This thesis deals with the issue of historical vegetation reconstruction in semi-arid rangelands primarily through the application of stable carbon isotope techniques. The ecological history of most rangeland environments is poorly documented and for this reason it is difficult to quantify the impact of European land use. Desertification, degradation and associated vegetation change is occurring throughout the semi-arid and arid rangelands of the world. However, our understanding of the extent to which these systems have changed and the interactions between factors affecting changes is limited due to a lack of detailed historical monitoring. This thesis presents some new techniques, and builds on others recently developed, which can be used to recapture the past. In addition some of these techniques can be applied to contemporary monitoring of rangelands.
Stable carbon isotope analysis forms the fundamental tool used throughout this research as a means of quantifying past changes in the contribution of different photosynthetic types of plants (C3 and C4) to biomass production and to the diet of rangeland sheep over time. In rangeland environments of the tropics and subtropics the majority of grasses are isotopically distinct from most other plants because they fix carbon via the C4 photosynthetic pathway. Because the isotopic signature of plants remains relatively unchanged as it is incorporated into soil of the tissues of animals the relative contribution of C3 and C4 plants to biomass production and the diet of animals can be traced. In this thesis I have investigated the use of stable carbon isotope analysis in soil organic matter, sheep faeces deposited beneath shearing sheds and in wool as a means of reconstructing past vegetation change.
This research supports the general conclusions made in previous studies that isotopic analysis of soils can provide a coarse view of the changing contribution of C3 and C4 plants to soil carbon over time. My results showed a trend to more depleted o13C (stable isotope) values near the soil surface at locations where it is believed that C3 woody vegetation has increased in historical times. However, the results suggested that variability in the isotopic composition of different soil size fractions (a proxy for carbon pools with different turnover rates) was not related to the presence or absence of a change in vegetation, as has been reported or assumed in previous work. I proposed, although tentatively, that such variability may result from a run down in the biological activity of those soils due to degradation. Such a change may result in a biased incorporation of some organic compounds into particular fractions, while other fractions remain relatively resistant to new inputs. Soil isotope analysis is a useful means of determining broad shifts in vegetation. However, as a technique for reconstructing historical vegetation change for periods of decades to a few centuries it is rather limited as a consequence of poorly constrained temporal resolution.
Deposits of sheep faeces contain a rich record of environmental change, which until now has remained untapped. The unique design of Australian shearing sheds means that hidden beneath their floorboards can be a continuous deposit of sheep faeces potentially spanning the life of the shed. These deposits document the historical diet of sheep, at the time of shearing, over several decades and occasionally into the last century. For the field of rangeland ecology, such deposits are akin to cave deposits which contain long tenn archaeological information on material, cultural and occasionally environmental change spanning thousands of years...........