Soil temperatures recorded with thermocouples and temperature-sensitive paints were quantified during Florida sand pine scrub prescribed fires at Ocala National Forest and Archbold Biological Station in May 1993. Thermocouples and glass petri dishes painted with temperature-sensitive paints and containing seeds of
Seeds that survive fires in the soil contribute to postfire recruitment to varying degrees among fire-influenced communities [
A large volume of field research and laboratory research has been conducted on temperatures reached during natural fires [
Researchers generally have used either temperature-sensitive paints or thermocouples to quantify soil temperatures reached during fires. Although recorded temperatures have varied among ecosystems and with differences in burning conditions, several patterns have been consistent across studies. Maximum soil temperatures generally decrease as depth in the soil increases, but durations of elevated temperatures increase [
In laboratory germination studies, plant species from several ecosystems have demonstrated positive germination responses after being exposed to temperatures typically reached during fires. Taxa or functional groups with this response include legumes in the southeastern United States, California, and Australia [
This study documents soil temperatures during fires, and seed germination responses to elevated temperatures in sand pine scrub, a community occurring primarily in Florida on well-drained sandy soils of former Miocene to Pleistocene dunes. Sand pine scrub is dominated by woody shrubs including several oak species (
Natural fires are lightning-initiated, and usually occur during the wet growing season during the months of April through July. Although frequency of lightning strikes is extremely high in Florida [
As in other fire-influenced communities, plant species in sand pine scrub vary in their responses to fire. Most woody shrub species resprout and do not establish seedlings immediately following fires. In contrast, seedling establishment occurs primarily or exclusively after fire for some plant species that are killed by fire (e.g.,
Although heat- and smoke-induced germination have not been documented in sand pine scrub as in some other fire-influenced communities [
Temperatures reached during fires and germination responses to those temperatures are relatively unknown for sand pine scrub. Only one other study has measured temperatures at the soil surface during a sand pine scrub fire [
The first of two objectives of this study was to measure soil temperatures at different depths and in different microsites during fires in sand pine scrub. Lower peak soil temperatures and longer temperature durations were expected with increasing depth in the soil, as documented in other studies. Spatial patterns of combustible fuel in sand pine scrub are defined primarily by a widespread matrix of woody shrub vegetation underlain by moderate amounts of ground litter, interspersed with canopy openings with little or no litter. In this study, soil temperatures in microsites under vegetation (e.g., shrubs) were expected to be higher than those in openings.
The second objective of this study was to quantify germination responses of two small-statured perennial species after exposing the seeds to temperatures typically reached during sand pine scrub fires. Small-statured perennial species were chosen for study because much of the postfire seedling establishment response in sand pine scrub is attributable to species from this general group [
Study areas were sand pine scrub sites at Ocala National Forest in Marion County, Florida (
Controlled burns were conducted at the Ocala and Archbold sites on May 10, 1993 and May 25, 1993, respectively. Before each fire, a datalogger (21X Micrologger, Campbell Scientific, Inc.) connected to 16 copper-constantan, 0.51 mm diameter thermocouples via insulated leads was buried in the site to be burned. The thermocouple leads were 1.5–4 m long, and were buried approximately 2 cm deep; the datalogger was placed in a fiberglass case and buried approximately 50 cm deep.
Each thermocouple was placed either at the soil surface or at 2 cm depth, and either in an open microsite or under vegetation. The 2 cm depth was chosen because seeds typically are found at this depth [
At Archbold the sides of each petri dish were painted with 19 temperature-sensitive paints (Tempilaq, Tempil Division, Big Three Industries, Inc.). Manufacturer-specified melting temperatures of paints ranged from
During each fire, the datalogger recorded thermocouple temperatures every five seconds. Temperature recording occurred from 90 minutes before the fire until 90 minutes after the flame front went through at the Ocala site, and from 90 minutes before the fire to 60 minutes after the flame front went through at Archbold.
After each fire, the datalogger and all petri dishes were recovered. Intact seeds were extracted from soil in the petri dishes, and were classified as charred or noncharred. All noncharred seeds were placed on moist filter paper in clean petri dishes, and observed for germination for one month. Viability of non-germinated seeds was assayed with a tetrazolium test [
Data on thermocouple maximum temperatures and temperature durations were analyzed separately for the Archbold and Ocala fires. Analyses of variance were conducted on rank-transformed maximum thermocouple temperatures to detect differences due to depth in the soil and microsite [
Maximum temperatures during the Archbold fire of petri dishes placed at the soil surface and at 2 cm depth were compared using a chi-square test, and a
Seeds of
Experiments were conducted from December 8–17, 1993, and from December 14, 1995 to January 7, 1996 for
To determine times required to melt temperature-sensitive paints on petri dishes, trials were conducted in which a petri dish painted with a temperature-sensitive paint was placed in a muffle furnace (Thermolyne, Sybron Corporation) with the door open, and timed until the paint melted. A minimum of five trials was conducted for each of 13 paints, with manufacturer-specified melting temperatures ranging from
Weather conditions and fire characteristics were similar for the two fires, but flame lengths in the Archbold fire were greater than those in the Ocala fire (Table
Weather conditions and fire characteristics for Florida sand pine scrub fires at Ocala National Forest and Archbold Biological Station during May 1993.
Ocala | Archbold | |
---|---|---|
Date of fire | May 10, 1993 | May 25, 1993 |
Time of measurements | 1150–1205 | 1259–1305 |
Air temperature | 28 | 29 |
Relative humidity | 46% | 52% |
Wind speed | 0.89 m/s | 0.54 m/s |
Wind direction | SE | SE |
Flame length | 3–5 m | 6–15 m |
Rate of spread | 12 m/min | 12 m/min |
ANOVA of the effects of soil depth and microsite on maximum temperatures recorded during a controlled burn in sand pine scrub at Ocala National Forest, Florida. Temperatures were recorded with copper-constantan thermocouples placed at the soil surface and at 2 cm depth, and in open and vegetated microsites. Data were rank-transformed prior to the ANOVA.
Source | SS | |||
---|---|---|---|---|
Depth | 182.3 | 1 | 19.5 | |
Microsite | 25.0 | 1 | 2.7 | |
Depth | 20.3 | 1 | 2.2 | |
Error | 112.5 | 12 |
ANOVA of the effects of soil depth and microsite on maximum temperatures recorded during a controlled burn in sand pine scrub at Archbold Biological Station, Florida. Temperatures were recorded with copper-constantan thermocouples placed at the soil surface and at 2 cm depth, and in open and vegetated microsites. Data were rank-transformed prior to the ANOVA.
Source | SS | |||
---|---|---|---|---|
Depth | 121.0 | 1 | 11.2 | |
Microsite | 64.0 | 1 | 5.9 | |
Depth | 25.0 | 1 | 2.3 | |
Error | 130.0 | 12 |
Temperature durations of copper-constantan thermocouples placed at the soil surface and at 2 cm depth in the soil. Temperatures were quantified during prescribed fires conducted during May 1993 at Ocala National Forest and Archbold Biological Station, Florida. Temperature duration was defined as length of time that (0.75
Site | Depth in soil (cm) | Temperature duration (s) (median (range)) |
---|---|---|
Archbold | 0 | 105 (20– |
2 | 2020 (160– | |
Ocala | 0 | 80 (65–1555) |
2 | 2918 (675– |
Soil temperatures at the soil surface during a prescribed sand pine scrub fire at Ocala National Forest, Florida on May 10, 1993. Temperatures were recorded with copper-constantan thermocouples placed in vegetated (a) and open (b) microsites. Each microsite location was replicated four times.
Soil temperatures at 2 cm depth during a prescribed sand pine scrub fire at Ocala National Forest, Florida on May 10, 1993. Temperatures were recorded with copper-constantan thermocouples placed in vegetated (a) and open (b) microsites. Each microsite location was replicated four times.
Soil temperatures at the soil surface during a prescribed sand pine scrub fire at Archbold Biological Station, Florida on May 25, 1993. Temperatures were recorded with copper-constantan thermocouples placed in vegetated (a) and open (b) microsites. Each microsite location was replicated four times.
Soil temperatures at 2 cm depth during a prescribed sand pine scrub fire at Archbold Biological Station, Florida on May 25, 1993. Temperatures were recorded with copper-constantan thermocouples placed in vegetated (a) and open (b) microsites. Each microsite location was replicated four times.
Temperatures in vegetated versus open microsites differed from predicted patterns, however. Although temperatures during the Ocala fire tended to be higher in vegetated than in open microsites, a large range of temperatures within microsites resulted in no statistically significant difference in temperatures between microsites (Table
As with thermocouple-quantified temperatures, patterns of melted paints on petri dishes at the Archbold site showed that maximum soil temperatures were higher at the surface than at 2 cm depth (
Maximum temperatures at 0 cm and 2 cm soil depth, and in open and vegetated microsites, during a prescribed fire in sand pine scrub at Archbold Biological Station, Florida. Temperatures were quantified with copper-constantan thermocouples and with temperature-sensitive paints placed on adjacent glass petri dishes.
Depth | Microsite | Maximum Temperature | |
Thermocouple | Paint(a) | ||
0 cm | open | 621.7 | 163.7 |
0 cm | open | 436.2 | 107.2 |
0 cm | open | 383.5 | 289.8 |
0 cm | open | 367.8 | |
0 cm | vegetated | 308.1 | 260.0 |
0 cm | vegetated | 59.8 | 287.8 |
0 cm | vegetated | 319.8 | 232.2 |
0 cm | vegetated | 56.9 | 162.8 |
2 cm | open | 63.0 | |
2 cm | open | 90.4 | |
2 cm | open | 52.0 | |
2 cm | open | 141.7 | |
2 cm | vegetated | 218.3 | |
2 cm | vegetated | 63.0 | |
2 cm | vegetated | 45.6 | |
2 cm | vegetated | 43.6 |
Frequency distributions of maximum petri dish temperatures during a prescribed sand pine scrub fire at Archbold Biological Station, Florida on May 25, 1993. Temperatures were quantified with temperature-sensitive paints on the petri dishes; temperature intervals for the frequency distributions represent melting points of the paints. Petri dishes were placed at the soil surface in vegetated (a) and open (b) microsites, and at 2 cm depth in vegetated (c) and open (d) microsites; each depth-microsite combination was replicated 12 times.
Melting times (mean ± S.E.) of temperature-sensitive paints (Tempilaq, Tempil Division, Big Three Industries, Inc.) on glass petri dishes. The number above each symbol represents the number of melting trials conducted for that paint. In each trial a petri dish was placed in a muffle furnace set at the melting temperature of the paint on the dish, and melting time was quantified.
Most non-charred
Number of petri dishes from which charred
Soil depth/microsite | Petri dishes with charred seeds | Petri dishes without charred seeds(a) | Total non-charred seeds |
---|---|---|---|
0 cm/open | 17 | 0 | 0 |
0 cm/vegetated | 16 | 1 | 10 |
2 cm/open | 3 | 14 | 154 |
2 cm/vegetated | 3 | 14 | 156 |
No seeds germinated during trials using non-charred seeds recovered after the fires. Tetrazolium tests on seeds that were collected at the same time as seeds used in the experimental fires indicated that viability was
In both
Number of petri dishes from which
Seed species | Temperature ( | Number of petri dishes(a) |
---|---|---|
25 | 4 | |
100 | 0(b) | |
25 | 4 | |
100 | 0(b) |
Number (mean ± S.E.) of
Seed species | Temperature ( | Germinated seeds (mean |
---|---|---|
25 | ||
60 | ||
25 | ||
60 |
Depth in the soil has a strong influence on magnitude and variability of temperatures during Florida sand pine scrub fires, and thus influences seed survival during fires. As expected, maximum temperatures at the soil surface were higher and peak temperature durations were shorter than at 2 cm depth. Less predictable, however, were (1) effects of microsites at the soil surface on maximum temperatures; and (2) detailed spatial patterns of temperatures at the soil surface, and their implications for seed survival.
The effect of microsites on soil temperatures was different from that predicted, and was different between the two fires. Differences in overstory pine species and resulting litter characteristics between the sites contributed to the different microsite effects. Sand pine, the only tree species at Ocala, produces sparse litter consisting of short needles, but south Florida slash pine has much longer and more flammable needles than sand pine, and produces more litter. Because south Florida slash pine was present only at Archbold, different types and amounts of pine litter had accumulated in open microsites at the two sites; litter in vegetated microsites consisted primarily of sclerophyllous leaves with comparatively low flammability.
During the Ocala fire, a high variance in temperatures within microsites precluded detection of a microsite effect. During the Archbold fire a microsite effect was detected, but the pattern of temperatures between open and vegetated microsites was the reverse of the expected pattern. Higher maximum temperatures in open microsites were probably due to the well-aerated, relatively flammable slash pine litter in open microsites burning more completely and at higher temperatures than the more compacted, less flammable oak and hickory litter in vegetated microsites [
Not only was the effect of microsites on soil temperature unpredictable, but temperatures at the soil surface in general were variable when compared to temperatures at 2 cm depth. Soil surface temperatures, in addition to having greater variation among experimental locations than at 2 cm depth, varied widely both in time and in space within experimental locations, as demonstrated by temperatures recorded with thermocouples and with temperature-sensitive paints. Temperatures recorded by thermocouples at the soil surface generally increased to extremely high, brief maxima, thereby producing many within-location combinations of temperatures and temperature durations. As a consequence of this high variation in temperatures over time, temperature-sensitive paints on petri dishes usually underestimated maximum temperatures of adjacent thermocouples at the soil surface. The maximum temperatures recorded by temperature-sensitive paints apparently were the highest temperatures that persisted long enough to melt the corresponding paints. As might be predicted by the variation in patterns of thermocouple temperatures versus time, differences between maximum temperatures of petri dishes and adjacent thermocouples varied widely.
Differences between petri dish and adjacent thermocouple temperatures at Archbold also indicated that soil temperatures varied spatially within experimental locations. Because maximum temperatures recorded by temperature-sensitive paints often differed from those expected based on temperature patterns of adjacent thermocouples (Table
Because virtually all maximum temperatures at the soil surface were
Seed survival is more likely and more predictable at 2 cm depth in the soil, because soil temperatures during fires were relatively low, and varied little from site to site. Although higher proportions of seeds should survive through fire at this depth in the soil, some seed mortality must occur, because lethal temperatures were occasionally reached. Surprisingly, in this study all
For species that rely on recruitment from seeds that survive through fire, a tradeoff exists between higher probability of survival and increasing difficulty of seedling emergence with increasing depth in the soil. Seedlings from larger seeds should emerge from 2 cm depth with little difficulty, as has been demonstrated for several legume species with seed masses near 30 mg [
The extremely limited capacities for
This study not only showed that patterns of temperatures differ between the soil surface and 2 cm depth, but also demonstrated differences in relative strengths and weaknesses of two temperature quantification methods. Temperature-sensitive paints on petri dishes were much less precise than thermocouples in quantifying maximum temperatures; therefore, results from temperature-sensitive paints were reliable only at 2 cm depth, where temperature durations were long. Nevertheless, temperature-sensitive paints are adequate for documenting large differences in temperatures between depths in the soil, and for recording maximum temperatures beneath the soil surface, where relatively large proportions of seeds should survive fires.
In addition to maximum temperatures, temperature duration influences seed survival through fires. Since (1) variation in temperature duration and spatial variation in maximum temperatures at the soil surface are potentially relevant to seed survival and (2) thermocouples can quantify temperature duration and complex spatial patterns in temperatures, thermocouples should be used in studies of temperatures relevant to seed survival during fires to quantify temperature patterns at the soil surface.
Results from this study suggest that small-scale spatial variation in temperature exists at the soil surface during fires, and that low-temperature patches are essential for seed survival on the soil surface. Additional studies are needed to document small-scale patterns of temperatures, temperature durations, and seed survival at the soil surface during fires. Seed survival through fire and germination after fire, however, are only a small part of the process of postfire recruitment. Future research on topics such as effects of postfire microsite conditions on seedling establishment, and the relevance of postfire microsite conditions to the evolution of seed germination characteristics, will contribute to a comprehensive characterization of postfire recruitment.