Flood variability and spatial variation in plant community composition and structure on a large arid floodplain

ARTICLE IN PRESS Journal of Arid Environments

Journal of Arid Environments 60 (2005) 283–302 www.elsevier.com/locate/jnlabr/yjare

Flood variability and spatial variation in plant community composition and structure on a large arid floodplain S.J. Capona,b,1, a

Cooperative Research Centre for Freshwater Ecology, Australia Centre for Riverine Landscapes, Griffith University, Nathan, Ql 4111 Australia

b

Received 26 February 2003; received in revised form 29 March 2004; accepted 9 April 2004 Available online 19 June 2004

Abstract Spatial variation in plant community composition and structure in relation to flood frequency was explored on a large hydrologically variable arid floodplain in central Australia. Flooding was found to have an overriding effect on species richness, total cover and cover amongst most major plant groups. At a landscape scale, frequently flooded sites were similar while rarely flooded sites were more divergent from each other, suggesting that local and regional impacts on diversity vary across the flooding gradient. Overall, the results indicate that variable flooding plays an important role in maintaining spatial heterogeneity in plant community composition and structure in this arid floodplain landscape. r 2004 Elsevier Ltd. All rights reserved. Keywords: Drylands; Wetlands; Vegetation dynamics; Zonation; Australia; Cooper Creek

1. Introduction In large temperate and tropical floodplains, spatial variation in plant community composition and structure is determined primarily by flooding (Hupp and Tel: +61-3-9905-5608; fax:+61-3-9905-5613. 1

E-mail address: [email protected] (S.J. Capon). Present address: School of Biological Sciences, Monash University, Clayton, Vic. 3800, Australia.

0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.04.004

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Osterkamp, 1985; Walker et al., 1986; Menges, 1986; Blom et al., 1990; Pautou and Arens, 1994; Trebino et al., 1996; Hughes and Cass, 1997; Ferreira, 1997; Ferreira and Stohlgren, 1999). These floodplains receive regular, often annual, seasonal flood pulses that cause predictable changes in the abiotic character of plant habitats, e.g. soil anoxia and moisture availability. Plant species inhabiting these environments usually posses attributes which enable either tolerance of these changes, through physiological or morphological traits (e.g. ability to develop adventitious roots), or avoidance via life history traits (e.g. timing of reproduction) (Blom et al., 1990; Blom and Voesenek, 1996). Given a potential pool of species, plant community composition at any position on a floodplain therefore reflects both local hydrological conditions and the flood tolerance of individual species. Variations in flood frequency, duration and depth create complex spatial gradients across floodplains, often related to elevation or distance from the channel, along which levels of resource availability (e.g. soil moisture) and stress (e.g. soil anoxia) vary (Huston, 1994). Distinctive plant communities can often be found at predictable locations along these gradients (Junk et al., 1989). In frequently flooded areas, plant communities are structured predominantly by abiotic factors (Lenssen et al., 1999) and are generally dominated by flood tolerant and annual species which can complete their life cycles between flood events (Menges, 1986; Bren, 1992; Trebino et al., 1996). Biotic factors (e.g. competitive exclusion) have been found to be more important in determining plant community composition in floodplain areas that are rarely flooded (Blom et al., 1990; Lenssen et al., 1999). This zonation of vegetation along gradients of flood frequency or duration is often perceived as a spatial representation of successional phases with early successional communities being maintained in dynamic equilibrium in frequently flooded areas (Walker et al., 1986; Junk et al., 1989; Gregory et al., 1991; Trebino et al., 1996). In many cases, maximum plant species richness occurs in areas of intermediate flood frequency (Robertson et al., 1978; Wilson and Keddy, 1988; Bornette and Amoros, 1996; Pollock et al., 1998) as predicted by the Intermediate Disturbance Hypothesis (Connell, 1978). Unlike temperate and tropical floodplains, large arid floodplains rarely receive regular flood pulses, their salient feature being hydrological variability (Walker et al., 1995; Puckridge et al., 1998, 2000). Large arid floodplain river systems of central Australia, for example, are considered to be amongst the most hydrologically variable in the world (Puckridge et al., 1998, 2000) and conditions shift between extended periods of drought and large flood events which occur unpredictably both temporally and spatially. Flood pulses in these catchments vary not only in magnitude or duration but also in their seasonal timing with large events occurring historically in both summer and winter months (Walker et al., 1995). Junk et al. (1989) suggest that floodplain organisms are unlikely to exhibit adaptations to such highly variable flood pulses. Given the high hydrological variability, factors other than flooding (e.g. soil characteristics, local rainfall, etc.) may be more important in determining spatial variation in plant community composition and structure in large arid floodplains. Whilst flood-related spatial gradients are well described for both woody and herbaceous vegetation in many temperate (e.g. Robertson et al., 1978; Menges, 1986;

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Blom et al., 1990; Pautou and Arens, 1994; Trebino et al., 1996) and tropical floodplains (e.g. Bowman and McDonough, 1991; Ferreira, 1997; Ferreira and Stohlgren, 1999), few studies have explored the occurrence of similar patterns in arid regions. These few have tended to focus predominantly on tree distributions (e.g. Hughes, 1988; Stave et al., 2003) despite herbaceous plant life often being more widespread in these environments (Higgins et al., 1997). Consequently, the role of flooding in determining spatial patterns in plant community composition and structure of hydrologically variable arid floodplains is poorly understood. This paper investigates the effects of flooding on spatial variation in plant community composition and structure on a large arid floodplain in central Australia: the Cooper Creek floodplain. Results are presented from a large-scale spatial survey of vegetation from 45 sites distributed across three broad flood frequency zones and spread along a longitudinal gradient stretching over 300 km. The major aims were to determine if, and how, plant community composition and structure varies spatially in response to flood frequency or whether local factors such as rainfall or soil type that might vary on a longitudinal gradient are more significant in this hydrologically variable arid floodplain. In particular, it was sought to identify spatial patterns in specific groups of plants and their relationships with flooding.

2. Materials and methods 2.1. Study area The Cooper Creek catchment is located in the Lake Eyre Basin of central Australia (Fig. 1). The majority of the catchment receives less than 400 mm average annual rainfall and is therefore predominantly arid or semi-arid (Davies et al., 1994). Hydrologically, the Cooper Creek is one of the world’s most variable rivers (Puckridge et al., 1998) and annual discharges fluctuate between zero flow and huge flood pulses (Fig. 2). Between 1951 and 1952, for example, 21 months of no flow occurred whilst in 1974, 23 million ML were recorded at Currareva (Fig. 1.) with a highest instantaneous flow of over 25,000 m3 s
1 and a flood peak height of 8.48 m at Windorah (Fig. 1) (Queensland Department of Natural Resources, 1998). When high flows do occur, widespread inundation is facilitated by a network of braided channels and the catchment’s extremely low topographic gradient. Due to the variability in flood pulse magnitudes (Fig. 2.), spatial inundation areas vary greatly between different sized flood events resulting in floodplain areas that are flooded at contrasting intervals (Fig. 1, Table 1). Some areas, for example, are likely to be flooded reasonably frequently, i.e. once every 2–5 years, while others, at the floodplains edge, may be inundated less than once every 10 years. 2.2. Site selection The study focused on the area of floodplain between Currareva and the border of Queensland and South Australia (Fig. 1). Three broad flood frequency zones were

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Fig. 1. Map of the study area illustrating location of study sites, longitudinal regions and flood frequency zones. Inset shows location of study area within the Lake Eyre Basin, Australia. N.B. The low flood frequency zone corresponds to the combined inundation extents of the 1990 and 1991 flood events. The medium flood frequency zone is equivalent to the area inundated by the 1986 event and the high flood frequency zone to the 1984 event (see Table 1).

delineated using maps of inundation extents obtained from the Queensland Department of Natural Resources. These were created from spectral analyses of Landsat TM images taken following four flood events of different magnitudes

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Fig. 2. Annual discharge of the Cooper Creek at Currareva gauging station between 1958 and 1998 (shown in millions of Megalitres). N.B. No discharge data are available after this period due to the closure of the gauging station.

Table 1 Hydrological characteristics and inundation areas of four flood events of different sizes in the Cooper Creek below Currareva (Source: Queensland Department of Natural Resources, 1998) Date of flood peak at Currareva

Peak height at Currareva (m)

Peak discharge (mL/day)

Area of inundation (km2)

Estimated flood frequencya

1984 1986 1990 1991

3.90 6.32 7.95 6.70

26,100 178,000 1,460,000 457,000

3400 9000 25,900 15,300

1 in 2–5 years 1 in 5–10 years p1 in 10 years p1 in 10 years

(December) (February) (April) (February)

a Broad estimates of flood frequency are based on anecdotal evidence provided by landholders in the catchment as hydrological data for the catchment are extremely limited and are intended as a rough indication only.

(Fig. 1, Table 1). The dates were selected as being representative of flood events of broadly differing frequencies (Queensland Department of Natural Resources, 1998), although due to the lack of accurate hydrological data for the catchment these can only be approximated (Table 1). For the purposes of this study, floodplain areas which were inundated by all four events were classified as the high flood frequency zone. Areas which were inundated by the three larger events, i.e. 1986, 1990 and 1991, but not the smallest event of 1984, were designated the medium flood frequency zone. The low flood frequency zone then included those areas only inundated by the two larger events of 1990 and 1991 (Fig. 1). In order to survey areas that potentially differed in factors other than flood frequency (e.g. soil type and local rainfall), the study area was further divided into three longitudinal regions: upper, middle and lower. Five sites were selected from each flood frequency zone within each longitudinal region providing a total of 45 sites (Fig. 1). Sites were selected on the basis of accessibility using maps of farm tracks.

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2.3. Vegetation survey Most vegetation on the Cooper Creek floodplain is classified as short grass and forb associations (Boyland, 1984). A shrub layer comprising Muehlenbeckia florulenta (lignum) and Chenopodium auricomum (Queensland bluebush) is also present in places. Woody vegetation is mainly restricted to channel margins, although scattered Eucalyptus coolabah (coolibah) and Acacia cambagei (gidgee) also occur on the floodplain. As trees were found to be extremely rare during this survey and are likely to respond to the flow regime at different temporal scales from herbaceous vegetation due to their longer life-spans (Menges, 1986; Pautou and Arens, 1994), they were excluded from consideration in this study. The field survey was conducted in September and October, 2001. Prior to the survey, all sites were last inundated in March 2000 by a flood pulse comparable in size to the 1990 event (Fig. 1, Table 1). A smaller flood event also occurred in early 2001 and it is likely that many of the sites in the high flood frequency zone were additionally inundated by this event prior to the field survey. Consequently, the time since last inundation was equivalent between all sites in the medium and low flood frequency zones, but less in the sites from the high flood frequency zone. All sites, however, had experienced 6 months of drought prior to the survey. In the field, sites were located accurately using a Global Position System (GPS) and Geographical Information System (GIS) on a lap-top computer. At each site, tape measures were used to delineate 50 m  50 m quadrats. Ten 1 m  1 m quadrats were then randomly selected from within this area. Care was taken to position these beyond the area directly influenced by the canopy of any trees if present. Within the 1 m  1 m quadrats, all plant species were then identified and their percentage of cover recorded. 2.4. Data analysis Prior to any data analysis, species were assigned to major plant groups based on life-form (i.e. shrubs, sub-shrubs, monocots and forbs) and life-span (i.e. annual and perennial) using descriptions contained in Cunningham et al. (1992). This produced a total of seven major plant groups including: (1) shrubs (perennial), (2) annual subshrubs, (3) perennial sub-shrubs, (4) annual monocots, (5) perennial monocots, (6) annual forbs and (7) perennial forbs. An index of cover was calculated for each species at each site by summing the cover percentages recorded in all ten 1 m  1 m quadrats. Cover indices were then calculated for each of the major plant groups by summing those belonging to the relevant species. This was also done at the family level within each major plant group. An index of total cover for each site was similarly calculated from the cumulative total of all cover percentages recorded from a site. The use of these cover indices was deemed necessary to avoid working with percentages greater than 100 resulting from vertically overlapping plant layers. Species richness was calculated as the total number of species recorded within each 50 m  50 m quadrat.

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Initial inspection of the data showed that transformation, ½log10 ðx þ 1Þ, of all cover variables was necessary to meet the assumption of normality required by analysis of variance (ANOVA) (Zar, 1999). The effects of flood frequency zone (high, medium and low) and longitudinal region (upper, middle and lower) on species richness (untransformed) and cover indices for total cover, each of the major plant groups and families within each of these (transformed) were then tested using twoway ANOVAs. Means which differed significantly (po 0:05) were then determined using Tukey’s b test. All univariate analyses were conducted in SPSS (SPSS, 2001). To further explore spatial patterns in plant community composition between flood frequency zones and longitudinal regions, an ordination of the data was conducted in PATN (Belbin, 1995) using a matrix of untransformed species cover indices. Species with cover indices totalling below 100 across all sites were considered to be rare and were excluded from this analysis. Bray–Curtis association measures were then used to calculate a semi-strong hybrid (SSH) ordination in three dimensions. In order to calculate species vectors which were significantly correlated with this ordination space, principal axis rotation (PCR), principal axis correlation (PCC) and Monte-Carlo randomization (MCAO) procedures were also employed (Belbin, 1995). Finally, mean Bray–Curtis distances between sites in each flood frequency zone were calculated as a measure of the degree of similarity between sites in each flood frequency zone. ANOVA and Tukey’s b post hoc test were then used to test for significant differences between these.

3. Results 3.1. General floristics Fifty-nine species from 24 families were recorded during the vegetation survey (Appendix A). Marsilea drummondii (nardoo), the sole representative of the Marsileaceae family in the perennial forb plant group, was the most commonly occurring species and Poaceae in the perennial monocot plant group the most abundant family. Annual forbs, particularly members of the Asteraceae family, were also frequently recorded. The shrub plant group contributed substantially to vegetation cover and comprised two species belonging to two different families, i.e. Muehlenbeckia florulenta (Polygonaceae) and C. auricomum (Chenopodiaceae). Members of the Chenopodiaceae family in the perennial sub-shrub plant group also had high cover. Thirty-one species were recorded from sites in the high flood frequency zone, 36 from the medium flood frequency zone and 38 in sites from the low flood frequency zone. Seventeen species were recorded at sites from all three flood frequency zones, nine species from only high and medium flood frequency zones and four species from only medium and low flood frequency zones. Five species, including three annual forbs and two perennial forbs, were restricted to the high flood frequency zone. Three annual forb species and three perennial forb species were recorded only from medium flood frequency sites and a total of 17 species mostly belonging to the

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perennial sub-shrub plant group and the Poaceae family in the perennial monocot plant group were restricted to sites from the low flood frequency zone. 3.2. Effects of flood frequency on plant community composition and structure Species richness and total cover both decreased significantly (po 0:05) with decreasing flood frequency and were also both significantly lower in sites from the middle longitudinal region (Fig. 3, Table 2). Flood frequency had a significant effect on cover in all of the major plant groups with the exception of annual forbs and perennial monocots (Fig. 4, Table 2). Cover of annual monocots was significantly higher in sites from the high flood frequency zone and cover of perennial forbs and shrubs also tended to decrease with decreasing flood frequency (Fig. 4, Table 2). Conversely, cover of annual and perennial sub-shrubs exhibited an inverse relationship with flood frequency (Fig. 4, Table 2). Longitudinal region also had a significant effect on cover in the annual monocot, perennial monocot and shrub plant groups (Fig. 4, Table 2). However, a significant interaction term was only found for the perennial sub-shrub plant group (Table 2), indicating that flood frequency had an overriding effect in most cases. Within the shrub plant group, the two families present both exhibited significant shifts in cover with relation to flood frequency (Fig. 5, Table 3). Chenopodiaceae, represented by the single species C. auricomum, had highest cover in sites from the medium flood frequency zone. Cover in the Polygonaceae family, comprising M. florulenta, was greatest in the high flood frequency zone. Sites from the upper

Fig. 3. Mean (7S.E.) (A) species richness and (B) total cover (untransformed) with relation to flood frequency zone and longitudinal region (indicated by shading).

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Table 2 F-statistics obtained from two-way ANOVAs of species richness (untransformed), total cover and cover of major plant groups (transformed) with flood frequency zone and longitudinal region as factors and showing homogeneous subsets determined by Tukey’s b post hoc test (po 0:05) Parameter

Flood frequency zone (df.=2, 36)

Species 7.011 richness Total cover 29.507 Annual forbs 2.526 Annual 6.021 monocots Annual sub5.590 shrubs Perennial forbs17.441 Perennial 2.579 monocots Perennial sub- 4.649 shrubs Shrubs 30.985

Longitudinal region (df.=2, 36)

Flood frequency  region Homogeneous (df.=4, 36) subsets for flood frequency H

M

L

4.101

1.588

a

ab

b

6.176 0.257 6.955

2.253 1.135 0.104

a a a

b a b

c a b

1.115

0.840

a

ab

b

0.708 4.701

1.358 2.009

a a

a a

b a

0.041

3.956

a

ab

b

6.477

0.721

a

a

b

H=high flood frequency zone, M=medium flood frequency zone, L=low flood frequency zone. * Significant at po 0:05.

longitudinal region had significantly lower cover of this species but there was no significant interaction with flood frequency (Table 3). All of the families present in the annual sub-shrub (i.e. Chenopodiaceae) and perennial sub-shrub plant groups (i.e. Chenopodiaceae and Frankeniaceae) increased in cover with decreasing flood frequency (Fig. 5, Table 3). The exception was in sites from the upper longitudinal region where Chenopodiaceae cover from the perennial sub-shrub group was greater in the medium flood frequency zone than in the low flood frequency zone, resulting in a significant interaction term (Fig. 5, Table 3). Frankeniaceae cover was also found to be significantly higher in sites from the lower longitudinal region and, as this family was absent from sites in the upper longitudinal region, there was also a significant interaction term (Fig. 5, Table 3). The annual monocot plant group consisted of species belonging to the Cyperaceae and Poaceae families. Cover in both of these families exhibited significant shifts with flood frequency but was not affected by longitudinal region (Fig. 6, Table 3). In the Poaceae family, cover was significantly higher in the high flood frequency zone and cover in the Cyperaceae family decreased significantly with decreasing flood frequency. In the perennial monocot plant group, only cover in the Cyperaceae family was significantly influenced by flood frequency as it was only present in the high flood frequency zone (Fig. 6, Table 3). Poaceae cover in the perennial monocot

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Fig. 4. Mean cover (7S.E.) of major plant groups in relation to flood frequency zone in sites from the (A) upper, (B) middle and (C) lower longitudinal regions. N.B. untransformed data are shown.

plant group was not affected by flood frequency but was significantly higher in the upper longitudinal region (Fig. 6, Table 3). In the annual forb plant group, only two families were significantly affected by either flood frequency or longitudinal region: Amaranthaceae and Tiliaceae (Fig. 7, Table 3). Cover in both of these decreased significantly with decreasing flood frequency with Tiliaceae only occurring in the upper longitudinal region and Amaranthaceae exhibiting significantly higher cover in the middle longitudinal region (Fig. 7, Table 3). Three families in the perennial forb plant group exhibited significant shifts in cover with flood frequency (Fig. 7, Table 3). In the Asteraceae family, cover decreased with increasing flood frequency and in Marsileaceae, increased with increasing flood frequency (Fig. 7, Table 3). Goodeniaceae cover was highest in the medium flood frequency zone (Fig. 7, Table 3). Longitudinal region only had a significant effect on cover in the Nyctaginaceae family in this plant group as this family was present only in the upper longitudinal region (Table 3).

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Fig. 5. Mean cover (7S.E.) of families belonging to the shrub and sub-shrub plant groups including (A) Chenopodiaceae in the shrub group, (B) Polygonaceae in the shrub group, (C) Chenopodiaceae in the annual sub-shrub group, (D) Chenopodiaceae in the perennial sub-shrub group and (E) Frankeniaceae in the perennial sub-shrub group. N.B. untransformed data are shown.

The results of the ordination indicate that plant community composition exhibits a gradient broadly related to flood frequency (Fig. 8A). Within the ordination space, sites from the low flood frequency zone are reasonably distinct, whilst considerable overlap is evident between sites from the high and medium flood frequency zones. There is no clear separation of sites on the basis of longitudinal region. Species vectors which correlate significantly (po 0:05) with the ordination axes (Fig. 8B, Table 4) indicate that plant community composition in sites from the high flood frequency zone was characterized by high cover in M. florulenta (Polygonaceae) from the shrub plant group, the perennial forb M. drummondii (Marsileaceae), the perennial monocot Eleocharis spp. (Cyperaceae) and the annual monocot Echinochloa turneriana (Poaceae). Perennial sub-shrubs from the Chenopodiaceae family (e.g. Sclerolaena muricata, Sclerolaena lanicuspis and Atriplex angulata), and Frankenia serpyllifolia from the Frankeniaceae family, correlated strongly with the position of sites from the low flood frequency zone. Overall, plant community composition in sites from the high flood frequency zone tended to be most similar. Conversely, sites from the low flood frequency zone appeared to be the most divergent. This was found to be statistically significant as indicated by the analysis of Bray–Curtis dissimilarity measures (F 2;312 ¼ 94:185, po 0:001) (Table 5).

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Table 3 F-statistics obtained from two-way ANOVAs of cover of families within major plant groups (transformed) with flood frequency zone and longitudinal region as factors and showing homogeneous subsets determined by Tukey’s b post hoc test (po 0:05) Plant groups

Flood frequency zone Longitudinal region (df.=2, 36) (df.=2, 36)

Flood frequency  Region Homogeneous (df.=4, 36) subsets for flood frequency H

M

L

b b

Shrubs Chenopodiaceae 6.022 Polygonaceae 43.566

1.211 4.191

0.170 1.221

a a

a b

Annual sub-shrubs Chenopodiaceae 5.590

1.115

0.840

a

ab b

Perennial sub-shrubs Chenopodiaceae 4.383 Frankeniaceae 6.301

0.068 3.439

3.719 3.439

a a

ab b a b

Annual monocots Cyperaceae 5.638 Poaceae 5.201

2.766 2.579

1.035 0.302

a a

ab b b ab

Perennial monocots Cyperaceae 11.736 Poaceae 1.626

3.164 4.779

3.164 1.584

a a

b a

Annual forbs Amaranthaceae 5.092 Tiliaceae 5.857

4.261 13.744

0.303 5.857

a a

a b ab b

Perennial forbs Asteraceae 4.694 Goodeniaceae 4.051 Marsileaceae 22.120 Nyctaginaceae 1.942

3.175 1.258 1.279 7.528

0.403 1.095 0.479 1.942

a ab a a

a a b a

b a

b b c a

N.B. only families significantly influenced by either flood frequency zone or longitudinal region are shown. H=High flood frequency zone, M=medium flood frequency zone, L=low flood frequency zone. * Significant at po 0:05.

4. Discussion Spatial variation in plant community composition and structure on the Cooper Creek floodplain appears to be closely related to flooding, despite the high hydrological variability. The results of this survey indicate that floodplain plant communities are structured primarily on a spatial gradient of broad flood frequency and that species composition varies along this with some degree of predictability. Although longitudinal region was found to have some effect on composition, flood history appears to have an overriding influence on plant community composition

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Fig. 6. Mean cover (7S.E.) of families belonging to the annual and perennial monocot plant groups including (A) Cyperaceae in the annual monocot group, (B) Poaceae in the annual monocot group, (C) Cyperaceae in the perennial monocot group and (D) Poaceae in the perennial monocot group. N.B. untransformed data are shown.

Fig. 7. Mean cover (7S.E.) of families belonging to the annual and perennial forb groups including (A) Amaranthaceae in the annual forb group, (B) Tiliaceae in the annual forb group, (C) Asteraceae in the perennial forb group, (D) Goodeniaceae in the perennial forb group, (E) Nyctaginaceae in the perennial forb group and (F) Marsiliaceae in the perennial sub-shrub group. N.B. only families significantly influenced by either flood frequency zone or longitudinal region are shown, untransformed data are shown.

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Fig. 8. (A) SSH ordination of sites by cover indices of species (stress=0.19) and (B) species vectors significantly correlating with the ordination space. Symbol shape in (A) indicates flood frequency zone of site; ’=high flood frequency zone, K=medium flood frequency zone and m=low flood frequency zone. Symbol shading signifies longitudinal region; open symbols=upper region, grey symbols=middle region and black symbols=lower region. Species indicated by each number in (B) are provided in Table 4.

Table 4 Species correlating significantly to the ordination (Fig. 8A) as indicated by the Monte Carlo randomization procedure ID # (Fig 8B)

Species name

Plant group

Family

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Iseilema spp.

Annual monocot Annual forb Annual forb Perennial sub-shrub Perennial sub-shrub Perennial sub-shrub Perennial sub-shrub Annual sub-shrub Annual forb Shrub Annual forb Perennial monocot Perennial monocot Annual monocot Perennial forb Shrub Perennial monocot

Poaceae Portulacaceae Asteraceae Frankeniaceae Chenopodiaceae Chenopodiaceae Chenopodiaceae Chenopodiaceae Asteraceae Chenopodiaceae Asteraceae Poaceae Cyperaceae Poaceae Marsileaceae Polygonaceae Poaceae

Portulaca oleracea Calocephalus platycephalus Frankenia serpyllifolia Sclerolaena muricata Atriplex angulata Sclerolaena lanicuspis Salsola kali Calotis hispidula Chenopodium auricomum Senecio glossanthus Eragrostis eriopoda Eleocharis spp. Echinochloa turneriana Marsilea drummondii Muehlenbeckia florulenta Sporobolus mitchellii *

Significant at po 0:05. Significant at po 0:01. *** Significant at po 0:001. **

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Table 5 Mean (7S.E.) Bray–Curtis dissimilarity measures for sites from each flood frequency zone showing homogeneous subsets determined by Tukey’s b post hoc test (po 0:05) Flood frequency zone

Mean Bray–Curtis distance

Homogeneous subset

High Medium Low

0.56151770.017609 0.71079370.015986 0.86546970.013044

a b c

and structure. This is best demonstrated by the ordination (Fig. 8A) which indicates a gradual shift in composition from frequently to rarely flooded sites. Rather than exhibiting a marked zonation, the composition of sites within different flood frequency zones shows considerable overlap which is probably due to the arbitrary nature of these categories for the purposes of this study and indicative of the spatially and temporally variable nature of floodwater distribution in this catchment. Flow variability occurs along a continuum, rather than in discrete zones, and consequently promotes landscape-scale diversity as a result of increased habitat heterogeneity (e.g. differences in inundation extent, depth and duration) (Ferreira and Stohlgren, 1999; Stromberg, 2001). The results of this study suggest that the landscape-scale effects of flood frequency have a stronger influence on plant community composition in frequently flooded zones, while local factors (e.g. soil type, local rainfall, etc.) appear to be more significant at low flood frequencies. Plant community composition at frequently flooded sites was found to be very similar throughout the study area and sites at the extremes of the longitudinal gradient were more comparable to each other than sites in adjacent medium and low flood frequency zones (e.g. Fig. 4). Furthermore, divergence between sites within a zone increased with decreasing flood frequency (Table 5). The results indicate that, at a landscape scale, fewer species were present within the study area which can persist in frequently inundated areas and that these are reasonably consistent throughout the catchment. It is likely that there is a larger pool of species which are adapted to the drier conditions of the infrequently flooded areas. More species were found to be restricted to the low flood frequency zone, although most of these were limited in their distribution within it. These species may have colonized from neighbouring land systems and therefore their presence at a particular site would depend on its position within the broader landscape (e.g. proximity to sand dune communities). This would partially account for the greater divergence observed amongst the composition of low flood frequency sites. The gradual zonation of plant community composition along the flood frequency gradient in the Cooper Creek floodplain broadly concurs with observations of temperate and tropical floodplain vegetation zonation (Menges, 1986; Blom et al., 1990; Friedman et al., 1996; Trebino et al., 1996; Hughes and Cass, 1997). Frequently flooded zones in these areas are generally dominated by flood tolerant species and annuals which complete their life cycles between flood events (Menges, 1986; Bren, 1992; Trebino et al., 1996). In the Cooper Creek floodplain the frequently flooded zone was characterized by high species richness and total cover and significantly

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higher cover of annual monocots. Species belonging to the Amaranthaceae and Tiliaceae families of the annual forb plant group also exhibited greater in this zone as did perennial species with known flood tolerance, e.g. M. florulenta (Polygonaceae) in the shrub plant group, M. drummondii (Marsileaceae) in the perennial forb plant group and Eleocharis spp. (Cyperaceae) in the perennial monocot plant group (Craig et al., 1991; Cunningham et al., 1992; Roberts and Marston, 2000). The higher cover of species belonging to the annual plant groups observed in the high flood frequency zone may also reflect the shorter time since last inundation in these sites compared with those from the medium flood frequency zone. Strong variations in the cover of longer-lived perennial species, e.g. M. florulenta, between these flood frequency zones, however, suggest that many of the observed differences in plant community composition and structure can be attributed to flood frequency in addition to time since last inundation. Flood frequency is likely to be a major factor contributing to the observed variation in plant community composition and structure between the low and medium flood frequency zones, as time since last inundation did not differ between sites within these zones. The low flood frequency zone in the Cooper Creek floodplain had significantly lower species richness and total cover and was dominated by perennial monocots in the upper longitudinal region of the floodplain and by perennial sub-shrubs in the middle and lower regions. Studies in temperate floodplains have also found perennial species to be more abundant in rarely flooded zones and their dominance has often been attributed to competitive advantages (Menges and Waller, 1983; Menges, 1986). In an arid floodplain such as that of the Cooper Creek, these species might be considered as drought tolerant rather than competitive. Characteristics of plant communities in moderately flooded areas of floodplains have tended to be overlooked by the majority of studies and are generally assumed to be intermediate to high and low flood frequency zones (Blom and Voesenek, 1996). In this study, plant communities in the medium flood frequency zone were found to be transitional in terms of species richness and total cover. Some plant groups, however, exhibited their highest cover in these sites, i.e. C. auricomum (Chenopodiaceae) from the shrub plant group and the Goodeniaceae family from the perennial forb plant group suggesting some degree of tolerance amongst these groups to both flooding and drought. Evidence in support of the Intermediate Disturbance Hypothesis (Connell, 1978) was not found, however, as species richness declined significantly with decreasing flood frequency (Fig. 3). This study has illustrated that plant communities of an arid and hydrologically variable floodplain are structured predominantly by flooding at a landscape level. Furthermore, plant community attributes of this arid floodplain appear to vary predictably along a flood frequency gradient in a manner similar to that described in less hydrologically variable temperate and tropical floodplains. The management implications of this is that anthropogenic changes to flooding regimes in highly variable arid catchments may have as severe an effect on floodplain vegetation as has been recorded elsewhere (Conner et al., 1981; Bren, 1992; Friedel et al., 1993; Kingsford and Thomas, 1995; Stromberg, 2001; Tockner and Stanford, 2002). Possible impacts of alterations to the flooding regime may include a streamward migration of vegetation

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zones (Hughes and Cass, 1997) and replacement of flood tolerant species by more mesic or drought tolerant species (Bren, 1992). In the case of the Cooper Creek floodplain, a shift in the distribution of dominant shrub species could be a likely consequence of altered flow regimes. For instance, a reduction in the size of frequent inundation events may lead to a decrease in the area currently occupied by the shrub M. florulenta and subsequent invasion of these areas by C. auricomum or perennial sub-shrub species. Landscape-scale heterogeneity could also be reduced through alterations to the flood regime and homogenization of plant community composition across the floodplain would certainly follow. Such changes in vegetation structure would be likely to have widespread implications for the broader ecological functioning of arid floodplain ecosystems as well as their socio-economic value.

Acknowledgements This work was undertaken as part of a Ph.D. project supported by Land and Water Australia scholarship, GRU26. The preparation of this paper was greatly enhanced by the advice and kind support of Drs. Margaret Brock, Fran Sheldon and Wade Hadwen and Prof. Stuart Bunn. Invaluable assistance was also provided by the people of the Cooper Creek catchment during the field survey, particularly Mr. Sandy Kidd and Mrs. Anne Kidd of ‘Our Del’.

Appendix. A List of species recorded in the vegetation survey can be seen in Table 6.

Table 6 List of species recorded in vegetation survey Major plant group

Family

Species name

Species authora

Annual forbs

Aizoaceae Amaranthaceae Apiaceae

Trianthema triquetra Alternanthera nodiflora Daucus glochidiatus

Rottb. Ex Willd. R. Br. (Labill.) Fisch., C.A. Mey and Ave-Lall. F. Muell. (F. Muell.) F. Meull.

Asteraceae

Brassicaceae Campanulaceae Cucurbitaceae Fabaceae

Eryngium plantagineum Calotis hispidula Calotis multicaulis Calotis porphyroglossa Leiocarpa brevicompta Rhodanthe floribunda Senecio glossanthus Lepidium sagittulatum Wahlenbergia gracilis Cucumis myriocarpus Cullen cinereum

F. Muell. Ex Benth. (F. Muell.) Paul G. Wilson (DC.) Paul G. Wilson (Sond.) Belcher Thell. (G. Forst.) A.DC. Naudin subsp. Myriocarpus (Lindl.) J.W. Grimes

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Table 6 (continued ) Major plant group

Annual monocots

Annual subshrubs Perennial forbs

Family

Species name

Species authora

Plantaginaceae Portulacaceae Tiliaceae

Trigonella suavissima Plantago cunninghamii Portulaca oleracea Corchorus trilocularis

Lindl. Decne. L. L.

Cyperaceae

Cyperus difformis

L.

Poaceae

Chloris pectinata Echinocholoa turneriana Eragrostis tenellula Iseilema sp. Salsola kali

Benth. (Domin) J.M. Black (Kunth) Steud.

Calocephalus platycephalus Streptoglossa adscendens Goodenia fascicularis Scaevola sp. Teucrium racemosum Malvastrum americanum Sida rohlenae Marsilea drummondii Boerhavia dominii Boerhavia schomburgkiana Solanum esuriale Verbena officinalis Eleocharis sp.

(F. Muell.) Benth. (Benth.) Dunlop F Muell and Tate

Chenopodiaceae Asteraceae Goodeniaceae Lamiaceae Malvaceae Marsileaceae Nyctaginaceae

Perennial monocots

Solanaceae Verbenaceae Cyperaceae Poaceae

Perennial subshrubs

Chenopodiaceae

Aristida jerichoensis var. subspinulifera Astrebla lappacea Eragrostis eriopoda Eragrostis setifolia Eriochloa crebra Eulalia aurea Panicum decompositum Sporobolus actinocladus Sporobolus mitchellii Atriplex angulata Atriplex spongiosa Malacocera albolanata Sclerolaena lanicuspis Sclerolaena longicuspis Sclerolaena muricata

Shrubs a

Frankeniaceae Chenopodiaceae Polygonaceae

Sclerolaena tricuspis Frankenia serpyllifolia Chenopodium auricomum Muehlenbeckia florulenta

Nomenclature follows Henderson (2002).

L.

R.Br. var. racemosum (L.) Torr. var. americanum Domin subsp. Rohlenae A. Braun Meikle and Hewson Oliv. Lindl. L. Henrard (Lindl.) Domin Benth. Nees S.T. Blake (Bory) Kunth R. Br. var. decompositum (F. Muell.) F. Meull. (Trin.) C.E. Hubb. ex S.T. Blake Benth. F. Muell. (Ising) Chinnock (F. Muell.) F. Meull. ex Benth. (F. Muell.) A.J. Scott (Moq.) Domin var. muricata (F. Muell.) Ulbr. Lindl. Lindl. Meisn.

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