Litter flammability in oak woodlands and shrublands of southeastern France

Forest Ecology and Management 261 (2011) 2214–2222

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Litter flammability in oak woodlands and shrublands of southeastern France Thomas Curt ∗ , Alice Schaffhauser, Laurent Borgniet, Claire Dumas, Roland Estève, Anne Ganteaume, Marielle Jappiot, Willy Martin, Aminata N’Diaye, Benjamin Poilvet Cemagref - UR EMAX Ecosystèmes méditerranéens et risques, 3275 route Cézanne - CS 40061, 13182 Aix-en-Provence cedex 5, France

a r t i c l e

i n f o

Article history: Received 28 July 2010 Received in revised form 30 November 2010 Accepted 1 December 2010 Available online 24 December 2010 Key words: Litter Flammability Point-source ignition Fire ignition hazard Quercus suber Shrubland

a b s t r a c t Characterizing the flammability of litter fuels is of major importance for assessing wildland fire ignition hazard. Here we compared the flammability of litter within a mosaic of Quercus suber (cork oak) woodlands and shrublands in a Mediterranean fire-prone area (Maures massif, southeastern France) to test whether the characteristics and the flammability of litter vary with the vegetation types. We tested experimentally the ignitability, the sustainability, the combustibility and the consumability of undisturbed (=non-reconstructed) litter samples with a point-source mode of ignition. Although the frequency of ignition was similar between all the vegetation types, we distinguished four groups having litter of specific composition and flammability: low and sparse shrublands dominated by Cistus species, medium shrublands with cork oak, high Erica shrublands with sparse cork oak woodlands, and mixed mature oak woodlands with Q. suber, Q. ilex and Q. pubescens. As these vegetation types corresponded to a specific range of past fire recurrence, we also tested the effect of the number of fires and the time since the last fire on litter flammability. Litters of plots recurrently burned had low ability to propagate flames and low flame sustainability. We discuss how the recent fire history can modify vegetation and litter flammability, and thus the fire ignition hazard. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Mediterranean forest ecosystems (MFEs) are periodically disturbed by wildfires that shape plant populations (Pausas, 1999) and affect every compartment of the ecosystem, including soil and litter (Pyne et al., 1996). Fires affect vegetation, which, in turn, affects the behavior of the fires (Malamud and Turcotte, 1999) since the plant species that provide the fuel for wildfires have specific flammability and combustibility (Dimitrakopoulos and Papaioannou, 2001; Fernandes, 2009). The forest ecosystems of the Maures massif in south-eastern France are dominated by cork oak (Quercus suber L.) populations and protected by the European Union in the frame of the Habitat directive (92/43/EEC) due to their high ecological value. Cork oak woodlands have long been favored for cork production, but most trees have not been debarked and most stands have not been shrub cleared since the 1960s owing to the collapse of silviculture and grazing, and competition from the Portuguese and Spanish cork industry. In the Maures massif cork oak is associated with the evergreen Holm oak (Quercus ilex) on southern slopes, and with the deciduous downy oak (Quercus pubescens) that has predominated on Northern slopes throughout the Holocene period as the fire regime was less intense (Bergaglio et al., 2006). The present

∗ Corresponding author. Tel.: +33 4 42 66 99 24; fax: +33 4 42 66 99 23. E-mail address: [email protected] (T. Curt). 0378-1127/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2010.12.002

landscape is a mosaic of cork oak woodlands, shrublands, mixed oak woodlands (Q. suber, Q. pubescens and Q. ilex), and small remnants of mixed oak-pine (Pinus pinaster) woodlands. This mosaic is periodically reshaped by severe wildfires that preferentially affect shrublands and sparse cork oak woodlands with a shrubby understory (Schaffhauser et al., 2008). Shrubs have expanded into cork oak woodlands due to the abandonment of traditional practices including shrub-clearing and grazing (Curt et al., 2009). In turn, shrubland expansion generally facilitates fires as demonstrated in Corsican ecosystems (Mouillot et al., 2003). Low shrublands dominated by Cistus species are more frequent on the most recurrently burned sites while medium-high shrublands dominated by Erica arborea and Calycotome spinosa predominate under longer fire-free intervals (Curt et al., 2009). In this shrubland–forest mosaic as in many ecosystems (Hely et al., 2000; Behm et al., 2004), characterizing the flammability of litter fuels of the main ecosystems is of major importance for assessing the fire ignition hazard. In addition, information on litter is critical for modeling fuel loading and fire effects (Pyne et al., 1996). Litter is the surface fuel consisting of freshly fallen leaves, needles, bark or acorns (Pyne et al., 1996). They are especially important in wildland fire dynamics as they constitute the ‘receiving’ fuel that may ignite and initiate the fire. Once ignited, litter fuels may propagate fire horizontally and vertically to the upper vegetation layer (Plucinski and Anderson, 2008). The flammability of litters is hypothesized to rely on their composition, biomass,

T. Curt et al. / Forest Ecology and Management 261 (2011) 2214–2222

and physical–chemical properties of the main constituents such as leaves (Zylstra, 2006; Scarff and Westoby, 2006). But flammability should also depend upon the characteristics of the whole litter, in particular its bulk density (Pyne et al., 1996) since the litter bed ventilation is critical for the burning process (Scarff and Westoby, 2006). Litter composition should reflect the composition of the overlying vegetation, and one can expect that similar ecosystems or genus would have similar litter flammability (e.g. Behm et al., 2004). Litter abundance (i.e. fuel load) and bulk density should vary with vegetation composition but also with the stand dynamics (i.e. stand age, density, light regime, interactions between plants) (Hely et al., 2000; Tanskanen et al., 2005), site conditions that can favor or limit the production and the decomposition of dead materials, and the disturbance regime (Olson, 1963). Actually, litter biomass and bulk density result from a balance between litter accumulation due to litterfall and its decomposition (Quézel and Médail, 2003). Litter accumulation is controlled by vegetation type, ecosystems productivity, season and local disturbances since litter decomposition is controlled by leaching, physical weathering, faunal activity and microbiological consumption (Olson, 1963). In MFEs, fires strongly disturb litters to a greater or lesser extent depending upon fire regime (fire intensity, fire recurrence, fire season). In forests, low fire recurrence favors the maturation of vegetation and the accumulation of litter (Cseresnyes et al., 2006). In comparison to many oak and pine communities, Q. suber communities produce low amounts of leaves and their litters mineralize rapidly, probably in relation to high site fertility and biological activity of soil (Caritat et al., 2006). Overall, mean annual litterfall is about 250–300 g m2 in Spanish or French Q. suber stands whereas it is 250–700 g m2 for Q. ilex forests, 500–600 g m2 for most Mediterranean deciduous oaks, and 400–500 g m2 for Pinus halepensis forests (Quézel and Médail, 2003). In shrublands, low fire recurrence mostly favors the senescence of shrubs and the production of high amounts of twigs and leaves, that may increase both vegetation combustibility and litter flammability (Baeza et al., 2002). Recurrent fires, especially when intense, destroy litters and may drastically limit postfire litter build-up (Fernandes et al., 2008). Shrublands recurrently burned often have low and sparse vegetation (e.g. Cistus species) that provide few amount of small leaves that decay rapidly, and may hardly ignite and carry fire. Since Anderson (1970) then Martin et al. (1994) it has been widely accepted that the flammability of wildland fuels is a combination of four components: ignitibility, sustainability, combustibility, and consumability. Ignitability can be defined as the time until ignition of a material exposed to a heat source, temperature or heat flux. Combustibility reflects the rapidity with which a fire burns (e.g. the rate of fire-spread) and the energy released by fire (e.g. flame height and temperature). Sustainability refers to the capacity of a fire to sustain itself once ignited while consumability is the proportion of mass or volume consumed by fire (Martin et al., 1994). A hypothesis is that the characteristics of litters (i.e. composition, biomass, depth, bulk density) inherited from those of vegetation and site conditions could entail differences of flammability. Flammability experiments on litters or duff have generally focused on ignitability, and especially on the assessment of ignition thresholds in relation to fuel moisture content (FMC). FMC was generally used as the main predictor of flammability since it affects ignition and can be easily measured and controlled experimentally (Plucinski and Anderson, 2008). As most litters offer a simple (=horizontal) geometric fuel layer, most flammability studies have used point-source ignition such as wood pieces or bark (Guijarro et al., 2002), cotton ball with methylated spirits (Plucinski and Anderson, 2008), matches (Tanskanen, 2002), cigarettes (Xanthopoulos et al., 2006) or an electrically heated coil (Frandsen, 1997). Ignition success has been proved highly variable according to the type of ignition source and the experimental conditions such as litter FMC

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or wind (Plucinski and Anderson, 2008). In comparison to those for ignitability, data on the sustainability, combustibility and consumability of litters are less abundant. In this study we compared the flammability of litters in a mosaic of ecosystems including Q. suber woodlands, mixed oak woodlands (Q. suber with Q. ilex and Q. pubescens), and varied shrublands. As these vegetation communities range roughly across a gradient of fire recurrence (Curt et al., 2009) our objective was to investigate the extent to which past fires have affected the litters, and if this controls their flammability. We especially aimed at characterizing the flammability of forest litters versus shrubland litters. Under our assumption, recurrently and recently burned shrublands would have low-flammable litters due to low amounts of dead fuel, while high senescent shrublands and mixings of shrublands and oak woodlands would have high-flammable litters. 2. Materials and methods 2.1. Study site and sampling schedule Our study area is located in the Maures massif in southeastern France (43◦ 3N, 6.3◦ E), which is the largest area of cork oak forest in France (44,330 ha). The Maures massif is composed of a granitic and metamorphic basement covered with acidic Cambisols. The climate is typically Mediterranean and classed as sub-humid xerothermic. The fire recurrence within the study area is known thanks to a comprehensive and georeferenced fire database dating back to 1959. A spatiotemporal analysis of wildfires (Curt et al., 2008) indicated that two large and intense summer wildfires burned about 25,000 and 13,000 ha in the Maures massif in 1990 and 2003, respectively. The majority of the burnt areas was composed of Erica–Cistus shrublands dominated by Q. suber, with few scattered P. pinaster. The mosaic of vegetation studied here included three types of shrublands and two types of woodlands (Curt et al., 2009): - Recurrently burned shrublands dominated by the seeder Cistus (C. monspeliensis, C. albidus, C. salvifolius) with a low mean height (1 m), with some scattered Q. suber individuals and patches of bare soil and herbaceous species (called LowShrub); - Medium shrublands (mean height 2 m) dominated by Cistus species, with C. spinosa and E. arborea, subject to recurrent burning but generally after 1990. They had two subtypes: the first is composed of a discontinuous shrub layer with low biomass and a low density of cork oak (called MedShrub Low) while the second has a dense and continuous shrub layer with high biomass and a high density of cork oak (called MedShrub High); - High shrublands (3–5 m height) dominated by a dense, mature or senescent E. arborea cover with some Q. suber trees, unburned since 1990 (called HighShrub); - Sparse cork oak woodlands with some scattered Q. ilex individuals and a shrubby understory, unburned since 1959 or burned before 1990 (called SparseWood); - Submature or mature mixed oak stands (Q. suber, Q. ilex and Q. pubescens) that were generally unburned or burned before 1990 (called DenseWood). Although there is not a direct correspondence between vegetation types and fire recurrence, each type generally matched with a limited range of fires (Table 1). Our sampling schedule comprised plots that had remained unburned since at least 1959 (=control plots), and plots burned one to four times since 1959 with the date of the last fire being 1990 or 2003. Each vegetation type was replicated to account for the variation existing in the field (Table 2). As vegetation composition and structure is assumed to affect litter composition and abundance, we first extensively described vegeta-

tion composition and structure in 20 m × 20 m plots. We assessed the covering, height and density of all vegetation layers including the ground layer (herbaceous species), the understory layer (shrubs and small trees) and the overstory layer (trees), and the vertical connectivity between shrubs and trees, as described in Curt et al. (2009). 2.2. Field data collection and litter sorting We collected two sets of litter samples. Large samples (circular, diameter 38 cm) were used for the main flammability experiments including flame spread, and small samples (20 cm × 20 cm) were used for additional experiments of ignitability and vertical propagation of flames. Large litter samples (diameter 38 cm) were collected between May and July (2006, 2007 and 2008) just before the main fire season. We collected undisturbed (=nonreconstructed) samples to avoid modifying the microstructure and bulk density of litters, which may affect their flammability (Plucinski and Anderson, 2008). Litter collection was done using a 40 cm × 40 cm iron plate sunk into the soil at ca. 5-cm depth in order to collect the whole undisturbed litter. After field collection, each 40 cm × 40 cm litter sample was cut with a metallic circle of diameter 38 cm. All the samples were dried in a ventilated oven for three days at 60 ◦ C until the FMC value did not change, so they were considered as anhydrous. This is expected to mimic very low FMC values (5 ± 2%) similar to those predominating during periods of high fire risk. Small litter samples (20 cm × 20 cm) were submitted to the same operating mode. To account for the high spatial variability of litter depth due to the spatial heterogeneity of vegetation and microsites, we measured the litter depth within each 20 m × 20 m plot. Measurements were operated every 2 m on parallel transects distant from each other of 2 m. Then, three samples were selected randomly per plot, one in each category of litter depth, i.e. <2 cm, 2–4 cm, and 4–6 cm.

P = 0.0008*** P = 0.0155* P = 0.2193NS 292 1.136 ± 0.039 2.9 ± 0.1 36.2 ± 1.1

P = 0.0030** P = 0.0022** P = 0.0296* P < 0.0001**** P = 0.0045** KW t = 0.0210* 49 22.6 26.2 6.4 56.2 17.5 2.8 9 34.0 ± 5.8 c 42.3 ± 10.6 c 14.9 ± 6.0 c 28.1 ± 5.0 a 12.0 ± 3.9 ab 2.1 ± 0.3 a

45 1.309 ± 0.112 cd 2.9 ± 0.3 bc 42.2 ± 3.6 NS = non significant, *P < 0.05, **P < 0.01; ***P < 0.001; ****P < 0.0001.

9

100 1.198 ± 0.058 cd 3.1 ± 0.2 bc 35.0 ± 1.8

5 6 11

8 9 7 3 13 9 49

27 1.394 ± 0.142 d 3.4 ± 0.3 c 38.9 ± 3.9

Total

7 6 2

54 1.092 ± 0.100 bc 2.8 ± 0.2 bc 35.6 ± 2.7

2003

1 3 5 3 8 3 27

36 0.898 ± 0.103 ab 2.5 ± 0.2 ab 32.1 ± 2.9

1990

30 0.794 ± 0.081 a 2.0 ± 0.2 a 35.1 ± 3.0

1959

Litter samples Nr litter samples Litter biomass (kg m−2 ) Litter depth (cm) Litter bulk density (kg m−3 )

LowShrub MedShrub Low MedShrub High HighShrub SparseWood DenseWood Total

Date of the last fire

13 31.4 ± 5.1 bc 36.4 ± 5.4 b 12.3 ± 4.7 bc 59.6 ± 7.9 b 26.6 ± 5.1 c 2.5 ± 0.2 a

(B) Vegetation types

3 10.3 ± 5.2 ab 15.3 ± 8.6 ab 3.3 ± 3.3 abc 90.0 ± 1.0 c 33.6 ± 13.8 c 4.0 ± 0.6 c

1

Total/Mean

8 9 7 3 13 9 49

7 18.3 ± 5.0 ab 19.7 ± 5.1 a 0.0 ± 0.0 ab 65.7 ± 3.7 b 27.3 ± 7.6 bc 3.3 ± 0.3 b

1 13

Total

1

9 10.6 ± 2.5 a 10.6 ± 2.6 a 1.1 ± 1.1 ab 89.4 ± 1.3 c 8.3 ± 4.1 a 3.1 ± 0.3 b

4 1 13

4

8 17.4 ± 3.2 ab 19.0 ± 3.5 a 0.0 ± 0.0 a 23.8 ± 3.1 a 4.7 ± 3.1 a 3.0 ± 0.2 b

5 3

7 2 2 1

DenseWood

5 5 10

3

SparseWood

2 2 2 4 2 12

2

HighShrub

1

MedShrub High

0 LowShrub MedShrub Low MedShrub High HighShrub SparseWood DenseWood Total

MedShrub Low

Number of fires

LowShrub

Vegetation types

Dendrometric variables Number of plots Stand density (n/ha−1 ) Stand basal area (m2 ha−1 ) Mean overstory covering (%)OC Mean understory covering (%)UC Covering by Erica arborea (%) Vertical connectivityVC

(A)

Vegetation types

Table 1 Number of plots sampled in reference to the fire recurrence since 1959 (A: number of fires; B: date of the last fire). In B, the date of the last fire ‘1959’ has been given to all plots unburned since at least 1959. LowShrub corresponds to the low shrublands (<1 m) dominated by Cistus species, recurrently and generally recently burned. MedShrub Low corresponds to medium shrublands (1–3 m) with a low shrub biomass understory and MedShrub High to the medium shrublands with a dense shrub understory. HighShrub stands for high shrublands (>3 m) dominated by Erica arborea. SparseWood corresponds to sparse and submature cork oak woodlands on shrubland. DenseWood corresponds to mature, dense and mixed woodlands dominated by cork oak, downy oak and Holm oak.

Tests of comparison

T. Curt et al. / Forest Ecology and Management 261 (2011) 2214–2222 Table 2 Dendrometric characteristics of vegetation types and main characteristics of litter. Data include mean values and standard errors (SE). Comparisons between vegetation types were done using ANOVA with an LSD procedure (95% confidence interval). KW is the non-parametric Kruskal–Wallis test. When necessary, data were log-transformed to meet the criterion for normality. OC The mean overstory covering is the covering by trees over 10 m high. UC The mean understory covering is the covering by shrubs from 1 to 3 m height. VC The vertical connectivity index relates to the connection between trees and the understory shrubs (high values indicate low distance, i.e. high connection).

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P = 0.167NS P = 0.0484* P = 0.0161* P = 0.0018*** P = 0.0197* P = 0.0484* *P < 0.05, ***P < 0.001.

Tests of comparison Total

49 5.2 ± 11.1 31.1 ± 18.2 14.1 ± 11.6 24.0 ± 17.6 22.3 ± 15.8 1.7 ± 4.6 9 10.8 ± 18.6 b 42.5 ± 10.1c 10.2 ± 9.2 a 12.2 ± 10.2 a 29.7 ± 14.2 c 0.5 ± 1.0 a

DenseWood SparseWood

13 2.8 ± 4.9 ab 31.6 ± 19.0 abc 10.3 ± 7.0 a 14.6 ± 13.2 a 27.1 ± 13.0 bc 1.7 ± 3.7 ab 3 3.7 ± 5.5 ab 21.5 ± 17.1 ab 25.8 ± 19.2 b 26.4 ± 5.9 ab 23.9 ± 10.0 bc 0.0 ± 0.0 a

HighShrub MedShrub High

7 7.3 ± 8.2 ab 36.0 ± 22.2 bc 9.9 ± 9.8 a 21. 6 ± 17.3 ab 24.4 ± 20.9 bc 0.3 ± 1.1 a 9 1.0 ± 1.5 ab 17.8 ± 10.5 a 17.0 ± 12.2 ab 45.7 ± 33.3 b 16.5 ± 14.1 ab 2.1 ± 4.7 ab 8 0.1 ± 0.2 a 33.8 ± 16.9 abc 21.5 ± 11.1 ab 30.8 ± 8.4 ab 8.0 ± 8.1 a 6.0 ± 9.3 b

MedShrub Low LowShrub

We performed two complementary flammability experiments. The first experiment was dedicated to the comparison of litter flammability between vegetation types and as a function of fire recurrence. To do this we selected a random subset of the abovementioned undisturbed litter samples (diameter 38 cm) collected from all the vegetation types, with replicates (Table 3). All the samples were oven-dried as described above. A domestic fan fixed onto a stand produced a hot (35 ◦ C), oblique (45◦ ) and constant wind speed of 9.8 ± 0.1 km h−1 measured across the surface of the samples. We have chosen an oblique and constant air flow because this allowed comparisons between experiments. To which extent this mimics the real direction of air flow in the field has not been tested. However, our measurements of the wind speed in the field indicated that a value of ca. 9.8 km h−1 at the immediate vicinity of soil surface can be considered as high. We measured the air temperature and humidity throughout the experiment period (June to August, 2008), but this did not affect flammability (Fisher’s LSD test, P > 0.05). The flammability experiments were performed at the INRA Avignon facility. Each litter sample was placed on a 40-cm diameter round tray made of non-flammable hardiflex to avoid overheating of the flammability device. Before each ignition test we measured litter depth, biomass and bulk density. The burning apparatus was calibrated (accuracy 0.1 g) to monitor mass loss rate during the burning process. The ignition setup was operated according to a standardized protocol (Guijarro et al., 2002), using a standard Pinus sylvestris dead wood piece (2 cm × 2 cm × 1 cm) which was left at room equilibrium to about 12% moisture content. The wood piece was placed on a 500 W epiradiator at a constant temperature of 415 ◦ C and emitting a constant 7.5 W cm−2 radiation (UNE 23729-90-1R) until ignition. We waited for the total extinction of the flames, then we placed the glowing wood piece (i.e. the ember) in the center of the litter sample: this was called below ‘glowing wood piece’ mode of ignition. For each litter sample, up to five successive ignition trials were performed until the sample ignited. A variety of definitions of ignition success exist in literature including the complete combustion of the sample (Frandsen, 1997) or a minimum area burned (Lawson et al., 1996). In our study ignition was considered successful if a flame lasted at least 10 s to ensure that ignition was sufficient allow propagating flames (see Plucinski and Anderson, 2008). We measured several flammability parameters: (i) the frequency of ignition (FIG) was computed as the number of successful ignitions relative to the number of trials for a same vegetation type; (ii) the time-to-ignition (TTI, in s) corresponding to the time necessary for the appearance of a flame after the firebrand had been placed on the sample (Anderson, 1970); (iii) the number of opposite directions of the round tray reached by flames (NBS, 0–4) and the rate of spread (ROS, in cm/s) calculated from the mean time necessary for the flames to reach up to the four opposite directions marked on the tray; (iv) the maximal (HFX, in cm) and mean flame height (HFM, in cm) estimated from video recordings taken throughout the experiments: flame height ‘maximal flame height’ was assessed visually every 2 s to the near-

Number of samples Oak broad leaves Sclerophyllous oak leaves Fine particles of shrubs (<2.5 mm) Medium particles of shrubs (2.5–6 mm) Large particles (>6 mm) Dead grass

2.3. Flammability experiments

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Vegetation types

In order to test the effect of type and size of litter particles on litter flammability (see in Pyne et al., 1996; Dupuy and Larini, 2000) we sorted them on one 50-g subsample collected randomly within each plot. We finally regrouped the particles of different types and sizes into six groups that were the most abundant: broad-leaves (mostly large and lobed leaves of Q. pubescens), sclerophyllous leaves (mostly Q. suber and Q. ilex), fine particles of shrub species (diameter < 2.5 mm, according to see Valette, 1990), medium particles of shrubs including mainly leaves and small twigs (2.5–6 mm), large flammable particles (>6 mm, mainly oak acorns, bark, twigs and branches), and dead grasses (mostly graminoids).

Table 3 Composition of the litter according to the main groups of particles. Data are relative frequencies in biomass (mean value ± standard error). Comparisons between vegetation types were done using ANOVA with an LSD procedure (95% confidence interval). Oak broad-leaves are mostly large and lobed leaves of Q. pubescens. Sclerophyllous leaves are mostly those of Q. suber and Q. ilex. Fine particles of shrub species (diameter < 2.5 mm, according to see Valette, 1990) mainly include fragments of leaves and small twigs. Medium particles of shrubs include mainly leaves and small twigs (2.5–6 mm). Large flammable particles (>6 mm) are mostly composed of twigs and branches of all shrub and tree species, oak acorns, and bark fragments. Dead grasses mostly correspond to graminoid species.

T. Curt et al. / Forest Ecology and Management 261 (2011) 2214–2222

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T. Curt et al. / Forest Ecology and Management 261 (2011) 2214–2222

est cm using a graduated ruler as a landmark. The corresponded to the 95% upper percentile of the distribution of flame heights; (v) the mean (TMM) and maximal (TMO) temperature throughout the whole combustion process using k-type thermocouples (accuracy 0.2 ◦ C at 1000 ◦ C) placed at 10, 20 and 40 cm above the litter sample; (vi) the mass loss rate (CMB, dimensionless) calculated as the ratio between the consumed mass and the initial litter mass; and (vii) the area of the litter consumed at the end of each burning (SRF, %) using an image analysis. We acknowledge that the rate of spread of flames (ROS) is only indicative and cannot be directly compared to values of ROS measured in real wildfires or computed with fire simulators since our samples are small and fire propagation cannot reach a steady state (see McAlpine and Wakimoto, 1991). The second experiment was dedicated to assessing the ability of litter to propagate flames vertically to suspended shrubs. To this end we used samples of each litter type with a representative mix of shrubs. Undisturbed 20 cm × 20 cm litter samples (n = 20 per litter type) were used that had been oven-dried at 60 ◦ C for three days. A mix of shrubs representative of the ecosystems investigated was suspended (i.e. laid) on steel wire netting at a height of 10 cm above the litter. The wire netting had large square holes (2 cm × 2 cm) to allow the hot air and flames to circulate freely. The shrub mix was composed of 17 g Cistus (C. monspeliensis, C. albidus, C. salvifolius), 17 g E. arborea, and 17 g C. spinosa. All these shrubs were ovendried to get standard moisture (5 ± 2%) and to mimic harsh summer conditions and then allow comparisons between all the ignition trials. Ignition was tested by placing the glowing wood piece on the litter as explained for experiments 1 and 2. We used the same wind speed as described above. We measured successively: (i) the timeto-ignition of the litter; (ii) the successful ignition of the suspended shrubs; (iii) the time-to-ignition of the suspended shrubs; and (iv) flame duration in the suspended shrubs. The ignition of the mixed shrubs was considered successful if a flame lasted more than 10 s.

2.4. Data analysis For the first experiment we compared the flammability variables versus the litter type using one-way analysis of variance (ANOVA) with a Fisher’s LSD test (least significant difference, 95%). The normality of data was tested for each variable and the data were log-transformed when necessary. We also checked for the equality of variances using the Levene test (Sokal and Rohlf, 1995; Zar, 1999). When the variances were unequal, we used the nonparametric Kruskal–Wallis test (KW). For the second experiment we used ANOVA’s to test the difference between the ability of litters to ignite the suspended shrubs. Statistical analyses were performed with Statgraphic® and the R software (R Development Core Team, 2005). Co-inertia analysis (Doledec and Chessel, 1994; Dray et al., 2003) was used to examine the association between the litter’s characteristics and the flammability variables. The complete matrix of data was transferred to the statistical package under R 2.5.1 (R Development Core Team, 2005) then analyzed using the ADE-4 package (Thioulouse et al., 1997). Co-inertia is a statistical method commonly used to analyze the relationship between species and environmental variables (e.g. Alard et al., 2005; Moretti and Legg, 2009). The first step of the co-inertia analysis (ter Braak and Schaffers, 2004) was to conduct a correspondence analysis (CA) on the litter’s characteristics, then a principal component analysis (PCA) on the flammability variables. A factorial plane was thus created and enabled a new ordination of each data set. The statistical significance of each effect or combination of effects has been tested using a Monte-Carlo permutation test with 1000 permutations using the ‘coin’ package on R. High sums of eigenvalues on the main axes indicate high correlation among datasets.

3. Results 3.1. Litter characteristics The biomass and the depth of litter differed between the vegetation types whereas the bulk density did not vary significantly (Table 2). High Erica shrublands and mixed oak woodlands had maximal litter biomass and litter depth, whereas low Cistus shrublands that corresponded to recurrent and recent fires had minimal values. The composition of litter varied significantly between vegetation types (Table 3) with shrublands having logically higher abundance of shrub particles while woodlands (SparseWood + DenseWood) had more oak leaves. Litter composition also varied within shrublands, with low shrublands having a high proportion of biomass made of fine shrub particles, sclerophyllous leaves (i.e. cork oak leaves), and dead grasses. The proportion of medium and large particles of shrubs increased with shrublands height, that is to say from medium to high Erica shrublands. Sclerophyllous leaves (mostly Q. suber and Q. ilex) were the most frequent in cork oak woodlands (SparseWood) and in shrublands with a high cork oak density (MedShrub High) while the mixed and mature oak woodlands (DenseWood) had a high proportion of Q. pubescens leaves and large of particles such as branches. Fire recurrence affected litter composition: plots that have not burned since at least 1959 had higher proportion of broad leaves (data not shown, P < 0.0001) and large particles (P = 0.0003) but lower proportion of grass (P = 0.0010) and medium shrub particles (P = 0.0209) than the other plots. The co-inertia analysis indicated that the vegetation types coincided for a part with the characteristics of litter and the flammability, and allowed distinguishing four main groups (Fig. 1). The sums of eigenvalues for the first two axes 1 and 2 were 59.4% and 34.4%, respectively. This analysis shows a major discrimination among plots, which corresponds to the main vegetation types. The abundance of shrubs and cork oak in vegetation corresponds to the abundance of cork oak leaves and shrub debris in litter, and to longer time to ignition and high combustibility (high temperature and flames). On the opposite side along the axis 1, mixed and mature oak woodlands correspond to high abundance of deciduous Q. pubescens leaves, high rate of fire spread and high fire propagation capacity of litter. Sparse cork oak woodlands on shrublands (SparseWood) corresponded to high litter biomass, high frequency of ignition, and sustained combustion. 3.2. Litter flammability The frequency of ignition of oven-dried litters was very high irrespective of the type of vegetation: on average 90% of the litter samples ignited (Table 4). Likewise, the time-to-ignition did not vary significantly between vegetation types although the litter of DenseWood tended to ignite rapidly. The propagation of flames (NBS) varied significantly with the type of vegetation, with minimal values for low Cistus shrublands and greatest values for mature woodlands and high Erica shrublands (Table 4). The maximal temperature and mass loss rate were maximal in high and medium shrublands and minimal in low shrublands. The previous fire regime impacted in some flammability components of litters (Fig. 2). High fire recurrence significantly decreased the flame propagation (NBS), the rate of fire spread (ROS), the mass loss rate and the percentage of area of litter burned. The time since the last fire had contrary effects, with recent fires (2003) corresponding to low NBS, ROS, mass loss rate and area burned. The capacity of litter to propagate flames vertically to the suspended shrub layer differed significantly between vegetation types (Fig. 3). The mean frequency of ignition ranged from 12.5% in low Cistus shrublands to ca. 50% in medium with dense cork oak cover and high shrublands. The time-to-ignition of suspended shrubs

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Fig. 1. Co-inertia analysis comparing the distribution of the characteristics of the litter samples and their flammability parameters. The main figure indicates the groups of plots having similar litter and flammability characteristics. The grey ellipses include 95% of the plots of a specific group. Abbreviations for lower left figure (litter characteristics): DEP (litter depth), QUS (leaves of Quercus suber), QUP (leaves of Quercus pubescens), UND (fine shrub particles, diameter < 2 mm), TWI (shrub twigs, diameter 2–6 mm), SHR (large shrub particles, >6 mm), NEE (needles of Pinus pinaster), HER (dead herbaceous particles). Abbreviations for lower right figure (flammability variables): ROS (rate of spread), NBS (number of directions reached by flames), SRF (% area burned), TTI (time to ignition), IGN (frequency of ignition), CMB (flame duration), TMM (flame maximal temperature), TMO (flame mean temperature), BUR (mass loss), HFX (maximal flame height), and HFM (mean flame height).

followed a similar pattern (P = 0.0022) while the flame duration followed an inverse pattern (P = 0.0011). 4. Discussion 4.1. Interactions between fire recurrence, vegetation characteristics and litter flammability The flammability of oven-dried litter varied with the type of vegetation and the fire recurrence along the past decades, mostly for variables describing the combustibility and consumability, and the ability to ignite the suspended shrubs. In general, higher rate of spread, number of sides reached by flames and mass loss corresponded to the denser vegetation types with high cover by shrubs and trees. These types were also those less recurrently burned, which allowed litter accumulation. Conversely, the ignitability and sustainability remained quite constant among the vegetation types, with oven-dried samples having similar frequency of ignition and time to ignition. This probably results from the fact that almost all litter samples are likely to ignite at very low moisture content. Only 10% of these samples did not ignite during the first trial, corresponding mainly to those having low litter amount (LowShrub, MedShrub Low). This may also result from the use of a small wood

piece to provide a point-source ignition: the ignition success was highly dependent on the litter composition at the location of the wood piece. Greater differences have been shown with the logistic regression of ignition versus litter FMC. The combination of flammability experiments and the coinertia analysis allowed distinguishing four main groups of litter. Mixed and mature oak woodlands constitute a first group with high rate of spread and flame propagation (NBS), high moisture extinction threshold (FME) but average capacity to propagate and sustain fire in suspended shrubs. These woodlands have generally remained unburned for several decades, thus allowing a heavy litter accumulation and the establishment of maturation of the shade mid-tolerant and deciduous Q. pubescens. The flammable and large lobed leaves of Q. pubescens burn rapidly and propagate flames efficiently. This fits with the results of Kane et al. (2008) for US southeastern oaks: large lobed oak leaves of Quercus falcata and Q. laevis burned better than the entire leaves of evergreen oaks such as Q. virginiana. At the opposite, a second group corresponds to litters of low biomass, low sustainability and combustibility, regrouping low Cistus shrublands and medium shrublands with low biomass that are recurrently and/or recently burned. These litters have high time to ignition, low ROS, and poor capacity to propagate fire to suspended shrubs. This may be described as a fuel-limited stage

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Fig. 2. ANOVA for three main flammability variables (number of opposite directions reached by flames, NBS; mean rate of spread, ROS; and percent of mass loss during the experiment,) according to the number of fires occurring since 1959 and to the date of the last fire. The date of the last fire is fixed at 1959 if the plot did not burn since 1959. For the number of fires, category 0 indicates plots unburned since at least 1959. In category 3 4, all the plots that had burned three or four times since 1959 were pooled. Different letters in the same figure indicate statistically significant difference (P = probability of the Kruskal–Wallis non-parametric test).

(sensu Guyette et al., 2002) due to frequent consumption of litter during the 2003 wildfire, an incomplete postfire shrub fuel buildup and the low litterfall owing to the sparsity of these cork oak stands. In many of low Cistus shrublands litter biomass was likely to be below the minimal fuel load necessary for fire ignition and sustainability, thus explaining the very low FMC values necessary for ignition (4%). The third group includes litters with high combustibility and an average ability to propagate and to sustain fire in the suspended shrubs. It corresponds to medium shrublands with high cork oak density. The fourth group of litters was highly combustible (high ROS, high flames, high flame duration, high temperature) and had a very high capacity to propagate and sustain fire in suspended shrubs. It corresponded mainly to litters from high Erica shrublands unburned for about 15–18 years and to sparse cork oak woodlands with a shrubby understory. Under these conditions, thick litter accumulates with high amounts of particles from senescent shrubs (E. arborea, but also Calycotome and Cistus). These shrub species have small leaves, densely branched and small twigs, thus favoring ignition then fire spread driven by strong ventilation (Rothermel and Philpot, 1973; Scarff and Westoby, 2006).

Experiments on plant flammability that have been performed on small samples composed of leaves of a single species (see Dimitrakopoulos and Panov, 2001) focused on how the chemical and physical properties of leaves result in differential pyric properties. Such studies have shown the impact of leaf thickness and surface-to-area ratio (Brown and Simmerman, 1986; Papio and Trabaud, 1990; Dimitrakopoulos and Panov, 2001), the proportion of volatile compounds (Owens et al., 1998; Alessio et al., 2008; ˜ et al., 2009; De Lillis et al., 2009), and lignin content (Owens Ormeno et al., 1998; Scarff and Westoby, 2006; Shan et al., 2008; De Lillis et al., 2009). This allowed segregating species into groups having different litter flammabilities; for example, southeastern US oaks have been classed as ‘fire facilitators’ or ‘fire impeders’ as a function of the flammability properties of their leaves (Kane et al., 2008). Our study confirms that the plants’ leaf characteristics influence the flammability of litters: in particular, the lobed leaves of the deciduous oak Q. pubescens are ignitable in comparison to the leaves with entire margins of the evergreen Q. suber and Q. ilex. The abundance of twigs, woody or non-flammable particles can modify the flammability parameters of a litter. Likewise, the flammability of

Flammability Values

P = 0.3291NS P = 0.0434* P = 0.818NS P = 0.717NS KW = 0.117NS KW = 0.0334* P = 0.0098** P = 0.261NS P = 0.099NS

60 50 40 30 20

d

d

oo W e_ D en s

_W Sp

ar

se

h_ ig H

ru Sh ed M

oo

b Sh

H b_

b_ ru Sh ed M

ru

ig

w Lo

ru Sh w Lo

Litter/Vegetation Types Fig. 3. Ignition frequency (mean value), time-to-ignition and flame duration of a suspended shrub mix of Cistus, Calycotome and Erica as a function of litter/vegetation types. The suspended shrubs were oven-dried (FMC = 5 ± 2%). The lines indicate the mean value for each litter/vegetation type and the small vertical bars are standard errors. The number of replicates is 20 per litter type.

a litter relies on its biomass, depth, and bulk density (Pyne et al., 1996). 4.2. Implications for the forest and landscape management

*P < 0.05, **P < 0.01. Values in bold are statistically significant with P < 0.05.

36 83.9 33.0 ± 6.0 a 0.62 ± 0.11 ab 10.5 ± 1.0 23.7 ± 2.0 a 39.7 ± 2.9 a 53.8 ± 9.0 ab 2.2 ± 0.2 bc 133 ± 12 a 61 ± 30 a 30 86.7 35.1 ± 8.1 a 0.39 ± 0.05 a 10.5 ± 1.5 a 24.3 ± 3.1 a 33.1 ± 1.3 a 43.7 ± 8.5 a 1.9 ± 0.3 a 115 ± 14 a 57 ± 30 a

h

0 b

292 87.4 29.4 ± 32.1 0.74 ± 0.67 11.1 ± 0.8 24.2 ± 15.6 42.4 ± 6.0 56.1 ± 34.7 41.2 ± 22.3 136 ± 88 65 ± 30 a 45 86.7 24.1 ± 2.5 a 0.89 ± 0.10 b 11.7 ± 0.8 a 25.3 ± 1.5 a 41.3 ± 3.1 a 77.8 ± 10.3 c 2.6 ± 0.1 cd 145 ± 9 a 69 ± 29 a 100 91.3 28.5 ± 3.6 a 0.80 ± 0.11 b 11.1 ± 1.5 a 22.2 ± 2.4 a 51.1 ± 6.8 a 56.5 ± 16.5 ab 2.4 ± 0.2 bcd 152 ± 15 a 65 ± 31 a 27 92.9 36.3 ± 5.2 a 0.63 ± 0.08 ab 9.7 ± 1.1 a 21.5 ± 2.1 a 41.5 ± 8.4 a 62.3 ± 16.7 bc 2.9 ± 0.2 d 132 ± 13 a 74 ± 25 a

Tests of comparison Total/Mean DenseWood SparseWood HighShrub

Frequency of Ignition (%) Time to Ignition (s) Flame Duration (s)

70

10

54 86.1 31.4 ± 5.1 a 0.74 ± 0.17 ab 12.3 ± 1.8 a 26.5 ± 3.4 a 46.7 ± 9.1 a 55.5 ± 9.8 ab 1.9 ± 0.3 a 115 ± 14 a 59 ± 30 a

MedShrub Low LowShrub

Nr litter samples Ignition frequency (%) FIg Time to ignition (s)TTI Rate of spread (cm s−1 ) ROS Mean flame height (cm) HFM Maximal flame height (cm)HFX Mean flame temperature (◦ C)TMM Maximal flame temperature (◦ C)TMO Number of sides reached (n) Flame duration (s)CMB Mass loss (%)

MedShrub High

2221

80

Vegetation types

Table 4 ANOVA for the flammability variables according to the vegetation types. Data include mean values and standard errors (SE). Comparisons between vegetation types were done using ANOVA with an LSD procedure (95% confidence interval), or with the non-parametric Kruskal–Wallis KW test. FIg The frequency of ignition is calculated as the proportion of successful ignitions as compared to the whole number of trials. TMO The flame temperature is the mean flame temperature in the center of the flammability apparatus, measured 40 cm above the litter surface. ROS The rate of spread is calculated as the mean value in four opposite directions on the round tray. HFM The mean flame height was assessed using image analysis throughout the flammability experiments: the mean flame height is the mean height of flames sampled every 2 s. HFX The maximum flame height corresponds to the upper 95% percentile of all the flame heights measured for a sample.

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In the Maures massif, the recent fire history has impacted on the composition and structure of vegetation (Schaffhauser et al., 2008) and, in turn, on the characteristics of litter. Recurrent fires destroy a part of litters and favor the development of low Cistus shrublands with limited litter ignitability. The highest fire ignition hazard is likely to correspond to high Erica shrublands with moderate fire recurrence (mean fire interval 15–20 years) because they have thick and flammable litters made of senescent Erica particles. Mature Erica shrublands are also highly combustible and support high-intensity and rapid fires (Fernandes et al., 2000). The huge expansion of Erica shrublands along the past decades may challenge the conservation of the cork oak Habitat of the Maures massif as they can ignite easily and support intense fires that affect cork oaks (Pausas, 1997), but also because they limit oak regeneration from seeds (Curt et al., 2009). This type of shrubland may persist for decades due to the longevity of Erica (Mesleard and Lepart, 1989). Shrub clearing has been applied for the conservation of cork oak populations but to which extent it may limit the flammability of litter remains to be studied. Acknowledgements This study has been funded by the European Commission through the Integrated Project Fire Paradox FP6-018505. We greatly acknowledge J.C. Valette, J. Maréchal and D. Mortier (INRA Avignon facility) for their very valuable help during the flammability experiments and for their scientific and technical advices. References Alard, D., Chabrerie, O., Dutoit, T., Roche, P., Langlois, E., 2005. Patterns of secondary succession in calcareous grasslands: can we distinguish the influence of former land uses from present vegetation data? Basic and Applied Ecology 6, 161–173. Alessio, G.A., Penuelas, J., Llusia, J., Ogaya, R., Estiarte, M., De Lillis, M., 2008. Influence of water and terpenes on flammability in some dominant Mediterranean species. International Journal of Wildland Fire 17, 274–286. Anderson, H., 1970. Forest fuel ignitability. Fire Technology 64, 312–319. Baeza, M.J., De Luis, M., Raventos, J., Escarre, A., 2002. Factors influencing fire behaviour in shrublands of different stand ages and the implications for using

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