Dendroecological testing of the pyroclimatic hypothesis in the central Great Basin, Nevada, USA

Abstract

Abstract. In the Great Basin region of western North America, records of past climate and wildfire variability are needed not only for fire use, but also for understanding the mechanisms behind the centurylong expansion of piñon-juniper woodlands. The Mt. Irish area (Lincoln County, south-eastern Nevada) is a remote mountain ecosystem on the hydrographic boundary between the Great Basin and the Colorado River Basin. Non-scarred ponderosa pines (Pinus ponderosa C. Lawson var. scopulorum Engelm.) and singleneedle pinyons (Pinus monophylla Torr. & Frém.) 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 was obtained from the PDSI reconstruction and from explicitly assumed relationships between climate and wildfire occurrence. A census of fire-scarred trees was then sampled at the study area, and crossdated firescar 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 single year. Numerical summaries were computed for the period 1550-2006, when recorder trees ranged from 16 to 169, using a total of 360 fire scars on 176 sections. Up to 1860, the time of Euro-American settlement, fires that 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 relatively rare (minimum fire interval: 40 years, mean: 66, median: 50, Weibull median: 63, maximum: 123). Fire frequency remained high during the 1780-1840 period, when fire was reduced or absent in other areas of the western United States. Both the ''expected'' and the ''observed'' fire history showed lower fire frequency after 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 climatic conditions, not post-settlement fire management, were responsible for reduced wildfire occurrence in the modern era.

Long‐term survivorship of single‐needle pinyon (Pinus monophylla) in mixed‐conifer ecosystems of the Great Basin, USA

“…Irish were consistent with a pre-settlement fire regime characterized by frequent low-severity events (Biondi et al 2011), as trees older than 200 years were found at most plots and gridpoints. Such old-growth individuals are also consistent with a long fire rotation.…”

“…Roughly at the same time, the reduction in wildfire frequency uncovered at both study areas (Biondi et al 2011;Kilpatrick et al, in press) was the main factor that allowed the increased survivorship of pinyon and ponderosa pine. After the early-2000s drought, and despite the greater density they acquired over the past century, the pinyon-juniper populations we studied have not experienced the dramatic mortality observed where P. edulis is present.…”

“…The period since Euro-American settlement during the late 19th and early 20th century coincides with a period of wetter conditions in the Great Basin (Biondi et al 2011), possibly associated with the end of the Little Ice Age (Grayson 2011). Roughly at the same time, the reduction in wildfire frequency uncovered at both study areas (Biondi et al 2011;Kilpatrick et al, in press) was the main factor that allowed the increased survivorship of pinyon and ponderosa pine.…”

“…Using PRISM (parameter-elevation regression on independent slopes model; Daly et al 2008) climate data for the study area, obtained from the average of two 2.5-arc minute grid cells, the average total annual precipitation was 28.8 6 9.6 cm and the average annual temperature was 9.0 6 0.88C. Additional details on this site, including its wildfire history pre-and postsettlement, are available in the peer-reviewed literature (Biondi et al 2011, Cheek et al 2012. Based on those studies, the year 1860 was chosen as the date of Euro-American settlement for this location.…”

“…Few studies to date have focused on long-term stand dynamics and demographic patterns of such mixed-conifer ecosystems, but their conservation requires disentangling the impacts of Euro-American land uses (fire suppression, open-range grazing, wood cutting for early mining operations) from natural events (climate, disturbance) on vegetation dynamics. Fire scars on ponderosa pines growing in mixed conifer associations with pinyon and juniper have already showed that low-intensity fires at relatively high frequencies (mean fire interval , 10 years) were the most common pre-settlement events (Biondi et al 2011;Kilpatrick et al, in press). In the same areas, assessments of fuel loads and potential fire behavior has uncovered a transition over time from surface to crown fire conditions (Cheek et al 2012).…”