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Paleobotany |
2Institut F. A. Forel, Université de Genève, 10 route de Suisse, CH-1290 Versoix, Switzerland; 3The Cambridge Phytolith Project, The McDonald Institute for Archeological Research, University of Cambridge, Downing Street, Cambridge CB2 3ER, UK; 4Centre Alpien de Phytogéographie, Fondation J.-M. Aubert, CH-1938 Champex and Conservatoire et Jardin botaniques de la Ville de Genève, 1 ch. de l'Impératrice, CH-1292 Chambésy, Switzerland; 5Institute of Plant Science, University of Bern, Altenbergrain 21, Ch-3013 Bern, Switzerland
Received for publication May 15, 2001. Accepted for publication August 9, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Alps • aluminum • Coniferae • Cyperaceae • Ericaceae • Gramineae • opal • X-ray microanalysis
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Fredlund and Tieszen, 1997
; McClaran and Umlauf, 2000
). Several other phytolith morphotypes are known to be diagnostic at different taxonomic levels in many economic plants (e.g., Ollendorf, Mulholland, and Rapp, 1987
; Piperno, 1988
; Mulholland and Rapp, 1992
). This is so, for example, of phytoliths produced in the epidermal tissues of palm (diagnostic at the family level) and banana leaves (genera Musa and Ensete) and in leaf and inflorescence tissues of Oryza sativa (rice) (for a review see M. Madella, unpublished manuscript). However, the taxonomic resolution of phytoliths is limited because of the intraspecific and interspecific variability of the silicified cells. Unquestionably, phytoliths are often the cast of cells that do not show diagnostic morphologies and their anatomical characteristics are redundant in different taxa (e.g., epidermal or mesophyll cells). Therefore, phytoliths with similar morphology can occur in plants that are not taxonomically related and/or can occur in different organs of the same plant. This intrinsic uncertainty in phytolith analysis has been obviated by the employment of different research approaches. It has been shown, for instance, that the taxonomic diagnostic power of a single phytolith morphotype is enhanced if the morphometric variability was measured and taken into account (Ball, Gardner, and Anderson, 1999
). Moreover, the concept of phytolith assemblages (for a review on this subject see Power-Jones, Padmore, and Gilbertson, [1989]
), in which the combination of a statistically meaningful pool of the phytolith morphotypes present in a record is considered (instead of the presence of a single morphotype), was successfully employed as a proxy to describe present plant communities (Powers-Jones, Padmore, and Gilbertson, 1989
; Powers-Jones, 1994
; Madella, 1997
; Fredlund and Tieszen, 1997
) and also to interpret the fossil record (Power-Jones et al., 1989
; Fredlund and Tieszen, 1994
; Power-Jones, 1994
; Madella, 1997
; Barboni et al., 1999
).
In the present work, it is suggested that the elemental characterization of biogenic silica through X-ray microanalysis paired with the phytolith morphology can be employed as a new tool to resolve taxonomic deadlock in the identification of phytoliths. Investigations into the chemical nature of phytoliths evidenced that aluminum is co-deposited in the phytoliths of certain species in the form of aluminosilicate (AS) and/or hydroxyalumino silicate (HAS) (Bartoli and Wilding, 1980
; Hodson and Sangster, 1999
). They also demonstrated that most of the taxa with Al present in the biogenic silica reticule were woody species (Hodson and Evans, 1995
). From these data, we hypothesized that phytoliths formed in the tissues of conifers and arboreal dicotyledons might be characterized by the co-deposition of aluminum and silica.
A reference collection of phytoliths from plants occurring in meadows, heaths, and forests of the Valaisan Swiss Alps (Carnelli, Madella, and Theurillat, 2001)
was used to test this hypothesis. The analysis was run on 20 species from this collection. We focused in particular on phytoliths with nonidiomorphic shapes, generally irregular polyhedrons, as they could not be identified by a repetitive and clearly defined morphological type (Figs. 1–8). These were tested for the presence of aluminum in the biogenic silica reticule to check if Al presence was diagnostic at any taxonomic level. Polyhedrons most commonly occur in dicotyledons and gymnosperm leaf and wood tissues, but morphotypes with a polyhedral outline are also found in the tissues of monocotyledonous plants (e.g., fragmented epidermal cells) (Figs. 6–8).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. The observation at the scanning electron microscope (SEM) does not allow distinction between opal and cristalline silica. The degree of purity of the silica extracted from the plant was previously estimated at light microscopy by counting the number of optically isotropic and anisotropic particles in three microscope fields (Carnelli, Madella, and Theurillat, 2001)
. Since oxalates and carbonates had been removed during the extraction procedure, optically anisotropic particles were considered to be crystalline silica, alkali feldspar, or micas (the most common minerals in the soil and parent rock in the sampling area) adhering to the plant surface and not removed by the cleaning process. The percentage of particles constituted of opal biogenic silica was very high: on average 96% for monocotyledons, 81% for dicotyledons, and 90% for conifers (Carnelli, Madella, and Theurillat, 2001)
. Although it is not possible to exclude the presence of traces of cristalline silica, only the polyhedrons that could be visually recognized as cast of plant cells were selected for the X-ray analysis.
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5 x 10–3 g) was extracted from each sample and spread with the help of a paintbrush on double-sided tape that was mounted on aluminum stubs. The stubs were coated with a gold layer (Balzers SCD 004 Sputter coater, Balzers Hochvakuum GMBH, Wiesbaden-Nordenstadt, Germany) and examined in a JEOL JSM 6400 scanning electron microscope operating at 15 kV (JEOL EUROPE S.A., Croissy-sur-Seine, France). An Oxford Link-Isis 300 (Oxford Instr. Sarl Analytical, Saclay, France) connected to the SEM carried out a semi-quantitative analysis of the pointed phytolith with a beam current of 50 nA, a working distance of 15 mm, and a live time of 50 sec. An Si(Li) detector type and an S-ATW window type (organic film) were used. During the analyses, the standard vertical distance and scale were kept constant in order to allow comparison of the data. The presence of Al peaks was visually estimated and considered positive when distinctly higher than the background radiation. Blank trials were run on the coated stubs to check for the presence of contaminants and to verify that the aluminum from the stubs was not interfering with the analysis. No extraneous Al peak was detected. Photographs were taken with Agfa Pan APX 100 ASA professional film. A total of 100 irregular polyhedral phytoliths were analyzed for each species. As these morphotypes can have a different histologic origin (epidermal cells, tracheary system, mesoderm), phytoliths extracted from the leaves and from the ligneous tissues of woody taxa were analyzed separately. The phytoliths were selected on the basis of their morphology only from a randomly selected transect on the stub. This approach reflects the conditions of a soil-born fossil phytolith assemblage of unknown origin.
The standard deviation was calculated employing the binomial distribution. If out of n tests, the presence of aluminum was detected in r cases, the standard deviation of r is: SDr = [n x p(1 – p)]1/2, where p = r/n. The percent rate was then calculated as SD% = [n x p(1 – p)]1/2 100/n.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Monocotyledons
Eight species of monocotyledons belonging to the Gramineae and Cyperaceae families were investigated. In general, Al phytoliths were rare or completely absent (Table 1). In four species Al phytoliths were present in up to 10% of the irregular polyhedron morphotypes: Carex curvula (10%), Nardus stricta (6%), Calamagrostis villosa (1%), and Poa alpina (2%).
Dicotyledons
The seven species analyzed belong to the Ericaceae family sensu latu (s.l.). Frequencies of Al phytoliths in the leaves and in the ligneous tissues were different in all the species (Table 1). In the phytoliths extracted from the leaves, aluminum was detected in most of the irregular morphotypes from Calluna vulgaris (95%), Loiseleuria procumbens (86%), Arctostaphylos uva-ursi (89%), and Empetrum nigrum (72%). In Rhododendron ferrugineum, aluminum was present in only 32% of the leaf phytoliths. The percentage of Al phytoliths extracted from wood was generally very high, ranging from 84% to 95%. The woody tissue from Vaccinium myrtillus was the exception, in which the polyhedral phytoliths containing aluminum were only 33% of the total. The Ericaceae had the highest combined (leaves and wood) frequency of Al phytoliths from this data set, with a mean of 76.5%.
Conifers
The majority of the phytoliths extracted from the needles of Juniperus nana and Pinus cembra contained Al (97% and 93%, respectively) (Table 1). Aluminum phytoliths were detected in 56% of the needles of Pinus mugo. Co-deposition of Al was detected in low proportion in Picea abies (5%) and was absent in Larix decidua. In contrast, phytoliths extracted from the conifers' woody tissues always contained Al in proportions ranging from 67% to 90% (Table 1). The mean frequency of Al phytoliths in wood is 81% and the combined (leaves and wood) frequency was 66%.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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]). In vascular plants, the endodermis acts as a barrier into the root, but is not completely effective as Al can be detected in the shoots of some species (Hodson and Sangster, 1999
). There is evidence that silicon mitigates Al toxicity (Hodson and Evans, 1995
), and the mechanism involved may consist of the co-deposition of Al and Si to form insoluble aluminosilicates (Cocker, Evans, and Hodson, 1998
).
Hodson and Sangster (1999)
reported that in shoot tissues of cereals there were undetectable or very small amounts of Al. In the cereal species, Al is probably sequestered in the plant roots' apoplast as microanalytical investigations in maize, sorghum, wheat, and oat would seem to suggest (for a review see Hodson and Evans [1995
]). These findings are consistent with the results of the present research, which highlighted an absence of aluminum in phytoliths from aboveground tissues of grasses.
The data on the uptake of Al and Si in conifer shoots (Hodson and Evans, 1995
; Hodson and Sangster, 1999
) show that conifer species allow far more Al into the root cortex and transport more to the shoot. These findings seem to be indirectly confirmed by the present results on the composition of conifer phytoliths.
By means of in situ X-ray microanalysis, the co-deposition of Al and Si was detected in the needles of four conifers (Pinus strobus, Larix laricina, Abies balsamea, and Picea glauca). In these species, Al was invariably present in the silicified epidermal walls of the needles and, in some species, also in the hypodermis, mesophyll, and endodermis (Hodson and Sangster, 1999
).
Among the conifers here examined, Larix decidua and Picea abies have the most heavily silicified needles (biogenic silica is 1.09% and 0.85% of plant dry mass respectively; Carnelli, Madella, and Theurillat, 2001)
but the lowest percentages of Al phytoliths. This indicates that Al and Si in conifer needles are not invariably co-localized as was the case for the species analyzed by Hodson and Sangster (1999)
.
Aluminum is not generally present in grasses; however, a study on Al/Si co-deposition in Sorghum bicolor using microanalytical techniques revealed that HAS/AS are deposited in the outer tangential wall of the root epidermis (Hodson and Sangster, 1993
). More evidence on the production of biogenic silica in belowground plant tissues is needed before any estimate of Al phytolith production in grass roots can be attempted.
Aluminum in phytoliths
The frequency of Al phytoliths was remarkably higher in Ericaceae and conifers (up to 97%) than in Cyperaceae and Gramineae (maximum of 10%). Phytoliths from wood always contained Al in a proportion as high as 80% (even in the case of Vaccinium myrtillus, which has less lignified tissue, a fact which explains its low 33% value).
Monocotyledonous species produce the highest quantity of biogenic silica (Table 1); however, most of the taxa analyzed did not contain Al phytoliths at all or only contained a small percentage. Comparative data concerning the co-deposition of Al in biogenic silica from monocotyledons are scanty. The few data available revised by Hodson and Evans (1995)
concern three grass species and are not directly comparable as they were obtained by quantitative chemical analysis. The content of Al was measured in the phytoliths obtained from the leaves of the grasses Festuca sylvatica (1.9% on dry mass basis), Sasa sp. (0.03%), and Miscanthus sp. (0.01%). The present study revealed that Al was primarily deposited in the phytoliths of Ericaceae and conifers. Quantitative chemical analyses performed by other authors confirm these results to some extent: in the phytoliths extracted from the leaves of the grass Festuca sylvatica, aluminum accounted for 1.9% of the dry mass, in Calluna vulgaris for 2.1%, and in Pinus sylvestris for 4.4% (Bartoli and Wilding, 1980
). For these species, it is reported that between 60 and 95% of leaf Al was incorporated into phytoliths.
In the present data set, Al phytoliths were detected in all woody tissues, even in species that were taxonomically unrelated. A functional interpretation of this evidence goes beyond the purpose of the present work; however, it may be argued that it is the result of a time-integrated co-deposition. It is known that the concentration of several elements (among them Al) increases in plant tissues with time (e.g., in Norway spruce needles; Wyttenbach and Tobler, 1988
) and should therefore be higher in tissues with a longer life span such as wood. Further investigations are needed to test if this trait is common to wood phytoliths from other taxa.
The physical-chemical properties of opal were investigated in detail on the phytoliths of Pinus sylvestris, Fagus sylvatica, Calluna vulgaris, Abies alba, and Festuca sylvatica (Bartoli, 1985
). The Si/Al ratios for whole phytoliths and for the phytolith surface were compared, showing that the latter always had a higher Al content. In particular, pine and fir phytoliths revealed the greatest content of surface aluminum. It was speculated that their surfaces might be coated with an octahedrally coordinated form of Al (Bartoli, 1985
, p. 339). Isomorphous substitution of Al for Si in the internal tetrahedral network of opal is less common, and Al is normally deposited after Si deposition. Moreover, the lower solubility of conifer phytoliths in comparison with those of other species analyzed was attributed to the higher surface content of chemiadsorbed Al (Bartoli, 1985
).
Implications for palaeoecological studies
The hypothesis that the presence of Al phytoliths can be regarded as a potential tracer for woody taxa was verified for those species most commonly occurring in subalpine and alpine vegetation on siliceous bedrock in the Alps, i.e., the conifers that form the timberline, the ericaceous dwarf shrubs, and some grasses and sedges of the alpine grassland. This focus on the dominant and most common plants was undertaken with a view to applying this technique to the investigation of treeline and more specifically to subalpine-alpine fluctuations in the Alps during the Holocene. Although a few important subalpine species are not taken into account in the present study, such as Abies alba Mill. (conifer), Alnus viridis (Chaix) DC., Salix helvetica Vill. (dicotyledon shrubs), and Vaccinium uliginosum L. (Ericaceae), this is unlikely to make a difference to the results obtained according to the first analyses of their production of biogenic silica (Carnelli, Madella, and Theurillat, 2001)
.
Indeed, although the European alpine treeline is possibly the most intensively studied of all distributional tree boundaries, conclusive functional explanations are awkward (Körner, 1998, 1999
). It is acknowledged that treelines generally correspond to stress gradients (e.g., thermal, hydric, nutritional, and disturbance) but the factors that determine the present alpine treeline need to be investigated on a longer time scale by means of comparison of palaeoecological data. In the case of the European Alps, the locations of past treelines have been reconstructed mainly by means of pollen, plant macrofossil, and charcoal analysis (e.g., Burga, 1991
; Carcaillet and Thinon, 1996
; Tinner, Amman, and German, 1996
; Tinner and Wick, 1997
; Carcaillet, 1998
; Carcaillet, Talon, and Thinon, 1998
; Carcaillet and Brun, 2000
). The possibility of definitely assigning nonidiomorphic phytoliths to herbaceous or ligneous plants creates a new method of investigating the dynamics of the timberline during the last postglacial in the Alps.
Recently, we suggested that when investigating the history of the main plant communities of the subalpine-alpine ecocline, the production of biogenic silica should be considered in parallel with the morphological approach (Carnelli, Madella, and Theurillat, 2001)
. Indeed, we estimated that upper subalpine to low alpine swards have a mean silica production of one order of magnitude greater than heaths, both for a mesophilous (mean = 9.4 vs. 0.37 g·m–2·yr–1) and a thermophilous ecosystem (mean = 10.3 vs. 0.79 g·m–2·yr–1). However, the production of biogenic silica of true alpine swards with Carex curvula All. (mean = 1.2 g·m–2·yr–1) is close to that of upper subalpine to low alpine heaths and thus would not be distinguished definitely on a solely quantitative basis. In the same way, although the biogenic silica produced by the litterfall of an upper subalpine forest may be estimated to be of the order 1.5 g·m–2·yr–1 on average, the production of its grassy understory can be as high as 29.2 g·m–2·yr–1 and thus may not differ so much from the production of a subalpine grassland. As the production of Al phytoliths appears to distinguish woody taxa, different plant communities, characterized by herbaceous vs. arboreal taxa, should vary consequentially in the production of Al phytoliths. The phytolith fossil record present in a soil should thus be a valuable marker of vegetation and vegetation shifts. The potential production of Al phytoliths of a plant community can be estimated by calculating its annual biogenic input and the fraction of Al phytolith input into the soil. Subalpine and alpine grasslands are the most active accumulators of biogenic silica but Al-rich phytoliths in grasses are very rare, representing only 2% on average. In contrast, subalpine woody/arboreal vegetation accumulates less opal silica but, on average, 72% of the phytoliths from leaves and wood of conifers and woody Ericaceae contain Al.
In case further investigations of different taxa confirm that the co-deposition of Al and Si in phytoliths is more frequent in or characteristic of woody tissues, this technique could still find wider applications. Ecotonal shifts involving a change in life-form dominance occurred in many biomes worldwide during the history of vegetation (e.g., dense forest/savanna, forest/steppe). As the presence of Al phytoliths in conifers was confirmed for several species (Bartoli, 1980
; Hodson and Sangster, 1999
), it can be argued that this method should be useful in those cases where there is investigation of conifer species (e.g., Pinus and Ericaceae matorral in the Mediterranean region, boreal forest, and steppe-tundra ecotone in North Eurasia).
Finally, a further implication of the presence of Al in phytoliths is related to the interpretation of phytolith assemblages. The taphonomic processes undergone by phytoliths in the soil are little understood, though this aspect of the phytolith "life cycle" is crucial for the interpretation of opal silica in palaeoecological archives. The durability of phytoliths in the soil appears to be related to several physical and chemical factors of the medium as well as to their own intrinsic characteristics: the presence of Al seems to increase their resistance to dissolution (Bartoli, 1985
). The differential preservation of phytoliths should lead to a relative time-integrated enrichment of the morphotypes more resistant to dissolution in the deeper horizons of undisturbed soils (Alexandre et al., 1997a
). Therefore, data on the Al content of phytoliths in different taxa are of general interest in the correct interpretation of fossil assemblages.
Summary of conclusions
In the samples of biogenic silica from subalpine-alpine species, Al was always present in woody species phytoliths, on average in 72% of the cases (wood and leaf samples); it can be hypothesized that the presence of Al in phytoliths of unknown shape and origin in fossil assemblages would lead one to deduce that they were produced by woody species. The application of microanalytical techniques to phytolith analysis in palaeoecological investigations should enhance the diagnostic taxonomical power of such analysis. In the present work we focused on polyhedral morphotypes because of the difficulty in distinguishing them on a solely morphological basis; nevertheless, this technique may be applied to other morphotypes. The prospect of identifying phytoliths originating in woody taxa, not only by using the established morphological approach (e.g., Piperno, 1985
; Bozarth, 1993
; Alexandre et al., 1999
; Barboni et al., 1999
; Runge, 1999
) but also by microanalytical techniques, can clarify a part of the uncertainty arising from redundant production of phytolith morphotypes in distinct taxa. In this way, X-ray microanalysis on phytoliths can supply another proxy with a theoretically unlimited availability of sampling sites, as they can be extracted from any soil, and a relatively high spatial resolution power. Biogenic silica produced by species from vastly different kinds of vegetation should be tested for the presence of Al. If the presence of Al in woody species is confirmed, the chemical composition of biogenic silica may be a powerful tool in the investigation of past shifts in herbaceous and woody vegetation produced as a result of climatic variation and of human activities.
In the context of global change, palaeoecological data are needed to validate climate models to predict the response of the ecosystems to future climate changes. When the reconstruction of the history of vegetation is based on multi-proxy evidence, it provides more precise and quantitatively reliable estimates. This method could open a new avenue of phytolith investigation that could be of interest not only for the Alps but anytime we are confronted with the need to understand past environments and the shift of two major types of vegetation: grasslands vs. woodlands.
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FOOTNOTES |
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6 Author for correspondence (carnelli{at}terre.unige.ch
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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