Review of Palaeobotany and Palynology 135 (2005) 71 – 98 www.elsevier.com/locate/revpalbo
Phytoliths in plants and soils of the interior Pacific Northwest, USA Mikhail S. BlinnikovT Department of Geography, St. Cloud State University, St. Cloud, MN 56301-4498, USA Received 13 July 2004; received in revised form 14 February 2005; accepted 23 February 2005
Abstract Phytoliths are a useful paleoproxy in the arid environments. This modern analog study assessed variability of silica phytoliths in 38 species of plants and 58 modern soil samples from 24 locations in the interior Pacific Northwest. Phytoliths were grouped into 20 broadly defined morphotypes based on their 3D shapes under light microscope and presumed anatomical origin within the plant. Grasses (all C3) have most diverse forms. Most examined conifers, sedges and some shrubs produce identifiable phytoliths as well. Eight different community types can be distinguished based on their modern phytolith record in soils, including shrublands, four regional grassland types, and three forest types. Low percentages of grass phytoliths and high incidence of non-grass forms correspond to forest vegetation in the region today, while certain grass phytoliths allow further differentiation among different grasslands. Phytolith assemblages were further compared to 5 environmental variables, including elevation, mean annual temperature, mean annual precipitation, a moisture index and a growing-degree days index. Some morphotypes tend to occur within relatively narrow environmental windows, which could enable direct paleoenvironmental inferences from phytoliths in geological sediments from the region. D 2005 Elsevier B.V. All rights reserved. Keywords: climate; modern analogs; Oregon state; plant opal; vegetation; Washington state
1. Introduction Phytolith analysis is a powerful, yet relatively underutilized, method of paleoenvironmental reconstruction that can be used to supplement pollen and macrofossil analyses (Piperno, 1988; Pearsall, 2000). In North America, both archaeologists (Rovner, 1971; Mulholland, 1993) and paleoecologists (Kurman, 1985; Fredlund and Tieszen, 1997a,b; Kearns, 2001; T Tel.: +1 320 308 2263; fax: +1 320 308 1660. E-mail address:
[email protected]. 0034-6667/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2005.02.006
Blinnikov et al., 2002) used phytoliths to infer a range of paleoenvironmental conditions. Despite some early applications of phytoliths in paleoenvironmental and paleopedological work (Smithson, 1958; Witty and Knox, 1964; Twiss et al., 1969; Rovner, 1971; Norgren, 1973), the use of phytoliths in paleoecology remains uncommon (Piperno and Persall, 1993). Recent studies suggest that any paleoenvironmental reconstructions using phytoliths must begin with analyzing modern phytoliths distribution in plants and soils in the given region (Bowdery, 1998; Carnelli et al., 2001; Lu and Liu, 2003).
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In this paper I present results of a modern analog study of phytoliths in plants and soils of the US interior Pacific Northwest defined as states of Washington and Oregon east of the Cascade Range. This study provides a modern analog dataset required for any paleoenvironmental reconstruction of the late Pleistocene and Holocene vegetation of the region, including the previously published work (Blinnikov et al., 2001, 2002). In fact, only two other studies of phytoliths in plants and modern soils are available from the Pacific Northwest (Witty and Knox, 1964; Norgren, 1973). The former mostly focused on phytoliths in soils at a forest-grassland ecotone in central Oregon, while the latter focused on Douglas-fir forests of the western Cascade Range and grasslands of north-central Oregon. Together they provide data on phytoliths from ca. 15 species in the region, mostly conifers and grasses. Neither study provided an adequate sampling design to compare phytolith distributions among different vegetation types. In temperate North America, phytoliths have been mostly studied in grasses (Twiss et al., 1969), sedges (Walter, 1975; Ollendorf, 1992), conifers (Klein and Geis, 1978), deciduous trees (Geis, 1973), and some dicotyledonous shrubs and herbs (Bozarth, 1992). Their distribution in grasses has been studied most extensively within Alberta (Blackman, 1971), North Dakota (Mulholland, 1989), central Great Plains (Twiss et al., 1969; Fredlund and Tieszen, 1994), northern Great Plains (Brown, 1984), southeastern states (Lanning and Eleuterius, 1987; Lu and Liu, 2003), and northern Arizona (Kearns, 2001). The number of modern soil studies on phytoliths on the continent is even smaller. Two important examples include reports of Bozarth (1993) from Alberta and Fredlund and Tieszen (1994) from the northern Great Plains. Earlier soil studies included work of Beavers and Stephen (1958) in Illinois, Verma and Rust (1969) in Minnesota, and Kurman (1985) in Kansas, but none of these provide sufficient data for interregional comparisons. The following questions are addressed in this paper: 1. What is the overall diversity of phytoliths in the main phytolith-producing plants in the interior Pacific Northwest? Which taxa can be identified
on the basis of their phytoliths and at what level (e.g., species, genus, family)? 2. What is the pattern of modern phytolith distribution in soils under present-day vegetation? Can different vegetation types be distinguished on the basis of their phytolith record in modern soils? 3. What is the relationship between modern phytolith assemblages in soils and climate?
2. Area description Franklin and Dyrness (1988) describe 15 physiographic provinces in Oregon and Washington within the study area (Fig. 1), of which two are most important. The Columbia Basin Province is flat and low (300–600 m elevation) and is covered with extensive basalt flows of 7–15 Ma (million years BP) in age (Baker et al., 1991). The Blue Mountains Province includes the Ochoco, Blue, and Wallowa mountains and has a more variable relief, ranging from 750 m elevation to 3000 m in the Wallowas. The Pacific Northwest lies in the path of westerly storm tracks coming from the Pacific Ocean (Bryson and Hare, 1974). The position of these storm tracks shifts seasonally from 608 N in summer to about 358 N latitude in winter as a result of shifts in the position and strength of the polar jet stream. Westerly winds during fall, winter, and spring bring most of the precipitation. The summers are sunny and dry. The study area, located on the leeward side of the Cascade Range, sits in a pronounced rain shadow with distinctly more continental climate than west of the range. In my study, the mean annual temperature (MAT) ranged from 0 8C at subalpine sites in the Wallowas, OR (WA on Fig. 1) to +12 8C at the Boardman Range, OR (BR on Fig. 1). The mean annual precipitation (MAP) ranged from semi-desert (172 mm) at the Boardman Range to 1492 mm at subalpine sites in the Wallowas. The climate data for the research sites were interpolated using an appropriate topographic adjustment from datasets in Thompson et al. (2000). In the dry interior of the Pacific Northwest, vegetation zones are often discontinuous, with distribution determined by available moisture (Franklin and Dyrness, 1988). The driest sites are located at low elevations in the western Columbia Basin near the Cascade Range. The wettest sites are located at high
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Fig. 1. Location of modern phytolith sample sites in the interior Pacific Northwest, USA. Physiographic province boundaries are after Franklin and Dyrness (1988). See Table 4 for the explanation of site codes.
elevations on the windward western slopes of the Blue Mountains (Fig. 1). Although the landscape today is altered by agriculture, the potential natural vegetation (PNV) was reconstructed by Daubenmire (1968, 1970). The shrub steppe, represented by the Artemisia tridentata–Agropyron spicatum association, occupies the driest sites. As precipitation increases to the east, the PNV changes to dry perennial bunch grassland with A. spicatum– Poa sandbergii and, higher up, more mesic A. spicatum–Festuca idahoensis association. The latter is in turn replaced by F. idahoensis–Symphoricarpos albus meadow steppe association in the Blue Mountains foothills. Festuca-dominated meadow steppe is in turn replaced by Pinus ponderosa zone common between 600 and 1200 m. Abies grandis–Pseudotsuga menziesii dark coniferous forest zone occupies higher
elevations (900–1500 m). The summits of the Blue Mountains above 1500 m elevation support Abies lasiocarpa–Picea engelmannii association. In the Wallowa Mountains, subalpine parklands of Pinus albicaulis and A. lasiocarpa and shrub communities of A. tridentata ssp. vaseyana are found above 2000 m (WA on Fig. 1). True alpine communities exist at the highest elevations (N 2500 m) with Festuca viridula grassland being the most common association.
3. Phytolith extraction and data analysis methods I analyzed phytoliths from 38 species of plants and 58 surface soil samples collected in 1996–1998. Above-ground biomass of grasses and herbs was collected to include both vegetative and generative
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organs. Leaves, needles, and twigs of trees and shrubs were included, but no attempt was made to collect underground parts. For each species, about 10 g of dry plant matter was obtained from a minimum of 3 different plants at each location. No attempt was made to quantify opal content of various species, although grasses were clearly the most heavily silicified overall. Samples of topsoil were collected at 24 locations (Fig. 1). To ensure that samples were collected from relatively undisturbed vegetation, most sampling was done in research natural areas, designated wilderness areas, and the Nature Conservancy and state preserves. Aggregated soil samples were obtained by ten random pinches for a total of about 20 g of topsoil (0–2 cm) on plots of approximately 4 4 m within each community. Some litter was present in the topsoil samples. Some locations had more than one plot thus sampled for a total of 58 soil samples. Vegetation descriptions were made in field recording dominant plants and their percent cover. Each of the samples was then assigned to one of the eight recognized vegetation types, based on the composition of dominants. Phytoliths were extracted using modified wet oxidation technique of Piperno (1988). Plant material was carefully rinsed with distilled water, cut into small
fragments of about 5 5 mm, and placed in a sand bath in glass beakers. Approximately 50 ml of 70% nitric acid and a pinch of KClO3 was added. Digestion was allowed to proceed near boiling point for 1.5 h. The remaining residue was washed twice with distilled water, kept in warm hydrochloric acid (10%) for 15–20 min to remove carbonates, washed twice again, and dried. The phytoliths were stored in ethyl alcohol. Soil samples were treated using the approach of Blinnikov (1994). Approximately 10 g of soil was sieved through a 700-Am mesh sieve to remove pebbles and large plant fragments. The organic material was then removed by wet oxidation in 70% nitric acid for 1.5 h. To remove carbonates, samples were additionally treated with 10% hydrochloric acid for 15 min in a warm bath. Samples were then subjected to deflocculation in 5% sodium hexametaphosphate solution for 1 h. Clays were removed by gravity sedimentation repeated twice (Soil Survey Laboratory Methods Manual, 1996). The remaining residue was subjected to heavy liquid flotation (potassium and cadmium iodide solution and/or sodium polytungstate) using the density of 2.3 g/ cm3. The samples were mixed thoroughly with a glass rod and centrifuged for 5 min at slow speed
Plate I. Light microscopy digital photographs of the phytolith morphotypes used in the study. Scale bar = 10 Am. Morphotypes 1–13 are grass phytoliths, 14 and 15 are grass or non-grass and 16–20 are non-grass phytoliths. The morphotype name, 2-letter shorthand, and source species are listed for each morphotype. All are shown in top view, unless otherwise noted. See Table 2 for full description of grass morphotypes according to the International Code for Phytolith Nomenclature (Madella et al., 2003): 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Plate rectangular, straight edges (pr), oblique view, Poa sandbergii. Plate wavy, short — 3–4 undulations (ws), P. sandbergii. Plate wavy, long — 5+ undulations (wl), Calamagrostis rubescens. Long cell, smooth, parallel sides (ls), Festuca idahoensis. Long cell indented (li), Stipa comata. Long cell deeply indented (ld), F. idahoensis. Long cell angular/nonparallel sides (la), Koeleria cristata. Dendritic long cell from seed epidermis (sd) Elymus cinereus. Rondel oval/elongated (ro), oblique view, Agropyron spicatum. Rondel rounded, keeled (rk), side view, F. idahoensis. Rondel rounded, horned (rh), side view, A. spicatum. Rondel pyramidal (rp), side view, S. comata. Bilobate Stipa-type (bs), Oryzopsis hymenoides. Silicified trichome (ht), C. rubescens. Silicified hair or hair base (hh), Sitanion hystrix. Blocky (bl), Picea engelmannii. Spiked of Pinus ponderosa (pp). Epidermal polygonal (ep), Artemisia tridentata. Asterosclereid, Pseudotsuga menziesii, from bother coniferousQ morphotype group. Unevenly thickened cell walls, Larix occidentalis, from bother coniferousQ morphotype group.
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(approximately 1000 rpm). The floating phytoliths were collected from the top 5 mm of the solution, transferred to clean test tubes and sunk by adding distilled water in proportion of 3 : 1. The phytolith-rich residue was stored in ethyl alcohol. Phytoliths were counted in immersion oil type A (refractive index 1.52) under Zeiss optical microscope ( 400) to examine 3D shapes under rotation. Morphotypes were tallied using the standard approach of Pearsall (2000). Between 200 and 300 phytoliths were counted per slide. Phytolith morphotypes were documented by hand drawings, light microphotographs, SEM, and permanent reference slides. All identifiable phytoliths larger than 5 Am were counted, not only short cells (rondels, bilobates, polylobates, and saddles), but also long cells and other grains of identifiable shape. I followed my own shorthand classification system, modified from Twiss et al. (1969) and Fredlund and Tieszen (1994) in describing grass morphotypes, and followed Piperno (1988) and Bozarth (1993) in describing non-grass morphotypes (Plate I, Fig. 2). I also provide descriptive code following the recently released Glossary for the International Code for Phytolith Nomenclature 1.0 (Madella et al., 2003) for each morphotype. Raw percentages of morphotypes were plotted in the final diagram for 58 soil samples. Detrended correspondence analysis (DCA) using PC-ORD (1997) was performed to examine the indirect environmental gradients implied by the phytolith data (Ter Braak and Prentice, 1988). The scores for samples and phytolith morphotypes were shown on the same scatterplot to illustrate the degree of distinction between different vegetation types. Boxplots (MINITAB, 1998) were used to further explore variability of the phytolith percentage data within and between vegetation types as a form of direct gradient analysis. To evaluate the relationship between the phytolith data and climate variables, scatterplots were drafted in MINITAB (1998) showing the correlation between selected phytolith types and climate variables. Each scatterplot has a locally weighted regression (LOESS) curve to show trends between phytoliths and climate variables (Cleveland, 1993). Because climate variables are usually highly correlated with each other, Principal Components Analysis of the climate data was performed to identify climate variables best characterizing regional climate patterns
(Ter Braak and Prentice, 1988). Climate variables for each site were derived from climate stations’ data (1951–1980) interpolated onto a 25-km equal-area grid with a locally fitted regression trend-surface model with latitude, longitude, and elevation as predictors (Bartlein et al., 1994). The predicted climate values at each grid point, the values at sea
Fig. 2. Hand drawing of the phytolith morphotypes used in the study. Scale bar = 10 Am. Morphotypes 1–13 are grass phytoliths, 14 and 15 are grass or non-grass and 16–20 are non-grass phytoliths. Morphotypes 1–13 re are shown in top view and side view with variants, if available. See Plate I for the digital microscopy pictures of all of these forms and Table 2 for the full description of grass morphotypes according to the International Code for Phytolith Nomenclature (Madella et al., 2003): (1) Plate rectangular, straight edges (pr). (2) Plate wavy, short — 3–4 undulations (ws). (3) Plate wavy, long — 5 or more undulations (wl). (4) Long cell, smooth, parallel sides (ls). (5) Long cell indented (li). (6) Long cell deeply indented (ld). (7) Long cell angular/nonparallel sides (la). (8) Dendritic long cell from seed epidermis (sd). (9) Rondel oval/ elongated (ro). (10) Rondel rounded, keeled (rk). (11) Rondel rounded, horned (rh). (12) Rondel pyramidal (rp). (13) Bilobate Stipa-type (bs). (14) Silicified trichome (ht). (15) Silicified hair or hair base (hh). (16) Blocky (bl). (17) Spiked of Pinus ponderosa (pp). (18) Epidermal polygonal (ep). (19) Asterosclereid of Pseudotsuga menziesii from bother coniferousQ morphotype group. (20) Unevenly thickened cell walls, Larix occidentalis, from bother coniferousQ morphotype group.
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level (a constant), and the local lapse rates determined in the regression analysis allowed bilinear interpolation of the climate values from the four-grid points surrounding each sample site. The interpolated values of monthly temperature and precipitation were used to calculate December–February and June–August temperature, MAT, MAP, and a few other climate characteristics. Growing-degree days from 0 and 5 8C bases (i.e. the accumulated growing-season warmth) were obtained by calculating pseudo-daily temperatures from monthly temperature estimates for each site and then summing the difference between pseudo-daily temperatures and temperatures of 0 and 5 8C, respectively (Prentice et al., 1992). The moisture index a (the ratio of actual evaporation to potential evaporation) was calculated using either the Priestley–Taylor equation (Prentice et al., 1992) or the Thornthwaite–Mather approach (Willmott et al., 1985).
4. Results 4.1. Morphotypes used in the study Many dominant plant species of the Pacific Northwest contain abundant silica phytoliths (Table 1, Plate I). I studied dominant grasses and conifers, two species of upland sedges, a few species of shrubs and forbs, and one species of fern. All these taxa were expected to contribute to the phytolith assemblages. As expected, each of the grasses produces a few different morphotypes. Most species, however, have only two or three morphotypes representing about 75% of the total. In addition to the silicified short and long cells, grasses also produce abundant silicified hairs and trichomes, bulliform cells, and scutiform and dendritic phytoliths that come from seed epidermis. The short-cell phytoliths are divided into three large groups: rectangular plates with straight edges (morphotype 1 in Table 2), plates with sinuous edges, or wavy, (morphotypes 2 and 3), also known as bcrenatesQ (Fredlund and Tieszen, 1994), and rondels (morphotypes 10–13), also known as bshort trapezoidsQ (Brown, 1984), or bhatsQ (Smithson, 1958). All these phytoliths come from specialized silica cells in the grass epidermis. The long cell phytoliths (morphotypes 4–7 in Table 2) come from non-specialized long cells in the
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grass epidermis. Although the overall variation in this group is high, four morphotypes were distinguished for convenience: rectangular long cells, indented long cells, deeply indented long cells, and angular long cells. The sum of all long cells was also calculated to simplify the analysis and compare the results with less-detailed classifications. Category 8 in Table 2 combines two very different forms of phytoliths from seed epidermis: dendritic long cells and scutiform opal. Only the dendritic form is shown in Fig. 2. Scutiform opal drawings can be found in Smithson (1958) and Kaplan et al. (1992). Rondels (bhatsQ, short trapezoids) constitute another important group of grass phytoliths. They form in highly specialized short cells in the epidermis. Rondels were divided into four groups, following in part Mulholland (1989) and Fredlund and Tieszen (1994): rondels elongated in top view, rondels rounded in top view with keel underneath, rondels rounded with double protrusions, or horns, underneath, and rondels rectangular in top view and pyramidal in side view. This classification should be approached with caution, as there are transitional types. The sum of all rondels can be additionally used, because it minimizes the risk of confusing two similar morphotypes. Bilobates were found only in the Stipa-group (Stipa and Oryzopsis) and in Aristida longiseta. Of these species, the Stipa-group produces primarily Stipa-type bilobates, which appear bilobate in the top view, but are actually trapezoids in the side view (Mulholland, 1989; Fredlund and Tieszen, 1994) with few true bilobates. Aristida longiseta produces Aristida-type true bilobates with very long shafts, bilobate in both side and top views. Because no Panicoid grasses are common in the study area today, presence of bilobates in soil samples likely indicates presence of Stipa s.l. or Aristida. Trichomes (Mulholland, 1989), also called bpricklesQ (Pearsall, 2000) or bpoint-shaped phytolithsQ (Brown, 1984), are thick-pointed epidermal appendages developing on the margins of grass blades. Their base length is equal to three to four heights of the phytolith (#14 in Table 2). All trichomes were treated together as a single morphotypes in this study. Silicified hairs and hair bases constitute a separate morphotype (#15). These are longer and thinner appendages (height H base length)
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Table 1 Dominant trees, shrubs, grasses, sedges, and herbs of the Pacific Northwest analyzed for phytoliths Species
Phytoliths present?
TREES Abies amabilis Abies grandis Abies lasiocarpa Alnus sinuata Juniperus occidentalis Larix occidentalis Picea engelmanni Pinus albicaulis Pinus contorta Pinus ponderosa Pseudotsuga menziesii Thuja plicata
No No Yes Yes Yes Yes Yes No Yes Yes Yes No
SHRUBS Artemisia rigida Artemisia tridentata Chrysothamnus nauseosus Chrysothamnus viscidiflorus Purshia tridentata Tetradymia canescens
Yes Yes Yes Yes No No
Morphotypesa
Reference
16, 18, tracheids Anticlinal epidermis Tracheids 20 16, 18, sinuous
Klein and Geis (1978), Bozarth (1993)
Norgren (1973), Carnelli et al. (2004) Norgren (1973), Bozarth (1993)
18 17 19
Norgren (1973), Kearns (2001) Norgren (1973), Klein and Geis (1978)
16 16, 18, tracheids Segmented hairs Segmented hairs
GRAMINOIDS (SUBFAMILY shown for grasses) Agropyron dasystachyum POOIDEAE Yes Agropyron spicatum POOIDEAE Yes Aristida longiseta ARUNDINOIDEAE Yes Bromus tectorum POOIDEAE Yes Calamagrostis rubescens POOIDEAE Yes Carex geyerii Yes Carex rossii Yes Elymus cinereus POOIDEAE Yes Festuca idahoensis POOIDEAE Yes Festuca viridula POOIDEAE Yes Koeleria cristata POOIDEAE Yes Oryzopsis hymenoides STIPOIDEAE Yes Poa sandbergii POOIDEAE Yes Sitanion hystrix POOIDEAE Yes Stipa comata STIPOIDEAE Yes Stipa occidentalis STIPOIDEAE Yes Stipa thurberiana STIPOIDEAE Yes
See Table 2 See Table 2 See Table 2 See Table 2 See Table 2 4, 7, sedge conical 4, 7, sedge conical See Table 2 See Table 2 See Table 2 See Table 2 See Table 2 See Table 2 See Table 2 See Table 2 See Table 2 See Table 2
Blackman (1971), Brown (1984) Norgren (1973), Brown (1984) Brown (1984), Mulholland (1989)
FORBS Balsamorrhiza spp. Lupinus sericeus Pteridium aquilinum
Segmented hairs Rough blocky, hairs Epidermal anticlinal, tracheids
Bozarth (1992)
Yes Yes Yes
Blackman (1971), Norgren (1973) Norgren (1973), Ollendorf (1992) Ollendorf (1992) Norgren (1973) Blackman (1971), Norgren (1973) Brown (1984), Mulholland (1989), Brown (1984) Blackman (1971) Norgren (1973), Brown (1984) Norgren (1973), Barkworth (1981) Barkworth (1981) Barkworth (1981)
Plant nomenclature follows Hitchcock and Cronquist (1994). a Numbers correspond to the morphotypes shown on Fig. 2.
than trichomes. While trichomes are primarily found in grasses, Asteraceae and some other forbs can produce multicellular trichomes and microhairs.
Non-grass forms comprise the remaining morphotypes and will be discussed in greater detail below.
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Table 2 Morphotypes recognized in this study described according to the International Code for Phytolith Nomenclature (Madella et al., 2003) Name and reference number used in this study (Fig. 2)
Acronym
Shape descriptor
Texture and ornamentation
Anatomical origin
1. Plates rectangular with straight edges 2. Short wavy
pr
3. Long wavy
wl
4. Long cells rectangular
lc
5. Long cells indented 6. L. c. deeply indented
li ld
Elongate, flat Elongate, flat
7. L. c. angular
la
8. Dendrictic from seed Scutiform from seed 9. Oval rondel
sd sd ro
10. Keeled rondel
rk
11. Horned rondel
rh
12. Pyramidal rondel
rp
13. 14. 15. 16.
bs ht hh bl
Elongate, flat, non-parallel sides Elongate, flat Scutiform Trapeziform in 3D, oblong to oval in top view 2D Trapeziform in 3D, oval in top view 2D Same as 10, but with horned-like protrusions near bottom Truncated pyramidal 3D, square top view 2D Trapeziform in 3D Acicular/lanceolate Cylindric/conical Cubic or globose
Smooth (psilate), linear ridges common Smooth (psilate) top, sinuate margin Smooth (psilate) top, sinuate margin Smooth (psilate) to mildly aculeate Aculeate Echinate (N1/2 the width of cell) Smooth (psilate)
Epidermal short cell
ws
Parallelepiped/trapeziform in 3D, rectangular in 2D Trapeziform in 3D, lobateb4 lobes on each side in 2D Trapeziform in 3D, lobateN4 lobes on each side in 2D Elongate, flat, parallel sides
Bilobate of Stipa-type Silicified trichome Silicified hair/base Blocky
17. Spiked of P. ponderosa 18. Epidermal polygonal
pp ep
Epidermal anticlinal
ea
19. Asterosclereid of P. meniesi
co
20. Uneven cells Larix-type
co
Clavate Irregular polygonal in top view, very flat on the side Anticlinal in top view, very flat on the side Very large (N200 Am), star-shaped Cuneiform
4.2. Phytoliths in grasses Fifteen grass species common to the interior Pacific Northwest were examined in this study (Table 3). All produce abundant phytoliths and are discussed below in the alphabetical order of their scientific names. Thick-spike wheatgrass Agropyron dasystachyum Hook (Scribn.) is an important dominant in a rare
Epidermal short cell Epidermal short cell Epidermal long cell Epidermal long cell Epidermal long cell Epidermal long cell
Dendriform Papillate Smooth (psilate)
Epidermal long cell Papillae cell Epidermal short cell
Smooth (psilate) top, keeled bottom Smooth (psilate) top, horned bottom
Epidermal short cell
Smooth (psilate)
Epidermal short cell
Smooth Smooth Smooth Smooth
Tuberculate Smooth
Epidermal short cell Trichome Hair cell or hair base Unknown, probably epidermal or mesophyll Epidermal? Epidermal, non-grass
Smooth to sinuate margins
Epidermal, non-grass
Striate, very dark because of occluded carbon Facetate
Vascular cell
(psilate) (psilate) (psilate) (psilate) or cavate
Epidermal short cell
Epidermal cell
grassland association (A. dasystachyum–Oryzopsis hymenoides) on exceedingly dry soils in north-central Oregon, e.g., in The Nature Conservancy’s Lindsay Prairie Preserve. This species has a large diversity of Festucoid phytoliths with a mixture of wavy plates, long cells, and rondels (Table 3). Its assemblage also contains dendritic and scutiform phytoliths from seed, also common in cultivated grains with large inflorescences, e.g., barley and wheat.
80
Morphotypes Species
1
2
3
4
5
Plates Plates Plates Long cells Long rectangular short long rectangular cells with wavy wavy indented straight edges
Agropyron 6 dasystachyum Agropyron 10 spicatum Aristida longiseta 0 Bromus tectorum 4 Calamagrostis 10 rubescens Elymus cinereus 3 Festuca 6 idahoensis Festuca viridula 4 Koeleria cristata 2 Oryzopsis 5 hymenoides Poa sandbergii 8 Sitanion hystrix 10 Stipa comata 3 Stipa occidentalis 0 Stipa thurberiana 6
6
7 Sum of all long cells Long Long (4 + 5 + 6 + 7) cells cells deeply angular indented
8
9 10 11 12 Sum of all 13 14 rondels Dendritic Rondels Rondels Rondels Rondels Bilobates Trichomes (9 + 10 + 11 + 12) and elongated rounded rounded pyramidal scutiform keeled horned (seeds)
15 Hairs and hair-bases
Other Total types number of phytoliths counted
8
6
6
4
0
1
11
31
10
0
0.5
22
33
0
0
4
0
202
5
0
6
9
7
9
31
1
18
2
0.5
27
48
0
1
3
1
616
1 24 17
0 0 20
0 2 15
2 0 4
0 9 0
0 0 0
2 11 20
0 21 0
0 1 0
7 0 4
12 0 0
0 28 0
19 29 4
67 0 0
4 2 26
4 8 2
3 1 1
223 305 414
14 2
7 0
17 1
14 23
1 10
1 0
34 34
6 0
3 0
7 9
3 36
0 1
14 47
0 0
14 3
6 3
3 4
236 295
1 37 0
0 7 0
0 11 1
41 0 2
0 0 0
0 0 0
43 12 3
0 0 1
0 1 0
20 10 5
11 0 6
5 0 28
37 11 39
0 0 47
13 24 1
1 2 4
0 6 0
152 315 209
56 12 0 12 4
6 0 0 0 0
0 7 7 2 3
4 1 2 0 6
0 6 2 0 3
1 3 0 0 0
6 17 11 3 12
0 0 0 0 0
0 3 0 0 0
17 11 4 0 3
2 0 36 9 8
0 9 23 37 4
18 23 64 46 14
0 0 11 35 51
3 0 6 1 5
0 35 3 2 7
2 4 1 0 1
324 222 228 348 237
All counts rounded to the nearest percent of the total (0% for values b 0.5%). Morphotype numbers correspond to Fig. 2. Nomenclature follows Hitchcock and Cronquist (1994).
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Table 3 Percentages of different phytolith morphotypes in grasses of the Pacific Northwest. All counts rounded to the nearest percent of the total (0% for values b0.5%). Morphotype numbers correspond to Fig. 2. Nomenclature follows Hitchcock and Cronquist (1994)
M.S. Blinnikov / Review of Palaeobotany and Palynology 135 (2005) 71–98
Bluebunch wheatgrass (Agropyron spicatum (Pursh) Scribn. and Smith) is one of the three most common grasses in the Columbia Basin native grassland. It dominates dry Artemisia tridentata–A. spicatum sagebrush steppe association and slightly wetter A. spicatum–Festuca idahoensis grassland association (Daubenmire, 1970). Agropyron spicatum produces smooth or indented long cells (brodsQ), and some elongated rondels (Norgren, 1973). Deeply indented and angular long cells may help distinguish this species from other species, except Sitanion hystrix. These two species are in fact closely related and even hybridize (Hitchcock and Cronquist, 1994). The overall proportion of the long cells in S. hystrix is only half that of A. spicatum. Red threeawn (Aristida longiseta Steud.) is a rare C4 species in the study area. It contains over 66% of bilobate phytoliths. Many of these are of the diagnostic Aristida-type with very long shafts (Mulholland, 1989), that had also been recently reported from Aristida species in Australia (Bowdery, 1998) and South America (Piperno and Persall, 1998). The non-native cheatgrass (Bromus tectorum L.) can be readily distinguished from native grasses on the basis of the high content of hairs (8%) and seed phytoliths (21%) that come from inflorescences. Cultivated annual grains and annual Bromus species with relatively large inflorescences are likely to have a high proportion of such forms as well (Ball et al., 1999). The dominant grass of pine forests in the study area is pinegrass (Calamagrostis rubescens Buckl.). It produces wavy plates with more than five undulations (20%). This morphotype (#3 in Fig. 2) is characteristic of the genus, similar phytoliths were found in Calamagrostis sp. in Russia (Blinnikov, 1994). Another abundant morphotype is trichomes (26%), but it is not genus-specific. Calamagrostis rubescens also has a large proportion of rectangular long cell phytoliths (15%), short wavy plates (17%), and rectangular plates (10%). It produces few rondels (b5%) or hairs (b2%). Because this species is common in pine forests today but not found in grasslands, it can be an important indicator of the past forested environments. Giant wildrye (Elymus cinereus Scrib. and Mer.) has limited distribution today in small seasonally wet depressions in non-forested areas. It produces diverse
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phytoliths making it hard to distinguish from other grasses, especially closely related Agropyron and Sitanion. Elymus assemblage has a high proportion of long cells (34%), including smooth rectangular (17%) and indented (14%) morphotypes. However, deeply indented and angular forms, typical of A. spicatum, are uncommon in this species. Idaho fescue (Festuca idahoensis Elmer) is a dominant species of the moderately dry Agropyron spicatum–F. idahoensis grassland, and it also dominates the more mesic F. idahoensis–Symphoricarpos meadow steppe (Daubenmire, 1970). Agropyron and Festuca can be distinguished based on their rondels. Agropyron spicatum contains mostly elongated and pyramidal forms of rondels (#9 and 12), while F. idahoensis contains mostly horned rondels of #11, similar to bconicalQ phytoliths of Fredlund and Tieszen (1994) (Table 3). Festuca idahoensis also produces long cell phytoliths (34%), most of them indented (23%). In contrast to A. spicatum, angular long cells appear to be absent from F. idahoensis, which may help differentiate these two species further. Green fescue (Festuca viridula Vasey) is an important grass of the alpine zone in the study area. It is one of the least silicified species and requires further research. Preliminary results show that F. viridula is similar to Festuca idahoensis in overall phytolith composition. Most phytoliths are indented long cells (41%) and rounded keeled (20%) and horned (11%) rondels. Junegrass (Koeleria cristata Pers.) is a species with circumpolar distribution. In the study area, it co-dominates with Festuca idahoensis in the F. idahoensis–Symphoricarpos meadow steppe. Its common phytoliths are wavy plates with 3 or 4 undulations. Some of these forms with diagonally slanted ends appear to be diagnostic (Kearns, 2001). In addition, K. cristata contains about 24% silicified trichomes, but few long cells (b 4%) or rondels (b 11%). The needlegrass species from the Stipae tribe (Oryzopsis hymenoides (R. and S.) Ryker, Stipa occidentalis Thurb., Stipa thurberiana Piper) are readily distinguished from other grasses on the basis of their Stipa-type bilobates (36–51%). These species grow in dry open grasslands or subalpine meadows, typically on soils with low effective moisture content.
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Stipa-type phytoliths in soils may thus indicate dry conditions. These three species contain few long cells or rectangular plates (b 12% each). Stipa comata Trin. and Rupr. belongs to a different section than S. occidentalis and S. thurberiana (Barkworth, 1981). Its phytoliths are primarily horned and pyramidal rondels (36% and 23%, respectively) with a lower percentage of bilobates (11%). Sandberg bluegrass (Poa sandbergii Vasey) is found in dry grasslands and scablands with Agropyron spicatum–P. sandbergii, Artemisia tridentata/A. spicatum and A. tridentata/P. sandbergii associations (Daubenmire, 1970). Poa sandbergii contains a large proportion of short wavy forms (56%, #2 in Fig. 2). Usually these phytoliths have more pronounced undulations than those of Koeleria cristata and pointed rather than diagonally slanted ends. Poa sandbergii has 18% rondels, mainly rounded keeled morphotype #10. Squirreltail (Sitanion hystrix Nutt. (Smith)) grows in disturbed grasslands in the study area. Its phytolith composition resembles that of the related Agropyron spicatum, Agropyron dasystachyum, and Bromus tectorum. It produces a high share of silicified hairs (N34%) and a lower share of wavy forms (12%). Although S. hystrix cannot be unambiguously identified, high proportion of silicified hair phytoliths in soil assemblages may indicate its presence. 4.3. Phytoliths in non-grasses Twenty-three species of non-grasses were analyzed for phytoliths, including common trees, shrubs and forbs in the study area (Table 1). Of these species, 17 contained silicified material. Among the conifer tree species (Pinaceae family), ponderosa pine (Pinus ponderosa Dougl.) and Douglas-fir (Pseudotsuga menziesii Mirbel (Franko)) contain abundant diagnostic phytoliths. Pinus ponderosa produces spiked forms in its needles (Norgren, 1973; Kearns, 2001; cf. Pinus banksiana form in Bozarth, 1993). It also produces potentially confusing wavy phytoliths, similar to wavy plates of grasses (#2 and 3 in Fig. 2, Klein and Geis, 1978). Pseudotsuga menziesii contains large branched silicified asterosclereids (Norgren, 1973; Klein and Geis, 1978). Other common pines in the region, for example, lodgepole (Pinus contorta Dougl.) and whitebark
(Pinus albicaulis Engelm.) do not appear to produce phytoliths of any kind, although some cell wall silicification was observed in P. contorta. Among other conifers, two species with the greatest potential to be recognized in soils are western larch (Larix occidentalis Nutt.) and Engelmann spruce (Picea engelmannii Parry). Larch needles contain abundant cell wall phytoliths of uneven thickness (cf. Larix decidua in Carnelli et al., 2004). Spruce features polyhedral endodermal cells (blocky phytoliths), found also in other spruces and firs (Klein and Geis, 1978; Bozarth, 1993), and flat epidermal forms characteristically sinuous on all four sides (cf. Picea glauca in Bozarth, 1993). Of the three examined fir species (Pacific silver fir Abies amabilis (Dougl.) Forbes, grand fir Abies grandis (Dougl.) Forbes, and subalpine fir Abies lasiocarpa (Hook.) Nutt.), only the latter contains identifiable phytoliths—endodermal blocky #16, elongated blocky forms (#18), flat polygonal epidermal morphotype #19, and some silicified tracheids (Klein and Geis, 1978). Big sagebrush Artemisia tridentata Nutt. and stiff sagebrush Artemisia rigida (Nutt.) Gray (Asteraceae) are the most common sagebrush species of the Columbia Basin (Daubenmire, 1970). They produce similar phytoliths: epidermal flat polyhedral cells, blocky forms, silicified tracheids, and a limited amount of silicified hairs. Another common dominant shrub Chrysothamnus, or rabbitbrush, is poorly silicified. Two species Chrysothamnus nauseosus Pall. (Britt.) and Chrysothamnus viscidiflorus (Hook.) Nutt. produce limited amount of segmented hairs, common in other Asteraceae. Phytoliths were not found in bitterbrush Purshia tridentata (Pursh) DC. (Rosaceae) or Tetradymia canescens DC. (Asteraceae). Sitka alder (Alnus sinuata (Regel) Rydb., Betulaceae) contains silicified epidermal cells with anticlinal (bjigsaw puzzleQ) walls. Two upland sedges (Carex geyerii Boott and Carex rossii Boott) contain abundant conical phytoliths, diagnostic of genus Carex and Cyperaceae family (Ollendorf, 1992), as well as some smooth elongated forms similar to the rectangular long cells of grasses. The balsamroot, Balsamorhiza spp., widespread in the Columbia Basin, produces distinct segmented hairs, diagnostic of Asteraceae. Another common forb, silky lupine (Lupinus sericeus Pursh, Fabaceae),
M.S. Blinnikov / Review of Palaeobotany and Palynology 135 (2005) 71–98
produces thin silicified hairs and also verrucate blocky forms (not shown). The only examined fern (Pteridium aquilinum L.) produces abundant anticlinal epidermal phytoliths (bjigsaw puzzleQ) and silicified tracheids. 4.4. Phytoliths in modern soils Fig. 3 illustrates the range of regional variability in the phytolith record from 58 modern soil samples. Table 4 provides information on the geographic coordinates, elevation, and vegetation composition for each sample. The samples were grouped into eight vegetation types (see Table 4 footnote for details). The vegetation types are necessarily subjective, but they generally correspond to the common associations found in the study area (Daubenmire, 1970; Franklin and Dyrness, 1988). Samples from sagebrush steppe were distinguished by high percentages of blocky forms (24 F 4%, n = 11) and epidermal polygonal phytoliths (17 F 3%), both probably produced by Artemisia tridentata, but the latter may also be from other dicot shrubs or herbs. Anticlinal epidermal cells, and some Asteraceae phytoliths (e.g., perforated plates and segmented hairs, Bozarth, 1992) were also found in a few samples. Among grass phytoliths, the most common were different kinds of long cells (18 F 4%) probably contributed by Agropyron spicatum. Short wavy phytoliths with 3–4 undulations (#2 on Fig. 2) were from Poa sandbergii (12 F 4%). Sample 3 (A. tridentata/P. sandbergii shrub steppe) and sample 9 (Artemisia rigida/P. sandbergii shrub steppe) contained about 30% of this morphotype, which is consistent with P. sandbergii being the only grass present in the sampling plot. Dendritic and scutiform phytoliths from seed epidermis were present in samples where Bromus tectorum cover was high. Samples from grasslands with Stipa or Oryzopsis species present were easily distinguished from other vegetation types by the presence of Stipa-type bilobates (9 F 2%, n = 9). In other samples, this morphotype (#13, Fig. 2) never exceeds 3%. Other common grass morphotypes are long cells (23 F 4%) and rondels (17 F 6%). Of the former group, rectangular smooth were the most common morphotype (common in Stipa comata). Among the rondels, the most common morphotype was rounded horned rondel (#11), which is also common in S. comata (N 36%),
83
and well represented in other species of Stipa. This morphotype, however, is also found in Festuca, so it cannot be a reliable indicator of Stipa or Oryzopsis presence. Blocky forms were also found in soil samples from this vegetation type, but in lower percentages than in those from sagebrush steppe (4 F 0.5%). Most of these phytoliths probably came from Artemisia, because the Stipa grassland sampling sites were located near Artemisia stands. As expected, dendritic and scutiform phytoliths from seeds (#8) were found in the samples with high presence of Agropyron dasystachyum and/or Bromus tectorum. There also was a large proportion of both epidermal polygonal (12 F 3%) and anticlinal (3 F 2%) phytoliths in this vegetation type. Rabbitbrush (Chrysothamnus spp.) and some Asteraceae forbs, e.g., Balsamorhiza and Centaurea, were the likely contributors of these phytoliths. The single sample of bother grasslandQ (sample 21, Fig. 3) came from a dense patch of Elymus cinereus. The most abundant forms in the Elymus grassland were the long cells (34%) and trichomes (14%), consistent with the species’ assemblage (see above). Samples from Agropyron grassland contain equal proportions of short wavy morphotype (23 F 5%), long cells (26 F 6%), and rondels (22 F 4%). The first morphotype (#2, Fig. 2) was apparently contributed by Poa sandbergii, while the other two came from Agropyron spicatum and, when present, Festuca idahoensis. Rectangular plates were more abundant in samples from Agropyron grassland than in those from sagebrush steppe or Stipa grassland. Among non-grass phytoliths, blocky and epidermal polygonal morphotypes were present in samples near sagebrush steppe or forest boundary. Samples from Festuca grassland contained the same morphotypes as those of the Agropyron grassland, but could be distinguished based on different phytolith proportions. Short wavy morphotype (#2) made up 12 F 3% (n = 9), long cells (#4–7) 26 F 8%, and rondels (#9–12) 31 F 8%. The proportion of wavy phytoliths thus was lower (due to less Poa in this grassland), while the proportion of rondels was higher, reflecting the added contributions of Festuca and Agropyron. When individual rondel and long cell morphotypes were examined (Fig. 3), samples from Festuca-dominated grasslands could be differentiated from those from Agropyron-
84
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BR 2 LP 4 KR 6 SH 8 WF 10 LP 12 LP 14 LP SH 16 WA WA 18 WA WA 20 LP LP 22 U4 SH 24 SH LG 26 LG MA 28 HR U1 PA 30 U5 32 LG HR 34 HR PA 36 PA PA 38 WA SC 40 MR WS 42 WS WF 44 PA RS RS 46 WA U6 48 U3 50 U3 DL FG 52 DL 54 DT DT 56 DT DT 58 U2 SH KR LG RS LP
Veg.Type Code SS
SG OG AG
FG
PF OF SF 1000 2000 3000
20
20
40
20
20
40
60
20
20
40
20
20
20
20
20
40
60
20
40
20
20
40
20
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60
ma.s.l.
Fig. 3. Percentage of phytolith morphotypes in 58 modern soil samples from the interior Pacific Northwest. Vegetation codes: SS — sagebrush steppe, SG — Stipa grassland, OG — Other (Elymus) Grassland, AG — Agropyron Grassland, FG — Festuca Grassland, PF — Pinus ponderosa Forest, OF — other (Abies grandis and Pseudotsuga menziesii) forest, SF — Subalpine Forest. See Table 4 for the explanation of site codes and sample ID numbers.
M.S. Blinnikov / Review of Palaeobotany and Palynology 135 (2005) 71–98
te Sa m El ple ev ID ati on
m
ModernPhytolith Assemblages from the PNW:Morphotype Percentages
M.S. Blinnikov / Review of Palaeobotany and Palynology 135 (2005) 71–98
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Table 4 Surface sample sites in the Pacific Northwest Site Site name code
Latitude (N)
HR
43856V00W 121802V00W 1230
FG WS SC MR SH LG MA WA
Horse Ridge Research Natural Area (RNA), OR Frog Camp Rd., Willamette National Forest, OR West of Sisters, OR, on Hwy. 242 Stevens Canyon Rd., Deschutes National Forest, OR Metolius RNA, Deschutes National Forest, OR Sheep Rock Trail, the Peninsula, OR Lawrence Memorial Grassland Preserve (TNC), OR Hwy. 97 near Madras, OR Mt. Howard, Eagle Cap Wilderness Area, OR
Longitude (W)
Elevation Vegetation (m)
44818V00W 121837V00W 1025 44821V00W 121832V00W 1000
Juniperus/Artemisia/ Agropyron–Festuca woodland Pinus contorta–Abies lasiocarpa forest Pinus ponderosa/Agropyron forest Pinus ponderosa/Festuca forest
42, 43 (PF) 40 (PF)
44830V00W 121837V30W 1000
Pinus ponderosa/Festuca forest
41 (PF)
44831V30W 121817V30W 700
Juniperus/Artemisia/ Agropyron–Festuca–Stipa woodland Agropyron–Festuca grassland and Artemisia rigida–Poa scabland Artemisia–Agropyron shrub steppe Pinus ponderosa forest,
24, 25 (AG), 16 (SG) 9 (SS), 26, 27 (AG), 33 (FG) 28 (AG) 49 (PF). 17, 18, 19, 20 (SG), 39 (FG)
44812V00W 121852V30W 1475
44856V30W 120848V00W 1020 45810V00W 120850V00W 1000 45815V30W 117810V30W 1710, 2450
45844V00W 119838V00W 170 46805V00W 117852V00W 1450
1,2 (SS) 48 (PF)
46805V30W 117851V30W 1450
Abies grandis–Pseudotsuga forest
50, 51 (OF)
46809V00W 117828V00W 1750
Abies lasiocarpa forest
Lindsay Prairie Preserve (TNC), OR
45835V30W 119839V00W 275
WF
Rd. 46 at Rd. 080, Wallowa-Whitman National Forest, OR Boardman RNA, OR Robinette Mtn., Columbia Co., WA Rd. 64 at Chase Mtn., Umatilla National Forest, WA Devil’s Tailbone Rd., Umatilla National Forest, WA Prairie in Walla Walla Co., WA Bundy Hollow Cemetery, WA Pataha River Canyon, Umatilla National Forest, WA
45837V00W 117813V00W 1275
U3 DT U4 U5 PA
RS U2 KR SH U1 DL
Rose Springs, Umatilla National Forest, WA Sagebrush Scrub, Benton Co., WA Kahlotus Ridgetop Natural Area, WA Sand Hills, near Hampton Rd., WA Dry Prairie, Adams Co., WA Dewey Lake, near Mt. Rainier National Park, WA
29 (AG) 34, 35 (FG) 52 (SF)
Festuca viridula and Stipa occidentalis grasslands Agropyron dasystachyum– Oryzopsis grassland Artemisia–Stipa shrub steppe, Elymus grassland Artemisia–Poa scabland, Pinus ponderosa– Calamagrostis forest Artemisia–Agropyron shrub steppe Pinus ponderosa forest
LP
BR U6
Sample numbers (vegetation typea)
46812V00W 118815V00W 500 46812V00W 118805V00W 650 46816V00W 117831V30W 1400
46816V30W 117833V00W 1400 46817V00W 46842V00W 46847V30W 46850V00W 46851V15W
119830V00W 118833V00W 118831V00W 118830V00W 121829V00W
250 475 450 450 1550
12, 13, 14, 15 (SG) 22 (AG) 4 (SS) 21 (OG) 10 (SS) 44 (PF)
55, 56, 57, 58 (SF) Agropyron–Festuca grassland 23 (AG) Festuca grassland 32 (FG) Festuca–Koeleria grassland 36, 37, 38 (FG) Agropyron–Festuca grassland 30 (AG) Pinus ponderosa–Calamagrostis forest 45 (PF) Artemisia rigida–Poa shrubland 11 (SS) Pinus ponderosa–Carex forest 46, 47 (PF) Artemisia/Agropyron shrub steppe 3 (SS) Artemisia/Agropyron–Festuca 6, 7 (SS) Artemisia shrub steppe 5, 8 (SS) Agropyron–Festuca grassland 31 (FG) Tsuga mertensiana– 53, 54 (SF) Abies lasiocarpa forest
Site codes correspond to Fig. 1. a Eight vegetation types were distinguished based on the dominant species found within the 4 4 m plots: SS = Artemisia-dominated Sagebrush Steppe, AG = Agropyron-dominated Grassland, FG = Festuca-dominated Grassland, SG = Stipa/Oryzopsis-dominated Grassland, OG = Other Grassland (Elymus-dominated), PF = Pinus ponderosa Forest, OF = Other Forest (Abies grandis–Pseudotsuga menziesii), SF = Subalpine Forest (Abies lasiocarpa and/or Tsuga mertensiana-dominated).
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dominated grasslands based on the higher percentage of rounded horned rondels (12 F 3% versus 1 F 0.5%), and higher percentage of indented long cells (14 F 6% versus 6 F 1%). One sample from this vegetation type (39) came from an alpine stand of Festuca viridula growing at 2550 m elevation, and was very different in its composition. Additional samples from other high-elevation sites are necessary to more reliably identify such alpine communities in the soil phytolith record. The occurrence of conifer phytoliths in some samples of Festucadominated grassland can be explained by proximity to Pinus ponderosa forest zone. Pinus ponderosa forest was identified by the presence of the diagnostic spiked morphotype of P. ponderosa (#21, Fig. 2). Spiked phytoliths were abundant in soils under present-day P. ponderosa forest (28 F 6%, n = 10). Two groups of samples can be distinguished within this vegetation zone (Fig. 3): samples from low-elevation dry forests of P. ponderosa/Agropyron spicatum and P. ponderosa/Festuca idahoensis associations (samples 40–44), and samples from more mesic P. ponderosa/Calamagrostis rubescens association at higher elevations (samples 45–49). The former group had few wavy or rectangular phytoliths, but abundant rondels and long cells. The latter group had a high proportion of wavy forms, including those diagnostic of Calamagrostis, and trichomes. Two samples were taken from bOther ForestQ vegetation type, sample 50 from an almost pure Abies grandis stand and sample 51 from A. grandis– Pseudotsuga menziesii forest with some Pinus ponderosa nearby. Unstudied forest grasses probably contributed their phytoliths to both samples. Subalpine forest samples could be readily distinguished from those from other vegetation types by the high proportion of blocky forms (probably Picea and Abies, 9 F 3%, n = 7), epidermal polygonal phytoliths (13 F 2%) probably contributed by Abies, and a suite of bother coniferQ phytoliths (41 F 22%). The bother coniferQ suite included silicified tracheids, phytoliths with unevenly thickened cell walls of Larix, and phytoliths with four sinuous sides (Klein and Geis, 1978; Bozarth, 1992) from Picea engelmannii and Abies lasiocarpa. The variability in the bother coniferQ group was quite high, and further research of the phytoliths from subalpine forests is needed. Sam-
ple 54 had a high proportion of conical phytoliths of Carex (15%) and some Stipa-type bilobates. This sample came from a subalpine bog on the shore of Dewey Lake near Mt. Rainier National Park. Stipa occidentalis and Carex both grew at the site. Detrended correspondence analysis (DCA) was performed in PC-ORD (1997) on 58 modern soil samples to test how well vegetation types could be differentiated based on phytolith assemblages, and if any gradients could be extracted from the data by this indirect method. In the resulting scatterplot (Fig. 4, only first two axes are shown) samples from all vegetation types grouped together well, except samples from bother forestsQ, which appeared similar to grasslands. The main reason for this was lack of phytolith-producing conifers in this community, which made phytoliths from grasses excessively important. Artemisia shrub steppe, Pinus ponderosa forests and subalpine forests appeared as the most distinct vegetation types. The other four vegetation types were less well differentiated. Samples from Stipa-dominated grasslands contained high proportion of bilobate Stipa phytoliths. Agropyron spicatum-dominated grasslands contained high percentage of long cells and wavy forms, while those from Festuca-dominated grasslands were distinguished by the high proportion of rondels and silicified hairs. Axis 1 in Fig. 4 can be interpreted as an artificial gradient stretching from the assemblages with the high proportion of spiked morphotype (#21 on Fig. 2) to the assemblages with the high proportion of the bother coniferQ morphotypes or blocky forms, with grassland assemblages (usually N 80%) taking up the middle. Axis 2 apparently represents the main environmental moisture gradient extending from very dry communities (assemblages from Artemisia steppe) near bottom through moderately dry grasslands to mesic Pinus ponderosa forests, and finally to the moist subalpine forests. The exceptions to this pattern are the samples from the more mesic Festuca idahoensis-dominated grasslands that were placed below, not above, drier Agropyron spicatum and Stipa-dominated grasslands on Axis 2. Boxplots allow further direct examination of the phytolith data by plotting each morphotype abundance for the eight vegetation types (Fig. 5).
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Axis 2, SD conical
55
+2 47
45 other conifer
trichomes
+1 43
39
49
bilobates
34
spiked
41 44 42 46
0
40 36
-1
57
17 18 21 48
long cells
hairs
7 seed
54 52 58
13
epidermal Asteraceae 50 19 12 24 14 30 29 25 27 wavy short 15 20 plates 4 3738 3 32 31 16 22 23rondels26 3 28
33
2
anticlinal
5
epidermal polygonal
Artemisia steppe Stipa or Oryzopsis grassland Elymus grassland Agropyron grassland
11 9 1
Festuca grassland Pinus ponderosa forest
6
P.menziesii-A.grandis forest
10
-2
subalpine forest phytolith morphotypes
blocky
Axis 1, SD
8
-2
-1
53
56
wavy long
0
+1
+2
+3
Fig. 4. Detrended Correspondence Analysis scatterplot showing the first two axes of the simultaneous ordination results of phytolith surface samples and morphotypes from the interior Pacific Northwest. Both axes are in units of standard deviations of DCA scores from the mean sample score, which was assigned a zero value. Phytolith morphotypes are from Fig. 2 (#9–12 rondels and #4–7 long cells were merged into single categories). Shape symbols represent 8 recognized vegetation types. Numbers correspond to 58 modern soil sample IDs from Table 4.
4.5. Phytoliths and climate Interpolated values of 5 selected environmental variables from 30 sampling sites provide a basis for direct phytolith–climate comparisons. Principal Components Analysis was performed on the climate variables and elevation to explicitly characterize climatic gradients between the sites and to reduce the number of variables for the subsequent analysis. The first two components resulting from PCA explained 90.8% of the variance in the data. PC1 (72.5% of the total variation) represented the main environmental gradient in the data dependent on elevation. As expected, a strong negative correlation was found between elevation and temperature, and a strong positive one between elevation and precipitation. Loadings for PC1 were highest for growingdegree days above 0 8C (0.254), mean annual temperature (0.253), mean July temperature (0.253), June–August temperature (0.253), and mean temperature of the warmest month (0.253). Loadings were lowest for June–August precipitation ( 0.251), July precipitation ( 0.248), and elevation ( 0.250).
PC2 appeared to describe seasonality of precipitation (18.3% of the total variance). PC2 loadings were highest for July precipitation to annual precipitation ratio (0.442) and January–July temperature range (0.351), and were lowest for the January precipitation to annual precipitation ratio ( 0.423), and the absolute minimum temperature ( 0.405). Five variables were then chosen for subsequent analysis against phytolith data, elevation, mean annual temperature (MAT), mean annual precipitation (MAP), number of growth degree-days with temperatures above 0 8C, and the moisture index of the ratio of actual evaporation to potential evaporation (Thornthwaite–Mather method). The first 3 variables highlight the main environmental gradient apparent in PC1, while the latter two were chosen as useful bioclimatic variables representing two main parameters essential for plant growth: amount of useful energy and amount of available moisture (Thompson et al., 2000). Scatterplots (MINITAB, 1998) display abundances of 13 selected morphotypes (Fig. 6) against the five selected environmental variables. Rare morpho-
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types were excluded from the analysis. After initial analysis of all 58 samples, a few alpine sites, the only bother grasslandQ and two bother coniferQ forest sites were dropped, leaving a total of 45 samples for the direct gradient analysis. Despite the high degree of variation in the data, several trends are evident: Rectangular plates, common in all grasses, were more abundant at low elevations and most abundant (about 13%) at approximately 500 m above sea level. They did not show a strong relationship with any factor. Short wavy morphotype had higher values at sites at intermediate elevations, with higher annual temperature, lower precipitation, and intermediate number of GDD (N 2000). These were the dry grassland sites dominated by Agropyron spicatum with high presence of Poa sandbergii, where values close to 30% were observed with MAT of 7 8C and MAP of 300 mm. Overall, the presence of short wavy phytoliths suggests presence of dry conditions with a very weak negative linear trend for MAP (Fig. 6, R 2 = 3%). Long wavy cells share increased with increasing temperature and decreasing precipitation. The main contributors of these phytoliths were grassland species of Agropyron spicatum and Festuca idahoensis. Over 30% long cells in a sample may indicate moderately dry conditions (b 600 mm MAP). Few plots contained these, and thus no strong linear trend was evident. A sum of all long cells from grasses demonstrated a weak negative trend with increasing elevation (Fig. 6, R 2 = 9.2%), MAP (Fig. 6, R 2 = 23%)and AE / PE (Fig. 6, R 2 = 9.7%), and a weak positive trend with MAT (Fig. 6, R 2 = 12%) and GDD N 0 8C (Fig. 6, R 2 = 9.1%). These are weak regression values, but stronger than for individual long cell or for wavy morphotypes. Long cells seem to be a good overall indicator of grasslands. The abundance of rondels (brondel sumQ in Fig. 6) decreases with increasing precipitation (R 2 = 5.5%) and barely increases with increasing temperature
(R 2 = 2.9%). Samples with the highest proportion of rondels (N30%) come from moderately moist Festuca idahoensis-dominated grasslands with less than 670 mm MAP. Bilobate phytoliths of Stipa-type had a bimodal distribution for all environmental variables except annual temperature range. These forms were found in both cool and moist conditions (MAT b 3 8C, MAP N 1500 mm) at subalpine sites, where Stipa occidentalis is common, and in dry warm conditions on the plains, where Stipa comata, Stipa thurberiana, and Oryzopsis are found (MAT N 10 8C, MAP b 300 mm). Increase in GDD N 0 8C had the strongest positive linear trend (R 2 = 11.1%), but because of the bimodal distribution, quadratic regression would be more appropriate (R 2 = 13.6%). Trichomes were most abundant in samples from middle elevations (ca. 1300 m), moderately warm temperatures (MAT 5 8C), and moderately moist conditions (MAT 790 mm). These came largely from Pinus ponderosa forests with Calamagrostis (N 5% of trichomes). Thus, trichomes may indicate presence of forested, primarily Pinus-dominated, environments. In contrast, silicified hairs were common in samples from low elevations (b1000 m), with high MAT (N 8 8C for samples with N 10% hairs) and low MAP (b 670 mm for samples with N 10% hairs). Although the relationship was again weak (e.g., R 2 = 9% for MAT and 7.5% for MAP), such phytoliths were definitely more common in dry, warm environments of grasslands and shrub steppe. The sum of all grass phytoliths (bgrass sumQ in Fig. 6) showed some of the strongest linear trends of all morphotypes: a positive linear trend with increasing MAT (R 2 = 11.7%) and GDD N 0 8C (R 2 = 7.0%) and a negative one with elevation (R 2 = 8.9%), MAP (R 2 = 23.7%), and AE / PE (R 2 = 3.3%). Blocky phytoliths were most common in samples from dry and warm sites at low elevations (shrub steppe sites at b 700 m, N 10 8C MAT, b 300 mm MAP; R 2 = 4.9%). A few samples with abundant blocky forms came also from subalpine forests at N 1500 m elevation.
Fig. 5. Boxplots showing different phytolith morphotypes found in modern soils of the interior Pacific Northwest under eight vegetation types arranged from the driest (steppe) to the wettest (subalpine forest). Vegetation codes: 1 — sagebrush steppe, 2 — Stipa grassland, 3 — Other (Elymus) Grassland, 4 — Agropyron Grassland, 5 — Festuca Grassland, 6 — Pinus ponderosa Forest, 7 — other (Abies grandis and Pseudotsuga menziesii) forest, 8 — Subalpine Forest. Percentage value of morphotype concentration in a sample is shown as dots. The left and right sides of each box indicate the 25th and 75th percentiles respectively, the median is shown as the vertical line near the middle of the box.
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Fig. 5 (continued).
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Fig. 5 (continued).
They exhibited a positive trend with increased number of GDD N 0 8C (R 2 = 6.8%), and a negative trend with elevation (R 2 = 7.6%), MAP (R 2 = 2.4%) and especially AE / PE index (R 2 = 12.2%). Many different taxa contribute epidermal polygonal cells to the soil assemblages. No apparent trend was found in the climate data for this morphotype. It does exhibit a bimodal distribution with elevation with mid-elevation pure grassland sites having the lowest proportion (quadratic trend’s R 2 = 17.1%). In contrast, spiked phytoliths of Pinus ponderosa had a very narrow distribution, since as a diagnostic form of P. ponderosa, they were restricted to P. ponderosa forests. These phytoliths were found only in samples from higher elevations (1000–1400 m), with 7–8 8C MAT and 600–1000 mm MAP. Because of the narrowness of distribution, no linear trend was apparent, except for AE / PE index (R 2 = 19.8%). bOther coniferQ suite of morphotypes was restricted to the subalpine forests at even higher elevations (1600–2000 m), with lower temperatures (3–4 8C
MAT, R 2 = 30%) and higher precipitation (900–2000 mm MAP, R 2 = 27.2%).
5. Discussion 5.1. Phytoliths in plants Identifiable phytoliths were found in many dominant plants in the interior Pacific Northwest (Table 1). Phytoliths from two Artemisia shrubs, two Chrysothamnus shrubs, Bromus tectorum, Carex rossii, Festuca viridula, and Lupinus sericeus are described here for the first time. Characteristic phytolith shapes from four species of Pinus, two species of Abies, two of Larix, three of Picea, two of Tsuga, and Pseudotsuga were described earlier from New York, Arizona and the European Alps (Klein and Geis, 1978; Kearns, 2001; Carnelli et al., 2004). My findings confirm conclusions about diagnostic value of asterosclereids of Pseudotsuga menziesii and bendodermal polyhedronsQ (i.e., blocky) phytoliths of Picea (Norgren, 1973; Bozarth, 1993).
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Fig. 6. Scatter diagrams (MINITAB, 1998) showing the abundance of selected phytolith morphotypes plotted against selected environmental variables. Long cell sum includes morphotypes #4–7, rondel sum — #9–12, grass sum — #1–13. Environmental variables: MAT—mean annual temperature in 8C, MAP—mean annual precipitation in mm, AE / PE – actual to potential evapotranspiration index of Thornthwaite–Mather and GDD N 0 — growing-degree days with base N0 8C. LOWESS curves show the locally weighted regression of the relationship of phytoliths to climatic variables.
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Klein and Geis (1978) and Carnelli et al. (2004) reported that Larix sp. produces abundant epidermal cell wall fragments, also found in my study. Spiked phytoliths of Pinus ponderosa were previously described by Norgren (1973) and Kearns (2001) and appear similar in form to the spiny irregular bodies diagnostic of Pinus banksiana (Bozarth, 1993). In the same study, a four-sided wavy plate diagnostic of Picea glauca was reported, which I found in Picea engelmannii, and elongated polyhedrons in Abies balsamea, corresponding to my blocky form in Abies lasiocarpa. Some conifers in the study area contained little (Juniperus occidentalis, Pinus contorta) or no silicified material (Abies amabilis, Abies grandis, Pinus albicaulis, Thuja plicata). The paucity of phytoliths in these trees can be attributed to the overall low level of silicification in these species. Given the high diagnostic value of the examined conifer phytoliths, a thorough study of phytoliths for all Holarctic conifers is desirable. As reported in Geis (1973) and Bozarth (1992), few deciduous trees in North America produce abundant distinct phytoliths. Exceptions are Quercus, Ulmus, and Acer, but these genera are not prominent in my study area today. Deciduous tree species produce polygonal and anticlinal (bjigsaw puzzleQ) epidermal phytoliths, also produced by some dicot shrubs and forbs. Polygonal epidermal forms were reported from Quercus, Betula, Corylus, Populus, and Ulmus (Bozarth, 1992). Anticlinal forms were found in Acer, Platanus, Populus and Salix. In the study area, representatives of some of these genera are restricted to floodplain forests or upper treeline. Alnus sinuata, a shrub growing near the upper treeline, was found to contain some anticlinal phytoliths. Unlike trees, shrubs have received little attention with respect to their phytolith production. My study demonstrated potential utility of Artemisia phytoliths. Blocky phytoliths of Artemisia of unknown anatomical origin (perhaps from subepidermal tissue or bark) appear to be similar to blocky forms of Abies and Picea. Artemisia also produces epidermal polygonal phytoliths (morphotype #18) common to other Asteraceae. In their classic paper, Twiss et al. (1969) proposed a bthree-groupQ classification model that divided all grass phytoliths into Panicoids (i.e., bilobates and
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crosses), Chloridoids (i.e., saddles), and Festucoids (i.e., rondels and wavy forms). A thorough evaluation of this system can be found in Mulholland (1989). Because few Chloridoid or Panicoid species grow in the study area, I chose to develop a more detailed system of grass phytolith classification based primarily on common Festucoid forms. Patterns in Festucoid phytolith production in the PNW are similar to those in Alberta (Blackman, 1971). In particular, Blackman observed short wavy phytoliths in both Poa sandbergii (identified as Poa secunda) and Koeleria cristata, long wavy phytoliths in Calamagrostis rubescens, scutiform seed opal in Agropyron, and diverse wavy and smooth phytoliths in Elymus. Her illustrations of phytoliths from Stipa comata and three other species of Stipa appear similar to my pyramidal and Stipa-type morphotypes (Blackman, 1971). An earlier attempt at classifying grass phytoliths (Norgren, 1973) provided data and illustrations of phytoliths in seven species of grasses and one sedge from Oregon. He distinguishes three classes: brodsQ (corresponding to plates (#1) and long cells (#4–7) in this study), bhookbasesQ (apparently corresponding to trichomes (#14) and rondels (#9–12) in this study), and hairs (#15). The findings of Norgren (1973) closely match my results, although lack of clear definitions for some of his morphotypes makes direct comparisons difficult. For example, he found Agropyron spicatum to contain large proportion of bwavy rodsQ (around 27%), similar to my bdeeply indented rods and angular rodsQ (15%). Festuca idahoensis, on the other hand, contained mostly brough rods b (60%), apparently corresponding to my bindented long cellQ (24% in my study), and hairs (17% in Norgren (1973), 4% in my study). No distinction was made by Norgren (1973) between trichomes and rondels, both were treated as bhookbasesQ (i.e., silicified epidermal appendages), which confuses interpretation. For example, Stipa comata had about 40% bhookbasesQ (bspheroidal to peanut-shapedQ, Norgren, 1973, p. 57), apparently corresponding to pyramidal rondels and Stipa-type bilobates (this study), which anatomically are not epidermal appendages at all, but epidermal short cells. Surprisingly, Norgren (1973) does not describe any bhookbasesQ in F. idahoensis, which is hard to explain given the widespread occurrence of rondels in this species, as well as in other Festuca spp.
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worldwide (Smithson, 1958; Kiseleva, 1982; Blinnikov, 1994). Norgren (1973) mentions high presence of trichomes (blarge hookbasesQ) in Calamagrostis rubescens, which my study also confirmed. I also found a high proportion of bsmooth rodsQ (i.e., rectangular long cells) in Elymus cinereus. The only sedge mentioned in Norgren (1973) was Carex geyerii, which contained both brodsQ (i.e., long cells), and bsmall hookbasesQ (i.e., diagnostic conical forms of Carex). Brown (1984) examined 112 species of grasses common to central North America. Direct comparisons are difficult to make, however, because his system is very detailed. Overall, Brown’s results support my findings with respect to phytolith forms found in Agropyron spicatum and Agropyron dasystachyum, Calamagrostis rubescens, Festuca idahoensis, Koeleria cristata, Poa sandbergii, Sitanion hystrix, and Stipa comata. Mulholland (1989) identified a large number of bpolylobate sinuateQ forms in Koeleria cristata and Elymus canadensis from North Dakota (cf. short wavy plates in this study), entire rondels (apparently, pyramidal) in Stipa comata, and abundant Stipa-type bilobates in other Stipa species (cf. Stipa-type bilobates in this study). Kiseleva (1982) described phytoliths from arid Mongolian grasslands, including some from the genera found in my study area. Some of her matching observations include high abundance (about 80%) of rondels in seven closely related species of Festuca, the presence of Stipa-type phytoliths in six Stipa species (ca. 20%), presence of pyramidal rondels in Elymus and short wavy forms in Koeleria cristata, and a high percentage of long wavy forms in Calamagrostis macrolepis. Thus, different species of the same genera of Festucoids in Mongolia contain phytolith morphotypes similar in appearance and proportion to the North American species. This conclusion is further corroborated by Blinnikov (1994), who reported many of the same phytolith morphotypes in the same genera of Festucoid grasses from the Caucasus (e.g., Calamagrostis, Festuca, Poa). In a new study from Europe, Carnelli et al. (2004) analyzed 21 species from the Swiss Alps. Their results confirm many of my findings both with respect to grass and non-grass forms. While their
classification is considerably more detailed (e.g., they distinguish 18 types of trichomes, while I lump them all together), ample illustrations make comparisons easy. Their analysis of another Calamagrostis species (Calamagrostis villosa) corroborate my findings that this genus tends to have high incidence of long wavy cells and trichomes (Blinnikov, 1994; this study). Three Festuca species tend to have high incidence of rondels (their btrapezoidQ category). Larix in their study contained spiky cells (cf. my morphotype #20), etc. 5.2. Phytoliths in modern soils Phytolith assemblages in soils can differentiate common vegetation types of the interior Pacific Northwest. Overall, the most distinct assemblages come from Pinus ponderosa forests and Stipa grasslands. Both could be identified on the basis of a single diagnostic morphotype, spiked of P. ponderosa (#17) and Stipa-type bilobate (#13) respectively. Stipa-type bilobates (Fredlund and Tieszen, 1994) and spiked phytoliths (Kearns, 2001) can persist for centuries in the modern soil record. Another diagnostic morphotype, the large silicified asterosclereids of Pseudotsuga menziesii, helped identify the presence of P. menziesii in the past (Norgren, 1973), but their extremely large size makes them rare in the silt fraction. Although Artemisia shrub lacks phytoliths diagnostic of the genus, Artemisia shrub steppe can be reliably identified on the basis of the high presence of blocky forms apparently from Artemisia and polygonal epidermal forms of other shrubs and forbs. Grasslands can be distinguished from either forests or shrub steppe on the basis of high proportion of grass phytoliths (N90%), particularly long cells and wavy forms in Agropyron-dominated grassland and rondels in Festuca-dominated grassland. Although both arid Agropyron-dominated and mesic Festucadominated grasslands have similar assemblages, they can be distinguished based on different types of rondels and long cells: high presence of short wavy forms indicate presence of Poa sandbergii or Koeleria cristata. Few modern reference studies of phytoliths in soils exist anywhere in North America. Norgren (1973) analyzed soil samples from ten different locations in Oregon, including three samples from the central
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Coast Range, two from the Cascades, one from eastern Oregon, three from the Columbia Plateau, and three from the Wallowa Mountains. Despite confusing terminology, the findings match my data reasonably well. For example, a Tolo series soil from Pinus ponderosa–Pseudotsuga menziesii–Larix occidentalis forest in the Wallowa Mountains, which contained 10% Calamagrostis rubescens and Carex geyerii understory, featured 35% trichomes (referred to as bhookbasesQ) of C. rubescens. In my study, 20% trichomes were observed in a sample from P. ponderosa–C. rubescens forest in the northern Blue Mountains in Washington State (sample 45 on Fig. 3). According to Norgren (1973), Agropyron-dominated grassland assemblages, such as Condon silt loam sample from northern Oregon near the Columbia River, contained about 80% of brodsQ (i.e., long cells and wavy and rectangular plates), and 12% bhookbasesQ (i.e., rondels and hair bases), similar to my study. A more recent example of a modern analog phytolith study comes from the Great Plains (Fredlund and Tieszen, 1994). Although only short-cell forms were considered, some interesting comparisons can be made. Most of the 50 assemblages studied in that work contained a high proportion of saddles, coming from the short-grass prairie species. None of my assemblages contained saddles, not surprisingly, given the fact that Chloridoid grasses were not found in the study area. Two samples from Stavely, Alberta, from Agropyron–Stipa–Danthonia community (Fredlund and Tieszen, 1994) are similar to some of the driest assemblages (Stipa-dominated grasslands with Agropyron dasystachyum) from Lindsay Prairie in the Columbia Basin (samples 12–14 on Fig. 3). The Stavely assemblages were dominated by three kinds of rondels (60%), Stipa-type bilobates (10%), and crenate (wavy/lobed plates in my classification) phytoliths (10%). My samples were likewise dominated by rondels (40%), Stipa-type (15%), and wavy forms (40%), if all the non-grass, long cell, and trichomes/hair phytoliths are excluded. It is also noteworthy that Fredlund and Tieszen (1994) bkeeledQ Q morphotype appear to be the same form as my belongated rondelQ, and is likewise found in a member of Agropyron genus (Agropyron smithii). The Stavely assemblages contained about 20% of this morphotype, while Agropyron-dominated grasslands in my study
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contained only 9% of this morphotype (median value), or about 14% if non-grass and long cell forms are excluded. Although it is true that long cell taphonomy and production is less known than that of the short-cell forms (Fredlund and Tieszen, 1994), the results of my research suggest that such forms should be considered in the phytolith analysis because they yield additional interesting data. Some further comparisons can be made with a modern analog study of phytoliths assemblages from subalpine and alpine communities in the northwestern Caucasus (Volkova et al., 1995). Despite different species, genera of grasses and conifers were the same, and some similarities can be pointed out. Phytolith assemblage from Pinus hamata–Calamagrostis arundinacea forest from Teberda Nature Reserve, Russia, is similar to the Pinus ponderosa–Calamagrostis rubescens assemblages from the Blue Mountains, Washington. Both assemblages contain a high proportion of diagnostic Pinus phytoliths, trichomes, and long wavy phytoliths of Calamagrostis. The more prominent role of Pinus phytoliths in the Blue Mountains may be explained by the higher phytolith production in P. ponderosa. As more modern phytolith studies are done elsewhere, there will be more opportunities for further interregional comparisons. A reliable internationally recognized nomenclature of morphotypes is being developed to facilitate such comparisons in the future (Madella et al., 2003). 5.3. Phytoliths and climate Poor understanding of phytolith production and preservation patterns hampers application of the phytolith analysis in direct paleoclimatic reconstructions (Fredlund and Tieszen, 1997b). Phytoliths are commonly thought to reflect subtle shifts in vegetation composition at the local scale, because they are primarily deposited in situ (Piperno, 1988). However, phytoliths are displaced from the original deposition site both vertically and horizontally as a result of fire, flood, grazing, burrowing, or another disturbance. Such mixing may blur the vegetation signal offered by phytolith assemblages in modern soils. Indeed, Fredlund and Tieszen (1994) found that phytoliths from a given geographic location appeared to reflect vegetation of a larger area than previously thought. They concluded that the phytolith assemblages were
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likely to reflect regional climate. For example, at the regional scale, grassland composition is closely associated with temperature and moisture gradients. Calibration of grass phytolith assemblages in climatic terms opened a way to the direct paleoclimatic reconstructions from phytoliths by developing response surfaces and transfer functions (Fredlund and Tieszen, 1997b). The phytolith data could thus supplement paleoclimatic reconstructions provided by pollen (Bartlein et al., 1986; Williams et al., 2001). More extensive and even sampling of different vegetation and climate zones would be required before quantitative paleoclimatic reconstructions could be made with phytoliths. My sampling strategy was to maximize the number of different native vegetation types visited, which necessitated sampling dissimilar communities near ecotones (e.g., a grassland site sampled in proximity to a forested site). In macroclimatic terms, such sites would have an identical climate, which would blur the phytolithderived signal. More even sampling of sites along a regular grid and over a larger geographical area, as was recently done with the regional pollen (Minckley and Whitlock, 2000), would better illustrate current phytolith–climate relationships.
6. Conclusions The objective of this study was to provide a basis for the interpretation of fossil phytolith assemblages recovered from the late Pleistocene loessal paleosols in the Columbia Basin Province of the Pacific Northwest. Phytoliths from 38 plant species and 58 samples of modern soils were examined. Using twenty phytolith morphotypes from modern soils (13 of which were from grasses) it was possible to distinguish eight vegetation types, including Artemisia-dominated shrub steppe, Stipadominated lowland and subalpine grasslands, Agropyron-dominated dry grassland, Festuca-dominated mesic grassland, Elymus cinereus grassland, Pinus ponderosa-dominated dry forest, Abies grandis– Pseudotsuga menziesii mesic forest and Abies lasiocarpa–Picea engelmannii subalpine forest. Shrub steppe, Stipa-dominated grasslands, P. ponderosa forests, and subalpine forests produced the most distinctive phytolith assemblages. Use of three
morphotypes within the group of long cells and four different rondels enabled further differentiation of grasslands. Grass phytoliths are the most diagnostic at the subfamily level, but can also distinguish certain genera of Festucoid grasses. For example, Agropyron, Calamagrostis, Festuca, Koeleria, Poa and Stipa may be distinguished based on their phytolith record. Non-grass phytoliths appear to be most distinctive at the family level, but some genera, and even species, could be identified, e.g., Pseudotsuga menziesii and Pinus ponderosa in my study area. Some morphotypes are redundant, e.g., rectangular plates are found in almost all grasses, polygonal epidermal cells are common in many conifers, shrubs and forbs, and silicified tracheids are common in most conifers. Study of direct phytolith–environmental relationships proves that certain morphotypes occur preferentially in certain climates and at certain elevations. For example, long cells as a group and short wavy morphotype tend to occur in higher proportion in drier and warmer habitats at lower elevations, while spiked and other conifer morphotypes occur under moist and cool conditions at higher elevations. Rondels tend to be more abundant in samples from moderately dry to moderately moist habitats. Bilobates of Stipa-type have a distinct bimodal distribution, occurring in either the dry and warm climate of the lowlands, or in the cool climate of the subalpine zone. Blocky forms are mostly restricted to dry and warm habitats in lowland areas, but are also found in subalpine forest samples. While the data show only weak climate– phytolith relationships, selection of a wider range of sites across longer environmental gradients could further improve our understanding of the phytolith– climate relationships. This study did not address the issues of variable production of opal by individual species and variability in preservation of various forms in soils. In general, paleoassemblages may be matched against their modern analogs based on the whole assemblage composition or indicator forms. Additional research of production and preservation is required, because some phytoliths may prove to be systematically underrepresented in the paleoassemblages based on either their low production today or low preservation rate in soils.
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Acknowledgements I. Blinnikova, A. Gubin and S. Sumstine provided help in the field. I. Blinnikova also greatly assisted with lab processing. P. Bartlein provided the modern climate dataset and helped with climate– phytolith interpretations. A. Busacca, P. Bartlein, P. MacDowell, C. Whitlock, and G. Retallack provided helpful comments on an earlier draft of this paper. The paper presents some of the results of a Ph.D. dissertation research supported by the Mazamas and Sigma Xi research grants and the Graduate Doctoral Research Fellowship of the University of Oregon.
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