OOS 33-3 - Drought responses of forage supply increased with increasing aridity in South American rangelands

Background/Question/Methods
The magnitude, frequency, and duration of droughts is increasing globally as a result of human activities. The impact of drought is particularly important in rangelands, where net primary production is closely linked to annual precipitation patterns. Consequently, the provision of valuable rangeland’s ecosystem services, such as the supply of forage for livestock production, can be severely affected. The objective of this study was to assess the sensitivity to drought of forage productivity in rangelands located in a wide aridity gradient in Argentina, South America, a region which economy depends mainly on agricultural products, and which is often underrepresented in scientific studies.

To achieve our goals, we established comparable drought treatments among a wide range of Argentine rangelands with contrasting climates (mean annual precipitation from 150 to1000 mm/yr), simulating dry years with the same low probability of occurrence (once every 100 years). We followed the International Drought Experiment protocol of reducing incoming precipitation during three consecutive years with rain-out shelters in field plots and compared them with control plots. We estimated changes in forage productivity at nine experimental sites during the peak growing season, considering the biomass accounted for by species frequently consumed by domestic grazers using to local knowledge.

Results/Conclusions
We found that arid and semi-arid rangelands were more severely impaired by drought in their forage supply than mesic rangelands. Forage productivity decreased on average by ca. 50% during three consecutive drought years in arid and semi-arid rangelands, whereas mesic sites did not differ significantly between drought and control treatments. Consequently, the sensitivity to drought of forage productivity increased across the aridity gradient. The negative impact on forage productivity of arid and semi-arid rangelands was mainly driven by severe reduction in the productivity of a few dominant forage species. In seven of the nine rangelands, we found detrimental effects on forage productivity during the first experimental year, and in five of those sites the effect of drought was accentuated with multi-year droughts.

Our main findings indicate that the imposed extreme drought halved forage supply for the more arid sites. This pattern highlights the urgent need to implement strategies to mitigate the detrimental consequences of expected water shortages, particularly in arid and semiarid rangelands, where forage provision is strongly associated with ecosystem conservation and human well-being. Our study incorporates local knowledge to improve our understanding of the relationship between the provision of ecological services and people’s perception.