Late Holocene Fire History Research Articles

Human influence on late Holocene fire history in a mixed-conifer forest, Sierra National Forest, California

BackgroundUnderstanding pre-1850s fire history and its effect on forest structure can provide insights useful for fire managers in developing plans to moderate fire hazards in the face of forecasted climate change. While climate clearly plays a substantial role in California wildfires, traditional use of fire by Indigenous people also affected fire history and forest structure in the Sierra Nevada. Disentangling the effects of human versus climatically-induced fire on Sierran forests from paleoecological records has historically proved challenging, but here we use pollen-based forest structure reconstructions and comparative paleoclimatic-vegetation response modeling to identify periods of human impact over the last 1300 years at Markwood Meadow, Sierra National Forest.ResultsWe find strong evidence for anthropogenic fires at Markwood Meadow ca. 1550 – 1750 C.E., contemporaneous with archaeological evidence for fundamental shifts in Indigenous lifeways. When we compare our findings to five other paleoecological sites in the central and southern Sierra Nevada, we find evidence for contemporaneous anthropogenic effects on forest structure across a broad swath of cismontane central California. This is significant because it implies that late 19th and early twentieth century forest structure – the structure that land managers most often seek to emulate – was in part the result anthropogenic fire and precolonial resource management.ConclusionWe consequently suggest that modern management strategies consider (1) further incorporating traditional ecological knowledge fire practices in consultation with local tribal groups, and (2) using pollen-based reconstructions to track how forest composition compares to pre-1850 C.E. conditions rather than the novel forest states encountered in the late 20th and early twenty-first centuries. These strategies could help mitigate the effects of forecast climate change and associated megafires on forests and on socio-ecological systems in a more comprehensive manner.

A lake sediment–based paleoecological reconstruction of late Holocene fire history and vegetation change in Great Basin National Park, Nevada, USA

AbstractAnalyses of macroscopic charcoal, sediment geochemistry (%C, %N, C/N, δ13C, δ15N), and fossil pollen were conducted on a sediment core recovered from Stella Lake, Nevada, establishing a 2000 year record of fire history and vegetation change for the Great Basin. Charcoal accumulation rates (CHAR) indicate that fire activity, which was minimal from the beginning of the first millennium to AD 750, increased slightly at the onset of the Medieval Climate Anomaly (MCA). Observed changes in catchment vegetation were driven by hydroclimate variability during the early MCA. Two notable increases in CHAR, which occurred during the Little Ice Age (LIA), were identified as major fire events within the catchment. Increased C/N, enriched δ15N, and depleted δ13C values correspond with these events, providing additional evidence for the occurrence of catchment-scale fire events during the late fifteenth and late sixteenth centuries. Shifts in the vegetation community composition and structure accompanied these fires, withPinusandPiceadecreasing in relative abundance and Poaceae increasing in relative abundance following the fire events. During the LIA, the vegetation change and lacustrine geochemical response was most directly influenced by the occurrence of catchment-scale fires, not regional hydroclimate.

Middle- to late-Holocene fire history and the impact on Mediterranean pine and oak forests according to the core RF93-30, central Adriatic Sea

The high-resolution Adriatic RF93-30 core shows changes in its microcharcoal record, which correlate to terrestrial fires from the last 7000 years. Pollen and microcharcoals were transported by wind and fluvial transport from the sedimentary basin, including the Po River and other rivers flowing into the sea off the Italian east coast. Charcoal particles and pollen were counted in the same samples, and the maximum breadth and length of charcoal particles were measured. Microcharcoals with large dimensions were taken as fire indicators occurring along the near coast, as they probably arrived from short distances, the nearest being in Apulia, in southern Italy. The age–depth model was developed within the multidisciplinary PALICLAS project. Several potential fire activity increases (PFAIs) were visible as peaks in the diagram. The oldest PFAIs occurred at the middle Holocene (approximately dated to c. 6730, 5430, 4150 cal BP), others occurred at the late Holocene ( c. 3760, 2660, 2240, 2030, 1930, 1510 cal BP) and during the last millennium ( c. 900–865, 530, 120–96 cal BP). The two oldest peaks in the diagram, occurring in the 7th–6th millennia, showed the highest contribution of charcoal corresponding to the highest values of arboreal pollen (AP) in the sedimentary record. Although the CHAR peaks did not represent a single fire event, the diagram suggests a good correspondence between paleofire activity and terrestrial vegetation biomass during this early phase. Pollen containing black particles was observed, which suggested some grains were transported in suspension with winds from burned woods. The main unambiguous anthropogenic fire causation would have occurred during the last four millennia. From 4.2 ka, it became hard to disentangle climate and Bronze Age actions. Technology and human activity probably improved the pace of fire events, especially involving oak woods, with evidence of an increase of CHAR during the last millennium.

Late Holocene fire history and charcoal decay in subtropical dry forests of Puerto Rico

BackgroundFire is an important disturbance that influences species composition, community structure, and ecosystem function in forests. Disturbances such as hurricanes and landslides are critical determinants of community structure to Caribbean forests, but few studies have addressed the effect of paleofire disturbance on forests in Puerto Rico, USA. Soil charcoal is widely used to reconstruct fire history. However, the occurrence and frequency of paleofire can be underestimated due to charcoal decay.ResultsWe reconstructed the fire history of subtropical dry forests of Puerto Rico based on the analysis of soil macrocharcoal numbers adjusted by the negative exponential decay function of charcoal. Twenty-one fire events occurred over the last 1300 yr in the subtropical dry forest of northeastern Puerto Rico, and 10 fire events occurred over the last 4900 yr in the subtropical dry forest of southeastern Puerto Rico. The average turnover time of charcoal in these subtropical dry forest soils of Puerto Rico was 1000 to 1250 yr. Soil charcoal decay leads to an underestimation of one to two undetected fire events during the Late Holocene in the subtropical dry forests of Puerto Rico. The peak of paleofire events for subtropical dry forests in northeastern and southeastern Puerto Rico was broadly similar, occurring between 500 to 1300 calibrated years before present (cal yr BP; before present is understood to mean before 1950 AD). Fire frequency of the subtropical dry forests in Puerto Rico decreased after the immigration of Europeans in the past 500 yr. The fire that occurred between 4822 and 4854 cal yr BP can be interpreted as either a natural fire or a new record of a native peoples settlement in southeastern Puerto Rico. Fire became a frequent disturbance in the subtropical dry forest of Puerto Rico after the development of cultigens by native peoples.ConclusionsOur data suggested that fire was a frequent disturbance and human activity was likely a dominant cause of these paleofires in the subtropical dry forests of Puerto Rico.

Late-Holocene fire history as revealed by size, age and composition of the soil charcoal pool in neighbouring beech and spruce forest landscapes in SE Norway

European beech and Norway spruce are late successional tree species that have become rapidly dominant in northern Europe in late-Holocene. The northern distribution limit for natural beech forests is in SE Norway, where beech forests and boreal spruce forests meet. Here we have estimated the size, composition and age of the macroscopic charcoal pool to infer past fire history and the establishment of neighbouring Norwegian beech- and spruce forests. To encompass landscape level scales of variations in the charcoal pool, we have analysed the charcoal record in 100 soil cores that were collected using a restricted random procedure. The sizes of the soil charcoal pools ranged from 2 to 1214 g m−2, and they were significantly more spatially variable in the beech forest landscape than in the spruce forest landscape. We show that today’s beech forests took over the dominance from Norway spruce in the landscape about 300 years ago, and that fire disturbances on the landscape level preceded the establishment of beech in the former spruce forest landscape. Interestingly, large-scale fire disturbances have not occurred ever since beech gained dominance. Conversely, we show that today’s spruce forests took over the dominance from Scots pine in the landscape, and that also the establishment of spruce was preceded by fire disturbances on the landscape level.

A Regional Perspective on Holocene Fire–Climate–Human Interactions in the Pacific Northwest of North America

Wildfire plays an important role in ecosystems of the Pacific Northwest, but past relationships among fire, climate, and human actions remain unclear. A multiscale analysis of thirty-four macroscopic charcoal records from a variety of biophysical settings was conducted to reconstruct fire activity for the Pacific Northwest (PNW) during the past 12,000 years. Trends in biomass burning and fire frequency are compared to paleoenvironmental and population data at a variety of temporal and spatial scales to better understand fire regime variability on centennial- to millennial-length time scales. PNW fire activity in the early Holocene is linked to climatic and vegetation changes; however, increased fire activity in the middle to late Holocene is inconsistent with long-term trends in temperature and precipitation. Two hypotheses are explored to explain the rise in fire activity after ca. 5,500 calendar years before present, including greater climate variability and increased human use of fire. Climatic changes such as increased El Niño/Southern Oscillation event frequency during the past approximately 6,000 years could have led to hydrologic shifts conducive to more frequent fire events, despite overall trends toward cooler and moister conditions. Alternatively, increasing human populations and their associated uses of fire might have increased biomass burning. Centennial-scale changes in fire activity, such as during the Medieval Climate Anomaly and the Little Ice Age, closely match widespread shifts in both climate and population, suggesting that one or both influenced the late-Holocene fire history of the PNW.

Climatic and human controls on the late Holocene fire history of northern Israel

Long-term fire histories provide insight into the effects of climate, ecology and humans on fire activity; they can be generated using accumulation rates of charcoal and soot black carbon in lacustrine sediments. This study uses both charcoal and black carbon, and other paleoclimate indicators from Lake Kinneret (Sea of Galilee), Israel, to reconstruct late Holocene variations in biomass burning and aridity. We compare the fire history data with a regional biomass-burning reconstruction from 18 different charcoal records and with pollen, climate, and population data to decipher the relative impacts of regional climate, vegetation changes, and human activity on fire. We show a long-term decline in fire activity over the past 3070years, from high biomass burning ~3070–1750calyr BP to significantly lower levels after ~1750calyr BP. Human modification of the landscape (e.g., forest clearing, agriculture, settlement expansion and early industry) in periods of low to moderate precipitation appears to have been the greatest cause of high biomass burning during the late Holocene in southern Levant, while wetter climate apparently reduced fire activity during periods of both low and high human activity.

Late-Holocene fire history in a forest-grassland mosaic in southern Brasil: Implications for conservation

Question: Is the diverse mosaic of forest/grassland (Campos) vegetation on the hills in the Porto Alegre region natural or of anthropogenic origin? What are the best approaches to management and conservation of forest/grassland mosaics in southern Brazil? Location: 280 m a.s.l., Rio Grande do Sul State (30°04'32''S; 51°06'05''W, southern Brazil. Methods: A 50-cm long radiocarbon dated sediment core from a swamp on Morro Santana was analysed for pollen and charcoal, and multivariate data analysis was used to reconstruct past vegetation and fire dynamics. Results: The formation of swamp deposits is related to a change to wetter climatic conditions since 1230 cal yr BP. The diverse forest/grassland mosaic existed already at that time and can be seen as natural in origin as it has been also shown from other studies in southern Brazil. Since 580 cal yr BP, forests expanded continuously. The marked higher occurrence of the pioneer Myrsine during the last 70 years, indicates a change in the disturbance regime. In the past, vegetation has been influenced by mostly anthropogenic fire, set first by Amerindians and later by European settlers. Conclusions: Management for conservation of forest/grassland mosaics should take into account, first, that grasslands are remnants of earlier drier Holocene periods and not a result of deforestation and, second, the history of disturbance by grazing and fire. Suppression of grazing and burning has likely resulted in a trend towards more woody vegetation under modern wet climatic conditions. If management for conservation excludes fire, the present grassland patches will tend to disappear due to forest expansion under the modern humid climate. Maintaining or reintroducing cattle grazing in conservation areas could be an alternative to fire.

Two proxy records revealing the late Holocene fire history at a site on the central coast of New South Wales, Australia

Abstract: The local fire history of a coastal swamp catchment in New South Wales was reconstructed using two proxy records of fire: sedimentary macroscopic charcoal and fire‐scar analyses of Xanthorrhoea johnsonii. The charcoal analysis provided a record of fire activity spanning the last 2800 years, while the Xanthorrhoea record covered the last approx. 300 years. The ability of each method to accurately record fire events was verified by cross referencing against the recent (post 1968) historic fire record. Fire history was then extrapolated beyond the historic record, to reveal an unprecedented level of fire activity in the last 35 years, which coincides with increased human activity in the area. In the prehistoric period charcoal and fire scars are comparatively rare, which is most parsimoniously ascribed to little fire activity, but perhaps represents skilful fire manipulation, as is often attributed to Aboriginal people. The comparatively minor fluctuations in macroscopic charcoal during the prehistoric period were approximately coeval with previous evidence of late Holocene environmental change in south‐eastern Australia, suggesting that fire frequency at the site responded to climatic variability. The longer temporal perspective of this palaeoenvironmental approach provides information for the contemporary management of fire in this conservation reserve.

Detecting local-scale fire episodes on pollen slides

Assessment of the impacts of future climate change on the boreal forest and forest–tundra biomes relies on a clear understanding of their past dynamics. Fire history information recorded in lake and peat sediments can be retrieved by counting charcoal particles on pollen slides, although it is still debated whether this is reliable at the scale of the watershed (i.e. local scale). Knowing that proportionately larger charcoal particles tend to settle closer to the fire compared to smaller particles, a new method of quantifying microscopic charcoal on pollen slides is presented, taking into account the size distribution of the charcoal fragments in each sample of a sediment core (the CSD method). The late-Holocene fire history of the surroundings of a small subarctic lake was reconstructed using this method and compared with results from four other methods (macroscopic charcoal counts, charcoal concentration, charcoal : pollen ratio, charcoal : Picea ratio). All methods were tested for accuracy by comparison with the fire history of the watershed reconstructed by dendrochronology and radiocarbon-dated soil charcoal. Only the CSD method allowed separation of local-scale fire episodes from background noise attributable to long-distance dispersal.