Alice Schaffhauser

Calculating long-term fire frequency at the stand scale from charcoal data

Fire frequency is a statistical metric used to evaluate long-term fire activity at stand and landscape scales. Fire frequency is defined as the number of fires occurring per unit time in a given area. In this study a method to calculate fire frequency at the stand scale is described, based on direct fire evidence of radiocarbon-dated macrocharcoal fragments (>2 mm diameter) at the soil surface and buried in the mineral soil. A jack pine (Pinus banksiana Lamb.) stand was used as a model site to calculate the long-term fire frequency. The number of fires recorded at the soil surface is a function of fire activity and residence time of charcoal, the fewer fires occurring in the site the longer the residence time of charcoal. The residence time of charcoal at the surface of the study site totals 1710 calibrated years (calibrated age in years before 2010). Fourteen fire events occurred over the last 1000 years, i.e., an average fire interval of 75 years, a situation facilitating the long-term maintenance of ja...

Recurrent fires and environment shape the vegetation in Quercus suber L. woodlands and maquis.

The effects of fire recurrence on vegetation patterns in Quercus suber L. and Erica-Cistus communities in Mediterranean fire-prone ecosystems of south-eastern France were examined on stands belonging to 5 fire classes, corresponding to different numbers of fires (from 0 to 4) and time intervals between fires since 1959. A common pool of species was identified among the plots, which was typical of both open and closed maquis. Fire recurrence reduced the abundance of trees and herbs, whereas it increased the abundance of small shrubs. Richness differed significantly between the most contrasting classes of fire recurrence, with maximal values found in control plots and minimal values in plots that had burned recurrently and recently. Equitability indices did not vary significantly, in contrast to Shannons diversity index which mostly correlated with richness. Forest ecosystems that have burnt once or twice in the last 50 years were resilient; that is to say they recovered a biomass and composition similar to that of the pre-fire state. However, after more than 3-4 fires, shrubland communities displayed lower species richness and diversity indices than unburned plots. The time since the last fire and the number of fires were the most explanatory fire variables, governing the structure of post-fire plant communities. However, environmental factors, such as slope or exposure, also made a significant contribution. Higher rates of fire recurrence can affect the persistence or expansion of shrublands in the future, as observed in other Mediterranean areas.

Forest soil paludification and mid-Holocene retreat of jack pine in easternmost North America: Evidence for a climatic shift from fire-prone to peat-prone conditions

Soil paludification is the main ecosystem process initiating the formation and development of most peatlands in the Northern Hemisphere. Sandy podzolic soils developed on coarse deposits are among a large variety of substrates overgrown by thick peat layers, particularly along the amphi-atlantic coasts of North America and Europe. Whether the podzolic soils beneath peat layers are remnants of former dry environments allowing the morphogenetic development of forest soils or the progressive outcome of natural succession towards full peatland growth is still debated. We have explored a part of this dual facet in documenting the interface between buried podzol profiles and basal peat. Two paludified sites located at the center and at the edge of an extensive plateau bog along the Gulf of St Lawrence were documented based on radiocarbon-dated tree and charcoal macrofossils. Paleosols beneath thick and relatively old (> 4000 cal. yr BP) peat were composed of slightly cemented, placic B horizons whereas those under thinner and younger (< 2500 cal. yr BP) peat were made of heavily cemented, ortstein B horizons. Forest soil paludification and peat growth at both sites commenced with the cessation of fire occurrence as evidenced by charcoal fragments in the paleosol matrix beneath Sphagnum peat layers devoid of charcoal fragments. Botanically identified charcoal fragments include several tree species, in particular jack pine (Pinus banksiana) presently absent from this part of the continent. The retreat of the species likely occurred after 5500 cal. BP with the cessation of fire occurrence, an indication of a shift in maritime Québec from dry to wetter conditions initiating peat growth and peatland expansion. It is concluded that the genesis and development of podzols with different degrees of soil cementation (placic and ortstein horizons) preceded the inception and development of the plateau bog which have been facilitated by wetter climatic conditions inimical to fire activity. The progressive lateral growth of large ombrotrophic peatlands during the Holocene is also an additional, possible factor influencing the natural occurrence and spread of fire.

The resilience ability of vegetation after different fire recurrences in Provence.

Literature on relationships between wildfires and vegetation in Mediterranean areas indicates high resilience ability for most ecosystems on calcareous soils, but data are scarce for acidic soils. The resilience ability of a “fire-driven” ecosystem is expected to depend both on the characteristics of the ecosystem and on the fire recurrence. We investigated the vegetation according to fire recurrence in a site with ecological and patrimonial stakes (the Maures massif, Provence, South Eastern France), in order to determine which ecosystems are more resilient and why. We sampled different modalities of fire recurrence (frequency of fires and interval of time between fires), and we investigated vegetation resilience to fire through different components such as composition, richness, diversity and structure. The sites corresponded to five modalities of increasing fire recurrence since 1959: control plots which had not burned since 1959, plots burned by a major and intense wildfire in 1990 (= reference fire), other plots burned twice or thrice before the 1990 fire and plots burned several times then and in 2003. The results confirmed that our Mediterranean ecosystems on acidic soils are mainly auto-successional, as vegetation composition remained rather constant after fire. Significant differences of composition existed between the control plots and the plots lastly affected by the 2003 fire. In contrast, significant differences of functional diversity (i.e. plant traits) existed among the five modalities, in particular an increase of ruderal species and a decrease of competitive taxa with increasing fire recurrence. Moreover, some types of vegetation were identified according to fire recurrence modalities. A variation partitioning procedure (partial CCA) indicated that the environmental data set explained better the plant distribution than the fire recurrence data set. The final objective of the study is to improve the understanding of the interrelationships between fires and the dynamics of vegetation, for a sustainable management of these Mediterranean ecosystems.