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Collingwood. Telephone: 61 3 9662 ... Paul R. Williams. Queensland Parks and Wildlife Service, PO Box 5597, Townsville, Qld 4810, Australia; email: Paul.
C S I R O

P U B L I S H I N G

Australian Journal

of

Botany Volume 48, 2000 © CSIRO 2000

An international journal for the publication of original research in plant science

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Published by CSIRO PUBLISHING for CSIRO and the Australian Academy of Science

Aust. J. Bot., 2000, 48, 651–658

Fire-stimulated rainforest seedling recruitment and vegetative regeneration in a densely grassed wet sclerophyll forest of north-eastern Australia Paul R. Williams Queensland Parks and Wildlife Service, PO Box 5597, Townsville, Qld 4810, Australia; email: [email protected]

Abstract. Details of plant regeneration, combined with soil seedbank data, are documented for a densely grassed wet sclerophyll forest in north-eastern Australia. The following two hypotheses were tested: (1) that established individuals of rainforest pioneer species are killed by low to moderate intensity fires and (2) that seedling recruitment of these species is evenly distributed throughout the intervals between fires. Both the sclerophyll and rainforest pioneer species displayed strong regenerative abilities as a response to low or moderate intensity fires. Most of the rainforest pioneer species were not killed by two recent fires but vegetatively regenerated. Alstonia muelleriana showed fire-enhanced vegetative expansion via root suckering. Both the sclerophyll and rainforest pioneer species were found to recruit seedlings primarily as a pulse in the first year or two after a fire, with limited recruitment after longer intervals between fires. This is consistent with suggestions that grass competition may limit tree recruitment. The germinable soil seedbank was dominated by rainforest pioneers, herbs and grasses, with heat treatment of the seedbank enhancing seed germination of two rainforest pioneer species. These results demonstrate the ability of rainforest pioneers to exploit the post-fire environment and indicate the complex nature of rainforest boundary dynamics. Further research into tropical rainforest expansion is required to examine the effects of fire regimes on vegetative and seedling regeneration across a range of sites.

Introduction Recent rainforest expansion into eucalypt-dominated wet sclerophyll forests in north-eastern Australia has been described (Unwin 1983; Unwin et al.1985; Ash 1988; Stanton 1989) and mapped (Harrington and Sanderson 1994). Harrington and Sanderson (1994) documented a rapid rate of rainforest expansion into these wet sclerophyll forests, with more than half of the area containing a continuous grass understorey in the 1940s having been invaded by rainforest species within the last 50 years. Charcoal and pollen evidence indicates that rainforest expansion has been occurring in this area over the last 9000 years (e.g. Kershaw et al. 1991 and Hopkins et al. 1993). Rainforest flora are routinely clumped together under the pyrophobic or fire sensitive labels (e.g. Ash 1988; Adam 1994). Fire has been shown to be a major factor determining rainforest–eucalypt forest boundaries at several sites in the Wet Tropics World Heritage Area of north-eastern Australia (described throughout this paper as simply the ‘Wet Tropics’) with regular fires inhibiting the encroachment of rainforest species into wet sclerophyll forests (Unwin 1983; Unwin et al. 1985; Ash 1988). Yet the impacts of fire on rainforest flora are complex, as many Australian tropical and subtropical © CSIRO 2000

rainforest species are known to coppice after disturbances such as cyclones, logging and fire. This has been documented repeatedly over the last two decades (Stocker 1981; Unwin 1983; Hopkins and Graham 1984; Unwin et al. 1985; Russell-Smith and Dunlop 1987; Olsen and Lamb 1988; Unwin et al.1988; Floyd 1990; Bowman 1991; Skull 1992; Fordyce et al. 1997; Russell-Smith et al. 1998; Williams 1998; Bowman 2000). The processes driving rainforest expansion in the Wet Tropics require scrutiny. The location of rainforest boundaries is affected by complex interactions between rainfall, substrate, topography and fire (Ash 1988). The current expansion of rainforest has been attributed to increased rainfall and changes in fire regime towards lower intensity and early dry season fires, often in association with cattle grazing (Ash 1988; Harrington and Sanderson 1994). Webb and Tracy (1981) noted that reduced fire frequencies were often associated with the expansion of rainforest in north Queensland. Hopkins et al. (1993) illustrated these observations through a model where rainforest established and expanded into eucalypt forests in the absence of fire. In contrast to the post-fire pulse of recruitment often reported for sclerophyll species (e.g. Purdie and Slatyer 1976; Bell et al. 1993; Benwell 1998), 10.1071/BT99020

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the recruitment of rainforest species occurs continuously throughout the interval between fires in this model. A subsequent fire would thin out the rainforest species. Although rainforest establishment throughout fire intervals is widely observed (e.g. Ash 1988), the competitive effect of grass cover has been proposed as a factor inhibiting tree seedling recruitment in the Wet Tropics (Hopkins and Graham 1984; Unwin et al. 1985). Enhanced germination of rainforest plants has been documented in post-fire environments. Unwin (1983) reported that experimentally sown seeds of Toona ciliata germinated better in the immediate post-fire wet sclerophyll forest environment than in adjacent unburnt wet sclerophyll forest and rainforest on the Atherton Tableland. Hill and Read (1984) found that seed germination, especially of Nothofagus cunninghamii, Acacia melanoxylon and Atherosperma moschatum, was much higher in recently burnt than in unburnt areas in a Tasmanian wet sclerophyll forest and associated rainforest. The availability and composition of seed reserves are important factors determining the composition of regrowth and rainforest succession in the lowlands of the Wet Tropics (Hopkins and Graham 1984). The seedbank of several rainforest pioneer species have been shown to tolerate or be enhanced by heating (Floyd 1976; Hopkins and Graham 1984; Skull 1992). However, Hopkins and Graham (1984) found the heating of soil seedbank caused seed mortality in many species and suggested that regular burning may restrict the accumulation of rainforest seed reserves in disturbed sites, thus limiting rainforest succession. The dynamics of the wet sclerophyll forests, especially in relation to rainforest expansion, has been identified as an important management issue within the Wet Tropics, including Lumholtz National Park located in the southern half of the

Wet Tropics (Smith 1993; Green 1998). To address this issue, research was undertaken to determine the effect of regular fires within a wet sclerophyll forest. The following two hypotheses were examined at a site with dense grass cover: (1) that the established individuals of rainforest pioneer species in wet sclerophyll forest are killed by low to moderate intensity fires and (2) that seedling recruitment of these species is evenly distributed throughout the intervals between fires. Alternatively seedling recruitment may be concentrated either immediately after fire or later in the fire-free interval. Methods Study site Lumholtz National Park is located north and west of Ingham, in the southern half of the Wet Tropics. It conserves an extremely diverse transect, from coastal mangroves and lowland woodlands, to rocky outcrop heath, rainforest, dry and wet sclerophyll forests on the Great Dividing Range, to dry sclerophyll forest and woodlands west of the range (Cumming and Thomas 1993). The study site is located in the Wallaman Falls section of Lumholtz National Park (18°36⬘S, 145°48⬘E), about 50 km west of Ingham, at 550-m altitude. Wallaman Falls contains soil derived from a granitic parent material and receives an average of about 2000 mm rainfall per annum. The site contains a Eucalyptus intermedia-dominated grassy forest with E. tereticornis, Allocasuarina torulosa and a mid-strata of rainforest trees such as Alstonia muelleriana and Alphitonia petrei. The area was mapped as mixed eucalypt species wet sclerophyll forest with a continuous grass layer or with a young rainforest understorey, by Harrington and Sanderson (1994). An important point regarding the study site is that it is a flat plateau containing a dense grass cover dominated by Themeda triandra and Imperata cylindrica. The rainforest surrounding this wet sclerophyll forest was described by Tracey (1982) as simple notophyll vine forest. The recent fire history of Wallaman Falls includes an almost annual burning regime for each fire management block, around August to September, from the late 1970s to the late 1980s when intervals between fires increased (Table 1).

Table 1. Fire history of Wallaman Falls fire blocks and location and recording times of permanent sites Fire historyA (dates of recent fires)

Sites present within fire block

1

1994, Nov. 1996

Site 1

2

1994, Nov. 1996

Site 2

3

1994, Dec. 1997

Site 3

Fire block

Site 4 4 5

A

1992, Dec. 1997 Last burnt in the late 1980s (exact date unknown)

Site 5 No site established (seedling recruitment survey only)

Data recording dates Oct. 1996, Apr. 1997, Oct. 1997, Apr. 1998, Oct. 1998 Apr. 1997, Oct. 1997, Apr. 1997, Oct. 1997 Apr. 1998, Oct. 1998 Oct. 1996, Apr. 1997, Oct. 1997, Apr. 1998, Oct. 1998 Apr. 1997, Oct. 1997, Apr. 1998, Oct. 1998 Dec. 1997 (before fire), Apr. 1998, Oct. 1998 Seedling recruitment survey in Oct. 1998

All blocks received an annual to biennial August–September burn, from the late 1970s to the late 1980s.

Fire-stimulated rainforest seedling recruitment and regeneration in northern Australia

Assessment Three means of assessment were used to collectively test the hypotheses outlined above. The ability of rainforest species to vegetatively regenerate after fire, and the timing of seedling recruitment were each documented at permanently located sites; seedling recruitment was surveyed across recently burnt and unburnt adjacent blocks; and germinable soil seedbank reserves were assessed, with an examination of the influence of heat on seed germination. Fire characteristics Post-fire scorch heights were documented for the November 1996 fire. More detailed fire characteristics data for the December 1997 fire was provided by CSIRO Tropical Forest Research Centre (unpubl. data). Permanent sites Five permanently marked 100-m2 sites were established within adjacent fire management blocks. The locations, establishment and recording times of the sites are given in Table 1. The number of trees and shrubs vegetatively regenerating after fire and the numbers of seedlings were recorded at each site. Nomenclature follows Henderson (1997), except that the proposed elevation to genera level of Corymbia by Hill and Johnson (1995) has not been used, as argued by Brooker (2000). Seedling recruitment surveys Seedling recruitment of trees and shrubs was surveyed across recently burnt and unburnt fire management blocks. A survey in October 1997 documented seedling recruitment within Block 2, burnt 11 months previously, and within Block 4 which had been unburnt for 5 years at that time. A second survey in October 1998 documented the number of seedlings remaining in Block 2, 2 years after fire; the number of seedlings recruited in Block 4, then 10 months after fire and also Block 5, unburnt for more than 5 years. The seedling surveys involved recording the density of seedlings within a 1-m2 quadrat, which was thrown in a haphazard fashion 100 times while walking a transect through the appropriate block, yielding mean seedling recruitment (n = 100). The small size of the quadrat allowed accuracy for density counts of seedlings. In recently burnt areas, seedlings were easy to distinguish from resprouting plants as the latter possess a thickened subsoil base, often with the remains of a burnt stem or were attached to roots. In longer unburnt areas, the seedlings were defined as plants less than 40 cm tall. These figures may overestimate recent seed germination in unburnt areas as some may have been stunted older plants. Germinable soil seedbank The soil seedbank was sampled in October 1997 from Blocks 2 and 4. Soil was collected from 30 locations across each block. These were haphazardly located along the transects used in the seedling recruitment survey. At each location, 250-cm3 subsamples from the top 5 cm of soil were collected from four points (a total of 120 points per block). The soil from Blocks 2 and 4 were kept in separate containers. Each was individually mixed then divided into 10 samples. Five samples from each block were placed into trays and put into a shade house with 70% shade cloth and watered daily. Seedlings were removed once they were large enough to be identified. The samples were assessed for 9 months. The remaining five samples from each block were placed onto trays and oven-heated at 85°C for 45 min before being placed in the shade house. The heat treatment was used in addition to untreated soil to stimulate the germination of as many species as possible and to detect the influence of heat on seed germination. The temperature used is expected to occur in the upper topsoil during the passage of fire (e.g. Bradstock and Auld 1995) which has been shown to be important for breaking seed dormancy (Auld and O’Connell 1991; Bell et al. 1993; Auld and Tozer 1995;

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Bell and Williams 1998; Bell 1999). The 45-min duration is longer than the expected duration of soil heating during the passage of a fire but was required to allow time for heat penetration into the soil samples that were spread across oven trays to about 2-cm depth. Auld and O’Connell (1991) indicated that this duration had no significant effect on the seed germination of Sydney legumes at temperatures below 120°C. Similar temperatures and durations have been used on tropical soil seedbanks with success by Hopkins and Graham (1984) and Skull (1992). The numbers of seeds germinating were converted to seedlings per square metre. As the total surface area for the soil of each replicate was 0.06 m2 (3000 cm3 per 5-cm depth), the number of seedlings germinating per replicate was multiplied by 16.67 to provide seedlings per square metre. Data were analysed by a factorial ANOVA test, with the heat treatments and blocks as factors. Due to the large variances and high number of zeros, the data were condensed by using a ln (x + 1) transformation.

Results Fire characteristics Mean scorch height after the November 1996 fire of Blocks 1 and 2 averaged 5.5 m around the permanent sites. The 1996 fire appeared to be of a lower intensity than the 1997 fire across Blocks 3 and 4, probably due to the proximity to recent rain and therefore moisture of the fuel. Fire characteristics data on the December 1997 fire (CSIRO unpublished results) include the following (means): oven dried fuel weight (grass and leaf litter) of 8.54 t ha–1, percentage curing of fuel of 62%, grass height of 0.7 m, wind speed of 3.35 km h–1, fire speed of 3.44 m min–1, maximum scorch height of 14.6 m, fire intensity of 890 kW m–1, which is a moderate intensity rating (Cheney 1981). Permanent sites A high percentage of all trees and shrubs resprouted after the fires, with the exception of Acacia cincinnata, Allocasuarina littoralis and Alphitonia petrei, which produced abundant seedlings (Table 2). While the larger sclerophyll plants (>2 m tall) were able to reshoot epicormically, the vegetative regeneration of the rainforest pioneers was consistently restricted to subterranean basal stem and root resprouting irrespective of size. Root suckering of Alstonia muelleriana, Banksia aquilonia and Duboisia myoporoides resulted in an increase in the number of stems. There was considerable mortality of root suckers, especially of A. muelleriana during the first 2 years after fire. Seed germination was prevalent after both 1996 and 1997 fires, with no seed germination observed in surveys before the fires (Table 2). There was considerable mortality of seedlings over the 2-year period after the 1996 fire (Table 2). The species with abundant seed germination were primarily rainforest pioneers, but many seedlings of sclerophyll trees were documented, especially Allocasuarina littoralis and Eucalyptus intermedia. Seedling recruitment surveys The seedling recruitment surveys documented a pulse of seed germination in the immediate post-fire period. The 1997 seedling survey documented high seedling recruitment of rainforest species in the recently burnt Block 2 and very

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Table 2. Percentage resprouting plants after fire and numbers of post-fire seedlings for trees and shrubs in permanent sites (100 m–2 each) at Wallaman Falls After Nov. 1996 fire Sites 1 and 2

After Dec. 1997 fire Sites 3, 4 and 5

Percentage resproutingA (no. individuals before fire)

Total post-fire seedlings (no. surviving by October 1998)

Percentage resprouting (no. individuals before fire)

Total post-fire seedlings (no. surviving by October 1998)

Sclerophyll species Acacia flavescens Allocasuarina littoralis Allocasuarina torulosa Alphitonia excelsa Banksia aquilonia Eucalyptus intermedia Eucalyptus tereticornis Eucalyptus torelliana Lantana camara Lophostemon suaveolens

69 (13) 33 (3) 100 (15) 100 (4) 100 (2) 100 (3) 100 (1) 100 (1) 0 (0) 100 (3)

11 (5) 0 (0) 2 (1) 0 (0) 3 (2) 3(0) 1 (1) 0 (0) 2 (2) 0 (0)

98 (43) 45 (13) 0 (0) 0 (0) 150B (2) 100 (12) 100 (8) 0 (0) 0 (0) 0 (0)

4 (1) ± 30 (29) 0 (0) 0 (0) 1 (0) 26C (21) 0 (0) 0 (0) 0 (0) 0 (0)

Rainforest pioneer species Acacia aulacocarpa Acacia cincinnata Alphitonia petrei Alstonia muelleriana Callicarpa pendunculata Canthium sp. Cissus opaca Commersonia bertramia Diospyros pentamera Duboisia myoporoides Glochidion sp. Guioa acutifolia Mallotus paniculatus Melastoma affine Melicope elleryana Neolitsea dealbata Polyscias australiana Rainforest sp. 1 Rapanea variabilis Rhodomyrtus trineura Sapinadaceae Trema tomentosa

0 (0) 7 (15) 0 (23) 133B (3) 100 (1) 1 resprout 0 (0) 100 (8) 1 resprout 0 (0) 100 (1) 4 resprouts 3 resprouts 91 (42) 0 (0) 1 resprout 1 resprout 0 (0) 0 (0) 83 (6) 0 (0) 100 (1)

28 (16) ± 166 (47) ± 730 (230) 6 (4) 2 (1) 4 (1) 1 (0) 6 (4) 0 (0) 0 (0) 2 (1) 0 (0) 0 (0) ± 77 (33) 5 (0) 0 (0) 1 (1) 0 (0) 3 (1) 4 (2) 33 (32) ± 145 (21)

0 (0) 0 (0) 0 (3) 164B (± 121) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 500B (2) 100 (1) 0 (0) 0 (0) 100 (9) 0 (0) 0 (0) 0 (0) 0 (1) 0 (0) 50 (2) 0 (0) 0 (0)

0 (0) 25 (19) ± 181 (176) 1 (1) 0 (0) 0 (0) 0 (0) 2 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 13 (13) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

A

No pre-fire data were collected for Site 2. For species present only in Site 2 numbers rather than percentage of resprouting plants are provided. B Alstonia muelleriana, Banksia aquilonia and Duboisia myoporoides have a higher percentage of resprouts than original plants due to rootsuckering. Mortality of root suckers occurred, and the percentage resprouting figures are based on surviving suckers at October 1998. C All eucalypt seedlings are listed under E. intermedia, even though a few may be E. tereticornis, as the distinction between them at a young age was difficult.

sparse seedlings in Block 4, which at that time was unburnt for 5 years (Table 3). After the 1997 fire, Block 4 contained considerable numbers of seedlings. By contrast, the unburnt Block 5 contained very few. As documented in the permanent sites, the seedling recruitment surveys detected considerably more seedlings of rainforest pioneer than sclerophyll species.

Germinable soil seedbank The germinable soil seedbank was dominated by rainforest trees, shrubs and herbs, plus grasses. Only 6 of the 34 trees and shrubs observed germinating in the field surveys were recorded in the seedbank, all of which were rainforest pioneers (Table 4). More than half of the species germinating were herbs

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Table 3. Mean (standard error) number of seedlings per square metre recorded within the seedling recruitment surveys (n = 100) October 1997 survey Block 2 Block 4 Last burnt 1996 Last burnt 1992

Block 2 Last burnt 1996

October 1998 survey Block 4 Block 5 Last burnt Unburnt for 1997 >5 years ago

Sclerophyll species Allocasuarina littoralis Allocasuarina torulosa Banksia aquilonia Eucalyptus intermedia Eucalyptus tereticornis Eucalyptus torelliana Lophostemon suaveolens Mean for all sclerophyll species

0.00 (0.00) 0.00 (0.00) 0.01 (0.01) 0.08 (0.04) 0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.1 (0.03)

0.00 (0.00) 0.02 (0.01) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.02 (0.01)

0.01 (0.01) 0.08 (0.04) 0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.01 (0.01) 0.02 (0.01) 0.13 (0.04)

0.12 (0.05) 0.27 (0.13) 0.00 (0.00) 0.03 (0.02) 0.25 (0.11) 0.00 (0.00) 0.13 (0.06) 0.8 (0.19)

0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.05 (0.02) 0.05 (0.02)

Rainforest pioneer species Acacia aulacocarpa Acacia cincinnata Acacia mangium Alphitonia petrei Alstonia muelleriana Callicarpa pendunculata Cissus opaca Commersonia bartramia Duboisia myoporoides Guioa acutifolia Mallotus paniculatus Melastoma affine Melicope elleryana Polyscias australiana Rapanea variabilis Rhodomyrtus trineura Sapindaceae Rainforest sp. 1 Rainforest sp. 2 Rainforest sp. 3 Trema tomentosa Mean for all rainforest species

0.00 (0.00) 1.47 (0.23) 0.00 (0.00) 7.84 (1.38) 0.08 (0.53) 0.02 (0.01) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.06 (0.03) 0.47 (0.13) 0.05 (0.04) 0.16 (0.14) 0.13 (0.04) 0.02 (0.01) 0.03 (0.02) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.33 (0.09) 10.66 (1.49)

0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.03 (0.02) 0.01 (0.01) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.02 (0.01) 0.03 (0.02) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.02 (0.01) 0.02 (0.01) 0.02 (0.01) 0.00 (0.00) 0.15 (0.04)

0.07 (0.03) 0.36 (0.08) 0.00 (0.00) 3.52 (0.52) 0.00 (0.00) 0.02 (0.01) 0.00 (0.00) 0.02 (0.01) 0.03 (0.02) 0.01 (0.01) 0.05 (0.02) 0.31 (0.08) 0.03 (0.02) 0.00 (0.00) 0.01 (0.01) 0.11 (0.06) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.16 (0.05) 4.7 (0.58)

0.04 (0.03) 1.39 (0.27) 0.12 (0.05) 2.90 (0.42) 0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.01 (0.01) 0.10 (0.04) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.02 (0.01) 4.6 (0.53)

0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.02 (0.01) 0.00 (0.00) 0.00 (0.00) 0.01 (0.01) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.06 (0.03) 0.03 (0.02) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.13 (0.04)

Mean for all species

10.76 (1.49)

0.17 (0.05)

4.83 (0.58)

5.4 (0.55)

0.18 (0.05)

or grasses. Oven-heating of the soil before lodging in the shade house significantly stimulated the seed germination of Acacia cincinnata and Alphitonia petrei, with no seed of these species germinating in the unheated soil (Table 4). However, the remaining species displayed highest seed germination in unheated soil. Soil seedbank germination of Melastoma affine, Rhodomyrtus trineura and grasses (Poaceae) was significantly reduced by the application of heat. There was a general consistency in germinable seedbanks between blocks, although the abundance of some species varied. While this difference was statistically significant only for Melastoma affine and the grasses, Alphitonia petrei and Acacia cincinnata also showed higher seed germination from Blocks 2 or 4, respectively.

Discussion The benefit of repeatedly measured, permanent sites for assessing vegetation dynamics has been discussed previously (e.g. Lonsdale and Braithwaite 1991; Russell-Smith et al. 1998) and is re-emphasised here. Although the site data section of this study involved only a limited number of plots, their permanency allowed the documentation of changes in populations due to mortality and recruitment. However, the multiple-stemmed, clumping nature of some species, especially Alstonia muelleriana, made the recounting of dense stands difficult and future survey work for this project will include tagging to define separate clumps. These results suggest that most of the rainforest pioneers at Wallaman Falls are able to regenerate vegetatively after a

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Table 4. Mean (standard error) seed germination per square metre from heated and unheated soil seedbanks of Blocks 2 and 4 Summary of ANOVA results for ln (x + 1) transformed soil seedbank germination. n.s., not significant at P = 0.05. Samples were collected in October 1997, 10 months after the last fire in Block 2 and 5 years after the last burn in Block 4 Block 2 Heated

Block 2 Unheated

Block 4 Heated

Block 4 Unheated

Sclerophyll species Convolvulaceae sp. 10.02 (10.02) Emelia sonchifolia 0.00 (0.00) Glycine sp. 0.00 (0.00) Lomandra sp. 10.02 (10.02) Phyllanthus virgatus 0.00 (0.00) Poaceae spp. 0.00 (0.00) Spermacoce brachyandrum 0.00 (0.00) Dicotyledon sp. 0.00 (0.00) Mean for all sclerophyll spp. 3.70 (1.75)

0.00 (0.00) 0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 3.33 (3.33) 30.01 (13.34) 16.67 (7.46) 3.33 (3.33) 17.04 (8.75)

6.67 (6.67) 3.33 (3.33) 0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 0.00 (0.00) 4.45 (1.44)

0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 0.00 (0.00) 3.33(3.33) 20.00 (16.16) 0.00 (0.00) 3.33 (3.33) 19.64 (6.67)

n.s. n.s. n.s. n.s. n.s. P = 0.012 n.s. n.s.

n.s. n.s. n.s. n.s. n.s. P = 0.005 n.s. n.s.

Rainforest pioneer species Acacia cincinnata Ageratum sp. Alphitonia petrei Gonocarpus sp. Hydrocotyle sp. Melastoma affine Rainforest herb sp. Rapanea variabilis Rhodomyrtus trineura Rubiaceae sp. Rubus molucannus Trema tomentsus Mean for all rainforest spp.

20.00 (9.72) 0.00 (0.00) 70.01 (27.59) 83.35 (44.73) 6.67 (4.08) 0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 15.06 (5.28)

0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 70.01 (9.72) 20.00 (13.34) 30.01 (16.16) 0.00 (0.00) 3.33 (3.33) 6.67 (4.08) 10.00 (6.67) 0.00 (0.00) 3.33 (3.33) 15.33 (5.00)

53.34 (15.28) 0.00 (0.00) 30.01 (3.33) 40.01 (15.46) 0.00 (0.00) 10.00 (6.67) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00) 3.33 (3.33) 3.33 (3.33) 14.79 (3.26)

0.00 (0.00) 3.33 (3.33) 0.00 (0.00) 30.01 (15.28) 30.01 (13.34) 56.68 (11.31) 0.00 (0.00) 3.33 (3.33) 23.34 (8.50) 10.02 (10.02) 3.33 (3.33) 10.00 (6.67) 15.59 (3.53)

P < 0.0001 n.s. P < 0.0001 n.s. n.s. P = 0.002 n.s. n.s. P = 0.012 n.s. n.s. n.s.

n.s. n.s.

Mean for all species

10.00 (3.10)

15.61 (4.54)

10.15 (1.99)

16.98 (3.37)

low or moderate intensity fire, thus Hypothesis 1 is rejected. Alstonia muelleriana has become a thicket in many areas of the field site. This is probably due to its ability to send up stems via root suckers in response to fire. The ability of Alstonia muelleriana to increase stem abundance vegetatively suggests that it has a capability to exploit frequently disturbed sites. However, the other two species observed to root-sucker after fire (Banksia aquilonia and Duboisia myoporoides) have not developed into dense thickets to date. In addition to the tolerance of standing sclerophyll and most rainforest pioneer plants to fires of low or moderate intensity at Wallaman Falls, a pulse of seedling recruitment, especially of rainforest pioneers, was recorded in the immediate post-fire period. Data from both the permanent sites and the seedling recruitment surveys suggest that seed germination of both rainforest pioneers and sclerophyll species in the densely grassed forest at Wallaman Falls is more abundant in the first year or two after fire than after longer intervals between fires, thus disagreeing with Hypothesis 2. The few species unable to resprout after fire, such as Alphitonia petrei, recruited large numbers of seedlings, which compensated for the death of adults. The results suggest that the seed germination of Allocasuarina littoralis and A. torulosa is enhanced by fire,

ANOVA comparisons Heat Block treatment variation

n.s. n.s. P = 0.03 n.s. n.s. n.s. n.s. n.s. n.s.

although the recruitment of A. littoralis can be abundant in the absence of fire (Lunt 1998). Also the germination of Eucalyptus intermedia and E. tereticornis appears to be increased in recently burnt areas; however, germination of E. intermedia along unburnt roadside areas suggests this species may be capable of significant germination in the absence of fire, given bare sites (Gill 1997). The limited number of trees and shrubs observed in the germinable soil seedbank, including a complete absence of sclerophyll trees, suggests either a general shortage of seed reserves at the time of collection or that seed germination requirements were not met by the treatment of the soil seedbank. However, the germinable seedbank results confirm that seed of rainforest species were present in Block 4 at the time of the 1997 seedling survey, when very limited seedlings were present in the field. It also shows considerable seed reserves of several species present in Block 2, demonstrating significant quantities of seed reserve are available one year after fire. This may be due to the germination of only a fraction of the seedbank after the 1996 fire and/or considerable replenishment may have occurred over that year. Although seed germination of many species appeared to be stimulated by fire, the heat provided by the fire may be the

Fire-stimulated rainforest seedling recruitment and regeneration in northern Australia

germination trigger for relatively few species, such as Acacia cincinnata and Alphitonia petrei. However, a different temperature and heating duration applied to the soil seedbank may stimulate the germination of other species. Separate factors such as smoke, changes in micro-site environment, or the removal of competition from the grass cover, may be stimulating the pulse of germination for many of these plants. Implications and further research There is accumulating evidence that many north-eastern Australian rainforest pioneer species possess the ability to vegetatively regenerate after low to moderate intensity fires. In addition, recruitment, particularly in wetter types of forests with dense grass cover, may often be as a pulse of seedlings in the immediate post-fire environment, with less recruitment occurring during later periods of fire intervals. A pulse of germination after fire, while the grass cover is thin, is consistent with the hypothesis of Hopkins and Graham (1984) and Unwin et al. (1985) in that grass competition may inhibit tree seedling recruitment. The rainforest expansion currently occurring across the Wet Tropics appears to be linked with changes in fire regimes (Ash 1988; Harrington and Sanderson 1994). Research into the individual aspects of fire regimes is desperately needed. This research needs to focus on obtaining data on issues such as (1) whether vegetative and seedling regeneration abilities of rainforest and sclerophyll species vary between sites and after different fire regimes; (2) detecting the period required for seedlings of resprouter species to become fire tolerant (primary juvenile period); (3) evaluating the potential barrier to woody species recruitment of dense grass cover (this may also be useful for developing techniques for enhancing natural regeneration in rainforest re-establish projects in previously cleared sites); (4) examining seedling recruitment in micro-sites under trees throughout fire intervals and their importance in the process of rainforest expansion; and (5) determining similarities between the expansion of tropical rainforest species and increased densities of native shrubs in semi-arid New South Wales and Queensland, which have also been linked to changed fire regimes (Hodgkinson and Harrington 1985). Acknowledgments I am grateful to the many people who have been involved in the collection of the data from the permanent sites and provided discussions and improvements on early drafts of this paper. Fred Thuler and Dave Green (Rangers in Charge of Wallaman Falls and Lumholtz National Park, respectively) provided both field support, useful discussions and background details on the recent fire history of Wallaman Falls. Dr Graham Harrington, Keith Sanderson and Mat Bradford of CSIRO Tropical Forest Research Centre, Atherton, provided field support for the establishment of several sites, the 1997 fire characteristics data and helpful discussions. Drs Peter

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Clarke, Malcolm Gill, Graham Harrington, Jeremy RussellSmith and an anonymous referee provided valuable advice that greatly improved earlier drafts of this paper. James Cook University students provided helpful discussions and assisted in the collection of data at the permanent sites in April 1997 and April 1998: Eleanor Collins, Annika Dahlgren, Hywel Evans and Rick Van Veen in 1997 and Erica Brown, Margaret Burton, Jasmine Foxlee and Veronica Sculac in 1998. Russell Cumming, Queensland Herbarium, provided useful discussions and identified several of the plant specimens. References

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Manuscript received 12 April 1999, accepted 2 February 2000

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