Table Of ContentMadrono, Vol. 53, No. 4, pp. 351-363, 2006
PHYTOLITH EVIDENCE FOR THE LACK OF A GRASS UNDERSTORY IN A
SEQUOIADENDRON GIGANTRUM (TAXODIACEAE) STAND IN THE CENTRAL
SIERRA NEVADA, CALIFORNIA
Rand R. Evett
Division of Ecosystem Science, Department of Environmental Science, Policy,
CA
and Management, University of California, Berkeley, 94720-3110
[email protected]
Roy a. Woodward^
CA
California Department of Parks and Recreation, Sacramento, 95814
Wayne Harrison
California Department of Parks and Recreation, Calaveras Big Trees State Park,
CA
Arnold, 95223-0120
James Suero
CA
California Department of Parks and Recreation, Sacramento, 95814
Patricia Raggio
California Department of Parks and Recreation, Calaveras Big Trees State Park,
CA
Arnold, 95223-0120
James W. Bartolome
Division of Ecosystem Science, Department of Environmental Science, Policy,
and Management, University of California, Berkeley, CA 94720-3110
Abstract
Treeringfire-scarsin Sequoiadendrongigcmteum(giantsequoia)standsrecordahighfrequency,low
intensity, prehistoric fire regime. Difficulties achieving short prehistoric fire return intervals with
prescribed burns at a S. giganteimi stand in Calaveras Big Trees State Park, Cahfornia, currently
characterized by dense tree cover with little understory vegetation due to over a century of fire
suppression, suggest that a prehistoric grass understory provided fine fuel required for frequent fire
spread. We used phytohth analysis to test this hypothesis. Phytohths, microscopic silica bodies found
inmanyplantsbutproducedin largequantitieswithdistinctivemorphotypesingrasses, arepreserved
for thousands ofyears in the soil. Soils under vegetation with extensive prehistoric grass cover retain
a high concentration of grass phytoliths regardless of historic vegetation changes. Phytoliths were
extracted from soil samples taken from pits dug at 14 plots throughout a S. gigcmteum stand in the
SouthGroveNaturalPreserve. Soilphytolithweight formostplots, currentlywithoutgrasscoverand
comprisingmost ofthe stand, was lessthan0.10%, consistent withreported values forforestswithno
grass in the understory. Soil phytolith weights for ridge-top plots and plots near the stream channel
weresignificantlyhigher, suggestinglocalized areaswith sparse grasscover. The hypothesis that there
was substantial prehistoric grass cover in this S. gigcmteum stand was rejected.
Key Words: Sequoiadendron gigcmteum, giant sequoia, phytoliths, fire regime, California grasses,
California paleoecology. Sierra Nevada, Calaveras Big Trees.
Fire is an important ecosystem process in DeBenedetti 1979; Mutch and Swetnam 1995).
Sequoiadendron giganteum (Lindley) Buchholz Analyses of tree-ring fire-scars from living trees
(giant sequoia) stands in the central Sierra and stumps in the Sierra Nevada indicate that
Nevada in Cahfornia. Sequoiagiganteum appears prior to the mid-1800s there was a composite
to be dependent upon ground fires for regener- mean fire return interval of approximately five
ation in many areas. Cones open following fire to years in S. giganteum stands ofthe mixed-conifer
release seeds; a few years following ground fire, forest type (Swetnam 1993; Caprio and Swetnam
newly germinated S. giganteum seedlings domi- 1995; Parsons 1995). This means that tree-ring
nate burned areas (Kilgore 1973; Parsons and fire-scars recorded a surface fire somewhere
within the sampling area (ranging from 13 to
69 ha [Swetnam 1993]) every five years, averaged
'Present address: Elm Road Generating Station, over a period of 2000 yr, with considerable
Bechtel Corporation, 11050 South Chicago Road, variability at the decadal and centennial scales
Oak Creek, WI 53154. and between groves (Swetnam et al. 1992). These
MADRONO
352 [Vol. 53
surfacefires occurred in a patchy, mosaicpattern, and Major 1978; Stephenson 1996; Stephenson
and because crown fires involving S. giganteum 1999). In the 1860s, livestock grazing, principally
were evidently rare (judging from the presence of large flocks of sheep, was introduced into the
trees in excess of 1500 yr old), caused little central Sierra Nevada (Swetnam 1993; Kilgore
mortality to large trees. Frequent, low intensity and Taylor 1979), suggesting there was a sub-
ground fires were probably important for forest stantial amount of forage available. Researchers
health and maintained S. giganteum groves in in many areas of the western United States,
open, park-like stands. particularly the monsoonal southwest, have
From about 1860 to 1900, natural ecosystem found that fine fuels, especially grasses, were
processes in S. giganteum stands, including fire, important for the spread of ground fires. The
were drastically altered by livestock grazing removal of these fuels through overgrazing by
(mainly sheep), cessation of aboriginal burning livestock beginning in the mid-1800s probably led
practices, limited logging, and suppression of to sharp declines in fire frequency (Swetnamet al.
natural or human-caused wildfires (Kilgore and 1991; Caprio and Lineback 2002).
Taylor 1979). Currently, many mixed conifer Prior to alteration ofnatural fire regimes in the
forest stands at Calaveras Big Trees State Park mid-1800s, S. giganteum and mixed conifer
are thickets of mid-sized trees, often dominated stands in the vicinity of Calaveras Big Trees
by Abies coneolor (Gordon and Glend.) Lindley State Park were probably composed of widely
(white fir) and Calocedrus decurrens (Torrey) spaced, large diameter, towering 5. giganteum,
Florin (incense cedar). There is little understory a few Pinus lambertiana Douglas (sugarpine) and
vegetation because the overstory canopy is Pinus ponderosa Laws, (ponderosa pine), and
almost closed and provides little opportunity for a lesser element ofA. coneolor and C. decurrens.
light to reach the forest floor. The conditions These stands were probably devoid of dense
wherein S. giganteum stands can self-perpetuate woody understory vegetation because offrequent
and flourish no longer exist. surface fires ignited by native people or lightning.
Based on tree-ring analysis, from 500 AD to The open nature of these stands may have
1900 AD, the longest recorded period without allowed sufficient sunlight to reach the forest
a fire in a S. giganteum stand was approximately floor to support an extensive understory ofnative
60 yr, while during centuries with high fire grass species, which during the dry season served
frequencies (generally periods of drought) the to carry frequent surface fires throughout the
longest non-burn interval per stand was typically forest stand (e.g., Swetnam et al. 1992). These
no more than 13 yr (Swetnam 1993). There are surface fires would have killed most small shrubs
currently some S. giganteum stands that have had and trees, removed smothering duff layers,
no fire for the past 100 yr. recycled nutrients, perpetuated the grass-domi-
Prescribed fire has been introduced into S. nated understory vegetation, and maintained
giganteum stands in many parks in the Sierra gaps in the stands for forest regeneration. Un-
Nevada, including Calaveras Big Trees State fortunately, little solid evidence exists either way
Park, as a means of restoring more natural regarding the existence of this prehistoric grass
conditions. Ground fires have typically been set understory.
under proper conditions in late summer or fall We used phytolith analysis to test the hypoth-
with goals of removing excessive fuels, killing esis that grasses were an important prehistoric
invading A. eoneolor and C. decurrens trees, and component of the understory of S. giganteum
restoring natural processes. Experience at Cala- stands. Opal phytoliths are microscopic particles
veras Big Trees State Park has revealed that for of silica formed in the cells of many plants and
up to 15 yr following initial prescribed burning, released into the soil during decomposition
the understory vegetation and overstory canopy (Piperno 2006). Phytoliths are highly resistant
conditions will not support extensive surface to weathering and in many soil environments will
fires. Maintaining a mean fire return interval of persist for thousands or even millions of years
five years has not been possible because not (Wilding 1967; Str5mberg2004). Grasses produce
enough fuel is available to adequately carry many more phytoliths than most other plant
surface fire throughout S. giganteum stands. To taxa, averaging 3% and commonly ranging from
approach the relatively short mean fire return 1-5% dry weight phytolith content (Witty and
interval suggested by the tree-ring fire-scar Knox 1964; Piperno 2006). Most conifer species
evidence, it appears there mayhavebeen different produce <0.5% dry weight phytolith content
overstory species, such as deciduous hardwoods, (Klein and Geis 1978; Hodson et al. 1997).
contributing fuel to the forest floor, or un- Consequently, soils formed under grassland
derstory shrubs, herbs, or grasses that grew more vegetation contain an order of magnitude more
densely and provided fuel for frequent fires. phytoliths (1-3%) than soils formed under forest
The precise structure ofpre-1850 S. giganteum vegetation with few grasses (Witty and Knox
stands, particularly the composition and biomass 1964; Jones and Beavers 1964; Wilding and Drees
ofunderstory plant species, is unknown (Vankat 1968; Norgren 1973; Miles and Singleton 1975).
2006] EVETT ET AL.: CALAVERAS BIG TREES PHYTOLITHS 353
Phytoliths often take the shape of the cell in distinctly seasonal and highly infiuenced by
which they are formed. Most phytolith shapes are elevation. The annual precipitation at the eleva-
produced by many different plant taxa, but a few tion ofthepark ranges from 100 to 170 cm, much
taxa produce distinctive phytolith morphotypes of it coming as snow; about 90% of the total
(Rovner 1971). Grasses produce short cell phy- precipitation occurs during the six month period
toliths in specialized silica accumulator cells that between November and April (Walfoort and
are diagnostic of the Poaceae family. Short cell Hunt 1982).
phytoliths, including rondels, bilobates, and Soils in the South Grove Natural Preserve
saddles, have been used to identify grasses to (Fig. 2) are dominated by the McCarthy series,
the subfamily level (Twiss et al. 1969; Twiss 1992; inceptisols characterized as gravelly sandy loam/
Mulholland and Rapp 1992). very gravelly sandy loam found on slopes of5 to
Several studies have used soil phytoliths to 60%. Soil pH ranges from 6.3 at the surface to 5.9
document changes in dominant vegetation over at 60 cm depth. Parent material is well-drained,
time. In the prairie peninsula in Illinois, Wilding basic, volcanic rock with rapid permeability.
and Drees(1968) used differencesin soil phytolith Overstory vegetation at Calaveras Big Trees
content (measured as percent dry weight) to State Park is largely pine/mixed pine and fir
identify areas ofprehistoric forest, grassland, and forest dominated by Pinus ponderosa (ponderosa
the forest-grassland ecotone. In Utah, Fisher et pine), Pinus lamhertiana (sugar pine), and Abies
al. (1995) used changes in phytolith morphotype coneolor(white fir) in either pure ormixed stands.
percentages in the soil profile to document a shift Calocedrus decurrens (incense cedar), Quercus
from grassland to shrubland with the introduc- kelloggii Newb. (California black oak) and
tion ofgrazing. Bartolome et al. (1986), working Quereus elirysolepis Liebm. (canyon live oak)
at Jepson Prairie in the California Central Valley, also occur within the park. Vascular plants that
used changes in the concentration of bilobate have been identified in the park include over 60
grass phytolith shapes with soil depth to infer famihes, 159 genera, and 219 species (Walfoort
a prehistoric shift from A^<3^^^//«-dominated and Hunt 1982). Common species are listed in
perennial grassland to the exotic annual grass- Table 1.
land present today. Bicknell et al. (1992, 1993) The Sequioadendron giganteum (giant sequoia)
used soil phytolith concentration to map the stands of the park are located in two groves,
extent of prehistoric grasslands at several state designated the North and South Groves. In the
parks in coastal California, finding many cur- never-logged South Grove, S. giganteum com-
rently forested areas were grasslands prior to prises 65.9% of the total basal area, followed by
European settlement and many current grass- A. coneolor at 21.7% (Walfoort and Hunt 1982).
lands were previously forested. In northern Grass cover is very sparse throughout the grove,
Arizona, the percent dry weight of phytoliths in with maximum 5% cover in patches near the top
the soil and the relative percent of morphotypes ofthe watershed's north ridge.
in the soil phytolith assemblagewere analyzed for
a P. ponderosa forest with a bunchgrass un-
derstory to determine the long-term stability of Phytolith Reference Collection
the current vegetation (Kerns 1999, 2001; Kerns
et al. 2001, 2003). High concentrations of pine To better understand the soil phytolith assem-
and grass phytoliths in soil surface and sub- blage, a phytolith reference collection, composed
surface samples indicated little change over time. ofphytoliths extracted from leafsamples from all
All of these studies have indicated that if there major plant species currently present in the study
was substantial prehistoric grass cover on a site, area (Table 1), was assembled. Phytoliths were
there will be a high concentration of grass extracted using a modified dry ashing technique
phytoliths in the soil. (Piperno 2006; Pearsall 2000; Kondo et al. 1994).
Samples were washed in distilled water to remove
adhering minerals, dried at 60 C and weighed,
Methods rinsed in 10% HCl to improve combustion, ashed
in a muffie furnace at 550 C for4 hr, rinsed again
Site Description in 10% HCl to remove acid-soluble residue,
washed 3 times in water, dried in ethanol, and
The study area is located in the South Grove the dry residue weighed. To view phytoliths, dry
Natural Preserve of Calaveras Big Trees State residue was resuspended in immersion oil so that
Park, 200 km east of Stockton, California phytoliths could be rotated and three-dimension-
(Fig. 1). The park is 2625 ha with an elevation al characteristics observed, mounted on a shde,
m
range from 1000 to 1700 m. Topography is and viewed under a phase-contrast light micro-
dissected, with a western slope overall, and NE- scope at 400X. Two slides for each plant were
SW
trending ridges between steep river canyons. scanned completely and potentially diagnostic
The climate of the western Sierra Nevada is phytolith types noted.
MADRONO
354 [Vol. 53
Fig. 1. Location of Calaveras Big Trees State Park and the South Grove Natural Preserve, georeferenced
UTM.
in
2006] EVETT ET AL.: CALAVERAS BIG TREES PHYTOLITHS 355
N
A' 285 570 2,280
[Meters
Fig. 2. Location of phytolith sampling plots in the South Grove Natural Preserve, Calaveras Big Trees State
Park, georeferenced in UTM.
Soil Phytolith Extraction particularly but not exclusively in locations
currently occupied by giant sequoia. To assess
Soil samples were collected froin soil pits dug within plot variation in soil phytoliths, two pits
m
on 14 plots (Fig. 2). Plots were selected to span within 100 ofeach otherweredug and sampled
the topographic gradient of the watershed, at each plot. Phytolith samples from both pits
MADRONO
356 [Vol. 53
Table 1. Plant Species Present in Calaveras Big Trees State Park, Sampled for the Phytolith
Reference Collection, with Leaf Silica Percentage by Weight and Observed Diagnostic
Phytolith Morphotypes.
Silica Diagnostic
Species Family Common Name (%) Morphotypes
Trees
Abies coiK'o/or (Gordon and Glend.) Pinaceae white iir A(J.1I none
Lindley
Ac€i' ))ioco}'p/i}'lliif7i Pursh Aceraceae big-leat maple U.J none
AInns rnomoijolia Nutt. Betulaceae white alder <0.1 none
Calocednis decurrens (lorrey) Morin Cupressaceae incense cedar <0.1 none
Cornusnuttcillii Audubon Cornaceae dogwood <0.1 none
Corylus coniuta Marsh var. californica Betulaceae Calilornia hazelnut A 1 none
(A. DC.) W. Sharp
Pinus lamherticma Douglas Pinaceae sugar pine <0.1 none
Piiiuspoiidci'osci Laws. Pinaceae ponderosa pine 0.5 spiny bodies, spiny
rods
Psendotsngci nicnziesii(Mirbel) Franco Pinaceae Douglas-fir 0.4 none
var. menziesii
Quercus kelloggii Newb. Fagaceae black oak 0.3 none
Sequoiadendron giganteum (Lindley) Taxodiaceae giant sequoia 0.1 none
Buchholz
Shrubs
Arctostaphylospatula Greene Ericaceae green-leaf <0.1 none
manzanita
Cecmothus cordiilatus Kellogg Rhamnaceae mountain 0.6 none
whitethorn
Ccciiiot/7iiis lutsgeii'iuius rUiook1. &P AArn. Rhamnaceae deer brush <^UA.1I none
ChciDiaehatiafoliolosa Benth. Rosaceae mountain misery 0.4 none
Chimaphila umbellata (L.) Bartram Ericaceae pipsissewa 0.1 none
Chrysolepissempervirens (Kellogg) Fagaceae bush chinquapin 0.1 none
Hjelmq.
Rhododendron Occidentale Ericaceae western azalea <0.1 none
(Torrey & A. Gray) A. Gray
Rosapinetorwn A.A. Heller Rosaceae pine rose 0.4 none
RubusparviJJonis Nutt. Rosaceae thimbleberry <0.1 none
Ribes roezlii Kegel Grossulariaceae Sierra gooseberry 0.2 none
Symp/wricarpiis mollis Nutt. Caprifoliaceae snowberry <0.1 none
Herbs
Adenocaulon bicolor Hook. Asteraceae trail plant 0.1 none
Aquilegiafonnosa Fischer Ranunculaceae crimson columbine 0.1 none
Asarum hartwegii S. Watson Aristolochiaceae wild-ginger <0.1 none
Carex sp. Cyperaceae sedge 4.3 conical cells
\^litllOrtttl Uf11/lota ^odlLlllCc>7 IVUlllll UllUCa UDIIIICL none
Fragaria vesca L. Rosaceae wood strawberry v.z none
Galium aparitie L. IVLiUldCCtlC goose grass Z.J
Hieracium albiflonmi Hook. Asteraceae hawkweed 0.2 none
Iris hartwegii Baker Iridaceae Hartweg's iris 0.2 none
Lonicera involucrata (Richardson) Banks Caprifoliaceae twinberry <0.1 none
Lupinus sp. Fabaceae lupine 0.1 none
Phacelia sp. Hydrophyllaceae phacelia 0.3 none
Pyrolapicta Smith Ericaceae white-veined 0.1 none
shinleaf
Stnilacina racemosa (L.) Link Liliaceae falseSolomon'sseal <0.1 none
Trientalis latifolia Hook. Primulaceae starflower <0.1 none
Viola lobata Benth. Violaceae pine violet <0.1 none
Grasses short cells, trichomes.
bulliforms
Achnatherum letntnonii(Vasey) Poaceae Lemmon's stipa 5.1 bilobate short cells
Barkworth
Broinus carinatus Hook. & Arn. Poaceae California brome 4.5 rondel short cells
Danthonia unispicata (Thurber) Poaceae one-spike oatgrass 1.9 bilobate short cells
Macoun
Elytnus elymoides (Raf.) Swezey Poaceae squirreltail 13.5 rondel short cells
Festuca rubra L. Poaceae red fescue 3.4 rondel short cells
Melica aristata Bolander Poaceae awned melic 3.2 bilobate short cells
Poa secunda J.S. Prcsl Poaceae one-sided bluegrass 6.1 rondel short cells
2006] EVETT ET AL.: CALAVERAS BIG TREES PHYTOLITHS 357
Table 2. Soil Phytolith Weight as a Percentage of Soil Weight and Percentage of Diagnostic
Grass Phytolithsin Soil Phytolith Assemblagein Relationto Soil Depthand Topographic Position
IN THE South Grove Natural Preserve, Calaveras Big Trees State Park.
yjyc t-value 9j% t-value
Soil Depth Sou Pnytolitn Confidence Grass Coniidence
Topographic Position (cm) Weight (%) Interval Phytoliths (%) Interval
Hillside plots (N=9) Surface 0.08 0.06-0.10 21 14-28
10 0.04 0.03-0.05 17 12-22
25 0.02 0.01-0.03 26 17-35
40 <0.01
55 <0.01
Ridge-top PIPO plots Surface 0.47 0.35—0.59 12 5-19
(N=6) 10 0.36 0.24^0.48 12 10-14
25 0.37 0.25-0.49 15 10-20
40 0.32 0.15-0.49 11 1-21
55 0.18 0.00-0.38 15 8-22
Lower channel-side plots Surface 0.31 0.25 0.37 43 37-49
(N=4) 10 0.52 0.38-0.66 31 20-42
25 0.53 0.39-0.67 38 30-46
40 0.44 0.33-0.55 35 18-52
55 0.43 0.35-0.51 42 28-56
were processed and counted for plots that had other plants in the reference collection, including
>0T0% soil phytoliths; otherwise, samples from simple trichomes, buUiforms, three types of long
only one pit were processed. Soil samples were cells, and short cells) out of total phytoliths in
collected from each pit at five depths, each depth each sample was calculated.
correspondingto a soil horizonbecause soilswere
similar throughout the site: mineral soil surface Results
(A horizon), 10 cm (upper B horizon), 25 cm
(middle B horizon), 40 cm (lower B horizon), and Extracted reference material from most plant
55 cm (C horizon). Because the soil horizon species contained small amounts of poorly
generally corresponded with soil depth, soil silicified biogenic silica but no diagnostic phyto-
phytolith data from each plot were aggregated liths (Table 1). Pinusponderosa was the only tree
by sample depth, which may be a rough approx- species with diagnostic phytoliths: spiny bodies
imation ofrelative sample age (e.g., Alexandre et and spiny rods (Norgren 1973; Kerns 2001) were
al. 1999), for analysis. isolated from needles. Several tree species pro-
Phytoliths were extracted from five gram soil duced phytolith morphotypes that were distinct
samples by standard procedures: deflocculation from grasses but not diagnostic beyond this level.
in Calgon, HCl wash, organic matter digestion in No shrubs contained diagnosticphytoliths. Carex
30% hydrogen peroxide, two heavy liquid flota- (sedge) species produced non-diagnostic long
tions in zinc bromide solution at specific gravity cells and diagnostic conical phytoliths (Ollendorf
2.30 (a third flotation for several samples pro- 1992). All grasses contained abundant phytoliths,
duced negligible additional phytoliths and was including long cells, trichomes, bulliforms, and
deemed unnecessary), and mounted on sUdes as short cells (Twiss et al. 1969; Twiss 1992).
above (Carbone 1977; Kondo et al. 1994). Achnatherum lemmonii (Vasey) Barkworth,
Because the biogenic silica residue contained Danthonia unispicata (Thurber) Macoun, and
diatoms and residual mineral silica particles, Melica aristata Bolander were the only grasses
phytolith weight for each sample was estimated sampled producing bilobate short cells. Other
by examining 10 microscope fields on a slide, grasses produced abundant rondel short cells.
estimating the percentage ofnon-phytolith mate- Soil phytolith weight averaged less than 0.55%
rial, and subtracting this percentage from the on all plots (Table 2), but varied according to
measured residue weight (Carnelli et al. 2001). topographic position in the watershed. Hillside
Phytolith morphological types were tallied for plots (Fig. 2: Plots 1-7, 11, 12), typical ofmost of
each soil sample until 400 total phytoliths were the sampled watershed, had very low soil
counted. Diagnostic types were tallied individu- phytolith concentration (<0.10%) at all depths.
ally by taxon. Non-diagnostic types were lumped Plots on or near the ridge (Plots 8-10), where P.
together; no attempt was made to identify or tally ponderosa was present in the overstory, contained
each individual non-diagnostic type. The percent- significantly more soil phytoliths (of which 42%
age of diagnostic grass phytoliths (i.e., morpho- were P. ponderosa spiny bodies) at all depths than
types observed in grasses but not observed in hillside plots. Plots in the lower part of the
I
MADRONO
358 [Vol. 53
Table 3. Estimated Percentage of Phytoliths in the Soil beneath Grassland and Forest
Vegetation from Study Sites in North America.
VCgClaIKJIl Csllllld-lUJll
J-jOCdLlOn Type (/C) iVlClIUJU jxCterence
Alberta Grassland 0.83-2.78 20-50 particle count Lutwick and Johnston 1969
Arizona Ponderosa pine- 0.9-1.4 5-250 dry weight Kerns et al. 2001
grassland
Illinois Grassland 0.77-1.23 20-50 particle count Beavers and Stephen 1958
Hardwood forest 0.19-0.28 20-50 particle count Beavers and Stephen 1958
Minnesota Oak forest and 1.54-3.63 5-50 particle count Verma and Rust 1969
grassland
North Oakhardwood and 0.94^0.99 all dry weight Knoepp et al. 1998
Carolina grass
Oregon Ponderosa pine 0.13-0.26 15-100 particle count Witty and Knox 1964
Grassland 1.92-2.89 15-100 particle count Witty and Knox 1964
Pennsylvania Grassland 1.19-1.75 20-50 dry weight Waltman and Ciolkosz 1995
Hardwood forest 0.07-0.33 20-50 dry weight Waltman and Ciolkosz 1995
watershed within <50 m of the stream channel result was the Pseudotsuga menziesii (Mirbel)
(Plots 13, 14), where no P. ponderosa was Franco var. menziesii (Douglas-fir) sample con-
observed in the overstory, also contained signif- tained no diagnostic asterosclereid phytoliths
icantly more soil phytoliths than hillside plots. observed by several others (Brydon et al. 1963;
All plots contained diagnostic grass phytoliths, Norgren 1973; Blinnikov 2005). One explanation
but topographic position influenced grass phyto- (although there is no supporting mineralogical
lith percentages (Table 2). Hillside plots had the data available) is that if the soils on the site are
lowest percentage of grass phytoliths; channel- rich in aluminum and/or iron oxides, silica in
side plots had significantly more grass phytoliths solution available to plants for phytolith forma-
than either of the other topographic positions. tion may be reduced considerably (Beckwith and
The dominant morphotypes comprising the grass Reeve 1963; Jones and Handreck 1965). Other
phytolith percentage were rondel short cells and plausible explanations are the needles collected
several types of elongate cells observed only in were too young to have much silicification or the
grasses in the reference collection. Extremely few asterosclereids may have been present but dam-
bilobate phytoliths (<0.1% on all plots) were aged in processing, broken into pieces that were
found, indicating none ofthe bilobate-producing unrecognizable.
grasses, A. lemmonii, D. imispicata, and M. Grassland soils have been found by various
aristata, had significant prehistoric cover on any researchers to contain at least 5-10 times more
of the plots. All ridge-top plots had >10% P. phytoliths by weight than forest soils, which
ponderosa spiny bodies and spiny rods through- generally have phytolith content between 0.1 and
out the soil profile, indicating long-term presence 0.5% by weight (Table 3). A site in the P.
of this species there. All plots contained sub- ponderosa-hunchgvdLSS vegetation type in north-
stantial percentages ofnon-diagnostic phytoHths, ern Arizona (7-50% grass cover in the understo-
dominated by several non-diagnostic types of ry), probably the closest phytolith analog for
elongate cells, but also including blocky polyhe- Sierra Nevada mixed conifer forests if they had
drals, tracheary elements, epidermal cells, meso- a prehistoric grass understory, showed total
phyll cells, and hair cells (Piperno 2006). Al- phytolith content of 1-1.4% (Kerns 1999; Kerns
though non-diagnostic phytoliths were counted, et al. 2001). Grass phytoliths made up approxi-
they were not quantitatively categorized because mately half of the total, P. ponderosa the other
the reference material suggested these types half(B. Kerns personal communication).
would not provide useful information due to Phytolith concentration on hillside plots in our
considerable redundancy between species. How- study was at the low end ofpublished data, even
ever, there were few obvious differences in the for forest plots. The paucity of phytoliths
assemblage composition of non-diagnostic phy- observed in most plant reference material species
toliths in relation to soil depth or between plots. (Table suggests that very few phytoliths would
Many of these non-diagnostic morphotypes may be depos1i)ted in the soil unless substantial coverof
have been produced by grasses. heavy phytohth producing plants not present on
the plots today {P. ponderosa, grasses, or sedges)
Discussion were present on the plot for a substantial period
of time in the past.
The lack ofdiagnostic phytoliths in most ofthe Ridge-top plots had phytolith concentrations
reference material was in agreement with other within the expected range for forest soils, but
studies of similar species. The only surprising considerably less than found at the P. ponderosa-
2006] EVETT ET AL.: CALAVERAS BIG TREES PHYTOLITHS 359
bunchgrass site in northern Arizona. Finns offine fuel required to carry frequent prehistoric
ponderosa and bunchgrasses are currently minor ground fires in this S. giganteum stand. Because
components of the vegetation on these plots; none ofthe other plant species examined (except
grasses (mostly Bromus ccirinatus Hook. & Arn. for sedges and P. ponderosa) produced diagnostic
and Elymus elymoides (Raf.) Swezey) are present phytoliths, the prehistoric presence of deciduous
with <5% cover. The phytolith evidence suggests trees, shrubs, or other herbaceous species in the
that P. ponderosa and bunchgrasses were pre- understory could not be determined from phyto-
historically present but have remained minor lith evidence. There is the possibility that more
components ofridge-top vegetation. detailed work, beyond the scope of this study,
Lower channel-side plots had higher grass analyzing many more samples of reference
phytolith concentration than expected for forest material from other sites, could identify more
soils, but less than half the concentration typical diagnostic phytoliths and signature phytolith
ofgrassland. There are currently very few grasses assemblages to shed more light on the presence
in this part of the watershed. One possible ofprehistoric understory species.
explanation is that there was a prehistoric Other research supports the conclusion that
stringer meadow associated with the stream that grass was not a substantial component of the
disappeared when the water table dropped due to giant sequoia forest understory. For example,
down-cutting. This interpretation is supported by even under the high levels oflight that exist after
data from phytolith sampUng of a small stringer logging, Biswell et al. (1966c/) found that grass-
meadow in the mixed-conifer forest in the Sierra like plants had less than 10% frequency in giant
San Pedro Martir in Baja California, that had sequoia forests. Phytolith data from a relatively
total phytoHth weight and percentage of grass open, mixed coniferforest in the Sierra San Pedro
phytohths comparable to these plots (R. Evett Martir, Mexico, suggest that grasses may not
et al. In Press). have been a major component of the understory
Phytolith migration and dissolution, alterna- (R. Evett et al. In Press).
tive explanations for low soil phytolith weights, Parsons (1978) found at Redwood Mountain
must be ruled out before the grass understory (approximately 100 km south of Calaveras Big
m
hypothesis can be rejected. Because they are Trees State Park and 550 higherelevation) that
mostly silt-sized particles, phytoliths are known forest floor fuel loads following the first pre-
to migrate downward in the soil profile, partic- scribed fire in a S. giganteum stand were similar
ularly in sandy soils (Starna and Kane 1983). to unburned stands 7 yr following burning;
Sampling at several depths in soil pits revealed no however, he attributed much of this fuel to
evidence of substantially fewer phytoliths at the material killed by the prescribed fire from A.
surface or accumulation of phytoliths at any concolor that had invaded the giant sequoia
depth (Table 2). Phytolith degradation and dis- stands during the past 100 yr when fire was
solution is known to occur under alkaline soil absent. Parsons predicted that a second pre-
conditions but only under the most severe acidic scribed fire would remove this fuel source; it was
conditions (Piperno 2006). The soil type sampled, unclear where additional fuel to support a short
the McCarthy series, is sHghtly acidic (pH 6.3- fire interval would come from. This is in contrast
6.4) (Walfoort and Hunt 1982), which provides to data from (Biswell 1966/?), who found that S.
ideal conditions for soil phytolith preservation. giganteum stands produced large amounts of
Phytolith migration and dissolution are not litter each year, —9000 lbs/acre.
plausible explanations for the lack of grass It is possible that deciduous hardwoods co-
phytoliths in South Grove soils. occurring with 5. giganteum provided sufficient
In the absence of phytolith migration, it is surface fuelsprior to the mid-1800's to achieve an
reasonable to expect increasing phytolith age and average five year fire interval. As a result of
decreasing phytolith concentration with increas- livestock grazing and the altered fire situation for
ing soil depth (Alexandre et al. 1999; Piperno the past 150 yr, hardwoods such as Quereus
2006). Although no dating was attempted (and kel/oggii (black oak) are now uncommon in S.
would be difficult to interpret because of giganteum stands (Biswell et al. 1966c/; Bonnick-
bioturbation issues), phytoliths from each sam- sen and Stone 1982). Further research is neces-
pled soil profile probably span at least the past sary to investigate the possibility ofhardwoods as
1500 yr, the age ofthe oldest living S. giganteum a fuel source.
in the stand, but more likely several thousand Parsons (1995) stated that fire in giant sequoia
years. stands stimulated shrub and hardwood growth.
The phytolith evidence indicates that grasses, The patchy nature ofnatural fires, with irregular
while certainly present, have not been a major shapes and varying rate of spread, and role of
component ofthe S. giganteum forest understory occasional intense fires (Stephenson et al. 1991)
in the South Grove Natural Preserve. We provides opportunity for shrubs to provide
conclude that grass was not likely to have been a significant amount of fuel. Common shrubs in
present with enough cover to be a major source the vicinity of giant sequoia stands include
MADRONO
360 [Vol. 53
Chcmiaebatiafoliolosa Benth. (mountain misery), (Miller and Urban 1999). According to the
Ceanothus cuneatus (Hook.) Nutt. (buckbrush), model, a five year accumulation of S. giganteum
Ceanothus integerrimus Hook. & Arn. (deer- litter should be enough to carry at least a patchy
brush), and Chrysolepis sempervirens (Kellogg) fire, but perhaps only under more extreme |
Hjelmq. (bush chinquapin). Ofthese, C. foliolosa weather conditions (low humidity, high temper-
seldom occurs in S. giganteum stands and would ature, high wind) that are not always accompa-
be displaced by frequent burning. Ceanothus nied by an ignition source. Many ofthe fires seen
species are known to invade areas following in the tree-ring fire-scar record may have been
disturbance but have never been commonly intentionally set by humans whenever extreme j
found within S. giganteum stands. C. sempervi- conditions were present, without concern for
rens burns well but is typically only dense on catastrophic fires because they knew the intensity
north-facing slopes. Because phytolith analysis would be very low. Because of safety concerns,
I
provides no information on these shrubs, re- prescribed burning typically occurs during peri-
solving the question of their abundance in S. ods with higher levels of fuel moisture when
giganteum stands remains problematic. Ceanothus effective fuel connectivity is much lower. Because
eordulatus Kellogg and Chamaebatia foliolosa, considerably more fuel accumulation is required
although lacking in diagnostic phytoliths, pro- to carry an extensive prescribed fire, a longer fire
duce enough biogenic silica (Table 1) to contrib- return interval would be expected.
ute considerable quantities of phytoliths to the Given the short prehistoric fire return interval
soil. Further study oftheir phytolith assemblages and more widely spaced trees, litter build-up was
in reference material may eventually produce probably very uneven on the forest floor, leading |
a soil phytolith assemblage signature for these to highly patchy, low intensity fires. This has ,
two shrubs. been observed in mixed conifer forests in the \
There also are several alternative explana- Sierra San Pedro Martir, where forest structure
tions, not requiring post-European settlement remains relatively open because of a continuing
changes in the forest understory, which could fire regime (Stephens et al. 2003). The mean fire
account for the lack of success achieving fire return interval, based on the frequency of |
return intervals approaching five years with occurrence of all fires on a sampled site, may be I'
prescribed burning at the South Grove Natural misleading if there are frequent, small fires that
Preserve. The simplest explanation is the five year burn very little area; burning every part of the
estimate is based on a composite of 2000 yr of stand may take considerably longer. A 25%
tree-ring fire-scar data from five S. giganteum composite mean fire return interval, calculated
groves along a 160 km transect in the Sierra from fires scarring >25% ofthe recordingtrees in
Nevada, with substantial variation between a stand, may be more useful to estimate the
groves and substantial variation at decadal and frequency of widespread fires in a highly patchy i
centennial scales correlated with fluctuating environment (Dieterich 1980; Swetnam and
climate (Swetnam 1993). The South Grove Baisan 2003). Another better measure in this
Natural Preserve is further north than any of patchy situation may be the fire rotation period,
the sampled groves and may be more mesic, while defined as the average length of time between
the current climate of the region may also be fires averaged for each fire-scar recording tree
more mesic than average (Cook et al. 2004, Grid (Baker and Ehle 2001). This gives an estimate of '
47), leading to a longer than averageexpected fire the maximum time required to burn an area the
i
return interval. Also, ifvery small fires (recorded size of the stand, and may more accurately
by only a single tree) are excluded, the mean fire represent temporal burning patterns in S. gigan- ,'
return interval for three of the five S. giganteum teum stands. Expecting a prescribed fire to burn
groves sampled is close to 10 yr (Swetnam et al. through an entire stand every five years is
1992). probably unrealistic. Mean fire return intervals
i
Miller and Urban (1999), based on a model of estimated using the 25% filter or fire rotation
the surface fire regime in mid-elevation conifer- period would be considerably longer than five ]
ous forests of the Sierra Nevada, suggested the years and probably more indicative of the time '
fire return interval depends on forest floor fuel required to burn the entire stand. The currently
load and fuel moisture. Their model indicated observed 15-20 yr period required for enough |
that even without a grass understory, there may fuel to accumulate between prescribed fires given |'
be sufficient fuel accumulation on the forest floor the present climate may be representative of the
:
to support a five year or less fire return interval fire regime under a similar climate in the past.
underconditions ofvery low(<5%) fuel moisture The management goal for Calaveras Big Trees
(Miller and Urban 2000). Their simulations State Park is to ensure natural processes are the |
showed that under average climate conditions, principal influence on S. giganteum stands so that
with no grasses or shrubs in the understory, the large, exceptional specimens can grow to matu- j
mean fire return interval at the elevation of the rity and the species can regenerate naturally,
South Grove Natural Preserve should be ~6 yr Current forest conditions, because ofpast logging j
i
f
