Causes and consequences of fire in the miombo woodlands of south-eastern Tanzania

Abstract

Disturbances such as fire and herbivory maintain the open tree canopies and grassy understoreys characteristic of southern African miombo woodlands. However, recent loss of woodland cover across the region via degradation threatens the future of these ecosystems and some of the services they provide to people. Understanding the causes and impacts of degradation is therefore a prerequisite to sustainable management of miombo woodlands. In this thesis I present a case study of Kilwa District, south-eastern Tanzania, to explore degradation at multiple scales, with a particular focus on fire. Fire in miombo woodlands has both detrimental and beneficial impacts – via complex effects on ecosystem structure, carbon emissions, local hazards and livelihoods. While fire is often assumed to be a major cause of degradation in inhabited landscapes, the empirical basis for this is weak and research exploring fire causes and impacts remains limited outside of protected areas. In this thesis I aim to assess causes and consequences of fire, their synergies and trade-offs, through three empirical research chapters. I ask: does fire drive degradation? What are the causes of fire? What are the impacts of fire on local peoples’ wellbeing?

In the first research chapter I establish tree demographic rates across Kilwa District and explore variables related to tree growth and mortality, based on the resurvey of a network of woodland plots consisting of ~1700 stems at 12 locations. Data were collected on large diameter (≥40 cm diameter at breast height) stems because large trees are keystone structures in miombo ecosystems with strong influences over biodiversity and carbon storage, but long-term studies exploring their demographics and potential threats have not been undertaken. I find that aboveground carbon storage and diversity of large trees remained stable over a period of nine years. Mortality was offset by stem growth and recruitment. Results from modelling and focus group discussions suggest that wounding, over-harvesting, and fire represent constraints on growth and/or survival of large diameter stems. Fire was not associated with increased mortality, but reduced growth rates of large trees. The evidence presented here shows that the current fire regime is not driving degradation in Kilwa, challenging prevalent narratives.

In the second research chapter, I explore human aspects of use and misuse of fire, including the motivations behind fire use and the extent to which human intentions drive the fire regime. Using data from interviews and focus group discussions in farming communities in combination with remote sensing, I identify intentions behind fire use and the perceived relative frequency and riskiness of fires set for different purposes. I found that the most common ignitions were intentional and important to livelihoods and that burning was optimised for the intended outcome. The largest land area burned during the late dry season when people burned for activities such as field preparation, and when environmental conditions encouraged fire spread. These findings offer an insight into how positive intentions shape the fire regime at the landscape scale.

The final research chapter focuses on the consequences of fire for local people. I used the capability approach to analyse, for the first time, fire impacts on multiple dimensions of wellbeing. I found that both controlled and uncontrolled fires were a means for improving wellbeing, particularly when used as a tool with a specific purpose. Fire was also a negative wellbeing converter, principally as a hazard unrelated to the original purpose, when it became uncontrollable. Most respondents considered these hazards as sometimes-unavoidable risks related to activities necessary to achieving broader wellbeing goals. Respondents reported that freedom and choice is a crucial component of wellbeing, supporting calls for integrated fire management which does not undermine the agency of those who inhabit fire-prone ecosystems, and who depend on their environment to improve their own wellbeing.