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Dendroecological testing of the pyroclimatic hypothesis in the central Great Basin, Nevada, USA
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In the Great Basin region of western North America, records of past climate and wildfirevariability are needed not only for fire use, but also for understanding the mechanisms behind the century-long expansion of pi ?non-juniper woodlands. The Mt. Irish area (Lincoln County, south-eastern Nevada) is aremote mountain ecosystem on the hydrographic boundary between the Great Basin and the ColoradoRiver Basin. Non-scarred ponderosa pines (Pinus ponderosaC. Lawson var.scopulorumEngelm.) and single-needle pinyons (Pinus monophyllaTorr. & Fre ?m.) were used to develop a tree-ring reconstruction of drought(mean PDSI for May?July from NV Climate Division 3) from 1396 to 2003. A hypothetical fire regime wasobtained from the PDSI reconstruction and from explicitly assumed relationships between climate andwildfire occurrence. A census of fire-scarred trees was then sampled at the study area, and crossdated fire-scar records were used to generate the fire history, independently of the pre-existing pyroclimatic model.Out of 250 collected fire-scar wood sections, 197 could be crossdated (about 89%from ponderosa pines),covered the period from 1146 to 2006, and contained 485 fire scars, 390 of which could be dated to a singleyear. Numerical summaries were computed for the period 1550?2006, when recorder trees ranged from 16to 169, using a total of 360 fire scars on 176 sections. Up to 1860, the time of Euro-American settlement, firesthat scarred at least two trees were very frequent (minimum fire interval: 1 year, mean: 4, median: 2,Weibull median: 3, maximum: 19), while fires that scarred at least 10%of the recorder trees were relativelyrare (minimum fire interval: 40 years, mean: 66, median: 50, Weibull median: 63, maximum: 123). Firefrequency remained high during the 1780?1840 period, when fire was reduced or absent in other areas ofthe western United States. Both the??expected??and the??observed??fire history showed lower fire frequencyafter Euro-American settlement, which most likely displaced Native people and any deliberate use of fire,but did not introduce publicly organized suppression in the area. Therefore, less favorable climaticconditions, not post-settlement fire management, were responsible for reduced wildfire occurrence in themodern era.
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