestock carrying capacity. Our results show that a rapid transition from a state dominated by unpalatable grasses to a state dominated by palatable grasses can be achieved by mechanical disturbance of mature stands of unpalatable grasses in combination with seeding of palatable grasses. The stability of mature stands of unpalatable grasses mainly originates in their functional ecological traits (e.g., large tussocks that accumulate huge quantities of senescent biomass that decompose slowly), rather than on edaphic conditions. This particularity enhances the probability of success of the "˜disked and seeded' treatment across sites in the studied grasslands. On the other hand, climate conditions after seeding may be critical for P. ligularis establishment. It is already known that successful establishment of perennial grasses native to central Argentina depend upon adequate soil moisture in fall and winter, which were the prevailing conditions after seeding in 2001. Finally, mechanical damage to the stand of unpalatable grasses seems to be an indispensable part of any restoration treatment because well-established stands of unpalatable grasses have proved to be impervious to invasion by palatable grasses. This, in turn, occurs because the former species easily out-compete seedlings of the latter species. Fires of moderate severity, commonly applied by ranchers in late summer - early fall, would not be an alternative to mechanical disturbance since, under the mild conditions of late summer - early fall fires, the mortality of unpalatable grasses is quite limited. Pre-fire cover values of unpalatable grasses will be rapidly restored since they show high regrowth capacity after defoliation. In the present study, unpalatable grasses maintained a high density of adult plants in the control after been burnt to eliminate standing dead biomass in March 2001.

Factors limiting recovery of the ecosystem:
The lack of replication in space and time and the omission of main treatments like seeding alone, represented design limitations in the project.

Socio-Economic & Community Outcomes Achieved

Economic vitality and local livelihoods:
The replacement of palatable by unpalatable grasses, induced by livestock overgrazing, results in the ecological degradation of rangelands. Unpalatable grasses possess traits such as leaf silicification, leaf sclerophylly, large size, and longevity that lead to reduced primary productivity, decelerated nutrient cycling, and decreased plant species diversity. Such floristic changes also lead to economic and social problems because of their negative impact on livestock carrying capacity. The results of this project showed that a rapid transition from a state dominated by unpalatable grasses to a state dominated by palatable grasses can be achieved by mechanical disturbance of mature stands of unpalatable grasses in combination with seeding of palatable grasses. The implications of more palatable grasses in the diets of livestock are that the more palatable grasses are higher in mineral and protein content overall.

Long-Term Management

The results of the project showed great promise to the restoration of more palatable grasses. The important practical implication for the project is the knowledge of the rapid transition from a state dominated by unpalatable grasses to a state dominated by palatable grasses can be achieved by mechanical disturbance combined with seeding. Further research is now the direction in which effort is necessary to determine the implications of restoring palatable grasses on rangeland carrying capacity and also to establish how grazing should be managed for long-term persistence.

Sources and Amounts of Funding

This research was supported by the Universidad Nacional del Sur, Consejo Nacional de Investigaciones Cientıficas y Tecnicas (CONICET) de la Republica Argentina and Agencia Nacional de Promocion Cientıfica y Tecnologica (ANPCyT) de la Republica Argentina.

Other Resources

Distel, Roberto A. et al. 2008. Restoration of palatable grasses: A study case in degraded rangelands of central Argentina. Journal of Arid Environments. In press.

Primary Contact

Organizational Contact