phytoliths of common grasses in the coastal environments in the usa

7/27/2019 Phytoliths of Common Grasses in the Coastal Environments in the USA

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Phytoliths of common grasses in thecoastal environments of southeastern USA

Houyuan Lu a,b , Kam-biu Liu b, *a Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825,

Beijing 100029, Chinab Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803, USA

Received 28 October 2002; received in revised form 8 May 2003; accepted 8 May 2003

Abstract

Thirty-four grass species were collected for phytolith analysis from a variety of coastal environments in the southeastern USA(Georgia, Florida, and Louisiana), including salt marshes, freshwater/brackish marshes, pine/oak forests, maritime hardwoodforests, and sand dunes. Phytoliths produced by these modern grasses include a large diversity of shapes and types. We proposea preliminary relationship between modern coastal plant communities and their predominant phytolith contents. The dominantgrasses of coastal sand dunes, such as Uniola paniculata , produce primarily at tower and two-horned tower phytoliths. Rondel/saddle ellipsoid phytoliths are mainly produced by Spartina alterniora , the most common plant in coastal salt marshes. Rondel andspool/horned tower phytoliths are common in brackish marsh grasses. Plants from interdune meadow produce primarily dumbbellphytoliths, as well as small cross and Cyperaceae-type phytoliths. These results provide a basis for the interpretation of fossilphytolith assemblages and the reconstruction of coastal environmental changes. 2003 Elsevier Ltd. All rights reserved.

Keywords: phytoliths; silica bodies; grasses; microfossils; coastal environments; Quaternary; southeastern USA

1. Introduction

Phytoliths are microscopic silica bodies that pre-cipitate in or between cells of living plant tissues. Theyoccur in many plant families ( Pearsall, 2000; Piperno,1988, 2001 ), but are especially abundant, diverse, anddistinctive in the grass family (Gramineae) ( Blackman,1971; Brown, 1984; Clifford & Watson, 1977; Grob,1896; Piperno & Pearsall, 1998; Prat, 1936; Twiss, 2001 ).Many taxa in Gramineae are characterized by phytolithswith specic morphological characteristics, hence theirtaxonomic signicance. Phytoliths are released fromplant tissues when they are decayed, burned, or digested.Released phytoliths thus become microfossils of theplants that produce them.

Phytolith morphology and taxonomy, as well asthe application of phytolith analysis to archaeologicaland paleoenvironmental research, have been the subjectof many studies ( Bowdery, Hart, Lentfer, & Wallis,2001; Horrocks, Deng, Ogden, & Sutton, 2000; Kondo,Childs, & Atkinson, 1994; Lu et al., 1996; Madella,1997; Meunier & Colin, 2001; Mulholland & Rapp,1992; Pearsall & Piperno, 1993; Piperno, 1988; Rosen,1992; Rovner, 1988; Wang & Lu, 1993 ). However, untilrecently very little attention had been paid to the useof phytoliths in the study of coastal environmentalchanges ( Fearn, 1998; Horrocks et al., 2000 ). In thisstudy, we conducted a pioneer investigation of phyto-liths in modern grasses growing in the coastal environ-ments of southeastern USA (Georgia, Florida, andLouisiana).

The coastal zones of the southeastern USA consist of a variety of ecological habitats and vegetation types.Chief among them are sand dunes and interdune

* Corresponding author.E-mail address: [email protected] (Kam-biu Liu).

Estuarine, Coastal and Shelf Science 58 (2003) 587600

0272-7714/03/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0272-7714(03)00137-9
mailto:[email protected]:[email protected]


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meadows, maritime or upland forests, swamps, freshmarshes, brackish marshes, and salt marshes ( Bertness,1999 ). These coastal environments pose special chal-lenges to Quaternary paleoenvironmental reconstruc-tion using conventional paleoecological techniques suchas pollen analysis (e.g. Clark, 1986; Clark, Overpeck,

Webb, & Patterson, 1986; Clark & Patterson, 1985;Davis, 1992 ). One of the constraints in the application of pollen analysis is that some key coastal environments,particularly salt and fresh/brackish marshes and to someextent sand dunes, are dominated by grasses, but, exceptfor Zeas mays (maize), Gramineae pollen cannot beidentied below the family level ( Fearn & Liu, 1997 ).Here we demonstrate that this constraint can be over-come by phytolith analysis, because characteristic grassesof different coastal environments produce differentphytolith assemblages.

2. Phytolith classication

At present, the classication of phytoliths is princi-pally based on the study of some modern Gramineae,Xylophyta, and a few other plants. Different researchershave suggested different terms and classication schemesbecause of the differences in materials, classicationcriteria, and study areas. So far, no uniform andconvenient classication scheme has been widely adop-ted for various conditions. Nevertheless, many impor-tant investigations have made a great contribution to

phytolith classication. Taking Gramineae phytoliths asan example, Prat (1936) divided the Panicoideae sub-family into two groups by studying the shapes of shortcells in the leaf veins of some genera of Gramineae.Twiss, Suess, and Smith (1969) classied Gramineaephytoliths into four groups after they summarized theachievements of Prat (1936) and other researchers.Brown (1984) classied Gramineae phytoliths into eightcategories including more than 130 types after studyingthe phytoliths from different parts of 112 taxa of Gramineae plants. Piperno (1988) presented an indextable of two different kinds of phytoliths. Mulhollandand Rapp (1992) proposed one morphological classi-cation of grass silica bodies after summarizing differentresearchers classications for the Gramineae family.Piperno and Pearsall (1998) summarized the distributionof short-cell phytoliths from the grasses, which weredescribed as circular to oval-(rondels), saddle-, bilobate-,or cross-shaped, and well-established diagnostic featuresof the leaf epidermis.

The above classication of phytoliths was mainlyproposed by European and American botanists. Japa-nese researchers such as Sase and Kondo (1974)developed a morphological classication, which addedthe following three classes: fan-shape, point-shape, and

bambusoid. Kondo et al. (1994) classied Gramineaephytoliths into nine classes based on a modied classi-cation of Kondo and Sase (1986) . In China, Wangand Lu (1993) classied grass phytoliths into 15 classes.Table 1 attempts to compare and reconcile the mainclassications of grass phytoliths proposed by various

researchers.The dumbbell (lobate) phytolith originates from theepidermal cells (shortcells) of Panicoideae and Oryzoi-deae, some Arundinoideae, and Chloridoideae subfami-lies (Brown, 1984; Fearn, 1998; Lu & Liu, 2003;Mulholland, 1989 ). Table 1 shows that different re-searchers mostly subdivided the dumbbell into threegroups: 1, dumbbell or bilobate, lobate; 2, complexdumbbell or multilobed, polylobates; and 3, cross.

The Chloridoid class ( Twiss et al., 1969 ) consists of only two types of saddle-shaped bodies; one is theChloridoid type ( Table 1 ), which originates fromepidermal cells (short cells) of Chloridoideae, and someBambusoideae and Arundinoideae subfamilies. Theyhave the form of short saddles (short seat) with non-wrinkly surface, and have been described elsewhere asbattle-axes with a double edge (Kondo et al., 1994;Prat, 1936 ). This type was referred to as the short saddletype, short plateau saddle type, or Chloridoid class byWang and Lu (1993), Piperno and Pearsall (1998), andKondo et al. (1994) , respectively. Another is the thinChloridoid type, which is saddle shaped, similar toChloridoid type, but longer. They may have a wrinklyand/or a non-wrinkly surface. This type was called thelong saddle type, tall collapsed saddle type, or Bambusid

class by Wang and Lu (1993), Piperno and Pearsall(1998), and Kondo et al. (1994) , respectively.

Pearsall (1989) and Pearsall and Trimble (1983, 1984)found six commonly occurring phytolith types, thenreferred to as horned towers, at towers regular spools,irregular spool, angular, and half rotated. Kondo et al.(1994) developed both Chionochloid class and phyto-liths from other short-cells class based on these shapes.It is difficult to relate these types to grass taxonomy andto infer moisture and temperature conditions from theiroccurrence ( Pearsall, 1989, 2000 ).

The Festucoid class ( Twiss et al., 1969 ) containsseveral geometrical types that are circular, rectangular,elliptical, or oblong. Kondo et al. (1994) and Pearsall(1989) used the same term of Festucoid class to denethe short-cell phytolith types as originating from theepidermal cells of the Pooideae subfamily. They alsoused different geometrical termsboat-shape, hat-shape, rectangular, and round/oblongto describeTwiss (1969) circular, rectangular, elliptical, and oblongtypes. Current groupings within the Festucoid classinclude two major geometrical types. One is trapezoids(Brown, 1984 ), which correspond to wavy trapezoid(Piperno & Pearsall, 1998 ), rectangular ( Mulholland &Rapp, 1992; Pearsall, 1989 ), boat-shaped ( Kondo et al.,

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Table 1Comparison of different grass phytolith classication systems

Lu et al., 1996 andWang and Lu, 1993

Piperno andPearsall, 1998(short cell) Twiss et al., 1969 Brown, 1984

Pearsall, 1989 andPearsall andTrimble, 1983

Sase andKondo,1974

Dumbbell Bilobate Panicoid class(dumbbell)

VI Bilobates Panicoid shapes(dumbbell, angular,

nodular, cross,crenate,half dumbbell)

Panicoid

Multilobed short cell (Complex dumbbell) VII Polylobates?

Cross (Cross) VIII Crosses

Long saddle Tall saddle/long saddle Chioridoid class(thin Chloridoid)

IV Saddles Chloridoid shapes(saddles)

Chloridoid

Collapsed saddle

Narrow-elliptate

Short saddle Short saddle (Chloridoid) Chl

Plateau saddleConical? Horned towers,

at towers, regular spoolsBilobate/saddle conicalirregular short cell

Half rotated, angular,irregular spool

Multiple tooth Wavy trapezoid Festucoid class(circular rectangularelliptical oblong)

V Trapezoids Festucoid shapes(square/rectangular)

Pooid(Festuoid)

Weakly tooth Hat Rondel

(circular to oval)III Double outlines (Round/oblong) (Ha

Fan type withraised ridge

I Plates (IB) Bulliform cells Fan-shaped Fan

Fan type without

raised ridgeSquareRectanglePlate-like bar Elongate class I Plates (IA) Long cells Elongate ElongPoint (joint) barSmooth-barLong-point II Trichomes Trichomes Point-shaped PointShort-point


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1994 ), and tooth-shaped ( Wang & Lu, 1993 ). Another isdouble outlines ( Brown, 1984 ), which correspond torondel ( Mulholland & Rapp, 1992; Pearsall, 1989;Piperno & Pearsall, 1998 ) and hat/cone-shaped phyto-liths ( Kondo et al., 1994; Wang & Lu, 1993 ).

Biogenic silica can also deposit in bulliform (motor)

cells, prickle hairs (Trichomes), long cells, stomata, andvascular tissues of grass epidermis. They can also formin subepidermal cells, forming easily recognizablebodies. The phytolith shapes from bulliform cells,prickle hairs, long cells, and non-short cells of leaveshave been described as fan, elongate, point, square, andplates by Twiss et al. (1969), Kondo et al. (1994), Wangand Lu (1993), and Brown (1984) . These forms do notpossess any subfamily or tribal characteristics, but havesecondary features allowing further subdivision ( Wang& Lu, 1993 ).

As for non-Gramineae plants, Kondo and Pearson(1981) and Kondo and Sumda (1978) classied phyto-liths of gymnosperm and monocotyledonous angio-sperm trees into six groups, and those of dicotyledonousangiosperm trees into eight groups based on studies of a wide range of native and exotic trees in Japan. A greatdiversity of phytoliths shapes from dicotyledon specieshas been described by Bozarth (1992), Geis (1973),Piperno (1988), and Runge (1999) . Ollendorf (1992)proposed a classication scheme for sedge (Cyperaceae)phytoliths.

While all these various classication systems haveadvanced the young science of phytolith study, signi-cant shortcomings and discrepancies remain. Some

classication systems are only suitable for detailed studyof modern plants. Some other classication schemes areonly applicable to regional oras.

Phytoliths produced in grass epidermis can be dividedinto two broad morphological classesbodies in longcells and bodies in short cells ( Metcalf, 1960 ). Long-cellsilica bodies are variable in shape but tend to beelongate with sinuous, often interlocking borders. Butlike their bulliform or prickle hair counterparts, theelongate phytoliths also do not posses any subfamily ortribal characteristics. By contrast, short-cell phytolithscan be generally classied into distinctive grass sub-families ( Piperno & Pearsall, 1998; Twiss et al., 1969 ).Although the previously published grass short-cellphytolith classication schemes have much in common,signicant differences in terminology and in emphasis onspecic morphological characteristics remain. In thisstudy, we use a descriptive name to describe eachmorpho-type. Each short-cell phytolith has been classi-ed into one of 11 basic morpho-types based on Pearsall(1989), Piperno and Pearsall (1998) , and this study. The11 morpho-types are dumbbell, cross, long saddle, shortsaddle, plateau saddle, rondel, at tower, two-hornedtower, spool/horned tower, rondel/saddle ellipsoid, andwavy trapezoid ( Fig. 1 ).

3. Materials and methods

3.1. Samples of modern plants for analysisof phytoliths

Sixty species of modern plants (including 34 species

of grasses) were collected for phytolith analysis from theAtlantic and Gulf of Mexico coasts of the southeasternUSA in the summers of 1999 and 2000. The principalsampling sites are Cumberland Island in southernGeorgia and the Gulf coastal regions of northwesternFlorida and Louisiana. The samples were collected froma variety of coastal or near-coastal environments, such assaltmarshes,pine/oakforests,maritimehardwoodforests,freshwater and brackish marshes, and sand dunes ( Table2). Each plant sample includes leaves, culms, inorescen-ces, and roots. Twenty-ve grass genera representing allsix subfamilies (Pooideae, Panicoideae, Chloridoideae,Bambusoideae, Oryzoideae, and Arundinoideae, seeGould & Shaw, 1983 ) were included in the samples.

Twenty-ve of the 34 grass species included in thisstudy belong to two subfamilies, Panicoideae andChloridoideae. Grasses in Panicoideae are distributedwidely in humid tropical or subtropical areas ( Gould &Shaw, 1983 ). The most suitable growing condition forgrasses of this subfamily is in the southeastern USA,while the poorest is in the northwest ( Gould & Shaw,1983 ). Although temperature is a primary limitingfactor, a few species are adapted to dry conditions,and moisture supply becomes an important secondarylimiting factor. The Chloridoideae is widely distributed

over the North American continent, but its frequency ishighest in the southwestern USA and northern Mexico(Gould & Shaw, 1983 ). There, under a warm and dryclimate, over 50% of the grass species are Chloridoid.Although in this study we have not collected all the grassspecies occurring in the coastal ora, we focused on thetypical or common grasses of the key coastal environ-ments in the southeastern USA. For example, oursamples include Uniola paniculata and Aristida desman-tha of the sand dunes, Spartina alterniora of the saltmarshes, Spartina patens of the brackish marshes,Arundinaria longifolia of the maritime forest edges, andPhragmites australis of the fresh marshes ( Bertness,1999; Gosselink, 1984; Wiegert & Freeman, 1990 ).

3.2. Methods of extracting phytoliths from modern plants

All the collected plant samples were cleaned withdistilled water in an ultrasonic water bath to removeadhering particles. Leaves and culms of each specieswere placed in 20 ml of saturated nitric acid for onenight to oxidize organic materials completely. Thesolutions were centrifuged at 2000 rpm for 10 min,



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Fig. 1. Hand drawings of principal morpho-types of short-cell phytoliths found in the 34 species of coastal grasses from the southeastern USA.

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decanted, and rinsed twice with distilled water. Theywere then rinsed with 95% ethanol until the supernatantwas clear. The phytolith sediments were transferred tostorage vials. The residual subsamples were mountedonto microscopic slides in Canada Balsam medium forphotomicrography and in liquid (glycerol) medium forcounting and line drawing. A minimum of 350 phytolithgrains were counted in each sample.

Light photomicrography at 400 magnication wasused to record typical types of phytoliths found in planttissues. A multitude of phytolith types were isolatedfrom the grass samples analyzed. A few of them havenot been reported previously in the phytolith literature.

4. Results and discussion

4.1. Distribution of short-cell phytolith typesobserved in coastal grasses

Eleven morpho-types of short-cell phytoliths werefound among the 34 grass species included in this study(Fig. 1 and Table 3 ). The different morpho-types of selected common coastal grasses are illustrated orphotomicrographed in Plates 13 . Our photomicro-graphic and hand-drawn illustrative key herein isdesigned to aid the identication of phytoliths com-monly found in Quaternary or archaeological deposits.

Table 2List of 34 modern grasses from the US Atlantic and Gulf coasts used for the phytolith analysis a

No. NameSubfamily (Gould& Shaw, 1983)

Ecology or distribution of plants (Allen, 1992; Gould &Shaw, 1983)

1 Andropogon glomeratus (Walt.) BSP Panicoideae Generally on wet sites2 Andropogon ternaries Michx. Panicoideae Edge of pine forest3 Anthaenatia rufa (Nutt.) Schult. Panicoideae Pine forest4 Aristida desmantha Trin. and Rupr. Chloridoideae Sandy soil along coast5 Arundinaria longifolia E. Fourn Bambusoideae Edge of forest6 Arundinaria gigantea (Walt.) Muhl. Bambusoideae Pine forest or edge of atwoods7 Avena sativa L. Pooideae Disturbed areas8 Cenchrus incertus M. Curtis Panicoideae Dry sand9 Chasmanthium laxum (L.) Yates. Arundinoideae Edge of forest

10 Chasmanthium ornithorhynchum (Steud.) Yates Arundinoideae Moist area11 Ctenium aromaticum (Walter) A.W. Wood Chloridoideae Edge of forest, interdune meadow12 Dactyloctenium aegyptium (L.) Willd. Chloridoideae Southeast and west USA13 Distichlis spicata (L.) Greene Chloridoideae Saline areas in coastal marshes and

islands, sandy beaches14 Eragrostis oxylepis (Torr.) Torr. Chloridoideae Sandy soil along the coast15 Eragrostis cilianensis (All.) Vignolo-Lutati Chloridoideae Sandy soil along the coast16 Erianthus strictus Spreng. Panicoideae Edge of pine forest and disturbed areas17 Eustachys petraea (Sw.) Desv. Chloridoideae Sandy soil along the coast18 Leersia oryzoides (L.) Sw. Oryzoideae Wet roadside ditches and edges of lakes,

streams, and other wet areas19 Panicum amarum Elliott Panicoideae Sandy soil along coast, interdune meadow20 Panicum dichotomiforom Michx. Panicoideae Disturbed areas, especially in moist

regions, throughout USA21 Panicum hemitomon Schult. Panicoideae Coastal marshes, wet areas and in the

inland part of coastal sites, interdune meadow22 Panicum verrucosum Muhl. Panicoideae Disturbed areas and edges of forests,

mostly in pine regions23 Panicum virgatum L. Panicoideae Edges of pine forests and remnant strips

in prairie regions; cheniers and spoilbanks in coastal freshwater marshes

24 Phragmites australis (Cav.) Trin. Ex Steud. Arundinoideae Fresh to brackish water marshes25 Poa annua L. Pooideae Disturbed areas throughout USA26 Saccharum officinarum L. Panicoideae Cultivated in tropical regions of the world27 Setaria sp. Panicoideae ?28 Sorgastrum nutans (L.) Nash Panicoideae Edge of forest and disturbed areas in pine and

prairie regions29 Sorghum halepense (L.) Pers. Panicoideae Widespread throughout the world30 Spartina alterniora Loisel Chloridoideae Salt marshes and sandy areas on coast31 Spartina patens (Ait.) Muhl. Chloridoideae Brackish to saline areas in coastal marshes32 Sporobolus virginicus (L.) Kunth. Chloridoideae Sandy soil along the coast33 Uniola paniculata L. Chloridoideae Sandy soil along the coast34 Zizaniopsis miliacea (Michx.) Doell and Aschers. Oryzoideae Edges of lakes, streams, wet roadside ditches

a All plant samples are preserved in the Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803, USA.



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Lobate phytoliths, including the dumbbell and crosstypes according to the traditional nomenclature, havebeen identied consistently as distinctive silica bodies(Lu & Liu, 2003 ). Dumbbells are characterized byhaving two somewhat rounded lobes connected bya shaft. Some (complex types) have one or two ad-

ditional lobes on the shaft. Crosses are shorter, nearlyequidimensional bodies with four distinct lobes. Allspecimens of plants from Panicoideae ( Fig. 1 and Plate1A, D ) and Oryzoideae ( Plate 1B ) grasses in this studyhave typical dumbbell type. The crosses are mostlyassociated with the Panicoideae subfamily. Despite

being the most characteristic markers of the Panicoideae(Twiss et al., 1969 ), dumbbell and cross phytoliths arealso found to be present in the Chloridoideae ( Aristidadesmantha and Ctenium aromaticum ) and Arundinoi-deae ( Chasmanthium laxum and Chasmanthium ornitho-rhynchum ) subfamilies in this study ( Table 3 ).

The term

dumbbell

was rst used by Metcalf (1960)as a morphological term for the shape of some inter-costal short-cell phytoliths. It has gradually become aname given to a loosely dened group of phytolithscharacterized by having two lobes joined by a shank.However, many subsequent researchers, including Brown

Table 3Distribution of short cell phytolith types observed in coastal grasses

Taxon Dumbbell CrossLongsaddle

Shortsaddle

Plateausaddle Rondel

Flattower

Two-hornedtower

Spool/hornedtower

Rondel/saddleellipsoid

Wavytrapezoid

Panicoideae

Andropogon glomeratus A*Andropogon ternaries A* CAnthaenatia rufa A* C*Cenchrus incertus A* R* RErianthus strictus A*Panicum amarum A* CPanicum dichotomiorum A* C* RPanicum hemitomon A* C* RPanicum verrucosum A*Panicum virgatum A* R CSaccharum officinarum A* CSetaria sp. A*Sorgastrum nutans A* R*Sorghum halepense A* A* C

ChloridoideaeAristida desmantha R? A C CCtenium aromaticum A? C RDactyloctenium

aegyptiumA* R R

Distichlis spicata R C C AEragrostis oxylepis A* REragrostis cilianensis A* R REustachys petraea A* R RSpartina alterniora R* ASpartina patens R R C A RSporobolus virginicus R R R AUniola paniculate A C C

BambusoideaeArundinaria longifolia A* R R R

Arundinaria gigantean A* R RPooideae

Avena sativa R* A*Poa annua C* R A*

ArundinoideaeChasmanthium laxum A C RChasmanthiumornithorhynchum

A R R

Phragmites australis R A* R C

OryzoideaeLeersia oryzoides A CZizaniopsis miliacea A R C

A, abundant; C, common; R, rare; * diagnostic forms present, characteristic of taxons subfamily.



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(1984), Fredlund and Tieszen (1994, 1997), and Pipernoand Pearsall (1998) have avoided using this term andfavored the alternative term bilobate . Recently Lu andLiu (2003) documented the variability of 25 diagnosticdumbbell (lobate) phytolith shapes occurring among 85modern grass species collected from a variety of environ-ments in China and the southeastern USA.

The short saddle morpho-type is produced in a highproportion by the Chloridoideae. The descriptive term,battle-axes with double edges (Prat, 1936 ), refers to theoutline of the base when the body is oriented in topview. Saddle-shaped phytoliths are the dominant class of the Chloridoideae subfamily ( Fig. 1 and Plate 3C ). Theyare also common in two species of the Bambusoideae

Plate 1. Illustrations of long-cell and short-cell phytolith morpho-types in four common coastal grass species: A, Panicum verrucosum (mostlydumbbells); B, Leersia oryzoides (mostly dumbbells, some at towers); C, Aristida desmantha (mostly tower-shaped phytoliths, some dumbbells);D, Panicum hemitomon (mostly dumbbells). One scale 10 l m.



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(Arundinaria longifolia , Arundinaria gigantea ), and arepresent in Phragmites australis (Arundinoideae) as well(Table 3 ).

A great quantity of rondel/saddle ellipsoid morpho-types are isolated from Spartina alterniora (Chloridoi-deae) ( Fig. 1 and Plate 2A ), with both round and saddletendencies. Spartina alterniora (cordgrass) is a tall,stiff-stemmed grass typically growing in salt marshes.On the other hand, the spool/horned tower type ( Fig. 1

and Plate 2C ) is produced primarily in brackish marshgrasses, such as Spartina patens (salt-meadow hay) andDistichlis spicata . This type is equal to the Chionochloidclass of Kondo et al. (1994) and the regular spools of Pearsall (1989) .

A great number of phytoliths called plateau saddles

are isolated from the leaves of Phragmites (Plate 3D ).More study is needed before they can be assigned genus-specic status, but they can be used as a marker of the

Plate 2. Illustrations and photomicrographs of different phytolith morpho-types in three common coastal grass species: A, Spartina alterniora(mostly rondel/saddle ellipsoid phytoliths); B, Uniola paniculata (mostly at towers, some two-horned towers and spool/horned towers); and C,Spartina patens (mostly spool/horned towers).



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Plate 3. Photomicrographs of principal phytolith morpho-types in four common coastal grass species: A, Arundinaria longifolia (mostly longsaddles); B, Avena sativa (mostly wavy trapezoid); C, Dactyloctenium aegyptium (mostly short saddles); and D, Phragmites australis (mostly plateausaddles) (bar unit in this plate is l m).



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Arundinoideae and to determine the possible presenceof Phragmites in soil phytolith assemblages. Kondo andSase (1986) distinguished the saddle from Phragmites asmiddle saddle , which is shorter than the long saddle

from Bambusoideae and longer than the short saddle

from Chloridoideae.

Bambusoideae contributes a large number of di-agnostic types at the family level, particularly the longsaddle phytoliths with a wrinkly and/or non-wrinklysurface ( Plate 3A ). The length of saddle from Bambu-soideae is longer than that from Chloridoideae.

Rondels (circular to oval phytoliths) and wavytrapezoids ( Mulholland, 1989 ) are most closely associ-ated with Pooideae. Wavy trapezoid ( Plate 3B ), inparticular, appears to be unique to Pooideae. Therondels are also found in the Chloridoideae ( Table 3 ).

Some types of phytoliths from coastal grasses do notlend themselves to easy description. For example, attowers, two-horned towers, and spool/horned towers(Fig. 1 and Plate 2B, C ) are typically small, irregularphytoliths. Although some researchers avoided usingthese descriptive terms (at towers, two-horned towers,and spool/horned towers), no uniform or standardterminology has been adopted in the literature. Thesephytoliths warrant further investigation, as some of them are likely to be of precise taxonomic value. Theyhave been isolated in great numbers from the Chlor-idoideae. In particular, at towers are produced in highproportions ( > 90%) by Uniola paniculata (sea oats)(Plate 2B ) , a C 4 grass typically occurring in primarydunes or other sandy substrates along the coast.

The sinuate elongate, smooth elongate, point, fan-shaped, square, and rectangle morpho-types ( Plate 1A,D ) are derived from epidermic long cells, trichome cells,and bulliform cells of grasses. They have no taxonomicsignicance in our classication.

Non-Gramineae families from coastal environmentscan also produce typical phytolith morpho-types. Forexample, the cone-shaped morpho-type is attributed tothe Cyperaceae (sedge) family, and the circular crenateto Sabal minor (saw palmetto).

Even though this study has not described all the shortcell phytoliths occurring in the coastal ora, our samplesinclude grasses that are common or diagnostic of mostof the ecologically important coastal environments inthe southeastern USA. More studies are needed thatwould include additional species in the larger genera of the Gramineae, as well as in other non-Gramineae taxa.

4.2. Typical assemblages of grass phytolithsin coastal environments

Grasses occur in nearly all habitats throughout thecoastal environments of the southeastern USA ( Bert-ness, 1999; Gosselink, 1984; Wiegert & Freeman, 1990 ).Vegetation in the coastal zone is strongly affected by

various environmental gradients. Perhaps the mostimportant factors are topography and salinity. Someof the drier habitats on the coast include beach ridgesand sand dunes, where Uniola paniculata (sea oats),Sporobolus virginicus (marshgrass, crabgrass) , Aristidadesmantha (curly threeawn), Distichlis spicata (salt-

grass), and Eragrostis cilianensis (stinkgrass) occur.The coastal marsh is characterized by large populationsof relatively few species. Spartina alterniora (cordgrass)is the dominant plant in salt marshes. Brackish marshes,developed extensively in estuaries and deltas, and on theperipheries of bays and lagoons, are often dominated bySpartina patens (salt-meadow hay). Other common grassspecies in brackish marshes include D. spicata , Leersiaoryzoides , and Panicum hemitomon . Fresh marshes aredominated by Panicum virgatum , P. hemitomon , andPhragmites australis . Swamps occupy the area betweenupland forests and coastal marshes. In these wetlandforests common trees include Taxodium distichum , Salixnigra , Sabal minor , Nyssa aquatica , and Quercus spp.Some grass species commonly found in the swamps arePanicum spp., Eragrostis spp., and Zizaniopsis miliacea .

Phytolith analysis of the grasses from differentcoastal vegetation communities shows signicant varia-tion in shapes and assemblages ( Fig. 2 ). Rondel/saddleellipsoid phytoliths are mainly produced by Spartinaalterniora , the most common plant in the salt marshes.Thus the predominance of rondel/saddle ellipsoidphytoliths in a sediment sample may be a good indicatorof salt marsh environment. Rondel and spool/hornedtower phytoliths are common in grasses from brackish

marshes, such as Spartina patens . Flat towers, two-horned towers, and short saddle-shaped phytolithsdominate the grass samples from sand dunes, especiallyUniola paniculata . Plateau saddle and dumbbell phyto-liths are common in the grass samples from freshmarshes, such as Phragmites australis . The grass samplesfrom coastal swamps produce many dumbbell phyto-liths. The long saddle phytoliths are exclusively derivedfrom Bambusoideae such as Arundinaria spp., whichcommonly occurs on the edge of upland forests ormaritime forests. They are distinctly larger than theplateau saddle phytoliths typically found in Phragmites .Trees from maritime forests also produce typicalphytolith shapes such as the circular crenate type frompalms ( Pearsall, 1989, 2000 ), and tracheid, polyhedronshape from other broad-leaved hardwood trees ( Kondoet al., 1994 ).

Although our study is based on the phytolithcontents of grass plants, the conclusions are supportedby the results of phytolith analysis of soil samplescollected from the corresponding coastal environments.In the 50 surface soil samples collected for phytolithanalysis from different environments along the Atlanticand Gulf coasts of the southeastern USA ( Lu & Liu,2001 ), we found that the modern phytolith composition



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in the primary dunes is well characterized by a highamount of two-horned towers (30 25%), at towers(13 7%), and short saddle (9 6%) phytoliths. Thephytolith composition from the surface samples ininterdune meadow shows very high values of dumbbellphytoliths (47 13%). Both the Palmae(circular crenate)morpho-types and other broad-leaved types account formore than 6080% of all phytoliths, and grass phytolithsaccount for less than 20% in the modern soils undermaritime forests ( Lu & Liu, 2001 ).

Our study demonstrates the high potential of phytolith analysis as a tool for identifying vegetationin coastal environments. Each of the key coastal en-vironments yielded abundant and distinctive phytolithassemblages.

5. Conclusions

Phytolith analysis of modern grasses from variouscoastal environments including salt marshes, brackishmarshes, freshwater marshes, pine/oak forests, maritimehardwood forests, and sand dunes in the southeasternUSA has revealed a large diversity of phytolith shapes.The dominant grasses from coastal sand dunes, such as

Uniola paniculata , and Eragrostis spp., produce primar-ily at towers and two-horned towers, as well as smallshort saddle and dumbbell-shaped phytoliths. Thegrasses and sedges from interdune meadows, such asPanicum spp., Cenchrus incertus , and Cyperaceae, pro-duce primarily dumbbell, small cross, and Cyperaceaephytoliths. The Pooideae and Bambusoideae subfamiliesof grasses, which are generally common along the edgesor on the understory of maritime forests, produce wavytrapezoid and long saddle phytolith, respectively. Thedominant grasses of swamps produce primarily dumb-bell and plateau saddle phytoliths. Rondel/saddleellipsoid phytoliths are almost exclusively produced bySpartina alterniora , the dominant plant in salt marshes.Thus the predominance of rondel/saddle ellipsoidphytoliths in a sediment sample may be a good indicatorof salt marsh environment. Rondel and spool/hornedtower phytolith are common in grasses from brackishmarshes.

This study provides a basis for the interpretation of fossil grass phytolith assemblages recovered fromcoastal sediments for the reconstruction of coastalenvironmental changes. Some coastal environments,such as salt marshes and brackish marshes, may not beeasily distinguishable based on their pollen assemblages

Fig. 2. An idealized transect of coastal environments in the southeastern USA showing major vegetation zones and their representative phytolithassemblages. The width of a black bar indicates relative abundance of each phytolith type.



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(e.g. see Chmura, 1994; Clark, 1986; Clark & Patterson,1985; Fearn, 1998 ). Other environments, such as sanddunes, are palynologically unidentiable because pollenis absent or rare due to poor preservation. Ourinvestigation demonstrates that these key coastalenvironments (salt marshes, brackish marshes, and sand

dunes) produce distinctive phytolith assemblages thatcan be used as a supplementary tool in coastalpaleoenvironmental reconstructions ( Lu & Liu, 2001 ).

Acknowledgements

We thank X.Y. Zhou and Carl A. Reese forproviding modern grass reference samples and forhelpful discussion. This work was supported by grantsfrom Risk Prediction Initiative (RPI) of the BermudaBiological Station for Research (RPI-00-1002), and bythe US National Science Foundation (SES-8922033,

SES-9122058, and BCS-0213884), and NSFC (40024202,40271117).

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