Tag Archives: plants

Preferential Preservation of Phytoliths

Scanning electron microscope image of an elephant grass phytolith after dry-ashing.[1]  Image credit: Benjamin Gadet (CC-BY-SA).
Scanning electron microscope image of an elephant grass phytolith after dry-ashing.[1] Image credit: Benjamin Gadet (CC-BY-SA).

As I was looking through the recently published papers in PLoS ONE (all open-access!), I came across an interesting article on the preservation of phytoliths.[2] It is an interesting and well-written paper, and is quite accessible—both in terms of copyright and of science content.

Plants often have little bits of rock in them, called phytoliths (phyto- plant, -lith rock). Phytoliths are formed within the plant by precipitating SiO2 in a non-crystalline form (opal). These microscopic stones can help maintain the structure of the plant, perhaps among other functions. They also preserve well, because SiO2 (glass, essentially) generally doesn’t react chemically with much in the environment.

Just like with fossilized bones or impressions of leaves, the size and shape of phytoliths can be used to identify the plant (or family of plants) which is producing them. If phytoliths are found in the geologic or archaeologic record, they can be used to determine what kinds of plants were in the area, or were being eaten. They also contain small traces of carbon, which can be used for radiocarbon dating (back to ~40 ka) or 13C isotope analysis.[3]

This paper is looking at what happens to various phytoliths in the archaeologic or geologic record, and whether there are preservation biases (some phytoliths being destroyed more easily than others).

The authors took samples of four different types of modern, living plants. These samples were then burned away in a 500°C furnace, leaving just ash and the microscopic rocky bits. With some further, relatively gentle treatment, they were able to isolate the phytoliths. Some of these phytoliths were mounted on microscope slides and counted to determine the relative abundance of different sizes and shapes.

Isolated phytoliths were partially dissolved for six weeks, and the Si content of the liquid was measured. The partially dissolved phytoliths were dried, mounted on microscope slides, and they too were counted to determine relative abundance of the different sizes and shapes after treatment.

Phytoliths which were small, and had a large surface-area-to-volume ratio, tended to be preferentially dissolved—this is not an unexpected result, but is important. The authors argue that based on the Si solubility, the degree of preservation can be assessed (high Si solubility means better preservation); in situations where the Si solubility is low, some of the more delicate phytoliths are likely to be missing, and a count of phytoliths under those circumstances would yield biased results.

But don’t take my word for it! Read the paper. It’s better written than my short explanation, and a fine example of scientific scholarship.

[1] Parr, J.F.; Lentfer, C.J. & Boyd, W.E. 2001, ‘A comparative analysis of wet and dry ashing techniques for the extraction of phytoliths from plant material’, Journal of Archaeological Science, vol. 28, no. 8, pp. 875-886. DOI: 10.1006/jasc.2000.0623

[2] Cabanes D. & Shahack-Gross R. (2015) Understanding Fossil Phytolith Preservation: The Role of Partial Dissolution in Paleoecology and Archaeology. PLoS ONE 10(5): e0125532. DOI:10.1371/journal.pone.0125532

[3] Looy, C.V.; Kirchholtes, R.P.J.; Mack, G.H.; Van Hoof, T.B. & Tabor, N.J. 2011, ‘“Ochoan” Quartermaster Formation of North Texas, U.S.A., Part III: First Sign of Plant Life‘ Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 383.

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Peer-Reviewed Research: Terrestrial Vegetation and Environments on Heard Island

Kerguelen cabbage (Pringlea antiscorbutica).  Image credit: B.navez, CC-BY-SA, via Wikimedia commons.
Kerguelen cabbage (Pringlea antiscorbutica). Image credit: B.navez, CC-BY-SA, via Wikimedia commons.

Previously I’ve mentioned rocks, glaciers, a volcano, penguins, and elephant seals, but what about plants on Heard Island? That has been well-covered (pardon the pun) by Bergstrom and Selkirk (2000), who published their findings in an open-access journal.[1]*

Vegetation on Heard Island is generally categorized into six groups (“communities”), which reflect the general microhabitats and species makeups of the area. Here are a couple of brief descriptions:

Poa cookii maritime grassland” is characteristic of nutrient-enriched, animal influenced environments (Hughes 1987, Scott 1988). This community is dominated by the small tussock grass Poa cookii (Hughes 1987), with the nitrophiles Callitriche antarctica and Montia fontana also common.

“Feldmark communities” are characterised by less than 50% vegetation cover. Hughes (1987) described feldmark as having high relative vasular plant diversity but low species abundance, with predominant plants being Azorella selago, Poa kerguelensis, Colobanthus kerguelensis, Pringlea antiscorbutica, bryophytes and lichens. Scott (1988) recorded feldmark on well-drained areas of high altitude/high wind exposure, areas of recent glacial retreat, flat valleys likely to be subject to cold air drainage, and geologically recent lava flows.

After establishing some terminology about what types of communities are there, the authors move into the environmental factors that influence the plants on Heard Island.

Animal-derived nutrients are one factor. “A nutrient gradient is apparent, diminishing away from coastal seal and bird-breeding, resting, and hauling-out areas. Areas affected by direct manuring by seals, penguins, cormorants and giant petrels are generally devoid of vegetation, reflecting toxic nutrient levels and physical damage to plants.”

The types of rocks present, and the geochemistry of those rocks, could control what plants will thrive there. However, that has not been studied in detail on Heard Island (yet! [as of 2000]).

Salt spray from the ocean can make life difficult for plants, as can debris blowing in the wind. The depth to which roots can be sunk varies greatly, from almost nothing on the lava flows to more than 50 cm on moraines and other areas of loose sediment. Water availability also is important; some areas with poor drainage form pools, while other areas of loose rocky material drain very quickly. Snowmelt provides water throughout the summer, although precipitation is frequent throughout the year.

Movement of the rock/soil surface, such as through landslides, frost-heaving, and sediment accumulation can disrupt plant activity. Animals can trample (and eat!) plants, in addition to “adding nutrients”. Of course, the general climate influences, such as temperature (warmer toward sea level) and sunlight (more clouds on west side, more sun on east side) also play a role.

Bergstrom surveyed the plant diversity and abundance quantitatively during the 1986/1987 Australian National Antarctic Research Expedition to Heard Island. Almost 500 quadrats (1×1 m squares) were surveyed, and included three main vegetated areas of the island: Laurens Peninsula (northwest), Spit Bay (southeast), and Long Beach (south-southwest). By placing these quadrats on random (or as random as practical) ice-free locations, representative population statistics can be tabulated. Some species occur primarily clustered with certain other species, and others (e.g. Azorella selago) are widespread.

This survey of terrestrial flora provides an excellent baseline from which to study the changes in populations as the climate warms and glaciers recede on Heard Island. Through this kind of work, scientists can find how the plants are responding to changing conditions and new areas to colonize.

Beyond this research are some big questions: how did plants first arrive on Heard Island? Where did they come from? Which species were first to arrive? When did they arrive, and how long had Heard Island been above the ocean when they came?

[1] Bergstrom, DM and Selkirk, PM (2000) Terrestrial vegetation and environments on Heard Island. Papers and Proceedings of the Royal Society of Tasmania, 133 (2). pp. 33-46. ISSN 0080-4703

* Open access journals are a great way to ensure that research is widely accessible. I am considering outlining my views on academic publishing in a later post (this footnote is only so large).