Habitat Segregation of Banksia Shrubs at Gibraltar Range National Park

2006·3.2 MB·English

by  Shanti Virgona

Habitat Segregation ofBanksia Shrubs at Gibraltar Range
National Park
Shanti Virgona, Glenda Vaughton and Mike Ramsey
Botany, School ofEnvironmental Sciences andNatural Resources Management, University ofNew England,
NSW
Aimidale 2351 ([email protected])
Virgona, S., Vaughton, G. andRamsey, M. (2006). Habitat segregation ofBanksia shrubs atGibraltar
RangeNationalPark. Proceedings oftheLinneanSocietyofNewSouth Wales 127, 39-47.
Eventsduringseedlingrecruitmentaffectspecies'distributions,causinghabitatsegregationofcongeneric
species within the same area. We documented the segregation of Banksia marginata and B. spinulosa
var neoanglica in adjacent swamp and woodland habitats at two sites by surveying adult and seedling
distributions. We also examined seed banks and seed characters as factors contributing to segregation.
Habitat segregation was pronounced, with 92% ofB. marginata adults located in swamps and 98% ofB.
spinulosa adults located in woodlands. Afterfire, 84% of5. marginata seedlings were in swamps, but 10
months laterthis increasedto 93%, indicatingthatalthough seeds dispersedto and germinated in adjacent
woodlands, most seedlings failed to establish. Seedlings of B. spinulosa were confined to woodlands,
indicatingthatseedsdidnotdisperseinto swamps orthat, iftheydid, seeds failedto germinateorseedlings
suffered early mortality. Canopy seed banks of both species were large (> 280 seeds/plant) and seeds
ofboth species possess membranous wings, allowing dispersal between habitats. Overall, neither limited
numbers of seeds nor limited seed dispersal are likely to cause habitat segregation. Instead, processes
occurring during earlyseedling growthareprobablymore influential.
Manuscriptreceived 1 May2005, acceptedforpublication 7 December2005.
KEYWORDS: Banksiamarginata,Banksiaspinulosa, fire,niche, Proteaceae, regeneration.
plants, providing substantial opportunities to explain
INTRODUCTION
the distribution patterns of different species (Grubb
1977).
Seedling recruitment is a critical stage in Habitat segregation of congeneric species in
the demography of plant populations. Seeds and distinct habitats within the same general geographic
seedlings typically experience high mortality rates, area is a common feature of coastal plains and
and events during recruitment potentially affect tableland areas of southern Australia (Siddiqi et al.
the distribution of species in the landscape (Harper 1972; Bowman et al. 1986; Keith and Myerscough
1977; Grime 1979; Silvertown and Chariesworth 1993; Myerscough et al. 1995; Clarke 2002), and
2001). When seeds are dispersed, they encounter a has been well documented in Banksia. On the Swan
variety ofabiotic and biotic conditions that affect the coastal sand plain near Perth, Banksia littoralis is
seed germination, seedling emergence, survival or restricted to swamp margins, whereas B. menziesii
growth stages ofthe life cycle. Interactions between andB. attenuataoccurmorewidelyindrierwoodland
these factors exert powerful effects on the spatial areas (Groom et al. 2001). Further north ofPerth on
patterns of recruitment, allowing regeneration to the Eneabba sandplain, B. hookeriana, B. prionotes
occur in some microhabitats but not others (Lament and B. attenuata are segregated along topographic
et al. 1989; Mustart and Cowling 1993; Schutz et al. gradients in the dune-swale system (Lamont et al.
2002; Castro et al. 2004). Grubb (1977) coined the 1989; Groeneveldetal. 2002). InNSW, onthecoastal
term "regeneration niche" to distinguish between the sand plains in the Myall Lakes area, B. oblongifolia
habitat conditions required for seedling recruitment and B. aemula are segregated into either wet heath
as opposed to adult survival and reproduction. The or dry heath occurring on the slopes and ridges,
regeneration niche is generally considered to be respectively (Myerscough et al. 1996). Finally, on
more complex than the niche experienced by adult thenorth coastofNSW,B. ericifolia is mostcommon
HABITAT SEGREGATION OF BANKSIA SHRUBS
in wet heath, while its congener B. aemula occurs in dispersal. Most seedling recruitment occurs in the
headland heath, dry heath and moist heath (Benwell first 12 months after fire.
1998). Although such patterns have been attributed Banksia spinulosa Sm. var. neoanglica A.S.
to physiological intolerance and competition at later George (5. cunninghamii Sieber ex Rchb. subsp.
life-cycle stages, increasing evidence points to the A; Harden 2002; hereafter B. spinulosa) is found
NSW
importance ofprocesses occurringduringrecruitment in northern and southern Queensland. Plants
in mediating segregation (Myerscough et al. 1996; have an underground lignotuber and are able to
Clarke et al. 1996; Williams and Clarke 1997). Using resprout after fire (i.e. are resprouting). Adult plants
m
field experiments Myerscough et al. (1996) and are multistemmed and grow to 2 in height. The
Clarke et al. (1996) demonstrated that segregation inflorescences are 6-15 cm long and bear hooked
of5. aemula andB. oblongifolia at Myall Lakes was styles. Flowers are self-compatible, but most seeds
related to processes operating during seed dispersal are outcrossed (Vaughton and Carthew 1993). The
when safe sites for seeds are required, and during follicles are strongly serotinous and have a single
establishment when resources are critical for early winged seed (Vaughton and Ramsey 2001). Seedling
seedling growth. recruitment occurs afterfire.
At Gibraltar Range National Park (GRNP),
sedge-heath communities occur in areas of impeded
Study sites
drainage on hillsides and on flats that are surrounded Two sites were chosen at GRNP that were
by a matrix ofsclerophyll woodland. Segregation of
burned by bushfires during November 2002: Waratah
congenericspeciesintoeithersedge-heathorwoodland
m
habitats is common (Sheringham and Hunter 2002). Trig (WT: 29^29' S, 152^19' E, 1050 a.s.l.) and
Here we quantified the distribution ofboth adult and Surveyors Creek (SC: 29o32' S, 152018' E, 1044 m
seedling populations of Banksia marginata and B. a.s.l.).Thesesitesoccuronseparatedrainagechannels
spinulosa var. neoanglica to determine whether the and are approximately 5 km apart. Both sites are
two species are segregated and whether processes floristically and structurally similar, comprising two
operating during recruitment mediate this pattern. associated vegetation types, sclerophyll woodland
We found pronounced segregation that is established and sedge-heath. The woodlands are dominated by
during recruitment, and to explain the observed Eucalyptus olida with a diverse shrub understorey.
distribution pattern we examinedthe seed production The sedge heaths are dominated by Lepidosperma
andseeddispersalstagesofthelifecycle. Specifically, limicolawith emergent shrub species occurring along
we examined seed bank size and seed attributes to the swamp margins (Sheringham and Hunter 2002).
determine whether both species produce viable seeds
and whether seeds have the potential to disperse
Distribution ofadults and seedlings
between habitats.
Forboth species, the distributions ofadultplants
were surveyed 3 months after the fire. Burnt adults
MATERIALSAND METHODS retained their cones, and were readily identified.
Seedling distributions were surveyedtwice, at 18 and
28 months after the fire. Seedlings ofthe two species
Study species were identified by differences in cotyledon and leaf
Banksia marginata Cav. is widely distributed traits.
along the coast andranges ofsouth-easternAustralia. We used stratified belt-transects for the adult
At GRNP, the species is at the northern limit of its and seedling surveys. Different transects were used
range (Harden2002). In this area, plants are killedby for each survey, ensuring samples were independent
m
fire andpopulations rely solely on seeds for recovery ofeach other. Transects sparmed 40 ofthe swamp
m
(i.e. areobligate seeders,VaughtonandRamsey 1998; habitat and 60 ofthe woodland habitat, and were
BensonandMcDougall2000).Adultplantsaresingle- haphazardlyplaced.Thisstratificationensuredthatwe
m
stemmedandgrowto2 inheight.Theinflorescences surveyed the distributions ofboth B. marginata and
are 5-10 cm long andbear straight styles. Flowers are B. spinulosa. Transects excludedthewetterregions of
self-compatible and two seeds are usually formed the swamp where banksias rarely occur (Sheringham
per follicle (Vaughton and Ramsey 1998, 2006). At and Hunter 2002). For adult and seedling surveys at
m m
this site follicles are strongly serotinous and open each site, four, 5 wide transects and six, 1 wide
only after exposure to high temperatures during transectswereused, respectively. Withintransects, all
fires. Seeds have amembranous wing, allowing wind banksiaswerecountedandrecordedasbeing ineither
40 Proc. Linn. Soc. N.S.W., 127, 2006
S. VIRGONA, G. VAUGHTON AND M. RAMSEY
Similarly, the number ofseeds perplantwas
estimated as the product ofcones perplant and
seedspercone.Plantdensitywasassessedusing
five haphazardly placed quadrats in each habi-
tat at each site. In swamp and woodland habi-
m m m m
tats, 5 X 5 and 10 x 10 quadrats were
used, respectively, to ensure both species were
adequately represented. To assess the number
ofcones per plant, we counted cones with fol-
licles on 50 plants ofeach species per site. For
the number ofviable seeds, we collected five
0-20 21-40 41-60 61-80 81-100
' ' cones fi-om each of 20 plants of each species
Transect locations (m) per site prior to the fire inAugust 2002. Seeds
"^
Swamp Woodland were extracted in April 2003 by heating cones
for30 min inan oven set at 110 °C. Seedswere
Figure 1. Vegetation profile oftlie swamp and woodland deemed viable if they were intact and plump.
association and the placement ofstratified belt-transects
used to examine tlie distribution ofB. marginata and B. Seed dispersal
WT
spinulosa at the SC and sites. The illustration is rep- To assess the dispersal potential of seeds,
resentative of the vegetation and is not drawn to scale. seed mass:seed area ratios (i.e. wing loading)
were calculated; larger wing loadings imply
shorter potential dispersal distances. Seeds
the swamp orwoodland habitat (Fig. 1).
were weighed and measured with the seed and wing
Toassesshabitatsegregationof5. marginataand
B. spinulosa, we calculated an analysis of deviance intact, using three seeds fi-om each of 10 plants per
using a logit model with a binomial error term and species fi-om each site. Seeds were weighed to the
a logit link fiinction. The three explanatory variables nearest 0.1 mg, and seed areawas determinedusing a
leafarea meter (A TArea Meter, Delta- T Devices).
were site (SC, WT), plant age (adults, seedlings at
18 and 24 months after the fire) and habitat (swamp,
Seed bank and dispersal analyses
woodland). The response variable was the number of
Data were analysed with two-way ANOVAs,
plants ofone species expressed as aproportion ofthe
with species as a fixed factor and site as a random
sum ofthe number ofplants ofboth Banksia species.
factor. When the site x species interaction was not
With this approach, the explanatory variables and
their interactions are interpreted as their effect on the significant{P>0.2), itwaspooledwiththe errorterm,
response variable in a similar fashion to ANOVA. In resulting in a more powerful test of the differences
between species (Quinn and Keough 2002). When
preliminary analyses, the three-way interaction (site
the site x species interaction was significant, we
x habitat x age) and the site x age interaction were
calculated tests of simple main effects comparing
not significant andwere omitted fi-om the final model
{P > 0.50). We also calculated the percentage ofthe the species at each site. To meet assumptions of
ANOVA, plant density, the numbers of cones, seed
total deviance explained by each term. Because the
mass and wing loadings were logio transformed.
age x habitat interaction was significant in the final
Other variables did not require transformation.
model, we also examined how the distributions of
adults and seedlings differed with respect to habitat
3x2
for each species at each site using contingency RESULTS
A
tables (G-tests). significant G-test indicates that the
distribution ofplants ofthe different ages depends on
habitat. WeusedasequentialBonferroni correctionto Distribution ofadults and seedlings
account for multiple tests. Of the 3610 plants found on transects, 15.5%
and 61.7% were B. marginata adults and seedlings,
respectively, and 3.2% and 19.6% were B. spinulosa
Seed bank
We estimated the seed bank (seeds/m2) of madoudletsl ianncdlusdeeeddltihnegst,hrreeespmeacitinveelfyf.ectTsh,eanfdinatlheloaggiet
each species as: (plants/m ) x (mean number of
X habitat and the site x habitat interactions (Table
cones/plant) x (mean number of viable seeds/cone).
1). The significant age x habitat interaction indicates
Proc. Linn. Soc. N.S.W., 127, 2006 41
HABITAT SEGREGATION OF BANKSIA SHRUBS
Table 1. Analysis of deviance (\ogit model) examining the dis- numerous than B. spinulosa plants
tribution of Banksia marginata and B. spinulosa adults and (Fii7= 173.67, P < 0.001; Table 3).
seedUngs (18 and 28 months after the fire) in swamp and For the number ofcones perplant and
woodland habitats at the SC and WT sites (n = 3610 plants). viableseedspercone,thesitexspecies
Habitat explained 97.7% of the total deviance. The site x interaction was significant (cones:
habitat x age and the site x age interactions were not signifi- Fi,i96= 16.85,P<0.001; seeds: Fii9g=
cant (P > 0.50), and they were omitted from the final model. 21.15,P<0.001),andweexaminedthe
simple main effects ofspecies at each
site. The number of cones produced
Source df A Deviance
by B. marginata was significantly
Site 32.71 < o.oooT^ greater than B. spinulosa at both sites
(WT: Fj 196= 103.49, P < 0.001; SC:
Habitat 2505 < 0.0001 Fi 196 =' 19.08, P < 0.001; Table 3).
For the number of seeds per cone, B.
Age 11.33 0.003
marginata produced more seeds than
WT = P
Age X habitat 12.34 0.002 B. spinulosa at (Fi ^^ 24.25,
< 0.001), but at SC seed production
Site X habitat 1 2.97 0.085 of both species was similar (Fj =
^g
P =
2.49, 0.109; Table 3). Overall, B.
that the relative frequencies of adults and seedlings marginata plants produced about 9-
varied between the two habitats (Table 1, Fig. 2). fold and 1.4-fold more seeds than did B. spinulosa
WT
Although the main effects of site, age and habitat plants at and SC, respectively. Similarly, the seed
were significant, habitat singulariy explained 97.7% bank (seeds/m ) of B. marginata was 182-fold and
WT
of the deviance in the model (Table 1), indicating 18-fold greater than that ofB. spinulosa at and
that the two species exhibited pronounced habitat SC, respectively, the large differences resulting from
segregation. Overall, most B. marginata plants ofall differences inplant density.
ages were found in the swamps (> 84%) and most
B. spinulosa plants were found in the woodland (> Seed dispersal
95%). For seed mass and seed area, the site x species
Fori?, marginata, adultandseedlingdistributions interactionsweresignificant(seedmass:Fiiig=11.13,
differed significantly at both sites (Table 2, Fig. P< 0.001; seed area: Fi 11^= 15.06, P < 6.001), and
2). Seedlings were distributed more widely than we examined simple main effects of species at each
adults 18 months after the fire, but by 28 months, site. Seeds ofB. spinulosaweighedatleast22%more
=
the distributions were similar. At 18 months, about than B. marginata seeds at both sites (WT: Fi ug
84%) and 16%) ofseedlings were found in the
swamps and woodlands, respectively, but 10 Table 2. Results of 3 x 2 contingency analyses examining
months later, about 93% and 1% ofseedlings the effects ofhabitat on the distribution ofplants ofdiffer-
were in the two habitats, respectively. ForB. ent ages. We compared the distributions ofadults (A) ver-
spinulosa, adult and seedling distributions at sus seedlings surveyed 18 and 28 months after the fire (SI
WT did not differ significantly (Table 2, Fig. and S2, respectively) between swamps and woodlands for
WT
2). At SC, however, adults were distributed Banksiamarginata andB. spinulosaatthe SC and sites.
more widely than seedlings (Table 2, Fig. G- and P-values are presented. G-values are significant
2). About 5% of adults were located in the following Bonferroni correction at P = 0.0125. All df = 2.
swamp, whereas > 99%) seedlings were
confinedto the woodland. Population B. marginata B. spinulosa
m
mH
Seed bank Avs Si vs S2 Avs Si vs S2
2 U
For plant density (plants/m ), the site x SC 1 17.54 10.07
species interaction was not significant (Fi,i6 "
= 0.48, P > 0.450), and it was pooled with p 0.0002 0.0065
the error term for the final analysis. Sites
WT G
did not differ (F, = 0.07, P > 0.750), but 23.27 0.15
j^
B. marginataplants were about 10-foldmore
P < 0.0001 0.926
42 Proc. Linn. Soc. N.S.W., 127, 2006
S. VIRGONA, G. VAUGHTON AND M. RAMSEY
1 1
SC adults WT adults
0.8-1 0.8-1
0.6
I 0.4 fh 0.4-
^
ffi m
0.2-1 0.2-^
~
I I i
In In
WT
SC seedlings 18 months seedlings 18 months
0.8 0.8
d
o
0.6 0.6-
o
0.4 0.4
-j
0.2 0.2
Q
—— —— — —— ——
V
r • T' ' T- T^ r ' T-
1-, 1-,WT
SC seedlings 28 months seedlings 28 months
i-
0.8 0.8-
0.6
O
0.4
a,
0.2-1 0.2
a
jn
^ ^
f .# J' <^ ^^
^ .A'-f .s^J' .uSJ' ^^ *" ^
Intervals (m) Intervals (m)
Figure 2. Distribution of Banksia marginata (open) and B. spinulosa (shaded) adults and seed-
WT
lings at the SC and sites. Adults were surveyed 3 months after the fire, and seedlings were sur-
veyed 18 and 28 months after the fire. Data are mean proportions of plants (± SE) at 20 m in-
m
tervals based on four transects for adults and six transects for seedUngs. The 0-20 and 21-40
m
intervals were in swamp habitats and the other three intervals were in woodland habitats.
Proc. Linn. Soc. N.S.W., 127, 2006 43
HABITAT SEGREGATION OF BANKSIA SHRUBS
WT
Table 3. Seed bank characters forBanksia marginata and B. spinulosa at the SC and sites. Data
are means ± SE. m
^1
WT
Characters
t
B. marginata B. spinulosa B. marginata B. spinulosa
2
Plants/m 0.9 ±0.2 0.1 ±0.1 1.0 ±0.3 0.1 ±0.1
Cones/plant 17.2 ±1.8 9.2 ±1.2 45.8 ±4.5 9.4 ± 0.9
Seeds/cone 24.4 ±3.8 33.3 ±3.8 57.7 ±3.5 30.0 ±4.7
Seeds/plant 420 ± 72 306 ±45 2642 ± 220 282 ±47
2
Total seeds/m 386 ±53 21±2 2736 ±271 15±2
15.74, P< 0.001; SC: Fiiig= 75.42, P< 0.001; Table concentrated in the swamp margins, whereas B.
4). By contrast, B. marginata seeds were at least 11% spinulosa plants were located in the woodland. In
greater in area than B. spinulosa seeds at both sites B. marginata, seedlings were more widely dispersed
(WT: Fi 1,6= 51.47, P< 0.001; SC: Fj ,i6= 2.84, P< than adults 18 months after fire, but by 28 months
0.01; Table 4). the distribution of seedlings had contracted so that
For wing loading, the site x species interaction it did not differ from that of adults. In B. spinulosa,
was not significant (Fj ],g = 1.51, P > 0.20), and it virtually all seedlings were confined to the woodland
was pooled with the error term for the final analyses. habitats. Our results are consistent with other studies
No differences in wing loadings were found between showing the importance ofthe regeneration niche in
sites (F, = 2.04, P = 0.156). Wing loadings of5. determining patterns of segregation of congeneric
117
spinulosa seeds were significantly greater than the species (Lamont et al. 1989; Mustart and Cowling
wing loadings ofB. marginata seeds, indicating that 1993; Myerscough et al. 1996; Clarke et al. 1996;
B. spinulosa shoulddisperse shorter distances thanB. Williams and Clarke 1997; Schutz et al. 2002).
marginata (Fj n^= 201.06, P< 0.001; Table 4). In B. marginata, processes operating during
seedling establishment appear to mediate habitat
segregation. The wider distribution of5. marginata
seedlings than adults 18 months but not 28 months
DISCUSSION
after fire, indicates that seeds dispersed into the
woodland and germinated, but seedlings failed to
Our results showed that B. marginata and B. establish. Other studies have shown that recruitment
spinulosa were segregated into different habitats, ofBanksia seedlings is strongly influencedby abiotic
and that this pattern was established during seedling conditions, especially drought (Lamont et al. 1989;
recruitment. Adult B. marginata plants were
Table 4. Seed characters relevant to dispersal forBanksia marginata andB. spinulosa at the
SC and WT sites. Data are means ± SE (n = 30 seeds).
Seed mass Seed area Wing loading
Site Species
("ig) (cm (mg/cm
) )
SC B. marginata 8.44 ±0.31 0.72 ± 0.04 11.95 ±0.49
B. spinulosa 12.85 ± 0.44 0.65 ± 0.07 19.56 ±1.04
WT
B. marginata 8.85 ±0.22 0.85 ±0.03 10.64 ±0.33
B. spinulosa 10.81 ±0.32 0.57 ± 0.04 18.51 ±1.07
44 Proc. Lirni. Soc. N.S.W., 127, 2006
S. VIRGONA, G. VAUGHTON AND M. RAMSEY
Myerscough et al. 1996; Lamont and Groom 1998). the swamps. Although B. spinulosa seeds had larger
At GRNP, soils in both the swamp and woodland wing loadings (greater mass but smaller area) than
habitats are of granitic origin. However, the swamp B. marginata seeds, this may not overly affect their
soils are fine textured and poorly drained compared dispersal potential. Using wind-tunnel and seed-
with the more well-drained soils of the woodland release experiments, Hammill et al. (1998) reported
(Virgona 2004). Segregation therefore may be driven thatBanksiaseedsweighingbetween 9 mgand70mg
by adaptation to the particular abiotic conditions dispersed similar distances. They found that seeds of
in the swamp and intolerance to conditions in the Banksia species with similar characteristics to those
woodland. Specifically, compared with B. spinulosa, of5. spinulosa andB. marginatawere mostabundant
B. marginata seedlings may be less tolerant of within 2-3 m of parent plants, but were readily
m m
fluctuating levels ofsoil moisture andthus the overall dispersed 9-12 and occasionally up to 40 from
drier soils in the woodland. Manipulative field and parents. Assuming that B. spinulosa seeds behave as
glasshouseexperimentsarenowneededto investigate in Hammill's study, the absence of seedlings in the
this possibility. swamps is unlikely to be due to the inability ofseeds
Asexpectedofanobligateseederthatreliessolely to disperse the short distance from the woodland to
on seeds for recruitment (Lamont and Groom 1998; the swamp margins.
Lamont andWiens 2003), B. marginata maintained a At SC, about 5% of B. spinulosa adults were
large canopy seedbank. ComparedwithB. spinulosa, located in the swamp, but no seedlings were found
adult5. marginataplantshad2-4timesmoreconesper in this habitat. These adult plants had canopy seed
plant, andatWT,moreviable seedspercone. Banksia banks and would have released their seeds directly
marginata adults also occurred at higher densities into the swamps. The lack ofseedling establishment
than did B. spinulosa, resulting in seed densities that in the swamps may be due to the inability of seeds
were 18-182 times greater in their preferred habitat. to find safe sites. At Myall Lakes, B. aemula, a dry
High seed densities provide maximum opportunities heath species, failed to establish in nearby wet heath,
for seedling recruitment and also allow colonisation even though experiments showed that seedlings were
of new sites, as evidenced by the occurrence of B. able to grow inthis habitat. The lack ofestablishment
marginata seedlings in woodland habitat 18 months in wet heath under natural conditions was attributed
after fire. Populations of obligate seeding plants, to insufficient soil disturbance, which reduced safe
however, are susceptible to either short or very long sites for seeds (Myerscough et al. 1996; Clarke et al.
fire intervals that can decrease the amount of stored 1996). Little or no recruitment in the swamps would
seeds for recruitment and hence threaten population be expected ifB. spinulosa seeds experience similar
persistence (Morrison et al. 1995; Enright et al. problems infinding safe sites.
1996). The effect of fire on demographic processes If seeds of B. spinulosa germinated in the
may therefore interact with other abiotic and biotic swamps, but seedlings died shortly afterwards, then
factors influencing segregation in fire-prone heaths they would not have been present when we surveyed
andwoodlands such as those occurring at GRNP. the swamps 18 months after fire. Early mortality
InB. spinulosa, > 99% ofseedlings were located couldhavebeenmediatedby abiotic conditions inthe
in the woodland 18 months after the fire. The lack swamps. Compared with B. marginata, seedlings of
ofrecruitment into swamps indicates that either seed B. spinulosa may lack the physiological capacity to
availability was limited, seeds did not disperse into cope with the poorly draining and often waterlogged
the swamps or seeds dispersed, but seedlings failed swamp soils. Further, early seedling mortality could
to establish. Although at present we are unable to result from competitive exclusion. Seedlings of B.
distinguishbetweenthesepossibilities,we suspectthe spinulosa grow more slowly than seedlings of B.
latter. First, recruitment by B. spinulosa in swamps marginata (Virgona 2004), and they may be unable
was unlikely to be limited by seed availability. to compete with the rapidly resprouting sedges
Compared with other resprouting Banksia species, in the swamps. Despite these potential obstacles,
adult B. spinulosa plants maintained a large store of establishment of B. spinulosa in the swamps must
seeds intheircanopypriorto thefire (~ 300 seeds for occuroccasionallyasevidencedatSCbythepresence
B. spinulosavs< 16 seeds, n= 7 species, Lamont and of the small number of adult plants in this habitat.
Groom 1998). Further, seedling recruitment occurred Given the ability ofB. spinulosa plants to persist by
in woodland but not adjacent to swamp habitats, resprouting, seedlings onlyneedtobe recruitedrarely
indicatingthatseedavailabilitywasadequate. Second, to maintain currentplant densities.
the inability ofseeds to disperse fi^om the woodland
is unlikely to account for the absence ofseedlings in
Proc. Linn. Soc. N.S.W., 127, 2006 45
HABITAT SEGREGATION OF BANKSIA SHRUBS
ACKNOWLEDGEMENTS (UniversityofNew SouthWalesPress, Sydney).
Harper, J.L. (1977). 'Populationbiology ofplants'.
We thank M. Campbell and P. Clarke for comments on the (Academic Press: London).
manuscript and thank S. Cairns for providing statistical Keith, D.A. andMyerscough, RJ. (1993). Floristics and
advice. Financial support was provided by UNE and a soil relations ofupland swamp vegetationnear
BotanyNCW Beadle Scholarship to S.V. Sydney.AustralianJournalofEcology 18, 325-344.
Lamont, B.B., Enright,N.J. andBergl, S.M. (1989).
Coexistence and competitive exclusionofBanksia
hookeriana inthepresence ofcongeneric seedlings
along atopographic gradient. Oikos56, 39-42.
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