Historical patterns of wildfire ignition sources in California ecosystems

Keeley, Jon E.; Syphard, Alexandra D.

doi:10.1071/wf18026

Cited by 108 publications

(87 citation statements)

References 41 publications

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“…Although we find that the observed increases in summer forest-fire area since 1972 are very likely explained by warming-induced increases in VPD, the mean baseline sensitivity of wildfire activity during this study period may have been elevated due to an artificial twentieth-century buildup of fuels due to fire suppression (e.g., Harris & Taylor, 2015;Marlon et al, 2012;Minnich et al, 1995). For example, the frequency of human-started wildfires in California declined over the past two to four decades despite increased ignitions from power-related infrastructure, and urban expansion has reduced the area and connectivity of burnable wildlands (Balch et al, 2017;Keeley & Syphard, 2018;Sleeter et al, 2011). On the other hand, humans may have counteracted the fire-promoting effect of recent warming in some cases.…”

Section: Nonclimate Factorsmentioning

confidence: 94%

“…There is high confidence in these historical records of burned area (Keeley & Syphard, 2018). We excluded all fires smaller than 0.1 ha, as these fires are inconsistently reported and contribute negligibly to total regional burned area.…”

Section: Wildfire History and Climate Datamentioning

confidence: 99%

“…Humans dominate the wildfire regime across much of the state by altering land cover (Sleeter et al, 2011;Syphard et al, 2018), supplying the vast majority of ignitions (Balch et al, 2017;Nagy et al, 2018), and attempting to suppress essentially all fires. Even under constant climate conditions, changes in California's fire activity over the past century would be expected as populations increased and cities expanded into surrounding wildlands (Radeloff et al, 2018), fire suppression strategies evolved (Stephens & Ruth, 2005), and frequency and type of humanignited wildfires changed (Balch et al, 2017;Keeley & Syphard, 2018). Even under constant climate conditions, changes in California's fire activity over the past century would be expected as populations increased and cities expanded into surrounding wildlands (Radeloff et al, 2018), fire suppression strategies evolved (Stephens & Ruth, 2005), and frequency and type of humanignited wildfires changed (Balch et al, 2017;Keeley & Syphard, 2018).…”

Section: Introductionmentioning

confidence: 99%

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Observed Impacts of Anthropogenic Climate Change on Wildfire in California

et al. 2019

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Recent fire seasons have fueled intense speculation regarding the effect of anthropogenic climate change on wildfire in western North America and especially in California. During 1972-2018, California experienced a fivefold increase in annual burned area, mainly due to more than an eightfold increase in summer forest-fire extent. Increased summer forest-fire area very likely occurred due to increased atmospheric aridity caused by warming. Since the early 1970s, warm-season days warmed by approximately 1.4°C as part of a centennial warming trend, significantly increasing the atmospheric vapor pressure deficit (VPD). These trends are consistent with anthropogenic trends simulated by climate models. The response of summer forest-fire area to VPD is exponential, meaning that warming has grown increasingly impactful. Robust interannual relationships between VPD and summer forest-fire area strongly suggest that nearly all of the increase in summer forest-fire area during 1972-2018 was driven by increased VPD. Climate change effects on summer wildfire were less evident in nonforested lands. In fall, wind events and delayed onset of winter precipitation are the dominant promoters of wildfire. While these variables did not change much over the past century, background warming and consequent fuel drying is increasingly enhancing the potential for large fall wildfires. Among the many processes important to California's diverse fire regimes, warming-driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date.Plain Language Summary Since the early 1970s, California's annual wildfire extent increased fivefold, punctuated by extremely large and destructive wildfires in 2017 and 2018. This trend was mainly due to an eightfold increase in summertime forest-fire area and was very likely driven by drying of fuels promoted by human-induced warming. Warming effects were also apparent in the fall by enhancing the odds that fuels are dry when strong fall wind events occur. The ability of dry fuels to promote large fires is nonlinear, which has allowed warming to become increasingly impactful. Human-caused warming has already significantly enhanced wildfire activity in California, particularly in the forests of the Sierra Nevada and North Coast, and will likely continue to do so in the coming decades.

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Section: Nonclimate Factorsmentioning

confidence: 94%

Section: Wildfire History and Climate Datamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Observed Impacts of Anthropogenic Climate Change on Wildfire in California

et al. 2019

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“…). More than 95% of fires are caused by humans (Keeley and Syphard ); thus, fire frequency has increased across much of the landscape in response to population growth and urban expansion, and intervals between fires are now much shorter than pre‐Euro‐American settlement conditions (Safford and Van de Water ). The conversion of chaparral to herbaceous vegetation has already been documented in some parts of the county (Keeley and Brennan , Lippitt et al.…”

Section: Methodsmentioning

confidence: 99%

“…The natural fire regime is characterized by periodic large, high-intensity crown fires driven by hot, dry Santa Ana winds that recur every autumn in addition to smaller fires that occur in the summer (Jin et al 2015). More than 95% of fires are caused by humans (Keeley and Syphard 2018b); thus, fire frequency has increased across much of the landscape in response to population growth and urban expansion, and intervals between fires are now much shorter than pre-Euro-American settlement conditions (Safford and Van de Water 2014). The conversion of chaparral to herbaceous vegetation has already been documented in some parts of the county (Keeley andBrennan 2012, Lippitt et al 2012).…”

Section: Study Areamentioning

confidence: 99%

Extent and drivers of vegetation type conversion in Southern California chaparral

Syphard

Brennan²,

Keeley

2019

Ecosphere

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The native chaparral shrublands of Southern California support exceptional biodiversity and provide critical ecological services, but increased fire frequency threatens to extirpate much of the chaparral due to long regeneration times needed between fires for many species. When short fire intervals inhibit shrub recovery, this favors invasion of exotic herbaceous species, and vegetation type conversion from woody shrubs to grassland is therefore a serious ecological concern in this biodiversity hotspot. Despite a history of field studies documenting the detrimental effect of short‐interval fire, the extent of vegetation type conversion and the conditions under which it occurs have not been documented at a landscape scale. Our objective was thus to provide an unbiased assessment of how and how much vegetation type conversion is occurring in Southern California chaparral. We used a chronosequence of aerial photographs to quantify percentage woody and herbaceous cover change from 1953 to 2016 across randomly sampled plots in San Diego County, then related conversion and decline to a range of explanatory variables including fire, proximity to human disturbance, and biophysical landscape characteristics. Within the 63‐yr study period, there was substantial net woody cover loss, and in the plots that were initially more than 75% woody cover in 1953, 59% experienced a decline, with a mean woody cover loss of 22.5%. Of these, 28% heavily type‐converted to the point that herbaceous vegetation covered more than 50% of the plot. The top drivers for woody conversion and decline included a fire interval shorter than 15 yr and total number of fires, actual evapotranspiration, and elevation. Although human land use variables were not strong independent contributors to either chaparral conversion or decline, 26% of the initial vegetated plots were directly converted to development or other human disturbance types. The combination of direct habitat loss and unintentional vegetation type conversion represents a substantial ecological impact in Southern California that can have far‐reaching impacts via loss of ecological services and by increasing the flammability of the landscape in general. Efforts to reduce fire frequency will be key to preventing further losses.

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Experimental seed sowing reveals seedling recruitment vulnerability to unseasonal fire

Miller

Fontaine

Merritt

et al. 2021

Ecological Applications

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Unseasonal fire occurrence is increasing globally, driven by climate change and other human activity. Changed timing of fire can inhibit postfire seedling recruitment through interactions with plant phenology (the timing of key processes, e.g., flower initiation, seed production, dispersal, germination), and therefore threaten the persistence of many plant species. Although empirical evidence from winter‐rainfall ecosystems shows that optimal seedling recruitment is expected following summer and autumn (dry season) fires, we sought experimental evidence isolating the mechanisms of poor recruitment following unseasonal (wet season) fire. We implemented a seed‐sowing experiment using nine species native to fire‐prone, Mediterranean‐climate woodlands in southwestern Australia to emulate the timing of postfire recruitment and test key mechanisms of fire seasonality effects. For seeds sown during months when fire is unseasonal (i.e., August–September: end of the wet winter season), seedling recruitment was reduced by up to 99% relative to seeds sown during seasonal fire months (i.e., May–June: end of the dry summer season) because of varying seed persistence, seedling emergence, and seedling survival. We found that up to 70 times more seedlings emerged when seeds were sown during seasonal fire months compared to when seeds were sown during unseasonal fire months. The few seedlings that emerged from unseasonal sowings all died with the onset of the dry season. Of the seeds that failed to germinate from unseasonal sowings, only 2% survived exposure on the soil surface over the ensuing hot and dry summer. Our experimental results demonstrate the potential for unseasonal fire to inhibit seedling recruitment via impacts on pregermination seed persistence and seedling establishment. As ongoing climate change lengthens fire seasons (i.e., unseasonal wildfires become more common) and managed fires are implemented further outside historically typical fire seasons, postfire seedling recruitment may become more vulnerable to failure, causing shifts in plant community composition towards those with fewer species solely dependent on seeds for regeneration.

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Historical patterns of wildfire ignition sources in California ecosystems

Cited by 108 publications

References 41 publications

Observed Impacts of Anthropogenic Climate Change on Wildfire in California

Observed Impacts of Anthropogenic Climate Change on Wildfire in California

Extent and drivers of vegetation type conversion in Southern California chaparral

Experimental seed sowing reveals seedling recruitment vulnerability to unseasonal fire

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