Fire in Australian savannas: from leaf to landscape.

Savanna ecosystems comprise 22% of the global terrestrial surface and 25% of Australia (almost 1.9 million km2) and provide significant ecosystem services through carbon and water cycles and the maintenance of biodiversity. The current structure, composition and distribution of Australian savannas have coevolved with fire, yet remain driven by the dynamic constraints of their bioclimatic niche. Fire in Australian savannas influences both the biophysical and biogeochemical processes at multiple scales from leaf to landscape. Here, we present the latest emission estimates from Australian savanna biomass burning and their contribution to global greenhouse gas budgets. We then review our understanding of the impacts of fire on ecosystem function and local surface water and heat balances, which in turn influence regional climate. We show how savanna fires are coupled to the global climate through the carbon cycle and fire regimes. We present new research that climate change is likely to alter the structure and function of savannas through shifts in moisture availability and increases in atmospheric carbon dioxide, in turn altering fire regimes with further feedbacks to climate. We explore opportunities to reduce net greenhouse gas emissions from savanna ecosystems through changes in savanna fire management.

Multiple livelihood strategies and high floristic diversity increase the adaptive capacity and resilience of Sri Lankan farming enterprises.

Globally farmers are challenged by increasing climate variability and frequency of extreme events. However, traditional farming enterprises demonstrate resilience. To understand the underlying reasons, traditional farming enterprises in Sri Lanka were explored. Eighty-five farming enterprises were sampled across nine locations of the Intermediate agroecological zone using mixed methods. Farming enterprises incorporate On- and Off-farm livelihood components (graphical abstract). On-farm refers to landholdings with land uses including tree-dominant forest gardens (FGs), paddy, cash crops, swidden plots (chenas), plantations and livestock. Off-farm includes employment, trading and grants. We investigated how farming enterprises remained resilient, and which land use had the greatest adaptive capacity and best fulfilled household needs? Farming enterprises were assessed with respect to water availability, farmers' perspectives of climate variability, their socioeconomic characteristics, and land uses in landholdings. Land uses were characterised and compared by floristic diversity, crop: utility benefits, food functions, and those consumed and sold. Results revealed that most respondents were women, had primary school education and engaged full-time in farming. Cultivation was mainly rainfed. Farmers' perceptions of climate variability were supported by meteorological data showing that interannual and seasonal rainfall variability prevailed in the reference and preceding years. Farmers withstood these challenges owing to FGs, which were the oldest and dominant of all land uses, with larger area, highest plant and crop species richness, and crop diversity. Greater numbers of primary and secondary FG crops and products provided multiple household benefits. High floristic diversity, tree-dominance and multifunctionality gave FGs strong adaptive capacity. Nevertheless, farmers adopted multiple land uses with diverse landscape designs and Off-farm livelihood strategies simultaneously because this combination offered greater opportunities, buffered risk and increased resilience in farming enterprises. A clear implication of this study is that policymakers should engage with farmers when planning for a resilient agriculture in a variable climate scenario.

Vulnerability of native savanna trees and exotic Khaya senegalensis to seasonal drought.

Seasonally dry ecosystems present a challenge to plants to maintain water relations. While native vegetation in seasonally dry ecosystems have evolved specific adaptations to the long dry season, there are risks to introduced exotic species. African mahogany, Khaya senegalensis Desr. (A. Juss.), is an exotic plantation species that has been introduced widely in Asia and northern Australia, but it is unknown if it has the physiological or phenotypic plasticity to cope with the strongly seasonal patterns of water availability in the tropical savanna climate of northern Australia. We investigated the gas exchange and water relations traits and adjustments to seasonal drought in K. senegalensis and native eucalypts (Eucalyptus tetrodonta F. Muell. and Corymbia latifolia F. Muell.) in a savanna ecosystem in northern Australia. The native eucalypts did not exhibit any signs of drought stress after 3 months of no rainfall and probably had access to deeper soil moisture late into the dry season. Leaf water potential, stomatal conductance, transpiration and photosynthesis all remained high in the dry season but osmotic adjustment was not observed. Overstorey leaf area index (LAI) was 0.6 in the native eucalypt savanna and did not change between wet and dry seasons. In contrast, the K. senegalensis plantation in the wet season was characterized by a high water potential, high stomatal conductance and transpiration and a high LAI of 2.4. In the dry season, K. senegalensis experienced mild drought stress with a predawn water potential -0.6 MPa. Overstorey LAI was halved, and stomatal conductance and transpiration drastically reduced, while minimum leaf water potentials did not change (-2 MPa) and no osmotic adjustment occurred. Khaya senegalensis exhibited an isohydric behaviour and also had a lower hydraulic vulnerability to cavitation in leaves, with a P50 of -2.3 MPa. The native eucalypts had twice the maximum leaf hydraulic conductance but a much higher P50 of -1.5 MPa. Khaya senegalensis has evolved in a wet-dry tropical climate in West Africa (600-800 mm) and appears to be well suited to the seasonal savanna climate of northern Australia. The species exhibited a large phenotypic plasticity through leaf area adjustments and conservative isohydric behaviour in the 6 months dry season while operating well above its critical hydraulic threshold.