Statistics for historical low-severity PMFI/FR in dry forests by state, based on the merged 342-site dataset.

<p>Sample sizes were 28 in AZ, 21 in CA, 65 in CO, 7 in ID, 12 in MT, 56 in NM, 24 in OR, 40 in SD, 76 in WA, 3 in WY, 9 in MX and 1 in BC.</p

Dry pine forests, high-severity fire rotations (FR), trends, and differences between recent or projected high-severity fire rotation and the range of historical high-severity fire rotations.

<p>Analysis regions are in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.g001" target="_blank">Fig 1</a>. All burn areas are corrected for missing small fires by dividing initial estimates by 0.95.</p><p>¹ Trends significant at α < 0.05 are starred (*), trends that are close to significant (p < 0.06) have a dark square (^▀). The p-values are from the Mann-Kendall trend test after the Benjamini-Hochberg correction for <i>n</i> = 88 trend tests.</p><p>² Differences between recent or projected high-severity fire rotations and the range of historical high-severity fire rotations are categorized as: (1) In range, if recent or projected high-severity fire rotation was within the range of available historical estimates, (2) too short, if recent or projected high-severity fire rotation was outside and shorter than the range of historical estimates, and (3) too long, if recent or projected high-severity fire rotation was outside and longer than the range of historical estimates. “Y” indicates there was a significant upward trend in area burned at high severity, and “N” indicates there was not.</p><p>³ The ratio of future area burned to recent area burned from the low and high projections by Yue <i>et al</i>. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.ref042" target="_blank">42</a>]</p><p>⁴ The total excludes 105,077 ha of dry pine forests not in the 23 analysis regions and not included in the analysis</p><p>⁵ This is the mean across the regions for which there is a projection</p><p>Dry pine forests, high-severity fire rotations (FR), trends, and differences between recent or projected high-severity fire rotation and the range of historical high-severity fire rotations.</p

Overall statistics for historical low-severity PMFI/FR in dry forests and by forest type, based on the merged 342-site dataset.

<p>Sample size was 342 overall, 223 in dry pine, 119 in dry mixed conifer.</p

Beginning year of analysis for the 331 sites with available data in the 342-site merged dataset.

<p>Beginning year of analysis for the 331 sites with available data in the 342-site merged dataset.</p

The 96 calibration cases and 252 prediction sites from the International Multiproxy Paleofire Database.

<p>Note that multiple plots were often done near one site, thus the number of dots is fewer.</p

Differences between recent (A.D. 1984–2012) high-severity fire rotation and historical range of high-severity fire rotations, recent trends, and recent fire rotations for high-severity fire in (a) dry pine forests and (b) dry mixed-conifer forests by analysis region.

<p>High-severity fire rotation (years), from Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t003" target="_blank">3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t004" target="_blank">4</a>, is printed over each region, and represents the expected time to burn, at high severity, an area equal to the region. Colors correspond with data in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t003" target="_blank">Table 3</a> for dry pine and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t004" target="_blank">Table 4</a> for dry mixed conifer forests. Differences are: (1) “In range,” if recent high-severity fire rotation was within the range of historical estimates, (2) “Too short,” if outside and shorter than historical estimates, and (3) “Too long,” if outside and longer than historical estimates. “Trend” indicates that a statistically significant upward trend in area burned at high severity was found in a region, and “No trend” indicates one was not found, with data shown in Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t003" target="_blank">3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t004" target="_blank">4</a>. Regions that lacked a statistically significant upward trend in area burned at high severity (Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t003" target="_blank">3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.t004" target="_blank">4</a>) have lighter shading. Several Bailey sections were merged or split to create the analysis regions.</p

Restoring and managing low-severity fire in dry-forest landscapes of the western USA

<div><p>Low-severity fires that killed few canopy trees played a significant historical role in dry forests of the western USA and warrant restoration and management, but historical rates of burning remain uncertain. Past reconstructions focused on on dating fire years, not measuring historical rates of burning. Past statistics, including mean composite fire interval (mean CFI) and individual-tree fire interval (mean ITFI) have biases and inaccuracies if used as estimators of rates. In this study, I used regression, with a calibration dataset of 96 cases, to test whether these statistics could accurately predict two equivalent historical rates, population mean fire interval (PMFI) and fire rotation (FR). The best model, using Weibull mean ITFI, had low prediction error and <i>R</i>²_adj = 0.972. I used this model to predict historical PMFI/FR at 252 sites spanning dry forests. Historical PMFI/FR for a pool of 342 calibration and predicted sites had a mean of 39 years and median of 30 years. Short (< 25 years) mean PMFI/FRs were in Arizona and New Mexico and scattered in other states. Long (> 55 years) mean PMFI/FRs were mainly from northern New Mexico to South Dakota. Mountain sites often had a large range in PMFI/FR. Nearly all 342 estimates are for old forests with a history of primarily low-severity fire, found across only about 34% of historical dry-forest area. Frequent fire (PMFI/FR < 25 years) was found across only about 14% of historical dry-forest area, with 86% having multidecadal rates of low-severity fire. Historical fuels (e.g., understory shrubs and small trees) could fully recover between multidecadal fires, allowing some denser forests and some ecosystem processes and wildlife habitat to be less limited by fire. Lower historical rates mean less restoration treatment is needed before beginning managed fire for resource benefits, where feasible. Mimicking patterns of variability in historical low-severity fire regimes would likely benefit biological diversity and ecosystem functioning.</p></div

Linear regression models for estimating PMFI/FR-total scarred trees/plots, based on the 96-case calibration dataset.

<p>All slopes (<i>ß</i>) were significant (<i>p</i> < 0.001) at α = 0.05.</p

High projection (to A.D. 2046–2065) of differences relative to the historical range of high-severity fire rotations, given 1984–2012 trends, and projected fire rotations for high-severity fire in (a) dry pine forests and (b) dry mixed-conifer forests by analysis region.

<p>Several analysis regions are omitted, because the high projections by Yue <i>et al</i>. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.ref042" target="_blank">42</a>] were not possible for those areas. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136147#pone.0136147.g003" target="_blank">Fig 3</a> for an explanation of figure contents.</p

Restoring and managing low-severity fire in dry-forest landscapes of the western USA - Fig 2

<p>Scatterplots showing the linear relationships between: (a) Weibull mean ITFI and fire rotation-total trees/plots, and (b) Fire rotation-total trees/plots and fire rotation-recorder trees.</p