Fire and arrival of humans on the California Channel Islands
Dr Mark Hardiman, University of Portsmouth, UK
Recent studies have suggested that the first arrival of humans in the Americas during the end of the last ice age is associated with non-trivial anthropogenic influences on landscape, in particular with the use of fire which would have given even small populations the ability to have broad impacts on the landscape. Understanding the impact of these early people is complicated by the dramatic changes in climate occurring with the shift from full glacial to full interglacial conditions. Despite these difficulties here we attempt to test the extent of anthropogenic influence using the Californian Channel Islands as a smaller, landscape scale test bed.
In 1954 Phil C. Orr discovered two human femora on Santa Rosa Island, California Channel Islands deposited in water lain sediments at the mouth of Arlington Springs Canyon which have become known as ’Arlington Springs Man’ direct and indirect radiocarbon suggest an age no younger than c.13,000 yrs BP. The probable stratigraphic positioning of the human remains has also been recently ascertained. This presentation places these existing investigations into a wider stratigraphic framework of the California Channel Islands. Over 20 field sections from Arlington Canyon coupled with >30 new radiocarbon dates (dating from c.20-11,000 yrs BP) has allowed a detailed chronostratigraphy to be developed for the Last Glacial-Interglacial Transition. Landscape changes and development both before and after ’Arlington Springs Man’ are discussed including possible changes in fire activity, vegetation, soil development and fluvial processes.
Fire, climate and humans in Western North America: Continental to mountain range-scale insights from tree rings
Professor Tom Swetnam, University of Arizona, USA
Forests and human communities in western North America are now extremely vulnerable to large, severe wildfires as a consequence of fire exclusion and other land use practices and increasingly warm and persistent droughts. Combined paleoecological and archaeological studies of climate, fire and human histories from the same landscapes can help reveal the relative roles of people and climate in driving past and modern trends in fire regimes. In this paper we evaluate human and climate controls of wildfire using (1) a mountain range-scale network of more than 1,000 fire scarred trees from the Jemez Mountains, New Mexico (~2,000 km2), and (2) a continental-scale network of fire history reconstructions from more than 800 sites in western North America (~3,000,000 km2), including 2,000 year-long fire histories from giant sequoia groves in the Sierra Nevada of California. The Jemez Mountains are an instructive case of the influences of long-time human occupation on fire-prone, forested landscapes. Here, in ponderosa pine and mixed conifer forests, we find that humans eliminated widespread surface fires during peak populations prior to 1650 CE, whereas many small fires still occurred at locations distant from villages and agricultural fields. After depopulation of forested areas circa 1680 CE, widespread surface fires became much more regular and these extensive fires were highly correlated with wet/dry switching of moisture regimes. Continental-scale fire climate patterns emphasize the consistently important roles of inter-annual to decadal-scale climate variations on spatial and temporal fire activity, regardless of human influences at finer spatial scales during recent millennia.
Fire and humans in Australian food webs from the Pleistocene to the Anthropocene
Professor David Bowman, University of Tasmania, Australia
Australia is an island continent with ecosystems renowned for their flammability, high endemism of plants and animals, the keystone role of marsupial herbivores and carnivores, ancient Aboriginal traditions of landscape burning, and more recently, extraordinarily high mammalian extinctions and irruptions of mammalian herbivores and carnivores introduced by Europeans.
There much debate about the causes and consequences of the recent extinctions in uncleared Australia landscapes. Several prominent theories have stressed, to differing degrees, the importance of change fire regimes following the breakdown of Aboriginal land management and the introduction of non-native herbivores and carnivores. Some of these theories have also highlighted the legacy effects of the initial impact of Aboriginal colonisation in the late Pleistocene, including the extinction of very large marsupial herbivores and carnivores, collectively known a ‘megafauna’.
It is possible that specific spatiotemporal patterns of landscape burning (pyrodiversity) influences habitat quality and hence mammal diversity and abundance. Recursively, pyrodiversity is reinforced by mammalian food webs via a range of ecological processes (such as nutrient cycling, soil turnover, plant regeneration and growth, plant species diversity because mammals affect ecosystems process via digging, trampling, phytomass off-take, seed and spore dispersal, and concentrating nutrients) all of which directly and indirectly influence fire regimes. Critically, humans are both keystone predators that regulate herbivores and affect pyrodiversity by controlling the frequency, extent and seasonality of landscape fire.
This human controlled food web – pyrodiversity hypothesis has substantial management implications for restoring ecosystems in the Anthropocene globally. While manipulating fire regimes, humans must simultaneously manage mammalian food webs, possibly by introducing new species to compensate for the extinction of herbivorous and carnivorous vertebrates that have occurred recent and more distant past.
The deforestation story: testing for anthropogenic origins of Africa's flammable grasslands
Professor William Bond, South African Environmental Observation Network and University of Cape Town, South Africa
The African continent has the largest extent of savannas and associated grasslands in the world. The savannas support frequent fires so that Africa also accounts for the largest fraction of the world’s annual burnt area than anywhere else. Thus Africa is an interesting test case for the influence of fire on vegetation and of humans in shaping fire regimes and thereby shaping the vegetation. For more than a century, Africa’s grasslands have been widely interpreted as the products of deforestation promoted by anthropogenic burning. The deforestation hypothesis has led to official policies of fire suppression in many African countries. It has motivated plans for large-scale ‘reforestation’ projects targeting, in particular, Africa’s humid savannas and grasslands. Here I review different lines of evidence for the age and origins of African savannas and the fires that maintain them. These point to ancient origins of the grassy biomes long before intensive human use of fire. Nevertheless, except for the last few decades, there is considerable uncertainty as to the influence of people in expanding the area of derived savannas at the expense of the forest they are supposed to have displaced. A better understanding of the attributes of ancient versus secondary grasslands is needed to evaluate human impacts on the spread of savannas.