Long-term perspective on wildfires in the western USA Marlon, Jennifer R; Bartlein, Patrick J; Gavin, Daniel G ...
Proceedings of the National Academy of Sciences,
02/2012, Letnik:
109, Številka:
9
Journal Article
Recenzirano
Odprti dostop
Understanding the causes and consequences of wildfires in forests of the western United States requires integrated information about fire, climate changes, and human activity on multiple temporal ...scales. We use sedimentary charcoal accumulation rates to construct long-term variations in fire during the past 3,000 y in the American West and compare this record to independent fire-history data from historical records and fire scars. There has been a slight decline in burning over the past 3,000 y, with the lowest levels attained during the 20th century and during the Little Ice Age (LIA, ca. 1400–1700 CE Common Era). Prominent peaks in forest fires occurred during the Medieval Climate Anomaly (ca. 950–1250 CE) and during the 1800s. Analysis of climate reconstructions beginning from 500 CE and population data show that temperature and drought predict changes in biomass burning up to the late 1800s CE. Since the late 1800s , human activities and the ecological effects of recent high fire activity caused a large, abrupt decline in burning similar to the LIA fire decline. Consequently, there is now a forest "fire deficit" in the western United States attributable to the combined effects of human activities, ecological, and climate changes. Large fires in the late 20th and 21st century fires have begun to address the fire deficit, but it is continuing to grow.
Western honey bees (Apis mellifera) are important pollinators in natural and agricultural ecosystems, and yet are in significant decline due to several factors including parasites, pathogens, ...pesticides, and habitat loss. A new beehive construction called the FlowTM hive was developed in 2015 to allow honey to be harvested directly from the hive without opening it, resulting in an apparent decrease in stress to the bees. Here, we compared the Flow and traditional Langstroth hive constructions to determine if there were any significant differences in the bee microbiome. The bee-associated bacterial communities did not differ between hive constructions and varied only slightly over the course of a honey production season. Samples were dominated by taxa belonging to the Lactobacillus, Bifidobacterium, Bartonella, Snodgrassella, Gilliamella, and Frischella genera, as observed in previous studies. The top ten most abundant taxa made up the majority of the sequence data; however, many low abundance organisms were persistent across the majority of samples regardless of sampling time or hive type. We additionally compared different preparations of whole bee and dissected bee samples to elaborate on previous bee microbiome research. We found that bacterial sequences were overwhelming derived from the bee guts, and microbes on the bee surfaces (including pollen) contributed little to the overall microbiome of whole bees. Overall, the results indicate that different hive constructions and associated disturbance levels do not influence the bee gut microbiome, which has broader implications for supporting hive health.
Wildfires can significantly alter forest carbon (C) storage and nitrogen (N) availability, but the long‐term biogeochemical legacy of wildfires is poorly understood. We obtained a lake‐sediment ...record of fire and biogeochemistry from a subalpine forest in Colorado, USA, to examine the nature, magnitude, and duration of decadal‐scale, fire‐induced ecosystem change over the past c. 4250 yr. The high‐resolution record contained 34 fires, including 13 high‐severity events within the watershed. High‐severity fires were followed by increased sedimentary N stable isotope ratios (δ¹⁵N) and bulk density, and decreased C and N concentrations – reflecting forest floor destruction, terrestrial C and N losses, and erosion. Sustained low sediment C : N c. 20–50 yr post‐fire indicates reduced terrestrial organic matter subsidies to the lake. Low sedimentary δ¹⁵N c. 50–70 yr post‐fire, coincident with C and N recovery, suggests diminishing terrestrial N availability during stand development. The magnitude of post‐fire changes generally scaled directly with inferred fire severity. Our results support modern studies of forest successional C and N accumulation and indicate pronounced, long‐lasting biogeochemical impacts of wildfires in subalpine forests. However, even repeated high‐severity fires over millennia probably did not deplete C or N stocks, because centuries between high‐severity fires allowed for sufficient biomass recovery.
Key uncertainties in anticipating future fire regimes are their sensitivity to climate change, and the degree to which climate will impact fire regimes directly, through increasing the probability of ...fire, versus indirectly, through changes in vegetation and landscape flammability. We studied the sensitivity of subalpine forest fire regimes (i.e. fire frequency, fire severity) to previously documented climate variability over the past 6000 years, utilizing pollen and macroscopic charcoal from high‐resolution lake‐sediment records in Rocky Mountain National Park, Colorado. We combined data from the four lakes to provide composite records of vegetation and fire history within a 200 km² study area. Rates of forest burning were relatively complacent to millennial‐scale summer cooling and decreased effective moisture. Mean return intervals between fire episodes, defined over 500‐year periods, generally varied between 150 and 250 years, consistent with tree‐ring‐based estimates spanning recent centuries. Variability around these long‐term means, however, was significantly correlated with variability in summer moisture (i.e. more burning with drier summers), inferred from existing lake‐level and supporting palaeoenvironmental records. The most pronounced change in fire regimes was in response to decreased subalpine forest density ca. 2400 cal. year BP, itself a response to regional cooling. This indirect impact of climate was followed by a decrease in charcoal production per fire, a proxy for crown‐fire severity, while the long‐term rate of burning remained unchanged. Over the last 1500 years, increased summer evaporation and drought frequency were associated with increased fire severity, highlighting a direct link between fire and climate. Synthesis. Subalpine forest fire history reveals complacency and sensitivity of fire regimes to changing vegetation and hydroclimate over the past 6000 years. Complacency is highlighted by non‐varying fire frequency over millennia. Sensitivity is evident through changes in biomass burned per fire (and inferred fire severity), in response to climate‐induced changes in forest density and, more recently, increased summer drought. Overall, the palaeo record suggests that (i) fire severity may be more responsive to climate change than fire frequency in Rocky Mountain subalpine forests and (ii) the indirect impacts of climate on vegetation and fuels are important mechanisms determining fire‐regime response to climate change.
A network of eight Holocene paleoenvironmental records from lakes in the Klamath Mountains of Northern California provides insights on how diverse coniferous forests are maintained in the face of ...climate change. Pollen data suggest that in most cases plants kept pace with climate change. The steep costal-to-inland precipitation gradient resulted in asynchronous responses to climate change with coastal forests responding before inland sites. This was likely due to the proximity to oceans, warm valleys, and the differential responses to changes in ocean upwelling. Plants growing on soils with heavy metals showed little response to Holocene climate variability, suggesting that they experienced stability during the Holocene, which helps explain the localized plant diversity on the harsh soils. Plant communities on soils without heavy metals adjusted their ranges along elevational gradients in response to climate change, however. Fires were a common occurrence at all sites and tracked climate; however, sites that were more coastal experienced fewer fires than inland sites. Fire severity remained similar through the Holocene at individual sites; however, it was low to moderate at southern locations and higher at more northern locations. The article highlights historical factors that help explain the diversity of plant species in the forests of Northern California and provides insights for managing these complex ecosystems.
Over a third of the world’s crops require insect pollination, and reliance on pollination services for food continues to rise as human populations increase. Furthermore, as interest in urban ...agriculture has grown, so has a need for studies of urban pollinator ecology and pollination. Analyzing pollinator assemblages along a rural-urban gradient provides powerful mechanistic insight into how urbanization impacts pollinators. Yet, studies examining pollinators along urban-rural gradients are limited and results vary. Since pollinators vary tremendously in life history characteristics and respond to urbanization differently, studies from different regions would improve our understanding of pollinator response to urbanization. This study documents different bee assemblages along a high-plains semi-arid urban-rural gradient in Denver, Colorado, USA. Percent impervious surface was used to define the extent of urbanization at 12 sites and local and landscape characteristics were estimated using field assessments and geospatial analysis. Wild bees were collected and the relationships between urbanization and bee communities were explored using linear modeling. Overall, bee abundance and diversity decreased with increasing urbanization, suggesting that urban areas negatively impact bee communities. However, all bee guilds responded positively to local floral richness and negatively to the degree of landscape urbanization, suggesting that different types of bees responded similarly to urbanization. These findings suggest that providing a greater diversity of floral resources is key to mitigating the negative impacts of urbanization on pollinator communities.
The influence of substrate on long-term vegetation dynamics has received little attention, and yet nutrient-limited ecosystems have some of the highest levels of endemism in the world. The diverse ...geology of the Klamath Mountains of northern California (USA) allows examination of the long-term influence of edaphic constraints in subalpine forests through a comparison of vegetation histories between nutrient-limited ultramafic substrates and terrain that is more fertile. Pollen and charcoal records spanning up to 15 000 years from ultramafic settings reveal a distinctly different vegetation history compared to other soil types. In non-ultramafic settings, the dominant trees and shrubs shifted in elevation in response to Holocene climate variations resulting in compositional and structural changes, whereas on ultramafic substrates changes were primarily structural, not compositional. Fire activity was similar through most of the Holocene with the exception of declines over the last 4000 years on ultramafic substrates, likely due to the reduction of understory fuels and cooler wetter conditions than in the middle Holocene. These results suggest that the tree and shrub distributions were more responsive to past climate changes on non-ultramafic substrates compared to those on ultramafic substrates. The combination of these dynamics may help explain high levels of plant diversity in the Klamath Mountains and provide insights for managing these complex ecosystems.
Summary
Key uncertainties in anticipating future fire regimes are their sensitivity to climate change, and the degree to which climate will impact fire regimes directly, through increasing the ...probability of fire, versus indirectly, through changes in vegetation and landscape flammability.
We studied the sensitivity of subalpine forest fire regimes (i.e. fire frequency, fire severity) to previously documented climate variability over the past 6000 years, utilizing pollen and macroscopic charcoal from high‐resolution lake‐sediment records in
R
ocky
M
ountain
N
ational
P
ark,
C
olorado. We combined data from the four lakes to provide composite records of vegetation and fire history within a 200 km
2
study area.
Rates of forest burning were relatively complacent to millennial‐scale summer cooling and decreased effective moisture. Mean return intervals between fire episodes, defined over 500‐year periods, generally varied between 150 and 250 years, consistent with tree‐ring‐based estimates spanning recent centuries. Variability around these long‐term means, however, was significantly correlated with variability in summer moisture (i.e. more burning with drier summers), inferred from existing lake‐level and supporting palaeoenvironmental records.
The most pronounced change in fire regimes was in response to decreased subalpine forest density ca. 2400 cal. year
BP
, itself a response to regional cooling. This indirect impact of climate was followed by a decrease in charcoal production per fire, a proxy for crown‐fire severity, while the long‐term rate of burning remained unchanged. Over the last 1500 years, increased summer evaporation and drought frequency were associated with increased fire severity, highlighting a direct link between fire and climate.
Synthesis
. Subalpine forest fire history reveals complacency and sensitivity of fire regimes to changing vegetation and hydroclimate over the past 6000 years. Complacency is highlighted by non‐varying fire frequency over millennia. Sensitivity is evident through changes in biomass burned per fire (and inferred fire severity), in response to climate‐induced changes in forest density and, more recently, increased summer drought. Overall, the palaeo record suggests that (i) fire severity may be more responsive to climate change than fire frequency in Rocky Mountain subalpine forests and (ii) the indirect impacts of climate on vegetation and fuels are important mechanisms determining fire‐regime response to climate change.
Fire is well recognized as a key Earth system process, but its causes and influences vary greatly across spatial and temporal scales. The controls of fire are often portrayed as a set of superimposed ...triangles, with processes ranging from oxygen to weather to climate, combustion to fuel to vegetation, and local to landscape to regional drivers over broadening spatial and lengthening temporal scale. Most ecological studies and fire management plans consider the effects of fire-weather and fuels on local to sub-regional scales and time frames of years to decades. Fire reconstructions developed from high-resolution tree-ring records and lake-sediment data that span centuries to millennia offer unique insights about fire's role that cannot otherwise be obtained. Such records help disclose the historical range of variability in fire activity over the duration of a vegetation type; the role of large-scale changes of climate, such as seasonal changes in summer insolation; the consequences of major reorganizations in vegetation; and the influence of prehistoric human activity in different ecological settings. This paleoecological perspective suggests that fire-regime definitions, which focus on the characteristic frequency, size and intensity of fire and particular fuel types, should be reconceptualized to better include the controls of fire regimes over the duration of a particular biome. We suggest that approaches currently used to analyze fire regimes across multiple spatial scales should be employed to examine fire occurrence across multiple temporal scales. Such cross-scale patterns would better reveal the full variability of particular fire regimes and their controls, and provide relevant information for the types of fire regimes likely to occur in the future with projected climate and land-use change.
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