Humans and their ancestors are unique in being a fire-making species, but ‘natural’ (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from ‘natural’ background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research.

The human dimension of fire regimes on Earth

SYNTHESIS The human dimension of fire regimes on Earth

Human use of land has transformed ecosystem pattern and process across most of the terrestrial biosphere, a global change often described as historically recent and potentially catastrophic for both humanity and the biosphere. Interdisciplinary paleoecological, archaeological, and historical studies challenge this view, indicating that land use has been extensive and sustained for millennia in some regions and that recent trends may represent as much a recovery as an acceleration. Here we synthesize recent scientific evidence and theory on the emergence, history, and future of land use as a process transforming the Earth System and use this to explain why relatively small human populations likely caused widespread and profound ecological changes more than 3,000 y ago, whereas the largest and wealthiest human populations in history are using less arable land per person every decade. Contrasting two spatially explicit global reconstructions of land-use history shows that reconstructions incorporating adaptive changes in land-use systems over time, including land-use intensification, offer a more spatially detailed and plausible assessment of our planet's history, with a biosphere and perhaps even climate long ago affected by humans. Although land-use processes are now shifting rapidly from historical patterns in both type and scale, integrative global land-use models that incorporate dynamic adaptations in human-environment relationships help to advance our understanding of both past and future land-use changes, including their sustainability and potential global effects.

https://www.pnas.org/content/pnas/110/20/7978.full.pdf

Used planet: a global history.

In many parts of the world forest disturbance regimes have intensified recently, and future climatic changes are expected to amplify this development further in the coming decades. These changes are increasingly challenging the main objectives of forest ecosystem management, which are to provide ecosystem services sustainably to society and maintain the biological diversity of forests. Yet a comprehensive understanding of how disturbances affect these primary goals of ecosystem management is still lacking. We conducted a global literature review on the impact of three of the most important disturbance agents (fire, wind, and bark beetles) on 13 different ecosystem services and three indicators of biodiversity in forests of the boreal, cool- and warm-temperate biomes. Our objectives were to (i) synthesize the effect of natural disturbances on a wide range of possible objectives of forest management, and (ii) investigate standardized effect sizes of disturbance for selected indicators via a quantitative meta-analysis. We screened a total of 1958 disturbance studies published between 1981 and 2013, and reviewed 478 in detail. We first investigated the overall effect of disturbances on individual ecosystem services and indicators of biodiversity by means of independence tests, and subsequently examined the effect size of disturbances on indicators of carbon storage and biodiversity by means of regression analysis. Additionally, we investigated the effect of commonly used approaches of disturbance management, i.e. salvage logging and prescribed burning. We found that disturbance impacts on ecosystem services are generally negative, an effect that was supported for all categories of ecosystem services, i.e. supporting, provisioning, regulating, and cultural services (P < 0.001). Indicators of biodiversity, i.e. species richness, habitat quality and diversity indices, on the other hand were found to be influenced positively by disturbance (P < 0.001). Our analyses thus reveal a 'disturbance paradox', documenting that disturbances can put ecosystem services at risk while simultaneously facilitating biodiversity. A detailed investigation of disturbance effect sizes on carbon storage and biodiversity further underlined these divergent effects of disturbance. While a disturbance event on average causes a decrease in total ecosystem carbon by 38.5% (standardized coefficient for stand-replacing disturbance), it on average increases overall species richness by 35.6%. Disturbance-management approaches such as salvage logging and prescribed burning were neither found significantly to mitigate negative effects on ecosystem services nor to enhance positive effects on biodiversity, and thus were not found to alleviate the disturbance paradox. Considering that climate change is expected to intensify natural disturbance regimes, our results indicate that biodiversity will generally benefit from such changes while a sustainable provisioning of ecosystem services might come increasingly under pressure. This underlines that disturbance risk and resilience require increased attention in ecosystem management in the future, and that new approaches to addressing the disturbance paradox in management are needed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/brv.12193

Natural disturbance impacts on ecosystem services and biodiversity in temperate and boreal forests.

Forest and savanna biomes dominate the tropics, yet factors controlling their distribution remain poorly
understood. Climate is clearly important, but extensive savannas in some high rainfall areas suggest
a decoupling of climate and vegetation. In some situations edaphic factors are important, with forest often
associated with high nutrient availability. Fire also plays a key role in limiting forest, with fire exclusion often
causing a switch from savanna to forest. These observations can be captured by a broad conceptual model with
two components: (1) forest and savanna are alternative stable states, maintained by tree cover-fire feedbacks,
(2) the interaction between tree growth rates and fire frequency limits forest development; any factor that
increases growth (e.g. elevated availability of water, nutrients, CO2), or decreases fire frequency, will favour
canopy closure. This model is consistent with the range of environmental variables correlated with forest
distribution, and with the current trend of forest expansion, likely driven by increasing CO2 concentrations.
Resolving the drivers of forest and savanna distribution has moved beyond simple correlative studies that are
unlikely to establish ultimate causation. Experiments using Dynamic Global Vegetation Models, parameterised
with measurements from each continent, provide an important tool for understanding the controls of these
systems.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1461-0248.2012.01771.x

What controls the distribution of tropical forest and savanna

Humans have altered natural patterns of fire for millennia, but the impact of human-set fires is thought to have been slight in wet closed-canopy forests. In the South Island of New Zealand, Polynesians (Māori), who arrived 700–800 calibrated years (cal y) ago, and then Europeans, who settled ∼150 cal y ago, used fire as a tool for forest clearance, but the structure and environmental consequences of these fires are poorly understood. High-resolution charcoal and pollen records from 16 lakes were analyzed to reconstruct the fire and vegetation history of the last 1,000 y. Diatom, chironomid, and element concentration data were examined to identify disturbance-related limnobiotic and biogeochemical changes within burned watersheds. At most sites, several high-severity fire events occurred within the first two centuries of Māori arrival and were often accompanied by a transformation in vegetation, slope stability, and lake chemistry. Proxies of past climate suggest that human activity alone, rather than unusually dry or warm conditions, was responsible for this increased fire activity. The transformation of scrub to grassland by Europeans in the mid-19th century triggered further, sometimes severe, watershed change, through additional fires, erosion, and the introduction of nonnative plant species. Alteration of natural disturbance regimes had lasting impacts, primarily because native forests had little or no previous history of fire and little resilience to the severity of burning. Anthropogenic burning in New Zealand highlights the vulnerability of closed-canopy forests to novel disturbance regimes and suggests that similar settings may be less resilient to climate-induced changes in the future.

/pdf/rapid-landscape-transformation-in-south-island-new-zealand-3rb14cdm4e.pdf

Rapid landscape transformation in South Island, New Zealand, following initial Polynesian settlement

We combine local knowledge of elders and environmental practitioners from two indigenous Māori communities and pollen evidence in soil cores from two islands and two mainland coastal sites to inform the planning of coastal ecosystem restoration initiatives in New Zealand. The Māori participants desired ecosystems that delivered cultural (e.g., support for identity), social (e.g., knowledge transfer), economic (e.g., agroecology) and environmental (e.g., biodiversity protection) outcomes to their communities. Pollen records identified three periods when vegetation was dominated by different taxa: (1) Pre-human (<AD c.1280) – forest dominated by native conifers, angiosperms and nīkau palm (Rhopalostylis sapida); (2) Māori settlement (AD c.1280–1770) – scrub and bracken fern (Pteridium esculentum); and (3) European settlement (post-1770) – Metrosideros excelsa forest with harakeke (Phormium sp.), raupō (Typha orientalis), grasses (Poaceae), exotic plantation conifers (Pinaceae), and agricultural weeds. A fourth, aspirational system that integrated human activities such as agriculture and horticulture with native forest was conceptualized. Our approach emphasizes the importance of placing humans within nature and the reciprocity of environmental and social well-being.

Mairie Fromont

Papers

Rapid landscape transformation in South Island, New Zealand, following initial Polynesian settlement

Looking Back for the Future: Local Knowledge and Palaeoecology Inform Biocultural Restoration of Coastal Ecosystems in New Zealand