Monsoon rain forest seedling dynamics, northern Australia: contrasts with regeneration in...

ORIGINALARTICLE

Monsoon rain forest seedling dynamics,northern Australia: contrasts withregeneration in eucalypt-dominatedsavannas

Jeremy Russell-Smith1,2* and Samantha A. Setterfield1,3

1Tropical Savannas Cooperative Research

Centre, Charles Darwin University, Darwin,

NT, 2Bushfires Council of the Northern

Territory, Winnellie, NT and 3Charles Darwin

University, Darwin, NT, Australia

*Correspondence: Jeremy Russell-Smith,

Tropical Savannas Cooperative Research Centre,

Charles Darwin University, Darwin 0909,

Northern Territory, Australia.

E-mail: [email protected]

ABSTRACT

Aim To explore: (1) the relative influences of site conditions, especially moisture

relations, on pathways and rates of monsoon rain forest seedling and sapling

regeneration, especially of canopy dominants, in northern Australia; and (2)

contrasts between regeneration syndromes of dominant woody taxa in savannas

and monsoon rain forest.

Location Four monsoon rain forest sites, representative of regional major habitat

and vegetation types, in Kakadu National Park, northern Australia.

Methods A decadal study involved: (1) initial assessment over 2.5 years to

explore within-year variability in seed rain, dormant seed banks and seedling

(< 50 cm height) dynamics; and (2) thereafter, monitoring of seedling and

sapling (50 cm height to 5 cm d.b.h.) dynamics undertaken annually in the late

dry season. On the basis of observations from this and other studies, regeneration

syndromes of dominant monsoon rain forest taxa are contrasted with comparable

information for dominant woody savanna taxa, Eucalyptus and Corymbia

especially.

Results Key observations from the monsoon rain forest regeneration dynamics

study component are that: (1) peak seed rain inputs of rain forest taxa were

observed in the wet season at perennially moist sites, whereas inputs at seasonally

dry sites extended into, or peaked in, the dry season; (2) dormant soil seed banks

of woody rain forest taxa were dominated by pioneer taxa, especially figs; (3)

longevity of dormant seed banks of woody monsoon rain forest taxa, including

figs, was expended within 3 years; (4) seedling recruitment of monsoon rain

forest woody taxa was derived mostly from wet season seed rain with limited

inputs from soil seed banks; (5) at all sites rain forest seedling mortality occurred

mostly in the dry season; (6) rain forest seedling and sapling densities were

consistently greater at moist sites; (7) recruitment from clonal reproduction was

negligible, even following unplanned low intensity fires.

Main conclusions By comparison with dominant savanna eucalypts, dominant

monsoon rain forest taxa recruit substantially greater stocks of seedlings, but

exhibit slower aerial growth and development of resprouting capacity in early

years, lack lignotubers in mesic species, and lack capacity for clonal reproduction.

The reliance on sexual as opposed to vegetative reproduction places monsoon

rain forest taxa at significant disadvantage, especially slower growing species on

seasonally dry sites, given annual–biennial fires in many north Australian

savannas.

Keywords

Australia, clonal reproduction, disturbance, Kakadu National Park, monsoon

forest, savanna, seed bank, seed rain, tropical rain forest.

Journal of Biogeography (J. Biogeogr.) (2006) 33, 1597–1614

ª 2006 The Authors www.blackwellpublishing.com/jbi 1597Journal compilation ª 2006 Blackwell Publishing Ltd doi:10.1111/j.1365-2699.2006.01527.x

INTRODUCTION

Monsoon rain forest occurs across northern Australia

typically as small (< 5 ha), floristically simple island-like

patches (mostly < 100 species per patch), in a vast expanse

of fire-prone eucalypt-dominated savanna (Russell-Smith,

1991). Patches occur on a variety of substrates and

perennially moist to seasonally arid site conditions, defined

by a number of floristic and structural types which describe

generally a subset of more complex and extensive rain forest

vegetation growing in lowland, climatically more equable

regions of north-eastern Australia (Webb & Tracey, 1981;

Russell-Smith, 1991; Bowman, 1999). Adult populations of

monsoon rain forest taxa in any one patch are mostly small

(< 50 individuals: Russell-Smith & Lee, 1992), with evidence

for populations of some species occurring as widely-

dispersed, genetically connected metapopulations (Shapcott,

1998a,b, 2000).

Fire frequencies over extensive areas of coastal to sub-coastal

northern Australia are often annual to biennial, are mostly

anthropogenic in origin, and occur mostly late in the seven-

month dry season under severe fire-weather conditions

(Williams et al., 2002; Russell-Smith et al., 2003a). Many

monsoon rain forest patches are afforded some level of fire

protection, occurring either at perennially moist sites (e.g.

springs) or, in seasonally dry situations, in rugged, rocky

terrain. Additionally, internal patch ground fuels are typically

compact and non-grassy, and may be relatively moist by

comparison with those in adjoining seasonal savannas

(Bowman & Wilson, 1988). Under fire regimes characterized

by frequent late dry season fires, critical issues at patch margins

include the capacity of component woody species to regenerate

(from seed, resprouting), and attendant rates of regeneration

(Russell-Smith & Stanton, 2002).

By contrast with tropical forest systems in perhumid

climates, it remains the case that very limited work has been

reported on the regeneration pathways and dynamics of

forest assemblages in the markedly seasonal tropics (Murphy

& Lugo, 1986; Gerhardt & Hytteborn, 1992). Available

observations suggest that: (1) fruiting phenological patterns

exhibit substantial variability between different study sites

(Bach, 2002); (2) seed banks are dominated by early

successional species, as per rain forests occurring under

more perhumid conditions (e.g. Garwood, 1989); (3)

seedling recruitment occurs mostly in association with

moist, rainy periods, and survivorship is affected markedly

by seasonal moisture deficit conditions (Lieberman & Li,

1992); (4) many woody species exhibit the capacity to

resprout following stem death (Murphy & Lugo, 1986;

Bowman, 1991a; Russell-Smith & Stanton, 2002); and (5)

clonal reproduction is likely to be more prevalent at

seasonally dry sites (Ewel, 1977; Lieberman & Li, 1992;

Russell-Smith, 1996).

Various of the above observations accord with similar

regeneration patterns observed for woody taxa in savanna

assemblages, for example: enhanced seedling recruitment

associated with consistently moist periods (Hoffmann, 1996;

Setterfield & Williams, 1996); substantial capacity of stems

to resprout following fire (Bond & van Wilgen, 1996;

Williams et al., 2002); and enhanced clonal reproduction

following burning (Lacey, 1974; Hoffmann, 1998). As with

monsoon rain forest patch margins, woody structure devel-

opment in fire-prone savannas is strongly dependent on

rates of stem growth and the frequency and intensity of fire

(Kellman, 1984; Braithwaite & Estbergs, 1985; Higgins et al.,

2000).

This paper reports a comparative decadal assessment of

seedling and sapling regeneration dynamics, focusing especially

on canopy dominants, at four sites representative of contrast-

ing site conditions (e.g. substrate moisture status, soil type), to

address: (1) the relative significance of seed rain, soil seed

banks, and clonal reproduction for monsoon rain forest woody

species recruitment; (2) the relative influences of different site

conditions, especially moisture relations, on rates of monsoon

rain forest seedling and sapling regeneration; and (3) in the

Discussion section, contrasts between the regeneration syn-

dromes of dominant regional monsoon rain forest and

savanna taxa.

Study sites

Four monsoon rain forest sites, each occurring in Kakadu

National Park, northern Australia, were selected for detailed

study based on prior numerical classification of a comprehen-

sive north Australian data set (Russell-Smith, 1991). The upper

levels of that classification hierarchy describe four major types

occurring in higher rainfall coastal and sub-coastal areas: (1)

rain forests associated with sites of perennial moisture, either

with neutral to basic, fine-textured soils (Groups 1, 2); or (2)

acidic, sandy soils (Groups 3–7); and (3) rain forests associated

with seasonally dry substrates, either in rugged sandstone

terrain (Group 8); or (4) occurring on a wide variety of other

substrate types (e.g. coastal dunes, granite or basalt outcrops,

lateritic landforms: Group 9). An analagous upper-level

classificatory hierarchy was reproduced from floristic

10 · 10 m quadrat data (n ¼ 303) for 33 northern Australian

monsoon rain forest patches, derived for the purposes of

characterizing habitat conditions of rain forest sapling banks

(Russell-Smith, 1996).

For the purposes of this study only one replicate of each of

the four floristic types was selected since: (1) previous

replicated studies (all one-off surveys, or of relatively short-

term duration) had already established major patterning in

forest typology (Russell-Smith, 1991), dormant seed banks

(Russell-Smith & Lucas, 1994), sapling banks (Russell-Smith,

1996), seed rain phenology (Bach, 2002); (2) the prime focus

of this study was to document the recruitment dynamics of

canopy dominants characteristic of the respective forest types

over a decadal period; (3) the four selected sites included

diagnostic dominant tree species (Russell-Smith, 1991) char-

acteristic of each of the respective four forest types (Table 1);

and (4) the logistic realities of sampling isolated replicate

J. Russell-Smith and S. A. Setterfield

1598 Journal of Biogeography 33, 1597–1614ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd

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Australian monsoon rain forest dynamics

Journal of Biogeography 33, 1597–1614 1599ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd

patches mitigated against being able to undertake a thorough

longitudinal (temporal) investigation across many sites.

Site names (and abbreviations as used throughout the

paper), locations, habitat conditions, and diagnostic species are

outlined in Table 1. Two sites were located at small evergreen

rain forest patches associated with perennial springs: site PL on

pH-neutral deep clay loam at the edge of a lowland freshwater

wetland, and PS on acid humic soils over deep sand in rugged

sandstone terrain. The other sites were associated with well-

drained, seasonally dry, acidic substrates: SS comprising

extensive semi-evergreen forest on a deep sandsheet in rugged

sandstone, and SL comprising a small patch of semi-deciduous

forest on deep sandy loam in the gently undulating lowlands.

In other continental settings, monsoon rain forest equates with

structural types ranging from tropical semi-evergreen to

deciduous forests (Walter, 1984), monsoon forests (Whitmore,

1984), and tropical dry forests (Holdridge, 1967; Murphy &

Lugo, 1986).

Available rain-year (July–June) rainfall data from the closest

recording stations to respective sites (Jabiru, c. 10 km from

both SS and PS; Wildman River Station, c. 20 km from both SL

and PL), is given for the 10-year period 1992/93–2001/02

(Fig. 1). Over 90% of rainfall is restricted to the summer

months, principally November through March, associated with

the arrival of the monsoon. While the amount of rainfall

received in any one area is highly variable from year to year,

the annual wet season is a highly reliable event (Fig. 1; Taylor

& Tulloch, 1985). Environmental relationships of northern

Australian rain forest assemblages are described in Russell-

Smith (1991).

METHODS

For the purposes of assessing the relative contributions of

seed rain and the soil seed bank to seedling regeneration

dynamics, an initial intensive study was undertaken of these

components over the first 2.5 years, commencing in late June

1993 and ending with a final field sampling in December

1995. Thereafter, seedling and sapling dynamics alone were

assessed once each year (Table 2). At each site a permanently

marked 50 · 10 m transect was established within intact

monsoon rain forest vegetation. Twenty-five 1-m2 permanent

quadrats were laid out systematically along the permanent

transects. Study components were sampled as detailed below.

Habitat features

At the commencement and conclusion of the study the

following two measurements were undertaken: d.b.h. of all tree

stems (> 5 cm d.b.h.) present in 50 · 10 m transects; canopy

cover, where 0 ¼ no canopy; 1 ¼ partial canopy; 2 ¼ > 50%

canopy. A Canopy Cover Index was calculated from canopy

cover data, comprising the sum of individual counts divided

by 50.

Seed rain

Five steep-sided plastic bins, each sampling a surface area of

59 · 37 cm and 28 cm deep, were randomly situated on the

ground within each transect. Small holes were drilled in

their bases for drainage. Sampling commenced at the end of

June 1993. Bins were emptied every 3 months, ending

December 1995. There was no evident sign of predation,

either from vertebrates or invertebrates. Large, readily

identified seeds were sorted, counted and recorded after

collection. After removing most of the leaf litter and other

detritus, all remaining materials, but including many-seeded

fruits of Ficus spp. and Nauclea orientalis, were stored at

room temperature in calico bags in a dry, well-ventilated

environment, and subsequently laid out in shallow trays on

a bed of sterile sand (oven-dried at 105 �C for 2 h) for

germination in a shade-house. Shade-house germination

trials of seed rain materials collected in any one year were

established in January of the year following. Germination

trials were conducted under ambient daylight conditions,

and daily overhead watering, over at least 6 months in all

assessment years. For each seed rain species sampled, the

final tabulation per sampling period used the maximum

numbers of seeds, or germinants, whichever was the larger.

It is possible that, under this sampling regime, seed rain of

woody rain forest species which fruited early in the year and

0

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Figure 1 Available rain-year rainfall data for recording stations

closest to study sites: Jabiru – PS and SS; Wildman River – PL and

SL (source: Australian Bureau of Meteorology).



had inconspicuous, small and short-lived seeds may not

have been detected.

Soil seed bank

Soils were collected at the end of June in 1993, 1994 and 1995.

Twenty surface soil samples (0–5 cm) were collected randomly

within transects, bulked together, mixed thoroughly, split into

5–10 sub-samples per sampling period, stored at room

temperature in calico bags, and subsequently laid out in shallow

trays on a bed of sterile sand for germination in a shade-house as

per the seed rain samples. All soil sub-samples were weighed

prior to setting out in germination trays, and small sub-samples

taken for determination of oven dry weight. Germination trials

of 10 soil sub-samples collected in 1993, 1994, 1995 were

conducted in the following January to assess dormant seed

banks (sensu Russell-Smith & Lucas, 1994). Additionally, for

soil sub-samples collected in 1993, germination trials of 10 sub-

samples were undertaken immediately after collection for

assessment of post-wet season seed banks, and 5 sub-samples

were taken in January 1996 for assessment of seed bank

longevity. Germination trials were conducted over at least

6 months in all assessment years, under similar conditions as for

seed rain germination trials. Soil seed bank data are presented as

numbers of germinants per kg of oven dry weight soil.

Seedlings and clonal sprouts

Seedlings (and clonal sprouts < 50 cm tall), and saplings (50–

200 cm tall; 200 cm tall to 5 cm d.b.h.) were identified to

species and counted separately in each of the 25 1-m2

permanent quadrats. Individuals were not tagged but, where

required for determination of clonal reproduction, seedling-

sized clonal sprouts were identified by non-destructive exca-

vation. Sampling was undertaken first in late September 1993,

then in April/May and September/October (before the rains of

the ensuing wet season) in 1994 and 1995. Thereafter,

sampling was undertaken only in September/October (before

the rains) of each year until 2002, given that data from 1994–

95 indicated that, typically, minimum annual survivorship of

woody seedlings occurred at this time (see Results).

Functional groups

Data are presented for six functional groups: rain forest trees,

rain forest shrubs, savanna trees, savanna shrubs, graminoids,

and herbs. Rain forest woody taxa are those as listed by Liddle

et al. (1994). Savanna woody taxa include Callitris, Eucalyptus,

Corymbia, Melaleuca spp. and Acacia spp., with the exception

of the rain forest taxon A. auriculiformis. Nomenclature follows

Cowie & Albrecht (2005). Tree taxa are defined as those

typically attaining > 8 m height. Woody lianes/vines are

included here as trees or shrubs, depending on their stature.

Graminoids comprise Cyperaceae and Poaceae. Herbs are

defined as non-woody non-graminoids.

RESULTS

Basal area, stem density and canopy cover was greater at

perennially moist sites (PL and PS) than at seasonally dry sites

(Fig. 2). Over the 10-year assessment period, basal area

increased in all plots by between 17% (PL) and 36% (PS),

while stem density and canopy cover increased in three plots

but decreased marginally at SS (Fig. 2).

Over the period of assessment, low intensity, late dry season,

typically patchy fires were experienced at sites as follows: SS –

all 25 sampling quadrats burnt in 1994, four quadrats burnt in

1997, 10 quadrats burnt in 2001; SL – all 25 quadrats burnt in

Table 2 Sampling regime for different study components, where sampling periods given in quarters as follows: 1 ¼ January–March;

2 ¼ April–June; 3 ¼ July–September; 4 ¼ October–December

Study component

Sampling period

Within-year dynamics Decadal dynamics

1993 1994 1995 1996 1996 1997 1998 1999 2000 2001 2002

Quarter Quarter

2 3 4 1 2 3 4 1 2 3 4 1 2 3 3 3 3 3 3 3

Seed rain assessment

Sampling · · · · · · · · · ·Germination · · · · · ·

Soil seed bank assessments

Sampling · · ·Non-delayed germination · ·Delayed germination (6 months) · · · · · ·Delayed germination (3 years) · ·

Seedling, sapling counts · · · · · · · · · · · ·Tree stem, canopy cover, inventory · ·



1993; PL – none burnt; PS – 18 quadrats burnt in 1993, after

late dry season sampling. The effects of these fires, mostly

minor, are noted where appropriate.

Within-year dynamics: July 1993–December 1995

Seed rain

Over the 2.5 years of the study, estimated total seed rain

(individuals m)2) was 4609 at PL, 1062 at SL, 841 at PS and

311 at SS. The contribution to this total from rain forest trees

ranged between 67% and 79% at three sites, and 30% at SL.

Contributions from rain forest shrubs ranged between 0% (PL)

to 3.7% (PS). Seed rain from savanna trees was marked at the

two lowland sites (42% at SL and 26% at PL), but not at

sandstone sites (7% at PS and 0% at SS). Seed rain from

savanna shrubs was negligible (£ 1%) at three sites, but was

6% at SS. Graminoid seed rain ranged between 15% (SS) and

< 1% (PL), and herb seed rain was from 8% (SL) to < 1%.

The seasonal pattern of seed rain for woody species

functional groups is given for each study site in Fig. 3(a–d).

With respect to rain forest tree seed rain, significant variability

was observed between sites, and between years within sites, e.g.

(1) distinct wet season peak inputs are evident at PS in 1993/94

and 1994/95, and PL in 1993/94; (2) wet season inputs are

extended to mid-year at SL in 1994 and PL in 1995; (3) major

dry season inputs occurred at SS in 1994 and SL in 1995; and

(4) at SS virtually no inputs were observed in 1993/94 wet and

1995 dry seasons (Fig. 3a). Similar levels of variability, both

with respect to seasonality and quantity, are evident for seed

rain inputs of other species functional types (Fig. 3b–d).

Soil seed bank

The sizes of soil seed banks from different species functional

groups varied markedly between sites, and between sampling

periods within sites (Fig. 4a–d). Overall, soil seed banks

from the 1993 non-delayed sampling and the 1993–95

delayed samples (i.e. samples 93, and 93D, 94D, 95D as

given in Fig. 4) were most abundant (expressed as number

of germinants kg)1 soil) at moist sites. At PL, soil seed

banks ranged from 794 to 2111 individuals, and comprised

mostly herbs (range: 80–93% in 93, 93D–95D samples), but

included no savanna trees or shrubs. At PS, soil seed banks

ranged from 117 to 301 individuals, and comprised mostly

graminoids (range: 31–62%), with significant numbers of

rain forest trees (range: 11–24%) and shrubs (range:

8–41%). At SL, soil seed banks ranged from 72 to 285

individuals, was dominated by herbs (range: 37–73%) and

graminoids (24–58%). At SS, soil seed banks ranged from 16

to 40 individuals, dominated by herbs (range: 32–48%) and

graminoids (24–6%), with similar proportional representa-

tion of rain forest trees and shrubs as at PL.

There were no consistent trends between the sizes of soil

seed banks for different species functional groups in soil

samples collected in June 1993 which were (1) germinated

soon thereafter (93 – July 1993), or (2) delayed (93D – Jan

1994) (see Fig. 4). Thus, for rain forest taxa, marked declines

in numbers of tree and shrub germinants were evident between

1993 and 1993D samples at PS, whereas at PL there was

marked increase of rain forest tree germinants; at other sites

small numbers of germinants also varied in the direction of the

general trend (Fig. 4a,c).

Rain forest taxa with dormant seeds dominated soil seed

banks of woody species in all 93 and 93D–95D samples. These

comprised mostly pioneer species, especially figs, Melastoma

affine and Nauclea orientalis. For example, of rain forest tree

germinants, figs comprised 100% at SS, 83% at PS, 20% at PL

and 9% at SL, and Nauclea comprised 80% at PL. Dormant

seed banks of canopy dominants were observed at SL (Callitris

intratropica and Vitex acuminata), whereas no germinants of

other diagnostic canopy dominants were recorded at SS

(Allosyncarpia ternata), at PS (Syzygium minutuliflorum), and

at PL (S. nervosum).

For soil samples collected in June 1993 and germinated in

January 1996, there were marked declines both in soil seed

bank diversity (only graminoids and herbs germinated) and

0

20

40

60

Bas

al a

rea

(m2

ha–1

)

0

20

40

60

80

No.

ste

ms

/ 0.0

5 ha

0.0

0.1

0.2

0.3

0.4

0.5

Can

opy

inde

x1993

2002

(a)

(b)

(c)

SL SS PS PL

Figure 2 Decadal comparisons between (a) basal area, (b) stem

(> 5 cm d.b.h.) density, and (c) canopy cover index (refer

Methods) at commencement (1993) and completion (2002) of

assessment period.



abundance. Small numbers of graminoids germinated at three

sites (range: 4–37 individuals kg)1 dry soil), but not at PL.

Relatively large numbers of herbs germinated at PL (481 kg)1

soil), but still much less than at PL in other treatments. A small

number of herbaceous germinants was also recorded from the

SS site.

Seedlings and saplings

Seedling densities varied markedly between sites and sampling

period (Fig. 5a). Seedling densities at perennially moist sites

(PL and PS) were substantially greater than at seasonally dry

sites (SS and SL). At all sites there was marked inter-seasonal

0

200

400

600

800

1000

1200

1400 Rainforest trees Savanna trees

0

5

10

15

20

25

No.

indi

vidu

als

m–2

(c) (d)

2 3 4 5 6 7 8 9 10 11 2 3 4 5 6 7 8 9 10 11Sampling period

(a)

Rainforest shrubs

(b)

Savanna shrubsFigure 3 Quarterly seed rain sampling,

July–September 1993 (sampling period 2) to

October–December 1995 (sampling period

11), at respective sample sites (SS, thin line;

SL, dashed line; PS, thick line; PL, thick

dashed line) for four functional groups (a–d)

as shown. Note: (1) seedrain expressed as

numbers of individuals per m2 of sampling

area; (2) wet season sampling periods

(October–December and January–March)

given in bold; (3) y-axis values differ between

each pair of graphs.

0

50

100

150

200

250 (a) Rainforest trees (b) Savanna trees

0

20

40

60

80

100

120

No.

ger

min

ants

per

kg

soil

(c) Rainforest shrubs (d) Savanna shrubs

Sample 93 93D 94D 95D 93 93D 94D 95D

Figure 4 Seed bank sampling, July 1993 to

July 1995, at each sample site (SS, thin line;

SL, dashed line; PS, thick line; PL, thick

dashed line) for four functional groups (a–d)

as shown. Note: (1) two samples for 1993,

where 93 refers to soil samples collected and

germinated in June 93, and 93D (delayed)

refers to samples collected in June 93 and

germinated January 1994; (2) samples 94D

and 95D refer to soils collected in June of

1994 and 1995, respectively, but germinated

6 months later; (3) y-axis values differ

between each pair of graphs.



dynamism in seedling densities, with substantially greater

densities observed at the end of the wet-season sampling

period. Sapling densities were relatively stable at three sites

over the sampling period, but at PS substantial numbers of

sapling-sized individuals were recruited over the 1994/95 wet

season (Fig. 5b).

Individual species

Seedlings of 23 woody rain forest shrub and tree species

increased from one observation period to the next in the

period 1993–1996 (Table 3). At three sites, increases of

dominant seedling species (Allosyncarpia ternata at SS, Syzy-

gium minutuliflorum at PS and S. nervosum and PL) were

associated with seed rain inputs only. Increases in seedling

numbers were not observed for a further 27 species, despite

observed seed rain inputs (7 taxa), dormant seed banks

(7 taxa), or both (13 taxa), at one or more sites (Table 3).

Decadal patterns: Sep 1993 to Sep 2002

Seedlings and saplings

Over the 10-year period woody seedling densities were

typically greater at moist sites than at seasonally dry sites.

Over the same time period there was a general increase in

densities of saplings 50–200 cm height at three sites, and very

substantial increase in densities of saplings > 200 cm at all sites

(Fig. 6). Compared with seedlings of shrubs and vines,

seedlings of tree species dominated late dry season seedling

banks at three sites, throughout the 10-year study period,

especially at PS, and for half the period at SL (Fig. 7). Similar

dominance of tree species saplings was evident at moist sites

(PL and PS), but not at seasonally dry sites where both shrubs

and vines were relatively abundant in some years at least

(Fig. 7).

Individual species

In general, at any one site (1) seedling densities of individual

species were often highly dynamic from year to year, (2) at any

one site, major seedling recruitment events often occurred in

different years for different species, and (3) overall, despite the

capacity of some listed species to reproduce clonally, the

contribution of clonal regeneration was very restricted. Clonal

regeneration was observed in four species only: the vines

Ichnocarpus frutescens (PL) and Sarcostemma hullsii (SS); the

small tree Santalum album (SL); and the shrub Helicteres

hirsuta (SL).

At SS, seedling regeneration of the dominant tree species,

Allosyncarpia ternata, collapsed as a result of burning in 1994,

and did not begin to recover until 4 years later; no recruitment

of saplings occurred over the 10-year period (Fig. 8). Fire also

affected seedling densities of three other tree and shrub species;

however, recruitment of small numbers of saplings was

observed for each of these species.

At SL, no seedlings or saplings of the dominant tree species,

Callitris intratropica, was observed over the 10-year period.

However, significant recruitment of saplings of the sub-

dominant species, Vitex acuminata (Fig. 8), and four other

tree and shrub species, was observed over that time.

At PL, seedling densities of the co-dominant tree species,

Syzygium nervosum, consistently comprised both a substantial

proportion of all woody species recruitment as well as

fluctuating markedly over time (Fig. 8). However, very few

seedlings of the other dominant tree species, Nauclea orientalis,

were observed. Whereas stem density of the palm Carpentaria

acuminata increased dramatically over time, there was evi-

dently little associated seedling recruitment; rather, there was a

clear trend for seedlings and saplings to advance into larger

size-classes with time. Conversely, sapling and stem densities of

the second palm species, Livistona benthamii, remained static

over most of the period, while seedling numbers fluctuated

markedly. Substantial fluctuations were observed also in

seedling-sized recruitment of the clonally reproducing shrubby

vine, Ichnocarpus frutescens.

At PS, relatively high densities of seedlings and saplings of

the dominant tree species, Syzygium minutuliflorum, were

observed over much of the study period (Fig. 8). Whereas

similar dynamism in seedling density with time was evident in

the sub-dominant tree, Pouteria richardii, patterns of seed-

ling recruitment were markedly occasional in some species

0

200

400

600

800

1000

1200

01993L 1994E 1994L 1995E 1995L

10

20

30

No.

indi

vidu

als

per

25 m

2

(b)

(a)

Sample

Figure 5 Densities of (a) seedlings (< 50 cm high), and (b)

saplings (50 cm high to < 5 cm d.b.h.), sampled in 25 m2 at

respective sample sites (SS, thin line; SL, dashed line; PS, bold line;

PL, bold dashed line), September 1993–September 1995. Note: (1)

E on x-axis refers to sampling in first quarter each year, and L

refers to sampling in third quarter each year; (2) y-axis values

differ between graphs.



Table 3 Responses of woody rain forest species at each study site, where: (a) seedlings (< 50 cm high) of listed taxa increased from one

observation period to the next over the period 1993–1996; and/or (b) seedrain and (c) seedbanks were observed in the period 1993–1995. After

species names # indicates taxa observed in this study, or elsewhere (Russell-Smith, 1996) to reproduce clonally; * indicates taxa observed

(Russell-Smith & Lucas, 1994) to possess dormant seedbanks. Family abbreviations given in parentheses, and given in full at end of table

Species

Seedlings Seedrain Seedbank

Site Site Site

PL PS SL SS PL PS SL SS PL PS SL SS

Acacia auriculiformis* (Mim.) + +

Aidia racemosa#* (Rub.) + + + +

Allosyncarpia ternata (Myrt.) + +

Alphitonia excelsa* (Rhamn.) + + +

Alstonia actinophylla* (Apoc.) +

Antidesma parvifolium# (Euph.) +

Breynia cernua#* (Euph.) +

Buchanania arborescens (Anac.) + +

Callitris intratropica (Cupr.) + +

Calophyllum sil (Clus.) + +

Canarium australianum* (Burs.) +

Canthium coprosmoides (Rub.) +

Carpentaria acuminata (Arec.) + + + +

Celtis philippensis (Ulm.) +

Choriceras tricorne# (Euph.) + +

Diospyros compacta# (Eben.) + +

Drypetes deplanchei# (Euph.) + + + +

Ficus leucotricha* (Mor.) + +

Ficus platypoda* (Mor.) + + +

Ficus racemosa* (Mor.) + + + +

Ficus scobina* (Mor.) + + + + + + +

Ficus virens* (Mor.) + + + + + + + +

Gmelina schlecteri (Verb.) +

Helicia australasica (Prot.) +

Helicteres hirsuta# (Sterc.) +

Horsfieldia australiana (Myrist.) + +

Ilex arnhemensis (Aquif.) +

Livistona benthamii (Arec.) + +

Lophopetalum arnhemica (Cel.) + +

Maranthes corymbosa* (Chrys.) +

Melastoma affine#* (Log.) + + + +

Melicope elleryana* (Rut.) + + + +

Nauclea orientalis* (Rub.) + + +

Notelaea microcarpa (Ole.) +

Polyalthia nitidissima (Ann.) + +

Pouteria richardii (Sap.) + +

Pouteria sericea* (Sap.) + + +

Rapanaea benthamiana (Myrs.) + +

Santalum album# (Lor.) +

Stenocarpus verticis (Prot.) + +

Strychnos lucida#* (Log.) + + + +

Syzygium minutuliflorum (Myrt.) + +

Syzygium nervosum (Myrt.) + +

Terminalia microcarpa* (Comb.) + + +

Timonius timon* (Rub.) + + +

Trema tomentosa* (Ulm.) + + + + + + +

Vavaea australiana (Sap.) + +

Vitex acuminata (Verb.) + + +

Vitex glabrata* (Verb.) + +

Xanthostemon eucalyptoides (Myrt.) + +

Abbreviations of family names: Anac. ¼ Anacardiaceae; Ann. ¼ Annonaceae; Apoc. ¼ Apocynaceae; Aquif. ¼ Aquifoliaceae; Arec. ¼ Arecaceae; Burs. ¼ Burseraceae;

Cel. ¼ Celastraceae; Chrys. ¼ Chrysobalanaceae; Clus. ¼ Clusiaceae; Comb. ¼ Combretaceae; Cupr. ¼ Cupressaceae; Eben. ¼ Ebenaceae; Euph. ¼ Euphorbiaceae;

Log. ¼ Loganiaceae; Lor. ¼ Loranthaceae; Mim. ¼ Mimosaceae; Mor. ¼ Moraceae; Myrist. ¼ Myristicaceae; Myrs. ¼ Myrsinaceae; Myrt. ¼ Myrtaceae;

Ole. ¼ Oleaceae; Prot. ¼ Proteaceae; Rhamn. ¼ Rhamnaceae; Rub. ¼ Rubiaceae; Rut. ¼ Rutaceae; Sap. ¼ Sapotaceae; Sterc. ¼ Sterculiaceae; Ulm. ¼ Ulmaceae;

Verb. ¼ Verbenaceae.



(Xanthostemon eucalyptoides and Rapanea benthamiana), or

restricted and steady in others (Carpentaria acuminata).

DISCUSSION

Recruitment processes

In general, seedling recruitment of monsoon rain forest woody

taxa was derived mostly from wet season seed rain with very

limited input from soil seed banks. Regeneration through

clonal reproduction was negligible. At moist sites, seed rain

inputs were coincident with significant rain forest tree species

seedling recruitment, especially the stand dominants Syzygium

nervosum (PL) and Syzygium minutuliflorum (PS), but also

certain others (Lophopetalum arnhemica, Pouteria richardii and

Xanthostemon eucalyptoides; all at PS). Given that none of these

species possesses capacity for extended seed longevity (Table 3)

(Russell-Smith & Lucas, 1994), it follows that increases in

juveniles emanated from seed rain.

Few seedlings of a variety of other rain forest taxa were

recruited, despite relatively large seed rain inputs particularly

from pioneer species that are observed here or elsewhere (e.g.

Russell-Smith & Lucas, 1994) to possess capacity for extended

seed dormancy (e.g. Acacia auriculiformis, Alphitonia excelsa,

Alstonia actinophylla, Ficus spp., Livistona benthamii, Melasto-

ma affine, Nauclea orientalis, Terminalia microcarpa, Timonius

timon and Trema tomentosa). Despite the occurrence of low

intensity fires at three sites over the period of study,

disturbance/light conditions evidently remained insufficient

for promoting significant germination in these taxa.

Increases in wet season seedling densities observed at all four

sites were not consistently coincident with seed rain. Peak seed

rain inputs of woody rain forest taxa were observed in the wet

season at both perennially moist sites, whereas there was a

greater tendency for inputs at both seasonally dry sites to

extend into, or peak in the dry season. This implies that

germination of many woody taxa at seasonally dry sites is

delayed until the start of the ensuing wet season period. Similar

observations were made by Bach (2002) in her 3-year study of

monsoon rain forest flowering and fruiting phenology at

twelve moist (floristically analogous to PL) and four seasonally

dry (floristically analogous to SL) sites in the Northern

Territory, Australia. She found that while fruiting both at

moist and seasonally dry sites was concentrated in the wet

season, at seasonally dry sites fruiting tended to peak later

than, and extend into, the dry season. Elsewhere, although rain

forest fruit production has been observed to peak in the wet

season under seasonal climatic regimes (e.g. Terborgh, 1986;

Hopkins & Graham, 1989; Kinnaird, 1992), fruiting peaks in

dry periods are also reported (Frankie et al., 1974; Bullock &

Solis-Magallanes, 1990).

Further differences in seedling recruitment patterns were

observed at seasonally dry sites. Whereas recruitment from

seed rain (albeit limited) at SS was observed for the rain forest

canopy dominant, Allosyncarpia ternata, which possesses non-

dormant seeds, at SL none was observed for the dominant

(essentially woodland) species Callitris intratropica, despite the

observation that seeds collected in seed trays remained viable

for at least 6 months following seed fall. With the exceptions of

two species (Micromelum minutum and Stenocarpus verticis),

all other woody species occurring at SS and SL possess

dormant seeds. Observed seedling recruitment in these latter

taxa thus may be attributable to seed rain and/or dormant

seeds.

In comparison with observations here that dormant seed

banks of woody rain forest taxa were greater at moist sites,

Russell-Smith & Lucas (1994) observed from a snap-shot

survey of 34 northern Australian sites that greatest densities

were associated with seasonally dry sites (equivalent to SL;

mean, 17 species m)2 soil), then moist lowland sites with fine-

textured soils (equivalent to PL; mean, 13 species m)2 soil),

and least from moist (equivalent to PS) and seasonally dry

(equivalent to SS) sites with sandy soils (mean, 9 species m)2

0

500

1000

1500

2000 (a) seedlings

0

10

20

30

No.

indi

vidu

las

per

25 m

2 (b) saplings 50 – 200 cm

1 2 3 4 5 6 7 8 9 10Year

0

5

10

15

20

(c) saplings >200 cm

Figure 6 Decadal patterns in late dry season (a) seedling

(< 50 cm high), (b) sapling (50–200 cm high), and (c) tall sapling

(> 200 cm high) densities, 1993–2002, at each of four sites

where: SS, thin line; SL, dashed line; PS, bold line; PL, bold dashed

line.



soil). We note, however, that generalizations from one-off

studies may be fraught given the considerable temporal

variability in dormant seed banks evident at all four sites. As

in this study, dormant soil seed banks of woody rain forest taxa

were dominated by pioneers, especially figs. Similar observa-

tions concerning the dominance generally of pioneer taxa in

rain forest soil seed banks have been widely reported

(Garwood, 1989). Data presented here indicate that longevity

of dormant seed banks of woody monsoon rain forest taxa,

including figs, is expended under field conditions in less than

3 years.

Clonal reproduction was observed in only four species

(three occurring at seasonally dry sites) and, while large

numbers of clonal sprouts were observed in one vine species,

Ichnocarpus frutescens, the overall contribution of clonal

reproduction observed over 10 years was negligible. By con-

trast, in an earlier survey of sapling banks at 33 north

Australian monsoon rain forest sites (Russell-Smith, 1996),

76 of 249 woody rain forest taxa (31 tree species, 21 shrubs,

21 vines) exhibited clonal (e.g. rhizomatous) reproduction in

some situations at least. In that study, clonal species were

particularly prevalent at non-sandstone, seasonally dry sites

(equivalent to SL). Significant levels of clonal reproduction on

seasonally dry forest sites in other tropical regions have been

reported by Ewel (1977) and Lieberman & Li (1992). While

clonality would thus appear to be more commonly expressed,

0

20

40

60

80

100(a) SS (b) SS

0

20

40

60

80

(c) SL (d) SL

0

20

40

60

80

Pro

port

ion

of in

divi

dual

s (%

)

(e) PL

(f) PL

1 2 3 4 5 6 7 8 9 10

Year

0

20

40

60

80

100

(g) PS

1 2 3 4 5 6 7 8 9 10

(h) PS

Seedlings Saplings

Figure 7 Proportions of woody tree, shrub

and vine seedlings (< 50 cm height) and

sapling (> 50 cm) at each of four sites, 1993–

2002, where bold line ¼ tree species, thin

line ¼ shrub species, dashed line ¼ vine/

liane species.



if variously, in seasonal situations, it is notable that none of the

monsoon rain forest canopy dominants studied here exhibits

clonality.

Recruitment dynamics

Seedling densities of woody rain forest taxa exhibited

substantial intra- and inter-annual variability at all four sites.

Seedling densities were consistently smaller in late dry season

samples, principally reflecting mortality due to declining

substrate moisture, as well as direct fire impact at one site

(see below). Notably, dry season seedling mortalities were not

consistently greater at seasonally dry sites (e.g. 49% mean

mortality at PS vs. 47% at SL, combining data presented in

Fig. 5 for 1994 and 1995), presumably reflecting differential

moisture tolerances of component seedling taxa. From a 2-year

study at Pinkwae, Ghana, with two wet and dry seasons per

annum (1100 mm mean annual rainfall), Lieberman & Li

(1992) likewise observed that mortality of seedlings (defined as

< 2 years old) was greater in dry periods under highly seasonal

rainfall conditions.

The severe impact of an extensive, but low intensity fire in

1994 on seedling-sized recruitment of the SS stand dominant,

Allosyncarpia ternata, is salutary given that, despite observed

seed input and limited further burning, no recruitment into

the > 50 cm height class was subsequently observed. Allosyn-

carpia, like the majority of Australian monsoon rain forest

woody plants, is a strong resprouter (Russell-Smith & Stanton,

2002), although stems of all sizes are vulnerable to intense fires

(Bowman, 1994). From observational and experimental

studies, Fordyce et al. (1997) showed that all 18-month

Allosyncarpia seedlings died following burning, whereas over

0

40

80

120

160

50

100(i) SSAllosyncarpia (85/89)

0

10

20

30

40

50

60

50

(iii) SLVitex acuminata (16/19)

0

2

4

6

8

50

(a) Seedlings (b) Saplings

(iv) SLVitex acuminata

0

100

200

300

400

500

Den

sity

25

m–2

50

(v) PLSyz. nervosum (46/50)

0

1

2

3

50

Proportion of total density (%

)

(vi) PLSyz. nervosum

1 2 3 4 5 6 7 8 9 10Year

0

500

1000

1500

2000

50

(vii) PSSyz. minutuliflorum (58/64)

1 2 3 4 5 6 7 8 9 10Year

0

4

8

12

16

50

(viii) PSSyz. minutuliflorum

0

1

2

3

4

5

0

50

100(ii) SSAllosyncarpia

Figure 8 Densities, and proportional representation, of seedlings and saplings of dominant tree species at each study site. Note: (1) in each

graph, bold line ¼ seedling density, thin line ¼ proportion of all tree seedlings/saplings per site; (2) numbers in parentheses after species

names: first number refers to species’ proportion of stand basal area in 1993, and second number gives proportion as of 2002; (3) data for

Callitris intratropica (dominant at SL) not given since no seedlings or saplings were recorded over entire study period.



half of a 3-year-old seedling cohort (established initially under

nursery conditions) recovered from lignotuber resprouts.

Under the relatively benign fire disturbance, and consistent

rainfall seasonality conditions that existed over the decade of

study, densities of saplings increased overall at all sites. At

moist sites there was substantial recruitment also into the

> 5 cm d.b.h. stem class. Conversely, at seasonally dry sites,

only one individual was recruited into this class at SL, and four

stems were actually lost at SS. Such differential recruitment

rates highlight the relative vulnerability of monsoon rain

forests growing under seasonally dry conditions in the face of

contemporary north Australian fire regimes.

Comparisons with eucalypt-dominated savanna

Available data concerning the regeneration ecologies/syn-

dromes of both monsoon rain forest dominants and dominant

northern Australian savanna eucalypts are summarized in

Table 4, derived from a variety of mostly recent studies. Major

contrasts are as follows:

Flowering and fruiting phenology

Flowering and fruiting phenology is strongly seasonal in both

monsoon rain forest and savanna dominants (Williams et al.,

1999; Bach, 2002), although there are significant differences in

timing of major phenological events. Flowering peaks before

the start of the wet season in rain forest at perennially moist

sites, and with the onset of the wet at seasonally dry sites.

Fruiting is concentrated in the wet season at moist sites and

peaks later at seasonally dry lowland sites (Bach, 2002; this

study). Fruiting in Allosyncarpia also occurs in the mid wet

season (Russell-Smith, 1986; Fordyce, 1998). By contrast,

flowering and fruit maturation of eucalypts occurs in the dry

season, with peaks in mid and late dry seasons (Williams et al.,

1999). Seed fall and resultant germination early in the wet

season facilitates establishment during the wet season before

rapid drying out of upper soil layers in the following dry

season (Setterfield & Williams, 1996; Williams, 2003). Newly

recruited seedlings of savanna dominants thus enjoy a longer

first wet season growth period than monsoon rain forest

counterparts at moist sites, and for Allosyncarpia at least in

seasonal forests.

However, this phenological pattern also results in eucalypt

flowering coinciding with current peak fire activity in northern

Australian savannas (Setterfield, 2002; Russell-Smith et al.,

2003a). Low intensity fires in the early dry season can cause a

marked reduction in ovule development and seed production

(Setterfield & Williams, 1996; Williams et al., 2003b). More

intense late dry season fires reduce floral and fruit reserves

across all savanna tree functional groups by more than 50%,

for 2–5 years (Williams et al., 2003b). Such fires also coincide

with early stages of floral primordial development in species

that flower in the early wet season (e.g. Terminalia ferdinan-

diana and Corymbia porrecta), or seed maturation in others

(e.g. Eucalyptus tetrodonta). Fires thus have a major impact on

savanna tree floral phenology, and contribute to the rarity of

sexual regeneration in dominant savanna woody species

(Setterfield, 2002; Williams, 2003). Nothing is known about

the effects of fires on monsoon rain forest reproductive

phenologies, although presumably such impacts would be

negative as for savanna dominants.

Seeds and seed banks

Despite such fire effects, adult eucalypts and other canopy

subdominants are capable of flowering and producing seed

in the season following fire (Setterfield, 2002; Williams et al.,

2003b), and providing annual input to the regeneration pool

even at frequently burnt sites (Williams, 1997; Setterfield,

2002). Dominant savanna species have short seed develop-

ment times (< 4 weeks in E. tetrodonta, E. miniata). The

dominant eucalypts are non-serotinous (Williams et al.,

1999), and seed banks are transient (Setterfield, 2002;

Williams et al., 2005). By contrast, most monsoon rain

forest canopy species have long fruit maturation periods

(Bach, 2002), longer fruiting periods (2–10 months; Bach,

2002), provide an important resource for regional frugivores

(Price et al., 1999; Palmer et al., 2000), and are widely

dispersed (Russell-Smith & Lee, 1992). While seed banks of

diagnostic dominant species are notably transient in three

regional monsoon rain forest types, those of many

co-occurring species exhibit extended longevity, particularly

pioneer taxa and those inhabiting seasonally dry sites.

Seedling density

As with savannas elsewhere, seedling regeneration occurs if

conducive environmental/moisture conditions coincide with

seed fall (Setterfield & Williams, 1996; Higgins et al., 2000).

Due to observed rarity of eucalypt savanna seedlings, however,

there have been few attempts to determine their density. After

8 years of monitoring the effects of fire on savanna at Cape

Cleveland, northern Queensland, Williams et al. (2003a) noted

only one recruitment event of the dominant overstorey tree,

Corymbia clarksoniana. A recent wet season survey for

Eucalyptus miniata seedlings within a 5-m radius of the

canopy of fecund adult trees (i.e. in the area of maximum

seedfall; S. Setterfield, unpubl. data) provided a density

estimate of 1.3 seedlings ha)1 (P. Clifton, unpubl. data). By

contrast, seedling densities of monsoon rain forest dominants

are typically substantially higher (c. 1 to >> 1 m)2), albeit

temporally highly variable, within close proximity to adults

(Russell-Smith, 1996; Fordyce, 1998; this study). Seedling

establishment of dominants is observed almost annually, with

very substantial recruitment in mast years (Fordyce, 1998; this

study).

Seedling growth rates

Vailable, if limited, observations indicate that seedling

growth rates of savanna dominants are substantially greater



Tab

le4

Reg

ener

atio

nec

olo

gyo

fd

om

inan

ttr

eeta

xain

no

rth

Au

stra

lian

euca

lyp

t-d

om

inat

edsa

van

na

and

mo

nso

on

rain

fore

sts

Ch

arac

teri

stic

Eu

caly

pt

sava

nn

aR

efer

ence

sM

on

soo

nra

info

rest

Ref

eren

ces

Sexu

al

Flo

wer

ing

ph

eno

logy

Pri

mar

ily

dry

seas

on

Sett

erfi

eld

&W

illi

ams

(199

6),

Wil

liam

set

al.

(199

9)

Wet

seas

on

Bac

h(2

002)

Fru

itin

g

Ph

eno

logy

Pri

mar

ily

dry

seas

on

Sett

erfi

eld

&W

illi

ams

(199

6),

Wil

liam

set

al.

(199

9),

Bro

oke

r&

Kle

inig

(200

4)

Wet

seas

on

inp

eren

nia

lly

wet

rain

fore

st;

late

wet

seas

on

tod

ryse

aso

nin

seas

on

ally

dry

rain

fore

st

Bac

h(2

002)

,th

isst

ud

y

Tim

eo

fse

ed

avai

lab

ilit

y

Lat

ed

ryse

aso

nSe

tter

fiel

d&

Wil

liam

s(1

996)

,

Wil

liam

set

al.

(199

9),

Wil

liam

s(2

003)

Wet

seas

on

-ear

ly/m

idd

ryse

aso

nB

ach

(200

2),

this

stu

dy

Seed

qu

anti

tySo

me

seed

avai

lab

lean

nu

ally

Sett

erfi

eld

&W

illi

ams

(199

6),

Wil

liam

set

al.

(199

9),

Wil

liam

s(2

003)

Som

ese

edav

aila

ble

ann

ual

ly

(at

leas

tfr

om

som

e

ind

ivid

ual

s);

irre

gula

rly

mas

t

Bac

h(2

002)

,th

isst

ud

y

Maj

or

dia

spo

rety

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than those of monsoon rain forest taxa. The savanna

dominant E. miniata grows an average of 20 cm within

1 year in the absence of fire, with some individuals

reaching over 50 cm within 18 months of establishment

(S. Setterfield, unpubl. data; P. Clifton, unpubl. data). By

contrast, seedlings of monsoon rain forest dominants attain

< 10 cm (cotyledons or first leaves only) by the end their

first wet season, with occasional individuals (particularly at

moist sites) attaining 50 cm after 3 years (Fig. 8). Growth

rates in Allosyncarpia ternata are even slower (Fig. 8) and,

even in the absence of fire, recruits spend many years as

small multi-stemmed plants (Fordyce, 1998). As adults,

tree growth (measured as d.b.h. increment) in monsoon rain

forest taxa has been reported to exceed that of var-

ious savanna species, including E. tetrodonta (Prior et al.,

2004).

Resprouting capacity

Most savanna woody species (Lacey & Whelan, 1976) and

monsoon rain forest woody species (Bowman, 1991a; Russell-

Smith et al., 1998) exhibit the capacity to resprout from

lignotubers or from other underground and stem basal tissues

following death of aerial stems. The adaptive value of

sprouting in fire-prone environments is well recognized (e.g.

Bond & van Wilgen, 1996; Peterson & Jones, 1997; Bond &

Midgley, 2003). Lignotubers develop rapidly in eucalypt

savanna dominants, and resprouting from lignotubers has

been observed in seedlings < 6 months old as a response to

insect attack or other stress (S. Setterfield, pers. obs.; P. Clifton,

pers. obs.). There have been few observations of the fate of

seedlings of known age following fire, although Setterfield

(2002) suggested that 3% of E. miniata seedlings survived an

early dry season fire within their first year, and Williams

(2004) noted that 7% of Corymbia clarksoniana seedlings

survived fire within a year of germination. Williams et al.

(2003b) suggested that an inter-fire interval of 2 years, possibly

more, appears necessary for longer-term recruitment of

seedlings into the sprout bank. Data from the long-term

savanna fire experiment at Munmarlary, northern Australia,

illustrate that (1) fire-free intervals of at least 5 years are

required for release of non-eucalypt species from the under-

storey (< 2 m), (2) no significant release of understorey

eucalypts occurs in intact eucalypt-dominated savanna,

independent of fire frequency (Russell-Smith et al., 2003b).

Experimental canopy removal studies undertaken by Fensham

& Bowman (1992) indicate that eucalypt overstorey root

competition plays a significant role in suppression of smaller

size-classes.

By contrast, the capacity of seedlings and saplings of the

mesic monsoon rain forest dominants Syzygium minutuliflor-

um and S. nervosum to resprout after all but the mildest fires is

very restricted, given an absence of lignotubers or similar

tissues. Similarly, Allosyncarpia recruitment is highly vulner-

able to even low intensity fires within at least the first few years,

despite possessing lignotubers (this study).Tab

le4

con

tin

ued

Ch

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teri

stic

Eu

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ase

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8)



Clonal capacity

Many Australian savanna species, including various eucalypt

dominants, reproduce clonally via sprouting from roots

(i.e. root suckers), rhizomes or stolons (Lacey, 1974; Lacey

& Whelan, 1976). Such vegetative regeneration offers obvious

advantages for recovery from, and recruitment of new shoots

following, burning – possibly at minimal developmental cost

(Peterson & Jones, 1997). Clonal eucalypts are found mainly

in tropical savannas, and apparently rarely elsewhere (Gill,

1997). Eucalyptus tetrodonta is the only eucalypt that can

produce aerial leafy shoots from the growth of root buds.

This trait is also exhibited by Erythrophleum chlorostachys,

another common savanna co-dominant. Superficially similar

reproduction is seen in the bloodwoods (Corymbia porrecta,

C. ptychocarpa and C. jacobsiana) but the aerial leafy shoots

develop from shallow rhizomes (Brooker & Kleinig, 2004). By

contrast, none of the monsoon rain forest dominants at the

four study sites possesses this regenerative capacity.

In summary, by comparison with dominant savanna

eucalypts, dominant monsoon rain forest taxa recruit substan-

tially greater stocks of seedlings, but exhibit slower aerial growth

and development of resprouting capacity in early years, lack

lignotubers in mesic species, and lack capacity for clonal

reproduction. Given annual–biennial fires in many north

Australian savannas, the study underscores the relative vulner-

ability of monsoon rain forest assemblages, especially those

comprising slower growing species on seasonally dry sites. As

noted by Hoffmann (1998) and Setterfield (2002), the reliance

on sexual as opposed to vegetative reproduction places such taxa

at significant disadvantage in fire-prone savanna environments.

ACKNOWLEDGEMENTS

Particular thanks are due to Diane Lucas for ongoing assistance

with field sampling, the staff of Kakadu National Park for

substantial logistical assistance, the traditional Aboriginal

owners of Kakadu National Park for permission to access

study sites, and J.S. Bach for inspiration. Professors Pat

Werner, Gordon Duff and two anonymous referees made

helpful comments on the text.

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59. CSIRO, Darwin.

Williams, R.J., Cook, G.D., Gill, A.M. & Moore, P.H.R. (1999)

Fire regime, fire intensity and tree survival in a tropical

savanna in northern Australia. Australian Journal of Ecology,

24, 50–59.

Williams, R.J., Griffiths, A.D. & Allan, G.E. (2002) Fire regimes

and biodiversity in the wet-dry tropical landscapes of

northern Australia. Flammable Australia: the fire regimes and

biodiversity of a continent (ed. by R.A. Bradstock, J.E. Wil-

liams and A.M. Gill), pp. 281–304. Cambridge University

Press, Cambridge, UK.

Williams, R.J., Muller, W.J., Wahren, C.-H., Setterfield, S.A. &

Cusack, J. (2003b) Vegetation. Fire in tropical savannas: the

Kapalga experiment (ed. by A.N. Andersen, G.D. Cook and

R.J. Williams), pp. 79–106. Springer-Verlag, New York.

BIOSKETCHES

Samantha Setterfield is a plant ecologist who has been

researching plant reproductive ecology in tropical savannas for

15 years. Her current research projects include studying the

impacts of introduced grasses on Australian savannas, and the

effect of fire on native savanna grass and trees.

Jeremy Russell-Smith is a consultant ecologist with over

20 years experience in northern Australia. He coordinates fire

research programs of the Bushfires Council of the Northern

Territory and the Tropical Savannas Cooperative Research

Centre and, as well, is involved with other natural resource

management projects in Southeast Asia. He has an abiding

interest in the ecology, biogeography and management of

monsoon rain forests.

Editor: Pauline Ladiges



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