Env. Mgmt. 24/3

RESEARCH Preliminary Evidence for Pollen as an Indicator of Recent Floodplain Accumulation Rates and Vegetation Changes: The Barmah-Millewa Forest, SE Australia CHRISTINE KENYON* Department of Geography and Environmental Studies University of Melbourne Parkville, Australia, 3052 IAN D. RUTHERFURD Cooperative Research Centre for Catchment Hydrology Civil Engineering, Monash University Clayton, Australia, 3168

ABSTRACT / Preliminary analysis of pollen in three shallow sediment cores demonstrates that pollen is preserved in the seasonally dry, vertically accreting Barmah-Millewa Forest floodplain of the Murray River, SE Australia. Deposition characteristics of a floodplain are a critical component of catchment sediment budgets, but it has proven difficult to identify this important stratigraphic point in floodplains using radio-

This paper reports on the utility of pollen analysis to discriminate between pre-European and European time periods in a sediment and geographical type not previously investigated by Australian analysts. The study sets out to investigate: (1) if pollen was preserved in floodplain sediments, (2) if exotic pollen can provide a chronostratigraphic marker in floodplain sediments, and (3) the rate of sediment accumulation in postEuropean times. Silts and clays are the most important components of floodplain sediment budgets (Meade 1982), but suitable techniques for dating such sediments in the range of 150 years to the present are often difficult to apply. Cultural artifacts found in a sediment sequence can provide minimum ages for stratigraphic breaks (Wasson and others 1987). Radiocarbon ( 14C) dating gives indeterminate results from about 200 years BP to the present (Stuiver and Pearson 1986, 1993). A number of radionuclide dating techniques are accurate over the KEY WORDS: Pollen; Floodplain; Vegetation; Deposition; Budget; Sediment; Australia; Fire *Author to whom correspondence should be addressed.

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nuclide dating techniques. Pollen in a floodplain, as opposed to that preserved in lacustrine settings, provides opportunities for investigating the impact of European land-use on both sediment deposition and floodplain vegetation. Pollen from exotic plant taxa identified in floodplain sediments provided a chronostratigraphic marker for the boundary between pre- and post-European sediments in the Murray River floodplain. A maximum deposition rate of about 80 mm per 100 years is estimated from the sediment history. The pollen record shows vegetation changes within the forest since European settlement. These included changes in the density of the Eucalyptus forest; and in the composition of understorey herbs, sedges, and grasslands. Pollen concentration and charcoal and organic content also exhibit postEuropean changes. Thus, pollen analysis provides a technique for determining changes in sediment budgets and identifying major vegetation changes in floodplains.

historical period, particularly lead-210 ( 210Pb) and cesium-137 ( 137Cs) (Wise 1980). However, within floodplains these techniques may be more useful in determining sediment source areas than in dating sediments (Longmore 1982, Wasson and others 1984, Bishop and others 1991). Thoms and Walker (1990), in the Barmah Forest, and Ivanovich and Harman (1983), showed that in dry environments low levels of unsupported 210Pb can be problematic in age determinations. Clark and Patterson (1984) have successfully correlated identifiable pollen horizons with 210Pb analyses to determine modern intertidal marsh sedimentation rates in North America. Determination of the first occurrence of pollen from introduced plant taxa in the stratigraphic record is a technique that can be used to provide a chronstratigraphic marker in sediments. Anthropogenic indicators identified in Northern Hemisphere pollen records have been described by Behre (1981), Janssen (1986), Vuorela (1977), Vorren (1986), and Baker and others (1993). Clark and Patterson (1984) stress the need to carefully identify anthropogenic indicators in the pollen record and to separate the local pollen component

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of the record that may obscure settlement horizons. In recent Australian studies, exotic pollen has been used to separate pre- and post-European pollen assemblages and to estimate post-European sediment deposition rates in Lake Burrinjuck, NSW (Clark 1986) and Lake Wellington, Victoria (Reid 1989, Grayson and others 1998). Studies of lake and swamp sediments are not comparable with seasonally dry, open floodplain systems. The latter are not considered optimal for pollen preservation and recovery because frequent wetting and drying of the surface has the potential to oxidize the pollen of some taxa (Jacobson and Bradshaw 1981, Sangster and Dale 1961). In an open forested floodplain system, the pollen catchment area through time is difficult to determine as source areas may change and flood waters will transport pollen from unknown distances to the site. Furthermore, floodplains are often subject to reworking and erosion of the sediments that will both mix and damage the pollen. Distribution of pollen grains within the floodplain sediment profile will be determined by sediment deposition rates, flood frequency, and aerial deposition of local and regional pollen. In floodplain sediments consideration must be given to the contribution of river-borne pollen deposited during flood events. Pollen and spores form a significant component of the particulate organic component of suspended sediments and reflect catchment and floodplain vegetation (Brown 1985, Dodson 1977, Pennington 1979). Furthermore, flood events, overland flow, and river channel erosion can rework sediment and be a source of pollen that is older than the modern pollen being deposited (Pennington 1979). Fortunately, to identify the pre- and post-European cultural boundary in the stratigraphic record only the presence or absence of pollen from exotic taxa is needed. Despite the potential problems with pollen records in floodplains, there are two major reasons why it would be useful to have an historical floodplain record. First, floodplain sediment deposition is a critical component of catchment sediment budgets that is not accurately represented by measures of sediment deposition in lacustrine settings. Second, the pollen record on floodplains is likely to be different from that recorded from lacustrine settings in that it directly reflects floodplain and catchment vegetation rather than regional vegetation.

Description of the Field Site The Barmah-Millewa Forest (44,500 ha) is located on the Murray River between Deniliquin and Tocumwal in

New South Wales, and Echuca in Victoria, an area with an annual rainfall of 400 mm/yr and potential evaporation of 1200 mm/yr (Figure 1). This river red gum (Eucalyptus camaldulensis Dehnh.) forest exists on a large floodplain subject to more-or-less annual inundation (Bren 1987). The floodplain has a slope of less than 0.00025 and is covered by a network of anabranching distributaries from the Murray River. The Murray River in this reach is about 60 m wide and 5 m deep, with a natural levee of less than 1 m. The Barmah-Millewa floodplain is vertically accreting, in contrast to laterally accreting floodplains in other reaches of the Murray River (Thoms and Walker 1990, Rutherfurd 1992). The predominance of vertical accretion is indicated by the absence of features such as meander cutoffs, ridges, and swales and by the low sinuosity of the river in this reach. As a result, sediment is not reworked by lateral migration of the river or by catastrophic floodplain stripping. Flooding usually occurs annually and persists for some months, so that accretion is gradual, resulting in a floodplain that is frequently wet. This increases the probability of pollen preservation. Drilling in the forest by the Rural Water Corporation of Victoria reveals that the floodplain is formed mainly of silty clays with lenses of sand, presumably related to anabranching channels. Samples of bank material from the study area are generally greater than 70% silt and clay (Thoms and Walker 1990, Rutherfurd 1992). Since 1934 the Murray River has been regulated, resulting in a decline in flood frequency and a change in season of flooding. Even with regulated flow, over 40% of the forest is flooded for more than four months of the year (Bren 1988). The Barmah-Millewa Forest begins to flood when discharge at Tocumwal reaches 140 m3/sec (Leitch 1988) and the majority of the forest is flooded when the flow at Tocumwal reaches 800 m3/sec (Bren 1988), which occurs less than 5% of the time under the present flow regime. In addition, the vegetation history of the forest has been documented by Curr (1883), Chesterfield (1986), and Bren (1992). This record allows us to test whether an accurate history of vegetation change can be reconstructed from pollen preserved in floodplains.

Vegetation and Land-Use History The vegetation of the Barmah Forest has been described by Chesterfield (1986), Bren (1992), and the Department of Conservation & Environment, Victoria (1992). Open Eucalyptus camaldulensis forest and woodlands are the main plant communities with grasses, sedges (Carex tereticaulis), or giant rush (Juncus ingens)

Barmah Forest Pollen

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Figure 1. Location of the core sample sites in the Barmah-Millewa Forest, South-eastern Australia.

dominant in the understorey depending on flood frequency. Extensive treeless Moira grass (Pseudoraphis spinescens) plains occur where prolonged flooding occurs. Box woodlands of Eucalyptus microcarpa and Eucalyptus melliodora occur where flood frequency is rare. Callitris and Allocasuarina previously occurred on the sand ridges but today few remain (Chesterfield 1986). Prior to European settlement the river, forest, and lagoons provided abundant food resources for the local aboriginal people (Curr 1883). Typha (cumbungi), Eleocharis (spike rush), and Phragmites (common reed) were some of the important plant foods of the forest (Beveridge 1889, Curr 1883). Displacement of the aboriginal community to missions began in the 1850s, but European encroachment and diseases reduced the population size and disrupted land-use practices before this time (Curr 1883). Summer grazing of sheep on the grass plains within the forest began in 1841 (Curr 1883), but by 1885 cattle replaced sheep. Rabbits were present by the 1880s and caused a reduction in river red gum and shrub seedling regeneration. Grazing of cattle and horses has continued to be an important activity in the forest (Dexter 1978). River red gum (Eucalyptus camaldulensis) has been extensively harvested for hardwood timber products since 1870. The forest has continued to be harvested and managed for its timber (Department of

Conservation & Environment, Victoria 1992). Callitris murrayii was extensively harvested by early settlers (Chesterfield 1986). The Royal Commission on State Forests and Timber Reserves (1899) and more recently Dexter (1978), Chesterfield (1986), and the Department of Conservation & Environment, Victoria (1992) have recorded vegetation changes since Europeans first passed through the region. Bren (1992) has documented red gum invasion of grasslands at War Plains in Barmah Forest since 1945.

Methods Three cores were taken by driving sharpened, PVC tubes, 50 mm diameter, into the floodplain sediments (Figure 2) with a sledge hammer. Compaction in the cores was measured by successively measuring the depth to the top of the core as it was driven in. The cores were collected from the floodplain sediments beneath the forest (Figure 2) from sites that are wet in late winter/ early spring and dry during the summer. Grass was removed from around the core site, and in the process, up to 20 mm of sediment was removed. In the laboratory, each PVC tube was cut lengthwise and sampled for pollen analysis. Core 1 (0.65 m), 800 m from the river, was taken at

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Figure 2. Sampling site for core 2 showing the typical open forest of river red gum (Eucalyptus camaldulensis).

the western edge of the Millewa State Forest (Mathoura 1:100,000 mapsheet, grid reference 153232.5). Core 2 (0.7 m) was taken near the top of the Murray River bank in the Barmah State Forest, some 20 m from the river edge. Core 3 (0.4 m) was also taken near the top of the Murray River bank, some 20 m from the river edge in the Millewa State Forest about 25 km to the east of core 2 (Tuppal 1:100,000 mapsheet, grid reference 328333). Core Sediment Analysis The exposed surface of each core was scraped clean and 2-cm3 samples were prepared for pollen analysis using the methods of Faegri and Iversen (1975). Core 1 was sampled at 2-cm intervals as this core suffered the least compaction (5% over 0.65 m). Organic content for each of the samples was estimated by loss-onignition. Core 2 was sampled at depths of 1, 10, and 20 cm and core 3 at 1, 5, and 10 cm. Cores 2 and 3 were not sampled further due to the degree of compaction of the sediments, making sedimentation rates difficult to determine. For each sample 200 pollen grains (the pollen sum) were counted at ⫻600 magnification. Preparations in which no exotic pollen grains were found were then scanned at low magnification to determine the presence or absence of pollen from introduced taxa. Poaceae pollen was sorted according to size, as it is difficult to identify different Poaceae species from the pollen (Andersen 1978). Values for each pollen taxon were represented as percentages of the pollen sum and a relative percentage pollen diagram was constructed for all taxa identified (Figures 3–5). Pollen concentration was calculated to assess changes in vegetation density and sedimentation rates. Core 1 pollen samples were analyzed for microscopic

charcoal using the point-count method of Clark (1983). In each pollen preparation, charcoal particles with diameters greater than 10 µm in 200 fields of view were counted. Within a floodplain microscopic, charcoal will have been transported by wind and water to the site from higher in the catchment. The presence of this charcoal in the pollen preparations is therefore likely to reflect the importance of, and changes in, regional fire regimes, although the temporal and spatial resolution of this method is poor as fire frequency and intensity in different areas of the catchment cannot be separated (Clark 1988).

Results and Discussion Presence of Pollen Pollen in low concentrations is preserved in all core sediments with the condition of the pollen grains varying between samples Figures 3–5. There was no obvious pattern of decreasing pollen preservation with increasing depth in core 1, although this is a possibility that would result in a biased pollen record. Pollen concentration increases above 8 cm in core 1 with a low value at the surface. In core 2 pollen concentration increases markedly at the surface and in core 3 values increase up the core. Pollen in the three cores is dominated by Eucalyptus pollen assumed to be Eucalyptus camaldulensis as this is the dominant floodplain species (Figures 3–5). In core 1 Eucalyptus values are constant at about 60% below 8 cm. Between 8 cm and 4 cm Eucalyptus pollen has a maximum value of 87%, while above 4 cm there is a steady decline in Eucalyptus percentage values to 35%. Eucalyptus pollen in core 2 increases toward the surface


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Figure 3. Percentage pollen, organic content and charcoal diagrams for core 1, Barmah-Millewa Forest.

but, in core 3 remains constant throughout. Cupressaceae, Chenopodiaceae, and Asteraceae (tubuliflorae type) decline in abundance in the top 8 cm of core 1. Cyperaceae decreases toward the surface in all three cores. Exotic Pollen Pollen from introduced taxa, including Pinus, Rumex, Hypericum, Plantago lanceolata, and Asteraceae (liguliflorae type), increase in the upper 8 cm of core 1. Exotic pollen is present at the surface of cores 2 and 3, but absent at 10 cm in core 2 and at 5 cm in core 3. Poaceae pollen ⬎35 µm occur only in the upper 8 cm in core 1 and not at all in cores 2 and 3. Poaceae pollen grains (⬍35 µm) are abundant in the core 1 record but decline above 8 cm while values decrease in the two

upper samples in core 3. Poaceae pollen (⬎35 µm) may represent introduced grasses (Behre 1981, Vuorela 1977, Vorren 1986) or a change in native grass species in response to grazing pressure and/or changes in hydrology on the floodplain after European settlement (Chesterfield 1986, Curr 1883, Beveridge 1889). Curr (1883) describes the demise of the reedbeds 35 years after he first arrived in the area. Grazing activities commenced in the forest in 1842 (Curr 1883), and it is estimated that exotic plant species were introduced at this time. Since many weed species are opportunistic, flowering within the first year after becoming established, it is probable that the first exotic pollen was deposited in the Barmah forest by 1843. The presence of introduced pollen taxa in the sediments presents a clear boundary between pre- and post-

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Figure 4. Percentage pollen diagram for core 2, Barmah-Millewa Forest.

Figure 5. Percentage pollen diagram for core 3, Barmah-Millewa Forest.

European settlement and provides unequivocal evidence of European presence in the region. Sediments The sediments of the three cores consisted of a sandy loam (⬎75% silt and clay). The cores were uniform in texture throughout their length except for occasional thin sandy lenses. Organic content in core 1 (Figure 3) is low throughout although it increases above 6 cm. In

cores 2 and 3 organic content is low and constant (Figures 4 and 5). Sedimentation rates of about 7–10 cm per 100 to 150 years (7 mm per 10 years), taking sediment compaction into consideration, are calculated for a maximum rate for the post-European period from core 1. Other long-term estimates of deposition in the BarmahMillewa Forest suggests slower rates than obtained in this study. Bowler (1978) dated the fluvial overbank unit


overlying the lacustrine Kanyapella facies at 6800 ⫾ 150 years. Assuming 2-m depth, this implies a deposition rate of 3 cm per 100 years. This rate of deposition is about three times slower than that suggested by the exotic pollen on the floodplain. Much lower deposition rates are suggested over longer time periods. Riley and Taylor (1978) estimated that the alluvial fans of the Namoi-Gwydir system (a tributary to the Darling River) are presently aggrading at a rate of 2 cm per 100 years. Pollen from exotic taxa appear at different depths in each core, suggesting that sediment deposition has varied across the Barmah-Millewa Forest floodplain since European settlement, although none of the cores would suggest greater than 10 cm of deposition since European settlement. Thoms and Walker (1990) estimated a sediment deposition rate in cores taken from the northwestern area of the Barmah State Forest (Figure 1) of approximately 60 cm in the past 30 years, based on a 137Cs peak. Although deposition rates can vary markedly across a floodplain (Birks and others, 1988), it is probable that the presence or absence of exotic pollen taxa is a less equivocal chronostratigraphic marker than the presence of 137Cs, which is subject to various problems, primarily vertical mobility (Longmore 1982). Thus, we favor this lower deposition estimate. Fire was an important component of the floodplain environment before European settlement in the region. Charcoal concentration decreases continuously above 6 cm in core 1, indicating a decrease in burning on the floodplain after European settlement. The lag time in the charcoal concentration decline after the first appearance of pollen from exotic plant taxa may be explained by the model of Clark (1988), where charcoal delivery to a site decreases over time since the last fire. The corresponding rise in organic matter above 6 cm may reflect a reduced frequency of burning with more vegetation left on the floodplain floor.

Conclusions We emphasize that this preliminary study was designed to determine whether pollen was preserved on the floodplain and would be a useful tool for identifying the pre- and post-European contact boundary in the stratigraphic record. The presence of exotic pollen has often been used as a chronostratigraphic marker, to identify post-European deposition in Australia. All such studies in SE Australia have previously been conducted in permanently wet lacustrine or bog settings. It has been assumed that pollen would not be preserved in floodplains because the cycles of wetting and drying would

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oxidize the pollen. On the contrary this study demonstrates that pollen deposited over the past 200 hundred years has been preserved in the vertically accreting floodplain sediments of the Barmah-Millewa Forest. Our major finding is that pollen can be used as an environmental indicator in fluvial settings subject to annual flooding. In addition, the clear break between the presence and absence of pollen from introduced taxa in core 1 provides a chronostratigraphic marker of the point of European arrival. This is important because it is difficult to date sediments approximately 100 years old by other techniques, and when developing historical sediment budgets in SE Australia, it is the historical period (1840–1940) that is of the most interest. In the three cores sampled, exotic pollen first appears between 5 and 10 cm, suggesting that deposition in the Barmah-Millewa Forest since the mid-1800s has been on the order of 10 cm although, it cannot be considered representative of deposition throughout the forest. A deposition rate of 1 mm/yr in the BarmahMillewa Forest is likely to be an overestimate of the general deposition rate. The presence of exotic pollen promises to provide valuable data for sediment budget studies that aim to identify the effects of European settlement on the Australian landscape. In addition, the identification of a minimum age for the post-European stratigraphic contact a possible increase in river red gum forest density, allows us to identify changes in vegetation communities following European settlement. There is a good historical record of post-European vegetation change in the BarmahMillewa Forest, and the utility of the pollen record can be compared against the historical record. The pollen record shows changes in the grasses and declines in herbaceous and aquatic taxa that correlate with historical records for the forest. Clearly, pollen records from floodplains offer considerable opportunities for defining historical deposition rates as well as reconstructing historical vegetation changes.

Acknowledgments We are grateful to Paul Bishop and Ian Thomas for their useful comments on the manuscript. Peter Gell’s thorough review of a previous manuscript has been valuable. Lawrence and Andrew Rutherfurd assisted with field work.

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