Review of Palaeobotany and Palynology 233 (2016) 1–11 Contents lists available at ScienceDirect Review of Palaeobotany and Palynology journal homepage: www.elsevier.com/locate/revpalbo Towards understanding the fossil record better: Insights from recently deposited plant macrofossils in a sclerophyll-dominated subalpine environment Giselle A. Astorga ⁎, Gregory J. Jordan, Timothy Brodribb School of Biological Sciences, University of Tasmania, Private bag 55, Hobart, Tasmania 7001, Australia article info abstract Article history: Accumulations of plant macrofossils in lake sediments and other sedimentary deposits are increasingly being Received 27 January 2014 used to refine our understanding of past vegetation history, ecological processes and related climate conditions. Received in revised form 20 June 2016 However, past vegetation studies based on the use of disarticulated plant structures need to consider the specific Accepted 23 June 2016 potential for fossilisation of different species and different plant organs. Such knowledge is available for many Available online 28 June 2016 systems, but the taphonomy of sclerophyll floras is very poorly known. Keywords: To provide understanding of the taphonomic processes affecting the representation of sclerophyllous plant Plant taphonomy species in fossil assemblages this study investigated the potential source vegetation of plant remains extracted Surface sediments from modern sediments of a subalpine lake in Tasmania, southernmost Australia. It was found that the vast Plant macrofossils majority of the leaf types represented in the sediments belong to broadleaf sclerophyllous species living in Plant megafossils close proximity to the lake, although the representation of species was not related to their values of leaf mass Megaflora per unit area. Leaf assemblages Additionally, a bias between the abundance of species in the standing vegetation and the number of leaves of the Sclerophyll vegetation same species in sediments was observed. Thus, small-leaved shrub species, such as many members of Ericaceae, Representation produce comparatively many more leaves and tend to be over-represented in sediments. In contrast, even though, large-leaved tree species such as Eucalyptus and Nothofagus are dominant in the standing vegetation, they produce substantially fewer foliar organs per ground area of vegetation. Accounting for these discrepancies, we developed an intrinsic representativity index that provides a more accurate picture of the relationship between the leaf assemblages incorporated in the sediments and the abundance of these species in the source vegetation. © 2016 Elsevier B.V. All rights reserved. 1. Introduction implications for the interpretation of plant macrofossil records. However, there have been very few such studies. Evergreen sclerophyll floras are widely distributed around the Accumulations of plant macrofossils (also known as megafossils) in world, especially in Mediterranean-type climates, where they represent different depositional environments, such as lakes or streambeds are the most diverse floras outside the tropics (Cowling et al., 1996). used to refine our understanding of past vegetation history, ecological However, plant macrofossil evidence indicates that diverse sclerophyll processes and related climate conditions (e.g. Allen and Huntley, floras existed under wet non-Mediterranean climates in the Cenozoic, 1999; Birks, 2001; Huntley, 2001; Collinson et al., 2010; Gee, 2005). and even as recently as the early Pleistocene, leading to questions about However, plant macrofossil assemblages can only be validly interpreted the link between sclerophylly and dry climates (e.g. Axelrod, 1975; in the light of the potential biases resulting from the differential preser- Chen et al., 2014; Hill, 2004; Palamarev, 1989; Schnitzler et al., vation of different organs and species. The analysis of plant macrofossils 2011; Sniderman et al., 2013). Plant taphonomic studies investigating from surface sediment samples, the recently deposited sediments in the potential for fossilisation of different plant organs and species in depositional environments such as lakes, can enhance the understand- sclerophyll-dominated environments may, therefore, have important ing of processes that determine the differential potential for fossilisation (Dieffenbacher-Krall and Halteman, 2000; Dieffenbacher-Krall, 2007; Spicer and Wolfe, 1987). ⁎ Corresponding author at: School of Biological Sciences (Life Science Building), The potential for fossilisation in plants may vary depending on both University of Tasmania, Private Bag 55, Hobart, TAS 7001, Australia. E-mail addresses: [email protected] (G.A. Astorga), intrinsic and extrinsic factors (Martin, 1999; Spicer, 1991). In particular, [email protected] (G.J. Jordan), [email protected] (T. Brodribb). intrinsic factors or individual characteristics of plant organs (e.g. the http://dx.doi.org/10.1016/j.revpalbo.2016.06.004 0034-6667/© 2016 Elsevier B.V. All rights reserved. 2 G.A. Astorga et al. / Review of Palaeobotany and Palynology 233 (2016) 1–11 degree of sclerophylly, number, weight, size and chemical composition macroremains collected by Hill and Gibson (1986), and information of leaves and reproductive structures) can be major determinants of the of present day vegetation within the catchment of the lake. Although likelihood of a species becoming fossilised. This is mainly because these Hill and Gibson (1986) made extensive macrofossil collections, they factors can affect the capacity of plant organs to be transported and made no analyses of reproductive structures or detailed consideration preserved in sediments (Ferguson, 1985, 2005; Gastaldo and Demko, of the potential source vegetation. Thus, this study aims to elucidate 2011; Spicer, 1989, 1991). Additionally, the potential for fossilisation is the modern representation of plant macrofossils within the sediments also affected by extrinsic factors (i.e. the natural characteristics of an of Lake Dobson, the identification of key taphonomic factors likely area). For instance, topographic features of the depositional environ- affecting the final representation of plant species within sediments, ment, the geographic distance of plant communities to the site of and to resolve whether the observed patterns are similar to those deposition, and the presence and ability of wind and/or flowing water found in other systems and geographic areas. to transport plant material (Ferguson, 2005; Gastaldo and Demko, 2011; Greenwood, 1992; Spicer, 1989; Spicer and Wolfe, 1987). Thus, 2. Materials and methods complex interactions of intrinsic and extrinsic factors may determine the final representation of plant parts within sediments resulting in fossil 2.1. Study site assemblages that rarely reflect their source vegetation in simple one to one relationships of abundance (Allen and Huntley, 1999; Birks, 2001, Lake Dobson (42°41′ S, 146° 35′ E) is located in the sub-alpine area 2013; Birks and Birks, 1980; Spicer, 1991; Spicer and Wolfe, 1987). of Mt. Field National Park, south-central Tasmania, Australia at an Plant taphonomic studies can usually provide insights into the altitude of 1034 m. The lake covers c. 5.7 ha, and occupies a glacial cirque relationship between the abundance of organs from a specific taxon bounded on the eastern side by a lateral moraine and on the west part present in the sediments, and the frequency of the same taxon in the by a headwall 250 m high rising at a slope of ~30° (Fig. 1). The lake is source vegetation (Birks, 2013; Birks and Birks, 1980; Spicer and Wolfe, irrigated mainly by two incoming streams: The Golden Stairs Creek 1987). However, in these studies the representativity (i.e. the proportion flowing from the Mawson Plateau down to the east, and Eagle Tarn of organs of a species in terms of the total number of organs preserved in Creek connecting Lake Dobson with Eagle Tarn to the north (Fig. 1). sediments) of any given species may vary markedly depending on the Prevailing winds in Lake Dobson area blow predominantly from west system in which it is measured. Partly because the representativity of a to east (Hill and Gibson, 1986). species is measured relative to the abundance of other represented Most of Tasmania has a temperate climate with mild summers or species, and is therefore non-independent of these other species. Thus, a cool summer in higher plateau areas (Stern et al., 2000). The park is more powerful approach is to use taphonomic studies to develop an located in a transitional area between a perhumid region of wet vegeta- understanding of the potential for fossilisation of different plant organs tion types, including extensive wet sedgeland/heaths and cool temperate and species in depositional settings. closed forests, extending to the west, and a subhumid region of Eucalyptus Taphonomic studies to date, including terrestrial and aquatic open forests and grasslands extending to the east (Harris and Kitchener, systems, especially in Europe (Allen and Huntley, 1999; Birks and 2005; Macphail, 1979; Read, 1999). Annual mean temperature at Lake Birks, 1980; Collinson, 1983; Greatrex, 1983; Spicer, 1981) and North Dobson is 6.2 °C with mean temperature of the warmest and coldest America (Demko et al., 1998; Dieffenbacher-Krall and Halteman, quarter 10.3 °C and 2.3 °C, and annual mean precipitation of 1454 mm. 2000; Dunwiddie, 1987; Spicer and Wolfe, 1987) have identified some Snowfalls are common between July and September. Climatic parameters general