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Savannas are characterized by a continuous grass layer with a discontinuous tree cover (Scholes & Archer 1997). These grasses, which constitute fine and well-aerated fuel in a microclimate that promote rapid curing, make savannas highly flammable (Biddulph & Kellman 1998; Stott 2000; Hennenberg 2006) . Moreover, their rapid post-fire re-growth promotes high fire frequency (Biddulph & Kellman 1998; Hély & Alleaume 2006). As a result, surface grass fire is one of the main disturbances, together with grazing, that interact to limit trees in savannas, allowing trees and grass to co-exist (Bond & Keeley 2005; Bond 2008; Staver et al. 2011). In New Caledonia (Jaffré et al. 1998; Ibanez et al. 2012), as in others places in the tropics (e.g. Cavelier et al. 1998; Veldman & Putz 2011), forests exposed to anthropogenic and environmental changes may shift to savanna, causing large losses of biological, ecological and economic resources (Scheffer & Carpenter 2003; Folke et al. 2004). Assessing fire tolerance of woody species growing in savannas is critical to understand forest–savanna dynamics and to manage and conserve biodiversity and associated ecological services.
During surface grass fires, fuel combustion transfers heat to the trees and injures exposed roots, boles, stems and crowns (see physical processes described in Michaletz & Johnson 2007). The degree of these injuries is determined by both fire behaviour and tree characteristics (Whelan 1995). Fire line intensity and fire residence time – describing the rate of heat or energy release by the fire front and the time of heating, respectively – are important fire behaviour characteristics widely used to predict stem mortality (e.g. Ryan & Reinhardt 1988; Williams et al. 1999; Michaletz & Johnson 2008). Bark thickness and height of the tree crown are considered as the most important fire-related traits because both insulate critical tissues from heat and are used as predictors of tree mortality (e.g. Pinard & Huffman 1997; Higgins et al. 2000; Hoffmann & Solbrig 2003; Bond 2008). Indeed, thick bark insulates the vascular cambium (lateral meristem) that produces vascular tissue (Gashaw et al. 2002; van Mantgem & Schwartz 2003; Dickinson & Johnson 2004; Bauer et al. 2010). Similarly, a high crown base removes the vegetative buds (apical meristems) that produces branches, buds, foliage and reproductive organs (van Wagner 1973; Michaletz & Johnson 2006) that are beyond fire heat effects, while below-ground meristems are insulated by the soil.
Bark thickness and tree crown base height constitute defences against fire that are largely dependent on stem diameter, so that large (mature) individuals will be less injured by fire than small (juveniles) ones (e.g. Hoffmann et al. 2003, 2009). Thus, during a savanna fire most small individuals are top-killed (i.e. their above-ground biomass is killed, including the crown) while the root system often survives and allows most of them to persist through basal resprouting (Bond & Midgley 2001). Juveniles have to reach the so-called ‘escape size’ that allows them to avoid fire top-kill and recruit to the mature size; however, in savannas that can burn every year, they may be caught in a ‘fire trap’ if the interval between two fires is too short to reach the necessary size (Higgins et al. 2000; Schutz & Bond 2009).
Confronted with such a fire disturbance regime, trees can either invest in growth to reach their ‘escape size’ and resist fire, or invest in below-ground reserves to allow them to resprout after top-kill and be resilient to fire (Gignoux et al. 1997; Vesk 2006; Schutz & Bond 2009). Although resprouting may allow species to persist in a fire-prone ecosystem, the time required to reach escape size, thus potentially allowing a species to reach its mature size in fire-prone ecosystems, is a critical trait (Gignoux et al. 2009).
Allometric relationships between stem diameter and sizes of fire-related traits (i.e. bark thickness and tree crown base height) offer a good framework to characterize and analyse differences in patterns of defence investment among species (Jackson et al. 1999). Escape sizes are widely determined by post-fire conformation of fire-related traits, and top-kill or mortality via survival analysis (e.g. Hoffmann & Solbrig 2003; Bond 2008; Lawes et al. 2011a). However, in ecosystems that have high fire frequency, post-fire mortality data may be difficult to obtain on a wide range of tree species and on stem diameters, as repeated fires limit the number of trees. Moreover, as mentioned above, top-kill, or mortality probability, is a function of tree characteristics and fire behaviour, the last being unknown for most post-fire studies.
In this study, we explored the fire tolerance of 11 woody species growing in New Caledonian open savannas and encompassing the dominant savanna species (Melaleuca quinquenervia) as well as early secondary forest species. We used an original combination of allometric relationships and models of fire behaviour with fire injuries to assess how these species differ in their tolerance to surface fires. We hypothesized that the fire tolerance of the studied species is highly variable due to species-specific traits that allow species to resist or avoid fire effects, e.g. differences in bark or height investment. We discuss the impacts of these differences in fire tolerance for savanna–forest dynamics and landscape management.
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According to the BehavePlus simulations, savanna trees were likely exposed to a wide range of fire line intensity (Fig. S2). Indeed, simulated values of I ranged from <10 to ca. 2000 kW·m−1 (95% CI) under the lowest wind speed condition, and from ca. 75.0 to ca. 13 500 kW·m−1 under the highest wind speed, with a rate of spread ranging from 0.1 to 3.4 and 0.8–27.4 m·min−1, respectively, and flame length from 0.2 to 2.5 m and 0.5–6.1 m, respectively.
Species and DBH explained fire-related trait variability
The morphological traits involved in fire tolerance (particularly bark thickness) varied significantly among species (Table 2). Indeed, species and DBH together explained ca. 75% of the variance observed in BT and TH, while they explained <50% of the variance in CBH. However, note that the species, which affects the architecture and development of the canopy, explained ca. 30% of the variance in tree CBH. Species also explained more of the observed variance in BT than did DBH (48.3% and 24.1%, respectively), while the opposite was found for variance in TH (20.1% and 53.9%, respectively).
Table 2. Covariance analyses on fire tolerance morphological traits based on tree species and diameter at breast height (DBH)
| || df ||Sum of Sq.||% Var.||f-value||P-value|
|Tree crown base height|
| ln(DBH)||1||3.0515||15.0||138.3615||*** |
Bark thickness, investment strategy and bole cambium injury
The 11 species had very contrasting capacities to avoid potential fire injury to bole cambium due to differences in bark investment patterns. On average, the normalized bark thickness (NBT) was 15.8 ± 4.3% (±SD) and ranged from 8.1 ± 2.5% to 22.9 ± 4.3% for Alstonia costata and Tabernaemontana cerifera, respectively (Fig. 2). The NBT tended to decrease with the estimated bark allometric coefficient, however the correlation was not significant (Spearman's rank test, P = 0.32). Overall, Geissois racemosa, M. quinquenervia and T. cerifera showed high investment in bark (high NBT and low αBT; Fig. 2) and thus potentially high capacity to avoid fire injury to the bole cambium, as compared to Achronychia laevis and Guioa villosa, which were likely the most vulnerable species. These two last species were the only two showing positive allometry (αBT > 1, concave form; Fig. 3), followed closely by Elattostachys apetala and Ficus habrophylla that had isometric allometry (αBT ≈ 1, linear form), while all other species had negative allometry (αBT < 1, convex form).
Figure 2. Normalized bark thickness (BT as percentage of stem radius) against bark allometric coefficients (bars represent 1 SE). The two-letter codes refer to species (see Table 1).
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Figure 3. Specific allometric relationships between bark thickness and stem diameter at breast height (DBH). The two-letter codes refer to species (see Table 1); grey horizontal lines represent estimated depth of necrosis for a fire of 60-s residence time and fire line intensity (I) of 1000, 2500 and 5000 kW·m−1 (dotted, dashed and full lines, respectively); arrows show minimum DBH required to avoid bole cambium injury. All represented relationships are significant (P < 0.05), bark allometric coefficients (αBT) are given ± 1 SD (*P < 0.05; **P < 0.01; ***P < 0. 001), different letters show significant difference (Student's t-test, P < 0.05).
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In the range of the studied DBH, and for fire with a 60 s residence time, the concave and linear patterns highlighted for A. laevis, G. villosa and E. apetala were the only ones for which no individuals are expected to avoid injury from a fire of 1000 kW·m−1 (Fig. 3) because the bark of these species was very thin (BT < 1.15 cm). This was due both to low bark investment (low NBT and high αBT) and low DBH, although for E. apetala, we were unable to sample the full range of DBH possible for the species. The range of minimum DBH required for the other species to avoid cambium injury during such fires was quite large, ranging from about 11 cm for T. cerifera to 42 cm for A. costata. G. racemosa, M. quinquenervia and T. cerifera showed particularly high capacity to avoid bole cambium injury as they could quickly reach the minimum DBH required to avoid fire of 1000 kW·m−1 (12.2, 13.9 and 11.1 cm, respectively), 2500 kW·m−1 (17.6, 21.0 and 14.2 cm, respectively) or 5000 kW·m−1 (23.3, ca. 30 and 17.2 cm, respectively). Codia albicans and F. habrophylla could also avoid these fires, however their minimum required DBH were noticeably higher (e.g. 18.6 and 20.4 cm for I = 1000 kW·m−1).
Tree height and crown injury
Allometric coefficients for TH growth (αTH) were significant for all studied species (P < 0.05; Fig. 4), but more surprisingly αTH were all <1, and of the same order for all trees: from 0.46 ± 0.04 to 0.64 ± 0.03 for C. albicans and G. racemosa, respectively, considered as large trees, and from 0.40 ± 0.15 to 0.47 ± 0.09 for A. laevis and Fagraea berteoroana, respectively, considered as small trees. Moreover, relationships between the CBH and DBH were weaker (not shown) and only seven species had significant αCBH (all large tree species but only two species among the six small trees). Therefore, small tree species, with low TH and low CBH growth, should be very exposed to scorching and associated crown injury, which was confirmed for even the lowest intensity fire tested (Fig. 4). Indeed, in the range of the studied DBH, most of the small trees were likely totally scorched by low-intensity fires (i.e. I = 1000 kW·m−1, potential scorch height ≈ 6 m), while for large trees the minimum DBH required to preserve half of the tree from scorch was wide, from ca. 10.5 cm for A. costata to 25.5 cm for M. quinquenervia (Fig. 4). However, even these most potentially resistant species were likely totally scorched by very intense fires (i.e. I = 5000 kW·m−1 and potential scorch height ≈ 17 m).
Figure 4. Specific allometric relationships between tree height (black circles and full lines) and tree canopy base height (white circles and dotted lines) against stem diameter at breast height (DBH). As in Fig. 3, letters code species; dotted, dashed and full grey lines represent estimated scorch height for each fire line intensity (1000, 2500 or 5000 kW·m−1, respectively) and arrows show minimum DBH required to prevent half of the tree from scorching (for I = 1000 kW·m−1). All represented relationships are significant (P < 0.05), tree height allometric coefficients (αTH) are given ± 1 SD (*P < 0.05; **P < 0.01; ***P < 0. 001).
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