Herbivory and vegetation openness in a pre-farming European landscape

The mode and tempo of forest compositional change during periods of rapid climate change, including the potential for the fire regime to produce nonlinear relationships between climate and vegetation, is a long‐standing theme of forest ecological research. In the old conifer forests of the coastal Pacific Northwest, fire disturbances are sufficiently rare that their relation to climate and their ecological effects are poorly understood. We used a 14 700‐year high‐resolution sediment record from Yahoo Lake on the Olympic Peninsula, Washington, USA, to examine vegetation (landscape vegetation from pollen and local vegetation from macrofossils) and fire (landscape fire from total charcoal and local fire from charcoal peaks) in conjunction with independent records of climate. We hypothesized that the successional stage of the local forest will exhibit alternate stable states over a range of fire activity, that species turnover will increase abruptly above a certain level of fire activity and that both responses would be more gradual at the landscape scale than the local scale. Supporting these hypotheses, at the local scale, we found strong evidence for alternate stable states of late vs. early successional communities and inertia of species turnover to changing fire activity. At the landscape scale, vegetation responded more gradually to changing fire activity. From 14 700 to 7000 years ago, high landscape vegetation turnover occurred along with high landscape fire activity, especially during the warm summers of the early Holocene. In several instances, local species turned over completely following fire events but several centuries after climate change. In contrast, during the last 7000 years, the local forest composition was dominated by late‐successional species with little species turnover, despite periods of moderate fire activity. We suggest that the relatively minor climate fluctuations of the past 7000 years were not sufficient to cause large‐scale species turnover after fire. The Yahoo Lake fire and vegetation record of the early Holocene provides a model for dramatic ecosystem change following an anticipated shift to warmer summer temperatures.

Abstract. Fire history reconstructions are typically based on tree ages and tree-ring fire scars or on charcoal in sedimentary records from lakes or bogs, but rarely on both. In this study of fire history in western Patagonia (47–48° S) in southern South America (SSA) we compared three sedimentary charcoal records collected in bogs with tree-ring fire-scar data collected at 13 nearby sample sites. We examined the temporal and spatial correspondence between the two fire proxies and also compared them to published charcoal records from distant sites in SSA, and with published proxy reconstructions of regional climate variability and large-scale climate modes. Two of our three charcoal records record fire activity for the last 4 ka yr and one for the last 11 ka yr. For the last ca. 400 yr, charcoal accumulation peaks tend to coincide with high fire activity in the tree-ring fire scar records, but the charcoal records failed to detect some of the fire activity recorded by tree rings. Potentially, this discrepancy reflects low-severity fires that burn in herbaceous and other fine fuels without depositing charcoal in the sedimentary record. Periods of high fire activity tended to be synchronous across sample areas, across proxy types, and with proxy records of regional climatic variability as well as major climate drivers. Fire activity throughout the Holocene in western Patagonia has responded to regional climate variation affecting a broad region of southern South America that is teleconnected to both tropical- and high-latitude climate drivers-El Niño-Southern Oscillation and the Southern Annular Mode. An early Holocene peak in fire activity pre-dates any known human presence in our study area, and consequently implicates lightning as the ignition source. In contrast, the increased fire activity during the 20th century, which was concomitantly recorded by charcoal from all the sampled bogs and at all fire-scar sample sites, is attributed to human-set fires and is outside the range of variability characteristic of these ecosystems over many centuries and probably millennia.

Holocene fire, vegetation, and climate dynamics inferred from charcoal and pollen record in the eastern Tibetan Plateau

Observing natural vegetation dynamics over the entire Holocene is difficult in Central Europe, due to pervasive and increasing human disturbance since the Neolithic. One strategy to minimize this limitation is to select a study site in an area that is marginal for agricultural activity. Here, we present a new sediment record from Lake Svityaz in northwestern Ukraine. We have reconstructed regional and local vegetation and fire dynamics since the Late Glacial using pollen, spores, macrofossils and charcoal. Boreal forest composed of Pinus sylvestris and Betula with continental Larix decidua and Pinus cembra established in the region around 13,450 cal bp, replacing an open, steppic landscape. The first temperate tree to expand was Ulmus at 11,800 cal bp, followed by Quercus, Fraxinus excelsior, Tilia and Corylus ca. 1,000 years later. Fire activity was highest during the Early Holocene, when summer solar insolation reached its maximum. Carpinus betulus and Fagus sylvatica established at ca. 6,000 cal bp, coinciding with the first indicators of agricultural activity in the region and a transient climatic shift to cooler and moister conditions. Human impact on the vegetation remained initially very low, only increasing during the Bronze Age, at ca. 3,400 cal bp. Large-scale forest openings and the establishment of the present-day cultural landscape occurred only during the past 500 years. The persistence of highly diverse mixed forest under absent or low anthropogenic disturbance until the Early Middle Ages corroborates the role of human impact in the impoverishment of temperate forests elsewhere in Central Europe. The preservation or reestablishment of such diverse forests may mitigate future climate change impacts, specifically by lowering fire risk under warmer and drier conditions.

Estimating natural forest fire return interval in northeastern Ontario, Canada

Long-term hydrological dynamics and fire history over the last 2000 years in CE Europe reconstructed from a high-resolution peat archive

Abstract. Vegetation and fire activity have dynamically changed in response to past variations in global and regional climate. Here we investigate these responses across the Neotropics based on the analysis of modern vegetation distribution and fire activity in relation to modern climate patterns, and a compilation of 255 vegetation records and 131 charcoal records encompassing the last 21 000 years before present (ka) in relation to past climate changes. Our analyses on the dynamics of past tree cover and fire activity focus on seven subregions: (1) northern Neotropics (NNeo); (2) tropical Andes (TrAn); (3) Amazonia; (4) northeastern Brazil (NEB); (5) central-eastern Brazil (CEB); (6) southeastern South America (SESA); and (7) extratropical Andes (ExTrAn). The regionalized assessment unveils spatial heterogeneity in the timing and controls of vegetation and fire dynamics. Temperature, atmospheric CO2 concentrations, and precipitation exhibit distinct and alternating roles as primary drivers of tree cover and fire regime changes with additional impacts from human activity. During the Last Glacial Maximum (LGM, here covering 21–19 ka), arboreal growth in high elevation sites (TrAn) and in sub- and extra-tropical latitudes (SESA and ExTrAn) was mainly limited by low temperatures and atmospheric CO2 concentrations, while fuel-limited conditions restrained fire activity. In warmer tropical regions (NNeo, Amazonia, CEB), moisture availability was likely the main controlling factor of both vegetation and fire, with the effects of low CO2 amplifying these constraints. Throughout the deglacial phase (19–11.7 ka), progressive warming and increasing atmospheric CO2 concentrations fostered a gradual biomass expansion, and together these changes led to intensified fire activity in the sub- and extra-tropical temperature-limited regions. Meanwhile, increased (decreased) precipitation associated with millennial-scale events favored increases (decreases) in tree cover in regions such as CEB and NEB (NNeo). Between 14–13 ka, most southern latitude subregions (Amazonia, CEB, SESA, ExTrAn) saw a rise in fire activity coeval with a second rapid warming, contrary to decreased fire activity in NNeo amid relatively wetter conditions. Throughout the Holocene, when temperature and atmospheric CO2 fluctuations were lower, shifts in precipitation became the primary driver of vegetation and fire dynamics across all the Neotropics. Changes in the Intertropical Convergence Zone and gradual intensification of the South American Summer Monsoon throughout the Holocene favored a continuous increase in tree cover over Amazonia, CEB, and SESA, but led to a forest cover decrease in NNeo and NEB. From the early- to the mid-Holocene, the strengthening of the Southern Westerly Winds promoted vegetation expansion and fire regime weakening in ExTrAn. In the late Holocene, human impacts became more pronounced, with a clearer effect on regional tree cover and fire activity, particularly in NNeo and TrAn.

AimDocumenting past vegetation dynamics and fire‐vegetation relationships at a regional scale is necessary to understand the mechanisms that control the functioning of the boreal forest, which is particularly sensitive to climate change. The objective of this study is to document these interactions in the Komi Republic during the Holocene.LocationYaksha, Vychegda river basin, Republic of Komi, Russia.TaxonPlantae, gymnosperms, angiosperms.MethodsTwo palaeoecological approaches are combined, based (1) on pollen (this study) and charcoal analysis (recomputed from our previous analysis) applied to cores from two peatlands and (2) on a REVEALS model (a part of the Landscape Reconstruction Algorithm “LRA”) applied to six regional pollen cores in order to obtain a regional estimate of vegetation cover during the Holocene.ResultsThe pollen diagram produced locally from Yaksha was compared with the regional vegetation cover determined by REVEALS. Taxa such as Abies sp. and Pinus spp. showed differences between the two approaches, but vegetation signals remain qualitatively consistent. From 10,000 to 6,000 cal. yr BP, the forest was mainly a light taiga (composed of Pinus sylvestris and Betula spp.) and low fire activity was recorded. From 6,000 to 3,500 cal. yr BP, a dark taiga (composed of Picea spp., Abies sibirica and Pinus sibirica) was established due to favourable climatic conditions, despite higher fire activity. From 3,500 cal. yr BP onwards, the continuous increase in fire activity allowed for a gradual return of light taiga, Betula spp., likely reinforced by human activities. The dynamics of Picea spp. and Abies sp. were asynchronous between the sites. For Picea spp., the hypothesis of local inter‐site expansion distributed along stream corridors is supported by the data. For Abies sp., a bias in REVEALS, and in climate cooling may explain disparities between sites.Main conclusionsWe found evidence that in the early and mid‐Holocene, vegetation dynamics were probably more influenced by climate, as fire activity was low. During the late Holocene, fire activity and geomorphology, eventually augmented by human activities, increased in influence on vegetation dynamics and led to the predominance of the light taiga forest up to the present.

Tropical savannas typically experience high fire frequencies, with prescribed fire commonly used as a management tool. Termites play an important role in the ecological functioning of tropical savannas, yet we have a limited understanding of how fire affects these important ecosystem engineers. To account for the effects of fire management on ecosystem structure and function, we need to understand the links between fire management and termite communities. This study used a long‐term (18‐year) fire experiment in a tropical savanna near Darwin, northern Australia, to investigate the effects of different fire regimes on termite species composition, abundance and activity. We measured termite abundance and activity using a combination of baiting and reduced transect survey methods and compared these with fire activity (summarised fire frequency and intensity) and woody cover. Termite species richness was similar across all fire treatments, and the level of fire activity had a minimal effect on species composition, which was more strongly influenced by woody cover. Wood‐feeding termite abundance and the consumption of wood baits were negatively correlated with fire activity and positively correlated with woody cover. Soil/wood interface‐feeding termites showed no correlation with fire activity but a positive correlation with woody cover. Significant negative mediation effects of fire activity through woody cover were detected on the abundance of wood‐ and soil/wood interface feeders and wood and straw bait consumption. Grass‐feeding termites were encountered too infrequently to draw conclusions about their correlation with fire activity and woody cover; however, straw bait consumption was positively correlated with fire activity. Synthesis and applications. The effects of fire on termite abundance and activity are primarily indirect, mediated through changes in vegetation structure. As high fire activity is associated with reduced woody cover, maintaining regimes of frequent, high‐intensity fires over the long term has the potential to affect ecosystem function. While minimising the occurrence of high‐intensity, late dry season fires is consistent with fire management goals in these savannas, care is still required to avoid the negative consequences of high fire frequencies.

A 1000-yr record of forest fire activity has been developed using three annually dated ice cores from Eclipse Icefield, Yukon, Canada. Forest fire signals were identified as NH4plus residuals above a robust spline and corroborated by an empirical orthhogonal function (EOF) analysis that identified a chemical association in the NH4plus, C2042macr and Kplus records similar to that observed in forest fire plumes. These statistical techniques yielded similar records of forest fire activity, although the EOF analysis provides more conservative identification of forest fire signals. Comparison of forest fire signals in the Eclipse ice cores with the record of annual area burned in Alaska and the Yukon demonstrates that 80% of high fire years in Alaska and 79% of high fire years in the Yukon are identifiable as NH4plus concentration residuals in at least one core from Eclipse Icefield, although any individual core records 36-67% of these events. The Eclipse ice cores record high fire activity in the AD 1760s, 1780s, 1840s, 1860s, 1880s, 1890s, 1920s-1940s and 1980s. Peak fire activity occurred in the 1890s, possibly reflecting anthropogenic ignition sources associated with the large influx of people to the Yukon during the Klondike Gold Rush. Periods of low fire activity are evident during the 1770s, 181Os-1830s, 1850s, 1950s and 1960s. Extending our proxy of fire activity to AD 1000 using annual NH4plus concentrations from our one core that extends back this far provides evidence of high fire activity from 1240 to 1410 during the waning stages of the ‘Mediaeval Warm Period’.

Human activities have altered fire regimes for millennia by suppressing or enhancing natural fire activity. However, whether these anthropogenic pressures on fire activity have exceeded and will surpass climate forcing still remains uncertain. We tested if, how and the extent to which seasonal fire activity in southern France has recently (1976–2009) deviated from climate-expected trends. The latter were simulated using an ensemble of detrended fire–climate models. We found both seasonal and regional contrasts in climatic effects through a mixture of drought-driven and fuel-limited fire regimes. Dry contemporary conditions chiefly drove fire frequency and burned area, although higher fire activity was related to wetter conditions in the last three years. Surprisingly, the relative importance of preceding wet conditions was higher in winter than in summer, illustrating the strong potential dependency of regional fire–climate relationships on the human use and control of fires. In the Mediterranean mountains, warm winters and springs favour extensive fires in the following dry summer. These results highlight that increasing dryness with climate change could have antagonistic effects on fire regime by leading to larger fires in summer (moisture-limited), but lower fire activity in winter (fuel-limited fire regime). Furthermore, fire trends have significantly diverged from climatic expectations, with a strong negative alteration in fire activity in the Mediterranean lowlands and the summer burned area in the mountains. In contrast, alteration of winter fire frequency in the Mediterranean and Temperate mountains has shifted from positive to negative (or null) trends during the mid-1990s, a period when fire suppression policy underwent major revisions. Our findings demonstrate that changes in land-use and fire suppression policy have probably exceeded the strength of climate change effects on changing fire regime in southern Europe, making regional predictions of future fires highly challenging.

Controls on variations in MODIS fire radiative power in Alaskan boreal forests: Implications for fire severity conditions

Biomass burning has become more frequent and widespread worldwide, with a significant proportion occurring in tropical Africa. Fire dynamics have been generally studied at global or regional scales. At local scale, however, fire impacts can be severe or catastrophic, suggesting local analyses are warranted. This study aimed to characterise the spatio-temporal variations of vegetation fires and identify the main fire hotspots in Côte d’Ivoire, a country of West Africa, one of the world’s burn centres. Using MODIS-derived fire data over a 10-year period (2007–2016), the number of fire days, active fires and fire density were assessed across the entire country. In the southern part dominated by forests, fire activity was low. Three main fire hotspots were identified between 2°30’–8°30’W and 7°00’–10°30’N in the North-West, North-East and Central areas all dominated by savannas. In these areas, Bafing, Bounkani and Hambol regions recorded the highest fire activity where fire density was 0.4±0.02, 0.28±0.02 and 0.18±0.01 fires/km²/year, respectively. At national scale, the annual fire period stretched from October to April with 91% of fires occurring between December and February, with a peak in January. Over the decade, there was a decreasing trend of fire activity. Fire density also was negatively correlated with rainfall >1000 mm for the synchronic analysis, whereas fire density was positively correlated with rainfall in the previous years. Results suggest that the positive relationship between the previous year’s rainfall and fire activity could operate on a cycle from 1 to 4 years. Significance: Three fire hotspots were found primarily in savanna vegetation, which burns more regularly than forestdominated vegetation. The fire season occurs over 7 months, the majority of active fires (91%) occurring in just 3 months (December-January-February) with a peak in January (39%). Fire activity has declined over the past decade with a return time of above-average fires from 1 to 4 years. Fire density is positively correlated to the amount of rainfall in preceding years, whereas fire density and rainfall of the same year were negatively correlated in the region of rainfall >1000 mm.

La importancia del estudio de la actividad del fuego en el pasado está en la necesidad de comprender los efectos que el Cambio Global puede tener sobre la misma, tanto en la actualidad como en el futuro. Estudiar cómo se relaciona el fuego con los ecosistemas y el clima en escalas temporales lo suficientemente largas permite conocer la respuesta de la vegetación a esta perturbación y definir los umbrales que provocan cambios irreversibles en la composición vegetal. Este trabajo estudia la dinámica del fuego durante el Holoceno en el entorno de la Basa de la Mora, lago de origen glaciar situado en el Macizo de Cotiella, Pirineo Central. La historia del fuego se ha reconstruido a partir de la identificación de partículas de carbón fósil a lo largo de la secuencia sedimentaria extraída del lago, que cubre los últimos 10 000 años de historia. Se ha observado que la mayor actividad del fuego se produce entre 8.1 y 4 cal Ka BP, seguida de una fase de menor actividad entre 3.2 y 1.7 cal Ka BP. Estas fluctuaciones se deben principalmente a dos razones: la cantidad de biomasa disponible y susceptible de ser quemada, y la existencia de un clima favorable para la ocurrencia de incendios. En ese sentido, se han observado correlaciones positivas de la actividad del fuego, por un lado, con Betula y Corylus como taxones ligados a la mayor producción de biomasa, y por otro, con agrupaciones de taxones adaptados a la aridez como indicadores de climas favorables a la ocurrencia de incendios. Por último, a partir de 1.7 cal Ka BP se registra en la secuencia una mayor incidencia del fuego, probablemente vinculada a la intensificación de la acción antropogénica a nivel regional.

Instrumental data show that while the impact of volcanic eruptions on their immediate vicinity is destructive, long-term consequences can be beneficial. However, beyond last millennia observational data and ancient oral history, the detailed insights into timescales and scopes of recovery remain largely unresolved. Here we illustrate the complex response of local and regional vegetation, aquatic ecosystem, and fire activity to volcanic eruptions in close connection to prevailing climate conditions and assess the recovery time in varve-years.We selected five volcaniclastic layers in the annually laminated sediments from Lake Van (Turkey). Analysed intervals cover glacial, interglacial, stadial and interstadial snapshots (spanning from Marine Isotope Stages 3 to 9e) and facilitate studying ecosystem’s responses under different climatic boundary conditions. Using high-resolution pollen data, non-pollen palynomorphs, and microscopic charcoal particles (>20 µm) we attempted to disentangle climatic and volcanic forcing of natural environmental disturbances. Our results highlight that the thickness of subsequent volcanic deposits and the respective climatic conditions strongly influence the impact on terrestrial and aquatic ecosystems. Similarily, the vegetation types predominant before the volcanic eruption have a decisive influence on subsequent pollen productivity and vegetation composition. On land, the most common response to ash deposition is a sudden shift towards steppe herbaceous taxa and abrupt fire activity. The affected herbaceous vegetation can recover to pre-eruption levels in as few as 20 to 40 varve-years. On the contrary, the lake water experiences intensified productivity due to subsequent nutrient input and significant short-term increase in aquatic taxa and non-siliceous microfossils.Our approach helps in understanding complex ecosystems subjected to a variety of influencing factors operating on different time scales. Our results show the importance of distinguishing between the impact of tephra deposition and volcanically-induced climate change for tracking short-term ecosystem changes superimposed on long-term trends.