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Keywords:

  • Carya alba ;
  • duration of heating;
  • ecosystem engineer;
  • fire temperature increase;
  • flammability;
  • longleaf pine;
  • mockernut hickory;
  • Pinus palustris ;
  • plant population and community dynamics;
  • survival and resprouting

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
  1. Fuels in the groundcover of frequently burned south-eastern pine savannas include shed leaves of trees. Flammable needles of longleaf pine (Pinus palustris) potentially increase maximum fire temperatures and durations of heating, negatively affecting other trees within the groundcover. Less flammable leaves that accumulate around the bases of understorey stems of hardwood trees such as mockernut hickories (Carya alba) in the fall potentially depress maximum fire temperatures and durations of heating, enhancing post-fire recovery.
  2. We experimentally manipulated amounts of pine and hickory leaves beneath understorey hickory stems in a pine savanna, measured temperatures during prescribed fires and assessed combustion of fuels and survival and regrowth of hickory stems.
  3. Pine needles increased fire temperatures and durations of heating relative to herbaceous fuels and increased combustion of hickory leaves. Hickory leaves, however, neither increased nor decreased fire characteristics relative to herbaceous fuels.
  4. All hickories survived fire by resprouting. When pine needles were absent, most hickories resprouted from buds located above-ground along the stem at heights inversely related to temperature increase. In contrast, resprouting occurred only from underground root crowns when pine needles were present. Such differences in locations of resprouts influenced sizes of stems at the end of the growing season.
  5. Synthesis. Groundcover fuels containing flammable leaves shed by pyrogenic species of savanna trees affect local fire characteristics and resprouting of non-pyrogenic understorey trees. Thus, local variation in flammable fuels produced by pyrogenic species can engineer landscape dynamics of other trees in savannas.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Fuels in fire-frequented savannas are comprised of vegetation close to the ground surface. These fuels contain live and dead plants in the groundcover, as well as litter shed from trees in the vicinity (Keane, Burgan & Wagtendonk 2001). As a result, fuel type and amount vary locally depending on the vegetation present, and this variation influences fire intensity (Thaxton & Platt 2006; Hiers et al. 2009; Wenk, Wang & Walker 2011). Because overstorey trees can produce large amounts of litter, especially in seasonal environments, they potentially have strong local effects on the intensity of surface fires, and hence the groundcover vegetation.

Leaves shed by trees vary in flammability. For example, needles of longleaf (Pinus palustris) and ponderosa (P. ponderosa) pines are more flammable than needles of sand (P. clausa) and lodgepole (P. contorta) pines (Fonda, Belanger & Burley 1998; Fonda 2001). Leaves of turkey oak (Quercus laevis) are more flammable than those of live oak (Q. virginiana) (Kane, Varner & Hiers 2008), but leaves of both oaks are less flammable than longleaf pine needles (Williamson & Black 1981). In general, more flammable fuels are produced by species with leaves that are large and loosely packed when shed, that have low moisture content or that contain volatile oils (Dimitrakopoulos & Papaioannou 2001; Behm et al. 2004; Scarff & Westoby 2006; de Magalhães & Schwilk 2012). Such species-level variation in flammability should influence local fire characteristics, especially intensity, in the vicinity of savanna trees.

Differences in local fire intensity resulting from fuels with different flammability should influence damage and survival of understorey plants. Mutch (1970), observing that many plants in fire-prone ecosystems had flammable tissues, hypothesized that flammability might be beneficial to plants in such ecosystems. Increased flammability of leaves shed by savanna trees might open space for recruitment and survival of their offspring, possibly by removing understorey competitors (Platt, Evans & Rathbun 1988; Bond & Midgley 1995). Increased flammability of leaf litter could also increase survival during fire by encouraging rapid consumption of fuels by fast-moving fires that are less damaging to below-ground plant tissues than slower, smouldering fires (Varner et al. 2005; ‘pyrogenicity as protection’ sensu Gagnon et al. 2010). In contrast to the Mutch hypothesis, however, reduced flammability might also increase survival by suppressing fire beneath trees producing such litter (Guerin 1993; Bradstock & Cohn 2002; Trauernicht et al. 2012). Reduced flammability of leaf litter may reduce fire intensity and allow these trees to survive fire intact, or allow resprouting trees to more quickly reach a size that can withstand fire (Grady & Hoffmann 2012; Hoffmann et al. 2012).

Plants indigenous to fire-frequented ecosystems have mechanisms for surviving fires. Overstorey trees tend to have thick or layered bark that reduces fire damage to the vascular cambium (Jackson, Adams & Jackson 1999). Many understorey plants are top-killed (meaning above-ground stems are killed), but resprout from dormant meristems at ground level or on underground storage organs (e.g. Keeley & Zedler 1978; Buchholz 1983; Ojeda, Maranon & Arroyo 1996; Peterson & Reich 2001; Drewa, Platt & Moser 2002; Higgins et al. 2007; Hoffmann et al. 2009; Werner & Franklin 2010). Local fire characteristics, influenced by the fuels present, should influence damage to dormant meristems and hence survival and regrowth of understorey plants.

Effects of flammability on fire characteristics, and hence survival of understorey plants, may differ among trees in longleaf pine savannas. Longleaf pines (Pinus palustris Mill.), whose flammable fuels encourage fire spread, usually survive fire, while understorey hardwoods (broadleaved woody plants) are top-killed and resprout (Platt, Evans & Rathbun 1988). Mockernut hickory [Carya alba (L.) Nutt., formerly Carya tomentosa (Lam.) Nutt.] is one hardwood common in the understorey of upland pine savannas in the south-eastern United States (Fig. 1a). After fires, hickories in the understorey resprout from root crowns typically located about 5 cm below the soil surface. The compound leaves of the hickory are among the largest tree leaves in pine savannas; once shed, these form a thick mat surrounding stems (Fig. 1b), possibly suppressing fires and protecting the hickory stem and root crown.

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Figure 1. (a) Representative photograph of the study site, overstorey dominated by longleaf pine, with an understorey of bluestem grasses and scattered resprouting hardwoods; photograph taken in September, approximately 4 months since fire. (b) Photograph of understorey hickories, showing natural litter accumulation, taken in April, 2 years since last fire.

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In this study, we explore effects of leaves shed by savanna trees on fire characteristics and survival and resprouting of understorey trees. First, we hypothesize that flammable and non-flammable fuels produce different fire characteristics. Based on prior study and field observations of fires burning pine needles and hickory leaves, we expect characteristics of fires to be augmented by pine fuels and suppressed by hickory fuels. We predict that increases in temperatures, durations of heating above ambient temperature and combustion of fuels at ground level will be modified by tree fuels added to background herbaceous fuels. When both pine and hickory fuels are absent and only herbaceous fuels are present, we expect elevation of temperatures for some time during fires (Fig. 2a). We expect lower temperatures and shorter durations of heating based on hypothesized fire suppression effects if hickory fuels, but no pine fuels, are added to the herbaceous fuels (Fig. 2c). We also expect further lowering of temperature increases and shortening of durations of heating as the amount of hickory fuels increases in the absence of pine fuels, as more fuels with low flammability should be further compacted and retain more moisture (Fig. 2e). In contrast, we hypothesize that pine fuels should increase temperatures and durations of heating above those resulting from herbaceous fuels, especially in the absence of hickory fuels (Fig. 2b). We expect temperatures and durations of heating between those for pine alone or hickory alone due to hypothesized opposing effects of hickory and pine fuels when both are present (Fig. 2d,f). We further expect these effects of fuels on temperature increases and duration of heating to be reflected in combustion of tree fuels: combustion of hickory leaves should be increased when pine needles are present, and combustion of pine needles should be reduced when hickory leaves are present.

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Figure 2. Predicted responses of temperature–time curves for various treatments: (a) when both hickory and pine fuels are absent, (b) when pine is present but hickory is absent, (c) when hickory is present but pine is absent, (d) when both pine and hickory are present in the same amounts, (e) when a greater amount of hickory is present but pine is absent and (f) when pine is present with a greater amount of hickory. Temperature increases (vertical arrows), and durations of heating (horizontal arrows) are derived from logger data on changes in temperature over time at one-second intervals during fires.

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Second, we hypothesize that fire characteristics produced by different fuels should in turn influence damage and post-fire responses of understorey plants in fire-frequented ecosystems. We expect survival and resprouting of hickory stems to be inversely related to temperature increases and durations of heating. As a result, hickory stems should be less likely to be damaged and more likely to survive fires with hickory fuels than fires with longleaf pine fuels.

We address these hypotheses using a field experiment. We manipulate fuel composition around hickory stems in a frequently burned pine savanna and measure fire characteristics and regrowth of hickory stems. We use results of this study to postulate how local heterogeneity in flammability of fuels produced by savanna trees might influence spatial heterogeneity in savanna vegetation.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We conducted our study in uplands at Girl Scouts Camp Whispering Pines (30°41′ N; 90°29′ W) in Tangipahoa Parish, Louisiana, at the western edge of the eastern Gulf Coastal Plain. Well-drained, fine Tangi–Ruston–Smithdale Pleistocene sands mixed with and capped by deposits of loess form a rolling topography 25–50 m a.s.l. (McDaniel 1990; Platt et al. 2006). Mean annual temperature is 19 °C, and mean annual rainfall is 1626 mm (Thaxton & Platt 2006). Current overstorey longleaf pines originated as natural regeneration within areas managed by fire for open-range grazing after widespread logging in the early 1900s (Noel, Platt & Moser 1998). Fire exclusion in the 1980s resulted in increased abundance of hardwoods. Since the mid-1990s, the site has been managed with biennial prescribed fires ignited during April–May to mimic timing of natural lightning fires (Platt et al. 2006). Longleaf pines at the camp have a mean density of 293.3 ± 34.6 stems ha−1, average diameter of 28.6 ± 1.5 cm and an average basal area of 18.2 ± 1.5 m2 ha−1 (Noel, Platt & Moser 1998; Platt et al. 2006; Carmichael 2012). The diverse understorey (≥30 species m−2, ≥100 species 100 m−2) includes grasses, herbs, shrubs and small trees (Noel, Platt & Moser 1998; Platt et al. 2006; Gagnon et al. 2012).

We selected an area of Camp Whispering Pines with longleaf pine in the overstorey and understorey hickories in a groundcover dominated by warm season grasses. This area, in a burn unit scheduled for a prescribed fire in May 2010, had last been burned in May 2008. The ground was relatively level, so any effects of topography on fire were small. The burn unit was split into two separate subunits to be burned sequentially in midmorning to early afternoon on the same day, so as to minimize likelihood of damage to campsites located within the burn unit.

We selected 30 hickory genets in January 2010, after they had shed leaves. All had a single dominant main stem, although many (80%) also had one or more smaller stems originating from the same root crown. Measurements were made on the largest stem of each genet. All stems were resprouts since the last prescribed fire, and average height of the tallest stem of each genet, measured before fire, was 75 ± 4 cm. No stem had thick bark that might protect the cambium from fires. Bark thickness of the largest stem was 2 mm, measured after the stem was top-killed by fire in 2011. Hickories were chosen far enough apart that plots would not overlap, but all 30 hickories were within an area about 70 m in diameter. Thus, general fire characteristics were expected to be similar for all hickories.

Litter treatments were based on observations of litter beneath hickory stems. The distance of shed hickory leaves from the stems indicated that most hickory litter was deposited within 40 cm of the stem; this distance was used to determine plot size for litter manipulations. All litter present beneath stems was then removed, on average 710 ± 46 g. When fuels were removed, we observed that most pine needles occurred beneath the layer of hickory leaves, a result of shedding occurring earlier in the dormant season for pines than hickories. We mimicked this layering when applying treatments.

We estimated amounts of fuels using groundcover biomass samples collected from plots of 40 cm radius around hickory stems. Based on mean fuel load estimates of 0.1 kg m−2 (Fig. 3), an average of 50 g of hickory leaf litter was expected within a 40 cm radius of hickory stems 2 years after fire, so this amount was used as the baseline for treatments. Fifty grams of pine needle litter was used for comparison with the average 50 g of hickory litter, although the average amount of pine litter beneath hickory stems at Camp Whispering Pines, based on a mean fuel load of 0.6 kg m−2, was closer to 300 g (Fig. 3). No wood was added to plots, as woody fuels are a minor and sporadic component of the natural fuels. Wood was not present beneath all hickory stems, but where present, a single cone or fallen branch made a disproportionately large contribution to fuel load. All treatments were within the range of natural variation and could be found at different locations at the study site.

image

Figure 3. Fuel loads beneath hickory stems 2 years after prescribed fires at Camp Whispering Pines. Fuels derived from trees were divided into wood and leaves. Hickory wood consisted of dead stems killed by the last fire, while pine wood included cones, bark and branches. The ‘other’ category includes herbs, lianas and other shrub leaves, and the ‘litter’ category includes partially decomposed fragments that were too small to sort, predominantly of pine and hickory leaves. Bars are back-transformed least-squares means ± standard error.

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Six different treatments were arranged in a 3 × 2 factorial design, with each treatment replicated five times. Circular plots with 40 cm radius (about 0.5 m2) were established around each hickory. One of three levels of hickory fuel was added to each plot: 0 g (0 kg m−2), 50 g (0.1 kg m−2) or 100 g (0.2 kg m−2). One of two levels of pine fuel was also added to each plot: 0 g (0 kg m−2) or 50 g (0.1 kg m−2). Treatments were applied in January, 4 months before fire, so fuels would have time to adjust to ambient conditions and become naturally compacted. Herbaceous plants regrew before fire, so all plots contained sufficient fine fuel to burn.

Prescribed fires were ignited on 24 May 2010, 22 days since the last rainfall. On the day of fire, 18 plots (three replicates of each treatment) were burned in the first burn unit, and 12 plots (two replicates of each treatment) were burned in the second unit. The use of two different fires allowed us to observe fire characteristics under a wider range of conditions. The first fire was ignited around 10:00 am, while fuels were still moist with dew, and the second fire was ignited around 12:30 pm when fuels were drier. Prescribed fires were ignited using a drip torch while walking around the burn units, first igniting backing and flanking fires, followed by head fire. Flame lengths were less than 50–100 cm, and rate of spread averaged 0.6 ± 0.2 m min−1 for both fires. During burning of the first unit, air temperature was 32 °C, with 66% relative humidity, and during burning of the second unit air temperature had risen to 34 °C and relative humidity dropped to 56%. Winds were light and variable during both fires, 0–5 km h−1 from the north during burning of the first unit and 0–8 km h−1 from the west during burning of the second unit. Weather data were recorded at Hammond, LA, about 18 km south of the study site, and downloaded from the National Climatic Data Center, operated by the National Oceanic and Atmospheric Administration (available online at http://www7.ncdc.noaa.gov/CDO/dataproduct).

During fires, data loggers were used to record temperatures at the soil surface beneath hickory stems at one-second intervals. Data loggers (U12-014) and connectors (SMC-K) were obtained from Onset Computer Corporation, Bourne, MA, and 0.81-mm thick insulated thermocouples (XCIB-K-2-3-10) were obtained from Omega Engineering, Inc., Stamford, CT. Three-metre thermocouple wires were connected to data loggers following general procedures in Grace, Owens & Allain (2005). Three probes were used at the base of each hickory because temperatures vary from point to point, and thermocouple wires have an average 7% fail rate. The day before fire, thermocouple probes were positioned on the soil surface beneath fuel treatments at 5 cm distances and angles of 0, 120 and 240 degrees from the dominant stem. On the morning of the fire, data loggers were attached to thermocouples, enclosed in plastic bags and buried well outside plots to protect the logger from temperatures experienced during fire. We retrieved loggers about 3 h after fires. We used graphs of each temperature–time record to determine maximum temperature increase and duration of heating. Temperature increase was calculated as the difference between the highest temperature during fire and ambient temperature before fire (Fig. 2, vertical arrows). Duration of heating was calculated as the amount of time that temperatures remained elevated above pre-fire ambient levels (Fig. 2, horizontal arrows).

After fire, we assessed survival and regrowth of hickory stems. We recorded resprout height and resprout location for each hickory. Resprout height was measured as the distance above ground of the terminal bud of the tallest resprout. Resprout location along the stem was measured as the distance from the ground to the base of the highest resprout; if all resprouts originated from the underground root crown, this distance was zero. Measurements were recorded in July and September, 2 and 4 months after fire in 2010. All hickories were top-killed by fire in the spring of 2011, so further measurements of growth were not possible.

In a separate experiment, the same six treatments were repeated in open areas of the pine savanna to measure combustion of pine and hickory fuels. Three replicates of each treatment were established in early March in the same burn unit. Pre-fire fuels thus were 0 and 50 g of pine needles and 0, 50 and 100 g of hickory leaves. After fire, unconsumed pine and hickory fuels were sorted from each pre- and post-fire sample, oven dried and weighed to obtain mass. Per cent combustion of pine and hickory fuel was then calculated as the ratio of unconsumed post-fire fuel mass to pre-fire fuel mass.

Analyses were conducted using SAS 9.1 (SAS Institute Inc. 2004). Temperature increases and durations of heating were analysed using Proc Mixed anova. We included a random effect for plot and fixed effects for treatments (pine and hickory) and fire unit to detect any differences between successive fires burned at different times of the day when fuel moistures were different. Denominator degrees of freedom were calculated using the Kenward–Roger method. Of the 90 data loggers (six treatments × five replicates × three loggers each plot), six malfunctioned and failed to record temperatures during fire and so were excluded from analysis. Residuals were examined for normality using Proc Univariate and the Shapiro-Wilk statistic. Temperature increase was natural log-transformed to correct for lack of normality, and no transformation was required for duration of heating. One additional outlier with respect to duration of heating was removed from analysis; this data point was about seven deviations from the mean when residuals were analysed, and temperatures at this data point remained elevated for nearly 40 min and could have resulted from burning wood falling on the thermocouple probe during fire. ancova in Proc Mixed was used to analyse relationships between durations of heating and temperature increases, with fuel treatments as covariates. Multiple regression in Proc Reg was used to detect relationships between hickory regrowth (resprout location, resprout height) and fire characteristics. We used backward variable selection to select fire variables that best explained survival and regrowth. One hickory that died before fire and one other outlier (five deviations from the mean) were removed from analyses of regrowth. Per cent combustion was also analysed using Proc Mixed anova. Combustion of pine litter was analysed with respect to hickory treatments, and combustion of hickory litter was analysed with respect to pine treatments. One outlier (over four deviations from the mean) was excluded from analysis; fire did not burn all the way across the plot of this outlier.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Characteristics of fires varied considerably among plots. Average ambient temperature at ground level recorded just prior to ignition of local fuels around thermocouples was 31 ± 0.35 °C. Maximum temperatures recorded at ground level during fires ranged from 32 to 561 °C. The average maximum temperature recorded across all treatments was 148 ± 11 °C, similar to values observed (147 ± 7 °C) at ground level in annually burned undisturbed Florida sandhill (Gibson, Hartnett & Merrill 1990). These maximum temperatures were lower than those recorded in other studies in pine savannas in which temperatures were recorded above ground level or with higher fuel loads (Williamson & Black 1981; Olson & Platt 1995; Drewa, Platt & Moser 2002; Kennard et al. 2005; Thaxton & Platt 2006). Durations of heating at ground level also varied among plots, ranging from 2 to 23 min, with an average of 12.9 ± 0.48 min across all treatments. These values are similar to those reported (13.3 ± 1.66 min) above the ground in wiregrass patches within Carolina sandhill (Wenk, Wang & Walker 2011). These durations measured in pine savanna are also similar to those recorded (13.0 ± 0.77 min) above the ground in Ohio oak–hickory forest (Iverson et al. 2004), but are much shorter than observed (219 min) at ground level in Spanish maquis (Molina & Llinares 2001). Fire characteristics are summarized in Table S1.

Characteristics of fires were influenced by pine fuel load but not by hickory fuel load. Mean maximum temperature increases at ground level were 60 °C (±1 SE ranged from 40 to 92 °C) when only herbaceous fuels were present (Fig. 4a). Addition of different amounts of hickory fuels did not significantly change mean maximum temperature increases (= 0.85; Table S2). In contrast, addition of pine fuels doubled temperature increases over herbaceous fuels, resulting in mean maximum temperature increases of 120 °C (±1 SE ranged from 79 to 182 °C; Fig. 4a); these effects were significant (= 0.04). Interactions between pine and hickory fuels had no significant effects on maximum temperature increases (Fig. 4a and Table S2).

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Figure 4. (a) Effect of pine and hickory fuels on maximum increase in temperatures at ground level during prescribed fires in May 2010. Bars are back-transformed least-squares means ± standard error. (b) Effect of pine and hickory fuels on durations of heating. Bars are least-squares means ± standard error. For both graphs, letters above bars indicate groupings based on the main effect of pine treatment.

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Durations of heating were also influenced by pine, but not hickory fuels. Temperatures were elevated above ambient levels for 11.0 ± 1.3 min when herbaceous fuels alone were present (Fig. 4b). Addition of different amounts of hickory fuels neither shortened nor lengthened durations of heating (= 0.82). Addition of pine fuels, however, significantly lengthened (< 0.001) durations of heating to 15.9 ± 1.3 min (Fig. 4b). Interactions between pine and hickory fuels were not significant (Fig. 4b and Table S3). Durations of heating were positively related to temperature increases (< 0.001; Table S4). Both temperatures and durations of fire were lower when pine fuels were absent (Fig. 6a and Table S4).

Successive prescribed fires in the two different burn units produced some differences in fire characteristics at ground level. Maximum temperature increases were significantly lower (= 0.03) in the earlier fire (typically about 50 °C above ambient temperatures) than in the later fire (typically >100 °C above ambient temperatures). Duration of heating was not significantly different between the two fires. There also was a significant interaction between timing of fire and pine treatments (= 0.003). During the first fire, when fuels were moist with dew, the presence of pine litter elevated fire temperatures, but during the second fire when fuels were drier, temperatures were generally higher regardless of the fuel present beneath hickory stems (Fig. 5). There were no significant interactions between hickory fuels and fire unit or combined interactions between pine, hickory and fire unit (Table S2).

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Figure 5. Interaction of fire with pine fuel treatment. Fire unit 1 burned in the morning when fuels were moist with dew, and fire unit 2 burned in the afternoon when fuels were drier. Bars are back-transformed least-squares means ± standard error. Letters above bars indicate groupings based on the interaction between fire unit and pine treatment.

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All hickories survived fire, but treatments affected resprout location along the stem. When pine fuels were present, all hickories were top-killed to the ground and resprouted from below-ground buds. In contrast, when pine fuels were absent, all stems were partially killed, but 78.6% resprouted from above-ground buds on the stem. When only herbaceous fuels were present, 80% of hickories resprouted from above-ground buds, on average 10.0 ± 1.0 cm above the ground. Addition of hickory fuels had no effect on resprout location. Resprout location also varied between the two fires. Half of the hickories burned during the first fire, and 18.2% of hickories burned during the second fire resprouted from above-ground buds. When pine litter was absent, all hickories burned during the first fire, and 40% of hickories burned during the second fire, resprouted from above-ground buds along the original stem. Hickories that resprouted from above-ground buds tended to experience both lower temperature increases and shorter durations of heating during fires (Fig. 6a).

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Figure 6. (a) Relationship between durations of heating and temperature increases in plots burned during prescribed fires in May 2010. (b) Relationship between location of resprouts along stems (distance above the ground) and average maximum temperature increase in the plot. (c) Relationship between resprout height, measured 4 months after fire, and temperature increase during fire. In all graphs, circles represent plots where pine fuels were absent, and squares represent plots where pine fuels were present. Filled symbols indicate plots in which hickory stems survived and resprouted from above-ground buds instead of the below-ground root crown.

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Resprout locations along the stem were related to fire characteristics. As maximum temperature at ground level increased, the likelihood of resprouting from above-ground buds decreased, and locations of above-ground resprouts shifted closer to the ground (Fig. 6b). The largest increase that resulted in above-ground resprouting was 73 °C above ambient temperature (Fig. 6b). The relationship between temperature increase and resprout location was significant (< 0.001, R2 = 0.55; Table S5). Four months after fire, total heights of resprouting stems were still correlated with fire temperatures (= 0.001, R2 = 0.36). Hickories that had experienced lower temperature increases during fire were generally taller 4 months later (Fig. 6c). Total resprout height was also somewhat correlated with resprout location (= 0.02, R2 = 0.19; Table S5). Stems that resprouted from above-ground buds were generally taller 4 months after fire (Fig. S1).

Pine litter was important for the combustion of hickory litter, but hickory litter had no effects on the combustion of pine litter. On average, 68 ± 6% of hickory litter burned when no pine litter was present, but when pine litter was added, per cent combustion of hickory litter increased to 94 ± 7% (Fig. 7a); this difference was significant (F1,9 = 9.11, P = 0.01). In contrast, hickory litter had no effect on pine combustion (F2,5 = 1.03, P = 0.42). On average, 83 ± 5% of pine needle litter burned regardless of presence or amount of hickory litter (Fig. 7b).

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Figure 7. (a) Effect of pine litter on combustion of hickory leaves and (b) effect of hickory litter on combustion of pine needles. For both graphs, bars are least-squares means ± standard error, and letters represent groupings based on main effects of litter treatments.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Our study demonstrates that shed needles of longleaf pine have large effects on fire characteristics and thus on understorey trees in pine savannas. Flammable needles (sensu Mutch 1970; Fonda 2001) increase fire temperatures, durations of heating and combustion of fuels, as well as top-kill understorey hickories, even when fuels are moistened by dew as in the morning fire at the study site. Most juvenile life cycle stages of longleaf pine survive groundcover fires, and so shed pine needles are likely to have greater effects on understorey hardwoods than on understorey pines (Platt 1999). Dispersal patterns of shed longleaf pine needles can be expected to involve most needles falling beneath trees. Some needles, however, have been observed to fall outside the crowns, producing distributions of needles somewhat similar to those of longleaf pine seed dispersal (Boyer 1963; Grace, Hamrick & Platt 2004). Because flammable needles are shed over areas larger than the crowns of trees, even low densities of needles away from trees should facilitate fires that maintain space suitable for recruitment of pines away from established trees (Platt, Evans & Rathbun 1988). Such engineering of fire characteristics by overstorey longleaf pines should influence both stand dynamics and spatial distributions of pines and hickories in south-eastern pine savannas.

Altered fire characteristics also potentially can influence groundcover plants. Pine fuels in the vicinity of overstorey pines, often much greater than the 50 g we used in our experimental study and documented to reach fuel loads of 600–700 g m−2 after tropical storms (Ellair 2012), should facilitate frequent fires with considerably elevated temperatures at ground level. Pine needles also increase durations of heating during fires, contrary to predictions of the pyrogenicity as protection hypothesis (Gagnon et al. 2010). In the absence of longer-burning coarse woody fuels and extreme accumulations of litter (duff), maximum temperatures and durations of heating should be positively correlated when fires are fuelled by pine needles. Increases to maximum temperature tend to occur rapidly, and the amount of time required to cool depends on the maximum temperatures reached (Fig. 2). In addition, the structure of needles and the layering of pine fuels on the ground appear to increase total time for complete combustion. Needles recently shed and caught in groundcover vegetation and hence located above the ground tend to combust quickly. Needles on the ground beneath vegetation (often those shed in prior years), in contrast, often combust slowly from one end to the other after the flaming front has passed, increasing the duration of heating at ground level (but not above the ground where maximum heating would have occurred during passage of the flaming front). Longer durations of heating at the soil surface should increase soil heating, negatively affecting perennial plants in the groundcover, including grasses (Gagnon et al. 2012), forbs (Wiggers 2011) and small trees (Thaxton & Platt 2006). We propose that the engineering of ground-level fire characteristics by pines should have effects on groundcover that increase with the amount of pine fuels present. Accordingly, we expect local variation in longleaf pine fuels to result in spatial variation in composition of the groundcover. Plant species with life cycle stages sensitive to elevated and extended heating of the soil surface or upper soil layers should likely be located away from pines.

Production of flammable fuels potentially could lead to increased local biodiversity of groundcover plants. Characteristics of pine populations (sizes, density and productivity) should determine amounts and distributions of flammable components of local fuels (i.e. local fuel heterogeneity; cf. Hiers et al. 2009). As a result, variation in contributions to local fuels could result in substantial variation in local fire characteristics (i.e. pyrodiversity; Martin & Sapsis 1991; Faivre et al. 2011) in south-eastern savannas, especially old-growth stands with variable densities and size classes of overstorey pines (Platt, Evans & Rathbun 1988; Noel, Platt & Moser 1998). Such pyrodiversity should increase variation in local post-fire microenvironments (e.g. the extent to which fuels are consumed, extent vegetation is damaged and killed, and thus, the extent to which soil is exposed post-fire). Such variation in fuels, and hence pyrodiversity, should in turn affect local composition, physiognomy and dynamics of the groundcover. Pyro-engineering by trees that produce flammable fuels thus could increase biodiversity in frequently burned plant communities, as suggested for other ecosystems (Keeley 1990; Martin & Sapsis 1991; Faivre et al. 2011; but also see Parr & Andersen 2006). We echo this idea, proposing that pyrodiversity driven by trees producing flammable fuels might enhance local heterogeneity and thus biodiversity in frequently burned, species-rich savannas such as those we studied.

Hickories did not engineer fire characteristics, in contrast to predictions that hickory fuels may suppress fires. According to the pyrogenicity as protection hypothesis, flammability should not benefit plants subject to large inputs of fuels from other species. Fuels deposited by these plants contribute less to fuel loads and are less likely to modify fire characteristics (Gagnon et al. 2010). This may be the case for hickories in pine savannas; there tend to be much more pine fuels than hickory fuels beneath hickory stems in the vicinity of large pines. Furthermore, flammability of fuel mixtures may be determined by the most flammable component of the mixture (de Magalhães & Schwilk 2012). In the presence of flammable fuels, fire top-kills hickory stems; hickory leaves provide no protection, either by burning rapidly or by suppressing fire when any flammable pine fuels are present. Nonetheless, hickory stems located well away from pines are likely to resprout from above-ground buds, at least when fires occur frequently and fuel accumulations remain low. Such hickories should become taller more quickly and might eventually reach a size that they could withstand low-intensity fires.

Savanna hardwoods are able to survive fires by resprouting under many different fire regimes. Hardwood cover may be reduced, but not eliminated, under short fire return intervals in savannas (e.g. Sankaran et al. 2005; Smit et al. 2010; Werner & Franklin 2010). Even after 20 years of annual growing season fires, some hardwoods survive by resprouting repeatedly (Waldrop, White & Jones 1992), and we note similar results at Camp Whispering Pines after almost two decades of biennial prescribed fires. Increases in the magnitude of fire characteristics can increase hardwood top-kill (Trollope & Tainton 1986), but higher likelihoods of mortality tend to result primarily from fuels that produce the largest increases in fire temperatures and burn for long periods of time (Thaxton & Platt 2006). Fires early in the growing season when hardwoods are investing in above-ground growth also are expected to limit resprouting, but not remove hardwoods from the groundcover (Glitzenstein, Platt & Streng 1995; Olson & Platt 1995; Drewa, Platt & Moser 2002; Smit et al. 2010). Although fire characteristics may damage or limit responses, once established, savanna hardwoods have a high likelihood of persisting for long periods of time in an ‘oskar’ life cycle stage (sensu Horvitz et al. 1998; an understorey plant in a suppressed juvenile state) by resprouting repeatedly. Similar patterns have been noted in other savanna trees as well (e.g. ‘gullivers’ sensu Bond & Van Wilgen 1996; ‘grubs’ sensu Peterson & Reich 2001 and other trees caught in the ‘fire trap’ sensu Hoffmann et al. 2009; Grady & Hoffmann 2012). Such suppressed hardwoods may allocate more to root biomass, facilitating storage of photosynthates used in resprouting after shoots are killed by fire (Schutz, Bond & Cramer 2009; Tomlinson et al. 2012).

Resprouting life histories may result in persistence of hardwoods such as hickories in south-eastern savanna landscapes dominated by ecosystem-engineering pines. The dynamics of populations of resprouting hardwoods potentially could operate on time-scales longer than the multiple-century life spans of reseeding pines. Establishment, then subsequent growth and survival of hardwoods should occur primarily in transiently open patches well away from overstorey pines, where fire characteristics are less likely to be influenced by shed pine needles (Rebertus, Williamson & Moser 1989; Rebertus, Williamson & Platt 1993). Once established, hardwoods should survive as resprouting shrubs until any pines present in the vicinity have died and conditions become favourable for growth into the overstorey. Once hardwoods reach a size large enough to escape the ‘fire trap’ (sensu Grady & Hoffmann 2012), they may be able to reproduce and eventually become large trees, as might be expected in open sandhills where recruitment of pines is limited (e.g. Greenberg & Simons 1999). Large overstorey hardwoods such as hickories also may have survived on adjacent mesic slopes where fires were of lower intensity, provided flammable fuels from savanna pines were not present (Platt & Schwartz 1990; Harcombe et al. 1993). Similar patterns are expected for other resprouting savanna trees (e.g. Higgins et al. 2007) and forest trees that may invade savannas during fire-free intervals (e.g. Hoffmann et al. 2009).

Moreover, once isolated hardwoods reach overstorey size, they may influence their local environment within the savanna landscape. The spreading branches and persistent (in the fall) canopy leaves of overstorey trees such as hickory may greatly reduce the amount of pine needles falling underneath the canopy, protecting bases and branches of such large trees from fire. The ground may become shaded enough to prevent longleaf pine establishment and reduce growth of herbaceous fuels, enabling hardwood seedlings such as hickories to become established and reach a size sufficient to survive fire. As a result, hardwoods that reach the overstorey and suppress pine regeneration potentially might generate local patches of hardwoods (e.g. thickets of Harcombe et al. 1993; oak domes of Guerin 1993), increasing local overstorey and understorey diversity in savannas. Similar processes might operate in other savannas, especially along savanna–forest boundaries (e.g. Mitchard et al. 2009; Hoffmann et al. 2012).

In summary, our study suggests that flammable fuels produced by trees could engineer aspects of local savanna landscapes through effects on fire regimes. Local variation within and among successive fires resulting from variation in fuels that influence temperatures produced and durations of heating, especially at ground level, should drive overstorey landscape dynamics involving resprouting (e.g. hickories and other hardwoods) and reseeding (e.g. longleaf pine) savanna trees. Because flammable fuels are shed into the groundcover (i.e. that component of savanna vegetation that combusts during fires), the potential exists for such engineering of fire characteristics to have large effects on the plants and animals that inhabit this layer. We propose that the vagaries of flammable fuel production and dispersal from the overstorey should increase the heterogeneity of fire characteristics (pyrodiversity), thereby introducing post-fire variation in environmental conditions that potentially enhance biodiversity in fire-frequented savannas. Moreover, we conceptualize life histories of non-flammable hardwood tree species like hickories as enabling them not just to persist in the presence of flammable tree species like longleaf pine, but to respond when and where flammable species are not present, and perhaps even to increase locally at the expense of flammable tree species. Similar patterns might be expected in other fire-frequented savannas worldwide.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We thank the Girl Scouts of Eastern Louisiana for use of Camp Whispering Pines as a study site. Larry Erhlich and David Brown conducted prescribed fires. Portions of this study were supported through NSF Award 0950302 (WJP, PI). Mindy Brooks, Becky Carmichael, Wynston Cormier, Daniel Depaula, Paul Gagnon, Leigh Griffin, Ellen Leichty, Anna Meyer and Heather Passmore helped with the study in various ways. Rae Crandall, Thomas Dean, James Geaghan, Tracy Hmielowski, Demetra Kandalepas, Kevin Robertson, Matt Slocum, Richard Stevens and Yalma Vargas-Rodriguez provided useful comments on the study and manuscript.

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  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
jec12008-sup-0001-FigS1-TableS1-S5.pdfapplication/PDF99K

Fig. S1. Relationship between resprout height and resprout location.

Table S1. Summary of fire characteristics.

Table S2. Results of mixed model anova on temperature increase.

Table S3. Results of mixed model anova on duration of heating.

Table S4. Results of mixed model ancova of duration of heating.

Table S5. Summary statistics from ancova and regression analyses.

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