RT Dissertation/Thesis
T1 Estimating grass productivity under different clipping frequencies and rainfall amount: implications for rangeland responses to climate change
A1 Kawo,Samuel Tuffa
WP 2017/10/16
AB Resilience and sustainable use of rangelands depend on pre- and post-degradation management. A sustainable rangeland use can be achieved by maintaining its productivity in the long run. Herbivory and drought are the two main stressors reducing the primary production of rangelands and, hence, related ecosystem functions and services as well as livestock production. In semiarid rangelands, herbivore populations can be kept at its varying carrying capacity through encouraging animal harvesting when forage production decreases to avoid rangeland degradation. Degraded areas can also be restored through reseeding with appropriate local species to enhance rangeland resilience, particularly given the current and projected impacts of climate change to cope with the rapid disappearance of species and ecosystem services. However, it is unclear how grass species currently used in the reseeding respond to combined effects of herbivory and drought and how grazing cattle populations change under the current and the predicted increasing drought frequency.
The Borana rangeland, Ethiopia, had been highly productive and an important forage resource for livestock. However, its productivity has been reducing as a result of degradation, mainly caused by recurrent drought, land use change, livestock overgrazing and bush encroachment. Reseeding as a management strategy for the restoration of degraded rangelands and their ecosystem services has been urgently recommended. This is particularly urgent as in the face of human population increase a high demand for meat as a protein source is expected and mitigation strategies to capture CO2 from the atmosphere in the face of climate change is needed. Rangeland restoration through reseeding of palatable grass species can improve both structural and functional vegetation characteristics, which will also enhance food security. The main rangeland ecosystem services include, amongst others, provision of animal feeds (biomass and nutrients) and carbon (C) storage to capture CO2 from the atmosphere. However, the post reseeding rangeland management is still lacking knowledge on grass biomass allocation, nutrient and C storage under the influence of herbivory and drought. In addition, the belowground biomass and C storage of grasses have been rarely studied in grasslands, and in this study, we quantified these two variables under the two major grass stressors, clipping and irrigation (simulated herbivory and rainfall regime, respectively). This dissertation aimed at filling this knowledge gap of pre- and post-reseeding rangeland management in order to inform policy makers for devising an appropriate strategy for a sustainable use of rangeland resources. Responses of two dominant perennial grass species (Cenchrus ciliaris and Chloris gayana) frequently used in reseeding to simulated herbivory and rainfall regimes were assessed in pot and field plot experiments on young grasses. Further, we addressed how herbivory influences biomass allocation, nutritive values and C storage in mature tufts of these two native grasses under ambient rainfall conditions.
The responses of biomass and C storage showed contrasting results across grass age as well as species. Generally, the clipping/grazing strongly triggered the belowground biomass allocation and enhanced C storage of C. ciliaris tufts while C. gayana tufts differed only slightly. In both mature grasses, however, clipping highly reduced aboveground biomass and C storage. In contrast, for the young grass seedlings, moderate and light clipping triggered regrowth and, hence, biomass and C storage in both above- and belowground parts. Meanwhile, reduced irrigation showed the same effect on biomass allocation and C storage in both study grasses. Lower irrigation highly reduced biomass and C in both above- and belowground parts.
The results presented in this dissertation highlight that the effects of herbivory and rainfall variability, as well as grass maturity, should be incorporated into the management of rangelands. Our experiments established the first interactive effect of herbivory and rainfall on the biomass allocation and C storage of mature and young grasses in the semiarid Borana rangelands, Ethiopia. Knowledge of these interacting factors is deemed essential for policy makers to develop a sound rangeland management policy that can enhance the C storage potential of degraded rangelands under climate change and, hence, the mitigation and adaptation strategies through improved post restoration of degraded areas.
We also modeled cattle population dynamics under a varying carrying capacity and stochastic environmental conditions, which has never been done before in the semi-arid Borana rangeland ecosystem. Modeling cattle population dynamics is essential for capturing changes in population responses to climate change in a variable social and ecological environment at a large temporal scale. We developed and evaluated a novel Boran cattle population trajectory model under different drought frequency events, using differences in vital rates among age- and sex-classes as well as a varying carrying capacity. Stochasticity was built into the model by allowing droughts to occur randomly within model runs in Berkeley Madonna software, with different long-term average drought frequencies characterizing the individual four model scenarios. The model result indicated that reduction in rainfall, i.e., increasing drought frequency - as predicted for Borana - leads to a high loss in cattle populations. The overall population size was highly sensitive to the sale of juvenile as well as mature female cattle when drought hit the system. The stochastic population modeling under varying carrying capacity in the face of increasing drought scenarios indicated the livelihood challenges ahead for the pastoral community. Cattle populations must be limited timely and grass productivity must be enhanced in a sustainable way in the face of climate change. Therefore, this dissertation aimed at rangeland pre- and post-degradation management suggestions by modeling cattle populations and through grass experiments, respectively.
Management should focus on lowering cattle herd crashes through increasing sale of mature males that increases feed availability to females during drought years in the Borana Rangelands as well as enhancing the resilience capacity of rangelands through maintaining healthy conditions and restoring degraded areas. Further, drought early-warning systems and market information must be strengthened so that pre-planned selling can be realized for the fair and sustainable use of the animal resource. Pastoralists would benefit from this approach as they could sell their animals before drought wipes out their cattle in huge numbers.
K1 Weideland 
PP Hohenheim
PB Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
UL http://opus.uni-hohenheim.de/volltexte/2017/1393