Aurélie Genries

Predictability of biomass burning in response to climate changes

Climate is an important control on biomass burning, but the sensitivity of fire to changes in temperature and moisture balance has not been quantified. We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo- fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.

Spatial Variability of Fire History in Subalpine Forests: From Natural to Cultural Regimes

Abstract: The goal of this study was to determine the effect of local and large-scale processes on fire frequency during the postglacial period in a subalpine ecosystem (Alps, France). Large-scale processes should produce homogeneous distribution of fire-free intervals and synchronicity of fire series, and dominance of local-scale processes, such as those triggered by differences in relief, slope aspect, human history, etc. should create heterogeneous fire regimes. Four ponds and peat were sampled at different elevations and exposures. Sedimentary charcoal was used as a fire proxy, and plant macroremains were used as a vegetation proxy. Synchronicity analysis was based on a transformed Ripleys K-function. Similar fire-free intervals during the early Holocene suggest that fire regimes were controlled at that time by large-scale natural processes such as climate and vegetation patterns and establishment. No fire was reconstructed before 9000 y cal BP. Infrequent fires occurred following establishment of the subalpine bio-climate belt. However, local-scale processes have dominated the pattern of fire intervals during the late Holocene, with more fires at lower elevation and on south-facing slopes. Although altitude, topography, and slope aspect certainly drove between-site differences during the early Holocene, these differences disappeared during the late Holocene, when fire frequency was related not to ecological features of the natural landscape but likely to human population density and activities, e.g., need for pastures (woody fuel suppression). Fires were certainly controlled at first by climate and vegetation (Pinus cembra), but human practices have affected the fire regime for centuries. A new fire epoch might result from both the current global warming and on-going land-use abandonment, which has led to a significant fuel build-up in the Alps. Nomenclature: Tutin et al., 1968–1993.

Vegetation limits the impact of a warm climate on boreal wildfires

Strategic introduction of less flammable broadleaf vegetation into landscapes was suggested as a management strategy for decreasing the risk of boreal wildfires projected under climatic change. However, the realization and strength of this offsetting effect in an actual environment remain to be demonstrated. Here we combined paleoecological data, global climate models and wildfire modelling to assess regional fire frequency (RegFF, i.e. the number of fires through time) in boreal forests as it relates to tree species composition and climate over millennial time-scales. Lacustrine charcoals from northern landscapes of eastern boreal Canada indicate that RegFF during the mid-Holocene (6000-3000 yr ago) was significantly higher than pre-industrial RegFF (AD c. 1750). In southern landscapes, RegFF was not significantly higher than the pre-industrial RegFF in spite of the declining drought severity. The modelling experiment indicates that the high fire risk brought about by a warmer and drier climate in the south during the mid-Holocene was offset by a higher broadleaf component. Our data highlight an important function for broadleaf vegetation in determining boreal RegFF in a warmer climate. We estimate that its feedback may be large enough to offset the projected climate change impacts on drought conditions.

Fires Control Spatial Variability of Subalpine Vegetation Dynamics During the Holocene in the Maurienne Valley (French Alps)

Abstract: Due to stresses resulting from their high altitudes, subalpine forests are sensitive to disturbances, including fire. This study analyzes the long-term relationships between fire and subalpine vegetation in the western Alps. High-resolution analyses of charcoal, pollen, macroremains, and other palynomorphs were performed on sedimentary cores from 2 small peaty ponds located above 2000 m asl. in the Maurienne valley, France. Results reveal similar long-term vegetation dynamics, with differences concerning the structure and composition of local and surrounding plant communities. The vegetation pattern appears partially related to local fire occurrence, which was most frequent between 8900 and 6500 cal. BP at one lake and between 4100 and 1800 cal. BP at the second. Fires notably triggered the development and occurrence of populations of Acer and Alnus incana-type during a 2000-y period and the asynchronous alteration of Pinus cembra forests at both sites. Results show that the low-competitive species, i.e., Larix decidua or Pinus uncinata, were never stimulated by increasing fire frequency. This highlights the past importance of local-scale processes such as fire, which favoured pioneer broad-leaved species but did not threaten the resilience of the subalpine forests dominated by the cembra pine. Nomenclature: Tutin et al., 1968–1993.

The effect of fire frequency on local cembra pine populations

It has been predicted that global climate change will lead to increasing drought in the Alps during the 21st century, as well as an increased fire risk, fires being currently rare in these mountains. Herein we describe fire frequency reconstruction using high-resolution analyses of macroscopic sedimentary charcoal, pollen, and plant macrofossils. Sediments were sampled from a subalpine pond within the dry western French Alps. Results show that the early-Holocene expansion of Pinus cembra (7200 calibrated years BP) occurred in Acer/Alnus incana/Betula woodlands, which were affected by fires with moderate mean fire-free intervals (MFFI = 173 +/- 61 yr [mean +/- SE]). Superposed Epoch Analyses show that the abundance of P. cembra macroremains decreased significantly after burning, although they never disappeared entirely. Statistics suggest that fires spread through cembra pine communities; they were not stand-replacing fires but mainly surface fires, probably killing nonreproductive pines. An increase in fire frequency occurred 6740 years ago, when four fires appear to have occurred within 140 years. These fires may have been associated with a regional drought and could have affected the composition of the subalpine forest by depleting the local P. cembra population in the short-term. The predicted increase in drought in the future could, therefore, affect the cembra pine ecosystem in the Alps if fire frequency is reduced to intervals of less than 80 years.

Sedimentary charcoal pattern in a karstic underground lake, Vercors massif, French Alps: implications for palaeo-fire history

Knowledge on processes of charcoal transportation is crucial for fire reconstruction based on sedimentary charcoal. Charcoal is susceptible to long-distance transport by water. A lake basin with a large and long catchment area is likely to accumulate charcoal from many fires, not only those produced by fires nearby the lakeshore. Here we test the potential of charcoal transportation by analysing sedimentary charcoal accumulated in an underground lake within a karstic massif. Fires cannot spread around the lake, nor within the karstic massif. Organic materials, including charred particles, are generated several kilometres from the lake on the karstic plateau above. The pattern of sedimentary charcoal shows that the underground lake records continuously produced charcoal by wild fires or human-made biomass burning (slash-and-burn, charcoal kilns) over centuries and millennia, but also stored charcoal from eroded soils. Although the charcoal series shows a certain high variability signal, fire frequency reconstruction cannot be performed owing to chronological uncertainties. The charcoal accumulation corresponds to a more or less regular background input. Such background input is empirically well described in palaeo-fire reconstruction, but was never experimentally displayed. This study provides evidence that the pattern (surface, length, slope, etc.) of catchment areas is crucial for interpreting sedimentary charcoal series. Large catchment areas draining long rivers are not suitable for high-resolution and spatially precise fire reconstructions.