School of Animal, Plant and Environmental Sciences

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Biological and geophysical feedbacks with fire in the Earth system
(Environmental Research Letters, 2018-03-06) Archibald, S.; Lehmann, C.E.R.; Belcher, C.M.; Bond, W.J.; Bradstock, R.A.
Roughly 3% of the Earth's land surface burns annually, representing a critical exchange of energy and matter between the land and atmosphere via combustion. Fires range from slow smouldering peat fires, to low-intensity surface fires, to intense crown fires, depending on vegetation structure, fuel moisture, prevailing climate, and weather conditions. While the links between biogeochemistry, climate and fire are widely studied within Earth system science, these relationships are also mediated by fuels-namely plants and their litter-that are the product of evolutionary and ecological processes. Fire is a powerful selective force and, over their evolutionary history, plants have evolved traits that both tolerate and promote fire numerous times and across diverse clades. Here we outline a conceptual framework of how plant traits determine the flammability of ecosystems and interact with climate and weather to influence fire regimes. We explore how these evolutionary and ecological processes scale to impact biogeochemical and Earth system processes. Finally, we outline several research challenges that, when resolved, will improve our understanding of the role of plant evolution in mediating the fire feedbacks driving Earth system processes. Understanding current patterns of fire and vegetation, as well as patterns of fire over geological time, requires research that incorporates evolutionary biology, ecology, biogeography, and the biogeosciences.
The consequences of replacing wildlife with livestock in Africa
(Scientific Reports, 2017-12-01) Hempson, G.P.; Archibald, S.; Bond, W.J.
The extirpation of native wildlife species and widespread establishment of livestock farming has dramatically distorted large mammal herbivore communities across the globe. Ecological theory suggests that these shifts in the form and the intensity of herbivory have had substantial impacts on a range of ecosystem processes, but for most ecosystems it is impossible to quantify these changes accurately. We address these challenges using species-level biomass data from sub-Saharan Africa for both present day and reconstructed historical herbivore communities. Our analyses reveal pronounced herbivore biomass losses in wetter areas and substantial biomass increases and functional type turnover in arid regions. Fire prevalence is likely to have been altered over vast areas where grazer biomass has transitioned to above or below the threshold at which grass fuel reduction can suppress fire. Overall, shifts in the functional composition of herbivore communities promote an expansion of woody cover. Total herbivore methane emissions have more than doubled, but lateral nutrient diffusion capacity is below 5% of past levels. The release of fundamental ecological constraints on herbivore communities in arid regions appears to pose greater threats to ecosystem function than do biomass losses in mesic regions, where fire remains the major consumer.
Demographics of Eucalyptus grandis and implications for invasion
(AOSIS OpenJournals Publishing AOSIS (Pty) Ltd, 2017-03) Musengi, K.; Archibald, S.
Alien invasive species can have negative impacts on the functioning of ecosystems. Plantation species such as pines have become serious invaders in many parts of the world, but eucalypts have not been nearly as successful invaders. This is surprising considering that in their native habitat they dominate almost all vegetation types. Available theory on the qualities that characterise invasive species was used to assess the invasive potential of Eucalyptus grandis - a common plantation species globally. To determine rates of establishment of E. grandis outside plantations, we compared population demographics and reproductive traits at two locations in Mpumalanga, South Africa: one at higher elevation with more frost. Eucalyptus grandis has a short generation time. We found no evidence that establishment of E. grandis was limiting its spread into native grassland vegetation, but it does appear that recruitment is limited by frost and fire over much of its range in Mpumalanga. Populations at both study locations this played characteristics of good recruitment. Size class distributions showed definite bottlenecks to recruitment which were more severe when exposed to frost at higher elevations. Generally, the rate of spread is low suggesting that the populations are on the establishing populations’ invasion stage. This research gives no indication that there are any factors that would prevent eucalyptus from becoming invasive in the future, and the projected increase in winter temperatures should be a cause for concern as frost is currently probably slowing recruitment of E. grandis across much of its planted range. Conservation implications: Eucalyptus plantations occur within indigenous grasslands that are of high conservation value. Frost and fire can slow recruitment where they occur, but there are no obvious factors that would prevent E. grandis from becoming invasive in the future, and monitoring of its rates of spread is recommended.
Fire ecology of C3 and C4 grasses depends on evolutionary history and frequency of burning but not photosynthetic type.
(Ecological Society of America, 2015-10) Ripley, B.; Visser, V.; Christin, P.-A.; Martin, T.; Osborne, C.; Archibald, S.
Grasses using the C4 photosynthetic pathway dominate frequently burned savannas, where the pathway is hypothesized to be adaptive. However, independent C4 lineages also sort among different fire environments. Adaptations to fire may thus depend on evolutionary history, which could be as important as the possession of the C4 photosynthetic pathway for life in these environments. Here, using a comparative pot experiment and controlled burn, we examined C3 and C4 grasses belonging to four lineages from the same regional flora, and asked the following questions: Do lineages differ in their responses to fire, are responses consistent between photosynthetic types, and are responses related to fire frequency in natural habitats? We found that in the C4 Andropogoneae lineage, frost killed a large proportion of aboveground biomass and produced a large dry fuel load, which meant that only a small fraction of the living tissue was lost in the fire. C3 species from the Paniceae and Danthonioideae lineages generated smaller fuel loads and lost more living biomass, while species from the C4 lineage Aristida generated the smallest fuel loads and lost the most living tissue. Regrowth after the fire was more rapid and complete in the C4 Andropogoneae and C3 Paniceae, but incomplete and slower in the C3 Danthonioideae and C4 Aristida. Rapid recovery was associated with high photosynthetic rates, high specific leaf area, delayed flowering, and frequent fires in natural habitats. Results demonstrated that phylogenetic lineage was more important than photosynthetic type in determining the fire response of these grasses and that fire responses were related to the frequency that natural habitats burned.
Strategies for managing complex social-ecological systems in the face of uncertainty: Examples from South Africa and beyond.
(Resilience Alliance, 2015-03) Biggs, R.O.; Rhode, C.; Archibald, S.; Ocholla, P.O.; Phadima, L.J.; Kunene, L.M.; Mutanga, S.S.; Nkuna, N.
Improving our ability to manage complex, rapidly changing social-ecological systems is one of the defining challenges of the 21st century. This is particularly crucial if large-scale poverty alleviation is to be secured without undermining the capacity of the environment to support future generations. To address this challenge, strategies that enable judicious management of socialecological systems in the face of substantive uncertainty are needed. Several such strategies are emerging from the developing body of work on complexity and resilience. We identify and discuss four strategies, providing practical examples of how each strategy has been applied in innovative ways to manage turbulent social-ecological change in South Africa and the broader region: (1) employ adaptive management or comanagement, (2) engage and integrate different perspectives, (3) facilitate self-organization, and (4) set safe boundaries to avoid system thresholds. Through these examples we aim to contribute a basis for further theoretical development, new teaching examples, and inspiration for developing innovative new management strategies in other regions that can help address the considerable sustainability challenges facing society globally.
Transplant Experiments Point to Fire Regime as Limiting Savanna Tree Distribution
(Frontiers Media, 2018-09-18) Stevens, N.; Archibald, S.; Bond, W.J.
Plant species range shifts are predicted to occur in response to climate change. The predictions are often based on the assumption that climate is the primary factor limiting the distribution of species. However the distribution of grassy biomes in Africa cannot be predicted by climate alone, instead interactions between vegetation, climate and disturbance structure the ecosystems. To test if climatic variables, as predicted by an environmental niche model, determine the distribution limits of two common savanna tree species we established a transplant experiment at a range of latitudes and altitudes much broader than the distribution limits of our study species. We planted seedlings of two common savanna trees, Senegalia nigrescens and Colophospermum mopane, at eight paired high and low elevation sites across an 850 km latitudinal gradient in South African savannas. At each site seedlings were planted in both grassy and cleared plots. After 2 years of growth, rainfall, temperature and location inside or outside their distribution range did not explain species success. Grass competition was the only variable that significantly affected plant growth rates across all sites, but grass competition alone could not explain the distribution limit. Species distributions were best predicted when maximum tree growth rates were considered in relation to local fire return intervals. The probability of sapling escape from the fire trap was the most likely determinant of distribution limits of these two species. As trees grew and survived 100 s of kilometers south of their current range limits we conclude that climate alone does not explain the current distribution of these trees, and that climate change adaptation strategies for savanna environments based only on climatic envelope modeling will be inappropriate.

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