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Wildfire is an important natural disturbance agent, shaping mixed conifer forest structure throughout the Southwestern United States. Yet, fire exclusion caused by late 19th century livestock grazing followed by human fire suppression has altered forest structure by increasing forest density, basal area, and canopy cover in recent decades. Changes in the abundance and vertical and horizontal continuity of fuels have increased the potential for high-severity fire, which construes a major regional forest management concern. In May 2016, the Coyote Fire burned through a network of permanent forest monitoring plots in Guadalupe Mountains National Park. This study employed a repeated-measures sampling design to quantify the effects of low- to moderate-severity wildfire on forest stand structure, species composition, fuels, and tree mortality using hierarchical cluster analysis, non-metric multidimensional scaling (nMDS), and paired t-tests. The 2016 Coyote Fire reduced live tree density in small-diameter size classes, but produced minimal changes in canopy stand structure and fuel loadings, despite nearly a century of fire exclusion and pre-fire tree densities that were four-times higher they were prior to last major wildfire in the early 1900s. Small-diameter surface fuel loadings (1 h and 10 h fuels) did not significantly change after fire, although 1000 h fuels increased significantly (p < 0.05), likely from the addition of new fuel from fire-caused tree mortality. While the wildfire reduced live tree density, the nMDS analysis indicated that the wildfire did not trigger major shifts in tree species composition. However, the wildfire triggered significant decreases in seedlings and small-diameter trees (<30 cm DBH) (p < 0.05). Although the fire thinned the forest, the persistence of fuels and increases in dead small-diameter trees heighten the need for additional fuel reduction treatments to mitigate the risk of future high-severity fire under extreme fire weather. Management of low-severity fire in this forest type may provide opportunities to reduce fuels and restore more desirable stand structure to enhance forest resilience to landscape fire. [ABSTRACT FROM AUTHOR]

Context: Spatial patterns of fire severity are influenced by fire-vegetation patch dynamics and topography. Since the late nineteenth century, fire exclusion has increased fuels and recent fire severity patterns may diverge from historical patterns. Objectives: We used data from a 2008 wildfire burning in a landscape with known nineteenth century fire severity patterns to answer the following questions: (1) Were the spatial patterns of fire severity and fire effects after the 2008 fire similar to those in the late nineteenth century? (2) What factors were most important in controlling spatial patterns of fire severity in 2008? Methods: Fire severity patterns in the late nineteenth century were identified by Beaty and Taylor (J Veg Sci 18:879, 2001) using dendroecology. Plots were remeasured after the 2008 fire and geospatial layers of vegetation type, topography, fire weather, daily fire extent and fire severity were used to identify controls on 2008 fire severity. Results: Fire severity in 2008 varied in ways similar to the nineteenth century. Tree mortality and bark char in plots were lowest on lower slopes and southwest facing slopes, intermediate on middle slopes, and highest on upper slopes and northeast slopes. At the landscape scale, vegetation type, elevation, slope aspect, slope position and weather were the variables controlling fire severity. Conclusions: Spatial patterns of fire severity persisted, despite more than a century of fire exclusion. Our findings suggest that wildfires burning under moderate conditions even with a warming climate can help reduce the fire deficit and promote forest resilience in fire prone landscapes. [ABSTRACT FROM AUTHOR]

Quantifying historical fire regimes provides important information for managing contemporary forests. Historical fire frequency and severity can be estimated using several methods; each method has strengths and weaknesses and presents challenges for interpretation and verification. Recent efforts to quantify the timing of historical high-severity fire events in forests of western North America have assumed that the “stand age” variable from the US Forest Service Forest Inventory and Analysis (FIA) program reflects the timing of historical high-severity (i.e. stand-replacing) fire in ponderosa pine and mixed-conifer forests. To test this assumption, we re-analyze the dataset used in a previous analysis, and compare information from fire history records with information from co-located FIA plots. We demonstrate that 1) the FIA stand age variable does not reflect the large range of individual tree ages in the FIA plots: older trees comprised more than 10% of pre-stand age basal area in 58% of plots analyzed and more than 30% of pre-stand age basal area in 32% of plots, and 2) recruitment events are not necessarily related to high-severity fire occurrence. Because the FIA stand age variable is estimated from a sample of tree ages within the tree size class containing a plurality of canopy trees in the plot, it does not necessarily include the oldest trees, especially in uneven-aged stands. Thus, the FIA stand age variable does not indicate whether the trees in the predominant size class established in response to severe fire, or established during the absence of fire. FIA stand age was not designed to measure the time since a stand-replacing disturbance. Quantification of historical “mixed-severity” fire regimes must be explicit about the spatial scale of high-severity fire effects, which is not possible using FIA stand age data. [ABSTRACT FROM AUTHOR]

• A century of fire exclusion caused forest carbon storage to more than double. • Carbon emissions from wildfire did not vary strongly by fire severity. • Fire exclusion and wildfire amplified spatial differences in carbon storage. • Protecting large trees and carbon-rich stands is crucial to carbon stability. Both fire exclusion and subsequent wildfires have strongly affected carbon storage in fire-prone dry forests, with implications for how carbon storage will change in the future. Using a reconstruction of forest structure in 1899 and pre- and post-fire field data, we quantified changes in carbon stocks in a 2125-ha old-growth mixed conifer forest landscape over a century of fire exclusion and emissions due to a 2013 wildfire. From 1899 to 2002 aboveground carbon storage in live trees increased 2.5-fold from 97 Mg/ha to 263 Mg/ha. Despite burning in an uncharacteristically severe wildfire, the forest still contained 169 Mg/ha of live aboveground tree carbon in 2014. Direct fire emissions were 72 Mg/ha and did not vary with canopy cover loss because emissions were largely driven by consumption of accumulated surface fuels. Areas that burned at low, moderate and high severity in the wildfire contained similar amounts of carbon in 1899, when the forest was still experiencing frequent low severity fire. By 2002 the low severity areas contained 80 and 86 Mg/ha more aboveground live tree carbon than moderate and high severity areas respectively. The wildfire reinforced and amplified these differences in carbon storage that arose during fire exclusion, such that carbon storage following the wildfire was more variable across the landscape. Additionally, the proportion of carbon stored in shade-intolerant, more fire-sensitive species increased. These changes in where and in what tree species carbon is stored, due to the combination of fire exclusion and wildfire, have implications for the potential future stability of these carbon stocks. [ABSTRACT FROM AUTHOR]