Implications of Prehistoric Burning for Management of Southwestern Forests
Investigators (most current known information)
In 2005, the International Arid Lands Consortium (IALC) funded a two-year research project to investigate the long-term relationships between prehistoric indigenous societies, climate change, and fire regimes in Southwestern ponderosa pine forests. The ultimate goal of this project was to evaluate hypotheses concerning the sustainability and resilience of indigenous human land-uses — including applied burning, inadvertent fire suppression, or no impact – to improve the context of contemporary decision-making. These project goals are consistent with IALC priorities, including the explicit investigation of human and historical dimensions (theme 5) of land-use and reclamation (theme 1) and ecosystem processes to enhance the management and restoration of ecological systems (theme 3) by demonstrating the utility of geoarchaeological methods for applied historical ecology in arid lands fire management (theme 4).
This research was guided by three fundamental hypotheses: 1) prehistoric, Ancestral Pueblo farmers and historic period Western Apache peoples used landscape burning for a variety of purposes, including agriculture, hunting, wild-plant management, ritual, and warfare, 2) indigenous, anthropogenic burning could be identified in multi-proxy paleoecological records from landscapes with known occupation histories, and 3) the long-term impact of anthropogenic burning would have altered the resilience of these landscapes to centennial scale climate change.
To evaluate these hypotheses, the research team required:
- An independent, millennial-length reconstruction of annual climate variability with known relevance to contemporary and historic fire activity.
- Fire history reconstructions with spatial and temporal scales relevant to prehistoric and historic period indigenous occupations and land-use.
For (1), the researchers combined two millennial length tree-ring based precipitation reconstructions for the southern Colorado Plateau. Second, the researchers calibrated the precipitation records with fire history data from more than 700 fire scarred ponderosa pine trees to reconstruct centennial scale variation in the frequency of interannual climate patterns analogous to those seen in the fire-scar and modern fire records.
For (2), the researchers adapted sedimentary charcoal-based fire history methods for use with other proxies from alluvial sedimentary records. The researchers surveyed four small (<40km 2 ), discontinuous, ephemeral stream valleys in the Black Mesa Ranger District of Apache-Sitgreaves National Forest and a fifth valley on the Fort Apache Indian Reservation in east-central Arizona. Each of these watersheds was located exclusively in ponderosa pine forest and had been previously surveyed by archaeologists for evidence of prehistoric and historic period human occupation. On the basis of archaeological and ethnohistoric evidence, these five watersheds represent a gradient of indigenous occupation.
- Two watersheds (Rocky Draw and Sharp Hollow) were not occupied.
- Two watersheds (Day Wash and Willow Wash) were occupied during the prehistoric period, depopulated by AD 1325 and probably used episodically or regularly by Western Apaches between AD 1550-1870.
- The Forestdale Valley was occupied prehistorically until AD 1400 and regularly used by Western Apaches after AD 1550.
A variety of samples were collected from cores or narrow, hand excavated trenches in exposed, alluvial sediments and soils at 18 stratigraphic localities in these five watersheds. Continuous, undisturbed soil and sediment monoliths were collected from 10 of these localities for sedimentary charcoal analysis and palynology. Opportunistic bulk and undisturbed samples were collected from lithologic and soil stratigraphic units for grain-size, carbonate, phosphorous, stable carbon isotope ratio, organic carbon analyses, and soil micromorphology.
Additional samples for charcoal, bulk soil analyses, palynology, and micromorphology were collected from a shallow alluvial channel fan that probably accumulated after the 1974 high severity Day Burn and the 2002 high severity Rodeo-Chediski fire. These samples served as analogs for identifying evidence for ancient high severity fires in our stratigraphic sequences.
Variation in biomass burning in the watershed was inferred using concentrations of macroscopic (>250µm) charcoal (related to the amount of biomass burned and the type of fuel) and soil phosphorous concentrations (related to the amount of biomass burned at low severity). Accelerator mass spectrometer radiocarbon measurements of detrital charcoal were used to provide age-control for all stratigraphic sequences. Fourier Transform Infrared Spectrometry (FTIR), and Differential Thermal/Thermogravimetric Analysis (DTA/TGA) of select charcoal samples were used to evaluate the post-depositional alteration of the sedimentary charcoal.
Variation in the relative frequencies with which particular patches were burned was inferred using pollen assemblages and stable carbon isotopes of soil organic matter. More frequently burned patches were expected to have higher influx of weeds, grasses, and other herbaceous plants into pollen assemblages relative to benchmark pollen studies in ponderosa pine forests. Many of these plants use a C4 photosynthetic pathway and were expected to enrich the carbon isotope ratios of alluvial soils and sediments, if the natural fire season was maintained.
Statistical modeling of climate-predicted fire frequencies between AD 572-1987 highlighted the period from AD 1300-1650 as a phase of reduced climate-driven fire frequencies. This period of time was expected to have elevated fuel loads on some landscapes, permitted increasing stand densities through recruitment of canopy species, and elevated the risk of these landscapes for larger patches of high severity fire during prolonged or severe droughts.
Field observations, micromorphology, and bulk soil data support the inference that the sampled stratigraphic sequences accumulated in channel fan, channel, and floodplain depositional contexts. Radiocarbon dating indicated that seven stratigraphic sequences accumulated over the period ca. AD 1100-1900. No unconformable boundaries were identified within these stratigraphic sequences and no prolonged hiatuses in sedimentation were inferred for these seven stratigraphic records. Three sequences that yielded radiocarbon dates during the early and middle Holocene (ca. 8,000-500 BC) were not included in the final analyses.
FTIR and DTA/TGA indicate that charcoal in carbonate rich sediments and soils from the Forestdale Valley may be diagenetically altered. Alternatively, charcoal from these deposits may have been combusted at lower temperatures.
Charcoal concentrations, radiocarbon dates, micromorphology, phosphorous, and pollen assemblages indicate that the records in unoccupied watersheds (Rocky Draw and Sharp Hollow) probably accumulated rapidly after watershed-altering, stand-replacing fires in the late AD 1300s or middle AD 1400s. By AD 1600, both watersheds had recovered to “natural” ponderosa pine forests and probably experienced the “natural” frequent, low-severity surface fire regime characteristic of the historic period.
Charcoal concentrations, radiocarbon dates, micromorphology, and pollen assemblages indicate that the record in the Day Wash watershed probably accumulated rapidly after stand-replacing fires in some portion “natural” of the watershed during the middle AD 1400s. By the 1500s, Day Wash had recovered to ponderosa pine forests and probably experienced the “natural” frequent, low-severity surface fire regime characteristic of the historic period. Between AD 1650-1850, elevated phosphorous and altered pollen assemblages indicate increased frequency of burning promoting fine, herbaceous understory plants. Pollen assemblages from this same period in Willow Wash indicate horticultural land-use of this area by Western Apaches, Yavapais, Hopis, and/or Zunis.
Charcoal concentrations, radiocarbon dates, micromorphology, phosphorous, and pollen assemblages from two localities in the Forestdale Valley indicate that increased fire activity above “natural” levels was associated with agricultural production during the prehistoric occupation (ca. AD 1150-1400). When the valley was unoccupied (ca. AD 1400-1550), charcoal, phosphorous, isotopes, and pollen assemblages were similar to “natural” fire regimes in the unoccupied watersheds. During the period of Western Apache occupation (after AD 1550), all three localities within Forestdale Valley had charcoal, pollen, and phosphorous evidence of increased fire frequency above “natural” levels that promoted wild grasses, herbs, and disturbance plants. Isotope and grain-size data suggest that some of these fires may have occurred during the fall or winter.
Both of the unoccupied watersheds and one of the two watersheds depopulated by prehistoric agriculturalists by AD 1325 experienced stand-replacing fires in all or part of their area during the period of climate-predicted vulnerability to such fires between AD 1300-1650. Specifically, prolonged droughts at the end of the 14th and in the middle of the 15th centuries were periods of increased fire severity in landscapes that were unoccupied or had been depopulated for more than a century.
In contrast, the Forestdale Valley had evidence for more than two centuries of prehistoric, agricultural burning until AD 1400. There was no evidence for increased fire severity in Forestdale Valley during prolonged and severe droughts of the late 14th, 15th , or 16th centuries.
Elevated fire frequencies and possibly altered fire seasonality between AD 1550-1900 in the Forestdale Valley and AD 1650-1850 in Day Wash and Willow Wash are consistent with the known practice of Western Apaches for fall burning to promote wild-food plants.
This study suggests:
- Anthropogenic burning by prehistoric Ancestral Pueblo and historic Western Apache people is identifiable in addition to natural fires in multi-proxy paleoecological data sets.
- Centennial-scale variation in climate-driven or “natural” fire regimes included increased areas of stand-replacing, high severity fires during prolonged or severe droughts that have followed a century or more of reduced surface fire frequencies.
- Anthropogenic burning reduced the vulnerability of Southwestern ponderosa pine forests to centennial scale climate change.
- But, the improved resilience of coupled human-natural ponderosa pine ecosystems did not persist more than a few decades after depopulation.
- Some ponderosa pine forests experienced a mosaic fire regime of late spring/summer fires and fall fires for more than three centuries of Western Apache occupation.
Management implications include:
- “Natural” prescribed fire may be insufficient to maintain resilient, open-canopied ponderosa pine forests in the face of predicted climate change.
- Management fires, in addition to thinning and prescribed “natural” fires may be necessary to achieve resilient, sustainable ponderosa pine forests and reduce vulnerability to large, high severity fires.
- A mosaic of management burning strategies, including fires during the early spring and during the fall, would be consistent with the recent fire history in some ponderosa pine forests in which Western Apache occupation is known.
Articles in Journals
Roos, C.I., T.W. Swetnam, V.T. Holliday, and B.J. Mills. Fire, climate, and land-use in the ancient southwest: historical ecology and implications for adaptive management. Sciencein October 2008 (to be submitted).
Roos, C.I, C. Nicholas M.A. Laluk, O.K. Davis, and I. Cohen-Ofri. Geoarchaeological evidence for protohistoric western apache landscape transformation in ponderosa pine forests of east-central Arizona. American Antiquity in September 2008 (to be submitted)
Roos, C.I., V.T. Holliday, P. Goldberg, and O.K. Davis. Evidence for climate-driven high severity fire in 14th and 15th century A.D. Southwest U.S. ponderosa pine forests. Palaeogeography, Palaeoclimatology, Palaeoecology in September 2008 (to be submitted).
Roos, C.I. and T.W. Swetnam. A comparison of two millennial-length reconstructions of climate predicted fire activity for ponderosa pine forests of the southern Colorado Plateau region, Southwest US. The Holocene in June 2008 (to be submitted).
Roos, C.I., A.P. Sullivan, III, and C. McNamee. 2008. Anthropogenic fire and long-term landscape management: paleoecological evidence for indigenous burning in the upland Southwest. (with A. P. Sullivan and C. McNamee). The Archaeology of Anthropogenic Environments, ed. R. Dean. Center for Archaeological Investigations, Southern Illinois University, Carbondale IL (in review).
Dissertations and Theses
Roos, C.I. 2008. Fire, climate, and social-ecological systems in the ancient Southwest: alluvial geoarchaeology and applied historical ecology in east-central Arizona . Unpublished PhD dissertation, University of Arizona, Tucson AZ.
Roos, C.I. 2008. Archaeological historical ecology of ponderosa pine fire regimes in east-central Arizona. Paper presented at the 2008 Association of Fire Ecology Symposium Fire in the Southwest: Integrating Fire into Management of Changing Ecosystems , 28-31 January 2008, Tucson AZ.
Roos, C.I. 2008. Archaeologically-informed applied historical ecology: fire, climate, and land-use in east-central Arizona. Invited lecture presented as part of the IGERT Seminar on Archaeological Sciences, 29 February 2008, University of Arizona, Tucson AZ.
Roos, C.I. 2008. Fire, climate, and coupled human-natural systems in east-central Arizona. Poster presented at the 20th Southwest Symposium, 17-19 January 2008, Tempe AZ.
Roos, C.I. 2008. Fire, climate, and indigenous people in ancient southwest us forests. Invited paper presented at the Kavli I nstitute of Theoretical Physics Miniconference on "Pyrogeography and Climate Change, " 27-30 May 2008, Santa Barbara CA.
Roos, C.I. 2008. Spatially explicit historical ecology of fire regimes, climate, and indigenous occupation in east-central Arizona. Invited paper presented at the 2008 annual meeting of the Association of American Geographers , 15-19 April 2008, Boston MA.
Roos, C.I. 2007. Fire and Socioecological Systems in East-Central Arizona. Invited poster presented at the 72nd annual meeting of the Society for American Archaeology , 25-29 April 2007, Austin TX.
Roos, C.I., A.P. Sullivan, III, and C. McNamee. 2007. Anthropogenic fire for long-term landscape management: geoarchaeological evidence for systematic burning in the upland Southwest. Paper presented at the Center for Archaeological Investigations' Visiting Scholar Conference, The Archaeology of Anthropogenic Environments, Southern Illinois University, Carbondale IL.
Roos, C.I. 2006. Geoarchaeology of fire and culturally modified environments in Arizona. Invited lecture presented as part of the Boston University Department of Archaeology Brown Bag series, 28 November 2006.
Roos, C.I. 2006. Human-modified fire regimes of the Forestdale Valley since AD 1150. Paper Presented at the 14th Mogollon Archaeology Conference , Tucson AZ. 14-15 October 2006.
Roos, C.I. 2006. Multi-proxy evidence for human and climatic influences on fire regimes of the Mogollon Rim region since AD 1150. Paper Presented at the 2 nd Archaeological Sciences of the Americas Symposium, 13-16 September 2006, Tucson AZ.
Roos, C.I. 2006. Variation in fire-conducive climate over the last 1400 years. Paper Presented at the 50th Annual Meeting of the Arizona-Nevada Academy of Science, Tucson AZ.
Altaha, M., N.C. Laluk, and C.I. Roos. 2006. Apache cultural values and research for landscape preservation. Poster presented at the 71st annual meeting of the Society for American Archaeology, San Juan PR.
Roos, C.I., N.C. Laluk, and M. Altaha. 2006. Fire and the ancient management of the Forestdale landscape. Invited poster presented at the 71st annual meeting of the Society for American Archaeology, San Juan PR.
Roos, C.I. 2007. Were wildland fires "natural" prior to late 19th century Euroamerican settlement of the eastern Mogollon Rim Region? Glyphs 57(13): 5, 9.
Roos, C.I. and N.C. Laluk. 2008. Environmental Archaeology, Climate Change, and Traditional Cultural Values, In preparation. SAA Archaeological Record in May 2008 (to be submitted).