Using a spatially explicit ecological model to test scenarios of fire use by Native Americans: An example from the Harlem Plains, New York, NY
William T. Bean and Eric W. Sanderson. 2007. Using a spatially explicit ecological model to test scenarios of fire use by Native Americans: An example from the Harlem Plains, New York, NY. Ecological Modelling 211 (2008) pp. 301–308.
Full text [here]
Selected excerpts:
Abstract
It is unclear to what extent Native Americans in the pre-European forests of northeast North America used fire to manipulate their landscape. Conflicting historical and archaeological evidence has led authors to differing conclusions regarding the importance of fire. Ecological models provide a way to test different scenarios of historical landscape change.We applied FARSITE, a spatially explicit fire model, and linked tree mortality and successional models, to predict the landscape structure of the Harlem Plains in pre-European times under different scenarios of Native American fire use. We found that annual burning sufficed to convert the landscape to a fire-maintained grassland ecosystem, burning less often would have produced a mosaic of forest and grasslands, and even less frequent burning (on the order of once every 20 years) would not have had significant landscape level effects. These results suggest that if the Harlem Plains had been grasslands in the 16th century, they must have been intentionally created through Native American use of fire.
Introduction
The use of fire by Native Americans in northeast North America has been the subject of much debate shared among a broad group of ecologists, archaeologists and environmental historians. Some like Day (1953), Cronon (1983) and Krech (1999) believe that Native Americans used fire often to manipulate their landscape, and that these manipulations may have taken place over broad extents in the pre-European forests.
Skeptics admit that the rate of forest fires around a village might have been elevated over a background rate because Northeast Indians were using fire for cooking and pottery. However, they find little evidence that fires were widespread or intentionally set (Russell, 1983). Early settlers rarely offer first-hand accounts of fires and fewer still tell of intentional burning. These, Russell says, might be attributed to escaped fires.
Intentional burning has many potential benefits for hunting and gathering peoples: frequent fires can clear tangled vegetation, making it easier to travel through and to clear for horticulture (Lewis, 1993); fire can create vegetation mosaics that are attractive to deer and other game species, and make hunting easier (Williams, 1997); sometimes people set fires just for fun (Putz, 2003).
Of course different fire regimes have different effects on the ecology of Northeast forests. A frequent fire regime would favor a grassland with lingering oaks, a fire-tolerant genus (Swan, 1970; Abrams, 1992, 2000). Less frequent fire would lead to regenerating forests (Abrams, 1992). Understanding Native American use of fire is important for understanding the structure and function of pre-European forests.
As part of a larger project to reconstruct the past landscape structure of Manhattan Island (Sanderson, 2003),we examined the use of fire by Native Americans in the Harlem Plains of what is now New York City, NY. Although records are sparse, a few accounts of pre-settlement Harlem exist. De Rasieres describes in 1624 “a large level field, of from 70 to 80 morgens (140–160 acres) of land, through which runs a very fine fresh stream; so that the land can be ploughed without much clearing” (de Rasieres, 1990). Historical maps consistently refer to this area as a plain.
Numerous historical accounts describe grasslands (the most famous of which were the Hempstead Plains of Long Island) (Svenson, 1936; Stalter and Lamont, 1987; Stalter et al., 1991) as well as patchier areas (Wood, 1824) in the New York region. But while the grasslands of the Hempstead Plains appear to be the result of primarily edaphic conditions –- specifically dry, sandy soil (Svenson, 1936) –- the Harlem Plains seemed to have quite fertile soil, underlain by calcareous bedrock (Baskerville, 1994). Dutch settlers in New Netherlands chose it as one of the first spots to farm on Manhattan (Stokes, 1967). Perhaps the most similar modern-day equivalent to the early Harlem Plains is Floyd Bennett Field in Jamaica Bay, Brooklyn. Recent prescribed burns have maintained the grasslands there, but without constant maintenance, shrublands quickly appear and would soon succeed to forest (Lent et al., 1997).
The Native Americans who lived on Manhattan at the time of European settlement were the Lenni Lenape, or Delaware, people (Cantwell andWall, 2001). A number of small groups of Lenape Indians lived on Manhattan, from the southern tip to what is nowGreenwich Village, Inwood, and the Harlem Plains (Bolton, 1920). The Lenape had a pattern of land rotation, clearing an area and planting maize for a number of years, then letting the fields growback for approximately 20 years (van der Donck, 1841).
Presumably they, as with other Algonquin people, used fire to clear the land for horticulture. What is clear is that within years of removal of the Lenape from the area, much of the land in Harlem became overgrown and reverted to forest (Riker, 1881).
From this history, we hypothesize that if the Harlem Plains could “be ploughed without much clearing” as De Raiseres describes, they likely consisted of grasslands intentionally created by Lenape use of fire. To test this hypothesis, we used a combination of ecological models parameterized under different scenarios of Native American fire use to examine what different landscape configurations would result from different fire regimes. In short, how often would the Lenape have had to burn Harlem to maintain a grassland ecosystem?
Methods
To examine the landscape level consequences of Lenape fire use in Harlem, we constructed an interlinked set of spatially explicit models (Fig. 1). First, the fire model FARSITE (Finney, 1998) was used to model the distribution and intensity of fire under different fire frequency scenarios. Second, the results of FARSITE at each time step were fed into a tree-mortality model, yielding vegetation changes as a result of fire in a given year. Third, a successional model was applied to the entire landscape to represent regeneration over time. Our model closely follows Li’s (1999) suggested methods for reconstructing historic fire regimes, however we do not make the assumption that forest cover types remain the same before and after the fire and succession.
The model was run at annual time steps for 200 years and with spatial resolution of 10m2. 200 years was assumed to be long enough to clarify the emergent patterns of the burning regimes — that, whatever the initial conditions, the resultant landscape would emerge over a long enough time frame. Fires were assumed to be associated with agricultural clearance and therefore all to have occurred in early April.
Fire frequency scenarios were tested with ignitions every year, every 10 and 20 years. Four different input vegetation maps were used: forest, old savanna, young savanna and grasslands. Originally, we had intended to start each scenario as old growth forest (the assumed climax vegetation for Manhattan). In the process of running the models, it became clear that the Lenape may have been forced to initiate a number of fires in quick succession to clear the forest, but that after the first clearance, a longer interval between fires would have sufficed. Our question was not how the Lenape created an open grassland or savanna, but whether they would have been required to maintain it. …
Discussion
Our model results indicate that by controlling fire frequency in the pre-settlement Harlem Plains, the Lenape people could control the structure of the landscape. Van der Donck’s claim that they cleared the land every 20 years does not appear to be supported by our model results. For the land to be a “plain” or “grassland” the landscape would have had to been burned at least once every 10 years and, depending on initial condition, would have yielded a mosaic of vegetation types. Burning every year overwhelms succession through disturbance and keeps the landscape in a grassland steady state.
Thinking back to the putative reasons for Native American fire use, it seems that if the main goal was to create open views, then an annual fire frequency would be most appropriate. If the purpose was to create a mosaic of wildlife habitat and facilitate hunting with cover and open spaces, then burning once every 10 years would suffice. Burning every 20 years is not sufficient to change the landscape structure, though it still might be fun!
Our results suggest that the hardwood forests of the northeast required significant maintenance in order to keep them clear and open—escaped fires and lightning strikes would not suffice to maintain a savanna or grassland. We believe this adds to the consensus that, while individual historical accounts may be suspect, the evidence continues to suggest that Native Americans were using fire to control their landscape, not only in the western and plains states, but also in the northeast.
In a larger context, our results indicate the utility of ecological models to address historical questions of human–nature interactions. Though models are formulated largely for modern purposes, like FARSITE (used mainly to predict wildlife spread in the western US), there is no reason they cannot be applied in historical scenarios, given that the model inputs can be reconstructed for a past time.
Addressing historical and archaeological questions through ecological reconstructions provides a new way to gain insights about our past. While others have created fire succession models –- as reviewed in Keane et al. (2004) –- and used them to explore historical fire regimes –- Scheller et al. (2005) –- this is the first study undertaken to model pre-colonial anthropogenic fire disturbance in northeastern North America. …
