Fire Sensitivity of Casuarina torulosa in North Queensland, Australia

Fire Sensitivity of Casuarina torulosa in North Queensland, AustraliaAuthor(s): Martin KellmanSource: Biotropica, Vol. 18, No. 2 (Jun., 1986), pp. 107-110Published by: The Association for Tropical Biology and ConservationStable URL: http://www.jstor.org/stable/2388752 .

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Fire Sensitivity of Casuarina torulosa in North Queensland, Australia1

Martin Keliman Department of Geography, York University, Downsview, Ontario M3J 1 P3, Canada

ABSTRACT An examination of young Casuarina torulosa Ait. populations at two recently burned, wet sclerophyll woodland/savanna sites in North Queensland, Australia, showed that both consisted of a narrowly unimodal height-class structure containing a high proportion of resprouted stems. Stem mortality was virtually complete below modal scorch height in each stand, but decreased in stems above this height. A high proportion of plants with fire-killed stems had resprouted: those below 2 m in height did so by basal tillering, and those above this height did so epicormically. A growth chamber experiment showed little resprout capacity when seedling heights approximated 23 cm, but a much higher potential when heights approximated 42 cm. However, resprout capacity was rapidly lost after two subsequent burns at short intervals. These results suggest that this species maintains even-sized cohorts of young stems within sclerophyll woodland and savanna subject to moderate fire recurrence intervals by repeatedly resprouting basally. Occasional longer fire-free intervals permit passage of these cohorts beyond the fire-sensitive stage to form relatively dense stands of even-sized stems.

STUDIES OF THE POPULATION DYNAMICS of tree species have traditionally concentrated upon the role of density-depen- dent effects in population regulation (Harper 1977). While the role of density-independent effects in regulating the populations of short-lived plants occupying frequently disturbed habitats has received considerable attention, much less attention has been devoted to the response of long-lived woody plant populations to these effects, despite the increasing recognition that disturbance is a nor- mal component of woodland ecosystems (White 1979). Wildfire is one of the most widespread forms of natural disturbance to ecosystems (Kozlowski & Ahlgren 1974). Where average fire recurrence intervals are longer than the maturation period of a tree species, the species may successfully function as a post-disturbance opportunist. However, where fire occurs frequently, tree species must possess specialized adaptations to ensure both persistence of some members of the population through fires, and periodic recruitment to this fire insensitive group.

The sparsity of trees in frequently-burned environments in the tropics, and the expansion of their populations following fire cessation in these areas (Munro 1966, Rose Innes 1971), suggests that many tropical tree species do not possess these adaptations. However, exceptions can be found to this generalization, most notably in tropical Australia, where frequent fires are virtually universal (Gill et al. 1981) but where many taxa successfully maintain relatively high tree densities. Preliminary observations of Casuarina torulosa Ait. (Casuarinaceae) populations at recently burned sites along rainforest boundaries in north Queensland, suggested a stem size-specific fire sensitivity pattern that may provide some clue to the success of this tree species in this frequently burned environment. The

I Received 6 May 1984, revision accepted 29 November 1984.

data presented here comprise records of the sensitivity to fire of young C. torulosa subjected to two fire intensities in this area. These data are supplemented by a growth chamber experiment on the resprouting capacity of seedlings of this species.

METHODS Field observations took place at two sites in wet sclerophyll woodland/savanna adjacent to the rainforest in the Herberton Range west of Atherton, Queensland. Site 1 was located on a 35-percent southwest-facing slope at 1100 m elevation, and Site 2 was located on a 30-percent southeast-facing slope at 900 m elevation. Both sites had been prescription-burned for wildfire hazard reduction on Sept. 23, 1981, using air-dropped flares that produced multiple burned patches 50-100 m in diameter. Accord- ing to local Forestry personnel, Site 1 had been burned 2-3 yr previously, while Site 2 had not been burned for 5-6 years. Observations were made between five and six weeks after the burn.

At each site, the height of all Casuarina torulosa stems <2 cm DBH within one burned patch were measured and placed into one of three categories: alive; killed but resprouting; killed but not resprouting. At Site 2, the resprouting class was further subdivided into basal resprouting (below 20 cm on the stem) and epicormic resprouting (above 20 cm). The maximum scorch height was measured on all live stems whose lower foliage had been killed by the fire. The base of each plant was ex- cavated to determine whether the stem(s) was an original shoot, or basal resprout after a previous fire. Original fuel loads at the two sites were estimated by sampling grass and litter quantities in the area surrounding the burned patch with randomly-located 20 cm x 20 cm quadrats. Harvested fuels were oven-dried at 650C and weighed.

BIOTROPICA 18(2): 107-110 1986 107



SITE 1 SITE 2

400 1 Seed I i ngs Resprouts Seed I ings Resprouts

320 -

E 2401

I _ : :~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. : .:. : :.:: . :: ::.:. . . . . ...::: ..:. '::,:,:

:::::::::::4:::::.:::::::::::............. :: c . :: :::::-,.........:::::::::,

8- . s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 10 0 10 20 30 0 10 20 0 10 20 30 Individuals Individuals

FIGURE 1. Maximum stem height frequency distributions of all previously unburned Casuarina torulosa seedlings and resprouted genets at the two field sites. Only stems <2 cm DBH shown.

Seed collected from several Casuarina torulosaz trees in the Site 2 area was used in the growth chamber experiment. Ten seeds were sprinkled on sterile potting soil in 60 420 ml pots and mixed to a depth of 5 mm. Pots were placed in a growth chamber with 2 5/20?C day/ night temperatures and a light intensity of 6500 lux. After germination (at 9-10 days), seedlings were thinned to one per pot and the maximum height of seedlings measured at 2-week intervals. Seedlings were fertilized periodically with a modified Hoagland's solution (Epstein 1972). At six months seedlings were transferred tO a better illuminated growth chamber (20,000 lux).

A random sample of 24 seedlings were burned with a propane torch six months after planting when seedling height averaged 23.2 cm. Plant pots were covered with aluminum foil and shoots were heated until foliage ig- nited. Signs of resprouting and the maximum stem height of any successfully resprouting seedlings were recorded at two-week intervals thereafter. All surviving plants were repotted to larger 1575 -ml pots one year after planting. All remaining unburned seedlings, plus the one successful resprout from the first burn, were burned at 14 months when seedling height averaged 41.5 cm. Those successfully resprouting were burned again 90 days later and once again 70 days later. After each burn, resprouting sequence and height growth of the tallest shoot on each plant were monitored.

TABLE 1. Fuel loagds agt the two field sites (g/20-cm2 plot).

Site 1 Site 2

~~~~~~~~~. . . . . . . .

Mean 19.68 43.16 SD 7.52 9.76 N 30 27 t 10.22 (P < 0.001)

1 08 Kellman

960 -

880 -

800- SITE 1 SITE 2

720 -

640 -

560- E

480- .M

I 400-

320 - Live

240

~Dead 160 Lx 80 f-~ 0

0 10 200 20 40 60 0 10 0 20 40 60 Scorch Levels Stem No. Scorch Levels Stem No.

FIGURE 2. Frequency distributions of scorch heights and heights of all live and killed Casuarina torulosa stems (<2 cm DBH) at the two field sites.

RESULTS In the field, a high proportion of young C. torulosa stems proved to be resprouts: 92 percent and 42 percent of all genets enumerated at Sites 1 and 2, respectively. The maximum stem height frequency distribution of resprouting genets and true seedlings are compared in Figure 1. While having similar modal heights, the seedling populations tended to have a large proportion of stems shorter than the mode, whereas a large proportion of resprouts were larger than the mode.

Fuel loads at the two sites were significantly different (Table 1), indicating that Site 2 had experienced a much hotter fire. This was confirmed by the scorch height data (Fig. 2) which showed a single height mode at 160-200 cm at Site 1, but two modes at 240-280 cm and 560- 600 cm, with scattered scorch to 900 cm, at Site 2. Survival of stems at the two sites was closely associated with scorch heights. At Site 1, few stems shorter than the modal scorch height survived the fire, but the proportion of stems surviving above this level was positively corre- lated with scorch height frequency (Fig. 2). At Site 2, where most stems were below the lowest scorch height mode, few stems survived (Fig. 2).

A high proportion of the stems killed had resprouted basally or epicormically (Fig. 3). Basal sprouting was concentrated around the swollen root crown that is characteristic of this species. Those stems not resprouting were distributed throughout all height classes. The data from Site 2 indicated that stems taller than 200 cm tended to



SITE 1 SITE 2 400

320 -

Not esprouting Epicormic sprouts E 240 - 1 .3::::: 240 Resprouting Basal sprouts

16 0 .... ........ 1:::::::::::::::::::

: : ~~~~~~~~. ::. . . . . . . . . . : : : : : : :3.. . . . . . . . . . . . . .::::::::::: : :: :-:-::::-:::-:--:-:::::::::-:::::,............ . ........ . .

,,. . . . . . . . . . . . . . . . . . , ,. . . . . . . . . . . . . ....., 80...

............ ::1S 1:: Not resprouting

0 0 10 20 30 40 50 60 0 10 20 30 40 50

Stem No. Stem No.

FIGURE 3. Fate of all fire-killed Casuarina torulosa stems (<2 cm DBH) at the two field sites, classified by maximum stem height.

resprout epicormically rather than basally, despite the hot fire at this site (Fig. 3). Consequently, once stems achieve this height, they tend to persist through fires instead of being "recycled" to new basal sprouts by the burn. At both sites, basal resprouts tended to form a narrowly unimodal height class. Furthermore, despite the difference in age of the resprouts at the two sites, heights were very similar.

The first fire treatment in the growth chamber experiment, when seedling height averaged 23.2 cm, resulted in only one of 24 seedlings resprouting successfully. Several other seedlings resprouted temporarily, but shoots stagnated, then died when only several millimeters in length. By the second burn, when seedling height had almost doubled, 17 of 22 seedlings resprouted successfully. However, mean preburn seedling heights of the successful and unsuccessful resprouters were not significantly different (Table 3). The subsequent burn, after 90 days, resulted in many fewer successful resprouts (5 of 17), with the difference between preburn stem heights of successful and unsuccessful resprouters again not signifi- cant (Table 3). No plant successfully resprouted after the final burn when mean stem height was only 20.9 cm.

DISCUSSION Fire sensitivity of young C. torulosa stems appears to be closely linked to major differences (Fig. 2, Table 2), al-

TABLE 3. Comparison of shoot heights of successful and unsuccessful resprouters in the growth chamber experiment. (x ? SD, cm; comparison by 2-tailed t-test).

Time of Not burn (days) Resprouting resprouting t

178 27.0 (N= 1) 23.1 ? 2.9 427 41.6 ? 5.3 42.8 ? 5.2 0.44ns 517 34.4 ? 5.5 34.1 ? 8.2 0.08ns 587 20.9 ? 4.3

though not to minor variations (Table 3), in stem height. Stem height probably provides only an indirect measure- ment of two more specific properties determining resprout potential, carbohydrate reserves and bark thickness. Re- search on a variety of woody plants capable of resprouting after stem destruction has implicated carbohydrate reserves as the most likely resource contributing to resprout potential (e.g., Woods et al. 1959, Jones & Laude 1960, Bamber & Humphreys 1965). The inability of very young or frequently burned C. torulosa seedlings to resprout successfully in the growth chamber experiment is com- patible with this hypothesis. Furthermore, the larger proportion of plants failing to resprout at Site 1, which experienced a more recent burn than Site 2 (Fig. 3), may also reflect insufficient time for carbohydrate reserve accumulation. The requirement for a stem height of ca. 200 cm before epicormic sprouting can take place may also be linked to the accumulation of adequate carbohydrate reserves in stem tissue. However, a more plausible explanation is the likelihood of cambial destruction on very small stems with an insufficiently thick bark.

The species does not appear capable of withstanding the frequent multiple defoliations reported for some Eu- calyptus species by Chattaway (1958). However, the very frequent defoliations of small Eucalyptus shoots used by that author may be less exhaustive of stored reserves than a longer cycle of defoliation, and so give an unrealistic impression of successful resprouting capacity. The simi- larity in resprout shoot heights at the two sites in the present study, despite their differing age, suggests that rapid shoot growth takes place after burning, presumably deriving from stored carbohydrate reserves, followed by

TABLE 2. Summary of Casuarina torulosa seedling height and resprout frequency after burning in the growth chamber experiment.

Time from seed planting (days)

178 427 517 587

Seedlings burned 24 22 (+la) 15 (+la) 5 Seedling height (x ? SD, cm) 23.2 ? 2.9 41.5 ? 5.7 (40.1a) 33.6 ? 7.1 (+40.Oa) 20.9 ? 4.7 Successful resprouts 1 17 5

a Single resprout from 178 day burn.

Fire Sensitivity in Casuarina torulosa 109



much slower development, during which assimilates are presumably allocated to the root reservoir at the expense of further rapid shoot growth. As a consequence of this growth pattern, most resprouting shoots remain suscep- tible to fire for many years, but are capable of a rapid switch to fire insensitivity once their modal height sur- passes the minimum height for successful epicormic sprouting.

The resprout capacity of young C. torulosa imparts a flexible demographic potential to the species, fitting it especially well to a range of intermediate and long fire recurrence intervals that are probably characteristic of wet sclerophyll woodland. In the absence of fire, the species is apparently incapable of competing with, or establishing among, the rainforest taxa that normally invade moister sites. This presumably accounts for its rapid decline in pollen records concurrent with the appearance of rainforest pollen (Kershaw 1978). In contrast, under a regime of very frequent fires, population extinction is likely, as young plants suffer some mortality with each burn, and senesence of seed-producing stems eventually precludes new recruits. At fire frequencies between these two ex- tremes, the species is probably capable of indefinite persistence by virtue of its ability to rapidly take advantage

of occasionally longer fire-free periods. At such times, the cohort of even-sized resprout stems that is maintained within the stand can traverse the fire-sensitive stage en masse to form a new population of seed trees and so maintain high-density, if patchily distributed, stands of this species. The species thus successfully combines the multiple resprout capacity of a savanna shrub (Miyanishi 1984) with the fire resistance of a savanna tree. In so doing, it effectively broadens the range of fire frequencies under which it can survive, provided that such frequencies contain some stochastic variability. The success of many other Australian woody taxa (notably Eucalyptus) at com- bining these two tactics may help explain the prevalence of sclerophyll woodlands, rather than open savanna, in much of tropical Australia.

ACKNOWLEDGMENTS Field observations were made while the author was a Guggen- heim Fellow and Visiting Fellow at the Australian National University. The hospitality of Alan and Susan House while in Atherton, and the laboratory assistance of Trudy Stornebrink at York University are gratefully acknowledged. I have benefited from many discussions with Kiyoko Miyanishi about the resprouting of woody plants in burned environments.

110 Kellman

LITERATURE CITED BAMBER, R. K., AND F. R. HUMPHREYS. 1965. Variations in sapwood starch levels in some Australian forest species. Aust. For.

29: 15-23. CHATTAWAY, M. M. 1958. The regenerative powers of certain Eucalypts. Victorian Nat. 75: 45-46. EPSTEIN, E. 1972. Mineral nutrition of plants: principles and perspectives. Wiley, New York. GILL, A. M., R. H. GROVES, AND I. R. NOBLE (EDS.). 1981. Fire and the Australian biota. Australian Academy of Science,

Canberra. HARPER, J. L. 1977. Population biology of plants. Academic Press, London. JONES, M. B., AND H. M. LAUDE. 1960. Relationships between sprouting in chamise and the physiological condition of the

plant. J. Range Mgmt. 13: 210-214. KERSHAW, A. P. 1978. Record of last interglacial-glacial cycle from northeastern Queensland. Nature, Lond. 272: 159-161. KOZLOWSKI, T. T., AND C. E. AHLGREN (EDS.). 1974. Fire and ecosystems. Academic Press, New York. MIYANISHI, K. 1984. The effects of prescribed burning on the population dynamics of Miconia albicans and Clidemia sericea.

Ph.D. Thesis, York University, Toronto. MUNRO, N. 1966. The fire ecology of Caribbean pine in Nicaragua. Proceedings of the fifth annual tall timbers fire ecology

conference, pp. 67-83. Tallahassee, Florida. ROSE INNES, R. 1971. Fire in west African vegetation. Proceedings of the eleventh annual tall timbers fire ecology conference,

pp. 147-173. Tallahassee, Florida. WHITE, P. S. 1979. Pattern, process, and natural disturbance in vegetation. Bot. Rev. 45: 229-299. WOODS, F. W., H. C. HARRIS, AND R. E. CALDWELL. 1959. Monthly variations of carbohydrates and nitrogen in roots of sandhill

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Fire Sensitivity of Casuarina torulosa in North Queensland, Australia